New Plasmonic Photocatalysts for Fine Organic Synthesis · 2018-03-18 · New Plasmonic...

New Plasmonic Photocatalysts for Fine

Organic Synthesis

Submitted in fulfilment of the requirements for the degree

of

Doctor of Philosophy

Yiming Huang

B. Eng.; M. A. Sci.

School of Chemistry, Physics and Mechanical Engineering

Queensland University of Technology

2018

New Plasmonic Photocatalysts for Fine Organic Synthesis I

Keywords

Metallic photocatalysis; LSPR effect; Plasmonic metal; Visible-light; Gold nanoparticles;

Gold-Palladium Alloy; Air-stable copper nanoparticles, Organic synthesis; Unsaturated

Aromatic Hydrogenation; Aromatic alcohol oxidation; Epoxidation; Action spectra;

Photoexcited electron; Hybridised molecular orbitals

New Plasmonic Photocatalysts for Fine Organic Synthesis

II

Abstract

Photocatalysis is a rapidly growing research field aiming to direct utilisation of abundant,

non-polluting and renewable solar energy, preferably in the visible light range, as a driven force

to trigger organic synthesis. Metallic photocatalysis is a new class of photocatalysis which has

been developed in the last decade. Nanoparticles of transition metal nanoparticles (Gold and

Copper) as well as their alloy nanoparticles were found strongly adsorbing light energy within

the visible light range owing to the LSPR effect, and thus these three transition metals were

labelled as plasmonic metals. The unique optical property of plasmonic metals allows them to

harvest and transfer photonic energy into chemical energy, which is driving increasing research

interest in their application for the synthesis of fine chemicals. Challenges lie in the

development of new metallic photocatalysts with high catalytic efficiency for important

organic synthesis reactions. As a result, this thesis focuses on three types of metallic

photocatalysts and their applications in important organic synthesis reactions as well as

mechanism studies.

In the first part of this thesis, a new reaction system utilising supported Au nanoparticles

(NPs) was developed for the selective hydrogenation of C=C, C≡C, C=O, N=O and C=N bonds

in the presence of the aromatic ring. Reaction systems were designed with green chemistry

principles that operated in an aqueous solution under mild reaction conditions, and formic acid

was applied as the reductive agent without further additives. The reaction system exhibited

excellent photocatalytic activity with high substituent tolerance. The reaction selectivity is

tuneable by manipulating the incident light wavelength. The mechanism study revealed that

the LSPR induced photoexcited electrons are responsible for the reaction activity. Furthermore,

the cooperation of formic acid and water in the reaction system was investigated using the


III

isotope techniques. The result indicated that water participated in the reaction and acting as a

hydrogen source. Thus, a water related hydrogen evolution route was proposed, such theory

could potentially be beneficial to future researchers.

In order to further develop the photocatalytic applications in organic synthesis, Au was

alloyed with another metal forming an alloy NP photocatalyst. Palladium (Pd) was selected

due to its excellent catalytic activity for a wide range of organic reactions, thus alloying Pd

with Au to form a new type of photocatalyst could enlarge application range of photocatalysis

in organic synthesis. In the alloy photocatalytic system, Au predominantly played the role of

the light harvesting site while Pd is the main catalytic active site. The high photocatalytic

activity of Au-Pd alloy NP was observed for dehydrogenation of aromatic alcohols to

corresponding aldehydes at ambient temperatures under visible light irradiation. The molar

ratio of Au to Pd was found to be critical to the photocatalytic performance, and further

theoretical simulations suggested a surface charge heterogeneity in the alloy NPs owing to the

charge re-distribution between Au and Pd atoms. The DFT simulation results made a

compelling case that the catalytic performance is coupled with surface charge heterogeneity

which is determined by Au to Pd ratio. Moreover, the experimentally optimised Au to Pd ratio

in terms of best photocatalytic performance is in good agreement with the theoretical

simulation, and such agreement confirmed the critical role of surface charge heterogeneity in

the alloy NP based photocatalysis. This work not only presents a photolytic dehydrogenation

reaction system over Au-Pd alloy nanoparticle photocatalyst but also provides useful

information for the future design of plasmonic-transition metal alloy photocatalyst.

In addition to Au metal, the other two plasmonic metals (Ag and Cu) are promising

candidates for organic synthesis applications. Cu, in particular, is known to be a versatile

catalyst, yet it is the least studied plasmonic metal for photocatalysis due to the instability of


IV

Cu NPs in an oxidative environment. In this study, it has been demonstrated that titanium

nitride (TiN) support material can effectively prevent the attached Cu NPs from being oxidised

by air and therefore demonstrate the first air-stable TiN supported Cu photocatalyst. This

stability may be attributed to the significant charge exchange between Cu NPs and TiN support,

which was revealed by density functional theory (DFT) calculation. Selective epoxidation of

alkenes to corresponding epoxides is a class of important but difficult reaction because the

alkenes can be easily over oxidised to aldehydes. The air stable Cu photocatalyst was

successfully applied in the selective epoxidation of alkenes using molecular oxygen (O2) under

mild reaction conditions and exhibiting good to high photocatalytic activity with excellent

product selectivity. The Cu NPs were found to mostly remain in the metallic state after seven

reaction cycles and the recovered photocatalyst was easily reactivated without significant loss

of activity or selectivity using hydrogenation gas reductive treatment. In this work, the

plasmonic catalysis has been extending into the previously unachievable use of readily oxidised

metals. The stabilisation strategy could provide a solution for practical applications of many

other non-precious metal nanoparticles.

Finally, a mechanistic physical chemistry study was performed for metallic

photocatalysis to foster better scientific understanding. The action spectra of several organic

reactions were investigated using several different types of metallic photocatalysts. By

analysing the different trend in the action spectra, a photon energy threshold was proposed

existing in metallic photocatalysis. A photocatalytic chemical reaction can only be triggered,

when the energy level of the incident photon is higher than the energy level of the lowest

unoccupied molecular orbital. The predominant role of direct photo-electron excitation in

metallic photocatalysis is demonstrated which provide an answer to a debate in the metallic


V

photocatalysis research that whether photoexcitation or photothermal is the main reason for

metallic photocatalysis.

In summary, this thesis shows three new photocatalytic systems with a different type of

metals to expand the applications of metallic photocatalysis in organic synthesis. A mechanistic

study for general metallic photocatalysis was performed illuminating the energy transfer and

evolution between light, metal NPs and the organic molecules. Lastly, the perspective is

expressed to a greener process of organic synthesis via metallic photocatalysis based.


VI

List of Publications

Publications presented in this Thesis:

(1) Huang, Y.; Liu, Z.; Gao, G.; Xiao, G.; Du, A.; Bottle, S.; Sarina, S.; Zhu, H. Stable Copper

Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible Light. ACS

Catalysis, 2017, 7, 4975-4985.

(2) Huang, Y.; Liu, Z.; Gao, G.; Xiao, Q.; Martens, W.; Du, A.; Sarina, S.; Guo, C.; Zhu, H.

Visible light-driven selective hydrogenation of unsaturated aromatics in an aqueous solution

by direct photocatalysis of Au nanoparticles. Catalysis Science & Technology, 2018, 8, 726-

734.

(3) Sarina, S.; Jaatinen, E. A.; Xiao, Q.; Huang, Y.; Christopher, P.; Zhao, J.; Zhu, H., Photon

Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key Evidence from

Action Spectrum of the Reaction. The Journal of Physical Chemistry Letters, 2017, 8, 2526-

2534.

(4) Sarina, S.; Bai, S.; Huang, Y.; Chen, C.; Jia, J.; Jaatinen, E.; Ayoko, G. A.; Bao, Z.; Zhu,

H., Visible light enhanced oxidant free dehydrogenation of aromatic alcohols using Au/Pd

alloy nanoparticle catalysts. Green Chemistry, 2014, 16, 331-341.

Publications not presented in this thesis:

(1) Zhao, J.; Zheng, Z.; Bottle, S.; Chen, C.; Huang, Y.; Sarina, S.; Chou, A.; Zhu, H., Factors

influencing the photocatalytic hydroamination of alkynes with anilines catalyzed by

supported gold nanoparticles under visible light irradiation. RSC Advances, 2016, 6, 31717-

31725.


VII

(2) Zhao, J.; Ke, X.; Liu, H.; Huang, Y.; Chen, C.; Bo, A.; Sheng, X.; Zhu, H., Comparing the

Contribution of Visible-Light Irradiation, Gold Nanoparticles, and Titania Supports in

Photocatalytic Nitroaromatic Coupling and Aromatic Alcohol Oxidation. Particle & Particle

Systems Characterization, 2016, 33, 628-634.

(3) Zavahir, S.; Xiao, Q.; Sarina, S.; Zhao, J.; Bottle, S.; Wellard, M.; Jia, J.; Jing, L.; Huang,

Y.; Blinco, J. P.; Wu, H.; Zhu, H.-Y., Selective Oxidation of Aliphatic Alcohols using

Molecular Oxygen at Ambient Temperature: Mixed-Valence Vanadium Oxide

Photocatalysts. ACS Catalysis, 2016, 6, 3580-3588.

(4) Tana, T.; Guo, X.-W.; Xiao, Q.; Huang, Y.; Sarina, S.; Christopher, P.; Jia, J.; Wu, H.; Zhu,

H., Non-plasmonic metal nanoparticles as visible light photocatalysts for the selective

oxidation of aliphatic alcohols with molecular oxygen at near ambient conditions. Chem.

Commun., 2016, 52, 11567-11570.

(5) Liu, Z.; Huang, Y.; Xiao, Q.; Zhu, H., Selective reduction of nitroaromatics to azoxy

compounds on supported Ag-Cu alloy nanoparticles through visible light irradiation. Green

Chemistry, 2016, 18, 817-825.

(6) Xiao, Q.; Sarina, S.; Bo, A.; Jia, J.; Liu, H.; Arnold, D. P.; Huang, Y.; Wu, H.; Zhu, H.,

Visible Light-Driven Cross-Coupling Reactions at Lower Temperatures Using a

Photocatalyst of Palladium and Gold Alloy Nanoparticles. ACS Catalysis, 2014, 4, 1725-

1734.

(7) Sarina, S.; Zhu, H.-Y.; Xiao, Q.; Jaatinen, E.; Jia, J.; Huang, Y.; Zheng, Z.; Wu, H., Viable

Photocatalysts under Solar-Spectrum Irradiation: Nonplasmonic Metal Nanoparticles.

Angewandte Chemie International Edition, 2014, 53, 2935-2940.


VIII

Conferences and Presentations

(1) Oral presentation: Pacifichem 2015, Hawaii, Dec. 2015.

Presentation title: Novel Photocatalysed Aqueous Hydrogenation System for Unsaturated

Aromatics using Formic Acid.

(2) Oral presentation: NMS HDR symposium, Brisbane, Feb. 2015.

Presentation title: Supported Metallic Photocatalysts for Organic Synthesis.

(3) Oral presentation: 8th Pacific Symposium on Radical Chemistry (PSRC-8), Brisbane,

July 2017.

Presentation tile: Free Radical Induced Selective Oxidation of Alkyl Aromatics under

Sunlight


IX

Table of Contents

Keywords ................................................................................................................................... I

Abstract ..................................................................................................................................... II

List of Publications .................................................................................................................. VI

Table of Contents ..................................................................................................................... IX

List of Figures .......................................................................................................................... XI

List of Abbreviations ............................................................................................................. XII

Statement of Original Authorship ......................................................................................... XIII

Acknowledgements ............................................................................................................... XIV

Chapter 1 Introduction .......................................................................................................... - 1 -

Research Background .................................................................................................. - 2 -

Research Problems ...................................................................................................... - 5 -

Research Objectives and Aims .................................................................................... - 7 -

Significance and Contribution to the Knowledge Base .............................................. - 9 -

Thesis Outline and Linkage between Chapters ......................................................... - 11 -

Chapter 2 Literature Review ............................................................................................... - 12 -

Early Study of Photochemistry and Photocatalysis ................................................... - 12 -

LSPR Effect and Plasmonic Metallic Photocatalyst ................................................. - 13 -

Gold NPs Photocatalyst for Organic Synthesis ......................................................... - 19 -

2.3.1 Introduction to gold NPs Photocatalyst .............................................................. - 19 -

2.3.2 Au NPs Photocatalysed Organic synthesis ......................................................... - 20 -

2.3.3 Hydrogenation of unsaturated aromatics. ........................................................... - 22 -

Au-Pd alloy NPs Photocatalysed Oxidation of Aromatic Alcohols .......................... - 23 -

2.4.1 Introduction to Au-Pd alloy NPs Photocatalyst .................................................. - 23 -

2.4.2 Au-Pd alloy NPs photocatalysed organic synthesis reactions ............................ - 23 -


X

2.4.3 Oxidation of Aromatic Alcohols ........................................................................ - 25 -

Stabilised Copper NPs Photocatalyst ........................................................................ - 26 -

2.5.1 Introduction to Cu NPs photocatalyst ................................................................. - 26 -

Reference ............................................................................................................................ - 29 -

Chapter 3 Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous

Solution over Supported Au Nanoparticle under Visible Light.......................................... - 42 -

Introductory Remarks ................................................................................................ - 42 -

Article 1 ..................................................................................................................... - 44 -

Chapter 4 Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy

Nanoparticle Catalysts ........................................................................................................ - 79 -

Introductory Remarks ................................................................................................ - 79 -

Article 2 ..................................................................................................................... - 81 -

Chapter 5 Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper

Nanoparticles as Visible Light Photocatalysts .................................................................. - 125 -

Introductory Remarks .............................................................................................. - 125 -

Article 3 ................................................................................................................... - 127 -

Chapter 6 In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst ..... - 222 -

Introductory Remarks .............................................................................................. - 222 -

Article 4 ................................................................................................................... - 224 -

Chapter 7 Conclusions and Future Perspective ................................................................ - 259 -

Conclusions ............................................................................................................. - 259 -

Future Perspective ................................................................................................... - 263 -

Bibliography ..................................................................................................................... - 265 -


XI

List of Figures

Figure 2-1. Mechanism of a semiconductor photocatalysis. .............................................. - 13 -

Figure 2-2. The Localised Surface Plasmon Resonance (LSPR) effect. ........................... - 14 -

Figure 2-3. (a) Metal nanoparticles absorbing light from its vicinity area owing to the

surrounded electric field of metal nanoparticles, (B) the demonstration of extinction cross

section of an isolated metal nanoparticle at LSPR wavelength. ......................................... - 15 -

Figure 2-4. Schematic illustration of three surface plasmon dephasing processes. ........... - 16 -

Figure 2-5. Schematic of photoexcitation of adsorbate-metal complexes on metal NPs

surface. ................................................................................................................................ - 18 -

Figure 2-6. UV-Vis absorption spectrum of plasmonic metal nanoparticles. .................... - 19 -

Figure 2-7. The diagram of band structures of supported Au-NPs and the proposed

mechanism for photocatalysis using supported Au-NPs. ................................................... - 20 -

Figure 2-8. The mechanism for the photocatalytic reduction of nitroaromatic compounds. - 21

-

Figure 2-9. Reaction mechanism for Au-Pd alloy NP photocatalysed Suzuki reaction. ... - 24 -

Figure 2-10. One pot synthesis of ester from aliphatic alcohols. ....................................... - 25 -

Figure 2-11 TEM images (A and B; the scale bars are 30 and 10 nm respectively; the scale

bar in the inset of (B) is 2 nm) and XPS profile(C) of 5wt% Cu/graphene, and UV/Vis

absorption spectra of Cu/graphene photocatalysts with different Cu loadings (D). ........... - 27 -


XII

List of Abbreviations

CID Chemical Interface Damping

DFT Density Function Theory

FDTD Finite Difference Time-Domain

HOMO Highest Occupied Molecular Orbital

LSPR Localised Surface Plasmon Resonance

LUMO Lowest Unoccupied Molecular Orbital

NPs Nanoparticles

XPS X-ray Photoelectron Spectroscopy


XIII

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any higher education institution. To the best of my

knowledge and belief, the thesis contains no material previously published or written by

another person except where due reference is made.

Signature:

Date: February 2018

QUT Verified Signature


XIV

Acknowledgements

Here I would like to express my sincere gratitude to the following persons and

organisations that accompanied my PhD study.

First and foremost, to my principle supervisors: Prof. Huaiyong Zhu for his far-sighted

academic guidance, financial support and patience throughout the past few years. My progress

would never have been achieved without his drive and warm encouragement. The methodology

of doing research, he passed to me is the precious gift that would benefit my whole future

academic career. It was my privilege to complete PhD study under his supervision.

I would like to deeply thank my associate supervisor Prof. Steven Bottle, for his

excellent guidance, patience and valuable criticism towards the completion of my research and

publication.

Special thanks to senior colleagues from our research group: Dr Zhanfeng Zheng, Dr

Sarina Sarina, Dr Xingguang Zhang, Dr Jian Zhao, Dr Qi Xiao and Dr Sifani Zavahir for their

willing support and suggestions.

My gratefulness also goes to associate Prof. Aijun Du from QUT and his PhD student

Mr Guoping Gao for providing kind assistance to my research and paper editing.

I am grateful for the technical support from Central Analytic Research Facility (CARF)

of QUT and their friendly and professional technicians: Dr Jamie Riches, Dr Peter Hines, Dr

Sanjleena Singh, Mr Tony Raftery and Dr Hui Diao. Many thanks to the technicians who run

our research lab and common analytic instruments: Dr Lauren Butler, Mr Peter Hegarty, Mrs

Leonora Nebwby and the late Dr Chris Carvalho for the supportive environment they created.

I also offer gratitude to Prof. Godwin Ayoko, A/Prof. Esa Jaatinen, Dr Wayde Martens,

Dr Hongwei Liu and collaborators outside QUT: Prof. Chen Guo, Dr Gang Xiao for their

assistance in data analysis and paper revisions.

Thank you to my dear fellow HDR students: Arixin Bo, Gallage Sunari Peiris, Zhe Liu,

Pengfei Han, Erandi Sakunthala Peiris Prangige, Xiayan Wu, Chenhui Han, Tana Tana, Fan


XV

Wang for your support and the friendly atmosphere you created in the daily research

environment.

Lastly, the greatest gratitude to my parents for their love which has been accompanying

me throughout the good and bad days of this journey.

Introduction - 1 -

Chapter 1 Introduction

This thesis focuses on the fabrication of new plasmonic metal nanoparticle photocatalysts

and their applications in visible-light driving photocatalytic fine organic synthesis and the

accompanying photoexcitation mechanism.

This chapter briefly outlines the Research Backgrounds (Section 1.1); Research Problems

(Section 1.2); Research Objectives and Aims (Section 1.3) and Research Contributions to

Knowledge (Section 1.4). The linkage chart for this thesis is outlined in Section 1.5.

Introduction - 2 -

Research Background

Chemistry is one of the few essential pillars of the science and industry with wide

influence in almost every aspect of modern society. Wide applications of chemistry are

associated with the great energy consumption. According to data from U.S. energy information

administration, the energy consumption in the chemical industry accounts for 29% out of all

industrial manufacturing and only 15.17% of the energy consumed originated from the

renewable source.[1] The traditional approach to chemicals synthesise is thermal activation,

which is normally associated with the consumption of fossil fuel.[2] Thus, a long standing

challenge in chemistry and chemical engineering is to drive chemical transformations with

green energy sources: clean, renewable and ideally abundant.[3]

Solar energy is an abundant, non-polluting and renewable energy source and direct use

of solar energy to initiate chemical reactions is a promising research field that seeks to

eventually employ solar energy in the chemical industry. Long before chemists introduced solar

energy into their labs, photosynthesis was recognised as a light induced chemical process. In

general, plants, bacteria and other organisms convert light energy into chemical energy and

generates carbohydrate and oxygen from carbon oxide and water. Such a process is so powerful

that it helped shape the nature before the human race.[4]

Since the first introduction of photochemistry in 1912 by Giacomo Ciamician, [5] a door

was opened to the direct utilisation of light energy in the organic synthesis, the organic

molecules were found activated by short wavelength UV light. In 1972, Fujishima and Honda’s

reported a representative photocatalysis work over semiconductor photocatalyst. They reported

a water splitting over TiO2 photocatalyst under UV irradiation.[6] Great efforts have been made

in the semiconductor photocatalysis ever since.[7-10] However, the drawbacks of semiconductor

photocatalysts are the low quantum efficiency and low thermal resistance. Additionally, the

Introduction - 3 -

affinity of semiconductors to many organic molecules is relatively weak, which results in the

limited application in organic synthesis.[2] Regarding the drawbacks of conventional

photochemistry and semiconductor photocatalysis, the developments of new photocatalysts

which can efficiently utilize visible light and catalyse various organic reactions are in demand.

Gold metal nanoparticles were found to exhibit non-linear optical properties and the UV-

Vis spectrum of Au nanoparticles (NPs) suggests a strong light absorption in the visible and

near UV range which peaked at around 500-600 nm due to the localised surface plasmon

resonance (LSPR) effect.[11-13] The initial applications of Au nanoparticles were reported as an

enhancer to the performance of semiconductor photocatalysts in order to create a visible range

active photocatalyst, yet it does not play a leading role in the photocatalysis.[14,15] However, Au

nanoparticles were reported to be catalytically active in various organic reactions.[16] As a result,

the photocatalytic activity of Au nanoparticles under visible light irradiation was discovered

and the first application was reported by our group regarding the photocatalytic oxidation of

organic pollutants and selective reduction of the aromatic nitro group.[17,18]. Following this,

silver and copper nanoparticles were found to exhibit similar light absorption properties in the

visible range and therefore showed potential in photocatalysis.[19] Thus, three metals (Au, Ag

and Cu) were categorised as plasmonic metals and represented a new area of metallic

photocatalysts other than tradition semiconductors.

Among the three plasmonic metals (Au, Ag and Cu), Cu metal is the least studied

plasmonic metal compared with Au and Ag. The reason for this is attributed to the instability

of Cu in the presence of oxygen gas or other oxidants especially when Cu in the forms of the

nanostructure. Cu, in different forms, is a highly active element for a large number of catalytic

organic syntheses and enzymatic reactions., The history of Cu based catalyst in thermal-driven

organic synthesis and biological applications is extremely rich due to the good association

between Cu and various organic compounds.[20-23] The distinctive catalytic activity combining

Introduction - 4 -

with strong light absorption allow Cu NPs as a promising candidate in the direct photocatalysis.

However, from the material science point of view, previous studies and applications of CuNPs

are strictly limited because CuNPs can be readily oxidized by air, oxidizing support materials

and oxidants in the reaction environment, yet there is evidence that the metallic state of Cu is

crucial to its non-linear optical properties and catalytic selectivity in many cases.[24] Normally,

Polymer stabilizers and/or inert atmospheres have been employed to maintain Cu in the

metallic state.[25] Thus, developing a convenient and practical method to stabilise Cu

nanoparticles for the resistance of oxygen gas or other oxidative environment could largely

expand the applicability of Cu nanoparticles as plasmonic metal photocatalysts.

Numbers of organic synthesis reactions were reported to be catalysed by plasmonic

metals, however, the versatile applications of the metallic photocatalyst are restricted by the

limited numbers of plasmonic metals. There are numerous transition metals, which are

catalytically active to a wide range of organic reactions. Nevertheless, those metals show poor

light absorption property in the visible range.[26] Introducing other non-plasmonic but

catalytically active metals into an alloy of plasmonic metals, using the plasmonic metal as the

light harvesting site and the non-plasmonic metal as the catalytic active site, is able to enhance

the photocatalytic performance and applicability of plasmonic metal photocatalysts and

significantly promote the metallic photocatalysis to a broader field of fine organic synthesis.[27]

In addition, metal alloys exhibit various forms of nanostructure. Au-Pd core-shell

nanostructure, for instance, can alter their optical property and eventually influence the

photocatalytic performance.[28] The vast combinations of metal alloys and correspondingly

different nanostructures imply a promising opportunity for the metallic photocatalyst.

Apart from the research of metallic photocatalyst from a material point of view, the other

promising research focus is to explore the metallic photocatalysts in organic synthesis.[2] To

Introduction - 5 -

date, metallic photocatalysts were reported active in several types of fine organic synthesis

reactions including redox oxidation, degradation, cross coupling and addition reactions.[29]

The development of metallic photocatalysts and their applications in organic synthesis is

growing rapidly and that demands a comprehensive mechanism study for a better

understanding of the photocatalysis and illumination for future research. The investigation of

the energy transfer on the surface of metal nanoparticles at electron-scale provides fundamental

insights from both a physical and a chemical point of view. It is now well accepted that photon

excited electrons from the surface of metal nanoparticles are responsible for the initiation of

photocatalytic chemical reactions.[30,31] Further study reveals the adsorption of reactant

molecular on metal nanoparticles results in hybridised adsorbate-metal bonding and

antibonding states, the direct photoexcitation of electrons within such states is the major reason

for the photocatalysis.[32] [33] Nevertheless, it is also critical to investigate the behaviours of

different types of metallic photocatalysts under controlled light irradiance.[34] This research

contributed knowledge into the field of both photocatalysts design and organic synthesis

optimisation of metallic photocatalysis.

Research Problems

Photocatalytic organic synthesis over metal nanoparticle photocatalysts is a relatively new

class of photocatalysis, which differs from traditional semiconductor based photocatalysis.

Despite the increasing interest in this research direction, there is only a small proportion of

organic reactions that have been accomplished with metal nanoparticles under visible light

irradiation. Thus, great opportunities are still open for researchers to enrich its applicability in

fine organic synthesis. An intriguing direction is to develop new reaction system, this includes

the fabrication of novel metallic nanostructure photocatalysts and searching for high

demanding organic reactions.

Introduction - 6 -

Firstly, since the Au based metallic photocatalyst has been recognised, expanding its

application in visible light driven organic synthesis is an attractive research direction. Thus,

locating a new high demanding organic reactions over Au based photocatalyst is considered

long term research task. More importantly, a nature of photocatalysis is to build eco-friendly

organic process, therefore developing a green reaction system over Au based photocatalyst, for

example avoiding hazardous reaction solvent, additives and side products, would be a

promotion to metallic photocatalysis. Moreover, the application scope of Au based

photocatalyst can be widely spread by introducing other catalytic active metal into the Au-M

alloy nanoparticles. This is a promising strategy for the development of metallic photocatalysts

family.

In addition to Au based photocatalysts, Cu is the least studied plasmonic metal due to its

intrinsic instability in an oxidative environment. Fabrication of versatile air-stable Cu

nanoparticle photocatalysts is an appealing research focus and the achievement of high

photocatalytic activity in organic synthesis with Cu nanoparticles is expected. The Cu

nanoparticle stabilisation mechanism is to be thoroughly studied in order to propose a strategy

for the versatile applications of Cu nanoparticles photocatalyst.

Secondly, the reaction pathways and mechanisms for each photocatalyst and reaction

system are investigated by the mechanistic study. This part of the study will illustrate metallic

photocatalysis in detail including: (i) the energy transfer and evolution of light energy to

chemical energy through metallic photocatalysts and (ii) the behaviour of metallic

photocatalysts and the targeted reactant is being intensively studied, yet a widely accepted

consensus has not been reached. Thus, the in-depth investigation of such subjects can provide

insights into metallic photocatalysis from a scientific point of view.

Introduction - 7 -

Given the current research progress, the research gaps that require fulfilling are listed as

below:

What are the important reactions, which can be photocatalysed by Au nanoparticles

under visible light irradiation?

What efforts can be made to further promote Au nanoparticle photocatalysed organic

reaction toward green chemistry?

How does an Au based alloy nanoparticle photocatalyst perform in organic synthesis?

What are the roles of Au and the other alloyed transition metal in a photocatalytic

organic synthesis and their influence on each other?

How to fabricate air-stable Cu nanoparticles and what is the stabilisation mechanism?

What is the photocatalytic performance of Cu nanoparticle photocatalysts in organic

synthesis?

What is the energy alignment pattern in the plasmonic metals and their alloy

nanoparticle photocatalysed organic reactions?

Research Objectives and Aims

Firstly, photocatalytic organic synthesis reaction over Au nanoparticle photocatalyst is

to be further explored with respect to green chemistry. The research objective and aims to this

part as listed as below:

1. Synthesis of Au nanoparticles supported by inert materials.

2. Apply supported metal nanoparticles in the photocatalytic hydrogenation of

unsaturated aromatics and evaluate the photocatalytic performance.

3. Optimise the hydrogenation reaction system toward green chemistry: (1) visible light

source: (2) moderate reaction temperature; (3) aqueous reaction system; (4) eco-

friendly hydrogen source; (5) additive-free reaction system.

Introduction - 8 -

4. Investigate the reaction mechanism, including photocatalysis mechanism and organic

reaction path.

The next part of the thesis aims to expand the applications of supported Au nanoparticle

photocatalysts by alloying Pd with Au to form Au/Pd alloy nanoparticle photocatalysts.

1. Synthesis of supported Au/Pd alloy nanoparticles with different Au-Pd ratios.

2. Appling supported Au/Pd alloy nanoparticles in the dehydrogenation of aromatic

alcohols and evaluate the photocatalytic performance.

3. Investigate the influence of Au-Pd ratio on the photocatalytic activity and optimise

the Au-Pd alloy nanoparticle photocatalyst.

4. Investigate the role of Au and Pd in the photocatalytic dehydrogenation respectively

and their possible interactions.


reaction path.

In the third part of the thesis, the research focused on the fabrication of air stable Cu

nanoparticle photocatalysts and their application in the epoxidation of various alkenes.

1. Synthesis of supported Cu nanoparticles that can resist oxidation from the air in the

ambient environment.

2. Apply supported Cu nanoparticles in the epoxidation of various alkenes and evaluate

the photocatalytic performance.

3. Investigate the stabilisation mechanism for supported air stable Cu nanoparticles.


reaction path.

Lastly, this thesis investigated the mechanism of photocatalysis by exploring the action

spectra (the relationship between the irradiation wavelength and apparent quantum efficiency

Introduction - 9 -

of reactions under constant irradiance) of different reactions to clarify the photon-electron

excitations process in the direct photocatalysis of metallic photocatalysis.

1. Investigate the action spectra of Au and Au-Pd alloy NPs for the same reaction.

2. Investigate the action spectra of Au-Pd alloy NPs for different reactions.

3. Investigate the action spectra of non-plasmonic metal NPs for different

reactions.

4. Investigate the action spectra of Au NPs for different reaction temperature.

5. Data collection and analysis, computational calculation.

6. Propose overall mechanism.

Significance and Contribution to the Knowledge Base

This thesis contributes to the following knowledge gaps:

The first application of a supported Au nanoparticle photocatalyst in the

hydrogenation of a series of unsaturated aromatics. The reaction system is designed

in the principle of green chemistry that using solar energy as driving force, water as

a solvent and formic acid as the reductive agent without other additives. This

photocatalytic system produces only carbon dioxide and water as by-products.

Formic acid was found to cooperate with water, yielding an intermediate orthofomic

acid as a hydrogen source. The hydrogenation mechanism demonstrated in this work

provides a theoretical basis for future hydrogenation applications involving formic

acid aqueous solutions.

The first report of photocatalytic oxidation of aromatic alcohols over supported Au-

Pd alloy nanoparticles. The metallic photocatalysts have been extended by alloying a

transition metal (Pd) with a plasmonic metal (Au) for the photocatalytic oxidation of

an aromatic alcohol to the corresponding aldehyde. The surface charge heterogeneity

Introduction - 10 -

caused by bi-metallic nanostructure was found to control the photocatalytic

performance. Such an understanding may enlighten designs and applications of future

alloy photocatalysis.

The first report of fabrication of TiN support material stabilised Cu nanoparticles that

can resist oxidation from the air. The TiN supported Cu nanoparticles were found to

be stable after a few reaction cycles and can be easily reactivated even after catalyst

passivation. Revealing the stabilisation mechanism brings knowledge into the future

practical applications of Cu nanoparticles which is not limited to photocatalysis. The

first report of epoxidation of alkenes over Cu nanoparticle photocatalyst using

molecular oxygen as an oxidant in the assistance of cyclic ether solvent. The oxygen

activation process has been investigated to creating a versatile photocatalytic

oxidative reaction pattern.

This thesis also contributes to the fundamental and comprehensive mechanism

research of general metallic photocatalysis. The photo-electron excitation was

demonstrated as the dominant driving force and a photon energy threshold was first

proposed existing in general metallic photocatalysis. The understanding of the

interaction between photons, metal electrons and targeted organic reactions is crucial

to the guidance of future metallic photocatalysis.

Introduction - 11 -

Thesis Outline and Linkage between Chapters

The following figure shows that how this thesis is constructed

Literature Review - 12 -

Chapter 2 Literature Review

Early Study of Photochemistry and Photocatalysis

Large energy consumption and its associated environmental issues is one of the major

concerns in traditional chemical industry, thus continuous attention has been attracted to seek

alternative energy sources for chemical industry with minimised environmental and economic

impacts. Sunlight, a natural energy source, is an inexpensive, non-polluting, abundant and

endlessly renewable source of clean energy.[35] Therefore, direct and efficient utilisation of

solar energy in chemical applications could be a promising solution to reduce the current

energy consumption.[36]

The earliest study of photochemistry can be traced back to 1912 by Dr Giacomo

Ciamician who is well-known as a pioneer in this field. He described a new concept for organic

chemistry associated with solar energy in his famous paper “The Photochemistry of the

Future”.[5] However, at Dr Ciamician’s time and the subsequent few decades, the

photochemistry was limited to the direct but inefficient interaction between light and organic

compounds.[37,38] One fundamental impediment of the early photochemistry was that only

ultraviolet light with very short wavelength can activate organic molecules, yet the proportion

of such ultraviolet light in solar spectrum is low.

In 1972, a breakthrough was published by Dr Fujishima and Honda reporting splitting of

water into hydrogen and oxygen gas over ultraviolet radiation illuminated titanium dioxide

(TiO2). The TiO2 photocatalysed water splitting was known as a representative application for

semiconductor based photocatalysis. Semiconductors can absorb certain wavelength range of

light, usually in the ultraviolet range depending on the width of its band gap, and activate


reactant molecules due to the interband excitation effect as shown in Fig. 2-1.[39] This study of

heterogeneous photocatalysts has enlightened photocatalysis for the past forty years.[6]

Following Fujishima and Honda’s work, extensive researches were conducted on classic

semiconductors such as TiO2, ZnO and CdS. In the meantime, various semiconductor

photocatalysts had been developed for wide applications other than the classic water splitting.[7-

10] Despite the significant progress in semiconductor based photocatalysis, the catalytic

property of semiconductor photocatalysts still suffers from low quantum efficiency and low

thermal resistance owing to their intrinsic nature. Moreover, the applications of semiconductor

photocatalysts in organic synthesis are rarely reported due to the relatively weak affinity of

semiconductors to organic molecules.[2] Regarding the drawbacks of conventional

photochemistry and semiconductor photocatalysis, the developments of new types of

photocatalysts which can efficiently utilize visible light and catalyse various organic reactions

are in demand.

Figure 2-1. Mechanism of a semiconductor photocatalysis.[39]

LSPR Effect and Plasmonic Metallic Photocatalyst

Localised Surface Plasmon Resonance (LSPR) effect is a non-linear optical effect, it is

mostly observed on the metal nanostructure surface when irradiating with a certain wavelength


of light.[40] The collective oscillation of surface electrons from metal nanostructure stimulated

by incident light is the source of the LSPR effect, and it reaches the maximum when the

frequency of incident light is resonant with the inherent oscillating frequency of surface

electrons against the restoring force of positive nuclei (Figure 2-2).[30] The resonance effect

results in a high light absorption and eventually produces a high concentration of energetic

electrons at the surface of the nanostructure.[41,42]

Figure 2-2. The Localised Surface Plasmon Resonance (LSPR) effect.[30]

The LSPR effect can further allow the metal nanostructures to absorb incident light not

limited to its cross section but rather to a larger vicinity area at the LSPR wavelength. Garcia

found that when exposed to light irradiation, the conduction electrons of metal nanoparticles

resonance with the electromagnetic field of incident light resulting in an electric field

surrounding the metal nanoparticles that opposite to that of the incident light. [43] As illustrated

in Fig. 2-3A, a metal nanoparticle is surrounded by the enhanced electric field. Such electric

field can influence the light passing its vicinity, resulting in an extinction cross section larger

than the physical cross section of metal nanoparticles. Moreover, when a plasmonic metal

nanoparticle is irradiated with light at its LSPR wavelength, which is 530 nm for Au

nanoparticles, the conduction electron resonance reaches its maximum and thusly create the

largest electromagnetic field region reflected by the huge extinction cross section as shown in

Figure 2-3B. Incident light from wavelength other than LSPR wavelength, on the other hand,


cannot results in an expanded extinction cross section due to the weak LSPR effect. This light

energy concentration effect is one of the reasons that metallic photocatalysts are superior to

other photocatalysts including semiconductors.

Figure 2-3. (a) Metal nanoparticles absorbing light from its vicinity area owing to the

surrounded electric field of metal nanoparticles, (B) the demonstration of the extinction cross

section of an isolated metal nanoparticle at LSPR wavelength.[43]

In addition, the proximity of plasmonic metal NPs significantly enhances their

electromagnetic field. When two metal NPs are close, on the order of several nanometers, a hot

spot with over 106 times enhanced electromagnetic field is created between these two metal

NPs revealed by Finite-Difference Time-Domain (FDTD) simulations.[40,44,45] However, such

enhancement decays rapidly with increasing distance of two nanoparticles.[46]

The LSPR effect generates energetic electrons and transfers energy to adsorbed organic

molecules via three dephasing processes to initiate organic reactions: (i) Elastic radiative

reemission of photons; (ii) Non-radiative Landau damping and (iii) Chemical Interface


Damping (CID) effect.[32] In the first process, the plasmonic nanostructures reradiate intensive

photons which are adsorbed by organic molecules, the energy from reradiated photons can be

transfer to organic molecular through a vibronic energy exchange follow the Franck-Condon

principle. This energy transfer route is particularly similar to the surface-enhanced Raman

spectroscopy (SERS) except SERS does not aim to initiate chemical reactions. The second

proposed dephasing process through Landau damping relies on the plasmon generated

energetic charge carriers transfer from metal nanostructure to the unoccupied orbitals of

adsorbed molecules. In this theory, it is proposed that metal electrons below Fermi level are

excited to an energy level above Fermi level and results in electron/hole pairs in metal

nanostructures, the energetic electrons with sufficient energy are scattered into the molecular

orbitals of adsorbed molecules through transient charge transfer to trigger chemical reactions.

The third plasmon decay mechanism is similar to the Landau damping process in terms of

energetic electrons interact with adsorbate orbitals, the difference is whether energetic

electrons are directly injected (fast) or scattered (slow) into adsorbate orbital. The CID theory

takes particular adsorbate into account that the energetic electrons instantly injected into the

adsorbate orbitals, this dephasing process is ultrafast and influenced by the interaction between

plasmonic nanostructures and the particular adsorbate.

Figure 2-4. Schematic illustration of three surface plasmon dephasing processes.[32]


Recently, Linic et al proposed a new theory base on CID damping and it was further

comprehensively described by Kale et al, this theory suggested two possible photoexcitation

method for metallic photocatalyst to activate target organic reactants.[33,47] They categorised

metallic photocatalysis into three models: one indirect photoexcitation and two direct

photoexcitations distinguished by the degree of reactant chemisorbed onto metal NP surface.

In the indirect photoexcitation, the reactant is not chemically bonded to the metal NP surface,

which limited their energy transfer efficiency. The energy transfer occurs through the relatively

slow migration of energetic electrons to the unoccupied state of reactant (Fig. 2-5a). In the

contrast, the direct photoexcitation occurs when there is a chemisorption between metal NPs

and the reactant, such interaction can significantly enhance the energy transfer efficiency, and

it leads to the instant photoexcitation between Highest Occupied Molecular Orbital (HOMO)

and Lowest Unoccupied Molecular Orbital (LUMO) of the reactant (Fig. 2-5b) which mush

faster than that of indirect photoexcitation. In the third scenario, if the chemisorption between

metal NPs and the reactant is rather strong that a metal-adsorbate complex is created, then the

molecular orbitals of metal and adsorbate could be hybridised to form a reduced energy gap

between hybridised HOMO and LUMO (Fig. 2-5c). The photoexcitation in this scenario is

considered the most efficient.


Figure 2-5. Schematic of photoexcitation of adsorbate-metal complexes on metal NPs

surface.[47]

Based on the above knowledge, the LSPR effect could be introduced into the

photocatalytic organic synthesis, many studies have already been reported focused on

fabricating plasmonic photocatalysts and applying them in important organic reactions. There

are three well-known plasmonic transition metals: gold (Au), silver (Ag) and copper (Cu), Fig.

2-6 displays the their light adsorption for ~ 5 nm nanoparticles and LSPR peak are located at

530, 400 and 580 nm respectively, it is noteworthy that the LSPR wavelength varies with the

size and shape of plasmonic metal nanostructure.[48-51] Due to the outstanding visible light

absorption and stable chemical and physical properties of noble metals, the application of gold

and silver nanoparticles in photocatalysis was soon discovered and many articles have been

published on gold and silver NPs since 1990.[49] On the other hand, copper NPs can be readily

oxidised by air, therefore the applications of Cu NP photocatalysts has significantly lagged

behind. As the plasmonic metals are being studied in depth, the high cost of a noble metal is

not to be neglected, and thus researchers have started to search for low cost replacements. Many

notable materials have been discovered such as Graphene, nitride ceramics and tungsten oxide

materials etc.[52-56] Nevertheless, the LSPR absorption of most newly uncovered plasmonic

materials lies on infrared range, where the photon energy is insufficient to trigger organic

reaction. Therefore, the potentials of new plasmonic materials have yet to be realised.


Figure 2-6. UV-Vis absorption spectrum of plasmonic metal nanoparticles.[49]

Gold NPs Photocatalyst for Organic Synthesis

2.3.1 Introduction to gold NPs Photocatalyst

Au NPs have been long applied to adorn glass windows of medieval churches due to its

brilliant colours without knowing the mechanism.[57] Recently, the distinctive light absorption

was found responsible for the various colours of Au NPs.[2] As a consequence, the optical

property of Au NPs came into scientific sight and had been widely studied, researchers found

that the light absorption of Au NPs is a result of the LSPR effect. As shown in Figure 2-7, when

irradiated with visible light, electrons in the 6sp band of Au NPs are excited to high energy

level due to the LSPR effect. The adsorbed molecules gain energy from those excited electrons,

it results in chemical reactions which has been discussed in section 2.2. Meanwhile, the Au

NPs can additionally absorb UV light to trigger chemical reactions. The electrons from 5d band

of Au NPs can directly adsorb photons from UV light and be excited to higher energy level.

Similar to LSPR induced energetic electrons, those UV photon excited electrons are also

capable of initiating chemic reactions. Thus, Au NPs can efficiently utilise light energy from

the visible range and UV range for possible photocatalytic reactions. Moreover, the

nanostructure of Au was found effective in many thermally catalytic reactions at elevated


temperatures including the oxidation of various substrates and the reduction of nitrobenzene.[58-

60] Therefore, the combination of the optical properties and catalytic properties of gold NPs

results in a promising photocatalyst candidate. [30,43]

Figure 2-7. The diagram of band structures of supported Au-NPs and the proposed mechanism

for photocatalysis using supported Au-NPs.[2]

It has been experimentally demonstrated that Au NPs are capable of activating steady-

state photocatalytic reactions under low intensity visible light. In addition, they demonstrated

the mechanism by FDTD simulations and found that the electric field intensity at the surface

of an isolated particle is around 1000 times greater than the incoming photon flux. The field

enhancement results in high energy state electrons at the surface and the excited electrons are

able to active organic molecules and initiate chemical reactions.[31]

2.3.2 Au NPs Photocatalysed Organic synthesis

Au NPs photocatalysts were firstly applied by our group in the photocatalytic reduction

of nitrobenzene. Nitrobenzene reduction attracted wide research interest because the reductive

products of nitrobenzene are feedstock in the chemical and food industry.[61] It was reported


that inert ZrO2 nanopowder supported Au NPs can selectively convert nitrobenzene to

azobenzene under the irradiation of visible light and UV light. Isopropanol was found to

provide hydrogen atoms to bond with gold NPs and form an Au-H species, such species are

capable of acquiring oxygen atoms from nitrobenzene and releasing oxygen gas, the reaction

path of which is shown in Fig. 2-8.[17]

Figure 2-8. The mechanism for the photocatalytic reduction of nitroaromatic compounds.[17]

Au NP photocatalysts have also been employed in the photocatalytic oxidations. Zhang

et al. reported the use of zeolite material supported Au NPs as photocatalysts for the oxidation

of benzyl alcohol to benzaldehyde at ambient temperature under O2 atmosphere.[62] Further, in

2014, Zhang et al.reported another application of supported Au NPs in the esterification of

benzaldehyde with alcohol under visible light irradiation.[63] The mechanism study suggested

a hemiacetal intermediate is formed between benzaldehyde and alcohol.

In addition to photocatalytic redox reaction, Au NP photocatalysts have also been applied

in other types of organic syntheses, such as hydroamination reaction, delivers a convenient tool

for the C-N bond coupling yet usually requires high reaction temperature due to the high energy

barrier.[64]


2.3.3 Hydrogenation of unsaturated aromatics.

The photocatalytic reduction of nitrobenzene to azobenzene was reported as mentioned

above. On the other hand, a full reduction of nitroaromatics results in aromatic amine

compounds, and it also attracts particular interest because they are essential industrial

intermediates for pharmaceuticals, polyurethanes, herbicides, agricultural products and other

fine chemicals.[61,65] In general, synthesis strategies fall into two main categories, one is direct

amination of aryl compounds and the other is catalytic hydrogenation of aryl nitro

compounds.[66,67]

The direct amination of aryl compounds normally uses ammonia as a nitrogen source for

direct amination of aryl compounds.[68,69] The direct coupling of ammonia and aryl halides

process has been discovered over a century ago and was named the Ullmann-type aryl

amination.[70] However, the Ullmann-type aryl amination was greatly limited by its harsh

reaction conditions due to the inactivity of primary aryl compounds.[71]

Given the drawbacks of direct amination, there is a strong incentive to discover other

synthetic pathways. Nitroaromatic compounds are normally resistant to oxidation and to

hydrolysis owing to the electron withdrawing effect of the nitro group. However, reduction of

the nitro group to low valence state such as an amino group is practically achievable.[72] As a

result, hydrogenation of nitro-substituted aromatic compounds provides researchers with

another powerful tool for the synthesis of aromatic amines.[73] Current commercial synthesis

of aniline through nitrobenzene reduction is a typical direct hydrogenation process, and it is

carried out in the gas phase where hydrogen gas acts as a reducing agent and hydrogen source

and various metal based catalysts are applied.[74] Although high conversion yield is achieved,

it still requires a temperature over 200 oC and a pressure over 4 bar.[75-77] The harsh reaction

conditions block the application of nitroaromatic compounds reduction in fine chemical


synthesis field such as pharmaceutical industry which requires the mild condition to prevent

the destruction of other sensitive functional groups.[78] In a laboratory scale, various catalysts,

as well as reductive agents, were utilized to avoid the harsh reaction conditions. However, the

progress on thermal catalysis is slow and most reductive agents such as hydroboron, low

valence state metals, hydrosulfite salt and hydrazine are harmful to the environment.[79-90]

Moreover, the selective hydrogenation of various unsaturated aromatics is a class of

reaction fundamental in organic synthesis. Traditionally they are attained with pressurised

hydrogen gas or hazardous reductive agents under elevated reaction temperatures. Thus, there

is a strong motivation to design a new reaction system with minimized energy consumption

and eco-friendly reductive agent.[91] In this thesis, one of the projects aimed to use supported

Au NPs to photocatalyse hydrogenation of unsaturated aromatics in an environmental-friendly

system where water is used as a solvent and formic acid as a reductive agent. The system yields

carbon dioxide and water as the only by-products. The project is introduced in Chapter 3.

Au-Pd alloy NPs Photocatalysed Oxidation of Aromatic Alcohols

2.4.1 Introduction to Au-Pd alloy NPs Photocatalyst

Despite the reported progress of Au NPs in organic synthesis, the total number of

chemical reactions that can be catalysed by Au NPs is still small, especially compared with

reactions thermally catalysed by non-plasmonic metals such as Palladium (Pd).[92-94] Au can be

alloyed with Pd to form a bimetallic NP where light energy absorbed by Au to enhance the

catalytic activity of Pd. This strategy greatly expands the application of Au NPs based

photocatalyst to a wide range of organic synthesis.

2.4.2 Au-Pd alloy NPs photocatalysed organic synthesis reactions

Owing to the significant catalytic activity of Pd, Au-Pd alloy NP photocatalysts have

been applied in multiple photocatalytic reactions. For example, Pd metal is known as an


excellent catalytic site for various C-C cross-coupling reactions. Thus, Kevin et al reported the

photocatalytic application of Au-Pd alloy NPs in coupling reactions, such as Suzuki-Miyaura

coupling, Stille coupling, Sonogashira coupling and Buchwald-Hartwig coupling.[27] The

reaction mechanism is shown in Fig. 2-9.

Figure 2-9. Reaction mechanism for Au-Pd alloy NP photocatalysed Suzuki reaction.[27]

Moreover, the Au-Pd alloy photocatalysts have been employed in more challenging

organic synthesis reaction, which is the one-pot esterification of aliphatic alcohols.[95] The

activation of aliphatic alcohols is considered difficult. However, with visible light irradiated

Au-Pd alloy photocatalyst, 94% conversion was achieved at ambient temperature. The

illustration of the esterification reaction pathway is shown in Fig. 2-10.


Figure 2-10. One pot synthesis of the ester from aliphatic alcohols.[95]

2.4.3 Oxidation of Aromatic Alcohols

A typical example of a Pd catalysed characteristic reaction is the selective oxidation of

aromatic alcohols to corresponding aldehydes. This oxidative reaction is one of the most

fundamental transformations in synthetic organic chemistry since the carbonyl products can

serve as important and versatile intermediates in the fine chemical industry. Traditional

aromatic alcohol oxidation involves toxic oxidative agents such as permanganate and

dichromate which cause serious environmental issues.[96] In 1977, Blackburn and Schwartz

firstly used palladium to accelerate this reaction and many studies were subsequently focused

on this subject.[97-100] Supported Pd NPs were reported in the thermal catalytic oxidation of

aromatic alcohols where elevated temperature and/or high pressure were applied.[101-103] As a

result, utilising solar energy to enhance the oxidation of aromatic alcohol can promote this

reaction to a new level of green chemistry. Thus, Au-Pd alloys NPs catalysts were designed to

fulfil this task. This project is presented in Chapter 4.


Stabilised Copper NPs Photocatalyst

2.5.1 Introduction to Cu NPs photocatalyst

Copper is a transition metal with excellent electrical and thermal conductivity and is

relatively cheap compared with noble metals. Initially, copper was found to be widely

distributed in the enzymatic systems, and by studying the role copper played in enzymes,

researchers found copper exhibits promising catalytic potential.[104] Following this lead, a series

of copper based catalysts were developed with high catalytic efficiency and wide applicability.

First of all, , Cu salt such as CuI can be directly applied as catalyst in organic synthesis reactions

such as amidation of aryl halides.[21, 105] Secondly, the copper based complex is a large class of

catalyst for organic synthesis, for example Zall et al fabricated Cu(I) complex LCu(MeCN)PF6

successfully applied it in the hydrogenation of CO2 to formate.[106] In addition, the Cu metal

nanostructure catalyst was also developed for many reductive reactions such as the methanation

of CO2,[107] however the application of Cu nanostructures in oxidative reaction have been

overlooked and it will be discussed later. Moreover, the formation of C-C and C-N bonds are

fundamental reactions that of great importance in organic synthetic. This type of reaction is

normally catalysed by Pd catalysts, however cupric iodide associate with ligands and base

additives and was found exhibiting excellent catalytic performance. Hence, Cu catalysts appear

to be desirable alternatives for Pd especially considering the low cost of copper metal.[105]


Figure 2-11 TEM images (A and B; the scale bars are 30 and 10 nm respectively; the scale bar

in the inset of (B) is 2 nm) and XPS profile(C) of 5wt% Cu/graphene, and UV/Vis absorption

spectra of Cu/graphene photocatalysts with different Cu loadings (D).[110]

Copper NPs are promising plasmonic materials similar to gold and silver, which exhibit

strong light absorption at around ~580 nm wavelength depending on its size.[51,108] Therefore,

it is attractive to combine the optical and catalytic properties of Cu to develop an efficient and

low cost photocatalyst. However, copper NPs could be readily oxidised in atmospheric

conditions and oxidative reaction environment. Therefore, most of the current applications of

copper NPs require the association of a polymer stabiliser or noble gas protection.[109] As a

result, the study of copper NP photocatalysts was significantly restricted, thus there remains a

strong incentive to fabricate an air-stable copper photocatalyst. Recently, an inspiring work has

been reported that graphene can stabilise metallic state copper NPs (Fig. 2-11) [110] The copper

nanoparticles can be observed in TEM images and their metallic state is demonstrated by X-

ray Photoelectron Spectroscopy (XPS) technique. Moreover, the graphene supported copper

NPs were successfully applied in the oxidative coupling of nitroaromatics. However, the


stabilisation mechanism for Cu NPs is not clear and limits the applicability of this

photocatalytic system. Furthermore, graphene is a relatively expensive material, which cannot

be extended to large scale application. This thesis tends to create an efficient copper NPs

photocatalyst with other novel support and clarify the stabilisation mechanism. This work is

described in Chapter 5.

Reference - 29 -

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Photocatalytic Selective Hydrogenation of Unsaturated Aromatic in Aqueous Solution over

Supported Au Nanoparticle under Visible Light

- 42 -

Chapter 3 Photocatalytic Selective Hydrogenation of

Unsaturated Aromatic in Aqueous Solution over


Introductory Remarks

This chapter presents Article 1 (published in Catalysis Science & Technology, 2018, 8,

726-734.) reporting the direct photocatalytic hydrogenation of unsaturated aromatics over

supported Au nanoparticles under visible light irradiation. This work focus on the research

problem that applying Au nanoparticle photocatalyst for a new class of hydrogenation, the

reaction system was designed for a green process including using eco-friendly hydrogen source,

using water as solvent, avoid of additives and undesired side products. The mechanistic study

in this work deposited knowledge into both Au based photocatalysis process and hydrogenation

of unsaturated aromatics.

The selective hydrogenation of unsaturated aromatics is a type of fundamental reaction

among organic synthesis that traditionally is attained with pressurised hydrogen gas and an

elevated reaction temperature. In this paper, we use supported Au nanoparticles for the

photocatalyst selective hydrogenation of C=C, C≡C, C=O, N=O and C=N bonds in the

presence of aromatic rings under mild reaction conditions. To make this process greener, we

created the reaction system using formic acid as an environmentally friendly hydrogen donor

to avoid the suppressed hydrogen gas or pollutant hydrogen source. The formic acid can be

commercially produced from biomass and generates only carbon dioxide and water as oxidized

products. Furthermore, the hydrogenation reaction is taking place in an aqueous system where



- 43 -

only water is used as solvent without any other additives. The photocatalytic hydrogenation

reaction system exhibits excellent photocatalytic activity with high substituent tolerance. The

reaction selectivity was found to be tunable by varying the irradiated light wavelength. The

photocatalytic activity is owing to the photoexcited electrons from supported Au nanoparticles.

The mild reaction temperature and pressure are the reason to the high selectivity control. The

reaction pathway was studied with the assistance of the isotope tracking technique We

discovered that the cooperation of water and formic acid as hydrogen source, formic acid react

with water forming an intermediate named orthoformic acid which directly deliver hydrogen

to the Au nanoparticles surface yield active Au-H species that can reduce the unsaturated

functional groups. This work illustrates an example for a green photocatalytic process that

using solar energy as driving force, water as reaction solvent without other additives and

releasing only carbon dioxide and water as by-products. It also reveals that formic acid and

water cooperating together as hydrogen donor, which bring knowledge into the future

hydrogenation applications involving formic acid



- 44 -

Article 1

Statement of Contribution of Co-Authors

Publication title and date of publication or status:

Visible light-driven selective hydrogenation of unsaturated aromatics in an aqueous

solution by direct photocatalysis of Au nanoparticles

Yiming Huang, Sarina Sarina, Qi Xiao, Wayde Martens, Zhe Liu, Cheng Guo,* and

Huaiyong Zhu*

Manuscript published in Catalysis Science and Technology

Contributor Statement of contribution

Student Author:

Yiming Huang

Wrote the manuscript, experimental design,

conducted experiments and data analysis.

Signature

Date

Dr Sarina Sarina Corresponding author, aided experimental

design, data analysis.

Dr Qi Xiao Aided experimental design, data analysis.

Dr Wayde martens Aided experimental design, data analysis.

Ms Zhe Liu Conducted experiments, data analysis.



- 45 -

Prof. Dr Cheng Guo Aided data analysis

Prof. Dr Huaiyong Zhu The corresponding author, aided

experimental design, data analysis.

Principal Supervisor Confirmation

I have sighted email or other correspondence from all Co-authors confirming their certifying

authorship.

____________ _____________ ________________

Name Signature Date



- 46 -

Visible light-driven selective hydrogenation of unsaturated aromatics in an

aqueous solution by direct photocatalysis of Au nanoparticles

Yiming Huang,[a] Sarina Sarina,[a] Qi Xiao,[b] Wayde Martens,[a] Zhe Liu,[a] Cheng Guo,*[c]

and Huaiyong Zhu*[a]

[a] School of Chemistry, Physics and Mechanical, Faculty of Science and Technology,

Queensland University of Technology, Brisbane, Queensland 4001, Australia

[b] Ian Wark Laboratory, Commonwealth Scientific and Industrial Research Organisation,

Bayview Ave, Clayton, Victoria 3168, Australia

[c] College of Science, Nanjing University of Technology, Nanjing, Jiangsu 211800, China

*E-mail: [email protected]; [email protected]

Abstract: Herein we report a new visible light driven efficient and eco-friendly selective

hydrogenation of C=C, C≡C, C=O, N=O and C=N bonds over supported gold nanoparticle

(AuNP) photocatalyst under mild reaction condition. The reaction system exhibits high

substituent tolerance and tuneable selectivity by light wavelength. The photocatalytic

mechanism is proposed that photoexcited hot-electrons are the driven force for the

hydrogenation reaction. The hydrogenation pathway is investigated with isotope tracking

technique. We reveal the cooperation of water and formic acid (FA) as a hydrogen source and

its hydrogenation route through Au-H species on the Au NP surface.

Introduction

Photocatalysis using plasmonic metal (Au, Ag, Cu and Al) nanoparticles (NPs) has

attracted much attention as they directly utilize solar power and are usually associated with

mild reaction conditions.[1-8] The NPs of these metals strongly absorb visible light via the

mailto:[email protected]



- 47 -

localized surface plasmon resonance (LSPR) effect which is the light-induced collective

oscillation of the metal conduction electrons, established when the incident light frequency is

resonant with the frequency of metal electron oscillation in response to the restoring force of

the positive nuclei, it results in energetic electrons (so-called hot-electrons) from metal NPs.[9-

11] Chemical transformations can thus be mediated due to the direct charge excitation within

the metal -adsorbed organic molecular system.[12] In addition, metal electrons can directly

absorb photons of ultraviolet (UV) and infrared irradiation to be excited to higher energy levels

due to the continuous electron energy levels of metals, thus making the utilization of most solar

spectrum possible.[13,14]

The selective hydrogenation of unsaturated organics is a fundamental reaction in organic

synthesis that is traditionally attained with the assistance of redundant hydrogen gas under high

pressures, and elevated reaction temperatures (over 100oC).[15] Elevated temperature in

traditional hydrogenation may accelerate undesired side reactions, such as hydrogenation of

the aromatic rings of the reactants, wasting the reduction agent resulting in poor economic

viability.[16,17] In an attempt to moderate the reaction conditions, various homogeneous or

heterogeneous transition metal based catalysts including gold, nickel, ruthenium, cobalt,

copper or palladium are widely employed in the hydrogenation of unsaturated bonds of C=C,

C≡C, C=O, N=O and C=N toward benign processes.[18-21] However, challenges remain in

energy efficiency, product selectivity and catalyst recyclability.[22] Thus, the hydrogenation

reactions can be greatly promoted by visible light driven mild photocatalysis. Meanwhile, the

high corrosivity of hydrogen gas at high pressure and temperature causes hydrogen

embrittlement of pressurized metal reactors resulting in high safety risks.[23,24] In contrast, with

another reducing agent such as formic acid (FA), the necessity of high-pressure vessels, as well

as corresponding risks, can be avoided.[25] FA is an environmental-friendly liquid reducing



- 48 -

agent readily produced from biomass, is also a convenient hydrogen donor and it only generates

carbon dioxide and water as oxidized products.[26] Commercially supplied FA is always in the

form of an aqueous solution. In organic synthesis, however, water is not a widely used solvent

despite its low cost and low environmental toxicity.[27,28] For this reason, there have been few

reports involving an aqueous solution of FA in organic hydrogenation reactions, to the best of

our knowledge.[29-31]

In this work, we developed an environmental friendly selective hydrogenation system for

unsaturated aromatics. The reaction is driven by a renewable energy source visible light

irradiation, FA aqueous solution is employed as a green hydrogen source and the reaction takes

place in water solvent. The supported AuNP photocatalyst is prepared by the impregnation-

reduction method. Five representative types of hydrogenation reactions were investigated

under mild temperatures in aqueous FA solution without other additives. The reaction

mechanism is investigated by isotope study where we found that water plays more of a role

than simply being a solvent but also acts as a hydrogen source in cooperation with FA.

Results and Discussion

The photocatalytic hydrogenation reaction system was tested with five types of

unsaturated aromatics, the catalytic activities of model reactions as well as control reactions,

presented with product yields, are shown in Table 1. The significant difference in the yields

between the reactions under irradiation and in the dark clearly demonstrates the contribution

of light. The explanation is that the hot electrons on the AuNP surface, generated by light

irradiation, are able to provide the activation energy necessary for a reaction of the reactant

adsorbed onto the AuNPs. Such a contribution is quantitatively reflected by the 73% decrease

of apparent activation energy between light irradiated styrene hydrogenation and that of the



- 49 -

same reaction in the dark. In the case of hydrogenation, the decrease was 35.3% decrease

(Figure S1).

The broad functional group tolerance of the new photocatalytic system is demonstrated in Table

1. The irradiation promotes the hydrogenation with excellent tolerance to various functional

groups, especially reducible substituents such as ketones, aldehydes and alkenes. It is worth

noting that FA itself, having a carbonyl group, should be active for the condensation with

amines and alkenes at elevated temperatures (over 70 °C).32 The low operating temperature

allows our photocatalytic reaction system to effectively avoid such side reactions and achieving

high chemo-selectivity.

Table 1. Performance of Au@ZrO2 catalyst for five hydrogenation reactions. The red numbers

are the product yield under visible light irradiation, and the black numbers in the parentheses

are the product yield for the control reaction in the dark.

Hydrogenation of Aromatic Olefins[a]

Hydrogenation of Aromatic Nitro-compounds



- 50 -

Hydrogenation of Aromatic Aldehydes[a]

Hydrogenation of Aromatic Alkyne[d]

Hydrogenation of Aromatic Imine[a]

Reaction conditions: 50 mg catalysts, 1 mmol reactant, 2 mL of 85% FA mixed with 2 mL

deionized water as solvent, light intensity 0.5 W/cm2, 40°C, 1 atm argon, reaction time 8 h. [a]

reaction time 16 h. [b] 80°C, reaction time 24 h. [c] UV light with a peak wavelength at 365



- 51 -

nm, 24 h. [d] 60 °C, 16 h. [e] combined yield of both styrene and ethylbenzene. The products

were identified by mass spectrometry (MS) and yields were measured by gas chromatography

(GC) using external standard.

Compared to aromatic aldehyde (Table 1, entry 21), we found a poor yield (17%) for

aliphatic aldehyde at 40 °C whereas negligible activity was observed in the dark (Table 1, entry

27). The hydrogenation of aliphatic aldehyde demands greater reduction power since the

reduction potential of aliphatic aldehydes is generally more negative than that of aromatic

aldehydes. For example, the reduction potential of acetaldehyde is -1.7 eV and that of

benzaldehyde is -1.36 eV (Table S1). When the hydrogenation of aliphatic aldehyde was

conducted under UV irradiation (365 nm) at 40 °C, a much higher yield of 53% was achieved.

Apart from LSPR excitation, electrons from Au NPs can be directly excited by UV light

through a photon excitation process which also results in energetic electrons in Au NPs, the

energy of UV photons is greater than that of visible photons, meaning the UV photons are able

to generate hot electrons with higher energies than the ones generated by LSPR absorption of

Au NPs in the visible range. These higher energy hot electrons are capable of reducing

compounds with more negative reduction potentials.33 In addition, when the reaction

temperature was raised to 80 °C and the reaction was prolonged to 24 h, the yields for visible

light irradiated reaction and reaction in the dark were 42% and 4%, respectively. Therefore, we

conclude that AuNPs can combine photonic energy (light) and thermal energy (heat or IR

irradiation) because metals have a continuous electron energy level. Above results reveal an

important feature that the reduction power of the reported system is tuneable by regulating the

irradiation wavelength and reaction temperature.



- 52 -

An unexpected finding is that the ratio of water to FA in the FA aqueous solution has a

significant impact on the reductive activity of the photocatalytic system. For example, in our

reaction system, phenylacetylene was reduced to styrene and ethylbenzene along with side

product of acetophenone, which is attributed to the hydration of phenylacetylene in the

presence of a hydroxyl group.34 As shown in Table 2, increasing water/FA ratio can

significantly enhance the yields of reductive products, while the production of acetophenone is

inhibited. In the cases of benzaldehyde and styrene hydrogenation (Table S2), increasing

water/FA ratio can accelerate the reaction rate. These results imply that the role of water is

more than merely acting as a solvent.

Table 2. Impact of water/FA ratio on the reductive performance of phenylacetylene

hydrogenation

Entry H2O/FA

(Volume ratio)

Conversion

%

Selectivity

a b c

1 1.5:8.5 100 0 0 100

2 3.5:6.5 100 0 10 90

3 6:4 100 25 59 15

4 8:2 100 22 73 5

Reaction conditions: 50 mg Au@ZrO2 photocatalyst, 0.3 mmol phenylacetylene as the

reactant, formic acid mixed with H2O as a solvent, 0.5 W/cm2 irradiance, 60 °C, 1 atm argon

gas and 16 h.



- 53 -

Prior to the mechanism investigation, the Au@ZrO2 photocatalyst was characterized.

Figure 1a indicates 3 wt% AuNPs, with a mean size of 7 nm (Figure 1d), are well dispersed on

the ZrO2 surface and predominantly exhibit crystal face (111) (Figure 1b). The light absorption

peak of Au@ZrO2 at 520 nm (Figure 1c) is attributed to the LSPR effect of AuNPs as ZrO2

support exhibits negligible visible light absorption. AuNPs also absorb UV light because

electrons of metal can be directly excited by photons from the ground state to high energy states.

Figure 1. Characterization of Au@ZrO2 photocatalyst. (a) TEM image of Au@ZrO2, (b) High-

resolution TEM image of Au@ZrO2, (c) Diffuse reflectance UV-Visible spectra of Au@ZrO2

and ZrO2; (d) Particle size distribution of Au@ZrO2.

To clarify the reaction pathway, isotope tracking was applied by using D2O and H218O.

We found the H-D ratio in the product ethylbenzene was 28% to 72%, which is close to 1:2.5.

Such an H-D ratio in the product cannot be rationalized by cation exchange between FA and

water (the excessive amount of water in FA solution should causing H-D ratio 1:4 in a typical

reaction). Pure water was found no activity in the hydrogenation. Hence, we hypothesise that

hydrogen atoms are transferred from water to the reactant with the assistance of FA. The role

of water could be overlooked although FA aqueous solution was previously reported as



- 54 -

hydrogen donor.35 Within gaseous products, C18O2 was detected with a 16O:18O ratio

approximately 2:1 when H218O was used. A rational deduction is that FA reacts with H2

18O

producing an intermediate which provides hydrogen to the styrene hydrogenation and itself is

oxidized to C18O2 and H2O. The possible intermediate is orthoformic acid which has the

formula HC(OH)3.36 It is an unstable hydrate, consisting of one water and one FA molecule,

and has not been isolated to date.

The proposed complete route is illustrated in Scheme 1. The hydration of FA (step 1,)

yields orthoformic acid, which is then oxidized on the surface of AuNPs (step 2), yielding H-

Au surface species.37 The orthoformic acid is oxidized possibly following the same principle

as the oxidation of a primary alcohol to carboxylic acid and eventually to carbon dioxide and

water.38 H-Au surface species are capable of reducing olefins by adding two hydrogen atoms

into the C=C double bond basically following Horiuti-Polanyi Mechanism (step 3 and 4,),39

the reaction equation is as follow:

These two reduction steps consume 2 H-Au species and restore the AuNP surface for

subsequent catalytic cycles (step 5,). This inference is supported by the fact that the reduction

is inhibited when removing these surface species by adding 0.1 equiv. of 2, 2’, 6, 6’-

tetramethylpiperidine N-oxyl (TEMPO) to the photocatalytic reaction system, which can

abstract hydrogen atoms from the AuNP surface,40 and found only trace amounts of

ethylbenzene in the product.



- 55 -

Scheme 1. Mechanism for the photocatalytic hydrogenation of styrene.

The proposed reaction mechanism is supported by step to step isotope study. According

to step 1 in Scheme 1, the H-Au (or D-Au) surface species form on the AuNP surface with the

H (or D) atoms from the orthoformic acid and the H or D atom should be eventually found in

the product ethylbenzene. When D2O was used, the orthoformic acid formed in the reaction

has two D atoms and one H atom available to yield the H-Au surface species (product of step

2). Therefore, the ratio of H to D in the product ethylbenzene should be 1:2, which is very close

to our observation mentioned above. Similarly, when H218O was used, the content of 18O atoms

in the product CO2, is in agreement with the experimental observation.

Table 3. Isotope abundance of deuterium and 18O in the products of nitrobenzene

hydrogenation.

Entry Water Conversion (%) Product Abundance (%)

1 D2O 87 PhNH2 H D



- 56 -

34 66

2 H218O 74 CO2 16O 18O

63 37

O2 100 0

Reaction conditions: 50 mg Au@ZrO2, 1mmol nitrobenzene, 2 ml Formic acid mixed with 2

ml water as a solvent, light intensity 0.5 W/cm2, 40 ℃, 1 atm Argon, 8 h. The products were

identified by mass spectrometry MS and yields were measured by GC using external standard.

We also verified the proposed mechanism with the hydrogenation of nitrobenzene.

Similar results were received, shown in Table 3, except pure 16O2 was observed in gaseous

product suggesting a source differ from H218O. We investigated the compositional evolution of

the reaction (Figure 2a). In the first two hours, the nitrobenzene was consumed rapidly and

aniline formed correspondingly. A notable side-product formanilide was detected after 2 hours,

resulting from the condensation of aniline and FA and it was confirmed by a control experiment

where aniline was used as an initial reactant. To our surprise, no azo or azoxy compounds were

detected, suggesting that our reaction system does not strictly follow the Haber’s mechanism.

In the Haber’s mechanism, the nitro group is reduced stepwise first to nitrosobenzene and then

the hydroxylamine, hydroxylamine can easily react with nitrosobenzene yielding

azoxybenzene and then further reduce to azobenzene and eventually aniline, details see Figure

S2. However, when nitrosobenzene was employed as reactant in our system, azobenzene was

produced predominantly along with the consumption of substrate in the first 5 hours, it was

gradually converted to aniline in the rest of reaction period without detection of other

intermediates (Figure 2b), and such process is in harmony with Haber’s mechanism. In addition,

we received a similar result in the reduction of hydroxylamine and azobenzene as shown in



- 57 -

Table S3. Thus, we can conclude that the reduction of nitrobenzene directly yields aniline

instead of flowing a stepwise reduction process, accordingly a tentative mechanism for the

photocatalytic reduction of nitrobenzene is proposed in Figure S3.

The influence of light wavelength on photocatalysis was studied by investigating action

spectra of hydrogenations of styrene, nitrobenzene and benzaldehyde. In an action spectrum,

the photocatalytic efficiency is plotted against light wavelength. Quantum yield (Φ) which was

converted from reaction rate is used to present the photocatalytic efficiency. It was calculated

as follows:

Φ = [the number of converted reactant molecules×100]/[the number of incident photons].

We observed two types of action spectrum as shown in Figure 3. The result indicates that

AuNPs can most efficiently drive the hydrogenation of nitrobenzene by the LSPR effect as the

highest Φ value was observed at 530 nm (Figure 3c). For light spectrum other than LSPR

wavelength, the photocatalytic activity is attributed to the hot electrons directly excited by

photons.



- 58 -

Figure 2. Time-conversion plot. (a) nitrobenzene hydrogenation and (b) nitrosobenzene

hydrogenation. Reaction condition: 200 mg Au@ZrO2 dispersed into a solution of 15 mL of

85% formic acid and 15 mL deionized water, light intensity 0.5 W/cm2, 40 ℃, 1 atm argon,

reaction time 8 h, 0.5 mL specimen was taken every hour and analysed by mass spectrometry

(identifying the species) and GC (determining the concentration of the species using external

standard).



- 59 -

Figure 3. Action spectra of selected hydrogenations. Hydrogenation of styrene (a),

benzaldehyde (b) and nitrobenzene (c) catalysed by Au@ZrO2. The purple line represents the

diffuse reflectance UV-vis spectrum of Au@ZrO2, blue marks represent the quantum yield of

each wavelength.

These hot electrons produced by the photons of shorter wavelength possess higher energy

levels but less population. It explains the relatively high quantum yield of 400 nm in the

hydrogenation of nitrobenzene. The light of 590 nm and 620 nm are neither triggering LSPR



- 60 -

effect nor delivering photons with sufficient energy, therefore resulting in low reaction rate. In

the styrene and benzaldehyde’s case (Figure 3a-b), the reaction demands more reduction power,

meaning hot-electrons with higher energy, due to their much more negative reduction potential

(Table S1). Although a strong LSPR absorption can generate a large population of hot

electrons,[20] many of them do not have sufficient energy to enable absorbed styrene to cross

the activation energy barrier. On the other hand, photons of 400 nm wavelength can generate

hot electrons with sufficient energy to drive the reduction of styrene and benzaldehyde. More

hot electrons generated by LSPR absorption have sufficient energy to drive the benzaldehyde

reduction compared with those in the styrene reduction as the reduction potential of

benzaldehyde is higher than that of styrene.

The dependence of the reaction yield on the irradiance was investigated as well. As

shown in Figure 4, the yield of hydrogenation increases with increasing irradiance for all four

reactions. It is direct evidence that irradiation can significantly promote the hydrogenation

reactions. In addition, the relative contribution of the irradiation in comparison to the thermal

contribution for the hydrogenation reactions is clarified. The grey part of a column is the yield

of the reaction in the dark at the same reaction temperature, which represents the contribution

of thermal effect. When irradiation was applied, the yield increased significantly and almost

linearly. It is also noted that at higher irradiance, the contribution of the irradiation to the overall

reaction rate is greater. For example, when the irradiance was 0.1 W/cm2 the contribution of

light to the yield of nitrobenzene hydrogenation was merely 25.6%, while the contribution was

79% when irradiance was raised up to 0.5 W/cm2. These results indicate that the irradiation is

the predominating force driving the hydrogenation reactions. Generally, higher irradiance can

produce more hot electrons resulting in a higher reaction rate. The hot electrons that have

insufficient energy to induce the reduction will relax and release their energy to heat the lattice



- 61 -

of the AuNP. Such a photo-thermal effect also accelerates the reaction. Therefore, higher

reaction rates are always observed at higher irradiance.

Figure 4. Dependence of the photocatalytic activity of Au@ZrO2 for hydrogenation on

irradiance. The grey part of a column represents the contribution of thermal effect and green

column represents the contribution of light. Hydrogenation of nitrobenzene (a), styrene (b),

benzyl aldehyde (c) and imine (d).

Experimental Section

Materials. Zirconium(IV) oxide (ZrO2, particle size <100 nm, TEM), gold (III) chloride

trihydrate (HAuCl4·3H2O, ≥99.9% trace metal basis), sodium borohydride powder (NaBH4,

≥98.0%), Deuterium oxide (D2O, 99.9 atom% D), Nitrosobenzene (C6H5NO, ≥97%),

azobenzene (C12H10N2, ≥98%), styrene (C8H8, ≥99.9%), N-(phenylmethylene)- Benzenamine



- 62 -

(C13H11N, ≥ 96%) and L-lysine (2,6-diaminocaproic acid, ≥ 97%), were purchased from

Sigma-Aldrich (Australia). H218O (≥96 atom% 18O) was purchased from HuaYi Isotopes Co.

(China). Formic acid solution (HCOOH, 85%), ethanol (CH3OH, ≥95%) and dichloromethane

(CH2Cl2) were of analytical grade. All chemicals were used as received without further

purification unless otherwise noted. The water used in all experiments was prepared by an

Ultrapure Water System from Merck Millipore Co.

Synthesis of Au@ZrO2 photocatalyst. ZrO2 supported AuNPs (Au@ZrO2) was prepared by

the impregnation-reduction method. For example, to prepare 3wt% Au@ZrO2, ZrO2 powder

(2.0 g) was dispersed into an aqueous solution of HAuCl4 (32.7 mL, 0.01 M) under magnetic

stirring at room temperature, followed by addition of a lysine aqueous solution (10 mL, 0.53

M) while it was vigorously stirred for 30 min. The pH value of the mixture was 8-9. To this

suspension, a freshly prepared aqueous NaBH4 (10 mL, 0.35 M) was added dropwise. The

mixture was aged overnight, and then the solid was separated by centrifugation, washed with

water (three times), ethanol (once), and was dried at 60 °C in a vacuum oven for 24 h.

Characterization. The morphology and elemental composition of photocatalysts were studied

using a JEOL 2100 transmission electron microscopy (TEM) coupled with an energy

dispersion X-ray (EDX) spectrometer (X-MAXN 80TLE, OXFORD Instruments). The

accelerating voltage of TEM was 200 KV. Diffuse reflectance UV-visible spectra of the

catalysts were collected with a Varian Cary 5000 spectrometer with BaSO4 as a reference.

General procedure for photocatalytic reactions. In a typical activity test, a 25 mL Pyrex

round-bottom flask was used as a container, and after 1 mmol reactants and 50 mg catalyst had

been added, the flask was filled with 1 atm argon gas and sealed in order to isolate the reaction

from the air. The flask was then stirred magnetically and irradiated with a halogen lamp (from



- 63 -

Nelson, 500W, and wavelength in the range of 400-750 nm). The irradiance was 0.5 W/cm2

unless otherwise specified. The reaction temperature was carefully controlled at 40 oC by using

an air conditioner unless otherwise specified. The reaction system in the dark was conducted

using a water bath placed above a magnetic stirrer, and the reaction flask was wrapped with

aluminium foil to isolate the contents from the influence of light. The reaction temperature was

maintained at the same temperature as the corresponding reaction under irradiation. At the end

of reaction time, the product was firstly extracted with an equivalent amount of

dichloromethane, and then 2 mL aliquots were collected and filtered through a Millipore filter

(pore size of 0.45 µm) to remove particulate matter. The clear liquid-phase products were

analysed with an Agilent 6980 gas chromatography (GC) using a HP-5 column to analyse the

change in the concentrations of reactants and products. An Agilent HP5973 mass spectrometer

was used to identify the products. In the analysis of gaseous products, prior to the photocatalytic

reaction, the reaction tube was purged with argon gas and sealed to isolate the reaction from

the air. After the reaction, a 1 mL gas sample was taken from the atmosphere above the reaction

suspension and analysed using mass spectrometry.

Action spectrum Test. An action spectrum indicates the dependence of reaction rate on the

wavelength of irradiation, which provides evidence for the mechanism of how the photocatalyst

responses to different light wavelengths and activates the reactant molecules. Action spectrum

experiments were conducted with light emitting diode (LED) lamps (Tongyifang, Shenzhen,

China) with wavelengths of 400 ± 5 nm, 470 ± 5 nm, 530 ± 5 nm, 590 ± 5 nm, and 620 ± 5 nm.

The light intensity was measured to be 0.50 W/cm2 using an energy meter (CEL-NP2000) from

AULTT Company and other reaction conditions were maintained identical to those of typical

reaction procedures.

Acknowledgements



- 64 -

Authors gratefully acknowledge financial support from the Australia Research Council.

(DP150102110).

Keywords: Photocatalysis; Visible light; Plasmonic metal nanoparticles, Selective

hydrogenation; Formic acid

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Supporting Information

Visible light driven selective hydrogenation of unsaturated aromatics in aqueous

solution by direct photocatalysis of supported Au nanoparticle

Yiming Huang,[a] Sarina Sarina,[a] Qi Xiao,[b] Wayde Martens,[a] Zhe Liu,[a] Cheng Guo,*[c] and

Huaiyong Zhu*[a]

[a] School of Chemistry, Physics and Mechanical, Faculty of Science and Technology,

Queensland University of Technology, Brisbane, Queensland 4001, Australia

[b] Ian Wark Laboratory, Commonwealth Scientific and Industrial Research Organisation,

Bayview Ave, Clayton, Victoria 3168, Australia

[c] College of Science, Nanjing University of Technology, Nanjing, Jiangsu 211800, China





- 68 -

Table of contents:

1. Apparent activation energy of nitrobenzene hydrogenation

2. Reduction potential of relevant substrate

3. Impact of formic acid-water ratio on the photocatalytic performance

4. Scheme of Haber’s mechanism

5. Time resolved evolution of hydroxylamine and azobenzene hydrogenation

6. Mechanism for the photocatalytic hydrogenation of nitrobenzene

7. Reference



- 69 -

1. Apparent activation energy of light irradiated and dark hydrogenation reaction

Figure S1-1. The apparent activation energy of styrene hydrogenation. 50 mg

Au@ZrO2 photocatalyst, 1 mmol nitrobenzene as reactant, 2 ml formic acid mixed with

2 ml H2O as a solvent, 0.5 W/cm2 irradiance, 1 atm argon gas and 16 h. The apparent

activation energy of catalytic styrene hydrogenation was estimated by using the

Arrhenius equation and kinetic data at different temperatures: 40 °C, 50 °C, 60 °C and

70 °C.



- 70 -

Figure S1-2. The apparent activation energy of nitrobenzene hydrogenation. 50 mg

Au@ZrO2 photocatalyst, 1 mmol nitrobenzene as reactant, 2 ml formic acid mixed with

2 ml H2O as a solvent, 0.5 W/cm2 irradiance, 1 atm argon gas and 14 h. The apparent

activation energy of catalytic styrene hydrogenation was estimated by using the

Arrhenius equation and kinetic data at different temperatures: 30 °C, 40 °C, 50 °C and

60 °C.



- 71 -

2 Reductive potential of relevant substrate

Table S1 Reductive potential of relevant substrates

Entry Substrate Reduction potential (eV)

11 Acetaldehyde -1.7

21 Benzaldehyde -1.36

32 Styrene -2.65

43 Nitrobenzene -0.356



- 72 -

3 Impact of water/formic acid ratio on photocatalytic performance

Table S2-1. Impact of water/formic acid ratio on the photocatalytic performance of

benzaldehyde hydrogenation.

Entry H2O:Formic Acid* Conversion

Volume ratio %

1 1.5:8.5 8

2 3.5:6.5 11

3 6:4 28

4 8:2 42

5 10:0 0

Reaction condition: 50 mg Au@ZrO2 photocatalyst, 1 mmol benzaldehyde as a reactant,

formic acid mixed with H2O as a solvent, 0.5 W/cm2 irradiance, 70 ℃, 1 atm argon gas and

16 h.



- 73 -

Table S2-2. Impact of water/formic acid ratio on the photocatalytic performance of styrene

hydrogenation.

Entry H2O:Formic Acid* Conversion

Volume ratio %

1 1.5:8.5 10

2 3.5:6.5 30

3 6:4 44

4 8:2 41

5 10:0 0

Reaction condition: 50 mg Au@ZrO2 photocatalyst, 1 mmol styrene as a reactant, formic acid

mixed with H2O as a solvent, 0.5 W/cm2 irradiance, 50 ℃, 1 atm argon gas and 5 h.



- 74 -

4 Scheme of Haber’s mechanism

Figure S2. Haber’s mechanism for nitrobenzene reduction

According to Haber’s mechanism,4 the nitro group is reduced stepwise first to

nitrosobenzene and then the hydroxylamine. Hydroxylamine can easily react with

nitrosobenzene yielding azoxybenzene, which is further reduce to azobenzene and

eventually aniline.



- 75 -

5 Time resolved evolution of hydroxylamine and azobenzene hydrogenation

Table S3. Time resolved performance of the hydrogenation using hydroxylamine and

azobenzene.

Reactant Time Conv. %

Select. %

Aniline Azobenzene

Hydroxylamine

1h 100 54 46

2h 100 50 50

4h 100 52 48

Azobenzene 1h 68 100 -

2h 100 100 -

4h 100 100 -

Reaction condition: 50 mg Au@ZrO2 dispersed into a solution of 4 mL of 85% formic acid and

15 mL deionised water, light intensity 0.5 W/cm2, 40 °C, 1 atm argon, reaction time 4 h, 1 mL

specimen was taken every hour and analysed by MS (identifying the species) and GC

(determining the concentration of the species).



- 76 -

6 Mechanism for the photocatalytic hydrogenation of nitrobenzene

Figure S3. Mechanism for the photocatalytic hydrogenation of nitrobenzene.

The first step is that orthoformic acid forms via the hydration of formic acid (step 1), which

can provide hydrogen to yield H-Au species (step 2). This step should appear in all the reactions

in the present study. These H-Au surface species are capable of interacting with the oxygen

atom of the N-O bonds (step 3), yielding OH-Au species on the surface of AuNPs while a

hydrogen atom from another H-Au species is added to the nitrogen atom forming a N-H bond

(step 4). Further reaction of the intermediate with H-Au surface species could yield aniline and

HO-Au surface species (step 5)

In the previously reported photocatalytic system of AuNP catalyst and KOH in isopropanol,[5]

which has weaker reduction power, the one N-O bond in the -NO2 group of nitrobenzene react

with the H-Au surface species yielding nitrosobenzene. The subsequent conversion of



- 77 -

nitrosobenzene to azoxybenzene, azoxybenzene to azobenzene are much easier than that from

azobenzene to aniline, and azoxybenzene was observed. As we did not observe the Ph-(HNO)

and other intermediates in the present study, we cannot exclude that the two N-O bonds in -

NO2 group react simultaneously with the H-Au surface species. In a difference from the

hydrogenation of styrene, this reduction of a nitrobenzene molecule consumes 4 H-Au species

and produces two OH-Au species on the surface of AuNPs. The two OH-Au species release an

oxygen molecule (oxygen gas was detected in the reaction system) and yield two H-Au species

in the subsequent process (step 6). As expected, the addition of TEMPO also stopped this

reaction, further supporting the role of H-Au species in the catalytic cycle.



- 78 -

Reference

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Photocatalytic Dehydrogenation of Aromatic Alcohols Using Au-Pd Alloy Nanoparticle

Catalysts - 79 -

Chapter 4 Photocatalytic Dehydrogenation of

Aromatic Alcohols Using Au-Pd Alloy Nanoparticle

Catalysts


This chapter presents Article 2 (published in Green Chemistry, 2014, 16, 331-341) first

reporting the dehydrogenation of aromatic alcohols using Au-Pd alloy nanoparticle catalyst

under visible light irradiation. Following the work of article 1, in this work, the applications of

Au based photocatalysts are further expanded to selective dehydrogenation of aromatic

alcohols by alloying Au nanoparticles with Pd. This work responds to the research problem

which is the development of new metallic photocatalyst design strategy. Introducing Pd, which

is non-plasmonic metal, into the visible light driven photocatalysis is a step towards the better

applicability of metallic photocatalysts in organic synthesis. The mechanistic study in this work

reveals the roles of each metal component in photocatalysis, investigation of the interaction

between two metal component help understand the bi-metallic photocatalysts and bring

knowledge that would benefit future photocatalysts design.

The selective oxidation of alcohols to the corresponding aldehydes is an essential reaction

type from organic synthesis, however, an Au nanoparticle photocatalyst was found to be

ineffective in the activation of alcohol molecules. Thus, we introduced the transition metal Pd

into the photocatalysts to create supported Au-Pd alloy nanoparticle photocatalysts in which

the Au metal predominantly played the role of light harvesting site whereas the Pd metal was

mainly the catalytic active site. In this work, we applied Au-Pd alloy nanoparticles

photocatalysts in the oxidant-free dehydrogenation of substituted aromatic alcohols at


Catalysts - 80 -

moderate reaction condition (45 oC, 1 atm Argon) under visible light irradiation. The Au:Pd

ratio was found to be critical to the photocatalytic performance, optimised Au-Pd molar ratio

was determined to be 1:1.186. By using density functional theory (DFT) simulation technique,

we revealed that the surface charge heterogeneity on the Au-Pd alloy particle is the dominant

factor which can enhance the chemical adsorption of reactant and alloy nanoparticles. The

theoretical simulation results match the experimental results regarding the optimised Au-Pd

molar ratio. The photoexcited electrons from alloy nanoparticles were the direct energy source

to this photocatalytic action, meanwhile, the strong binding of reactant onto alloy nanoparticles

ensured the energy transfer. This work extended the plasmonic metals to alloy nanoparticle

photocatalyst and thus represents a step toward versatile applications. In addition, the proposed

theory of surface charge heterogeneity in Au-Pd alloy nanoparticle is useful for the future

design of plasmonic metal with other transition metal alloy nanoparticle photocatalysts for

broader applicability.


Catalysts - 81 -

Article 2



Visible light enhanced oxidant free dehydrogenation of aromatic alcohols using Au–Pd alloy

nanoparticle catalysts

Sarina Sarina, Sagala Bai, Yiming Huang, Chao Chen, Jianfeng Jia, Esa Jaatinen, Godwin A.

Ayoko, Zhaorigetu Bao* and Huaiyong Zhu*

Published in Green Chemistry, 2014, 16, 331-341.


Student Author:

Yiming Huang

Fabrications of catalyst, characterisations

including UV-Vis, TEM analysis, EDX

mapping, XRD analysis. Operation of

photocatalytic activity test and followed

data analysis including light wavelength and

intensity dependence experiments.

Participated in the manuscript drafting and

revision related to the conducted

experiments.

Signature

Date


Catalysts - 82 -

Dr Sarina Sarina First author, wrote the manuscript,

experimental design and theoretical

calculation.

Dr Sagala Bai Revise the manuscript and data analysis.

Mr Chao Chen Conducted experiments

A/Prof. Esa Jaatinen Aided experimental design, data analysis.

Prof. Godwin Ayoko Aided experimental design, data analysis.

Prof. Zhaorigetu Bao Aided experimental design, data analysis.

Prof. Huaiyong Zhu The corresponding author, aided




authorship.

____________ _____________ ________________

Name Signature Date


Catalysts - 83 -

Visible light enhanced oxidant free dehydrogenation of aromatic alcohols

using Au–Pd alloy nanoparticle catalysts

Sarina Sarinaa, Sagala Baib, Yiming Huanga, Chao Chena, Jianfeng Jiac, Esa Jaatinena,

Godwin A. Ayokoa, Zhaorigetu Bao*b and Huaiyong Zhu*a

a School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Brisbane, QLD4001, Australia

b School of Chemistry, Inner Mongolia Normal University, Hohhot, China.

c School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China.


Abstract: We find that visible light irradiation of gold-palladium alloy nanoparticles supported on

photocatalytically inert ZrO2 significantly enhances their catalytic activity for oxidant-free

dehydrogenation of aromatic alcohols to the corresponding aldehydes at ambient temperatures.

Dehydrogenation is also the dominant process in the selective oxidation of the alcohols to the

corresponding aldehydes with molecular oxygen. The alloy nanoparticles strongly absorb light and

exhibit superior catalytic and photocatalytic activity when compared to either pure palladium or gold

nanoparticles. Analysis with a free electron gas model for the bulk alloy structure reveals that the

alloying increases the surface charge heterogeneity on the alloy particle surface, which enhances the

interaction between the alcohol molecules and the metal NPs. The increased surface charge

heterogeneity of the alloy particles is confirmed with density functional theory applied to small alloy

clusters. Optimal catalytic activity was observed with an Au : Pd molar ratio of 1 : 186, which is in

good agreement with the theoretical analysis. The rate-determining step of the dehydrogenation is

hydrogen abstraction. The conduction electrons of the nanoparticles are photo-excited by the

incident light giving them the necessary energy to be injected into the adsorbed alcohol molecules,




Catalysts - 84 -

promoting the hydrogen abstraction. The strong chemical adsorption of alcohol molecules facilitates

this electron transfer. The results show that the alloy nanoparticles efficiently couple thermal and

photonic energy sources to drive the dehydrogenation. These findings provide useful insight into the

design of catalysts that utilize light for various organic syntheses at ambient temperatures.

Introduction

Photocatalytic processes are generally able to drive reactions at ambient temperature and

pressure. As a result, product selectivity is improved and unstable intermediates of thermal reactions

may be produced as products of photocatalytic reactions. However, the bulk of all reported

photocatalytic studies focus on semiconductor photocatalysts such as TiO2, ZnO and CdS and their

application in decomposing organic pollutants,1 the development of new solar cells and the

production of hydrogen and oxygen from water.2 To date, only limited progress has been reported

in applying TiO2 and Nb2O5 semiconductor photocatalysts to synthesize organic chemicals.3–5 This

is largely because of the limited light absorption and low efficiency of these photocatalysts. Due to

their band structure, the well-known TiO2 photocatalysts only show significant light absorption of

ultraviolet (UV) light.1,2,6,7 Consequently, the efficiency of the semiconductor photocatalysts,

expressed as photon yield, is typically low.7 To overcome this, various methods have been

developed to produce efficient visible light photocatalysts from semiconducting materials.8 In recent

years, photocatalysis with nanoparticles (NPs) of plasmonic metals, such as gold (Au), silver (Ag)

and copper (Cu), has attracted significant interest because of their exceptional absorption of visible

light and thus their potential as visible light photocatalysts.9–17 Plasmonic metal nanoparticles exhibit

strong visible-light absorption due to the localised surface plasmon resonance (LSPR).18–20 In

addition they also strongly absorb ultraviolet (UV) light due to inter-band electron transitions (e.g.

for Au, between the 5d and 6sp bands).21–23 The NP conduction electrons gain the irradiation energy,

resulting in high energy electrons at the surface. These energetic electrons, which remain in an

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Catalysts - 85 -

excited ‘hot’ state for up to 0.5–1 ps,24 can promote chemical reactions of molecules on the surface

of the NPs.25,26

The underpinning photocatalytic mechanisms of the Au and Ag NPs supported by insulating

solids with very wide band gaps are quite distinct from those of semiconducting photocatalysts.9–

12,25,26 These highly efficient NP photocatalysts have several important advantages over their

extensively studied semiconductor counterparts. (1) The NP conduction electrons gain light energy,

resulting in high energy electrons at the NP surface which is advantageous for activating molecules

adsorbed on the NPs for chemical reactions. (2) Since both light harvesting and the catalysing

reaction take place on the NPs, the photon efficiency is not significantly affected by photo-excited

charge migration. (3) The density of the conduction electrons at the NP surface is much higher than

that at the surface of any semiconductor, so once photo-energized more electrons are available to

drive reactions. (4) The metal NPs have a strong affinity for some reactants, such as CO and organic

compounds, making the NPs superior photocatalysts for organic synthesis reactions compared to

semiconductor photocatalysts.

However, the total number of chemical reactions that can be catalysed by the three plasmonic

metals is relatively few when compared to those thermally catalysed by non-plasmonic transition

metals. To develop catalysts for light driven synthesis of a broad range of organic chemicals, we

presented a unique but viable approach: alloying gold and a transition metal such as Pd, which is

thermally catalytically active for many reactions.13 Thus, the light energy absorbed by the gold can

enhance the intrinsic catalytic activity of Pd at moderate temperatures. A typical example is the

selective (or partial) oxidation of aromatic alcohols to the corresponding aldehydes, which can be

catalysed by Pd catalysts but not Au catalysts. Here we show that Au–Pd alloy NPs exhibit superior

catalytic activity when exposed to visible light irradiation at ambient temperatures than that

displayed by NPs made from either pure component metals.







Catalysts - 86 -

Selective oxidation of alcohols is widely considered one of the most fundamental

transformations in both laboratory and industrial synthetic chemistry because the product carbonyl

compounds can serve as important and versatile intermediates for fine chemical synthesis.27–30

Permanganate and dichromate have been traditionally employed31,32 but they are expensive and/or

toxic. For environmental and economic reasons, metal-catalysed reactions using molecular oxygen

as an oxidant are particularly attractive. Many of the reactions are conducted under heating and/or

high pressure to achieve a better reaction efficiency. Since the first successful example of palladium-

catalysed aerobic oxidation of alcohols in 1977 by Blackburn and Schwartz,33subsequent efforts

have extended the substrate scope and efficiency of palladium catalysts. However, the reported Pd(II)

salt based homogeneous catalysts34–37 require high catalyst concentration and an excess of ligands

or bases. On an industrial scale, the problems related to corrosion and plating out on the reactor wall,

handling, recovery, and reuse of the catalyst represent serious limitations of these processes.38 Solid

catalysts active in the liquid phase under mild conditions have a much broader application range.

Relatively few heterogeneous Pd (supported Pd nanoparticles and Pd II)) are available. For example,

Pd on hydrotalcite,39,40 carbon,39,41 Al2O3,39 SiO2,39 pumice,42 SiO2–Al2O3 mixed oxide,43 TiO244 and

polymer supported Pd.45 Those catalysts, both in the metallic NPs or immobilized ionic state, can

catalyse the benzyl alcohol oxidation at elevated temperatures and/or high pressure. Recently it was

reported that activation of molecular oxygen is the key step in the selective oxidation of aromatic

alcohols using TiO2 as photocatalysts under UV irradiation.

The use of Au and Pd alloy NPs as visible light photocatalysts for the selective oxidation of

aromatic alcohols will have an underlying mechanism different from those aforementioned

processes. It is known that during selective oxidation, the aromatic alcohols may undergo abstraction

of a hydrogen atom bonded to the α-carbon atom (the carbon atom of the methylene group bonded

to the hydroxyl group), which is denoted as α-H, followed by abstraction of the hydrogen atom from















Catalysts - 87 -

the hydroxyl group.46,47 If the first hydrogen abstraction is the rate-determining step and can take

place via a photocatalytic process, high reaction temperature and high oxygen pressure are not

necessary for selective oxidation. In the present study, we find that light irradiation can drive the α-

H abstraction of aromatic alcohols with Au–Pd alloy NPs and thus the transformation from alcohol

into the corresponding aldehyde can be achieved in an oxidant free environment at ambient

temperatures. The α-H abstraction is the rate-determining step of the selective oxidation. Theoretical

calculations by two independent methods show that the alloying of gold and palladium enhances the

interaction between the alcohol molecules and the alloy NPs. The strong interaction facilitates the

transfer of light excited electrons of the alloy NPs to the alcohol molecules adsorbed on the NPs,

and such an electron transfer enables the hydrogen abstraction under moderate conditions.

Understanding this mechanism is useful for developing photocatalytic processes for other important

syntheses.

Results and discussion

Performance of the photocatalysts

In the present study, Au and Pd alloy NPs with varying relative ratios were prepared on

a ZrO2 support (abbreviated as Au–Pd@ZrO2, and details are given in ESI†). The metal

nanoparticles on the support were well dispersed avoiding aggregation of the particle. ZrO2 has

a wide band gap (5 eV) and exhibits no visible light absorption. Hence, the support does not

contribute to the photocatalytic activity. Fig. 1 shows the photocatalytic performance of the

Au–Pd@ZrO2 photocatalysts with various Au : Pd mass ratios in catalysing the

dehydrogenation of aromatic alcohols. All experiments were conducted under an Ar gas

atmosphere after a strict freeze–pump–thaw degassing process to remove O2. The conversion

rates achieved under light irradiation (in blue) are compared with those obtained in the dark (in

black).


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http://pubs.rsc.org/en/content/articlehtml/2014/gc/c3gc41866a#imgfig1


Catalysts - 88 -

Fig. 1 Dehydrogenation of aromatic alcohols with the Au–Pd@ZrO2 catalysts of various Au : Pd

mass ratios under visible light irradiation (blue bar) and in the dark (black bar). All of the data

represent the averages of triplicate runs with a mean variation of less than ±3%. The quantum yield

(Q.Y., %) and its calculation method are given in ESI.† A(1) Benzyl alcohol dehydrogenation using

the catalyst of 3% Au–Pd@ZrO2; the reaction proceeded for 5 h. A(2) 4-Methyloxy benzyl alcohol

dehydrogenation, reaction for 2 h. A(3) 1-Phenylethanol dehydrogenation, reaction for 22 h.

Reaction conditions: 2 mmol of the reactant, 50 mg of catalyst in trifluorotoluene solvent at 45 °C

and under 1 atm of Ar after O2 removal using freeze–pump–thaw degassing. B(1) Benzyl alcohol

dehydrogenation, 2% Au–Pd@ZrO2, reaction for 16 h. B(2) 2-Phenylethanol dehydrogenation, 2%

Au–Pd@ZrO2, reaction for 16 h. B(3) 3-Phenylpropanol dehydrogenation, 2% Au–Pd@ZrO2,

reaction for 16 h. Reaction conditions: 1 mmol of the reactant, 50 mg of catalyst in trifluorotoluene

solvent at 45 °C and under 1 atm of Ar after O2 removal using freeze–pump–thaw degassing.



Catalysts - 89 -

The results in Fig. 1 demonstrate that visible light irradiation increased the product yield of

the dehydrogenation product of aromatic alcohols – corresponding aldehydes. For example, the

conversion rate of benzyl alcohol with the Au–Pd@ZrO2catalyst with an Au : Pd mass ratio of 1 :

1 is 100%. In contrast, the rate is 43% when the reaction was conducted in the dark. Similar trends

are observed in other reactants, for example, the conversion rate of 4-methoxybenzyl alcohol is 99%

under light irradiation but 54% in the dark at the same reaction temperature. Blank experiments

without metal NPs were also conducted with just the photocatalytically inactive ZrO2 supports

dispersed in a toluene solution of benzyl alcohol. Under otherwise identical conditions, no alcohol

conversion was observed under visible light irradiation or in the dark. We found that at identical

reaction temperatures and identical reaction periods, the conversion rates of the oxygen-free

dehydrogenation of aromatic alcohols (under an argon atmosphere) are comparable to those of the

selective oxidation conducted under an oxygen atmosphere. This demonstrates that the rate-

determining step of the transformation from alcohols into the corresponding aldehydes with Au–

Pd@ZrO2 photocatalysts is dehydrogenation, and light irradiation significantly enhances the

dehydrogenation.

Compared to the Au–Pd@ZrO2 catalyst containing 3% of alloy NPs, the Au–Pd@ZrO2

catalyst with lower metal loading (2%), the reaction time required to achieve the same conversion

rate with identical reaction conditions was longer, as shown in Fig. 1B. The impact on the

photocatalytic activity of varying mass ratios of the two metals with 2% metal loading has the same

trend as that observed with the catalysts with 3% metal loading.

As can be seen in Fig. 1, the alloy NP catalysts exhibited significantly higher activity than

pure Au NPs (Au : Pd ratio of 1 : 0) or pure Pd NPs (Au : Pd ratio of 0 : 1) for the dehydrogenation

of alcohols under visible light irradiation (>420 nm). This fact implies that Au–Pd@ZrO2 catalysts





Catalysts - 90 -

are not collections of Au NPs and Pd NPs; instead, Au and Pd exist as binary alloy particles in these

samples.

Transmission electron microscopy (TEM) analysis of the NPs (Fig. 2) shows that the mean

diameters of the Au–Pd alloy NPs are less than 10 nm. Fig. 2B is a line profile of the energy

dispersion X-ray (EDX) spectrum of a typical Au–Pd alloy NP in Fig. 2A, which shows the

elemental distribution along the radial direction of the metal NP. The line profile indicates that the

NP consists of both Au and Pd distributed spherically around a common centre. This means that the

two metals exist as binary alloy NPs in this sample. No diffraction peaks corresponding to either

metallic Au or metallic Pd were observed from X-ray diffraction (XRD) patterns of the Au, Pd and

Au–Pd samples (not shown) because of the low metal content of the samples.

Fig. 2 (A) TEM image of 1.5% Au–1.5% Pd@ZrO2 catalyst. The arrows indicate Au–Pd NPs. (B)

High resolution TEM images of an alloy particle in the catalyst. (C) EDX spectrum line profile





Catalysts - 91 -

analysis of a typical Au–Pd NP indicated along the blue arrow in Fig. 2A, providing information of

the elemental composition and distribution of the NP.

The formation of Au–Pd alloy NPs is also supported by the significant change of light

absorption properties of the sample as shown in Fig. 3.48–50 Therefore, the catalytic properties of this

sample resulted from Au–Pd alloy NPs rather than a collection of discrete Au NPs and Pd NPs.

Fig. 3 Diffuse reflectance UV-visible (DR-UV-vis) spectra of the photocatalysts and ZrO2 support.

ZrO2 has a band gap of about 5 eV,51 and exhibits weak visible light absorption with no charge

carriers generated under irradiation of light with wavelengths above 400 nm (Fig. 3). Therefore, the

ZrO2 support by itself does not contribute to photocatalytic activity. The absorption peak at 520 nm

in the spectrum of the pure Au (3 wt%) sample is due to the LSPR absorption of the Au NPs.21,25,26,48–

50,52–55 The presence of the support and its interaction with the Au NP can shift the resonance to

longer wavelengths and broaden the LSPR absorption peak. The LSPR absorption band of Pd NPs

is in the UV range at a wavelength of 330 nm.18 However, this absorption is not observed in the

extinction spectrum of the pure Pd NPs on ZrO2.










Catalysts - 92 -

Interestingly, we observed a clear light absorption near the Pd absorption band (350 nm) in

the spectrum of Au–Pd alloy particles on ZrO2 (trace b, Fig. 4B), which is believed to be associated

with Au–Pd alloy NPs.18 In the spectrum of the alloy sample, the characteristic LSPR absorption

peak of Au NPs at 520 nm is much weaker when compared to the spectrum of the pure Au sample,

but larger than the absorption observed for the pure Pd sample.

Fig. 4 (A) The dependence of the catalytic activity of the Au–Pd@ZrO2 catalyst for the benzyl

alcohol dehydrogenation on the wavelength of light irradiation. Both the light-driven reaction and

the reaction in the dark were conducted at 45 °C ± 1 °C. (B) Action spectrum based on the data in

(A), and the light absorption spectra of Au–Pd@ZrO2 and Au@ZrO2 are given for comparison.




Catalysts - 93 -

The influence of light irradiation

The wavelength and intensity are important irradiation energy parameters and, hence, they are

the critical factors influencing the performance of photocatalysts in reactions. The dependence of

the photocatalytic dehydrogenation of benzyl alcohol on the wavelength and intensity was

investigated. The results are illustrated in Fig. 4.

Optical filter glasses were applied to block irradiation below specific cut-off wavelengths.

Therefore, we were able to tune the wavelength of the light to clarify the influence of the wavelength

range on the catalytic activity of the Au–Pd@ZrO2 catalyst for benzyl alcohol oxidation. When light

with wavelengths in the full 400 nm to 800 nm range irradiated the reaction system a 100% reaction

conversion was observed. Applying a filter that blocked wavelengths below 490 nm resulted in a

decrease in the conversion of the reaction to 83%. Increasing the cut-off wavelength to 550 nm and

then 600 nm resulted in the conversion decreasing to 65% and 46%, respectively. Given that the

thermal conversion rate at this temperature is 44%, the contribution of light irradiation in the

wavelength range of 400–800 nm to the overall catalytic activity is 56%.

From the results acquired from the irradiation of tuned wavelength we can estimate the

contribution of the light in a narrow wavelength range. For example, the yield of the reaction is 83%

when the light with wavelengths below 490 nm was cut off. Since the reaction temperature was held

constant, the contribution of the thermal reaction remains constant regardless of the filter used.

Therefore, the observed decrease in the yield of 17% (= 100%–83%) can be attributed to light

irradiation by wavelengths in the 400 nm–490 nm range. The enhancement in the yield due to the

light irradiation by wavelengths in the 490 nm–800 nm range is 39% (= 83%–44%), which accounts

for 47% of the overall yield achieved (83%). When the system was irradiated with light with

wavelengths in the 600 nm–800 nm range, the enhancement in the yield decreased to 4%. As shown

by the results in Fig. 4A, light irradiation in the wavelength range of 490 nm–600 nm results in the




Catalysts - 94 -

largest enhancement in yield, accounting for 66% of the total light irradiation enhancement

(conversion rate difference between photocatalytic reaction and that observed in the dark), 32%

results from the light irradiation with wavelengths between 490 nm and 550 nm, 34% from light

between 550 nm and 600 nm, while the light with wavelengths between 400–490 nm and 600–800

nm account for the remaining 34%. The total light energy absorbed by the NPs was estimated from

the overlap of the absorption spectrum of the Au–Pd@ZrO2 catalyst, and the spectral distribution of

the light irradiated. It was found that 36.2% of the total light energy absorption by the NPs was in

the 490 nm–600 nm wavelength range. Plotting the enhancement caused by light irradiation from

the different wavelength ranges (the vertical axis on the right hand side) against the light wavelength

reveals the action spectrum (Fig. 4B). It shows which light wavelengths are most effectively used in

specific chemical reactions. Given that the LSPR peak of Au NPs is in the wavelength range

between 500 nm and 600 nm, these results suggest that for Au–Pd@ZrO2 photocatalysts, it is the

gold that harvests visible light and that the gold nanostructure's LSPR plays an important role in

enhancing the reaction yield in alloy NP catalysed reactions.

A different wavelength range is found to produce the most significant enhancement in

performance of the pure Au NP photocatalyst from that observed for Au–Pd@ZrO2. In our recent

study on selective reactions catalysed by the Au NPs under visible light, acetophenone

hydrogenation to 1-phenyl ethyl alcohol and styrene oxide reduction to styrene,14,15 light irradiation

in the wavelength range between 490 nm and 550 nm made the most important contribution to

driving the reaction. The contributions from light in this wavelength range for the two reactions are

41% and 65%, respectively. The contribution of light in the wavelength range of 550–600 nm is

much less (24% and 0% for the two reactions, respectively), while for the Au–Pd@ZrO2catalyst in

the present study, the contribution of light within the wavelength range of 550–600 nm is even more

significant than that of light in the range of 490–550 nm. In other words, the effective wavelength




Catalysts - 95 -

range of Au–Pd alloy NPs is broader (490–600 nm) than that of Au NPs. This phenomenon can be

attributed to the formation of the alloy NPs, and it also reveals that Au–Pd alloy NPs have the

potential to utilize light energy from a wider portion of the visible light spectrum for enhancing

reactions than pure Au NPs.

The impact of the light intensity on the catalyst performance was investigated while keeping

other experimental conditions unchanged. Fig. 5 shows the rate of benzyl alcohol dehydrogenation

over the Au–Pd@ZrO2 catalyst with an Au : Pd molar ratio of 1 : 1 as a function of light intensity at

45 °C ± 1 °C and 60 °C ± 1 °C, respectively. When the light intensity increased (the reaction

temperature of the reaction mixture was controlled at 45 °C; the only parameter changed is light

intensity), the conversion of benzyl alcohol oxidation increased linearly up to a light intensity of 0.8

W cm−2. Further increase in light intensity results in much greater rate increases and the relation

between light intensity and reaction rate becomes nonlinear. This is a feature of the chemical

processes driven by the light excited electrons of metals.16 It is also possible that when the light

intensity is very high, multi-photon absorption occurs, increasing the number of excited metal

electrons with sufficient energy to drive the reactions.




Catalysts - 96 -

Fig. 5 Light intensity dependent photocatalytic activity of Au–Pd alloy NPs for the transformation

from benzyl alcohol into benzaldehyde at 45 °C (A) and 60 °C (B).

The light induced enhancement on the conversion was calculated by subtracting the observed

conversion of a reaction performed in the dark from the conversion observed under light irradiation

performed at the same temperature. This allows the photo-induced and thermal contributions to the

conversions to be determined and expressed as a percentage for each process, as shown in Fig. 6. It

shows clearly that higher light intensities result in a larger light enhanced contribution to the total

conversion rate.



Catalysts - 97 -

Fig. 6 Light intensity dependent activity of Au–Pd@ZrO2 photocatalysts on oxidant free

dehydrogenation of benzyl alcohol.

The influence of reaction temperature

Increasing operating temperature was observed to increase the photocatalytic rate of benzyl

alcohol oxidation. Fig. 7 shows that at a constant light intensity (we applied 0.4 W cm−2 as an

example), increasing operating temperature results in a significant increase of the photocatalytic

reaction rate. There are two critical aspects of the temperature effect on the photocatalytic rate. First,

the relative population of the adsorbed reactant molecule with excited states increases according to

the Bose–Einstein distribution at higher temperatures, which means that the reactant molecule

(benzyl alcohol or oxygen molecule) will require less energy to overcome the activation barrier, and

this “less energy” could be provided by light irradiation.16,56 Second, at higher temperature more




Catalysts - 98 -

electrons of the alloy NPs are in higher energy levels which can be excited by light irradiation

yielding more electrons with sufficient energy to induce reactions in the alcohol molecules adsorbed

on the alloy NPs.

Fig. 7 Dependence of the photocatalytic rate of benzyl alcohol oxidation on reaction temperature

at a constant light intensity.

The influence of the Au : Pd component ratio

As shown by the results in Fig. 1, the Au : Pd mass ratio of the alloy particles is a key

influencing factor on the catalytic performance of the oxidant free dehydrogenation of the aromatic

alcohols both under light illumination and in the dark. All reactions achieved the highest yield of

target products when the Au : Pd mass ratio of the alloy NPs was 1 : 1 (molar ratio of 1 : 1.86). Alloy

NPs with other Au : Pd mass ratios proved to be much less active. When the reactions were

performed in the dark, the same dependence on the Au : Pd mass ratio was observed but with much

lower conversion efficiencies. Catalytic processes driven by heating Au–Pd alloy catalysts have

been reported in the literature. It was found that Au–Pd alloy NPs could catalyse the hydrogenation

of cinnamaldehyde to cinnamyl alcohol.57 Toshima et al. reported that Au–Pd alloy NPs were more

active in catalysing the hydrogenation of 1,3-cyclooctadiene than either gold or palladium alone.58,59





Catalysts - 99 -

A possible explanation for the superior catalytic activities of Au–Pd alloy NPs to NPs

consisting of either a pure component is that Au can isolate active Pd sites within bimetallic

systems.60 In the present study, to confirm the significant role that the Pd sites at the alloy interface

play in the catalytic processes, a sample of NPs with a Pd core and an Au shell was prepared by

reducing HAuCl4 on the Pd-ZrO2 support with H2. This sample has almost no Pd sites at the NP

surface and exhibited low activity for selective oxidation of benzyl alcohol (10% conversion

compared to 100% achieved by the Au–Pd alloy NPs with a similar Au : Pd ratio). However, the

existence of Pd sites at the alloy surface does not explain why the optimal catalytic activity was

observed when the Au : Pd molar ratio is 1 : 1.86. The underlying cause of this dependence on alloy

composition is believed to be related to the higher surface charge heterogeneity of the alloy NPs,

compared with those of either pure component NPs.13,61 Pure palladium has a slightly larger work

function (ΦPd ∼ 5.6 eV) than pure gold (ΦAu ∼ 5.3 eV), so once the two metals are in contact,

electrons will redistribute between gold and palladium until equilibrium is reached and the chemical

potentials are equal everywhere in the NPs (see Fig. 8). This electron redistribution between Au and

Pd enhances surface charge heterogeneity of the NPs (the surface charge of an NP of pure metal is

not homogeneous). The greater surface charge heterogeneity results in an enhanced interaction

between the alcohol and the NP. The enhanced interaction may lower the activation energy of the

oxidation and thus enhance the catalytic activity. Furthermore, the Fermi level in alloy NPs, Φalloy,

is higher than that in pure Pd NPs, ΦPd, so that the transfer of electrons at the Fermi level of the alloy

NPs to the benzyl alcohol molecule adsorbed on the NPs is easier compared with that from the Fermi

level of pure Pd NPs to the adsorbed molecule. The electron transfer causes the transformation of

benzyl alcohol molecules as discussed later. The light absorption of gold results in energetic

conduction electrons, which are in an even higher energy level Φalloy* and have a better ability to





Catalysts - 100 -

migrate to the Pd sites on the surface. This further increases the possibility of electron transfer from

the alloy NPs to the reactant molecules.

Fig. 8 Schematic profiles showing the impact of alloying and visible light irradiation of an Au–Pd

alloy NP. (A) The surface electronic properties of the alloy NPs are different from those of pure gold

NPs as there are Pd islets on the alloy NP surface. The Pd sites are electron-rich because Pd has a

slightly larger work function than gold and electrons will flow from gold to palladium until

equilibrium is reached (the chemical potentials of the electrons are equal in the two metals, being

Φalloy). (B) The light absorption by gold results in energetic conduction electrons, which migrate to

the Pd sites on the surface. The Fermi level of the alloy NPs under light irradiation is higher (Φalloy*)

than that without irradiation (Φalloy), which increases the charge transfer possibility from the NP to

the reactant molecule. Thus the surface Pd sites with energetic electrons could exhibit significantly

enhanced catalytic activity even at ambient temperatures. Since in such an alloy NP structure the

energy of incident light is very efficiently utilised to enhance the intrinsic catalytic activity of

palladium, efficient photocatalysts may be developed from it for the synthesis of organic chemicals.

(C) Electron transfer from gold to palladium in the alloy NPs, expressed as ΔN, varies with the


Catalysts - 101 -

composition of the alloy NPs (the curve). ΔN reaches a maximum when the atomic ratio of Au and

Pd in alloy NPs is 1 : 1.86 (mass ratio 1 : 1). The information on gold content of the alloy NPs

(horizontal axis) and photocatalytic conversion (vertical axis on the right hand side) of the reactions

in the present study is also given in the figure (the symbols). The photocatalytic efficiency of the

alloy NP photocatalysts is strongly correlated to the electron transfer ΔN.

Since the increased surface charge heterogeneity of the alloy NPs is due to the electron

redistribution between Au and Pd, we estimated the increase (details are provided in ESI†) by using

a free electron gas model.51 The analysis reveals that the number of electrons transferred between

the two metals, (ΔN), is maximum when the molar ratio of the two metals in the alloy NPs is 1 :

1.86. A plot of the electron transfer (ΔN; refer to the axis on the left hand side) predicted by the

model as a function of the gold mole fraction (%) in the Au–Pd alloy NPs is shown in Fig. 8C. The

catalytic conversion rates (refer to the axis on the right hand side) of the photocatalysts for oxidant

free dehydrogenation of benzyl alcohol are also given (the symbols; refer to the axis on the right

hand side). The results in Fig. 8C demonstrate a strong correlation between ΔN and the catalytic

conversion efficiency of reactants over the alloy NP catalysts. Note that this electron transfer ΔN

analysis uses parameters of bulk particles and is independent of the size of alloy particles.

We also carried out simulations using the density functional theory (DFT) for electron states

with and without light irradiation; the irradiation wavelength range between 532 and 535 nm is

chosen, which is around the LSPR absorption of Au. Calculation capacity limitations of our DFT

simulation necessitated the examination of a Pd32, Au32, and Au12Pd20 cluster. The Au : Pd ratio of

the Au12Pd20 cluster is 1.67 close to the ratio of 1 : 1.86 for the optimal Au–Pd@ZrO2 photocatalyst.

The detailed calculation method and the calculated Mulliken charge distributions are given in ESI.†

The DFT simulation results confirm that charge heterogeneity exists even in the monometallic Pd

clusters and monometallic Au clusters (Fig. 9), and the alloy structure of Au and Pd increases the








Catalysts - 102 -

charge heterogeneity of the NP surface. This result is consistent with that of the free electron gas

model analysis and previous reports.62,63 Furthermore, the comparison of simulation results without

irradiation (blue lines) with the results under irradiation (red lines) suggests that light irradiation also

promotes the charge heterogeneity in Au–Pd alloy NPs.

Fig. 9 The optimized geometry and the natural charge distributions of the Au32 cluster (A) and

Au12Pd20 clusters (B) in the ground state and the considered excited state. (C) Apparent activation

energy reduction of benzyl alcohol dehydrogenation caused by the light irradiation on the Au–Pd

alloy NP photocatalyst.

The apparent activation energies of the oxidant free dehydrogenation of benzyl alcohol under

light irradiation and in the dark were derived from the kinetic data of the reaction at different

temperatures (details are provided in ESI†) using the Arrhenius equation. The difference between

the activation energies under light irradiation and in the dark (ΔEa) indicates the contribution of the

light irradiation to reducing the apparent activation energy.14,15 For example, as illustrated in Fig.

9C, the apparent activation energy for the dehydrogenation in the dark is ∼74.5 kJ mol−1 and it is

∼58.7 kJ mol−1 for the reaction under visible light illumination. The apparent activation energy in

the dark is in agreement with those reported by Bavykin et al.64 (79 kJ mol−1 for the ruthenium-

catalysed oxidation of benzyl alcohol), and by Ilyas et al.65 (77.8 kJ mol−1 for a Pt/ZrO2 catalyst









Catalysts - 103 -

system). Visible light irradiation reduces the activation energy of the partial oxidation by 15.8 kJ

mol−1, which represents 21% of the activation energy.

It was reported that the first step of alcohol oxidation with a Pd catalyst was alcohol

dehydrogenation, which resulted in adsorbed hydrogen on the Pd surface and the formation of

aldehyde.66 The rate-determining step of the alcohol oxidation on gold catalysts is believed to be the

hydrogen abstraction from alcohol and the formation of Au–H species; the role of oxygen in this

reaction is to remove hydrogen from the gold surface, leading to the catalytic cycle.67,68 In order to

clarify the reaction mechanism we investigated the hydrogen abstraction from alcohol to the surface

of Au–Pd NPs. 2,2′,6,6′-Tetramethylpiperidine N-oxyl (TEMPO) is a good hydrogen abstractor, and

can abstract hydrogen from the surface of metals to form hydroxylamine but not from alcohol

molecules.69 If the addition of TEMPO can result in alcohol dehydrogenation in the absence of

oxygen, then it proves that hydrogen transfer from the alcohol to Au–Pd NPs occurs in the reaction

because the TEMPO can only abstract the hydrogen on the NP surface. Here TEMPO plays a role

similar to oxygen: removing hydrogen from the NP surface to complete the catalytic cycle so that

the catalytic conversion of alcohol to the corresponding aldehyde could proceed.

Benzyl alcohol dehydrogenation with Au–Pd@ZrO2 (Au : Pd ratio of 1 : 1.86) catalyst at

45 °C under light irradiation was conducted under a N2 atmosphere with 300 mg of TEMPO added

to the reaction system. After visible light irradiation, 21% of the benzyl alcohol was converted to

benzaldehyde. No conversion was observed in the dark at 45 °C. When the reaction was conducted

at 80 °C in the dark, 30% of the benzyl alcohol was converted by the TEMPO. Evidently, in the

absence of oxygen, TEMPO could abstract hydrogen atoms and drive alcohol oxidation either under

visible light irradiation or in the dark at a higher temperature. Blank experiments of TEMPO addition

without metal NPs (ZrO2 support only) were also conducted at both 45 °C and 80 °C. No conversion





Catalysts - 104 -

was observed even under visible light, indicating that TEMPO could not abstract hydrogen atoms

directly from the alcohol but could capture them from the surface of Au–Pd NPs.

It is known that light excited electrons of plasmonic metal NP can populate unoccupied

orbitals of the molecules adsorbed on the NPs yielding a transient anion species.16,25,70 Results of a

DFT simulation (the detailed calculation method is given in ESI†) show that in a benzyl alcohol

molecule, the distances between the α-C and the two H atoms are 1.098 and 1.100 Å, respectively,

while in the transient benzyl alcohol anion species one C–H distance remains at 1.100 Å while the

other elongates to 1.104 Å. The energy required to break one of the C–H bonds at the α-C in the

molecule is 371 kJ mol−1, but only 242 kJ mol−1 is required to break the longer C–H bond of the α-

C in the transient anion species. Hence the light irradiation can facilitate the hydrogen abstraction

from the α-C through the excitation of NP electrons to the benzyl alcohol molecules adsorbed on

them. Therefore, the irradiation reduces the apparent activation energy of the reaction.

On the basis of these facts, a tentative mechanism for selective benzyl alcohol oxidation is

proposed as depicted in Scheme 1. The results of the influence of light intensity and the action

spectrum indicate that the rate determining step of the partial oxidation takes place on the surface of

the supported alloy NPs. The first step should be the abstraction of α-H atoms from the alcohol

molecules. Once this abstraction is completed the subsequent abstraction of the hydrogen atom from

the hydroxyl group of the transient anion species proceeds readily producing aldehyde as the final

product while the negative charge of the transient anions returns to the alloy NPs. The charge

injection to the reactant on the plasmonic metal NPs and the charge return to the NPs after reaction

are known processes.25,26 Oxygen or TEMPO takes away the hydrogen on the NP surface yielding

water or hydroxylamine, respectively, and thus, the NPs’ ability for dehydrogenation of alcohol is

restored.



http://pubs.rsc.org/en/content/articlehtml/2014/gc/c3gc41866a#imgsch1



Catalysts - 105 -

Scheme 1 Proposed mechanism of aromatic alcohol oxidation over Au–Pd alloy NPs under

visible light irradiation.

The conduction electrons of the alloy NPs can gain the energy of the incident visible light via

the LSPR effect and inter-band transition. These energetic electrons are available to Pd sites at the

NP surface because of electron collisions and electron redistribution between Au and Pd. The Pd

sites have good affinity to the aromatic alcohol molecules and the interaction between the alcohol

molecules and the NP surface is enhanced by the surface charge heterogeneity of the alloy NPs. The

strong interaction facilitates the transfer of the light excited electrons of the NPs to the adsorbed

alcohol molecules. As a result, the catalytic activity of alloy NPs is significantly enhanced at ambient

temperatures, which allows the alloy NPs to efficiently catalyse the selective oxidation of aromatic

alcohols to the corresponding aldehydes and ketones. This explains our observations that the

conversion of a reaction under light irradiation is always higher than that of the corresponding


Catalysts - 106 -

reaction in the dark, and that the selective oxidation of benzyl alcohol could proceed at moderate

temperatures (e.g. <45 °C) under irradiation but not in the dark.

Conclusions

In summary, it was found for the first time that visible light could significantly enhance the

performance of Au–Pd alloy NPs supported on ZrO2 for the dehydrogenation of aromatic alcohols

to yield the corresponding aldehydes in the absence of oxygen at ambient temperature. The rate

determining step of the dehydrogenation is the abstraction of α-H atoms from the alcohol molecules.

The dehydrogenation is also the rate determining step of selective oxidation of the alcohols to the

corresponding aldehydes with molecular oxygen. The results of both the free electron gas model

analysis and DFT simulation indicate that the alloy structure of Au and Pd increases the charge

heterogeneity of the NP surface, which enhanced interaction between the alloy NPs and the alcohol

molecules adsorbed on the NPs. The combination of light absorption of alloy NPs, the enhanced

interaction and the intrinsic catalytic activity of the transition metal leads to a unique structure where

the absorption of visible and UV radiation can yield energetic electrons available at catalytically

active transition metal sites on the NP surface promoting the reaction of the molecules adsorbed on

the NPs. The optimal activity for the alloy NPs was observed with an Au : Pd molar ratio of 1 : 1.86,

being in good agreement with the simulation results. The Au–Pd@ZrO2 is an example of the alloy

NPs formed by gold and a catalytically active transition metal, which can be used as a new superior

catalyst for fine organic chemical synthesis under light irradiation. Since little input energy is

consumed by other components of the reaction system, such as the solvent, support of the NPs, the

atmosphere or container, this catalyst structure is highly efficient for driving various chemical

reactions with sunlight. The knowledge acquired in this study is useful for designing suitable

photocatalysts made from gold alloyed with other transition metals and may inspire further studies


Catalysts - 107 -

on new efficient photocatalysts of gold and other transition metals for a wide range of organic

synthesis driven by sunlight.

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Catalysts - 112 -

Electronic Supplementary Information for

Visible light enhanced oxidant free dehydrogenation of aromatic alcohols

using Au–Pd alloy nanoparticle catalysts

Sarina Sarinaa, Sagala Baib, Yiming Huanga, Chao Chena, Jianfeng Jiac, Esa Jaatinena,

Godwin A. Ayokoa, Zhaorigetu Bao*b and Huaiyong Zhu*a



b School of Chemistry, Inner Mongolia Normal University, Hohhot, China.

c School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China.





Catalysts - 113 -

Table of Content

1. Experimental Section

2. Table S1. A, Dehydrogenation of aromatic alcohols with the 3% Au-Pd@ZrO2

catalysts of various Au:Pd mass ratios under visible light in air and in argon gas

atmosphere. B, Dehydrogenation of aromatic alcohols with the 2% Au-Pd@ZrO2

catalysts of various Au:Pd mass ratios under visible light in argon gas atmosphere.

3. Text S1. The calculation method of quantum yield.

4. Text S2. Estimation of Au-Pd alloy NPs’ ionic property by free gas model.

5. Text S3. Density function theory (DFT) calculation of charge distribution in Au - Pd

alloy nanoparticle.

6. Text S4. Calculation of apparent activation energy for all reactions.

7. Text S5. DFT simulation of PhCH2OH- transient anion.


Catalysts - 114 -


Materials and Methods

Photocatalyst preparation: Catalysts with 3wt% of pure gold nanoparticles on ZrO2

(labelled 3%Au), 3wt% of pure palladium nanoparticles on ZrO2 (3%Pd) and three Au-

Pd@ZrO2 photocatalysts with different Au:Pd ratios on ZrO2 were prepared by

impregnation-reduction method. For example, 1.5%Au-1.5%Pd/ZrO2 was prepared by the

following procedure: 2.0 g ZrO2 powder was dispersed into 15.2 ml of 0.01 M HAuCl4

aqueous solution and 28.3 ml of 0.01 M NaPdCl3 aqueous solution (0.05g of PdCl2 was

dissolved in 28.3ml of 0.02M NaCl solution under stirring) were added while magnetically

stirring. 20 mL of 0.53 M lysine was then added into the mixture with vigorous stirring for 30

min. To this suspension, 10 mL of 0.35 M NaBH4 solution was added dropwise in 20 min,

followed by an addition of 10 mL of 0.3 M hydrochloric acid. The mixture was aged for

overnight and then the solid was separated, washed with water and ethanol, and dried at

60 °C. The dried solid was used directly as catalyst. Catalysts with other Au:Pd ratios were

prepared in a similar method but using different quantities of HAuCl4 aqueous solution or

NaPdCl3 aqueous solution.

Catalyst Characterization: TEM study and Line profile analysis by energy dispersion X-ray

spectrum technique of the photocatalysts were carried out on a Philips CM200 TEM with

an accelerating voltage of 200 kV. The Au and Pd content of the prepared catalysts were

determined by EDX technology using the attachment to a FEI Quanta 200 Environmental

SEM. The element line scanning was conducted on a Bruker EDX scanner attached to JEOL-

2200FS TEM with scanning beam diameter down to 1.0 nm. X-ray diffraction (XRD)

patterns of the sample powders were collected using a Philips PANalytical X’pert Pro


Catalysts - 115 -

diffractometer. CuKα radiation (λ= 1.5418 Å) and a fixed power source (40 kV and 40 mA)

were used. DR-UV-vis spectra of the sample powders were examined by a Varian Cary 5000

spectrometer.

Activity Test: The information of reaction system is given briefly as footnotes of Figure1.

The suspension of catalyst powder, solvent and the reactant was placed in a chamber in which

a 500 W Halogen lamp (from Nelson, wavelength in the range 400–750 nm) was used as a

light source and the light intensity was usually 0.40 W/cm2 (except for the experiments

investigating the impact of the intensity), 50W high power LED lamps are applied as high

intensity light source in the experiment of investigating impact of light intensity (0.6~1.2

W/cm2). At given irradiation time intervals, 2 ml aliquots were collected, centrifuged, and then

filtered through a Millipore filter (pore size 0.45 μm) to remove the catalyst particulates. The

filtrates were analysed in a Gas Chromatography (HP6890 Agilent Technologies) with a HP-5

column to measure the concentration change of alcohol and products.


Catalysts - 116 -

Table S1. A . Dehydrogenation of aromatic alcohols with the Au-Pd@ZrO2 catalysts of

various Au:Pd mass ratios under visible light at oxygen gas atmosphere and argon gas

atmosphere.


Catalysts - 117 -

Table S1. B, Dehydrogenation of aromatic alcohols with the 2% Au-Pd@ZrO2 catalysts of

various Au:Pd mass ratios under visible light in argon gas atmosphere.


Catalysts - 118 -

Text S1. The calculation method of quantum yield

The light intensity measured at the reaction system was 0.30 W/cm2 (which included

both the absorbed and scattered light). The overall energy of the photons of the irradiation on

the reaction system was derived from the product of the light intensity and section area of

the reactor under irradiation. The overlap of the light source and the absorption spectrum of

catalysts provide the distribution of the absorbed photons over the wavelength range between

400 nm and 800 nm, as shown in figure below. We could estimate the mean wavelength of

the absorbed photons from the distribution (after being normalized). The mean energy of the

photons could be calculated from the mean wavelength. The number of the photons

introduced in the reaction system in our study was calculated from the ratio of the overall

energy of the photons and mean energy of the photons. The number of molecules formed

was determined during the reaction course. Thus the apparent quantum yield was from the

ratio of the number of molecules formed to the number of the photons introduced in the

reaction system.

Figure. Absorption intensity of Au-Pd alloy NPs on ZrO2 (a) and irradiation intensity of

incandescent light (b). The overlapped area indicated the distribution of the absorbed

photons.


Catalysts - 119 -

Text S2. Estimation of Au-Pd alloy NPs’ ionic property by free gas model.

The electron redistribution of the Au-Pd bond is dependent on the magnitude of the

electron transferred between the two metals. An estimate of magnitude of the charge

transferred can be obtained with the free electron gas model (1), with the change in the

number of electrons given by:

(1)

where D(εF) is the density of electron states at the Fermi energies for the two metals and:

(2a)

(2b)

(2c)

where ФPd and ФAu are the work functions of pure palladium and gold, respectively, and

Ф* is the work function of the alloy once charge equilibrium is reached.(see Scheme 2).

Effectively Δa and Δb give the shift in Fermi level (chemical potential) of the two metals at

their interface upon contact. The density of states of a free electron gas at the Fermi level is

given by (3):

𝐷(𝜀F,Au ) =3𝑁𝐴𝑢

2𝜀F,Au

where N is the number of electrons, so for the two metals the densities are:

𝐷(𝜀F,Au ) =3𝑁𝐴𝑢

2𝜀F,Au (3a)

𝐷(𝜀F,Pd ) =3𝑁𝑃𝑑

2𝜀F,Pd (3b)

Combining Equations 3a and 3b with Equation 1 the ratio Fermi level shift is given by:


Catalysts - 120 -

∆𝑎

∆𝑏=

𝑁𝑃𝑑

𝑁𝐴𝑢 𝜀F,Au ,

𝜀F,Pd (4)

In the alloy systems in this study, the relative concentration of Pd and Au is varied. If

the relative concentration of Pd in the alloy is x, then that of Au will be 1-x and Equation 4

becomes:

∆𝑎

∆𝑏=

𝑥

1−𝑥 𝜀F,Au ,

𝜀F,Pd

(5)

By combining Equation 5 with Equations 2c and 1, the total change in electron

concentration can be evaluated:

(6)

where K is a constant of proportionality. Therefore, the net increase in electron concentration

on the Pd outer-shell of the nanoparticle will be:

(7)


Catalysts - 121 -

Text S3. Density function theory (DFT) calculation of charge distribution in Au - Pd alloy

nanoparticle.

An Au32 cluster and a corresponding Au12Pd20 alloy cluster were constructed to mimic

the Au and Au-Pd nanoparticles. The geometry of the Au32 and Au12Pd20 were optimized by

PBE1 method of density functional theory implemented in CP2K2 code. The Molecular

optimized double zeta-valence Shorter- Range basis sets3 with a polarization function was

used to describe the valance orbitals and Goedecker-Teter-HutterPseudo-potential4 was used

to describe the core electrons. The excited state calculations on as optimized structures were

performed in the framework of Time-Dependent density functional theory with B3LYP5,6

functional provided by Gaussian09 package7. In this stage, Lanl2dz basis set8was selected

to describe the atomic orbital of Au and Pd atoms. The excited states with excited

wavelength of 534 nm for Au32 and 532 nm for Au12Pd20were considered in our

calculations. The optimized geometry of the Au32 and Au12Pd20 clusters and the natural

charge distributions9 of them in ground state and considered excited state were depicted in

Figure 8.

References:

1. Perdew, J. P; Burke, K; Ernzerhof, M., Physical Review Letters, 77 (18), 3865-3868 (1996).

2. CP2K version 2.4, the CP2K developers group (2013), http://www.cp2k.org/.

3. D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem.

Phys.1993, 98, 5648- 5652.

4. VandeVondele, J; Hutter, J. J. Chem. Phys., 127 (11), 114105 (2007).

5. Krack, M., Theoretical Chemistry Accounts, 114 (1-3), 145-152 (2005).

6. Lee, C., Yang, W., and Parr, R. G., Phys. Rev. B, 1998, 3, 785-789.

7. Hay, P. J. and Wadt, W. R., J. Chem. Phys.1985, 82, 299-310.

8. Frisch, M. J., Trucks, G. W., Schlegel, H. B., G. E. Scuseria, M. A. Robb, J. R.

Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji,

http://www.cp2k.org/


Catalysts - 122 -

M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.

Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.

Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E.

Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.

Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S.

S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.

Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,

A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V.

G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,

O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian 09,

Revision C.01, Gaussian, Inc., Wallingford CT, 2010.

9. Reed, A. E., Weinstock, R. B., and Weinhold, F., J. Chem. Phys., 83 (1985) 735-46.


Catalysts - 123 -

Text S4. Calculation of apparent activation energy

Panel a shows the Arrhenius plots for benzyl alcohol dehydrogenation in dark (Thermal

reaction) and under light irradiation (Photo-reaction).1,2 The conversion rates of the catalytic

oxidation were used for the calculation of the reaction rate k. Arrhenius equation is applied

for calculating apparent activation energy based on reaction rate k: k=Ae-Ea/RT. Panel b

schematically illustrates the difference in activation energy between the dark reaction and

the reaction under light irradiation.

Reference

1. K. Yamada, K. Miyajima, F. Mafune, J. Phys. Chem. C 111, 11246 (2007)

2. Kittle, Introduction to Solid State Physics, 8th ed. Wiley and Sons, New York (2005).


Catalysts - 124 -

Text S5. DFT simulation of PhCH2OH- transient anion:

To model the PhCH2OH- transient anion, all the associated species were optimized at the

level of density functional theory (DFT) with Becke’s1 three-parameter exchange and Lee-

Yang-Parr correlation functional2 implemented in Gaussian 09 package3. 6-311++G(d,p) basis

set was employed to describe the orbital of all atoms involved. The energy to break the bond

between C and α-H in PhCH2OH was calculated favouring the reaction of PhCH2OH =

PhCHOH + H, while in PhCH2OH- favouring the reaction of PhCH2OH- = PhCHOH + H-.

References

1. A. D. Becke, J. Chem. Phys. 98, 5648 (1993).

2. C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 37, 785 (1988).

3. M. J. Frisch, et al., Gaussian 09, C.01, Gaussian, Inc., Wallingford CT (2010).

Epoxidation of Alkenes with Molecular Oxygen Using Air-Stable Copper Nanoparticles as

Visible Light Photocatalysts - 125 -

Chapter 5 Epoxidation of Alkenes with Molecular

Oxygen Using Air-Stable Copper Nanoparticles as

Visible Light Photocatalysts


This chapter presents Article 3 (accepted manuscript on ACS Catalysis, 2017, DOI:

10.1021/acscatal.7b01180), in which supported Cu NPs are employed for the highly selective

epoxidation of various alkenes. This work expands the perspective of metallic photocatalysis

from tradition Au metal to another plasmonic metal and also introduced a new type of organic

into photocatalysis. Moreover, this study overcomes an intrinsic and long-standing challenge

of Cu NPs photocatalysts, which is its instability in the oxidative environment. The mechanistic

study of Cu NPs photocatalysts and the epoxidation revealed a new oxygen activation process

on the Cu NPs in the assistance of light irradiation, and therefore illustrated the potential of

photocatalysis in a wide range of oxidative organic synthesis.

Selective epoxidation of alkenes is a difficult and important reaction in organic synthesis

requiring fine catalyst design and reaction condition control to avoid the over oxidation of

alkenes to aldehydes. Copper based catalysts have been proven active for this reaction and the

metallic state of Cu is found crucial to the epoxidation selectivity. However, the instability of

Cu metal in an oxidative environment hinders the practical application of Cu metal based

catalysts. Meanwhile, metallic state Cu nanoparticles are strong light absorbers in the visible

range due to the LSPR effect. Therefore, in this part of the thesis, we studied the least

investigated plasmonic metal - Cu nanoparticle photocatalysts. We managed to stabilise Cu

nanoparticles by using titanium nitride (TiN) support material and the Cu nanoparticles were



found to be stable when exposed to air in the absence of additional polymer stabilisers. Through

DFT simulation technique, we illustrated that the metallic state of Cu nanoparticles is

maintained owing to the significant charger transfer loop between Cu and TiN. Next, the air

stable Cu nanoparticle photocatalyst was employed in the epoxidation of various alkenes using

oxygen gas or even air as a benign oxidant in the cyclic ether solvent. A good to high yield and

excellent selectivity were received with multiple types of alkenes. The photocatalysis

mechanism study reveals the strong chemical adsorption of alkene to Cu nanoparticles resulting

in Cu-alkene surface complexes. Such complexes can be activated under light irradiation. The

further investigation of epoxidation path tells us that the cyclic ether solvent plays a key role

in the reaction cycle. When illuminated with light, the ether interacts with O2 on the Cu

nanoparticles surface to yield active oxygen adatoms able to convert alkenes to corresponding

epoxides. The reusability test of TiN supported Cu nanoparticles suggested a long-lasting

stabilisation effect after several cycle runs of epoxidation experiments. Moreover, the

passivated Cu nanoparticles after several reaction cycles can be easily recovered by reductive

hydrogen gas treatment without significant activity or selectivity loss. In summary, this paper

introduced a convenient method for fabrication of metallic state Cu nanoparticles and reported

a successful application in the photocatalytic epoxidation of alkenes. Apart from photocatalysis,

it also could benefit future applications that require the metallic state of Cu nanostructure.



Article 3



Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible

Light

Yiming Huang, Zhe Liu, Guoping Gao, Gang Xiao, Aijun Du, Steven Bottle, Sarina Sarina*,

and Huaiyong Zhu*,

Accepted manuscript on ACS Catalysis, 2017, DOI: 10.1021/acscatal.7b01180


Student Author:

Yiming Huang

Wrote the manuscript, experimental design,

conducted experiments, and data analysis.

Signature

Date

Ms Zhe Liu Aided experimental design, conducted

experiments and data analysis.

Mr Guoping Gao Aided experimental design, conducted

computational simulation.

Dr Gang Xiao Aided experimental design.



A/Prof. Dr. Aijun Du Aided experimental design, conducted

computational simulation.

Prof. Dr Steven Bottle Aided experimental design and data

analysis.

Dr Sarina Sarina Corresponding author, aided experimental

design and data analysis.

Prof. Dr Huaiyong Zhu Corresponding author, aided experimental

design and data analysis.



authorship.

____________ _____________ ________________

Name Signature Date



Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible

Light

Yiming Huang,† Zhe Liu,† Guoping Gao,† Gang Xiao,‡ Aijun Du,† Steven Bottle,† Sarina

Sarina*,† and Huaiyong Zhu*,†

†School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and

Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia

‡Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing,

100029, P. R. China.


Abstract: Selective epoxidation of various alkenes with molecular oxygen (O2) under mild

conditions is a long standing challenge in achieving syntheses of epoxides. Cu based catalysts

have been found to be catalytically active for selective epoxidations. However, the application

of copper nanoparticles (CuNPs) for photocatalysed epoxidations is encumbered by the

instability of CuNPs in air. Herein we report that CuNPs supported on titanium nitride (TiN)

without additional stabilizers, not only are stable in air but also can catalyse selective

epoxidation of various alkenes with O2 or even air as benign oxidant under light irradiation.

CuNPs remain in the metallic state due to the significant charge transfer that occurs between

CuNPs and TiN. The epoxidation is driven by visible light irradiation at moderate temperatures,

achieving good-to-high yields and excellent selectivity. The photocatalytic process is

applicable to the selective epoxidation of various alkenes. In this photocatalytic system,

reactant alkenes chemically adsorb on CuNPs forming Cu-alkene surface complexes and light

irradiation can activate the complexes for reaction. The cyclic ether solvent also plays a key

role, reacting with O2 on the surface of CuNPs under light irradiation, yielding oxygen adatoms.

The activated surface complexes react with the adatoms, yielding corresponding epoxides.



Analysis of the influence of irradiation wavelength and intensity on the epoxidation suggests

that light-excited electrons of CuNPs drive the reaction. The adatoms formed react with alkenes

producing the final product epoxides. We also observed interesting product stereo-selectivity

predominantly generating the trans-isomers for the epoxidation of stilbene (up to 97%). The

findings reported here not only provide an effective and selective reaction system for alkene

epoxidations but also are a step towards demonstrating the practical use of CuNPs as

photocatalysts for various applications.

Introduction

Selective epoxidation of alkenes is a reaction of great interest because epoxides are

versatile building blocks in organic syntheses, the pharmaceutical industry and in materials and

life sciences.1-3 At the forefront of existing challenges is the development of processes that use

a benign oxidant, have broad substrate scope while maintaining high selectivity, high atom

economy and reduced environmental impact.4

The use of metal-based homogeneous catalysts has a rich history and has been

comprehensively reviewed,5 however, such catalysts can be costly and not eco-friendly.6,7 In

the last decade, heterogeneous metal catalysts from group IB (Au, Ag, Cu etc.) have been found

to be highly effective in epoxidations in the presence of activated oxygen atoms. It is generally

accepted, although the exact details remain to be determined, that an oxametallacyclic (OME)

intermediate is the key species in the catalytic cycle that drives the epoxidation.8-13

In the past decade, the direct utilization of solar energy over various forms of

photocatalyst to promote organic synthesis has been recognized as prospective strategies.14-17

Among such, Au, Ag, Cu and Al, as plasmonic metals, their nanoparticles (NPs) exhibit strong

visible light absorption due to the localized surface plasmon resonance (LSPR) effect.18-22 Free

electrons of such NPs can also absorb ultraviolet, visible and infrared photons because these



metals have continuous electron energy levels. Thus NPs of these metals can utilize light

energy arising from most of the solar spectrum for catalysis.23 The light irradiation generates

photoexcited electrons and these electrons can transfer energy to molecules absorbed on the

metal surface, inducing a range of chemical transformations.24-32

Consequently, the NPs of group IB metals represent excellent candidates for the

development of new photocatalysts for epoxidations. Copper has the lowest cost among the IB

metals, whereas it also exhibits the highest selectivity.13 Studies and applications of CuNPs are,

however, strictly limited because CuNPs can be readily oxidized by air, oxidizing support

materials and oxidants in the reaction environment, yet there is evidence that the metallic state

of Cu is critical for both the optical properties of the catalyst and the epoxidation selectivity.33

Polymer stabilizers and/or inert atmospheres have been employed to maintain copper in the

metallic state.34 Nitrobenzene reduction over a Cu@graphene photocatalyst has been

reported,35 in which CuNPs stay in the metallic state on the graphene support. As yet there has

been no in-depth study to establish the basis of the stabilization effect. In this work, we

demonstrate that CuNPs can be prevented from oxidation when titanium nitride (TiN) is used

as a support, on which copper is dispersed as nanoparticles. Density function theory (DFT)

calculations suggest that the significant charge transfer loop between CuNPs and TiN support

provides the resistance of CuNPs towards oxidation.

Another challenge for selective epoxidation is using molecular oxygen as the oxidant for

the epoxidation of alkenes at mild temperatures. Oxygen, especially if delivered in air,

represents the ideal oxidant, superior even to the well-known “green” oxidant hydrogen

peroxide.36,37

In addition, low reaction temperatures are usually employed for photocatalysis processes,

which not only reduces the energy consumed by the reaction but also allow the process to be



applied to temperature sensitive substrates. Despite these attractions, the use of O2 as a selective

oxidant and Cu-based catalysts under mild conditions has been faced with some challenges.38,39

In this study, we overcame many of these obstacles by using a novel copper supported

photocatalyst (Cu@TiN) and a cyclic ether solvent, successfully achieving epoxidation of a

range of alkenes with molecular oxygen under visible light irradiation at moderate reaction

temperatures. Preliminary investigations of the photocatalytic epoxidation mechanism suggest

a process involving adsorbed oxygen interacting with solvent to transfer oxygen atoms to the

alkene.

Results and discussion

Metallic state of air-stable CuNPs. The photocatalyst is readily prepared by

impregnation-reduction methodology to load CuNPs onto TiN powder (with particle size ~30

nm). In the as-prepared sample, copper exists at mixed oxidation states according to the XPS

spectrum (shown in Figure S1a in Supporting Information, SI). This sample was labelled as

CuO/Cu@TiN. The CuO/Cu@TiN was further treated under a hydrogen atmosphere at 300°C

to secure the NPs in metallic state (Cu@TiN), this is confirmed by the XPS result shown in

Figure 1). The binding energies at around 952.0 eV and 932.5 eV are indicative of the existence

of Cu(0), and peaks for other oxidation stated are notably absent.



Figure 1. XPS spectrum of Cu@TiN photocatalyst suggests the metallic state of air-stable

CuNPs. Air-stable metallic CuNPs supported by TiN substrate.

Transmission electron microscopy (TEM) images indicate that well-dispersed CuNPs

(with a mean size of 4 nm) are distributed on the TiN surface (Figure 2a). Scanning electron

microscopy (SEM) images and energy dispersive spectrometer (EDX) mapping shown in

Figure S3 confirm the elemental composition of Cu@TiN is as designed. A representative

single-crystal Cu NP with (111) lattice planes predominantly exposed is illustrated in Figure

2b. It has previously been reported that the (111) planes of Cu are favoured for the absorption

of reactants such as styrene.40 O2 can also be absorbed easily onto the surface of the (111) plane

and this likely to represent the first step of oxygen activation.13 XRD patterns indicate that

CuNPs have negligible impact on the TiN crystal structure presumably due to the low copper

loading (Figure 2c).



Figure 2. TEM and XRD characterizations of Cu@TiN. (a) TEM image of Cu@TiN, CuNPs

are well dispersed on TiN substrate. (b) High-resolution TEM image, single crystal CuNP was

form on the surface of TiN substrate with clear (111) index face. (c) XRD patterns of the

Cu@TiN, CuO/Cu@TiN and TiN. Cu@TiN exhibits identical XRD peaks compared with TiN

substrate indicating negligible influence of CuNPs loading on the TiN substrate.

The light absorption of TiN and Cu@TiN were examined by diffuse reflectance UV-Vis

(DR-UV/Vis) spectra (Figure 3). Distinctly different light absorption in the visible range is

observed between TiN and Cu@TiN material. We further analyzed the UV-Vis spectrum of



Cu@TiN by using TiN spectrum as background and obtained a spectrum attributed to isolated

CuNPs as shown in the insert in Figure 3. This indicates that CuNPs exhibit strong visible light

absorption with peak at around 580 nm being the characteristic LSPR absorption of CuNPs.35,41

Figure 3. DR-UV-Vis spectra of Cu@TiN, TiN and isolated CuNPs (insert); Cu loading has a

significant impact on the optical properties of the TiN substrate; the isolated spectrum of

CuNPs was obtained by measuring Cu@TiN using TiN as background, peak at 580 nm is

attributed to the LSPR peak of CuNPs.

To provide an in-depth understanding of the stabilization mechanism for CuNPs

supported on the TiN substrate, we have carried out systematic DFT calculations for the

Cu/TiN interface. Figure 4 presented a side view of 3D plot of charge density difference for

CuNPs on the TiN surface. Yellow and cyan iso-surfaces represented charge accumulation and

depletion in the 3D space with an iso-surface value of 0.005 e/Å3.

300 400 500 600 700

450 600 750

No

rma

lize

d a

bs

orb

an

ce

(a

.u.)

TiN

Cu@TiN

Wavelength (nm)

580 nm

Cu NPs



Figure 4. Structure and charge density difference plot at the CuNPs/TiN interface simulated

using DFT method. Charges are transferred from Cu atoms to N atoms and a similar degree

of charge transfer from Ti atoms back to Cu results in the stabilization of CuNPs on the TiN

substrate. The oxidation state of Cu@TiN was calculated to be +0.02.

Clearly, there is a significant charge transfer from the CuNPs to the N atoms of the TiN

substrate, which can be the major reason for CuNPs being more stable. Importantly, an equal

amount of charge is donated back from the Ti atoms of TiN to the CuNPs. Such a charge

exchange between the CuNPs and the TiN support consequently gives rise to a slightly

positively charged state (+0.02) for the CuNPs, thus suggesting a negligible oxidation state of

Cu atom. The stabilization of CuNPs is also confirmed by the large negative formation energy

(-4.08 eV) of Cu@TiN.

In the investigation of photocatalytic performance of Cu@TiN for epoxidation of alkenes.

We found that the new Cu@TiN photocatalyst exhibits excellent activity for selective

epoxidation of various alkenes using air or molecular oxygen as the oxidant under visible light

irradiation at 60°C. Table 1 shows the results of the representative epoxidation of styrene,

trans-stilbene and norbornene (Table 1, entries 1, 4 and 5). The photocatalytic epoxidation

using air as an oxidant proceeds smoothly over the Cu@TiN photocatalysts under

moderate/mild reaction conditions. The significant difference in both conversion and



selectivity between light irradiated reactions and the same reactions in the dark (numbers in

parentheses) confirms that light absorption is the major driving force for the reactions.

We also observed only a certain degree of photocatalytic-performance from the TiN

support material alone (Table S1). The TiN support exhibits light adsorption (Figure 3) and

results in 40% conversion for oxidizing styrene to benzaldehyde yet no epoxides product was

detected, which is a critical issue in the epoxidation of terminal alkenes. In the contrast, with

CuNPs loaded TiN photocatalyst, we observed 100% conversion with 89% selectivity towards

epoxides product, demonstrating the support makes little contribution to the overall conversion

and selectivity of the Cu@TiN catalyst.

The epoxidation over Cu@TiN is inhibited when air or pure O2 is replaced by argon

(Table 1, entry 3; Table S2) indicating that molecular oxygen is required as the oxidant in the

process. It also reveals the fact that Cu@TiN can cooperate with O2 while maintaining its

metallic state.

The important role of the metallic state CuNPs in epoxidation is also demonstrated in

Table 1. The precursor powder CuO/Cu@TiN comprises both metallic and oxidized Cu, as

confirmed by XPS measurements (Figure S1a). We applied CuO/Cu@TiN in photocatalytic

epoxidations as well as CuO@ZrO2 (XPS spectrum in Figure S1b), commercial Cu oxides and

salts for comparison. It should be noted that Cu oxides are p-type semiconductors with narrow

band gaps and can also absorb visible light (see the reflectance UV-Vis extinction spectra in

Figure S2a).42 Thus, as expected, we observed some light induced conversion with

CuO/Cu@TiN, CuO@ZrO2 and commercial CuO and Cu2O powder (with much higher Cu

content); although none of these materials proved as effective as metallic Cu@TiN (Table 1,

entries 6-11). It has been previously reported that copper salts, complexes or oxides exhibit

catalytic activity for epoxidation of alkenes in the assistance of stronger oxidants than



molecular oxygen.43,44 This may explain the very low conversion that we observed with copper

oxide catalyst using molecular oxygen in the dark. Thus, on the basis of the results in Table 1,

we conclude that metallic state CuNP plays an important role in the activation of molecular

oxygen, whereas copper oxides are much less effective.

Table 1. High performance epoxidation of alkenes over Cu@TiN.[a]

Entry Catalyst Atmosphere Substrate

Conv.

[%]

Select. [%]

2 3

1 Cu@TiN Air[b] 1a 100(6) 89(n.d) 11(100)

2 Cu@TiN O2 1a 100(5) 81(n.d) 19(100)

3 Cu@TiN Argon 1a n.r. n.d. n.d.

4[c] Cu@TiN Air[b] 1b 100(14) 100(100) n.d(n.d)

5[d] Cu@TiN Air[b] 1c 100(23) 100(100) n.d(n.d)

6 CuO/Cu@TiN O2 1a 85(13) 44(19) 56(81)

7[c] CuO/Cu@TiN O2 1b 36(4) 100(100) n.d(n.d)

8[d] CuO/Cu@TiN O2 1c 72(20) 100(100) n.d(n.d)

9 CuO@ZrO2 O2 1a 74(25) 32(20) 68(80)



10 CuO[e] O2 1a 87(7) 53(n.d) 47(100)

11 Cu2O[e] O2 1a 79(4) 54(n.d) 46(100)

12 Cu(NO3)2[e]

O2 1a 7(3) n.d(n.d) 100(100)

13 None O2 1a 6 n.d 100

Reaction conditions: 20 mg catalyst, 0.1 mmol substrate and 3 mL 1,4-dioxane as solvent.

Selected gas was bubbled for 5 min and then reaction tube was sealed. The reaction mixture

was stirred under visible light irradiation (0.5 W/cm2) at 60°C for 4 h. Conversion were

determined by using an Agilent 6980 gas chromatography coupling with an Agilent HP5973

mass spectrometer equipped with a HP-5 column (GCMS). Numbers in parentheses are results

of reactions in the dark. [a] (n.r.= no reaction; n.d.=not detected); [b] air was bubbled for 5 min

prior to the start of reaction and every hour after; [c] reaction for 8 h; [d] reaction for 16 h; [e]

the Cu loading of the catalyst is 40 times of that in Cu@TiN.

More importantly, in the epoxidation of terminal alkenes, where aldehyde can be easily

formed as by-product, the work of Marimuthu et al had proved that metallic state CuNPs are

superior to copper oxides in respect to epoxidation selectivity,33 a possible reason is the direct

electron photoexcitation between hybridized orbitals of metal-organic as such effect does not

occur on copper oxides (comparing the results of entries 2, 6 and 9 in Table 1), this theory will

be further discussed later. Cu(NO3)2 showed negligible activity in both light irradiated and dark

reactions (Table 1, entry 12) demonstrating the ineffectiveness of Cu2+ ions in epoxidation of

alkenes. The blank reaction of styrene epoxidation was also tested in the absence of any catalyst

showing a 6% conversion with only benzaldehyde product as shown in (Table 1, entry 13).



As shown in Table 2, Cu@TiN photocatalyst exhibits wide substrate scope while

maintaining excellent selectivity, being active on linear, aromatic, terminal, electron deficient,

electron rich and conjugated alkenes. For example, good to excellent yields were achieved for

epoxidation of terminal alkenes. This is of particular interest as the 1,2-epoxide products are

often key intermediates in organic synthesis (Table 2, entries 1, 2, 4 and 5 etc.).45-47 Epoxidation

of electron-deficient alkenes, which is regarded to require strong oxidants,5 was achieved with

excellent selectivity using Cu@TiN (Table 2, entries 7 and 8).

It has been reported that terminal alkenes are less active in epoxidation reactions than

cyclo-alkenes,48 because the low electron density of terminal alkenes has a negative impact on

the electrophilic oxygen transfer and results in reduced reactivity.49 In the present study, poor

conversion is observed for cyclohexene and its derivative (Table 2, entries 9 and 10), while the

reaction of 1-hexene achieved a good yield (Table 2, entry 20).

We also found that increasing the strain energy of cycloalkanes can enhance the reaction

efficiency. Two examples, cyclooctene and norbornene are shown in Table 2 (entries 11 and

12). Nonetheless, low selectivity for the epoxide product is observed for propene epoxidation

over Cu@TiN although the propene conversion is high. The predominant product, in this case,

is the ketone (Table 2, entry 13) rather than the epoxide, the reason has not been understood

yet.

Cu@TiN photocatalyst also exhibited very good performance for reactants with terminal

conjugated double bonds (Table 2, entries 14-16) due to the strong adsorption of π-conjugated

molecules on to the Cu surface. Such adsorption lowers the LUMO energy of the adsorbate.50

Poor yields are observed with relatively inactivate aliphatic long-chain alkenes (Table 2, entries

18 and 19). Nevertheless, employing a stronger oxidant such as H2O2 can effectively enhance



the reaction efficiency (see entries 18 and 19 under conditions d), indicating the Cu@TiN can

interact with other oxidants to transform less reactive alkenes to the corresponding epoxides.

Table 2. Scope of the Cu@TiN catalyzed epoxidation reaction of alkenes. The performance

is expressed in conversion of the reactant and selectivity to the corresponding epoxide

product (percentage in parentheses)

72%(83%) 100%(78%) 100%(100%) 100%(53%)

25%(63%) 3%(100%)[b] 100%(100%) 45%(100%)[a]

15%(100%)[a] 7%(100%)[a] 50%(100%)[a] 100%(100%)[b]

100%(5%)[b] 85%(100%)[b] 100%(65%)[b] 100%(100%)



100%(15%)

32%(100%)[c]

65%(100%)[a][d]

14%(100%)[c]; 19%(100%)[a][d]

83%(100%)[a] 55%(100%)[a] 32%(100%)[a] 71%(100%)[a]

59%(100%)[a] 100%(89%)[a] 28%(100%)[a]

Reaction conditions: 20 mg Cu@TiN, 0.1 mmol reactant and 3 mL solvent. O2 was bubbled

for 5 min. and then the reaction tube was sealed. The reaction mixture was stirred under visible

light irradiation (0.5 W/cm2) at 60°C for 4 h [a] reaction time 16 h; [b] reaction temperature

40°C, reaction time 8 h; [c] reaction temperature 70°C, reaction time 48 h; [d] 2 equiv. H2O2

(0.2 mmol) was added as oxidant. Conversion (dark color numbers) and selectivity (red color

numbers) were determined by GCMS analysis. For entries 21-26, starting reactants are trans-

2-hexene, cis-2-hexene, trans-3-hexene, cis-3-hexene, trans-stilbene and cis-stilbene, for

possible enantiomer products, only one enantiomer is presented.

It is noteworthy that the photocatalytic epoxidation system favours trans-epoxide over

their cis-isomers (Table 2, entries 21-26). This stereo-selectivity is evident as the results in

Table 3 show that the trans-epoxide is predominant product no matter cis- or trans-stilbene

were used as reactant. Conversion of the cis-stilbene is substantially lower than that trans-

stilbene.



It is known that both photo induced isomerizations from cis-stilbene to trans-stilbene and

from trans-stilbene to cis-stilbene can take place.51 But there is a larger barrier (~3 kcal/mol)

for the trans- to cis-isomerization than for the reverse reaction (the barrier is negligible). Trans-

stilbene should be less reactive than cis-stilbene from the point of view of energy. In the

contrary, results in Table 3 are against the inference. This indicates the stereo-selectivity over

Cu@TiN photocatalyst is not determined solely by bond energy. Importantly, under the

photocatalytic conditions of the present study, we did not observe direct trans- to cis-

isomerization of alkene reactants in experiments. Therefore, we deduce that the

stereoselectivity is integrated into the epoxidation process. Similar results have been reported

with Au/C catalyst, it was believed that the adsorption of olefins onto metal surface may create

steric constraints, resulting in a specific reaction pathway.52 Another possible explanation is

that the isomerization on CuNP surface is caused by light irradiation.53

Table 3. Stereoselectivity of stilbene epoxidation

Entry Substrate Light Conversion % Selectivity %

1 trans-Stilbene

Light 100 89 (trans)

Dark 12 98 (trans)

2 cis-Stilbene

Light 28 95 (trans)

Dark n.r. n.d.

3[a] mixed-Stilbene

Light 76 97 (trans)

Dark 5 99 (trans)



Reaction conditions: 20 mg Cu@TiN, 0.1 mmol substrate, 3 ml 1,4 dioxane as solvent, O2

bubbled for 5 min, 60°C, 0.5 W/cm2, 16 h. [a] mixed stilbene was prepared by mixing trans-

stilbene and cis-stilbene in 1:1 ratio. (n.r. = no reaction; n.d. =not detected)

To study the active sites of Cu@TiN, we measured infrared emission spectra (IES) of

styrene adsorbed on Cu@TiN photocatalyst and TiN support, respectively, at stepwise elevated

temperatures. As shown in Figure 5, the characteristic peaks of the aromatic C=C stretch

located at around 1400 cm-1 can be identified for styrene adsorbed on both Cu@TiN and TiN

at 50oC. It confirms the existence of styrene on Cu@TiN photocatalyst and TiN support

material. These peaks can still be observed from the spectra of styrene adsorbed on Cu@TiN

when the sample was heated to 450oC, as shown in Figure 5a, suggesting a strong

chemisorption of styrene on the sample. In contrast, as shown in Figure 5b, the characteristic

aromatic C=C peaks from the sample of styrene adsorbed on TiN are greatly weakened with

raised temperature and nearly vanish at 300oC.

Figure 5. Infrared emission spectra of styrene adsorbed on (a) Cu@TiN photocatalyst and (b)

TiN support. The infrared spectra were measured from 50oC to 450oC at every 50oC gap.



The results reveal that chemisorption of styrene on CuNPs is much stronger than that on

TiN metal. Thus, it is rational that CuNPs, rather than TiN support, are the active sites. It was

reported that chemisorption of styrene on Cu surface of (111) plane lowered unoccupied

molecular orbitals (LUMOs) and elevated occupied molecular orbitals (HOMOs).40 This is

caused by the hybridization of Cu d-band orbitals and styrene π1* and π2

* orbitals into bonding

and antibonding orbitals of Cu-styrene surface complexes. Electrons from such complexes can

be directly photo-excited between bonding and antibonding states, similar to the situation

reported by Christopher et al.54 Such excitation activates the double bond for epoxidation,

enhancing photocatalysis activity and well-controlled selectivity (Table 1, entry 1). This theory

can explain the fact that Cu@TiN exhibits photocatalytic performance superior to

CuO/Cu@TiN and other Cu(I) and Cu(II) photocatalysts.

The effect of light on the reaction was examined to provide insights into the

photocatalysis mechanism. Firstly, the irradiation wavelength has a crucial impact on the

photocatalytic epoxidation and can be directly reflected by action spectrum analysis, which

shows a variation of the photocatalytic activity (in quantum yield) as a function of the

irradiation wavelength.55 We found that the trend of quantum yields (orange dots in Figure 6)

does not follow the trend of light absorption shown by the DR-UV/Vis spectrum of Cu@TiN

(the dash lines in Figure 6). However, in the wavelength ranging from 365 nm to 490 nm, the

trend of the action spectra of all reactions match well to the light absorption spectrum of

isolated CuNPs (solid blue lines in Figure 6). The matching for cis-stilbene even extends to

530 nm. These results further demonstrate that CuNPs are the photocatalytic sites and TiN

surface sites have very limited contribution to the photocatalytic epoxidation. This is consistent

with reactant adsorption results obtained by IES spectroscopy. In this wavelength range,

stronger absorption by CuNPs results in a higher quantum efficiency.



Figure 6. Action spectrum of (a) styrene, (b) norbornene, (c) trans-stilbene and (d) cis-stilbene

epoxidation. Reaction rates presented as quantum efficiency are plotted against wavelength of

irradiated light at 400±5 nm, 470±5 nm, 530±5 nm, 590±5 nm and 620±5 nm, 0.2 W/cm2.

Nonetheless, the CuNPs can strongly absorb light in the range from 530 nm to 630 nm

according to the DR-UV-Vis spectrum of isolated CuNPs, the quantum efficiency of Cu@TiN

in this range is rather low and does not follow the trend of the LSPR adsorption of CuNPs. This

fact clarifies two key issues.

First, the dependence on wavelength indicates that the contribution from the photo-

thermal effect to the reaction rate is not important. Absorption of light by the NPs may cause a

short term temperature increase of the NPs, leading to the so-called photo-thermal effect.32 The

elevated temperature on NP surface could enhance the catalytic reaction on particle surface,



such temperature rising is stronger with longer light wavelength. However, the light source

with wavelength >530 cannot drive the epoxidation efficiently even with relatively high light

absorption at such range. For example, CuNPs exhibit strong absorption at 580 nm but give a

very low catalytic activity under irradiation at this wavelength. On the other hand, reaction

with 400 nm light source, which is weak in the photo-thermal effect, exhibits a much higher

reaction rate. Thus we conclude that the epoxidation was not driven by the photo-thermal effect

but rather by the direct photon energy.

Second, the steep drop of the catalytic performance under wavelengths >530 nm implies

that there is a threshold of photon energy required for the reaction. In Figure 6a,b and c, the

photons of long wavelengths (e.g. >570 nm) do not have sufficient energy to induce the

epoxidation; therefore negligible quantum yield was observed even though significant light

absorption occurs.56 However, in Figure 6d, we observed a higher quantum yield for cis-

stilbene epoxidation at wavelength 530 nm than that at wavelength of 470 nm. A possible

explanation for this is that the energy threshold for cis-stilbene is relatively low that photons

of 530 nm are sufficient to trigger epoxidation and, as a result, the high light adsorption at such

wavelength emerges to dominate the photocatalysis and leads to a quantum yield bounce.



Figure 7. Dependence of photocatalytic activity on the light irradiance. 20 mg Cu@TiN, 0.1

mmol styrene and 3 mL 1,4-dioxane as solvent. Oxygen gas was bubbled for 5 mins and then

reaction tube was sealed. The reaction mixture was stirred under visible light irradiation (400-

700 nm) at 60°C for 4 h. Conversion and selectivity were determined by GCMS analysis using

HP-5 column. The reaction rate super linearly increases with the increase of light irradiance.

[a] reaction time 3 h.

The relationship between photocatalytic performance and light irradiance (that is, the

photon flux) was investigated through the epoxidation of styrene. Since the photons absorbed

by the CuNPs induce the reactions, the light absorption should be proportional to the reaction

yield. We observed a super linearly increased photocatalytic activity with the increasing light

irradiance as shown in Figure 7 and Table S3, this is proved to be one of the experimental

signatures of photo-excited electron-driven reactions.57 In general, high irradiance gives greater

conversion and product yield, presumably through the more photoexcitation of the CuNPs.

Additionally, more than one photoexcited electron may deposit their energies in one adsorbed

reactant molecule at high light irradiance causing a superlinear reaction rate increase, which is

a subsequent photoexcited electron deposits its energy in the reactant molecular before the

dissipation of molecular vibration induced by another photo-excited electron.

We also investigated the influence of reaction temperature on the photocatalysis process.

The temperature was adjusted by external heating or cooling. It was found that a moderate rise

of reaction temperature can efficiently accelerate photocatalytic epoxidation of styrene (see

Table S4). High reaction temperatures enhance the vibrational state of adsorbed reactant

molecules,57 so that less energy is required from photoexcited electron to overcome activation

barrier. As a result, the Cu@TiN photocatalyst can utilize thermal energy to promote



photocatalytic epoxidation as well, which is an advantage to traditional semiconductor

photocatalysts.

To investigate the role of solvent for the epoxidation over Cu@TiN, we tested multiple

organic solvents listed in Table S5. First of all, the most common solvents such as toluene,

CH2Cl2, DMF and MeCN all give negative results, the results also illustrate that solvent

polarity does not exhibit any notable influence to the epoxidation. Moreover, high oxygen

solubility is not crucial for epoxidation as we do not observe notable conversion with acetone

and DMF. Solvent was found to be the determining factor to the ether structure because we

notice epoxidation over Cu@TiN only takes place in the presence of ether solvents, 1,4-

dioxane, THF and diphenyl methyl ether for instance (Table S5). Therefore, we conclude that

ethers structure, cyclic ethers, in particular, play a key role in the reaction mechanism and the

oxygen activation process as discussed in the subsequent section.

The epoxidation mechanism of the new photocatalytic system must involve two general

stages: 1) activation of molecular oxygen on CuNP surface; 2) selective epoxidation of C=C

bond. Previous work in this area has shown that oxygen adatoms (Oa) absorbed on CuNP, also

referred to as Cu-O species in many cases, have been identified, both theoretically and

experimentally, as the epoxidizing agent for alkene epoxidation.58 Adsorption of O2 on low

index faces of Cu is facile,59 60 yet it is difficult to convert the adsorbed O2 to Oa adatoms

weakly bonded onto Cu surface. This is supported by the fact that Cu@TiN cannot initiate

epoxidation in various solvents even those having high solubility of oxygen molecules (Table

S5, entries 1-5). Successful epoxidations were only observed with 1,4-dioxane and other ethers,

implying ether solvents played a crucial role in formation of Oa adatoms (Table S5, entries 6-

10).

Table 4. The formation of peroxide intermediate.



Entry Catalyst Light Atmosphere Peroxide intermediate

1 Cu@TiN Light Oxygen Detected

2 Cu@TiN Light Argon Not detected

3 Cu@TiN Dark Oxygen Not detected

4 No catalyst Light Oxygen Not detected

Reaction conditions: 20 mg of 3 wt% Cu@TiN, 3 ml 1,4-dioxane as solvent, 60oC, light

irradiance 0.5 W/cm2, selected gas bubbled for 5 min, reaction time 2 h.

Taking 1,4-dioxane as an example (Table 4), GCMS analysis suggests that a seven-

membered cyclic peroxide is produced from 1,4-dioxane. We found that the cyclic peroxide

was produced within 2 h, only in the presence of both Cu@TiN catalyst and light irradiation

as shown in Table 4. It means that 1,4-dioxane interacts with the adsorbed O2 molecules on

CuNP surface yielding the peroxide, and this process is predominantly driven by light

irradiation due to the significant difference between light reaction and dark reaction.

We separated the liquid phase and solid catalyst of the system of entry 1 in Table 4, they

are labelled as 1,4-dioxane-peroxide (the liquid) and Cu@TiN-peroxide (the solid),

respectively. Both of them contain the seven-membered cyclic peroxide. Then we studied the

function of the both, separately and in combination for styrene epoxidation reaction. The results

are shown in Table 5.



Combining Cu@TiN-peroxide with 1,4-dioxane-peroxide exhibited similar yields (Table

5, entry 2) to that of a typical reaction without the peroxide (Table 5, entry 1). When oxygen

was removed from reaction system, the typical reaction did not proceed (Table 5, entry 3).

However, 8% yield was observed with the combined system (Table 5, entry 4). This is the

evidence that the seven-membered cyclic peroxide provides the oxygen for epoxidation. We

also found that epoxidation can proceed with Cu@TiN-peroxide alone (Table 5, entries 5 and

6) and without an oxygen source (neither oxygen gas nor ether), but cannot with 1,4-dioxane-

peroxide alone (Table 5, entry 7). This means that 1,4-dioxane-peroxide does not react directly

with the C=C bond of styrene. Instead, there is an intermediate formed on the CuNPs, which is

the direct cause of epoxidation reaction. This intermediate is generated from the reaction

between from the seven-membered cyclic peroxide and the CuNP surface. Therefore, it is

rational that the peroxide on CuNPs decomposes to oxygen adatoms (Oa) and 1,4-dioxane, the

released oxygen adatoms act as an oxidant which directly react with the C=C bond of alkene

(which is activated by the photoexcitation of bonding electrons as afore discussed) as

schematically illustrated in Scheme 1. The releasing of Oa does not require light irradiation,

therefore we infer that light does not play critical role in this step.

Table 5. Roles of 1,4-dioxane-peroxide and Cu@TiN-peroxide in epoxidation reaction.

Entry Photocatalyst Solvent Atmosphere Yield %

1 Cu@TiN 1,4-dioxane Oxygen 53

2 Cu@TiN-peroxide 1,4-dioxane-peroxide Oxygen 56



3 Cu@TiN 1,4-dioxane Argon 0

4 Cu@TiN-peroxide 1,4-dioxane-peroxide Argon 8

5 Cu@TiN-peroxide Toluene Argon 4

6 Cu@TiN-peroxide 1,4-dioxane Argon 6

7 None 1,4-dioxane-peroxide Argon 0

8 Cu@TiN Toluene Argon 0

Reaction conditions: 20 mg of photocatalyst, 3 ml solvent, 60oC, light irradiance 0.5 W/cm2,

selected gas bubbled for 5 min, reaction time 2 h.

It has been reported that styrene lays on a CuNP with C=C bond parallel to surface,12,13

and Oa reacts with the activated C=C bond causing the insertion of an O atom and producing

the -C-C-O- structure. With such a configuration, two types of surface intermediates could be

yielded according to previous studies on epoxidation.11 One is a four-member oxametallacycle

(OME-4) which includes one Cu atom bonded to both C and O atoms (see Scheme 1). The

other is a five-membered OME (OME-5) involving a Cu-Cu fragment bonded to C and O atoms.

The intermediate OME-5 would be expected to undergo a ring-opening process leading to

undesired combustion products, while the -C-C-O- structure in OME-4 yields epoxide through

a ring-closure process.11 Thus, selectivity of epoxidation is determined in this step that occurs

with lower activation energy. The overall reaction mechanism is shown in Scheme 1.

Interestingly, the selectivity also depends on the wavelength (Figure S4). When styrene

is epoxidized with irradiation at 470 nm, we observed the highest selectivity, either shorter or

longer wavelength light results in decreased selectivity towards epoxides. Photons with short



wavelengths (<470 nm) are likely to lead to breakdown of the oxirane ring. This was confirmed

experimentally through irradiation of styrene oxide that was shown to be unstable using shorter

wavelength irradiation. On the other hand, photons with long wavelengths cannot deliver

sufficient energy to trigger the epoxidation process resulting decreased selectivity as well as

low conversions.

Scheme 1. Overall reaction mechanism of styrene epoxidation over Cu@TiN.

Oxygen molecules adsorb onto CuNPs first, and the consequent O2 activation process occur on

the surface of CuNPs in the assistance of cyclic ethers under visible light irradiation at mild

temperature (<60oC). The adsorbed O2 and cyclic ether yield cyclic peroxide, which then

release oxygen adatoms (Oa) on the CuNP and the ether. Styrene strongly chemisorb on the

CuNPs surface forming Cu-styrene surface complex, and the chemisorption results in

hybridization of Cu and styrene orbitals and formation of bonding and antibonding states.

Hence, the light irradiation could induce direct resonant photoexcitation of electrons from

hybrid bonding state to antibonding state to trigger the interaction between styrene and Oa

adatoms. The oxidation of styrene occurs on CuNP surface undergoes two competing reaction



pathways through two types of OME intermediates and eventually give epoxidation product

and combustion product respectively. EF denotes the system Fermi level.

Reusability study.

We also investigated the reusability of the Cu@TiN catalyst. The used photocatalyst was

recovered, washed simply with water and ethanol and then directly applied in cycling reactions

without further treatment or regeneration. Cu@TiN exhibits good reusability as shown in

Figure 8: the reaction conversion maintained 100% for seven cycles while selectivity towards

epoxide was 85% in the first reaction run and decreased to 70%. We therefore analyzed

Cu@TiN of 4 cycles runs with XPS as shown in Figure S5. Results suggest that CuNPs remain

mostly metallic state after one cycle run, the content of CuO in Cu@TiN is increasing with the

increase of cycle run number, this is the reason to the epoxidation selectivity dropping. Next,

the Cu@TiN photocatalyst was recovered and reactivated in hydrogen gas atmosphere at 200oC

for 10 mins, we found the reaction conversion drop to 95% whereas the selectivity towards

epoxide regained to 76%. The XPS spectra (Figure S5e) confirmed that the CuNPs are

completely reformed to the metallic state. In addition, the inductively coupled plasma-atomic

emission spectroscopy, (ICP-AES) analysis of after-reaction solution indicated only 0.09% of

Cu was leached after one reaction cycle as shown in Table S6. The negligible metal loss is the

reason to the good reusability of the novel photocatalyst. Meanwhile, it also demonstrated that

the TiN support material not only effectively stabilizes CuNPs in the metallic state but also

solidly bonds the NPs avoiding the loss of CuNPs.



Figure 8. The reusability of the Cu@TiN. Epoxidation of styrene was used as module, used

Cu@TiN photocatalyst was recovered, washed with water and ethanol and then directly applied

in cycle reactions without further treatment or regeneration in the first seven cycle runs. Prior

to the eighth cycle run, the used Cu@TiN photocatalyst was reactivated by hydrogen gas at

200oC for 10 mins. The bar chart represents the reaction conversion and the dot line represents

the selectivity towards epoxides.

Conclusions

In summary, we have developed a stable Cu@TiN photocatalyst for selective epoxidation

of alkenes with molecular oxygen as the oxidant. Our characterization and DFT simulations

indicate that the stability of metallic state of CuNPs in air is attributed to significant charge

transfer between CuNPs and TiN substrate. Photocatalyzed epoxidation processes are driven

by visible light irradiation under mild conditions. The novel photocatalyst exhibits with wide

substrate scope with various types of alkenes. Interestingly, stereo-selectivity to trans-isomer

product was observed with stilbene epoxidation. The solvent cyclic ether facilitates the reaction

by reacting with molecular oxygen on the surface of CuNPs, yielding oxygen adatoms. This

reaction process is driven by light irradiation at mild temperatures. The reactant alkenes

1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

C

on

ve

rsio

n (

%)

Runs



chemically adsorb on CuNPs, forming surface complex. The complex can be activated by

irradiation of visible light. The oxygen adatoms react with the activated alkene yielding final

product of corresponding epoxide. Light irradiation is the driving force of the epoxidation

process. Thus, the light intensity and wavelength, as well as the reaction temperature, are

influencing parameters on the performance of the photocatalytic epoxidation. The

photocatalyst exhibits excellent reusability and an overall reaction pathway for selective

epoxidation was proposed. Our results indicate a promising CuNP photocatalyst and

photocatalytic methodology at mild reaction conditions for alkene epoxidations by using

photocatalysis and molecular oxygen.

ASSOCIATED CONTENT

Supporting Information.

This material is available free of charge via the Internet at http://pubs.acs.org.

Calibration curve for conversion determination; Mass spectra for products characterization;

Characterization of photocatalyst: XPS, UV-Vis, SEM, EDX; Detailed reaction condition

optimization; ICP test for metal loss after reaction.

AUTHOR INFORMATION

Corresponding Author


ACKNOWLEDGMENT

Authors gratefully acknowledge financial support from the Australis Research Council.

(DP150102110)

KEYWORDS.

http://pubs.acs.org/



Selective epoxidation; Air-stable CuNPS; Photocatalysis; Direct electron photoexcitation;

Visible Light; Asymmetric synthesis.

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Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with

Visible Light

Yiming Huang,† Zhe Liu,† Guoping Gao,† Gang Xiao,‡ Aijun Du,† Steven Bottle,† Sarina

Sarina*,† and Huaiyong Zhu*,†

†School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and

Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia

‡Key Laboratory of Bioprocess, Beijing University of Chemical Technology, Beijing, 100029,

P. R. China

*Correspondence to: E-mail [email protected]; [email protected]





Table of Contents


Calculation of conversion rate and calibration curve

Characterization of epoxidation products

Characterization of epoxide diastereomers

XPS analysis

UV-Vis spectra

Photocatalytic activity of TiN substrate

The influence of reaction atmosphere

SEM and EDX analysis

Dependence of selectivity on the light wavelength

Influence of irradiance

Influence of Reaction Temperature

Influence of Solvent

Meal loss in cycle reactions.

XPS Analysis of Cycled Cu@TiN Photocatalyst

Temperature Control Experiment with single wavelength light source

Photocatalyst Dose Optimization

Reference




Materials and Chemicals

TiN nanopowder was purchased from Research Nanomaterials, Inc. USA. CuO, Cu2O

and Cu(NO3)2 were purchased from Chem-supply, were of AR grade and were used as received.

All other chemicals were purchased from Sigma-Aldrich and used without further purification.

Preparation of Catalysts. TiN supported CuNPs (Cu@TiN) were prepared by the

impregnation-reduction method. For example, to prepare 3 wt.% Cu@TiN, TiN powder (2.0

g) was dispersed in an aqueous solution of Cu(NO3)2 (47.2 mL, 0.01 M) under magnetic

stirring at room temperature, followed by addition of a lysine aqueous solution (10 mL, 0.53

M) to the suspension while it was vigorously stirred for 30 min. To this suspension, a freshly

prepared aqueous NaBH4 (20 mL, 0.7 M) was added dropwise. The mixture was aged

overnight, and then the solid was separated by centrifugation, washed with water (three times),

ethanol (once), and was dried at 60°C in a vacuum oven for 24 h. The as-prepared

CuO/Cu@TiN powder was reduced at 300°C for 20 mins in a flow of hydrogen gas under

argon gas protection; the obtained powder was labelled as Cu@TiN and used as prepared.

Characterization. The morphology and elemental composition of photocatalysts were studied

using a JEOL 2100 transmission electron microscopy (TEM) equipped with a Gatan Orius

SC1000 CCD camera, energy dispersion X-ray (EDX) spectrometer (X-MAXN 80TLE,

OXFORD Instruments) was coupled for elemental analysis. The accelerating voltage of TEM

was 200 KV. Scanning electron microscope (SEM) images and EDS mapping were obtained

with a ZEISS Sigma SEM at accelerating voltages of 20 KV. Diffuse reflectance UV-visible

spectra of the catalysts were collected with a Cary 5000 UV-Vis-NIR spectrometer from



Agilent company using BaSO4 as blank reference, the scanning scope was 200 nm to 800 nm.

X-ray diffraction (XRD) patterns were recorded on a Philips PANalytical X’Pert PRO

diffractometer using Cu Kα radiation (λ=1.5418 Å), the fixed power source was 40 kV and 40

mA. The diffraction data were collected from 5° to 75° at a scanning rate of 2.5o/min with

resolution of 0.01°. X-ray photoelectron spectroscopy (XPS) analysis was performed with a

Kratos Axis Ultra photoelectron spectrometer using mono Al Kα(1486.6 eV) x-ray.

Photocatalytic Activity Test. In a typical activity test, a 20 mL reaction tube was used as the

reactor, after reactant (0.1 mmol) and catalyst (20 mg) had been loaded, oxygen gas was

bubbled into the reaction solution for 5mins, the reaction tube was sealed in order to isolate the

reaction from air. The reaction tube was then stirred magnetically and irradiated with a halogen

lamp (from Nelson, 500 W, wavelength in the range of 400-750 nm). The irradiance was set to

0.5 W/cm2 unless otherwise specified. The reaction temperature was controlled by air

conditioner. The control reaction in the dark was conducted using an oil bath placed above a

magnetic stirrer, and the reaction tube was wrapped with aluminium foil to isolate the contents

from the influence of light. The temperature of dark reaction was maintained at the same

temperature as the corresponding reaction under irradiation. At the end of reaction time, 2 mL

aliquots were collected and filtered through a Millipore filter (pore size of 0.45 µm) to remove

particulate matter. The clear liquid-phase products were analyzed with an Agilent 6980 gas

chromatography (GC) using a HP-5 column to analyze the change in the concentrations of

reactants and products. An Agilent HP5973 mass spectrometer was used to identify the

products.

ICP Analysis

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was

performed using a Perkin Elmer 8300DV ICP fitted with an ESI SC-4DX auto-sampler and



PrepFAST 2 sample handling unit for online internal reference and auto-dilution of samples

and calibration references. Nitric acid, purified by sub-boiling distillation, was used for the

preparation of all references and blank solutions were used throughout the analysis.

IES Analysis

The IES analysis was conducted with a Digilab FTS-60A spectrometer equipped with a

TGS detector, the detector was modified by replacing the IR source with an emission cell.

Action Spectrum

Action spectrum experiments were conducted with light emitting diode (LED) lamps

(Tongyifang, Shenzhen, China) with wavelengths of 400±5 nm, 470±5 nm, 530 ± 5 nm, 590 ±

5 nm, and 620 ± 5 nm. The light intensity was measured to be 0.2 W/cm2 using an energy meter

(CEL-NP2000) from AULTT Company and other reaction conditions were identical to those

of typical reaction procedures.

The apparent quantum yield (AQY) was calculated as follows: apparent quantum yield

= [(Mlight−Mdark)/Np] × 100%, where Mlight and Mdark are the molecules of products formed under

irradiation and dark conditions, respectively, M = mole number of the reactant × conversion ×

6.02 × 1023 (Avogadro constant). Np is the number of photons involved in the reaction. Np =

Etotal/E1, Etotal (the total energy involved in the reaction irradiation) = intensity × light spot area

× reaction time, E1 (the energy of one photon) = h×c/ λ (h is Planck constant, c is light speed,

and λ is wavelength of the LED light).

Density Functional Theory (DFT) calculation

Geometry optimization and electronic structure calculations were carried out using

density functional theory plus long range dispersion correction under the TS method of DFT-

D as implemented in the Dmol3 software[1,2] with Semi-Core Pseudopots Potentials. Exchange-



correlation interaction is treated as generalized gradient approximation (GGA) with the Perdew,

Burke, and Ernzerhof (PBE) functional and electronic eigenfunctions are expanded in terms of

DND with a real-space cut-off of 4.4 Å. The convergence criteria for energy change, force, and

displacement during geometry optimization were set to be 2.0 × 10−5 Ha, 4.0 × 10−3 Ha/Å, and

5.0 × 10−3 Å, respectively. The differential charge density is obtained by following equation

(1):

∆𝜌 = 𝜌𝐶𝑢𝑁𝑃𝑠 𝑜𝑛 𝑇𝑖𝑁 − 𝜌𝐶𝑢𝑁𝑃𝑠 − 𝜌𝑇𝑖𝑁 (1)

where 𝜌𝐶𝑢𝑁𝑃𝑠 𝑜𝑛 𝑇𝑖𝑁 is the total electron density of the CuNPs supported on TiN, and 𝜌𝐶𝑢𝑁𝑃𝑠,

𝜌𝑇𝑖𝑁 is the eletron density of isolated CuNPs, and TiN respectively.

The formation energy of Cu13 cluster on TiN surface are calculated by following eq(1)

𝐸𝑓 = 𝐸𝐶𝑢13@𝑇𝑖𝑁 − 𝐸𝐶𝑢13 − 𝐸𝑇𝑖𝑁

Where 𝐸𝐶𝑢13@𝑇𝑖𝑁, 𝐸𝐶𝑢13,and 𝐸𝑇𝑖𝑁 is the total energies of Cu13 cluster adsorbed on the TiN

surface, Cu13 cluster and TiN surface, respectively.



Calculation of conversion rate and calibration curve

Conversion Calculation

In this study, the reaction conversion was determined based on the concentration

change of reactant and product. Taking styrene epoxidation as an example, the conversion

rate was calculated using followed equation:

Conv. %=(Cb-Ca)/Cb•100

where Cb is styrene concertation before the reaction, Ca is styrene concentration after

reaction. Ca and Cb were calculated using the equation obtained from calibration curve of

styrene:

C = 3-10•P + 0.05•10-2

where P is GC peak area, C is styrene concentration (mM). Reaction conversion was

calculated based on the concentration change:

Calibration curve of styrene

Concentration (mM) GC peak area

0 0

0.01 2.6E+07

0.02 5.5E+07

0.03 8.5E+07

0.04 1.22E+08



0.05 1.38E+08



Calibration curve of styrene oxide


0 0

0.01 1.8E+7

0.02 3.9E+7

0.03 6.3E+7

0.04 8.9E+7

0.05 1.2E+08

Calibration curve of norbornene




0 0

0.01 2.1E+7

0.02 3.7E+7

0.03 5.6E+7

0.04 7.2E+7

0.05 9.6E+7



Characterization of epoxidation products

The epoxidation products were identified using an Agilent 6980 gas chromatography

(GC) coupling with an Agilent HP5973 mass spectrometer equipped with a HP-5 column. Mass

spectra of epoxides involved in this study are listed below, it should be noted that spectra of

some of products may reflect the differences in instrument and ionization methods.

1. Styrene oxide. Table 1 entry 1, m/z for C8H8O is 120.15.



2. Oxirane, 2-(4-methyl phenyl)-. Table 2 entry 1, m/z for C9H10O is 134.17.



3. Oxirane, 2-(3-methyl phenyl)-. Table 2 entry 2, m/z for C9H10O is 134.17.



4. Oxirane, 2-methyl-3-phenyl-. Table 2, entry 3, m/z for C9H10O is 134.17.



5. Oxirane, 2-methyl-2-phenyl-. Table 2, entry 4, m/z for C9H10O is 134.17.



6. Oxirane, 2-(2-methylphenyl)-. Table 2, entry 5, m/z for C9H10O is 134.17.

The cluster of peaks of Oxirane, 2-(2-methylphenyl)- ~ 120 m/z which is not present in

the experimental spectrum may reflect different fragmentation pathways to those generated

using the Agilent HP5973 mass spectrometer. However, this fragmentation is unusual

compared with similar compounds such as Oxirane, 2-(3-methylphenyl)- and Oxirane, 2-(4-

methylphenyl)-.



7. Oxirane, 2-ethyl-3-phenyl-. Table 2, entry 6, m/z for C10H12O is 148.20.



8. 2-oxiranecarboxaldehyde, 3-phenyl-. Table 2, entry 7, m/z for C9H8O2 is 148.05.

The peaks in the experimental spectrum roughly match with the peaks in the reference

spectrum however weak ionization gives a noisy spectrum.



9. Oxirane, 2-(3-nitrophenyl)-. Table 2, entry 8, m/z for C8H7NO3 is 165.04.



10. 1,2-Cyclohexene oxide. Table 2, entry 9, m/z for C6H10O is 98.13.



11. 1,2-Epoxy-3,4-cyclohexene. Table 2, entry 10, m/z for C6H8O is 96.13.



12. 1,2-Epoxycyclooctane. Table 2, entry 11, m/z for C8H14O is 126.20.



13. 2,3-Epoxynorbornane. Table 2, entry 12, m/z for C7H10O is 110.15.



14. Propylene oxide. Table 2, entry 13, m/z for C3H6O is 58.08.



15. Oxirane, 2-methyl-2-(1-methylethenyl)-. Table 2, entry 14, m/z for C6H10O is 98.14.



16. Oxirane, 2-(1-methylethenyl)-. Table 2, entry 15, m/z for C5H8O is 84.11.



17. Oxirane, 2,2-dimethyl-3-(2-methyl-1-propen-1-yl)-, table 2 entry 16, m/z for C8H14O is

126.20.



18. 2,3-Oxiranedimethanol. Table 2, entry 17, m/z for C4H8O3 is 104.10.

The peaks in the experimental spectrum match with the peaks in the reference spectrum

at high m/z, however, the experimental result involves a noisier spectrum below 70 m/z which

may arise from the different sensitivity of the Agilent HP5973 mass spectrometer used for the

measurements.



19. Oxirane, 2-hexyl-. Table 2, entry 18, m/z for C8H16O is 128.21.


at high m/z. However, the experimental result involves a noisier spectrum below 70 m/z which


measurements. The additional reference spectrum from MassHunter gives well match to the

experimental spectrum.





20. Oxirane, 2-decyl-. Table 2, entry 19, m/z for C12H24O is 184.32.

In experimental spectrum, the peaks above 110 m/z are too weak to be observed, possibly

due to the poor ionization of the compound in the instrument. Additional reference spectrum

from MassHunter is provided for comparison:





21. Oxirane, 2-butyl-. Table 2, entry 20, m/z for C6H12O is 100.16.



22. Oxirane, 2-methyl-3-propyl-. Table 2, entry 21, m/z for C6H12O is 100.16.



23. Oxirane, 2-methyl-3-propyl-.Table 2, entry 22, m/z for C6H12O is 100.16.



24. Oxirane, 2,3-diethyl-. Table 2, entry 23, m/z for C6H12O is 100.16.




measurements. Additional reference spectrum from MassHunter is provided for comparison:





25. Oxirane, 2,3-diethyl-.Table 2, entry 24, m/z for C6H12O is 100.16.




measurements.



26. Oxirane, 2,3-diphenyl-.Table 2, entry 25, m/z for C14H12O is 196.24.



27. Oxirane, 2,3-diphenyl-, (2R,3S)-. Table 2, entry 6, m/z for C14H12O is 196.24.



Characterization of Epoxide Diastereomers

The epoxidation product diastereomers were identified using an Agilent 6980 gas

chromatography (GC) coupling with an Agilent HP5973 mass spectrometer equipped with a

HP-5 column. In this study, the difference in melting point of the two pairs of enantiomers,

trans-stilbene /cis-stilbene and trans-stilbene oxide/cis-stilbene oxide, are remarkable, as

shown below:

Entry Enantiomers Melting Point Boiling Point

1 trans-stilbene 124 °C [3] 307 °C [3]

2 cis-stilbene 5-6 °C [4] No experimental data

3 trans-stilbene oxide 67-69 °C [5] No experimental data

4 cis-stilbene oxide 38-40 °C [5] No experimental data

Thus, GCMS equipped with HP-5 column (non-chiral column) is capable of

distinguishing trans-stilbene/cis-stilbene and trans-stilbene oxide/cis-stilbene oxide. Two GC

spectra are shown below as examples.



1. GC spectrum of mixed-stilbene which is prepared by mixing trans-stilbene and cis-stilbene

in 1:1 ratio.



2. GC spectrum of epoxidation products from mixed-stilbene.



Trans-stilbene and cis-stilbene give identical mass spectrum.



XPS analysis

Figure S1. XPS analysis. XPS spectra of (a) CuO/Cu@TiN and (b) CuO@ZrO2.



UV-Vis Spectra

Figure S2. UV-Vis spectra. UV-Vis spectra of isolated (a) CuNPs and Cu oxides; (b)

CuO/Cu@TiN and TiN.



Table S1. Photocatalytic activity of TiN substrate.[a]

Entry Catalyst Atmosphere Substrate Conv. [%]

Select.

2 [%] 3 [%]

1 TiN O2 1a 40

(12)

n.d

(n.d)

100

(100)

2[b] TiN O2 1b 11

(n.r)

100

(n.d)

n.d

(n.d)

3 [c] TiN O2 1c 15

(2)

n.d

(100)

100

(n.d)

Reaction conditions: 20 mg catalyst, 0.1 mmol substrate and 3 mL 1,4-dioxane as solvent were

added. Selected gas was bubbled for 5 mins and then reaction tube was sealed. The reaction

mixture was stirred under visible light irradiation (0.5 W/cm2) at 60°C for 4 h. Numbers in

parentheses are results of reactions in the dark. [a] (n.r.= no reaction; n.d.=not detected); [b] 8

h. [c] 16 h.



Table S2. The influence of reaction atmosphere. The influence of different atmosphere on

the epoxidation of styrene over different photocatalyst

Entry Catalyst Atmosphere Conv. Select.

1 Cu@TiN O2 100 81

2 Cu@TiN Air 64 72

2 Cu@TiN Air[a] 100 89

3 Cu@TiN Argon n.r. n.d.

4 CuO/Cu@TiN Argon n.r. n.d.

5 TiN Argon n.r. n.d.

6 Cu@TiN Argon n.r. n.d.

7 Cu@ZrO2 Argon n.r. n.d.

8 CuO[b] Argon n.r. n.d.

17 Cu2O[b] Argon n.r. n.d.

Reaction conditions: 20 mg catalyst with 3 wt% Cu loading, 3ml 1,4-dioxane, 0.1 mmol styrene,

halogen lamp 0.5 W/cm2, selected gas was bubbled for 5 mins, 60 °C, 4h. Conversion and

selectivity were determined by GCMS analysis. [a] air was bubbled for 5 mins at the start of

reaction and every hour after. [b] 40 equiv. Cu loading compared with Cu@TiN.



SEM and EDX analysis

Figure S3. SEM and EDX analysis. Scanning electron microscopy (SEM) images and

corresponding energy dispersive spectrometer (EDX) mapping of (a) Cu@TiN and (b)

CuO/Cu@TiN.



Dependence of selectivity on the light wavelength

Figure S4. Dependence of selectivity on the light wavelength.

Reaction conditions: 20 mg 3 wt% Cu@TiN, 3ml 1,4-dioxane as solvent, 0.1 mmol styrene,

60oC, oxygen gas bubbled for 5 min, then reaction proceeded for 4 h. Light emitting diode

(LED) lamps (Tongyifang, Shenzhen, China) with wavelengths of 400±5 nm, 470±5 nm, 530

± 5 nm, 590 ± 5 nm, and 620 ± 5 nm were used as light source. The light intensity was measured

to be 0.2 W/cm2 using an energy meter (CEL-NP2000) from AULTT Company and other

reaction conditions were identical to those of typical reaction procedures, the reaction

selectivity was determined by product distribution which was measured with an Agilent 6980

gas chromatography (GC) equipped with an Agilent HP5973 mass spectrometer using a HP-5

column.



Table S3. Influence of irradiance. Influence of light irradiance on the styrene epoxidations.

Entry Irradiance (W/cm2) Conv. (%) Select. (%) Yield (%) Reaction

rate

(µmol/min)

1 0.30 39 21 8 0.1625

2 0.45 47 35 16 0.19

3 0.60 92 77 71 0.38

4 0.75 100 77 77 0.41

5[a] 0.90 90 75 67.5 0.50


halogen lamp, 60oC, oxygen gas bubbled for 5 min., reaction time 4 h, [a] reaction time 3 h.



Table S4. The Influence of Reaction Temperature

Entry Temperature(oC) Reaction time Light Conv. (%) Select. (%)

1 30 24h Light 32 26

2 24h Dark 0 0

3 40 24h Light 28 24

4 24h Dark 2 0

5 50 4h Light 25 23

6 8h Light 54 44

7 24h Light 99 68

8 24h Dark 0 0

9 55 2h Light 48 43

10 4h Light 63 52

11 8h Light 95 66

12 16h Light 100 62

13 24h Light 100 67



14 8h Dark 0 0

15 60 2h Light 36 43

16 4h Light 100 89

17 24h Light 100 74

18 4h Dark 5 0

19 65 2h Light 54 75

20 4h Light 100 77

21 5h Light 100 74

22 8h Light 100 76

23 4h Dark 0 0

24 70 2h Light 100 70

25 4h Light 100 67

26 24h Light 100 49

27 2h Dark 0 0

28 4h Dark 13 11


halogen lamp 0.5 W/cm2, oxygen or argon gas bubbled for 5 min. Numbers in red are data

from light reaction and those in dark are data from dark reaction at the same temperature.



Table S5. The influence of solvent. The influence of solvent on the epoxidation of styrene

Entry Solvent Light Conversion (%) Selectivity (%)

1 Acetone

Light 0 0

Dark 0 0

2 MeCN

Light Trace 0

Dark Trace 0

3 DMF

Light 0 0

Dark 0 0

4 Toluene

Light 0 0

Dark 0 0

5 DMSO

Light 4.3 0

Dark Trace 0

6 CH2Cl2

Light 0 0

Dark 0 0

7

Light 100 81

Dark 5 0



8

Light 70 80

Dark 8 0

9[a]

Light 30 50

Dark 12 0

10[a]

Light 0 0

Dark 0 0

11a]

Light 20 41

Dark 3 0

12[b]

Light 2 0

Dark 0 0

13[b]

Light Trace 0

Dark 0 0

14[a]

Light 62 0

Dark 20 0

15

Light 0 0

Dark 0 0



16 Ethyl acetate

Light 12 0

Dark 0 0

Reaction conditions: Cu@TiN. 20 mg 3 wt% Cu@TiN catalyst, 0.1 mmol styrene, 3 mL

solvent. Oxygen was bubbled into solvent for 5 mins. Halogen lamp 0.5 W/cm2, 60°C, 4 h.

Conversion and selectivity were determined by GCMS analysis. [a] 16 h, [b] 24 h.



Meal Loss in Cycle Reactions.

Table S6 ICP data

Sample Cu mg/L

Reaction solution after cycle one 0.181

Sample was collected after one reaction cycle and filtrated with 0.45 μm polymer filter.



XPS Analysis of Cycled Cu@TiN Photocatalyst

Figure S5. XPS spectra of Cu@TiN recovered from styrene epoxidation cycle run test, (a) 1

cycle; (b) 2 cycles; (c) 3 cycles; (d) 4 cycles and (e) reactivated Cu@TiN after 8 cycles using

H2 flow at 200 oC.



Figure S6. The temperature control experiment (50 oC, 60 oC, 70 oC) using the light source

with wavelength > 530 nm.

We conducted the styrene epoxidation at different temperature (50 oC and 70oC) using

light source 530 nm, 590 nm and 630 nm. The reaction rate shows a positive relationship to

temperature indicating a positive impact of system temperature on the yield.

520 540 560 580 600 620

0

10

20

30

70 oC

60 oC

50 oC

Yie

ld (

%)

Wavelength (nm)



Photocatalyst Dose optimization

3wt% Cu@TiN dose Conv. (%) Select. (%)

1 mg 13 65

10 mg 82 87

20 mg 100 90

50 mg 100 88


halogen lamp 0.5 W/cm2, oxygen gas bubbled for 5 min, 60 oC, 6 h.

The result suggests 20 mg Cu@TiN gives 100% conversion with 90% selectivity to

epoxides. On the other hand, 82% conversion was obtained with 10 mg Cu@TiN. 50 mg

Cu@TiN can also completely convert styrene with 88% selectivity to epoxide, however owing

to the catalyst efficiency reaction, 20 mg is the optimal amount.



Reference

[1] B. Delley, J. Chem. Phys. 1990, 92, 508.

[2] B. Delley, J. Chem. Phys. 2000 113, 7756.

[3] J. Laane, K. Haller, S. Sakurai, K. Morris, D. Autrey, Z. Arp, W.-Y. Chiang, A. Combs, J.

Mol. Struct. 2003, 650, 57.

[4] D. S. Brackman, P. Plesch, J. Chem. Soc. (Resumed) 1952, 2188.

[5] A. C. Cope, P. A. Trumbull, E. R. Trumbull, J. Am. Chem. Soc. 1958, 80, 2844.

In-depth Mechanism Study of Metallic Nanoparticle Based Photocatalyst

- 222 -

Chapter 6 In-depth Mechanism Study of Metallic

Nanoparticle Based Photocatalyst


In this chapter presented is Article 4 (Published in The Journal of Physical Chemistry

Letters, 2017, 8, 2526-2534.) reporting fundamental mechanism research by analysing action

spectra (the photocatalytic performance of a metallic photocatalyst plot against the incident

light wavelength) that bring insights into the metallic photocatalysis.

In the past decades, the transition metal based photocatalysts have been realised as a new

class of promising photocatalysis candidates. Many organic synthesis reactions have been

reported over different types of transition metal and their alloy involved photocatalysts. Despite

the increasing effects being made in this area, little is known fundamentally regarding the

metallic photocatalysis mechanisms. The understanding of light induced chemical conversion

associated with energy transfer and evolution had been through the path of photothermal effect,

photoexcited charger carriers transfer (indirect photoexcitation), direct photoexcitation of

intramolecular orbitals and direct photoexcitation of the hybridised molecular orbitals of the

metal-substrate complex. Consensus has yet to be reached and proposed theories are still under

debate and being examined from both theoretical and experimental aspects. In this part of the

thesis, we investigated a number of action spectra obtained from multiple organic reactions

photocatalysed by different classes of transition metal or metal alloy NP photocatalysts. By

comparing the different action spectra trends, we reveal the versatile existence of photon

threshold energy in photoexcited electron induced metallic photocatalytic reactions. In addition,

we have further demonstrated that direct photon-electron excitation rather than photothermal


- 223 -

effect is the dominant driving force for the initiation of metallic photocatalytic reactions. In

summary, this work contributes to the construction of a comprehensive theory to the metallic

photocatalysis and could benefit the future work in this field.


- 224 -

Article 4



Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key

Evidence from the Action Spectrum of the Reaction

Sarina Sarina, Esa Jaatinen, Qi Xiao, Yi Ming Huang, Philip Christopher, Jin Cai Zhao and Huai

Yong Zhu

Published in The Journal of Physical Chemistry Letters, 2017, 8, 2526-2534.


Student Author:

Yiming Huang

Conducted catalyst fabrication and

characterisations, conducted action spectra

experiments of Au-Pd alloy nanoparticles

and responsible for related data collection

and calculation, participated in the overall

data analysis and mechanism development,

revised the manuscript to improve the

logical chain.

Signature

Date


- 225 -

Dr Sarina Sarina First author, wrote the manuscript,

experimental design, conducted experiments

and data analysis.

A/Prof. Dr. Esa Jaatinen Aided experimental design, data analysis.

Dr. Qi Xiao Aided experimental design, data analysis.

Dr Philip Christopher Aided experimental design, data analysis.

Prof. Dr Jin Cai Zhao Aided experimental design, data analysis.

Prof. Dr Huai Yong Zhu The corresponding author, aided




authorship.

____________ _____________ ________________

Name Signature Date


- 226 -

Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles: Key

Evidence from the Action Spectrum of the Reaction

Sarina Sarina,† Esa Jaatinen,† Qi Xiao,†‡ Yi Ming Huang,† Philip Christopher,§ Jin Cai Zhao∥

and Huai Yong Zhu*†

†School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Brisbane, Queensland 4001, Australia

‡CSIRO Manufacturing, Bayview Avenue, Clayton, Victoria 3168, Australia

§Department of Chemical & Environmental Engineering, University of California, Riverside,

Riverside, California 92521, United States

∥Key Laboratory of Photochemistry, Institute of Chemistry, The Chinese Academy of

Sciences, Beijing 100190, China

*E-mail: [email protected]

Abstract: By investigating the action spectra (the relationship between the irradiation

wavelength and apparent quantum efficiency of reactions under constant irradiance) of a

number of reactions catalysed by nanoparticles including plasmonic metals, nonplasmonic

metals, and their alloys at near-ambient temperatures, we found that a photon energy threshold

exists in each photocatalytic reaction; only photons with sufficient energy (e.g., higher than

the energy level of the lowest unoccupied molecular orbitals) can initiate the reactions. This

energy alignment (and the photon energy threshold) is determined by various factors, including

the wavelength and intensity of irradiation, molecule structure, reaction temperature, and so

forth. Hence, distinct action spectra were observed in the same type of reaction catalysed by

the same catalyst due to a different substituent group, a slightly changed reaction temperature.

http://pubs.acs.org/author/Sarina%2C+Sarina

http://pubs.acs.org/author/Jaatinen%2C+Esa

http://pubs.acs.org/author/Xiao%2C+Qi

http://pubs.acs.org/author/Huang%2C+Yi+Ming

http://pubs.acs.org/author/Christopher%2C+Philip

http://pubs.acs.org/author/Zhao%2C+Jin+Cai

http://pubs.acs.org/author/Zhu%2C+Huai+Yong

http://pubs.acs.org/author/Zhu%2C+Huai+Yong



- 227 -

These results indicate that photon–electron excitations, instead of the photothermal effect, play

a dominant role in direct photocatalysis of metal nanoparticles for many reactions.

Nanoparticles (NPs) of transition metals, such as Au, Ag, Cu;(1-7) Pd, Pt, Rh, Ir, Ru, and

their alloys,(8-10) dispersed on optically and catalytically inert materials (ZrO2, Al2O3, etc.) have

been found as efficient photocatalysts. The NPs exhibit strong optical absorption over the entire

solar spectrum and efficiently channel the photon energy into molecules that are adsorbed on

their surfaces and initiate chemical transformations.(4, 11) This process can significantly promote

catalytic activity at near-ambient temperature and pressure.(5) Direct photocatalysis by metal

NPs has inspired rapid expansion of the field of green photocatalysis, which is a promising

alternative to the many heat-driven reactions currently using thermal catalysts.(1-18) As a result,

it is of great interest to fully understand the mechanisms underpinning direct photocatalysis on

metal NP surfaces.

When the incident photon energy is less than the work function of the metal, Φ, the

incident light excites conduction electrons in metal NPs to higher energy levels by two

pathways depending on the wavelength. Single-photon excitation (one incident photon excites

one metal electron–hole pair) occurs in all metal particles. The excited metal electrons will

rapidly thermalize by successive electron–electron scattering with the high density of free

electrons in the metal.(9, 19) This results in an initial distribution of hot electrons with energies

in the range of EFermi < Ee< EFermi + Φ, where EFermi is the Fermi level of the metal. Single-

photon excitation and subsequent electron–electron scattering occur regardless of the metal

NP’s physical properties (particle size, morphology, etc.) due to the near-continuum of electron

energy levels possessed by the metal. The maximum energy reached by the hot electrons and

the rate at which the Fermi–Dirac electron distribution cools via electron–electron scattering,

however, is determined by the incident photon energy or wavelength. Irradiation can also


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induce a collective oscillation of the conduction electrons in some metal NPs when the light

frequency is resonant with the metal’s plasma oscillation frequency, with the electrons gaining

light energy through the localized surface plasmon resonance (LSPR) effect.(1-7) The LSPR

absorption depends strongly on the properties of metal NPs, such as the dielectric coefficient

and particle morphology.(20) NPs made from Au, Ag, Cu, and Al strongly absorb visible light

through the LSPR effect, characterized by a strong absorption peak, and as a result, these metals

are often referred to as plasmonic metals.(18, 20-27) Following a very short coherency lifetime of

the LSPR, plasmons will decay, resulting in electron excitations, similar to the single-photon

excitation process described above, albeit with much higher cross sections.

Over time, the hot free electron gas relaxes to lower energy levels through electron–

phonon scattering that distributes the absorbed energy to the larger thermal mass of the NP

lattice. When high-intensity light is applied, plasmonic metal NPs absorb a significant amount

of light energy and the resulting hot NP has advantageous uses in some applications such as

the photothermal effect, plasmonic photothermal therapy, and plasmonic autoclaves.(22, 27-29)

Initially, the photothermal effect was considered as the responsible or sole driving mechanism

for direct photocatalysis involving metal NPs.(30) In our earlier study, we also assumed that

reactions on Au NPs could have been driven predominantly by a photothermal effect.(1)

However, further analysis reveals that when a metal NP 5 nm in diameter is exposed to

moderate light intensities (∼0.5 W·cm–2, about five times of mean irradiance of sunlight, which

is usually applied to photocatalytic reactions) and assuming that the whole photon energy

absorbed by this NP is converted to heat, the resulting temperature increase is about 1 K.(31, 32)

This suggests that the photothermal effect plays a minimal role in direct photocatalysis of metal

NPs when using low-intensity continuous-wave excitation sources.


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It is often noted that the photocatalytic performance of metal NPs depends exponentially

on reaction temperature,(4, 5, 8, 15, 31) while semiconductor photocatalysts generally do not show

this dependence. Temperature can positively influence catalysis by providing an increased

driving force to overcome activation barriers by populating higher-lying vibrational states

along the reaction coordinate. For semiconductors, the minimal influence of temperature is

because increasing the temperature also increases the rate of electron–hole pair

recombination.(33) The direct photocatalysis by supported metal NPs is only influenced by

temperature through increased population of high-lying vibrational states because higher

temperature does not significantly modify hot electron and hole lifetimes, thus minimizing the

required photocatalytically mediated non-adiabatic energy gain required to drive the

reaction.(15) Nevertheless, the temperature dependence of photocatalytic performance of metal

NPs is often confused with the photothermal effect, causing difficulty in understanding the

mechanism of photocatalysis of the metal NPs.

While it is known that direct photocatalysis of metal NPs is influenced by multiple

effects,(31) the underlying mechanisms are not fully understood, and significant confusion exists.

Experimentally, distinguishing the role of the photothermal effect, hot electron transfer, and

photon–electron excitation of hybridized metal–molecule states will help shed light on the

reaction mechanisms but is a challenging undertaking.

It is known that the hot electrons at higher energy levels of the Fermi–Dirac distribution

or nonthermal distribution at very short time scales after photoexcitation of the metal could

transfer the necessary energy to the adsorbed species on metal NPs and initiate reactions of the

species.(2, 4, 8, 12, 15) We note that energy alignment is required for the reactions mediated by the

hot electrons. Such an energy alignment requirement is different from reaction to reaction and

depends closely on reaction conditions that influence the rate-limiting step of the reaction, for


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which the energy state population will drive the system along the reaction coordinate. Because

the energy state of the photoexcited electrons is mainly determined by the irradiation

wavelength, it is possible to gain insight into the mechanism of direct photocatalysis of metal

NPs from analysis of the impact of the irradiation wavelength on the photocatalytic reactions

catalysed by metal NPs.

In this study, we attempt to clarify the situation by investigating the dependence of

photocatalytic activity on the irradiation wavelength for many reactions directly photocatalysed

by a variety of metal NP photocatalysts strictly under the regime of low-intensity continuous-

wave excitation.

We analyse the dependence of photocatalytic activity in many photocatalytic reaction

systems under illumination of different wavelengths, using six catalysts in total (NPs made

from six different metals, including Au, Pd, Pt, Rh, Ir, and Au–Pd alloy). The performances

and structure information on some of the catalysts were reported in our previous studies.(8, 34,

35) To avoid interference from light absorption by the support solids, metal NPs were supported

on photocatalytically inert support materials (such as ZrO2, etc.) that have negligible light

absorption at the wavelengths used.

The metal NP catalysts were prepared by reducing the corresponding metal salt with

NaBH4 in the presence of the support solids, abbreviated as M@support. Light-emitting diodes

(LEDs) with narrow emission bands (365 ± 5, 400 ± 5, 470 ± 5, 530 ± 5, 590 ± 5, and 620 ± 5

nm) are used as the light sources for all reactions. The reaction temperature and irradiation

intensities are kept constant for the reactions to ensure that the impact of the external heating

on each reaction remains identical.

We use action spectra (the wavelength dependence of the photocatalytic apparent

quantum efficiency, AQE) in the present study to analyse the contribution of the different


- 231 -

effects. By measuring the wavelength dependence of the AQE, we reveal the photon energy

threshold, above which significant photocatalytic activity is observed. Analysis of the action

spectra and the shape of the spectral dependence allows us to distinguish reactions mediated

by energetic electrons and those mediated by plasmon-induced heating, as the spectral

dependence of heating-induced catalytic processes should be identical regardless of the

reaction or conditions. The action spectra shapes also shed light on the nature of how energy is

transferred to molecular species.

The photocatalytic reaction rates under illumination are converted into the AQE. The

AQE is the number of reactant molecules converted by each photon absorbed by the metal NPs,

expressed as a percentage at each wavelength

Where Ylight and Ydark are the number of reactants converted with and without light irradiation,

respectively. This metric directly measures the influence of the light only as the number of

reactants converted in the dark at the same temperature is deducted.

Action Spectra of Plasmonic Metal NPs. It is generally expected that the photocatalytic

performance of a metal NP catalyst will follow its light absorption spectrum: the greater the

light absorption, the higher the photocatalytic activity. However, we found that the action

spectra of one reaction can differ due to small changes in moieties on the reactants. Figure 1

shows the action spectra of Sonogashira cross-coupling reactions of phenylacetylene and

iodobenzene with different substituent groups.

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Figure 1. Different action spectra shown on Au−Pd alloy NP photocatalysts. The dashed line

is the light absorption of the Au−Pd alloy NP photocatalyst, and the solid line is the absorption

of Au NPs dispersed on the same ZrO2 support. Left axes are the normalized absorption intensity

of metal NP photocatalysts (lines), and the right axes are the normalized AQE of reactions

(symbols). (a) Sonogashira coupling of phenylacetylene with 1-iodo-4-methoxybenzene (4-

OCH3−PhI) catalyzed by Au−Pd alloy NPs showing an action spectrum that follows the light

absorption of Au− Pd alloy NPs. (b) Sonogashira coupling of phenylacetylene with 1-iodo-4-

nitrobenzene (4-NO2−PhI) catalyzed by Au−Pd alloy NPs that exhibits an action spectrum that

does not follow the light absorption of Au−Pd alloy NPs but follows absorption of the Au NP.

Reaction conditions are listed in the Experimental Section in the Supporting Information.

In these two reactions, light absorption by the catalysts (Au–Pd alloy NPs) is the same

(the dashed line) and all other reaction conditions are identical, but the reactant aryl iodides

possess different substituent groups. When we use 1-iodo-4-methoxybenzene, the action

spectrum matches to the light absorption of Au–Pd alloy NPs (dashed line, Figure 1a).

Nonetheless, when 1-iodo-4-nitrobenzene instead of 1-iodo-4-methoxybenzene was the

reactant, the action spectrum did not follow the light absorption spectrum of the Au–Pd NPs

(dashed line in Figure 1b). The highest AQE was observed at 530 nm, even though light

absorption of the Au–Pd alloy NP (dashed line) at this wavelength was weaker than that at

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shorter wavelengths. We found that this action spectrum matches the light absorption of Au

NPs instead of the Au–Pd alloy, which shows an intensive LSPR absorption peak at about 530

nm. Evidently, in the Sonogashira coupling of 1-iodo-4-nitrobenzene (Figure 1b), the LSPR

absorption of the catalyst plays a more dominant role than the absorption at shorter wavelengths

(365 and 400 nm). In contrast, the shorter-wavelength photons more effectively drive the

reaction when 1-iodo-4-methoxybenzene is used as the reactant (Figure 1a).

The mismatch of AQE spectral dependences with the most intensive light absorption of

metal NP catalysts demonstrates directly that the major driving force of direct photocatalysis

in these cases is not a simple photothermal effect. The −NO2 group is an electron-withdrawing

group that increases the activity of iodobenzene in reaction with the nucleophilic reagent alkyne,

and −OCH3 is an electron-donating group that decreases the activity. We deduce that the

different action spectra are result of the existence of an energy threshold for inducing the

carbon–iodine bond cleavage, and the particular substituent group of the reactants influences

the threshold. Scheme 1 illustrates reactant molecules for the Sonogashira coupling with

reactants of 1-iodo-4-nitrobenzene and 1-iodo-4-methoxybenzene, respectively.

Scheme 1. Energy Levels of Hot Electrons Generated by a Short-Wavelength and a

Long-Wavelength Excitation of the Au−Pd Alloy NP, LUMO and HOMO Orbitals of 1-

Iodo-4- methoxybenzene (left hand side) and 1-Iodo-4-nitrobenzene (right hand side)

Moleculesa


- 234 -

a Longer-wavelength irradiation (at the LSPR absorption range, blue area) can drive hot

electron transfer to the LUMO orbitals of 1-iodo-4-nitrobenzene only, while shorter

wavelengths are required to produce hot electrons with sufficient energy to transfer to the

LUMO orbitals of 1-iodo-4-methoxybenzene. As the adsorption of reactant molecules reduces

the energy gap between their LUMO and HOMO orbitals,(22) the relative positions of the

LUMO and HOMO with respect to the Fermi level are only qualitative and schematically show

the relative positions for the reactants.

The energy of the lowest unoccupied molecular orbital (LUMO) of 1-iodo-4-

methoxybenzene is ∼2.1 eV higher than that of 1-iodo-4-nitrobenzene, and thus, light with

shorter wavelengths are required to efficiently drive the reaction with 1-iodo-4-

methoxybenzene. This indicates that shorter wavelengths are required to generate a significant

population of hot electrons that have sufficient energy to transfer to the LUMO of 1-iodo-4-

methoxybenzene. The result shown in Figure 1 demonstrates that the rate of direct

photocatalytic conversions at each different wavelength is determined not only by the

absorption spectrum of the photocatalyst but also by the energy level of the relevant molecular

orbitals of the reactants, which in turn are dependent on the chemical bonds being activated.

The photon energy threshold in reactions involving hot electron transfer depends on energy

level alignment between hot electron energy (and therefore photon energy) and molecular

orbitals of reactants.


- 235 -

Scheme 2. Hot Electron Distribution of a Plasmonic Metal or Its Alloy NPs under

Irradiation of Different Wavelengths and Their Contribution to AQE in Different

Reactionsa

(a) Irradiation with Light of 400 nm wavelength: only hot electrons located above the LUMO

level (area between two red dash line) are able to contribute to AQE. (b) Irradiation with light

of 530 nm wavelength (plasmonic wavelength of Au NPs in this study): the hot electron

distribution area above LUMO level is much smaller than that in (a). (c) Irradiated with a 400

nm wavelength on the NP and a reactant molecule with a lower LUMO: more hot electrons can

contribute to AQE. (d) Irradiated with a 530 nm wavelength on the NP and a reactant molecule

with a lower LUMO: the hot electrons able to contribute to AQE are the area above the LUMO,

involving hot electrons excited by both wavelengths.

The plasmonic metal NP catalysed photocatalytic reactions include two light absorption

mechanisms: (1) short-wavelength (e.g., 400 nm) absorption via single-electron excitation


- 236 -

(also called interband excitation); and (2) LSPR absorption (e.g., the adsorption at 530 nm for

Au NPs).(36) Reactant molecules on the metal NP surface with different LUMO energy levels

are indicated in Scheme 2: high-energy LUMO (a,b) and low-energy LUMO (c,d). When

irradiated with a shorter wavelength (e.g., 400 nm), single-photon excitation generates hot

electrons. Nonetheless, the number of hot electrons with sufficient energy to inject into the

higher LUMO (Scheme 2a) is much less than that able to inject into a lower LUMO (Scheme

2c). This is the simplest situation involving only light absorption of short wavelength by single-

electron excitation.

When irradiated with longer wavelengths that drive LSPR excitation in the metal NP, the

population of hot electrons is significantly increased, as shown in Scheme 2b,d, compared with

that produced from absorption of non-LSPR wavelength light (even at short wavelengths)

(Scheme 2a,c). The LSPR excitation is the collective excitation of the conduction electrons of

metal NPs by the resonant incident light. The number of hot electrons generated by the LSPR

can be very different from the number of photons of incident light. It is rational that more hot

electrons are generated by the same quantity of light energy absorbed by the LSPR effect than

that generated by single-photon excitation, but the hot electrons generated by the LSPR effect

are at lower energy levels and can only induce reactions with lower-energy thresholds. Hence,

in a photocatalytic reaction with a lower LUMO (Scheme 2d), hot electrons produced from

LSPR decay can effectively drive the reaction, and in this case, the action spectra of plasmonic

metal NPs are expected to align well with the light absorption spectra, with higher AQE values

observed at LSPR wavelengths, such as in Figure 1b. If the reactant molecule has a relatively

higher energy LUMO, as shown in Scheme 2b, most of the hot electrons excited by LSPR

absorption are unable to transfer to the LUMO as they have insufficient energy. As a result, we

observe a lower AQE at the long wavelength, as shown in Figure 1a. Therefore, for reactions

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with a high photon energy threshold, light at shorter wavelengths is more effective in driving

the reactions than light at the LSPR peak wavelength as most of the hot electrons yielded

though LSPR absorption have energies below the threshold. For reactions with a low photon

energy threshold, the observed AQE at the LSPR peak wavelength dominates because the

strong absorption yields a large number of hot electrons that have sufficient energy for transfer

to the reactant. Consistent with this, Toker et al. reported that excitation with the LSPR

wavelength on the Ag NP cannot increase photocatalytic activity to activation of the C–Cl bond

in ethyl chloride, while the short wavelength (266 nm for example) can.(37) This is due to the

fact that the ethyl chloride LUMO level is relatively high and able to be activated by the high

energetic electrons (excited by short wavelength) only. We achieved a much lower conversion

rate of bromobenzene and chlorobenzene on the Au–Pd alloy NP photocatalyst due to the

significant higher bond energies of C–Cl and C–Br bonds than the C–I bond. To investigate

the impact of irradiation wavelength on the reaction clearly, a considerable conversion rate of

reactant is necessary; thus, we chose iodobenzene as a model reactant.

In addition to the reactions shown in Figure 1, another pair of comparable reactions is

shown in Figure 2. The action spectra for Au–Pd alloy NP@ZrO2-catalyzed Sonogashira

coupling and Suzuki coupling (coupling reactions of iodobenzene and phenylboronic acid) are

distinctly different even though the reactant is 3-methyl-iodobenzene for both reactions.

Sonogashira coupling has a lower photon energy threshold for activation of the reaction. This

is attributed to the other reactant, phenylacetylene, having a strong affinity with Au.(13) Thus,

both reactants have an easier activation on Au–Pd alloy NPs. In comparison, the other reactant

of the Suzuki reaction, phenylboronic acid, has a relatively weak affinity to the NPs, and thus,

a higher energy threshold for activating this reaction is expected. The results indicate that the


- 238 -

threshold may depend on multiple factors, which are directly related to the energetic position

of the adsorbate orbital involved in charge transfer.

Figure 2. Different action spectra shown on Au−Pd alloy NP photocatalysts. (a) Sonogashira

coupling shows an action spectrum that follows plasmonic absorption of Au NPs (solid line).

(b) Suzuki coupling shows an action spectrum that follows light absorption of Au−Pd alloy

NPs (dashed line). Left axes are the normalized absorption intensity of metal NP

photocatalysts (lines), and right axes are the normalized AQE of reactions (symbols). Reaction

conditions are listed in the Experimental Section in the Supporting Information.

In a recent study of Au–Cu alloy NP-mediated photocatalytic reduction of nitrobenzene

and 4-CH2OH-substituted nitrobenzene (4-nitrobenzyl alcohol),(35) we also found that the

action spectra can be distinctly different even though the same catalyst was used. The above

argument on energy alignment requirement is applicable to these cases. The LUMO of 4-

CH2OH-substituted nitrobenzene is higher than that of nitrobenzene, resulting in a higher

threshold for activating the molecules. For reactions driven by the photothermal effect, there

should not be such a threshold because metal NPs have continuous electronic energy levels and

can absorb irradiation of all wavelengths to be heated.

We found that a photon energy threshold exists in many reactions. Data summarized in

Table 1show the wavelength dependence of AQE for several other reactions catalysed by Au

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NPs and Au–Pd alloy NPs. Irradiation with a 590 nm wavelength is not able to activate Cl-

substituted benzylamine for oxidation (entry 1) but is able to activate benzylamine (entry 2);

benzyl alcohol can be activated by the longest considered wavelength, 620 nm (entry 3), while

benzylamine cannot (entry 2). Furthermore, the highest AQE of benzyl alcohol oxidation (entry

3) and benzylamine oxidation (entry 2) is found at the LSPR wavelength (530–590 nm), while

that of Cl-substituted benzylamine is found at the short wavelength only (365 nm, see entry 1).

Table 1. AQE at Different Wavelengths in LSPR-Based Photocatalytic Reactions

Catalysed by Au−Pd Alloy and Au NPs

Entry Photocatalysts and reactions

AQE at different wavelengths (%)

365

± 5 400 ± 5

470 ± 5

530 ± 5

590 ± 5

620 ± 5

1 Au−Pd alloy @ ZrO2 benzylamine oxidative coupling (−Cl)

0.02 0.02 0.01 0.01 0 0

2 Au−Pd alloy @ ZrO2

benzylamine oxidative coupling 0.02 0.01 0.02 0.03 0.03 0

3 Au−Pd alloy @ ZrO2

benzyl alcohol dehydrogenation 1.12 0.56 0.98 1.00 1.05 0.26

4 Au NP @ CeO2

acetophenone hydrogenation 0.21 0.12 0.09 0.16 0.07 0

5 Au NP @ CeO2

styrene hydrogenation 0.15 0.11 0.05 0.17 0 0

For reactions that have a low photon energy threshold and are catalysed by Au NPs, as

shown in entries 4–5, the reaction rates under short wavelengths (365 and 400 nm) are relatively

high (entry 4) but comparable to that under the LSPR absorption wavelength, 530 nm (entry

5). This series of results is a clear demonstration that a complete understanding of the energy

of targeted molecular orbitals associated with driving rate-limiting steps in the reaction must


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be achieved before photocatalytically enhancing the processes. Christopher et al. theoretically

predicted a similar phenomenon in a small gas molecule–metal NP surface adsorbate system.(38)

These cases demonstrate convincingly that catalytic reactions are not driven by a

photothermal effect of the catalyst as the effect depends on the light absorption of

photocatalysts. If the pair of reactions was driven by the photothermal effect, the action spectra

should have been the same. Similarly, these cases directly show that the shape of action spectra

reflects the photocatalytic light absorption and the energetic position of the molecular orbital

that is accepting the hot electron.

Action Spectra of Nonplasmonic Metal NPs. Visible light irradiation also significantly

enhances the catalytic performance of nonplasmonic metal NPs,(8) and the wavelength

dependences of AQE for reactions catalysed by nonplasmonic metal NPs (Table 2) share a

common feature, which differentiates them from the plasmonic NPs: higher AQEs are always

achieved with shorter wavelengths. Light absorption cross sections of nonplasmonic metal NPs

vary only slightly in the visible light range as the LSPR absorption for these metal NPs is in

the UV region.(39) This implies that when exposed to low-intensity visible light, reactions on

nonplasmonic NPs are driven by neither LSPR light absorption nor the photothermal effect. As

shown in Table 2 (entries 1–4), the AQE values when irradiated with wavelengths of 530 nm

or longer are negligible. Appreciable reactions only take place when the irradiation

wavelengths are shorter than 530 nm. There exists a photon energy threshold for each

photocatalytic reaction, for example, the AQE values for reactions given by entries 2–4 at a

wavelength of 470 nm are two to seven times greater than that observed at 530 nm.

Table 2. AQE at Different Wavelengths in Various Photocatalytic Reactions Catalysed by

Nonplasmonic Metal NPsa

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- 241 -

Entry Photocatalysts and reactions

AQE at different wavelengths (%)

365 ±

5 400 ± 5

470 ± 5

530 ± 5

590 ± 5

620 ± 5

1

Pd@ZrO2

benzyl alcohol

dehydrogenation

0.17 0.08 0.07 0.03 0.02 0

2

Pt@ZrO2

benzyl amine oxidative

coupling

0.39 0.28 0.19 0.04 0.03 0

3

Rh@ZrO2

benzyl amine oxidative

coupling

0.50 0.15 0.14 0.02 0 0

4

Ir@ZrO2

benzyl alcohol

dehydrogenation

0.25 0.18 0.12 0.04 0 0

5 Pd@ZrO2

Heck coupling 0.15 0.09 0.07 0.06 0.04 0.03

a Reproduced with permission of ref 8.

Consider a reaction driven by hot electron transfer, which occurs when the interaction

between the reactant and metal NPs is relatively weak. The electron energy distribution of the

hot electrons of nonplasmonic metal NPs soon after photon absorption is schematically

depicted in Scheme 3 for two different photon energies. Hot electron transfer can be very rapid

and can take place in tens of femtoseconds (fs),(9, 19, 27, 40-43) which is much shorter than the

length of time that the population of electrons remains “hot”. Whether an electron transfer

occurs will depend on whether there are enough electrons with sufficiently high energies to

make the transition possible. As previously discussed, absorption of higher-energy incident

photons will increase the number of electrons that have energies aligned to the molecular

orbitals during the time that the electrons remain hot and thereby increases the probability of

electron transfer.(44, 45) Only hot electrons with energies higher than the LUMO of the reactant

molecule (area between two red dashed lines) are able to induce a reaction and contribute to




- 242 -

the AQE. For equivalent light intensities, a shorter wavelength (400 nm for example, as shown

in Scheme 3a) is able to excite more hot electrons to higher energy levels than the longer

wavelength (530 nm, Scheme 3b). Hence, when the LUMO energy of the reactant molecule is

relatively high, the hot electrons excited by shorter-wavelength photons are more effective at

activating the reactant.

Scheme 3. Hot Electron Distribution of Nonplasmonic Metal NPs under Different

Wavelength Irradiation and Their Contribution to AQEa

a(a) Irradiation with light of 400 nm wavelength: only the hot electrons located above the

LUMO of the reactant molecule (area between two red dashed line) are able to contribute to

AQE. (b) Irradiation with light of 530 nm wavelength: the hot electron distribution area above

LUMO is much smaller than that in (a).

This phenomenon occurs in all metal NPs, plasmonic and nonplasmonic, as all metals

have conduction electrons (including hot electrons) distributed over continuous electron energy

levels. Combined with the information given in Table 2, irradiation with longer wavelengths

does not induce measurable reaction activity, although the irradiation should cause a similar

photothermal effect on the metal NPs with shorter wavelengths. Evidently, the photothermal

effect is not the driving force for these reactions.




- 243 -

It is also important to note that the relative distribution of hot electrons is likely very

different comparing plasmonic (Ag, Au, Cu, and Al) metals and nonplasmonic metals.

Plasmonic metals have filled d bands that sit well below EFermi and most of the time below the

LSPR energy. This suggests that hot electron production from plasmon decay will occur by

intraband transitions within the sp band of the metal, producing a flat probability distribution

in the range EFermi < Ee < EFermi + Φ. On the other hand, nonplasmonic metals are characterized

by unfilled d bands, meaning that the EFermi cuts through the d band. This suggests that

photoexcitation of nonplasmonic metals will mostly produce hot holes and electrons with

energies only slightly above EFermi, where the empty d band density is positioned. This may

explain why long-wavelength excitation produces so little photocatalytic activity because the

hot electrons are not very hot and situated only just above EFermi.

Other Factors Influencing Action Spectra. We also noted that photon energy thresholds

in the reactions are influenced not only by the molecular structure of the reactants but also by

other factors such as temperatures. Figure 3 shows one case where the highest reaction rate was

observed at short wavelengths even if light absorption at the short wavelength was not as

intense as that at the LSPR absorption wavelength of the Au NPs. When the reaction

temperature was raised from 30 to 45 °C, the highest AQE of nitrobenzene (Ph-NO2) reduction

shifted from 365 nm (in the ultraviolet, UV, range) to 530 nm (typical LSPR absorption peak

of the supported Au NPs). The difference in reaction temperature of this reaction system is

sufficient to cause profound changes in the action spectrum. At lower temperature (30 °C), the

AQE does not match to the light absorption spectrum of Au NPs, while it matches at higher

temperatures (≥45 °C). This means that at 30 °C the light energy absorbed by the Au NPs

through LSPR absorption (peaked at 530 nm) is insufficient to surmount the activation barrier

of the reaction. As mentioned, the changes in molecule vibration states resulted from a rise of




- 244 -

reaction temperature and contribute to reduce the photon energy threshold. This inference can

explain spectra in Figure 3a,b. As the temperature rises, the energy threshold is reduced; at

45 °C, the hot electrons generated by LSPR absorption are able to induce this reaction.

Figure 3. Action spectra of nitrobenzene reductive coupling catalysed by Au NPs. (a) Catalysis

by Au NPs at 30 °C exhibits an action spectrum that does not follow plasmonic absorption of

Au NPs. (b) Reactions catalysed by Au NPs at 45 °C show an action spectrum following

plasmonic absorption of Au NPs.

In summary, analysis of the wavelength dependence of AQE indicates that there is a

photon energy threshold for many reactions photocatalysed directly by metal NPs. The

threshold is a feature of the photoinduced electron excitation-driven reactions. The reactions

are predominantly driven through interaction between the hot metal electrons with sufficient

energy and reactant molecules adsorbed on the metal NPs or by exciting the hybridized states

of metal electron states and orbitals of the reactant molecules strongly adsorbed on the metal

NPs. By changing the position of the orbital that must be populated to drive the chemical

reaction, the threshold varies and the action spectra are significantly modified. Thus, one can

see a molecular imprint on the action spectra. The contribution to the observed photocatalysis

from the photoinduced heating is not dominant in most cases when using low-intensity light

sources. The photon–electron excitation mechanisms are important as they can efficiently


- 245 -

channel the photon energy into the chemical bonds to be activated. They also reveal

opportunities to achieve efficient chemical transformations at near-ambient conditions by

tuning the wavelength and intensity of the irradiation as well as the reaction temperature. Such

photocatalysis may lead to discoveries in the selective organic synthesis reactions driven by

irradiation of the solar spectrum.

ASSOCIATED CONTENT

*Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at

DOI: 10.1021/acs.jpclett.7b00941. Experimental methods, TEM images of photocatalysts

(Figure S1), and photograph of the LED lamp setup (Figure S2)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing financial interest.

Acknowledgment

The authors gratefully acknowledge financial support from the Australian Research

Council (ARC DP110104990 and DP150102110). Q.X. acknowledges an Office of the Chief

Executive (OCE) Postdoctoral Fellowship from CSIRO. P.C. acknowledges funding from the

U.S. Army Research Office through Grant No. W911NF-14-1-0347. Electron microscopy

analysis was performed through a user project supported by the Central Analytical Research

Facility (CARF), Queensland University of Technology.

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Photon Energy Threshold in Direct Photocatalysis with Metal Nanoparticles –Key

Evidence from Action Spectrum of the Reaction

Sarina Sarina,a Esa Jaatinen,a Qi Xiao,a,b Yi Ming Huang,a Philip Christopher,c Jin Cai

Zhao,d Huai Yong Zhua,*



b CSIRO Manufacturing, Bayview Avenue, Clayton, VIC 3168, Australia

c Department of Chemical & Environmental Engineering, University of California, Riverside,

Riverside, California 92521, United States

d Key Laboratory of Photochemistry, Institute of Chemistry, the Chinese Academy of Sciences,

Beijing 100190, China.

*Corresponding author: Prof. Huai-Yong Zhu, E-mail: [email protected]



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Contents

Experimental section

Figure S1 TEM images of metal nanoparticle photocatalysts supported on ZrO2

Figure S2 The digital photograph of the LED lamps setup used in the wavelength

experiments


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Experimental Preparation of catalysts

Au NP@ZrO2: ZrO2 powder (1.0 g) was dispersed in a HAuCl4 (13mL, 0.01 M)

aqueous solutions under magnetic stirring at room temperature. An aqueous solution of

lysine (3 mL, 0.1 M) was then added to the mixture with vigorous stirring for 30 min,

and the pH was 8−9. To this suspension was added a freshly prepared aqueous solution of

NaBH4 (2 mL, 0.35 M) dropwise. The mixture was aged for 24 h, and then the solid was

separated by centrifugation, washed with water (three times) and ethanol (once), and

dried at 60°C in a vacuum oven for 24 h.

The same procedure is applied to other monometallic NPs, Ag, Pt, Pd, Rh and Ir onto

ZrO2 powder by reducing the corresponding metal salt with NaBH4 in the presence of ZrO2

powder.

Au-Pd alloy NP@ZrO2: 2.0 g ZrO2 powder was dispersed into 15.2 mL of 0.01 M

HAuCl4 aqueous solution and 28.3 mL of 0.01 M PdCl2 aqueous solution were added

while magnetically stirring. A total of 20 mL of 0.53 M lysine was then added into the

mixture with vigorous stirring for 30 min. To this suspension, 10 mL of 0.35 M NaBH4

solution was added dropwise in 20 min, followed by an addition of 10 mL of 0.3 M

hydrochloric acid. The mixture was aged for 24 h and then the solid was separated, washed

with water and ethanol, and dried at 60°C.

Characterization of Catalysts

The sizes, morphologies, and compositions of the catalyst samples were

characterized by TEM using a JEOL 2100 transmission electron microscope equipped

with a Gatan Orius SC1000 CCD camera and an Oxford X-Max EDS instrument. TEM

images are provided in Figure S1.


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Photocatalytic Reaction conditions

For all reactions: A 20 ml Pyrex glass tube was used as the reaction container with

seal of a rubber septum cap. The reaction mixture was stirred magnetically and irradiated

using Light-emitting diode (LED) lamps (Tongyifang, Shenzhen, China, setting image is

given in Figure S2) with wavelengths of 365±5, 400±5, 470±5, 530±5, 590±5, and 620±5

nm. After reaction 0.5 ml aliquots were collected at given irradiation time intervals and

filtered through a Millipore filter (pore size 0.45 µm) to remove the catalyst particulates.

The flask was purged with argon again for more than 3 min to remove air and then sealed.

The filtrates were analysed by an Agilent 6890 gas chromatograph with HP-5 column.

An Agilent HP5973 mass spectrometer was used to determine and analyse the product

compositions.

Reaction condition of data collection in Figure 1 and 2. Aryl iodide (1 mmol),

alkyl alkyne (1.2 mmol), photocatalysts (50 mg), cetyltrimethylammonium bromide

(CTAB) (1 mmol), and K3PO4 (2 mmol) were added to 10 mL of H2O. The reaction

temperature was 45±2 °C, under a 1 atm argon atmosphere, with a reaction time of 24 h.

Reaction condition of data collection in Table 1. Entry 1 and 2: 1 mmol of reactant,

50 mg (containing 3% of metals) of catalyst in CH3CN solvent at 45°C and 1 atm of O2,

reaction time 48 h. Entry 3: 2 mmol of the reactant, 50 mg (containing 3% of metals) of

catalyst in trifluorotoluene solvent at 45°C and 1 atm of O2, reaction time 5h. Entry 4

and 5: 1 mmol nitrobenzene in 2 ml isopropyl alcohol (IPA), 0.3 mmol KOH, and 50

mg of catalyst were added to the reaction tube, and the reaction was run at a required

temperature under a 1atm argon atmosphere for a reaction time of 18 h.

Reaction condition of data collection in Table 2. Entry 1 and 4: 100 mg catalyst,

the metal content is 3 wt%; 0.5 mmol reactant in 5 ml triflourotoluene solvent; 1atm Ar


- 256 -

atmosphere. Oxygen was removed from the reaction mixture prior to introducing Ar and

the reaction proceeded 48 h for all catalysts under irradiation of various light and in the

dark at 45±2 ºC. Entry 2 and 3: 1 mmol of reactant, 50 mg (containing 3% of metals) of

catalyst in CH3CN solvent at 45 °C and 1 atm of O2, reaction time 24 h. Entry 5 and 6:

catalyst 50 mg, iodobenzene 0.1 mmol, styrene 0.12 mmol, N,N-Dimethylformamide

(DMF) 2 mL, 1 atm Ar, sodium acetate (AcONa) 50 mg, reaction time 17 h, temperature

for (a) 50 ± 2 °C.


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Figure S1 TEM images of metal nanoparticle photocatalysts supported on ZrO2.


- 258 -

Figure S2 The digi ta l photograph of the LED lamps se tup used in the wavelength

experiments.

Conclusions and Future Perspective - 259 -

Chapter 7 Conclusions and Future Perspective

Conclusions

Overall, this thesis focus on the comprehensive study of new metallic photocatalysis

system for organic synthesis. Three types of organic reactions photocatalysed by different types

of metal NP photocatalysts under visible light irradiation are developed. This thesis also

includes an in-depth mechanism study to describe the photo-electron excitation scheme that in

general application to all metallic photocatalysis.

Through the direct utilisation of solar energy, the photocatalysis is capable of reducing

energy consumption in organic synthesis and therefore attracted interests, great efforts were

made to develop new photocatalysts, yet most researchers stopped at this point and omitted

further attempt towards a greener process. This project suggests another direction that one can

promote the application of a relatively traditional photocatalyst, Au nanoparticles for example,

with respect to sustainable chemistry. Chapter 3 presents a novel photocatalytic system for the

hydrogenation of five types of unsaturated aromatics using aqueous solutions of FA as the

reducing agent over a visible light irradiated supported Au nanoparticle photocatalyst at

ambient temperatures and pressures. The visible light induced LSPR effect of Au nanoparticles

provides driving force to the hydrogenation reaction. The photocatalytic system exhibits high

catalytic efficiency and broad substituent tolerance. One can tune the reduction power of this

system by regulating the irradiation wavelength. We have found that water plays an active role

in enhancing the reductive efficiency of FA. Water reacts with FA to yield orthoformic acid

which provides the hydrogen to yield H-Au surface species. These H-Au species react with the

groups to be reduced in the hydrogenation. This finding will improve the understanding of the

selective reductions and inspire the future application of FA as a hydrogen source. Another

important finding is that the hydrogenation of nitroaromatic compounds follows a mechanism


distinct from the well-known Haber mechanism, yielding corresponding anilines directly due

to the strong reduction power of the new photocatalytic system. This photocatalytic process is

green, in terms of reduction agent, and is feasible for a wide range of molecules.

Apart from pure Au nanoparticle photocatalyst, this thesis introduced an alloy

photocatalyst of Au and Pd metal and its applications in the dehydrogenation of aromatic

alcohols yielding corresponding aldehydes at ambient temperature. The wide applications of

LSPR effect induced photocatalysis in organic synthesis are restricted by the number of

plasmonic metals and their limited catalytic potential. This thesis proposes a strategy that

alloying plasmonic metal with other catalytically active metal to establish alloy nanoparticles

for photocatalysis. The alloy nanoparticles, owing to their bi-metallic structure, are found to be

efficient in both visible light absorption and affinity to organic compounds. The driving force

is also the photoexcited electrons, the energy level and the population of those electrons

determines the reaction rate, which is reflected by the influence of incident light wavelength

and irradiation. The optimal photocatalytic activity to aromatic alcohol dehydrogenation is

observed with Au to Pd molar ratio 1:1.186. As a result, we proposed that the photocatalytic

performance is controlled by different surface charge heterogeneity caused by different Au to

Pd molar and its enhancement to the metal-substrate chemisorption. This theory is supported

by the agreement of experimental results and free electron gas model analysis and DFT

simulations. In addition, this project indicates that a great number of organic reaction thermally

catalysed by metals can be photocatalysed by alloying the catalytic active metals with

plasmonic metals, and thus the current chemical industry to extent could be reshaped. The

knowledge that we acquired from this work is beneficial to the design and application of future

plasmonic-transition metal alloy photocatalysts.

In addition to Au based photocatalysts, this thesis also turned to the Cu based

nanoparticle photocatalyst and its application in organic synthesis. Cu is a promising


photocatalyst candidature with both strong LSPR effect and significant catalytic potential, yet

it is the least studied plasmonic metal compared with Au and Ag, its practical applications were

confined due to its low resistance to oxygen and oxidative environment. For the first time, we

fabricated a TiN supported Cu NP photocatalyst, in which TiN support material plays a critical

role in the stabilisation of Cu NPs when exposing in the atmosphere of air. DFT calculation

results revealed a considerable charge exchange loop between TiN support and Cu NP which

is the main reason to Cu nanoparticle stabilisation. The stability of supported Cu nanoparticles

was further tested in cycled reactions and showed good re-usability. Furthermore, the

passivated Cu nanoparticles can be reactivated after a convenient reductive treatment. Highly

selective epoxidation of various alkenes using molecular oxygen under mild reaction condition

was achieved with the new TiN supported Cu photocatalyst with the assistance of cyclic ether

solvent. The reaction system showed good to high reaction conversion depending on the

different alkenes substrate. Light irradiance induced electron excitation is the driving force for

the photocatalytic epoxidation reaction. Cyclic ether solvent participates in the oxygen

activation step, which transfers molecular oxygen to activated oxygen adatom on the surface

of Cu nanoparticles when illuminated with visible light. The oxygen adatoms can effectively

oxidise alkenes, which chemosorbed onto the Cu nanoparticles surface, and the different

chemisorption pattern of alkenes with Cu is the determining factor to epoxidation selectivity.

This work has the potential to extend the field of plasmonic catalysis into the previously

unachievable use of readily oxidised metals. Meanwhile, the concept on the method to inhibit

the oxidation of Cu nanoparticles with TiN may be applicable to many other non-precious

metal nanoparticles not limited to plasmonic metals such as Fe, Co, Ni and etc.

Finally, the energy transfer pattern and charge carrier flow route in plasmonic effect

induced photocatalysis have yet been clearly understood. Currently reported theories were

normally based on theoretical calculation and supported by molecular level experiments. In


this thesis, we managed to reveal the in-depth mechanism by comprehensive investigation of

action spectra of multiple photocatalytic systems. By comparing the action spectra trend in

different cases. It is proposed for the first time that there is a threshold that exists in the metallic

photocatalysis. Photoexcited electrons with energy level higher than LUMO of chemosorbed

organic molecular can trigger a photocatalytic reaction. Overall, the predominant role of

photoexcited electrons in metallic photocatalysis is further demonstrated. The finding extends

current understanding of energy transfer and charge carrier flow route


Future Perspective

Metallic photocatalysis is a rapidly growing research field. Despite the considerable

progress to date, continuous efforts are still being made by researchers to create a various

efficient photocatalytic system and reveal their physical and chemical mechanism. Based on

the results of this thesis as well as other’s work, the perspective to the future work of metallic

photocatalysis can be categorised into several potential directions as followed:

I. Development of new materials for photocatalysis. The first expected research focus

lies in the development of well-designed photocatalysts using novel materials. The

candidates for new metallic photocatalyst can be expanded from plasmonic metals to a

wide range of transition and non-transition metals. New forms of nanostructures

consisting of more than one metal component (plasmonic, transition or non-transition)

are promising research directions, the alloy combinations are vast and alloys can exhibit

various forms of nanostructure that facilitate the design of promising photocatalysts. In

addition, a combination of metal with other forms of catalysts such as functional

organic reagents is another field worth entering. In addition, the metallic photocatalyst

can be promoted by the modification of other catalytic components such as functional

organic reagents. Lastly, a new research direction could be developing non-metal

materials for LSPR induced photocatalysis. A series of ceramic materials and carbon

materials were found exhibiting LSPR in the visible and near-infrared region.

Introducing them into photocatalysis could extend the applications of metallic

photocatalysis.

II. Photocatalytic applications in organic synthesis and greener chemical processes.

The family of metallic photocatalysed organic reactions is still young, therefore the

application of metallic photocatalysts in more important organic synthesis reactions is

another major part of photocatalysis research, and in most cases, it is associated with


the development of new material photocatalysts. More importantly, tuning the

photocatalytic selectivity by manipulating the photocatalyst construction or

wavelength/irradiance of incident light is an intriguing and very attractive direction.

The metallic photocatalysis represents research efforts towards a greener organic

synthetic process.

III. Development of a comprehensive theoretical model for metal photocatalysis. The

metallic photocatalysis has been realised for less than one decade, thus the proposed

photocatalytic mechanism is still under broad debate and there are also unrevealed light

induced physical and chemical processes in quantum scale. More theoretical efforts, as

well as experimental illustration, are desired to further clarify the picture of

photoexcitation and resultant energy transfer and evolution in the photocatalysts-

substrate system. Research in this area is crucial for the future design of efficient

metallic photocatalyst and their applications in organic synthesis.

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