Thesis Luis Polanco

Copyright

by

Luis Ruben Polanco

2017

The Thesis Committee for Luis Ruben Polanco Certifies that this is the approved version of the following thesis:

Metal Oxide Support Effects on the Hydrogenation of Cyclohexene and Crotonaldehyde using Microwave Synthesized Rhodium and Iridium

Nanoparticles

APPROVED BY SUPERVISING COMMITTEE:

Simon M. Humphrey

Richard A. Jones

Supervisor:


Nanoparticles

by

Luis Ruben Polanco

Thesis

Presented to the Faculty of the Graduate School of

The University of Texas at Austin

in Partial Fulfillment

of the Requirements

for the Degree of

Master of Arts

The University of Texas at Austin August 2017

iv

Abstract


Nanoparticles

Luis Ruben Polanco, M.A.

The University of Texas at Austin, 2017

Supervisor: Simon M. Humphrey

Nanoparticles (NPs) are an exciting new class of materials with unique physical

and chemical properties, which have been studied for applications in semiconductors, drug

delivery, heavy metal sequestration, and heterogeneous catalysis. The last decade has seen

an exponential growth in noble metal NP catalysis research. The scarcity and price of these

metals has created a need for more highly efficient catalysts and the surface-area-to-volume

ratio of NPs can alleviate that demand. Highly selective catalysis is still dominated by

homogeneous catalysts, but their lack of recyclability makes them unusable for industrial

settings where durability is the top priority. The Humphrey group has pioneered the

synthesis of monometallic, core-shell, and alloyed noble metal NPs of different sizes and

morphologies, facilitated by microwave heating. However, support media effects have not

been studied in the group, as strong metal-support interactions (SMSI) and hydrogen

spillover have been shown to alter the observed catalytic activities.

v

Herein, mono metallic Rh NPs (~5nm) have been immobilized on amorphous metal

oxides (SiO2, Al2O3, TiO2, Nb2O5, and Ta2O5) to study the effects these supports play in the

hydrogenation of alkenes and the chemoselectivity hydrogenation of α,β-unsaturated

aldehydes. Cyclohexene was utilized as a model alkene to assess the reactivities of said

catalysts.

In addition, controlled growth of Ir NPs in aqueous media is in development.

Computational and experimental data has shown higher selectivity towards unsaturated

alcohol products from the hydrogenation of α,β-unsaturated aldehydes and ketones.

Unfortunately, not much research has been done Ir NPs due to their small particles sizes.

Viscous solvents are typically used in NP synthesis to avoid particle agglomeration, but Ir

since NPs don’t typically grow past 2 nm, other solvents can be used during synthesis. This

allows the use of less viscous, greener solvents, such as water. Herein, the synthesis of Ir

NPs in water is explored and the largest free-standing Ir NPs (2.98 nm) are presented. Also,

2.71 nm Ir NPs can be achieved after 1 minute, making them desirable for large sacel

synthesis of these materials.

vi

Table of Contents

List of Tables ....................................................................................................... viii

List of Figures ........................................................................................................ ix

Chapter 1: Introduction ...........................................................................................11.1 Nanoparticle Formation ............................................................................11.2 Noble Metal Nanoparticle Catalysts .........................................................2

1.2.1 Examples of Nanoparticle Catalysis .............................................31.2.2 Hydrogenation on Nanoparticle Surfaces .....................................41.2.3 Selective Heterogeneous Hydrogenation ......................................5

1.3 Microwave Synthesis of Noble Metal Nanoparticles ...............................6References .....................................................................................................10

Chapter 2: Metal Oxide Support Effects on the Hydrogenation of Crotonaldehyde using Microwave Synthesized Rhodium Nanoparticles ...............................142.1 Introduction .............................................................................................142.2 Synthesis of Amorphous Metal Oxide Supported Rhodium Nanoparticles

..............................................................................................................162.3 Results and Discussion ...........................................................................172.3 Conclusion ..............................................................................................222.4 Experimental Section ..............................................................................23

2.4.1 Materials .....................................................................................232.4.2 Methods.......................................................................................232.4.3 Characterization ..........................................................................242.4.4 Synthesis of Rh NPs.32 ................................................................242.4.5 Synthesis of polystyrene beads28 .................................................252.4.6 Synthesis of macroporous/mesoporous Al2O3 and SiO2

28 ...........262.4.7 Synthesis of mesoporous Ta2O5 and TiO2 ...................................262.4.8 Synthesis of amorphous Al2O3, Nb2O5, Ta2O5, and TiO2 ............26

References .....................................................................................................27

vii

Chapter 3: Synthesis of Microwave Irradiated Iridium Nanoparticles in Aqueous Media ............................................................................................................293.1 Introduction .............................................................................................293.2 Microwave Synthesis of Iridium Nanoparticles .....................................303.3 Conclusion ..............................................................................................343.4 Experimental Section ..............................................................................35

3.4.1 Materials .....................................................................................353.4.2 Methods.......................................................................................353.4.3 Characterization ..........................................................................353.4.4 Synthesis of Ir NPs7 ....................................................................36

References .....................................................................................................38

Bibliography ..........................................................................................................39Chapter 1 .......................................................................................................39Chapter 2 .......................................................................................................42Chapter 3 .......................................................................................................44

viii

List of Tables

Table 2.1: Activation energies and Rh loading percentages for metal oxide supported

Rh catalysts. ......................................................................................17

Table 2.2: Comparative table between the initial and steady state TOFs of each Rh

catalyst. .............................................................................................20

ix

List of Figures

Figure 1.1: Gibb free energy changes in NPs formation. ........................................2

Figure 1.2: Mechanism for the homogeneous Heck coupling reaction.31 ...............4

Figure 1.3: Ethylene hydrogenation mechanism on clean Pt surface. ....................5

Figure 1.4: Comparative catalysis between pure Au NPs (yellow), pure Rh NPs (red),

thin-Rh shell NPs (blue) and thick-Rh shell NPs (green) for cyclohexene

hydrogenation. ....................................................................................8

Figure 1.5: Cyclohexene hydrogenation of (a) RhAg and (b) RhAu) alloy NPs of

different compositions. .......................................................................9

Figure 2.1: PXRD pattern (left) and TEM image (right) of RhNPs. Inset on right

shows the size distribution and average particle size with standard

deviation. ...........................................................................................16

Figure 2.2: Comparative TOF data for the hydrogenation of cyclohexene to

cyclohexane: yellow = Rh/Ta2O5; purple = Rh/Nb2O5; red = Rh/SiO2;

green = Rh/Al2O3. .............................................................................19

Figure 2.3: TEM images of Rh NP catalysts before (blue) and after (red) cyclohexene

hydrogenation. ..................................................................................20

Figure 2.4: Comparative catalysis data for the hydrogenation of crotonaldehyde to

butyraldehyde: blue = Rh/Ta2O5; green = Rh/TiO2; yellow = Rh/Al2O3;

black = Rh/SiO2; red = Rh/Nb2O5. ....................................................22

Figure 3.1: PXRD pattern (left) and TEM image (right) of Ir NPs synthesized

following previously reported Rh NP procedure. Inset on right shows the

size distribution and average particle size with standard deviation. .30

x

Figure 3.2: PXRD patterns (left) of Ir NPs synthesized following previously reported

Rh NP procedure (black), addition of NaBH4 (purple), using 100 mg of

PVP (pink), 120 °C reaction temperature (blue), water as solvent at 100

°C (orange) and 80 °C (green). TEM image of Ir NPs using water as the

solvent at 80 °C (right). .....................................................................31

Figure 3.3: PXRD patterns of the attempted nucleation reactions for Ir NPs. ......32

Figure 3.4: TEM images of Ir NP nucleation at different reaction times. ............33

Figure 3.5: TEM images of Ir NPs after initial nucleation and second injection (20

ml/hr) at different reaction times. .....................................................34

1

Chapter 1: Introduction

1.1 NANOPARTICLE FORMATION

Nanoparticles (NPs) are an exciting class of materials that has gathered an

increased amount of interest in the last 20 years. This interest is primarily due to their

increased surface-area-to-volume ratios and quantum confinement effects.1,2 Perhaps the

most well-known example of particle size effects is the modulation of the plasmon

resonance in Au NPs.3–5 The effect was first observed by Michael Faraday in 1857, when

an aqueous solution of chloroaurate was reduced with phosphorus to produce a red

solution.5 The advent of electron microscopy allowed more in-depth studies of NPs and the

effects of size and shape on their physical properties. NPs are typically synthesized by one

of two general methods: 1) Top-down approaches, in which bulk scale materials are

mechanically milled or ground down to the desired nanoscale6,7, or 2) bottom-up methods

based on the reduction of molecular metal precursors in solution.2 Top-down methods are

advantageous due to the relative ease of use and minimal synthetic changes in the

production of materials. The downsides to this procedure are the poor size distribution and

lack of control in particle shape.6,7

The interest in NP shape and size control has increased efforts towards solution-

based reduction bottom-up processes, so a basic understanding in the nucleation

mechanism is important.8 Although mechanism for NP formation is still debated, the most

commonly accepted, and perhaps simplest, theory is based on homogeneous atom

clustering for electronic stabilization. Upon reduction, dissolved metal atoms will begin to

cluster to lower their free energies. At smaller particle radii, the newly created surface is

too high in energy and the particle will re-dissolve. If a particle forms with a large enough

size to a point where the cluster interactions can compensate for the high surface

2

Figure 1.1: Gibb free energy changes in NPs formation.

energy, the particle will remain and can then begin a growth phase by adding more atoms

to its surface (Fig. 1.1).

These relatively stable NP surfaces are still highly energetic and must be passivated

with capping agents or immobilized on support media. The simplest of these methods is

the reduction of a metal precursor of interest in the presence of a solid support. These

supports are typically metal oxides9–11 or carbon based materials, such as graphite12,13 or

carbon nanotubes14,15. Synthesis of free particles can be achieved with the addition of a

capping agent to avoid particle sintering. Common types of capping agents include small

anions, bulky organic molecules, ionic liquids (which also serve as solvents) and polymers.

These capping agents not only help prevent particle sintering, but also play a significant

role in shaping NPs.8 Although any solvent capable of dissolving the metal precursors can

be used in NP synthesis, polyols have often been employed due to their high viscosity

(slowing particle sintering), ability to dissolve a wide array of metal precursors, and

capacity to act as a reducing agent.

1.2 NOBLE METAL NANOPARTICLE CATALYSTS

Noble metals can be used as homogeneous or heterogeneous catalysts for a wide

range of reactions. Industrial-scale catalysis typically uses heterogeneous catalysts due to

3

their high reactivities and ease of recyclability, which originates from their enhanced

stability at high temperatures and pressures. Unfortunately, noble metals tend to be

expensive and low in abundance. For example, three-way catalysts (TWCs) were

implemented in the United States in the 1970’s to lower the toxic emissions of vehicles

(e.g., CO, NOx and unburned hydrocarbons) and minimize the negative impact of

automobiles on the environment.16 The steep price of these TWCs, however, makes them

inaccessible to developing countries and makes these catalysts less helpful to the global

environment. Since heterogeneous catalysis is surface mediated, increasing the proportion

of the noble metal available on the surface leads to a catalyst that can achieve the same

reactivity with less metal and therefore at lower costs. This is where the high surface-area-

to-volume ratios of NPs can be exploited.

1.2.1 Examples of Nanoparticle Catalysis

One of the first examples catalytic activity in NPs was the hydrogenation of

nitrobenzene using Pd and Pt NPs.17 These particles were reduced with H2 in aqueous

solutions of polyvinyl alcohol (PVA) and used directly as catalysts by adding the substrate

to the mixture. Although no recyclability studies were performed, it is highly unlikely these

particles would have retained their activities without immobilization to prevent

aggregation. A wide range of reactions have been studied using NPs as catalysts since then.

Hydrogenation17–23, oxidation10,13,24,25, C-C coupling26,27 and isomerization28,29 reactions are

just some examples of the types of reactions examined by some sort of NP catalyst. One of

the important breakthroughs in NP catalysis came in 1987 in the form of Au NPs supported

on oxides of Fe, Co and Ni.24 These Au NPs were used in the oxidation of CO in the

presence of O2 at low temperatures. This reaction is important because it opened the door

4

for NPs to be used in the removal of CO from petroleum based products, as this is a typical

side-product in these reactions.

Another instrumental study was the formation of C-C bonds using Pd NPs.30 In this

case, styrene was coupled to several halogenated benzene compounds to form a series of

stilbene derivatives through a Heck style cross-coupling (Fig. 1.2). This was one of the first

examples of fine chemical production in NPs, and showed that NP catalysts could

potentially be used in more than simple molecule activations.

Figure 1.2: Mechanism for the homogeneous Heck coupling reaction.31

1.2.2 Hydrogenation on Nanoparticle Surfaces

The seminal work on nitrobenzene hydrogenation17 paved the way for in depth

exploration of NP hydrogenation catalysts. Mechanistic studies of hydrogenation on a

Pt(111) surface using ethylene as a model substrate determined that the ethylene molecule

undergoes the following steps at room temperature: i) the ethylene molecule physisorbs

5

with the C=C bond parallel to the surface, ii) the ethylene chemisorbs to the surface by

breaking its p-bond to form two sigma bonds with a three-fold surface site, iii) sequential

hydrogenation through adsorbed hydrogen atoms and release of the newly formed ethane

molecule (Fig. 1.3).18 Although real heterogeneous hydrogenation is more complicated,

this mechanism has provided a handle for the process that can be applied to more complex

molecules. NPs have now been introduced as potential catalysts for numerous substrates,

such as alkenes18,32–34, alkynes35–38 and aromatic19,20,39–41 compounds. Recent studies on

hydrogenation are focused on increasing catalyst efficiencies and selectivities.

Figure 1.3: Ethylene hydrogenation mechanism on clean Pt surface.

1.2.3 Selective Heterogeneous Hydrogenation

Heterogeneous catalysts lack the defined active sites that their homogeneous

counterparts have, making modification and fine-tuning less intuitive. This results in the

formation of unwanted chemicals that must be separated, increasing costs and lowering the

total yield of the desired product. This leaves fine chemical-producing industries, like

pharmaceutical and cosmetics companies, with few options other than to rely on the

increased selectivity and tunability of homogeneous catalysts. Unfortunately, multi-step

syntheses of directing ligands, as well as the necessity to separate the catalyst from

products, still make the utilization of homogeneous catalysts an expensive endeavor. For

this reason, selective heterogeneous catalysts are a major goal in the field. Great progress

6

has been made in regio-, chemo-, stereo- and enantioselective heterogeneous

hydrogenation reactions. The Somorjai group published studies on the hydrogenation of

benzene where cubic Pt NPs only yielded cyclohexane, while cuboctahedral Pt NPs

produced both cyclohexene and cyclohexane. 19,20 Formation of cyclohexane, unlike the

formation of cyclohexene, is not sensitive to NP structure and is formed in both NP types.

Production of fine chemicals usually requires more involved methods. Catalyst

modification can be achieved by the addition of surfactants that promote preferential

binding by the desired part of the molecule or by using different support media to finely

tune the electronics of the NPs. The undefined active site in NPs makes the former approach

more common in stereo- and enantioselective catalysis by attaching chiral surfactants to

the catalyst surface.42 NP chemoselective hydrogenation typically revolves around two

types of compounds, nitroaromatics and a,b-unsaturated aldehydes and ketones.

Unsaturated alcohols are the desired product in the hydrogenation of a,b-unsaturated

aldehyde and ketones, but most catalysts preferentially hydrogenate the alkene moiety to

form saturated aldehydes or ketones. Most of the work on carbonyl hydrogenation has been

performed on cinnamaldeyde, likely due to the bulky aromatic group attached to the C=C

bond that can hinder alkene hydrogenation.

1.3 MICROWAVE SYNTHESIS OF NOBLE METAL NANOPARTICLES

Research in solution-based reduction of metal precursors for the formation of

NPs has explored many different conditions to obtain better control the reaction kinetics of

NP syntheses. The Humphrey group has pioneered the synthesis of monometallic (e.g., Rh,

Pt, Pd)32, core-shell (e.g., Au@Rh)21 and alloyed (e.g., RhAu, RhAg, PdAu, RhPd)22 noble

metal NPs of different sizes and shapes, facilitated by microwave heating (MwH). At first

glance, this minor change might not seem like a worthy endeavor, but the MwH method

7

works very differently from conventional heating (CvH). CvH relies on thermal conduction

between the hot-plate and the reaction flask, usually through some sort of heating bath, to

achieve the desired temperature. On the other hand, MwH works by rotationally exciting

molecules in solution through the oscillating electric field implemented by the microwaves.

The larger the dipole moment in the molecule, the bigger the excitation by the microwaves.

The energy released, through friction and breaking of intermolecular interactions that

occur, as the molecules rotate to align with the fast oscillating electric field produces heat.

The concentrated energy, known as “hot spots”, then disperse to heat the bulk solution. The

desired temperature can be tuned by changing the power and/or frequency of the

microwaves. Studies have shown that these “hot spots” can be much hotter than the

temperature of the bulk.43–45

Coupled with MwH, controlled precursor addition rate (using programmable

syringe pumps) during NP synthesis also leads to finer structural control. Although

syntheses for monometallic noble metal NPs using CvH exist, MwH yields more highly

crystalline NPs with a narrower size distribution.32 Microwave-heated NPs also

incorporated less capping agent, allowing higher reactivity without the need to pretreat the

catalyst through calcination. Such procedures can cause unwanted NP sintering and

carbonation of catalytically active surfaces. The fine control of this method also allowed

the synthesis of core-shell NPs of different shell thickness.21 From an economic standpoint,

core-shell NPs can increase atom efficiency when they are composed of an inexpensive

sacrificial core and a thin shell of the active metal. In this case, Au acted as the sacrificial

core and a thin shell (2-4 monolayers) of Rh was added onto the Au core NPs. The lattice

mismatch present at the interface of the two metals also seems to impart unusual reactivity

and likely becomes less pronounced as the shell becomes thicker (Fig. 1.4).

8

Interestingly, MwH also permitted the synthesis of classically immiscible alloys

(RhAu and RhAg NPs).22 In the bulk, these metals are only miscible when melted, and

begin to segregate upon cooling. Microwave irradiated RhxAu1-x and RhxAg1-x alloy NPs of

different compositions were then tested as cyclohexene hydrogenation catalysts, where

they performed with higher efficiency than monometallic Rh NPs despite Au and Ag being

inert toward hydrogenation under the tested conditions (Fig. 1.5). The ability to form these

alloys proves that the different heating method involved in MwH can lead to particles with

vastly different characteristics.

Figure 1.4: Comparative catalysis between pure Au NPs (yellow), pure Rh NPs (red), thin-Rh shell NPs (blue) and thick-Rh shell NPs (green) for cyclohexene hydrogenation.

9

Figure 1.5: Cyclohexene hydrogenation of (a) RhAg and (b) RhAu) alloy NPs of different compositions.

10

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(45) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Kubokawa, M.; Tsuji, T. Microwave-Assisted Synthesis of Metallic Nanostructures in Solution. Chem. - Eur. J. 2005, 11, 440–452.

14

Chapter 2: Metal Oxide Support Effects on the Hydrogenation of Crotonaldehyde using Microwave Synthesized Rhodium Nanoparticles

2.1 INTRODUCTION

Selectivity has been an issue for heterogeneous catalysts because modulation of

these materials is less intuitive on the nanoscale.34,46 The disadvantage of surfactant

incorporation to catalyst surfaces is the inherent loss of activity caused by the blockage of

a percentage of the materials’ active sites. High efficiencies are especially important in

hydrogenation catalysis because the active species is almost always scarce noble metals.

The ideal way to improve catalytic activity in these materials is by altering the electronics

of the particle. Metal oxides of different compositions have different oxygen vacancies,

oxidation states, and d-orbital energies that result in electronic changes in the NPs

supported on them, known as strong metal-support interactions (SMSI).47,48 Along with

SMSI, varying interactions with the substrate and the ability of these oxides to store

hydrides on their surfaces (hydrogen spillover) have been shown to alter the observed

catalytic activities and selectivities of noble metal NPs.29,49–51 Reducible supports have been

shown to have greater SMSI and hydrogen spill over, although defects on the surface of

supports with non-stoichiometric amounts of oxygen can allow them to also exhibit these

effects. The presence of hydrides at surfaces of the support could allow catalysis to occur

at the oxide surface. The oxophilicity of these materials, coming from the high oxidation

state of the metal centers, could allow preferential hydrogenations by orienting the polar

carbonyl bond of α,β-unsaturated aldehydes and ketones towards the surfaces.

As opposed to adsorbed surfactants, alteration of NP electronics through SMSI and

hydrogen spillover can lead to higher or lower overall catalytic activities. Computational

studies on the RhAg and RhAu alloy NPs, formed by the Humphrey group, showed a

synergistic effect between the two metals.22 Rh is the only active metal in each NP; Ag and

15

Au only serve to dilute the Rh, especially at the core. The presence of these non-active

metals though, attenuates the hydrogen binding energy of the Rh sites to reach a ‘happy

medium’. If the binding energy is too high, hydrogen is easily bound, but is sluggishly

released and turnover is slow. On the other hand, low binding energies means hydrogen

cannot bind and catalysis does not occur. Although supports may not have an effect as

extreme as in the alloy case, supports with different properties can play an important role

in catalysis. SiO2, Al2O3, Ta2O5, Nb2O5, and TiO2 were chosen based on their physical

properties and previous reports on their about their impacts on catalytic activity.28 SiO2 is

the most common support used, including the studies done by the Humphrey group, and

will serve as our control. SiO2 has stoichiometric amounts of oxygen and is a non-reducible

support while Al2O3 is a non-reducible support with non-stoichiometric amounts of

oxygen. Ta2O5, Nb2O5, and TiO2 are all reducible, with Ta2O5 and Nb2O5 being non-

stoichiometric and TiO2 their stoichiometric counterpart.

To test the supports’ effect on hydrogenation activity, Rh NPs were supported on

these materials and tested in the hydrogenation of cyclohexene and crotonaldehyde.

Cyclohexene is used as a model substrate to compare activities of each catalysts without

introducing the complexities of molecules with more than one functional group. Deciding

on a substrate for chemoselective hydrogenation is not a simple matter. Many studies have

been done using cinnamaldehyde, likely due to the steric hindrance the phenyl ring imposes

on the internal alkene. Steric hindrance can play a big role in the orientation of the molecule

when it makes contact to the surface. The phenyl group in cinnamaldehyde likely imparts,

at least in some part, the selectivity observed in some reports. Our catalytic setup is based

on vapor flow of the desired substrate via a carrier gas (in our case He/H2 mixture).

cinnamaldehyde has a very low vapor pressure even at elevated temperatures, so we must

resort to a smaller, more volatile aldehyde. For these reasons, crotonaldehyde was

16

introduced as the model α,β-unsaturated aldehyde to test the support effects on

chemoselective hydrogenation. The alkene moiety in crotonaldehyde is less sterically

hindered than the one present in cinnamaldehyde, which can make the monohydrogenated

crotyl alcohol more difficult, but will allow us to better ascertain the selective properties

of the supports.

2.2 SYNTHESIS OF AMORPHOUS METAL OXIDE SUPPORTED RHODIUM NANOPARTICLES

RhNPs (5.7 ± 1.2 nm) were synthesized via microwave assisted heating and

controlled precursor addition, following the previously reported procedure for RhNP

seeding (see experimental section 2.4.4).32 Powder X-ray diffraction (PXRD) confirmed

the presence of crystalline FCC Rh metal (Fig. 2.1 left) and the size of these particles was

confirmed through transmission electron microscopy (TEM, Fig. 2.1 right). These particles

were then loaded onto amorphous SiO2, Al2O3, Nb2O5, and Ta2O5 (synthesis in experimental

section) via incipient wetness impregnation.

Figure 2.1: PXRD pattern (left) and TEM image (right) of RhNPs. Inset on right shows the size distribution and average particle size with standard deviation.

17

2.3 RESULTS AND DISCUSSION

The Rh concentration of each catalyst was determined by inductively coupled

plasma optical emission spectroscopy (ICP-OES) and loadings summarized in Table 2.1.

Catalyst Activation Energy (kJ/mol) Rh loading (%, ICP-OES)

Rh/SiO2 25.29 1.07

Rh/Nb2O5 49.57 1.71

Rh/Ta2O5 38.56 3.20

Rh/Al2O3 54.37 0.44

Table 2.1: Activation energies and Rh loading percentages for metal oxide supported Rh catalysts.

Differences in loading percentages stem from the varying interactions between PVP and

the different metal oxides. Partial incorporation is most evident in the impregnation of

Al2O3 as the solution remained significantly dark after stirring overnight, indicating the

presence of NPs still in solution.

To assess the catalytic effects the different metal oxides have on the overall

hydrogenation activity of Rh NPs, milligram quantities of the Rh/support catalyst materials

were then studied in the vapor-phase hydrogenation of cyclohexene with H2 gas, using a

single-pass flow reactor. Analysis of flow stream was performed by gas chromatography

18

(GC) sampling of the exit mixture at 3.4-minute intervals. All reactions were studied at 25

°C without any pretreatment to remove the PVP capping agent (e.g., high-temperature

calcination). Previous studies of cyclohexene hydrogenation using Rh NPs in the

Humphrey group showed high activity without the need of high temperature

pretreatment.21,32,52–55 Although the presence of PVP could limit the interactions between

NP and support, calcination can lead to sintering and carbonation of particle surfaces and

loss of activity. Upon exposure to the reactant stream, each catalyst underwent a short

initial induction period, which was followed by a sharp increase in turnover frequency

(TOF) for the hydrogenation of cyclohexene to cyclohexane. Each catalyst was tested in

triplicate and the kinetic data was averaged to give the data shown in Fig. 2.2. The catalytic

data shown in Fig. 2.2 shows that even without catalyst pretreatment, the support changed

the activity of the Rh NPs. The Rh/Al2O3 and Rh/SiO2 catalyst had the highest activity at

steady state, followed by Rh/ Nb2O5 and Rh/ Ta2O5. Interestingly, despite having very

similar steady state activities, the Rh/Al2O3 catalyst had a significantly higher initial

activity compared to Rh/SiO2. In fact, all catalysts show a higher percent change between

their initial and steady state TOFs (Table 2.2). TEM images, before and after catalysis,

show no major difference in the sizes of Rh NPs attached to the different supports (Fig.

2.3), which likely means the changes in reactivities stems from the interactions between

the NPs and the support.

19

Figure 2.2: Comparative TOF data for the hydrogenation of cyclohexene to cyclohexane: yellow = Rh/Ta2O5; purple = Rh/Nb2O5; red = Rh/SiO2; green = Rh/Al2O3.

Catalyst Initial TOF Steady State TOF TOF Change (%)

Rh/SiO2 20.89 10.29 50.7

Rh/Nb2O5 10.98 3.89 64.6

Rh/Ta2O5 2.37 0.63 73.4

Rh/Al2O3 28.85 10.47 63.7

Table 2.2: continued next page.

20

Table 2.2: Comparative table between the initial and steady state TOFs of each Rh catalyst.

We initially hypothesized that the higher activities observed in the NPs supported

on SiO2 and Al2O3 were due to lower activation energies for the reaction. To test this,

temperature dependent catalytic studies were performed (Table 2.1). Significant

differences can be observed in the activation energies although no real correlation can be

made to the TOFs or the change in TOF over time.

Figure 2.3: TEM images of Rh NP catalysts before (blue) and after (red) cyclohexene hydrogenation.

We hypothesized that the difference in activation energies could help in selectively

hydrogenating the carbonyl double bond of the aldehyde moiety present in crotonaldehyde

over the carbon-carbon double bond. The flow reactor was then adapted for the

vaporization of crotonaldehyde. Unfortunately, no direct comparison could be drawn

between the cyclohexene and crotonaldehyde systems because no activity was observed

without pretreatment of the catalysts. The polar nature of crotonaldehyde could cause it to

21

interact more strongly with the PVP, preventing much of the substrate from reaching the

surface of the NPs. The catalysts were heated at 300 °C under a H2/He mixture for 3 hours

prior to the introduction of crotonaldehyde. The reaction temperature also had to be

adjusted from 25 °C to 100 °C, due to the higher boiling point of crotonaldehyde (104 °C

vs. 83 °C) and its lower vapor pressure (19 mmHg vs. 67 mmHg at 20 °C). Running the

reaction at 25 °C would most likely condense crotonaldehyde in the catalyst bed and alter

reaction kinetics.

The Rh/Nb2O5 catalyst was used to tailor the reaction parameters. At first, the

amount of catalyst used was similar to the amounts used in the cyclohexene reactions (~5

mg), but the activities were very low (<1% crotonaldehyde conversion). Increasing the

amount of catalyst did not improve the conversions significantly, which alluded to a

problem in the setup of the reactor or the activation of the particles. The low activities could

also be attributed to Rh/Nb2O5 having low activity in general for this system. The remaining

catalysts were then tested for this reaction, but none showed preferential hydrogenation of

carbonyl bonds over carbon-carbon double bonds. The activities for all catalysts were still

very low and the most prominent product (1.6-15%) was the singly hydrogenated carbon-

carbon bond, butyraldehyde (Fig. 2.4). It is likely that the high temperature pretreatment

inhibited the catalytic activity given that all systems were able to hydrogenate cyclohexene

in good yields without pretreatment. This reduction in activity is likely caused by

carbonation of the surfaces, or changes in NP structure, is facilitated by heat and an

alternate pretreatment should be considered moving forward.

22

Figure 2.4: Comparative catalysis data for the hydrogenation of crotonaldehyde to butyraldehyde: blue = Rh/Ta2O5; green = Rh/TiO2; yellow = Rh/Al2O3; black = Rh/SiO2; red = Rh/Nb2O5.

2.3 CONCLUSION

In conclusion, 5.7 nm Rh NPs were synthesized and supported on a series of metal

oxides to be used as hydrogenation catalysts and activation energies determined via

temperature dependent studies. Rh/Al2O3 catalyst showed the highest initial activity for the

conversion of cyclohexene to cyclohexane, although its steady state activity was very close

to that of Rh/SiO2.

Crotonaldehyde was then used as test reaction for chemoselective hydrogenation.

Unfortunately, no catalyst showed significant hydrogenation activity and no selectivity

23

towards the unsaturated alcohol product. This low conversion and selectivity could arise

from the high temperature catalyst pretreatment. Future work will involve activation of

catalysts through different methods to ensure higher performance as well as the use of

porous supports to improve support effects on the NPs.

2.4 EXPERIMENTAL SECTION

2.4.1 Materials

Rhodium(III) chloride hydrate (RhCl3�xH2O, 98%; Johnson Matthey), styrene

(C8H8,), divinylbenzene (C10H10,), aluminum isopropoxide (Al(OiPr)3, >98%, Sigma-

Aldrich), tetraethyl orthosilicate (Si(OCH2CH3)4,), titanium isopropoxide (Ti(OiPr)4,),

tantalum(V) chloride (TaCl5,), niobium(V) chloride (NbCl5,), poly(vinylpyrrolidone)

(PVP, Mw = 55 000; Sigma Aldrich), ethylene glycol ((CH2OH)2, 99.8%; Fisher

Scientific), and anhydrous cyclohexene (Alfa Aesar, g99%) were used as received. All

gases (Praxair) used in catalytic studies were 99.9995+% purity. All other reagents and

solvents (analytical grade) were employed without further purification unless stated

otherwise. TEM grids (200 mesh Cu/Formvar; Ted Pella, Inc.) were prepared by drop-

casting ethanol suspensions of materials that were evaporated to dryness in air.

2.4.2 Methods

Syntheses of metal oxides were performed under a N2 atmosphere using standard

Schlenk line techniques unless otherwise stated. A MARS 5 (CEM Corp.) microwave

system with a maximum power of 1600 W (2.45 GHz) was used to perform all microwave-

based reactions. Throughout the course of the experiment, the reaction temperature was

finely controlled by power modulation via a RTP-300þ fiber-optic temperature sensor,

located in a beaker of solvent identical to that employed in the reaction. A 50 mL round-

bottomed flask fitted with water-cooled reflux condenser was placed in the center of the

24

microwave cavity. The solvent and reactants were magnetically stirred, and Rh precursor

was then added directly into the stirred solvent through a disposable, fine-bore Teflon tube.

The rate of Rh precursor addition was controlled using an Aladdin programmable syringe

pump (WPI, Inc.) that was directly attached to the Teflon tubing. Prior to each new

reaction, glassware was thoroughly cleaned in base bath (3 M NaOH in i-PrOH/H2O,

overnight), followed by oven drying at 110 °C.

2.4.3 Characterization

Transmission electron microscopy (TEM) images were obtained from a FEI

Tecnai microscope operating at 80 kV. The samples were prepared by drop-casting a single

aliquot of nanoparticles dispersed in ethanol onto 200 mesh copper Formvar grids (Ted

Pella Inc.) and allowing for subsequent evaporation in air. Nanoparticle sizes and standard

deviations were derived by measuring a minimum of 400 individual particles per

experiment and by averaging multiple images from samples obtained from at least two

separate syntheses. Individual particles were measured using Image-J

(http://rsbweb.nih.gov/ij), which finds the area of each nanoparticle by pixel counting.

Powder X-ray diffraction patterns were recorded with a Rigaku R-Axis Spider

diffractometer with a curved image plate using a Cu Kα source (1.5418 Å) operated at 40

kV and 40 mA; spectra were collected using a scan speed of 10° min-1 with a step width of

0.02 (2θ).

2.4.4 Synthesis of Rh NPs.32

A solution of PVP (200 mg, 1.8 mmol) in ethylene glycol (15.0 mL) was prepared

directly in the reaction vessel and brought to 150 °C with stirring. A second solution of

RhCl3�xH2O (20.0 mg, 0.095 mmol) was prepared in the same solvent (2.5 mL) and loaded

into a fresh 10 mL disposable syringe. The precursor solution was injected into the hot

25

stirred PVP solution at a rate of 11.5 mmol h-1. The color of the solution rapidly became

black. The mixture was stirred for 30 min. at 150 °C and was then cooled rapidly by

transferring the reactor vessel to an ice/water bath. The Rh NPs were precipitated by adding

acetone (60 mL) to give a black suspension. The precipitate was then isolated by

centrifugation (5500 rpm, 5 min.), and the clear supernatant was decanted away to leave a

black solid. This was further purified to remove excess PVP and ethylene glycol by 3 cycles

of dissolution in ethanol (15 mL) followed by precipitation with hexanes (75 mL) and

isolation by centrifugation (5500 rpm for 5 minutes). The final products were dried in

vacuum desiccator overnight and stored at room temperature.

2.4.5 Synthesis of polystyrene beads28

The polystyrene beads were synthesized through emulsifier-free emulsion

polymerization. 650 mL of water were added to a 1L round bottomed reaction flask fitted

with a mechanical stirrer and N2 was bubbled through the water for ~2 hours while heating

at 70 °C. 70 g (0.67 mol) of styrene monomer and 3.5 g (26.9 mmol) of divinylbenzene as

a crosslinker were washed with 0.1 M NaOH solution and then with DI water to remove

inhibitors, in which each washing was repeated for 4 times. N2 was then bubbled through

the mixture of styrene and divinylbenzene for 20 minutes, and the mixture was then added

to the reaction flask. The mixture was allowed to equilibrate for 20 min., then potassium

persulfate (150 mg, 0.55 mmo,l dissolved in 30 mL of water) was added as an initiator for

the polymerization. After heating at 70 °C for 12 hours, the polystyrene beads were cooled

to -15 °C for 1 hour. Once thawed, the solution was filtered through glass wool to remove

any coagulated material and diluted with methanol. The solid was then collected through

centrifugation (8000 rpm, 3 hours) and dried in an oven at 60 °C overnight to yield a white

solid.

26

2.4.6 Synthesis of macroporous/mesoporous Al2O3 and SiO228

Macroporous/mesoporous oxides were prepared by using polystyrene beads. For

the preparation of macroporous/mesoporous alumina and silica, 4.0 g of Pluronic P123

((EO)20(PO)70(EO)20 triblock copolymer, EO = ethylene oxide, PO = propylene oxide,

Mw = ~ 5,800) was dissolved in 40 mL of ethanol for 4 hours under ambient conditions.

Separately, 8.16 g (40 mmol) of Al(OiPr)3 or 8.3 mL (37.5 mmol) of TEOS was dissolved

in 6.4 mL of 68-70 wt% nitric acid (12 M HCl for SiO2) and 20 mL of ethanol under

vigorous stirring. After the solids are dissolved completely, the precursor solution was

added dropwise into the P123 solution. The combined solution was stirred for 5 h and 4 g

of the dried polystyrene beads were added. The solvent of the mixture was evaporated at

60 °C for 48 h. The resulting flakes were calcined in a furnace at 700 °C for 6 hours (SiO2)

or 900 °C for 10 hours (Al2O3) in air yielding white solids.

2.4.7 Synthesis of mesoporous Ta2O5 and TiO2

For mesoporous Ta2O5, 10.74 g (30 mmol) of TaCl5 in 30 mL anhydrous ethanol

was added dropwise to 3 g of P123 dissolved in 30 mL of anhydrous ethanol. For

mesoporous TiO2, 1.5 mL (5.1 mmol) of titanium isopropoxide was added dropwise to 1g

of P123 dissolved in 5 ml ethanol and 1.6 mL of 12 M HCl. The combined solution was

stirred for 5 h. The solvent of the mixture was evaporated at 60 °C for 48 h. The resulting

flakes were calcined in a furnace at 700 °C for 6 h in air. After calcinations, solids of TiO2

and Ta2O5 were obtained.

2.4.8 Synthesis of amorphous Al2O3, Nb2O5, Ta2O5, and TiO2

Synthesis of amorphous metal oxides follows the synthesis of the porous versions

without the addition of P123 or polystyrene.

27

REFERENCES (1) Gallezot, P.; Richard, D. Selective Hydrogenation of α,β-Unsaturated Aldehydes.

Catal. Rev. 1998, 40, 81–126. (2) Dostert, K.-H.; O’Brien, C. P.; Ivars-Barceló, F.; Schauermann, S.; Freund, H.-J.

Spectators Control Selectivity in Surface Chemistry: Acrolein Partial Hydrogenation Over Pd. J. Am. Chem. Soc. 2015, 137, 13496–13502.

(3) Gross, E.; Somorjai, G. A. The Impact of Electronic Charge on Catalytic Reactivity and Selectivity of Metal-Oxide Supported Metallic Nanoparticles. Top. Catal. 2013, 56, 1049–1058.

(4) Matte, L. P.; Kilian, A. S.; Luza, L.; Alves, M. C. M.; Morais, J.; Baptista, D. L.; Dupont, J.; Bernardi, F. Influence of the CeO 2 Support on the Reduction Properties of Cu/CeO 2 and Ni/CeO 2 Nanoparticles. J. Phys. Chem. C 2015, 119, 26459–26470.

(5) Bhowmick, R.; Rajasekaran, S.; Friebel, D.; Beasley, C.; Jiao, L.; Ogasawara, H.; Dai, H.; Clemens, B.; Nilsson, A. Hydrogen Spillover in Pt-Single-Walled Carbon Nanotube Composites: Formation of Stable C−H Bonds. J. Am. Chem. Soc. 2011, 133, 5580–5586.


(7) Prins, R.; Palfi, V. K.; Reiher, M. Hydrogen Spillover to Nonreducible Supports. J. Phys. Chem. C 2012, 116, 14274–14283.

(8) Sata, S.; Awad, M. I.; El-Deab, M. S.; Okajima, T.; Ohsaka, T. Hydrogen Spillover Phenomenon: Enhanced Reversible Hydrogen Adsorption/desorption at Ta2O5-Coated Pt Electrode in Acidic Media. Electrochimica Acta 2010, 55, 3528–3536.




(12) García, S.; Anderson, R. M.; Celio, H.; Dahal, N.; Dolocan, A.; Zhou, J.; Humphrey, S. M. Microwave Synthesis of Au–Rh Core–shell Nanoparticles and Implications

28

of the Shell Thickness in Hydrogenation Catalysis. Chem. Commun. 2013, 49, 4241.

(13) Kunal, P.; Li, H.; Dewing, B. L.; Zhang, L.; Jarvis, K.; Henkelman, G.; Humphrey, S. M. Microwave-Assisted Synthesis of Pd x Au 100– x Alloy Nanoparticles: A Combined Experimental and Theoretical Assessment of Synthetic and Compositional Effects upon Catalytic Reactivity. ACS Catal. 2016, 6, 4882–4893.

(14) Kunal, P.; Roberts, E. J.; Riche, C. T.; Jarvis, K.; Malmstadt, N.; Brutchey, R. L.; Humphrey, S. M. Continuous Flow Synthesis of Rh and RhAg Alloy Nanoparticle Catalysts Enables Scalable Production and Improved Morphological Control. Chem. Mater. 2017, 29, 4341–4350.

(15) Piburn, G. W.; Li, H.; Kunal, P.; Henkelman, G.; Humphrey, S. M. Rapid Synthesis of RhPd Alloy Nanocatalysts. ChemCatChem 2017.

(16) Seraj, S.; Kunal, P.; Li, H.; Henkelman, G.; Humphrey, S. M.; Werth, C. J. PdAu Alloy Nanoparticle Catalysts: Effective Candidates for Nitrite Reduction in Water. ACS Catal. 2017, 7, 3268–3276.

29

Chapter 3: Synthesis of Microwave Irradiated Iridium Nanoparticles in Aqueous Media

3.1 INTRODUCTION

Ir NPs have shown increased selectivity toward unsaturated alcohol products in the

hydrogenation of α,β-unsaturated aldehydes and ketones, due to the expansion of the Ir d-

band spacing.1 Limited examples of free (non-immobilized) Ir NPs exist in literature, likely

derived from the minimal growth of these NPs (2 nm).2–4 Direct reduction of Ir precursors

on metal oxide supports does yield bigger particles, at the expense of particle shape and

size control.5 Typical NP syntheses utilize viscous solvents (e.g. ethylene glycol) and

capping agents (e.g. poly(vinylpyrrolidone)) in order to minimize particle agglomeration.

For NPs that have minimal growth, the use of less viscous, ‘green’ solvents, such as water,

can be employed.

Microwave irradiation has shown to produce larger Rh, Pd, and Pt NPs, likely due

to more efficient nucleation. Microwave heating would be an interesting way to develop a

systematic scheme for the growth of Ir NPs in water for chemoselective hydrogenation of

α,β-unsaturated aldehydes and ketones. The potential economic and environmental

benefits of using water as the reaction solvent, along with its ability to allow better Ir NP

growth, make these particles great candidates as catalysts. In addition to the potential

improvements Ir NPs can have on selective hydrogenation, studies have shown that Co3O4

can also yield more crotyl alcohol in the hydrogenation of crotonaldehyde.6 Combining

both Ir NPs along with mesoporous Co3O4 as a support, our aim to make an industrially

applicable catalyst with higher selectivity towards unsaturated alcohol products.

30

3.2 MICROWAVE SYNTHESIS OF IRIDIUM NANOPARTICLES

Initially, Ir NPs were synthesized following the general procedure for the synthesis

of MwH Rh NPs developed in our group.7 This procedure consisted of an initial seeding

stage that yielded NPs with an average size of 5.3 ± 1.2 nm, followed by an overgrowth

step that increased the average size to 5.8 ± 1.5 nm. The PXRD pattern for the Ir NPs (Fig.

3.1 left) showed a wide FCC {1,1,1} peak that potentially masked the {2,0,0} reflection,

alluding to very small particles. Transmission Electron Microscopy (TEM) confirmed an

Figure 3.1: PXRD pattern (left) and TEM image (right) of Ir NPs synthesized following previously reported Rh NP procedure. Inset on right shows the size distribution and average particle size with standard deviation.

average size of 1.6 ± 0.4 nm. This lack of growth could mean that the Ir precursor is difficult

to reduce or that the surfaces are sufficiently low in energy at small sizes. Adding a stronger

reducing agent, namely sodium borohydride (NaBH4), could help reduce the Ir precursor,

thus encouraging growth. No real change in the PXRD pattern was observed when adding

NaBH4, so the surfaces are likely passivated at small diameters enough to inhibit growth.

Lower concentrations of PVP could lead to more exposed surfaces, potentially aiding

growth, but again no change in the PXRD spectrum was observed. Higher temperatures

31

tend to promote more nucleation, so lower temperatures were then attempted in the hopes

of allowing NP growth over seeding. Unfortunately, no FCC peaks were seen below 100

°C, and only a very small {1,1,1} reflection was observed at 120 °C. Solvent viscosity

plays a role in particle growth by slowing diffusion. For this reason, the viscosity of the

reaction was lowered by moving from ethylene glycol to water as the solvent. Water is also

a ‘greener’ solvent that could reduce waste and lower costs when scaling the synthesis to

an industrial scale. Although water served well as a solvent, it does not have the reducing

ability of ethylene glycol. Instead, NaBH4 was added to the reaction as a reducing agent,

which could also help in the nucleation step since these reactions could not be heated past

100 °C due to the boiling point of water. The PXRD pattern for the solid recovered from

the reaction heated to 100 °C did not show much improvement, but when heated to 80 °C,

the higher order FCC reflections were more visible than previous experiments (Fig. 3.2

left). TEM images showed an increase in the average particle size to 2.2 ± 0.6 nm (Fig. 3.2

right). When the reaction temperature was lowered to 60 °C, no precipitation was observed.

Figure 3.2: PXRD patterns (left) of Ir NPs synthesized following previously reported Rh NP procedure (black), addition of NaBH4 (purple), using 100 mg of PVP (pink), 120 °C reaction temperature (blue), water as solvent at 100 °C (orange) and 80 °C (green). TEM image of Ir NPs using water as the solvent at 80 °C (right).

32

Figure 3.3: PXRD patterns of the attempted nucleation reactions for Ir NPs.

Even though there was evidence of growth from the changes made, it was clear that

better nucleation conditions needed to be defined before an overgrowth step was attempted.

The biggest change in PXRD pattern, aside from the exchange of EG to water, was seen

when halving the concentration of PVP. To better explore this change, the amount of PVP

used was lowered from 200 mg to 150, 100 and 50 mg (Fig. 3.3). A reaction was performed

in the absence of any capping agent, but powder was likely bulk Ir as it was not soluble in

ethanol and no characterization on this material was performed. The reaction with 50 mg

of PVP revealed improvements in the PXRD patterns, although still not ideal. NaBH4 is a

strong reducing agent, so it is possible that the IrCl3 is completely reduced at the beginning

of the reaction and no growth can occur after the initial seeding. Reducing agents with less

reducing abilities than NaBH4 were implemented by dissolving the Ir precursor in EG or

33

ethanol to be the sacrificial reductants. PVP could be inhibiting growth by binding too

strongly to the seed surfaces, so a copolymer of PVP and polyvinyl acetate (PVAc) was

used with no luck. The addition rate was also modified between 300-75 ml/hr (150 ml/hr

for optimal reaction). Overtime, NP ripening can help growth so the reaction time was also

changed. Even after 3 hours, no major improvements were seen. In the end, the best

conditions were determined to be 0.2 mmol of IrCl3 in 2.5 ml of H2O dispensed into a flask

with 50 mg of PVP and 6 eq. (40 mg) of NaBH4 at a rate of 150 ml/hr for 30 minutes. We

wanted to better understand the nucleation mechanism, so we decided to take aliquots of

the reaction at different time intervals. After 1 minute, the particles already seem well

formed and are an average size of 2.71 ± 0.64 nm. Strangely, after 5 minutes, the particle

size decreased to 1.67 ± 0.38 nm and at 15 minutes they have only grown back to 1.77 ±

0.40 nm. At 30 minutes, the particles are 2.2 nm ± 0.55 nm, smaller than the initial 2.71

nm (Fig. 3.4). It seems that the NPs go through an initial nucleation burst that forms

Figure 3.4: TEM images of Ir NP nucleation at different reaction times.

kinetically stable particles that over time decrease to their ideal size. A second injection of

IrCl3 at 20 ml/hr was added and 1 minute after the end of the addition (38.5 minutes total),

5 minutes (42.5 minutes total), 15 minutes (52.5 minutes total), and 30 minutes (67.5

minutes) the respective sizes were 2.98 ± 0.73 nm, 1.84 ± 0.44 nm, 2.56 ± 1.10 nm, 2.12 ±

0.53 nm (Fig. 3.5). To the best of our knowledge, free standing Ir NPs with a size of 2.98

34

nm do not exist. Larger Ir NPs have been observed by direct reduction of an Ir precursor

on metal oxides, but they exhibit a wide distribution of sizes. This result is specially

exciting since the reduction of Ir is performed in under an hour, while most Ir NP syntheses

tend to require long reaction times (3 hours-days).

Figure 3.5: TEM images of Ir NPs after initial nucleation and second injection (20 ml/hr) at different reaction times.

3.3 CONCLUSION

The facile synthesis of Ir NPs in aqueous media was explored. Optimal conditions

were found to yield particle with an average size of 2.71 nm with a short reaction time. If

the reaction was allowed to continue for 30 minutes, the size of the particles actually

diminishes to 2.2 nm. A subsequent Ir precursor injection yielded 2.98 nm particles, that

over time decreased in size to 2.12 nm, which indicated a kinetic NP growth that returns to

2 nm Ir NPs regardless of precursor addition. The facile synthesis of 2.7 nm particles in 1

minute could be beneficial for industrial applications, where a continuous-flow reactor

could be implemented to produce the desired amount of Ir NPs. Further stability tests on

these Ir NPs will include cyclohexene and crotonaldehyde to gain insight on the effect of

catalytic conditions on the NPs. Coupling Ir NPs with Co3O4, a support known for

enhancing hydrogenation selectivity, will also be studied in the near future.

35

3.4 EXPERIMENTAL SECTION

3.4.1 Materials

Iridium(III) chloride hydrate (IrCl3�xH2O, 98%; Johnson Matthey), Tetraethyl

orthosilicate (Si(OCH2CH3)4, Sigma Aldrich), poly(vinylpyrrolidone) (PVP, Mw = 55 000;

Sigma Aldrich), ethylene glycol ((CH2OH)2, 99.8%; Fisher Scientific), sodium

borohydride (NaBH4, Alfa Aesar) and anhydrous cyclohexene (Alfa Aesar, g99%) were

used as received. All gases used in catalytic studies were 99.9995+% (Praxair) purity. All

other reagents and solvents were employed without further purification unless stated

otherwise. TEM grids (200 mesh Cu/Formvar; Ted Pella, Inc.) were prepared by drop-

casting water or ethanol suspensions of materials that were evaporated to dryness in air.

3.4.2 Methods

A MARS 5 (CEM Corp.) microwave system with a maximum power of 1600 W

(2.45 GHz) was used to perform all microwave-based reactions. Throughout the course of

the experiment, the reaction temperature was finely controlled by power modulation via a

RTP-300þ fiber-optic temperature sensor, located in a beaker of solvent identical to that

employed in the reaction. A 50 mL round-bottomed flask fitted with a water-cooled reflux

condenser was placed in the center of the microwave cavity. The solvent and capping

agents were magnetically stirred, and Ir precursor was then added directly into the stirred

solvent through a disposable, fine-bore Teflon tube. The rate of Ir precursor addition was

controlled using an Aladdin programmable syringe pump (WPI, Inc.) that was directly

attached to the Teflon tubing.

3.4.3 Characterization

Transmission electron microscopy (TEM) images were obtained from a FEI

Tecnai microscope operating at 80 kV. The samples were prepared by drop-casting a single

36

aliquot of nanoparticles dispersed in water or ethanol onto 200 mesh copper Formvar grids

(Ted Pella Inc.) and allowing for subsequent evaporation in air. Nanoparticle sizes and

standard deviations were derived by measuring a minimum of 400 individual particles per

experiment and by averaging multiple images from samples obtained from at least three

separate syntheses. Individual particles were measured using Image-J

(http://rsbweb.nih.gov/ij), which finds the area of each nanoparticle by pixel counting.

Powder X-ray diffraction patterns were recorded with a Rigaku R-Axis Spider

diffractometer with a curved image plate using a Cu Kα source (1.5418 Å) operated at 40

kV and 40 mA; spectra were collected using a scan speed of 1° min-1 with a step width of

0.02 (2θ).

3.4.4 Synthesis of Ir NPs7

A solution of PVP (50-200 mg, 0.5-1.8 mmol) in ethylene glycol or water (15.0 ml)

was prepared directly in the reaction vessel and brought to desired temperature (60-197

°C) with stirring. NaBH4 (10-80 mg, 0.2-1.2 mmol) was added to serve as a reducing agent

in all water based reactions and in specified EG based reactions. A second solution of

IrCl3�xH2O (60.0 mg, 0.2 mmol) was prepared in the same solvent (2.5 ml) and loaded into

a fresh 10 ml disposable syringe. For the nucleation phase of the reaction, the Ir precursor

solution was injected into the hot stirred PVP solution at a rate of 12 mmol h-1. The color

of the solution rapidly became brown/black. The mixture was stirred for 1-180 min at the

desired temperature and was then cooled rapidly by transferring the reactor vessel to an

ice/water bath. The Ir NPs were precipitated by adding acetone (ca. 60 ml) to give a black

suspension. The precipitate was then isolated by ultracentrifugation (5500 rpm, 5 min), and

the clear supernatant was decanted away to leave a black solid. The PVP capped

nanoparticles using higher equivalences of PVP (>50 mg) were further purified to remove

37

excess PVP by 2-3 cycles of dissolution in ethanol (15 ml) followed by precipitation with

hexanes (75 ml) and isolation by centrifugation. The final products were dried in vacuum

desiccator overnight and stored at room temperature.

38

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CHAPTER 3 (1) Gallezot, P.; Richard, D. Selective Hydrogenation of α,β-Unsaturated Aldehydes.

Catal. Rev. 1998, 40, 81–126. (2) Hernández-Cristóbal, O.; Díaz, G.; Gómez-Cortés, A. Effect of the Reduction

Temperature on the Activity and Selectivity of Titania-Supported Iridium Nanoparticles for Methylcyclopentane Reaction. Ind. Eng. Chem. Res. 2014, 53, 10097–10104.

(3) Yu, J.; He, H.; Song, L.; Qiu, W.; Zhang, G. Preparation of Ir@Pt Core–Shell Nanoparticles and Application in Three-Way Catalysts. Catal. Lett. 2015, 145, 1514–1520.

(4) Fonseca, G. S.; Machado, G.; Teixeira, S. R.; Fecher, G. H.; Morais, J.; Alves, M. C. M.; Dupont, J. Synthesis and Characterization of Catalytic Iridium Nanoparticles in Imidazolium Ionic Liquids. J. Colloid Interface Sci. 2006, 301, 193–204.


(6) Kennedy, G.; Melaet, G.; Han, H.-L.; Ralston, W. T.; Somorjai, G. A. In Situ Spectroscopic Investigation into the Active Sites for Crotonaldehyde Hydrogenation at the Pt Nanoparticle–Co 3 O 4 Interface. ACS Catal. 2016, 6, 7140–7147.


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