Design, Synthesis, And Characterization Of Monometallic ...

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Theses and Dissertations

2018

Design, Synthesis, And Characterization OfMonometallic And Bimetallic CatalystsSonia EskandariUniversity of South Carolina

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DESIGN, SYNTHESIS, AND CHARACTERIZATION OF MONOMETALLIC AND

BIMETALLIC CATALYSTS

by

Sonia Eskandari

Bachelor of Science

Tabriz University, 2004

Master of Science

Amirkabir University of Technology, 2008

Master of Engineering

Clemson University, 2014

Submitted in Partial Fulfillment of the Requirements

For the Degree of Doctor of Philosophy in

Chemical Engineering

College of Engineering and Computing

University of South Carolina

2018

Accepted by:

John R. Regalbuto, Major Professor

John R. Monnier, Committee Member

John W. Weidner, Committee Member

Christopher T. Williams, Committee Member

Aaron K. Vannucci, Committee Member

Cheryl L. Addy, Vice Provost and Dean of the Graduate School

ii

© Copyright by Sonia Eskandari, 2018

All Rights Reserved.

iii

DEDICATION

Dedicated to my parents, Leili and Moharram, for trusting and believing me. Also

to my best friend and sister ever, Soheila for her nonstop support during this long journey,

her husband Rahim and their lovely son Ryne.

iv

ACKNOWLEDGEMENTS

I would like to acknowledge my supervisor, Dr. John R. Regalbuto for accepting

me in his wonderful group and giving me the opportunity to learn research. He is just

wonderful and patient in process of training every bit of research. I was so fortuned to work

with him. He just wants student to progress. I appreciate his support for letting me to attend

various conference over US to present my work and learn other researchers work.

I would like to thank Dr. Monnier, National Academy of Engineer member. He is a tough

but an awesome advisor to learn a lot from. Every day he gave me new insight related to

different projects. I appreciate his time for training me in different areas of catalysis with

patience.

I would also like to appreciate my committee members, Dr. Christopher T.

Williams, Dr. Aaron K. Vannucci and Dr. John W. Weidner for their advices after my

comprehensive exam and their time to review my work.

I would like to thank Dr. Feng and his group on doing EXAFS experiments, which

covers an important part of my second chapter.

The help of staffs in the Department of Chemical Engineering: Marcia Rowen,

Loretta Hardcastle, Carol Stork, Vernon Dorrell, Chase Ferch and Brian Loggans is truly

appreciated.

v

It is pleasure to thank my colleagues: Dr. Jadid E. Samad, Dr. John M. Tengco, Dr.

Shuo Cao, Dr. Kerry O’Connell, Dr.Ritubarna Banerjee, Dr. Bahareh Tavakkoli Al-Sadat,

Andrew P. Wong, Fahad Almalki, Jayson Keels, Greg, Sean Noble, Jeremiah W. Lipp,

Abolfazl Shakouri, Mozhdeh and Nathan for helping me to learn experimental work.

I sincerely appreciate my friends for their support and motivation.

I love to thank my favorable restaurant “OLIVE GARDEN” with its refill soup and

salad.

I am so happy of accomplishing my PhD at USC in Columbia and proud of that.

vi

ABSTRACT

Supported catalysts are used extensively in a variety of heterogeneously catalyzed

reactions for industrial processes. Techniques for preparing supported noble metal catalysts

are typically chosen to achieve high metal dispersions in order to obtain high activity for a

given metal loading. Enhancing the catalytic performance of heterogeneous catalysts can

be done by increasing active site count as well as modification of the physico-chemical

characteristics of the catalyst material. For supported metal nanoparticles this can be

achieved by decreasing particle size, thus increasing dispersion or metal utilization on the

surface of the particles, while modification of metal properties can be attained by addition

of a secondary metal that has a strong interaction to the primary metal, beneficial for a

given reaction. In this dissertation, the first two chapters cover preparation methods to

control metal particles size and the third chapter goes over evaluations of monometallic

and bimetallic catalyst.

In the preparation of supported metal catalysts, the methods of Strong Electrostatic

Adsorption (SEA), and its incipient wetness analog, Charge Enhanced Dry Impregnation

(CEDI), can yield supported metal nanoparticles with high dispersion and narrow size

distribution. Catalysts prepared by SEA and CEDI therefore are desirable as seeds for

addition of secondary metal using Electroless Deposition (ED), as the prepared bimetallic

catalysts should be of similar dispersion as the base catalyst. CEDI and ED methods were

used to demonstrate the preparation of monometallic and bimetallic catalysts containing

vii

noble and base metals which were then characterized and evaluated for hydrogenation

reactions. The first system used Pt, Pd, Co, Ni as the single metal, prepared by CEDI on

silica support. In the second system Ag-Ir bimetallic catalysts prepared by ED method were

characterized with hydrogenation reactions, FTIR and H2 temperature program desorption

(H2-TPD).

In the first work, it was demonstrated that sintering of metal nanoparticles can be

induced by the presence of residual balancing ions such as Cl-. We further show how

particle size can be drastically reduced by the removal of residual ions. X- ray diffraction

(XRD) was used to analyze the particle size.

In the second work, Pt(NH3)4(OH)2 as a model metal precursor has been studied to

investigate the effect of different anions such as Cl-, Br-, NO3-, and C6H5O7

-3- on the size

and polydispersity. Chloride, and bromide had largest impact on sintering and growth of

Pt particles along with wide size distribution, confirmed by XRD and STEM images.

In the third work, Ag-Ir bimetallic catalysts at different coverages of Ag were

studied. A direct method of reaction and two indirect methods including H2-TPD and FTIR

experiments were done to evaluate the unusual H2-uptake of these catalysts. Two different

hydrogenation reactions were used to evaluate catalytic properties of the Ir@Ag catalysts,

the hydrogenation of propylene (C3H6) and hydrogenolysis of methyl cyclopentane (MCP).

In situ transmission Fourier transform infrared spectroscopy (FTIR) of CO adsorption

indicates that the Ag is randomly deposited on all types of Ir surface sites during the ED

process.

viii

TABLE OF CONTENTS

DEDICATION ................................................................................................................... iii

ACKNOWLEDGEMENTS ............................................................................................... iv

ABSTRACT ....................................................................................................................... vi

LIST OF TABLES ...............................................................................................................x

LIST OF FIGURES ........................................................................................................... xi

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW ....................................1

INTRODUCTION ....................................................................................... 1

CHAPTER 2 EXPERIMENTAL METHOD....................................................................12

CATALYSTS PREPARATION ................................................................ 12

CATALYST CHARACTERIZATION ..................................................... 15

CATALYSTS EVALUATION (PROPYLENE HYDROGENATION

AND METHYL CYCLOPENTANE HYDROGENOLYSIS) .......... 19

CHAPTER 3 SYNTHESIS OF SUPPORTED METAL PARTICLES OF

CONTROLLED SIZE ...............................................................................................22

INTRODUCTION ..................................................................................... 22

MATERIAL AND METHODS ................................................................. 24

RESULTS AND DISCUSSION ................................................................ 26

CONCLUSION .......................................................................................... 36

ix

CHAPTER 4 NANOPARTICLE SYNTHESIS VIA ELECTROSTATIC

ADSORPTION USING INCIPIENT WETNESS IMPREGNATION......................37

INTRODUCTION ..................................................................................... 37

MATERIAL AND METHODS ................................................................. 39


CONCLUSION .......................................................................................... 58

CHAPTER 5 EVALUATION OF AG–IR/AL2O3 BIMETALLIC CATALYSTS

PREPARED BY ELECTROLESS DEPOSITION FOR HYDROGENATION OF

PROPYLENE AND HYDROGENOLYSIS OF METHYL CYCLOPENTANE .....59

INTRODUCTION ..................................................................................... 59

MATERIALS AND METHODS ............................................................... 63


CONCLUSION .......................................................................................... 77

REFERENCES ..................................................................................................................78

x

LIST OF TABLES

Table 2.1. Summary of chemicals and materials ...............................................................12

Table 3.1. EXAFS fitting data for Pt samples on SiO2. .....................................................34

Table 4.1. Metal loss after washing dried samples prepared by DI and CEDI methods. ..56

Table 4.2. Washing samples as wet without drying. ..........................................................57

Table 5.1 Bimetallic Ag-Ir Catalysts for C3H6 hydrogenation, and MCP hydrogenolysis69

Table 5.2. Selectivity to isomerization products of MCP, TOF (s-1) based on MCP reacted,

rate (µmol/g cat. Min), and Conversion (%) at reaction conditions of 300 °C and 1

atm, tot. flow of 50 sccm. .........................................................................................75

xi

LIST OF FIGURES

Figure 1.1. Impregnation method to prepare supported platinum nanoparticles 8. Modified

from Chan, K. Y.; et al. J. Mater. Chem. 2004, 14(4),505–516. Copyright 2004 Royal

Society of Chemistry.....................................................................................................4

Figure 1.2.(A) A schematic of the deposition precipitation procedure for the synthesis of

the Pt–Au/CeO2 catalyst. (B) TEM images and EDX spectra of the Pt–Au/CeO2

catalyst, respectively14. Modified from Hong, X.; et al. Catal. Sci. Technol. 2016, 6,

3606–3615. Copyright 2015Royal Society of Chemistry. ............................................6

Figure 1.3.(A) A schematic of the procedure for silver nanoparticles synthesized using

coreduction approach. (B) FEG-TEM images of silver nanoparticles16. Based on

Agnihotri, S.; et al. RSC Adv. 2014, 4(8), 3974–3983. Open access publication. .......7

Figure 1.4.An illustration of the synthesis of the polyol method. .......................................8

Figure 1.5. Steps of SEA mechanism: a) determination of PZC point of support by single

point method; b) metal precursor uptake to obtain optimal pH, c) adsorption of

precursor on support including hydration sheaths precursor at optimal pH. ...............9

Figure 3.1.(a) XRD profile and b) STEM image and particle size distribution of 4.5 wt %

Pt/SiO2 (PTAOH) prepared by CEDI. .......................................................................26

Figure 3.2. (a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing chloride

prepared by CEDI. .....................................................................................................27

Figure 3.3. Deconvolution of Cl-/Pt=1, PTAOH as precursor on silica support. ..............27

Figure 3.4. (a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing bromide

prepared by CEDI. ......................................................................................................28

Figure 3.5. Deconvolution of Br-/Pt=1, PTAOH as precursor on silica support. ..............28

Figure3.6.(a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing citrate

prepared by CEDI .......................................................................................................29

Figure3.7. Deconvolution of Cit3-/Pt=1, PTAOH as precursor on silica support. .............29

xii

Figure3.8(a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing nitrate

prepared by CEDI. ......................................................................................................30

Figure3.9. Deconvolution of NO3-/Pt=1, PTAOH as precursor on silica support.. ...........31

Figure 3.10 (a) reduction of dried 4.5% Pt(NH3)4(OH)2 in 10% H2, (b) He flow over the

previously reduced sample, (c) Repeating previous step. ...........................................32

Figure 3.11 (a) Temperature-programmed desorption of ammonia(NH3-TPD) profile of

silica (SiO2). ................................................................................................................32

Figure 3.12. Mass - TPR on dried samples prepared by CEDI by looking at the evolved

NH3 (mass 15) during reduction, (a) Cl-/Pt=1, (b) Br-/Pt=1, (c) NO3-/Pt=1, and (d)

citrate3-/Pt=1 by comparing them to anion free sample. .............................................33

Figure 3.13. EXAFS: the fitting plot of (a) Pt(NH3)4(OH)2 + SiO2 (physical mixture), (b)

Na2PtCl4+SiO2 (physical mixture), (c) Pt(NH3)4(OH)2 / SiO2 (CEDI), and (d)

Pt(NH3)4(OH)2 + NaCl / SiO2 (CEDI), dashed line shows Pt-ligand bond length. .....35

Figure 3.14. Schematic mechanism of NH3 ligand exchange with Cl-. .............................36

Figure 4.1. XRD patterns for PTAOH and PTACl samples: (a) CEDI – prepared series of

PTAOH , (b) CEDI – prepared series of PTACl, (c) DI - prepared series of PTACl,

and (d) Cl- free samples from CEDI – prepared PTACl samples after wash with

soultion at pH 10.5. .....................................................................................................42

Figure 4.2. Peak fitting for (a) 0.25 ML, (b) 0.50 ML, (c) 1.0 ML, (d) 1.5 ML, and (e) 2.0

ML Pt from Pt(NH3)4(OH)2 on SiO2 prepared by CEDI ............................................43

Figure 4.3. Deconvolution of XRD peaks for (a) 0.25 ML, (b) 0.50 ML, (c) 1.0 ML, (d) 1.5

ML, and (e) 2.0 ML Pt from Pt(NH3)4Cl2 on SiO2 prepared by CEDI. ......................44

Figure 4.4. XRD peak deconvolution for (a) 0.25 ML, (b) 0.5 ML, (c) 1.0 ML, (d) 1.5 ML,

and (e) 2.0 ML from Pt(NH3)4Cl2 on SiO2 prepared by DI.. ......................................45

Figure 4.5. X-ray diffraction pattern with deconvolution for different coverages of Pt after

washing PTACl samples for (a) 0.5 ML, (b) 1.0 ML, (c) 1.5 ML, and (d) 2 ML.. ....47

Figure 4.6. STEM images and corresponding particle distributions of reduced Pt catalyst

samples for: (a) 1ML PTAOH prepared by CEDI, (b) 1ML PTAOH prepared by SEA,

(c) 2ML post-washed samples from PTACl prepared with CEDI. .............................48

Figure 4.7. XRD patterns of silica supported catalysts, CEDI-prepared before wash and

after wash: (a,b) PdTACl, (c,d) CoHACl, (e,f) NiHACl. pH of solution was adjusted

at 10.5. .........................................................................................................................49

xiii

Figure 4.8. Metal particle size before and after wash with the metal loss (%) for the silica

supported catalysts, CEDI-prepared: (a) Pd(PdTACl)/silica, (b)

Co(CoHACl)/silica,(c) Ni(NiHACl)/silic ...................................................................50

Figure 4.9. Peak fitting of (a) 0.5 ML, (b) 1.0 ML, (c) 1.5 ML, and (d) 2 ML Co from

CoHACl precursor on silica prepared by CEDI... ......................................................51

Figure 4.10. X-ray diffraction pattern with deconvolution for different coverages of Ni

from NiHACl precursor before wash (a) 0.5 ML, (b) 1.0 ML, (c) 1.5 ML, and (d) 2

ML...............................................................................................................................52

Figure 4.11. Metal particle size before and after wash with the metal loss (%) for the silica

supported catalysts, CEDI-prepared: (a) Pd(PdTACl)/silica, (b) Co(CoHACl)/silica,

(c) Ni(NiHACl)/silica..... ...........................................................................................54

Figure 4.12. XRD of 1 ML prepared by DI and CEDI for before and after wash of (a,b)

PTACl, (c,d) PdTACl (e,f) CoHACl, (g,h) NiHACl. pH of solution was 10.5.. .......55

Figure 5.1.The H2 uptake of the Ag-Ir bimetallic catalysts remained higher

than the Ir monometallic catalysts after the elevated annealing

temperatures of 600 and 800°C. .................................................................................70

Figure 5.2. Effect of annealing temperature on TOF for C3H8 formation on (a) 1% Ir, (b)

0.14% Ag-1% Ir, (c) 0.24% Ag-1% Ir, and (d) 0.47% Ag-1% Ir. ..............................71

Figure 5.3. Comparison of initial TOF's for Ir and Ag-Ir catalysts. ..................................72

Figure 5.4. H2-TPD for before and after reaction of (a,b) 1% Ir, (c,d) 0.14%Ag, (e,f) 0.24%

Ag, and (g,h) 0.47% Ag. .............................................................................................73

Figure 5.5. Effect of Ag coverage on selectivity of MCP isomerization at 10% C3H6, 50%

H2, bal He at 100 SCCM flow, 6mg cat diluted to 60 mg with δ,θ-Al2O3, Temp =

100 °C and P=1 atm. ...................................................................................................74

Figure 5.6.Transmission FTIR spectra of CO adsorption on Ir/Al2O3, Ag/Al2O3 and Ag

Ir/Al2O3 bimetallic catalysts. Arrows highlight peak intensity losses as Ag wt.%

increases. .....................................................................................................................76

1

CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

INTRODUCTION

Supported metal catalysts play a vital role in many important chemical and energy

production which are highly favorable in environmental protection. The active component

of many catalysts is a supported noble metal such as platinum, palladium, or base metals

like Co, and Ni, and since only the surface of the metal is available to catalyze the reaction,

it is usually desirable to maximize the accessible metal surface area while decreasing the

amount used. For catalyst preparation, availability of surface sites of metal nanoparticle

catalysts is a critical aspect of its design, since overall activity of catalysts is largely

dependent on the number of active sites for molecular level reactions to occur on. For

supported nanoparticles, it is desired to expose the metals on the particle surface to decrease

the unused bulk nanoparticles. Therefore, smaller nanometals having a high ratio of surface

area to volume are preferred in catalyst preparation by maximizing utilization of the surface

nanoparticles. In chemical industry, metal particles are in the size of a few nanometers or

single atoms.

The ratio of exposed metal surface to the total number of metal atoms is defined as

dispersion which has a reciprocal relation with particle size. There are various techniques

to determine dispersion such as selective adsorption and measurement of an analyte gas or

titration of a gas chemisorbed on the surface of the metal. Based on chemisorption

2

stoichiometry for each metal, the amount of available metal on the surface of the

nanoparticles can be calculated from the measured gas by chemisorption. X-ray diffraction

(XRD) and electron microscopy (EM) are other methods for determining dispersion. In

these methods the amount of metal on the surface of nanoparticles can be calculated by

assumption of particle geometry.

Controlling sizes, shapes and the prevention of aggregation are the most important

challenges for catalyst preparation during synthesis process. Also, activity, selectivity, and

lifetime of the supported catalysts mainly depends on preparation method 1. According to

these criteria, a varied number of methods have been developed beyond the oldest, most

common method of impregnation and show a better degree of control of the catalyst

dispersion 2. In particular, strong electrostatic adsorption (SEA) and charge-enhanced dry

impregnation (CEDI) methods have been recently used as simple techniques to prepare

catalysts with more control over particle size and size distributions. However, other various

methods for making both monometallic and bimetallic catalyst have been developed such

as impregnation 3, deposition-precipitation 4, and reduction-oxidation 5.

1.1.1 Impregnation Methods

Among the many methods to prepare supported metal catalysts, impregnation is the most

prevalent catalyst preparation method in industry. By adding an amount of the precursor

solution in excess of the pore volume of the support which gives a thin slurry, is called wet

impregnation (WI). In wet impregnation, during filtration of impregnated support, excess

liquid containing any precursor that was not retained by the support is filtered out. This

leads to one more step to recycle the excess liquid to minimize loss of the precursor.

3

Dry or incipient wetness impregnation (DI) can be done by limiting the solution

amount to just fill the pore volume. In this method there is not strong metal-support

interaction and therefore leads to less homogenous metal deposition. However, this method

is known as simplest, quickest, and least expensive method of monometallic and bimetallic

nanoparticles preparation without loss of metals 6,7. Compared with Dry impregnation

method, WI requires longer time about several hours to reach equilibrium because diffusion

of metal precursor into pores in support is a slower process. Evaporation during water

removal can occur which causes precursor crystallization and metal precipitation and

wasting cause metal precursor. Due to lack of filtration step during DI synthesis of catalyst

any counterions from the metal precursor salt, such as the chloride from will be remained

in the dried catalyst. To remove these substances further processing is required. Figure 1.1

illustrates impregnation method for preparation of supported Pt catalysts.

Figure 1.1. Impregnation method to prepare supported platinum nanoparticles 8. Modified

from Chan, K. Y.; et al. J. Mater. Chem. 2004, 14(4), 505–516. Copyright 2004 Royal

Society of Chemistry.

There are two other methods for bimetallic catalysts synthesis beside “wet” and

“dry” impregnation: co-impregnation and successive impregnation. In co-impregnation

two or more metal precursors are simultaneously impregnated. In Successive impregnation,

first metal salt on a support is applied and then second metal on the monometallic catalyst

deposits by impregnation. In these methods like single metal analog pH is not controlled

4

and there is not strong interaction between precursors and support. Due to the lack of this

interaction, mixing between two metal precursor components is usually poor. Since there

is no control on the deposition of second metal on the core metal, random distribution of

monometallic and bimetallic particles occurs which are usually large in size.

1.1.2 Precipitation-deposition

The deposition-precipitation (D-P) method involves a process in which highly

soluble metal salt precursor precipitate selectively on the support and not in solution 9.

Typically, this process involves addition of a precipitation agent, addition of a reducing

agent, or change in the concentration of a complexation agent or pH of the solution. Sodium

hydroxide 10 and urea 11,12 are the most commonly used agents for this preparation. There

are two main conditions to make sure of precipitation only on the support instead of in

solution: controlled amount of the metal precursor in the solution and stablishing strong

interaction between support and metal precursor. Generally, solubility limit decreases in

the presence of the support to favor deposition on the support, while this amount increases

in only solution. To prevent precipitation in the solution the concentration of the metal

precursor should be kept between the solubility point and the super solubility point. In this

method there is not strong control on metal distribution and surface composition which doesn’t

allow to synthesize true bimetallic catalysts with well controlled compositions13. Figure 1.2 shows

deposition precipitation method for synthesis of Pt–Au/CeO2 (RDP) catalyst.

5

Figure 1.2. (A) A schematic of the deposition precipitation procedure for the synthesis of the Pt–

Au/CeO2 catalyst. (B) TEM images and EDX spectra of the Pt–Au/CeO2 catalyst, respectively14.

Modified from Hong, X.; et al. Catal. Sci. Technol. 2016, 6, 3606–3615. Copyright 2015Royal

Society of Chemistry.

1.1.3 Reductive Deposition

Liquid phase reduction method is one of the easiest procedures between various

procedures to prepare nanoparticles. In this method nanoparticles can be directly made by

various precursors soluble in a specific solvent15–18. To synthesize metal nanoparticles by

this method, metal precursors were dissolved in organic or aqueous solvent. Then reducing

agent was added to the solution and the reduction occurs which allows selectively

deposition of metal particles on supports. To reduce metal precursors usually thermal

decomposition at high-temperature or electrochemical routes are used. However, reducing

agents such as sodium borohydride, hydrazine, ethylene glycol, and ascorbic acid can also

be used in liquid phase for reduction of metal precursors. This method has some drawbacks

such as toxicity of reducing agents and need to provide additional electrochemical devices

6

for electrochemical routes. Poor control on the size of resulting nanoparticles is another

disadvantage of this method. Figure 1.3 demonstrates preparation of silver nanoparticles

using coreduction procedure.

Figure 1.3. (A) A schematic of the procedure for silver nanoparticles synthesized using

coreduction approach. (B) FEG-TEM images of silver nanoparticles19. Based on Agnihotri, S.; et

al. RSC Adv. 2014, 4(8), 3974–3983. Open access publication.

1.1.4 Colloidal

Colloidal syntheses methods consist of three major steps: (1) solving metal

precursors and protective agent like surfactant in a solvent, (2) letting colloids to deposit

on the support, and (3) chemically reduction of the mixture20. To prepare colloidal metals

stabilizing agents determine the type of preparation phase to be aqueous or organic. Main

advantage of colloidal procedure is forming very small particles. However, the presence of

surfactant and protective agents in this method requires washing catalyst in an appropriate

solvent for several times or treat at high temperatures to decompose these extra materials.

7

Thus, another method is preferred to use to make small metal nanoparticles in an easy way

without having difficulties of contamination due to protecting agents. Figure 1.4 illustrates

synthesis of supported Pt nanoparticles using colloidal method.

Figure 1.4. An illustration of the synthesis of the polyol method8.

1.1.5 Strong electrostatic adsorption (SEA)

Fundamental studies have been made in catalyst impregnation procedures. Brunelle

postulated in his work that the adsorption of noble metal complexes onto oxides supports

occurs due to electrostatically interaction21. Using concept of Brunelle’s work, a scientific

method, namely, strong electrostatic adsorption (SEA) was introduced to synthesize

heterogeneous metal nanoparticles with small particle size and tight size distribution3,22–26.

SEA is the same wet impregnation method with adjusted final pH in which the electrostatic

interaction is strongest. Figure 1.5 illustrates SEA mechanism occurs between support and

metal precursor.

Oxide and carbon surfaces terminate in hydroxyl groups and carboxylic acids can

be protonated or deprotonated by adjusting solution pH. In this way electrostatic

interaction can be established between the charged support and metal precursor of

8

opposite charges. At pH below the PZC of support anion precursors adsorbs on a

protonated surface, while cationic metal precursors will adsorb on a deprotonated surface

at pH higher than PZC of support.

Figure 1.5. Steps of SEA mechanism: a) determination of PZC point of support by single point

method; b) metal precursor uptake to obtain optimal pH, c) adsorption of precursor on support

including hydration sheaths precursor at optimal pH.

Induction of this strong columbic force prevents metal migration during thermal

treatment, and therefore results in metal nanoparticles with smaller size. Using SEA

method, ultra-small metal nanoparticles (1-2 nm) with high dispersion have been

synthesized on a variety of oxide and carbon supports27–31.

According Figure 1.5, the pH of solution containing support is adjusted depending

on PZC of support to establish electrostatic interaction; for supports with low PZC such as

silica by increasing pH of solution, higher amount of metal cationic precursors is adsorbed

on the support. To determine optimal pH for maximum uptake of metal precursor uptake

9

survey should be performed. Using ICP the amount of adsorbed metal precursor on support

in SEA process can be determined. Following formula shows the difference between initial

metal concentration, Ci, and final metal concentration, Cf, both in ppm unit. Surface loading

(SL) and Molecular weight (MWM) of metal precursor should be specified.

Metal uptake (μmoles/m2) = (Ci−Cf)[ppm]×1000 MWM

SL [𝑚2/liter]

Surface loading (SL) [𝑚2/𝑙𝑖𝑡𝑒𝑟] = Surface Area of support [𝑚2/g]×grams of support[g]

Volume of Precursor Solution[liter]

The metal uptake capacity is limited to numbers of hydration sheaths which gives

closed packed geometry of metal ion precursor (Figure 5.1c). Cationic metal precursor

retains two and anionic metal precursor retain one layer of hydration respectively. For

example, maximum adsorption capacity Pt(NH3)42+ and Pd(NH3)4

2+ precursors with two

hydration sheaths is ~0.84 mol/m2. However, for anionic precursors such as PtCl62- and

Ru(CN)64- with one hydration sheath the maximum uptake is around 1.6 mol/m2 28.

Adsorption capacity can be retarded at extreme pH values due to high ionic strength 27.

Therefore, using this method by induction strong electrostatic interaction between

precursor-support in a controlled manner small nanoparticle with tight size distribution can

be achieved.

1.1.6 Charge-enhanced dry impregnation (CEDI)

Due to lose metal using SEA method through filtration and limited amount of metal

loading, another method with the same concept has been introduced. Charge Enhanced Dry

Impregnation (CEDI) is the same incipient wetness impregnation method with controlled

10

pH of precursor solution32. Therefore, impregnating solution is basified or acidified to

make a columbic force between the support surface and the oppositely charged metal

precursor but applied on dry impregnation method. Due to higher amount of support and

its buffering effect, the amount of acid or base required to overcome the PZC point is

significantly large 33,34. Pt particles synthesized over silica, alumina, and titania have sizes

< 1.5 nm (the XRD limit of detection is 1.5 nm) 35.

While in this method electrostatic interaction between the support surface and the

oppositely charged metal precursor give very small particles, study has shown that the

remaining residual balancing ions from the precursor play effective role on nanoparticle

sintering. For example, counter ion of metal precursor such as chloride or nitrate plays an

important role on final metal particle size and dispersity during the synthesis of catalysts

via CEDI 32.

Therefore, removing these counterions from prepared samples is required to

prevent metal nanoparticles sintering. By washing away chloride counter anion from dried

samples which contains noble or base metal gives very small particles with tight size

distribution 36. Simultaneous SEA or co-SEA can be used to make bimetallic catalysts or

sequential SEA to prepare core-shell structure37,38.

1.1.7 Electroless deposition (ED)

Electroless deposition (ED) is another route to synthesize core-shell bimetallic

catalysts. In this method a shell metal is deposited with different coverages on a core

supported metal. This method includes an aqueous bath with a predetermined right pH, a

11

secondary metal precursor, and a reducing agent such as (Hydrazine (N2H4),

dimethylamine borane (DMAB), formaldehyde (HCHO), and hypophosphite (H2PO2-) 39.

For ED first monometallic core metal should be prepared with any monometallic

preparation method such as SEA, CEDI, or DI. Adjusting pH is required through ED

process to not let SEA occur on support. By this method second metal can exclusively

deposits on first metal core as a partial shell which offers the control over synthesis of true

bimetallic surfaces. ED can proceed catalytically or autocatalytically depending on the rate

of reaction. In the quick deposition there is the chance of deposition of metal over pre-

existing deposited metal as layers which is called autocatalytic. However, in slow reaction

rate with more control over deposition of second metal desired coverages of second metal

can be obtained which is called catalytic. Reducing agent in the solution phase is activated

only on the surface of metal particles. Thus, secondary metal can be deposited only onto

the base catalyst particles or pre-existing second metal. Several studies have been done to

synthesize various bimetallic catalysts using ED such as Cu−Pd 39, Ag−Pt40, Pt−Co41,

Au−Pd42, and Ag−Pd43. To successfully synthesize these bimetallic catalysts by ED

method, the base catalyst, secondary metal ion source, and reducing agent were carefully

determined and bath temperature, and pH were controlled. Ag-Ir bimetallic catalysts

prepared by ED method were evaluated in this study.

The objective of this work is to design, synthesis, and characterization of noble and

base monometallic and bimetallic catalysts using the rational catalyst synthesis methods

outlined earlier.

12

CHAPTER 2

EXPERIMENTAL METHOD

CATALYSTS PREPARATION

The metal salts, oxide supports, and other chemicals used in this thesis are

summarized in Table 2.1.

Table 2.1. Summary of chemicals and materials

Commercial name Formular/abbr. Supplier

Tetraamineplatinum(II) hydroxide Pt(NH3)4(OH)2/PTAOH Sigma Chem Co.

Tetraamineplatinum(II) chloride Pt(NH3)4Cl2/PTACl Sigma Chem Co.

Tetraaminepalladium(II) chloride Pd(NH3)4Cl2/PdTACl Sigma Chem Co.

Hexaamminecobalt(III) chloride Co(NH3)6Cl3/CoHACl Sigma Chem Co.

HexaamminenickelIII) chloride Ni(NH3)6Cl3/CoHACl Sigma Chem Co.

Sodium chloride NaCl Fisher Scientific

Sodium bromide NaBr Fisher Scientific

Sodium citarte Na3Citrate Fisher Scientific

Sodium nitrate NaNO3 Fisher Scientific

Ammonium hydroxide NH4OH BDH, 5N

Sodium hydroxide NaOH Sigma-Aldrich

Aerosil 300 SiO2 Evonik

δ,θ- alumina Al2O3 Sasol

potassium hexachloroiridiate (II) K2IrCl6 Alfa Aesar

Hydrazine N2H4 Sigma-Aldrich

13

Commercial powder supports were used as recieved. All chemicals including metal

precursors, acid/base solution to adjust pH are presented in the following table.

2.1.1 Catalyst synthesis by SEA

a) Determination of support PZC

Deionized water at 25 mL was added to one gram support in incipient wetness mode.

pH of the thick slurry was measured using a spear-tip pH meter.

b) Uptake surveys

Metal uptake-pH surveys were performed in 100-mL polypropylene bottles containing

55 mL of precursors with concentration of 200 ppm. Initial pH was adjusted in the range

of 2 to 13 by NH4OH for cationic metal precursors:PTACl, PdTACl, NiHA, and CoHA).

6 ml solution was taken out for determining initial concentration of metal using ICP

analysis (Cmetal,initial). Depending on desired surface loadings (500 or 1000 m2/L), amount

of supports were specified.

Solutions containing support and metal precursors were stirred on an orbital shaker for 1 h

to establish maximum adsorption. Solution pH values were measured using a standard pH

meter and 5 ML aliquot of solution was filtered for ICP analysis to determine final metal

concentration after adsorption on support (Cmetal,final). The metal surface density, Γ, can be

calculated with the following equation:

14

Γ(μmol

L) =

(𝐶metal,initial−𝐶metal,final)(μmol

L)

SLm2

L

.

2.1.2 Catalysts by CEDI

Using maximum uptake determined from SEA, metal loadings for different

coverages were specified to prepare CEDI catalysts. pH of solution was adjusted based on

maximum uptake taken from SEA. 1 g of PTAOH was dissolved into 2.8 ml solution with

pH 11.5 using 1M NH4OH. Sodium chloride (NaCl), sodium bromide(NaBr), sodium

citrate(Na3Citrate), and sodium nitrate(NaNO3) were added into the solution to achieve

anion/metal atomic ratio of 1 in a centrifuge tube. After tapping tube well and thorough

mixing, the thick slurries were oven dried overnight to remove excess water. The dried and

crushed powder was then reduced for 1 hour in 20% H2/He at 250 oC at a ramp rate of 5

oC/min.

PTACl, PdTACl, NiHACl, and CoHACl catalysts were synthesized with CEDI

method on silica support. Metal loading and right pH were determined from maximum

uptake measured by SEA. All samples were dried in oven at 120 oC overnight except

PTACl which was dried under vacuum at room temperature overnight.

2.1.3 Catalysts by DI

Alumina supported Ir monometallic catalysts were prepared at 1% metal loading

by dry impregnation (DI) method. The K2IrCl6 precursor was dissolved into the quantity

of deionized water needed to just fill the pore volume of 1 grams of alumina support. The

thick slurry was dried overnight at room temperature in air and reduced in a flow of 20%

15

H2/He at 400 oC for 1 hour. Ir particles sizes were below (<1.5 nm) which is the limit of

detection through XRD.

2.1.4 Ag-Ir/Al2O3 catalysts by ED

The DI prepared 1.0% Ir/Al2O3 was utilized as the core metal catalyst for ED.

Electroless Deposition (ED) was applied to the DI prepared Ir catalysts to synthesize Ag-

Ir bimetallic nanoparticles. The procedure was taken from earlier work which was done to

form bimetallic Ag-Ir catalysts structures by electroless deposition44. As the silver

precursor potassium silver cyanide, KAg(CN)2, and as the reducing agent hydrazine (N2H4)

were used. pH of ED bath was kept at 10 which was above the PZC of support, to prevent

any electrostatic adsorption of anionic Ag ion on the support.

CATALYST CHARACTERIZATION

2.2.1 pH probe meter

A standard pH electrode (Orion 3-star benchtop) was calibrated by 3-point

calibration using three pH buffer solutions (pH = 4.0, 7.0, 10.0) with standard deviation of

97% or higher. For determination of support PZC, pH of the thick slurry was measured by

a spear-tip pH meter from Fisher Scientific.

2.2.2 N2 physisorption (BET surface area)

To measure surface area of support BET measurements were performed by an

automated adsorption system (ASAP, 2100, Micromeritics). 0.1gram support was degassed

at 150 ºC, 10-3 Pa. After degassing step, the sample was charged in analysis port by N2 gas

at T=77 K with relative pressure ranging from 0~0.99. Specific surface area was measured

16

using the linear relation between P/P0 and 1/ [v/ (P/P0-1)] with 8 points from P/P0 values

of 0-0.35. Surface area of silica and δ,θ-Al2O3 was measured to be 280 and 37 m2/g

respectively.

2.2.3 Inductively coupled plasma spectrophotometry (ICP-OES)

ICP-OES from PerkinElmer was used to measure metal concentration before and

after SEA or after washing samples. Metal loading on support was calculated from the

difference between initial and final concentration of metal in solution. For all metal

precursors used here, a 5ppm Y solution was used as internal standard for ICP analysis.

For optical alignment magnesium solution was used. Depending on the expected metal

amount in solution different concentrations of standard metal solutions were used for

calibration. To get accurate data each analysis was repeated 3x times using an auto-

sampler. Acceptable fit for calibration was chosen to be larger than 0.999. After calibration

a quality check (QC) was performed at medium concentration to make sure of accuracy of

calibration.

2.2.4 Temperature program reduction (TPR)

To determine reduction temperature of samples, TPR was carried on Micromeritics

AutoChem II 2920 using a thermal conductivity detector. Samples were first dried in He

flow at 100oC for 1 hour to remove any moisture in sample. 10% H2/He was used for TPR

analysis and TCD signals were recorded from 40oC to 500oC at a ramp rate of 5oC/min.

17

2.2.5 X-ray diffraction (XRD)

XRD experiment was performed on a Rigaku Miniflex-II with a silica strip detector

(D/teX Ultra). Cu Kα radiation was used with λ = 1.5406 Å, at 15 kV and 30 mA. Patterns

were recorded in the range of 20°−80° 2θ, at a scan rate of 2.0° 2θ/min. Fityk software was

used to fit metal diffractions patterns using Gaussian to obtain FWHM values. Scherrer

equation was used to calculate metal particle sizes.

2.2.6 Pulse chemisorption

Micromeritics Autochem II 2920 instrument was used to run chemisorption

measurements. To do experiment, desired samples were reduced in situ in flowing H2 for

2 hrs at reduction temperature and then purged with flowing Ar for 30 min. Sample was

cooled to 40°C in Argon flow. After cooling catalyst was then contacted with 10% O2/bal.

helium for 30 min to form O-covered metal species. Ar for 30 min was flowed to remove

any residual physisorbed O2. Then sample was dosed in pulses of 10% H2/Ar at 4 min

intervals till all oxygen adsorbed on surface reacted with H2 to form H2O and metal−H

species. Each metal had one assumed stoichiometry. Cobalt and Nickel were not able to

chemisorb H2 or O2. Size of particle were determined from chemisorption by assumption

of hemispherical geometry.

2.2.7 Temperature programmed desorption (TPD)

ASAP 2920 was used to perform TCD- H2 TPD measurements. To run the

experiment each sample was reduced in situ with the flow of 10%H2/bal. Ar at 200 C for

2 hrs followed by flowing Ar at the same temperature for 2 hrs. After sample was cooled

down to room temperature H2 was flowed over sample to populate hydrogen on the

18

particles surface. After saturating particles surface with hydrogen temperature ramping

started along with Ar flow. H2 desorption pattern was recorded by increasing temperature.

2.2.8 Scanning tunneling electron microscopy (STEM)

Z contrast images were obtained using an aberration-corrected JEOL 2100F

scanning transmission electron microscope (STEM) equipped with a 200Kv field emission

gun. High angle annular dark-field (HAADF) STEM images were acquired on a Fischione

Model 3000 HAADF detector with a camera length such that the inner cut-off angle of the

detector was 50 mrad. For each sample, approximately 500 particles were counted over all

images for determination of size distribution.

2.2.9 Fourier transform infrared spectroscopy (FTIR)

Nicolet Nexus 4700 spectrometer equipped with a mercury–cadmium–telluride B

(MCT-B) detector was used to perform in situ FTIR spectra. At room temperature FTIR

spectra were collected for each sample with 40 mg in single beam absorbance mode with

a resolution of 4 cm-1. The samples were reduced in 5% H2/bal. He for 1 h at 200 °C and

then cooled to room temperature. Background spectra were taken in Ar flow before Co

exposure. Sample was saturated with 1% CO and then was flushed with pure Ar for 20 min

to remove weakly bonded CO species and gas phase CO. Deconvolution of FTIR peaks

were done for all samples based on spectrum of 1% Ir/Al2O3 to obtain peak position, width,

eight, and area of overlapped peaks.

19

CATALYSTS EVALUATION (PROPYLENE HYDROGENATION AND

METHYL CYCLOPENTANE HYDROGENOLYSIS)

The monometallic Ir/Al2O3 and the series of bimetallic Ag–Ir/Al2O3 were evaluated

for hydrogenation of propylene (C3H6) to propane (C3H8). Catalysts were placed in a single

pass, 0.19” ID tubular packed bed reactor (316 stainless steel). Reactor was loaded with

0.006 g of catalyst diluted by 0.054 g of the Al2O3 supported on glass wool in the middle

of the reactor. All samples were treated in situ at 200 °C in 10%H2/bal. He for 2 hrs; then

400 ◦C for 4 hrs and finally cool down to reaction temperature. To monitor the reaction

temperature a thermocouple was inserted into the catalyst bed. All catalysts were reduced

in situ at 200 ◦C in 10% H2/balance He for 2 h and then cooled to 100 ◦C to start the reaction.

All lines between the reactor outlet and the inlet of the GC were held at 120 °C to prevent

any condensation. Prior Using mass flow controllers for adjusting gas flows the reaction

feed stream were determined to be 10% C3H6, 50% H2, balance He at a total flow rate of

100 SCCM. To maintain differential conversion conditions, it was tried to keep the

conversion low for all set of reactions. An automated, on-line Hewlett-Packard 5890 Series

II gas chromatograph with flame ionization detection was used for analyzing both reaction

feed and products at every 1.0 h over the full length of the run. Due to transient behavior

for the first several hours on line all the reaction data reported here were based on stable

catalyst performance after 15 to 20 h on line.

The kinetic measurements for methyl cyclopentane hydrogenolysis/ hydrogenation

were studied using the same reactor system. To vaporize MCP from liquid phase, a vapor

liquid equilibrium (VLE) system was used. VLE was encased in a jacketed shell with liquid

inlet and exit ports at the bottom and top of the shell, respectively, which was connected to

20

an ethylene glycol/H2O recirculation bath to maintain isothermal behavior to give

convenient MCP vapor pressure for reaction. Calibration for feed and all ring opening

products including 2-MP, 3-MP, C1-C4, and n-C6 were done separately.

22

CHAPTER 3

SYNTHESIS OF SUPPORTED METAL PARTICLES OF CONTROLLED

SIZE

INTRODUCTION

Noble metal heterogeneous catalysts play an important role in the chemical

manufacturing, energy-related applications and environmental remediation36,45,46. In

heterogeneous catalysts, it is often desirable to have high metal dispersion (ultra-small

nanoparticle sizes) especially for expensive noble metals, so maximize the number of

active sites per mass of metal. Strong Electrostatic Adsorption (SEA) 3 and Charge

Enhanced Dry Impregnation (CEDI) 29 give much smaller particle size with very narrow

size distribution than the DI preparations in which agglomeration is likely caused by the

accumulation of metal in the solution phase during drying.

Metal particle size is an important parameter to be considered while designing a

rational catalyst for different reactions since particle size not only influences the activity

but also plays a crucial role in determining selectivity. It has been shown that catalysts

prepared by the SEA method have the highest catalytic activity for different reactions.

Klaigaew et al. used SEA method to make small Co/SF particles to get highest activity for

FTS reaction 47, and Zhang et al. synthesized Pd/CNTs particles around 1 nm by SEA to

increase specific activity value for Suzuki coupling reaction 48. Cao et al. 49 used CEDI to

23

prepare Pd/Mo carbon supported bimetallics. Their results for the oxygen reduction

reaction indicated increased activity of the CEDI-prepared samples compared to DI-

prepared catalysts.

Several research groups have shown that the TOF in Fischer-Tropsch (FT) reaction

decreased for catalysts with cobalt particle sizes smaller than 10 nm 50. Karim et al.

indicated that NH3 decomposition activity over Ru/γ-Al2O3 increases with the particle size

larger than 7 nm51. Matthias Bauer et al showed that Pd/ CNT at larger nanoparticles (∼15

nm) were selective (88%) toward propene at full propylene conversion 52. Kumar et al.

used Pt-SBA-15, for dehydrogenation of propane and showed larger particles are more

selective for propene formation 53. Some other researches also indicated that activity and

selectivity was improved by increasing metal nanoparticle size 44,54. Therefore, ability to

tightly control the particle size with narrow distribution of supported metal catalysts is

extremely important.

This can be done with colloidal route which allows for better control of the

particle size with narrow size distribution. However, this method still has disadvantages

such as washing surfactant and protective agents in an appropriate solvent several times;

or use high temperatures in an inert environment to decompose or remove these foreign

compounds 55.

In a preliminary work, we showed that adding salt to charge enhanced electrostatic

adsorption (CEDI) preparations increased particle size in a controllable way, however, the

resulting size distributions were wide32. In the current work, we undertook a broader study

of salt addition to investigate the mechanism of size control and to produce larger sized

24

particles with tighter size distributions. XRD and STEM gave consistent results, halides

had greatest effect, nitrate and citrate least effect.

MATERIAL AND METHODS

3.2.1 Catalyst preparation

As supports, SiO2 Aerosil 300 was used as received. PZC, BET surface area and

pore volume of the support were 3.5, 280 m2/g and 2.8 ml/g respectively.

Tetraammineplatinum(II) hydroxide (PTAOH), purchased from Sigma Aldrich, were used

as precursors without any purification. All samples were prepared by the method of Charge

Enhanced Dry Impregnation (CEDI) with optimal initial and final pH values as reported in

previous work 35. PTAOH was dissolved into NH4OH solution at an initial pH of 11.5,

which resulted, after 2.8 ml of the solution had been contacted with 1 g of the silica support,

in a final pH of 10.0. The amount of metal precursor placed in solution corresponds to the

amount able to be adsorbed on the respective surface by electrostatic adsorption, or about

0.86 micromoles/m2 (4.5wt%) for amorphous silica56. Sodium chloride (NaCl), sodium

bromide (NaBr), sodium nitrate (NaNO3), and sodium citrate (Na3C6H5O7) were added into

the solution to achieve anion/Pt atomic ratio of 1. The thick slurry/paste were oven dried

at 100 ◦C overnight to evaporate excess water. The dried powder was then reduced for 1 h

in 10% H2/He at the optimal temperature of 250 °C determined by temperature

programmed reduction (TPR), with a 5◦C/min ramp rate.

25

3.2.2 Catalyst characterization

Powder XRD analysis was performed using a Rigaku Miniflex-II system with a

silica strip detector (D/teX Ultra) with Cu Kα radiation (λ = 1.5406 Å), operated at tube

voltage of 15 kV and a current of 30 mA. Scans were made in the 30°−80° 2θ range with

a scan rate of 2.0° 2θ/min. Pt particle sizes were calculated by Scherrer equation using the

FWHM value obtained from Gaussian fitting.

Temperature programmed reduction was performed on a custom TPx System fitted

with an Inficon Transpector 2 Mass Spectrometer. 100 mg of dried sample was loaded in

a tubular TPx cell (6 mm, Pyrex) with an expanded bulb space fitted with a fritted glass

disc for holding powder samples. No pretreatment was done before reduction. The TPR

analysis was proceeded with 10% H2/ balance with a ramp rate of 5 °C/min. While the

spectra from 1 to 50 amu were recorded, the signals for ammonia was primarily monitored.

Z-contrast STEM imaging for particle size determination in the materials was

conducted with a JEOL JEM-2100F HRTEM with CEOSGmbH hexapole STEM probe

corrector. Approximately 500 particles were counted to determine averaged particle sizes

(volume averaged Dv, surface averaged Ds and number averaged particle size Dn).

Extended X-ray absorption fine structure (EXAFS) experiment was performed at

Stanford Synchrotron Radiation Lightsource (California, USA). The X-ray was

monochromated by a double Si (111) crystal. The samples were pressed into pellets and

mounted on the sample stage. The X-ray absorption signal was measured by a Ge 13-

channel detector under fluorescence model. The absorption spectrum of the Pt metal foil

reference was also collected and utilized as the energy calibration reference. The XAS data

26

files were analyzed and fitted using the Demeter software package. The amplitude factor

S02 was obtained by fitting the reference spectrum with full coordination in the 1st shell

and used in subsequent data fitting wherein the coordination number, bond length and the

Debye Waller factor were the fitted variables. The fitted k range was 3~12 Å-1 and the

fitted R range was 1~4Å.

RESULTS AND DISCUSSION

A 4.5 wt% Pt/SiO2 was made by CEDI using pH-adjusted PTAOH solution. XRD

characterization and STEM image of this sample is shown in Figure 3.1. Pt particle size

determined form XRD (Figure 3.1a) was 1.2 nm which agrees reasonably well with the

STEM volume size average (Figure 3.1b). The broad Pt3O4 (210) peak appearing in the

XRD pattern of Pt/SiO2 samples is due to spontaneous oxidation of small Pt nanoparticles

in ambient57.

Figure 3.1. (a) XRD profile and b) STEM image and particle size distribution of 4.5

wt % Pt/SiO2 (PTAOH) prepared by CEDI.

27

NaCl was added to the sample with the atomic ratio of Cl-/Pt=1 prepared by the

same procedure. XRD pattern of the reduced material is shown in Figure 3.2. With addition

of residual ion, Pt diffraction peaks became discernable. In Figure 3.2a Pt particle size

increased from 1.2 nm for the Cl- free control to 4.5 nm for the Cl- sample. STEM

characterization of the chloride containing silica supported sample supports the wider

distribution as well as the particle size trend (Figure 3.2b). Remaining NaCl in sample was

detected in the XRD pattern.

Figure 3.2. (a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing chloride

prepared by CEDI.

Pt (111) peak fitting in Figure 3.3 suggests that in the presence of chloride the particles

have bimodal size distribution.

NaCl was replaced by NaBr with the atomic ratio of Br-/Pt=1 in the pH-adjusted

PTAOH solution prepared by the same CEDI method. The XRD pattern and STEM image

of the reduced particles are shown in Figure 3.4. Pt particle size increased from 1.2 nm for

the Br- free control to 6.5 nm for the Br- sample shown in Figure 3.4a (deconvolution in

Figure 3.5).

28

Figure 3.3. Deconvolution of Cl-/Pt=1, PTAOH as precursor on silica support.

Figure 3.4. (a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing

bromide prepared by CEDI.

With the addition of bromide, not only is the average size larger, but the size distribution

is much broader (Figure 3.4b). The effect of bromide was stronger than that of chloride on

growth of particles and their polydispersity.

The presence of Cl-or Br-at the adsorption layer might reduce the strength of the precursor-

support interaction by locally increasing ionic strength. Therefore, anion of Citarte3- with

higher negative charge was chosen to see if higher ionic strength with much larger particles

will be obtained. Here, Na3Citarate was added to the solution with the atomic ratio of

29

Citrate3-/Pt=1 and catalysts sample was prepared by the same procedure. X-ray diffraction

peak and STEM image with size distribution analysis of the reduced sample.

Figure 3.5. Deconvolution of Br-/Pt=1, PTAOH as precursor on silica support.

Figure 3.6. (a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing citrate

prepared by CEDI.

is shown in Figure 3.6. As shown in Figure 3.6a Pt particle was estimated around 1.6 nm

with the appearance of Pt3O4 of 0.9 nm with peak fitting shown in figure 3.7. STEM image

30

Figure 3.7. Deconvolution of Cit3-/Pt=1, PTAOH as precursor on silica support.

of the sample in Figure 3.6b shows relatively tight size distribution compared to chloride

and bromide samples. This indicates that increasing ionic strength is not the main reason

for sintering of nanoparticles containing chloride or bromide.

CEDI method was extended to synthesize Pt nanoparticles using NaNO3. Silica

supported Pt particles was made using pH-adjusted PTAOH solution for NO3-/Pt=1. XRD

characterization of the sample is shown in Figure 3.8a.

Figure 3.8. (a) XRD pattern, and (b) STEM image of 4.5 wt% Pt/SiO2 containing nitrate

prepared by CEDI.

Pt particles are small around 2.3 nm with broad Pt3O4 peak. STEM analysis on the sample

was performed to gauge the effect of nitrate on particle size and on breadth of size

31

distribution. Representative STEM Z-contrast image and particle size distribution is shown

in Figure 3.8 b. The small average size of the NO3- sample with tight size distribution was

confirmed. XRD deconvolution for this sample is shown in Figure 3.9.

Figure 3.9. Deconvolution of NO3-/Pt=1, PTAOH as precursor on silica support.

To explore the number of species in the anion added samples mass-temperature

programmed reduction (Mass-TPR) was performed. Since ammonia is the indictor of

decomposition of the metal precursor during reduction, only evolution of NH3 was

evaluated. Prior to NH3-TPD experiments, the physically adsorbed moisture was desorbed

from the sample with He flow (25 mL/min) at 25 °C for 30 min. Figure 3.10.a shows the

temperature-programmed desorption of ammonia (NH3-TPD) profiles of Pt/SiO2

(PTAOH) catalyst. Reduction of the silica supported Pt precursor shows two peaks

centered at 240 and 400 °C. The first peak around 240 °C belongs to decomposition of

PTAOH during reduction. To investigate the source of second peak, the temperature-

programmed desorption of ammonia (NH3-TPD) of SiO2 was done in 10%H2/bal He. SiO2

support showed only one peak corresponding to weak acidic sites with Tmax at 220 °C

32

(shown in Figure 3.11) 58–61. Therefore, the second peak at the temperature of 400 °C was

assigned to reverse spillover of adsorbed ammonia on reduced Pt. Figure 3.10(b,c) shows

NH3 TPD of the reduced sample for successive TPD experiments in which second peak

disappeared after second TPD.

Figure 3.10. (a) reduction of dried 4.5% Pt(NH3)4(OH)2 in 10% H2, (b) He flow over the

previously reduced sample, (c) Repeating previous step.

Figure 3.11. Temperature-programmed desorption of ammonia(NH3-TPD) profile of

silica (SiO2).

33

Figure 3.12 shows the temperature-programmed desorption of ammonia (NH3-

TPD) profiles of NaCl, NaBr, Na3Citrate, and NaNO3 samples. On NH3-TPD curve of

applied chloride, and bromide in Figure 3. 12 a, and b multiple desorption peaks of

ammonia centered at 170, 230, 350, and 450°C were observed. The low temperature peaks

of 170, and 230°C belong to reduction of two different precursor species. The high

temperature peaks were assigned to reverse spillover of NH3 from reduced Pt to silica.

Figure 3.12 c, and d for nitrate and citrate samples show only desorption peaks similar to

anion free sample. First peak corresponds to decomposition of precursor at 230 °C and

second peak belongs to reverse spillover of ammonia from Pt to silica at temperature of

400 °C. Basolo62 using Grinbergs polarization theory showed that the metal(Pt)- ligand

Figure 3.12. Mass - TPR on dried samples prepared by CEDI by looking at the evolved

NH3 (mass 15) during reduction, (a) Cl-/Pt=1, (b) Br-/Pt=1, (c) NO3-/Pt=1, and (d)

citrate3-/Pt=1 by comparing them to anion free sample.

34

(NH3) bond strength can get weaker by polarization of Pt (II) by Cl- anion. In this way

NH3 group can be easily replaced by Cl-. The ligand exchange of the 5 wt.% Pt/SiO2

material prepared by CEDI was estimated using the Pt–ligand coordination number

obtained from EXAFS. Pt(NH3)4(OH)2 + SiO2 (physical mixture) as sample 1 and

Na2PtCl4+SiO2 (3.5% Cl physical mixture) as sample 2 were used as reference to get

amplitude of Pt-N and Pt-Cl, respectively. Coordination number of Pt-N in sample 1 and

Pt-Cl in sample 2 were both fixed as 4, due to their square planar structure. Both Pt-N and

Pt-Cl bond were used for the fitting of Pt(NH3)4(OH)2/SiO2 (CEDI) as sample 3 and

Pt(NH3)4(OH)2 + NaCl / SiO2 (1.8wt% Cl, CEDI) as sample 4, so that sample 3 can be seen

as reference compared with sample 4. SO2 of fitting of sample 3 and sample 4 were set as

0.0025. Coordination numbers obtained from EXAFS for the dried samples is given in

Table 3.1.

Table 3.1. EXAFS fitting data for Pt samples on SiO2.

Sample

# Pathway CN s.s

1 Pt(NH3)4(OH)2+SiO2(Physical mix) Pt-N 4a 0.0043

2 Na2PtCl4+SiO2 (Physical mix) Pt-Cl 4a 0.0035

3

Pt(NH3)4(OH)2+SiO2 (CEDI) Pt-N 4.3±0.241

0.0025b

Pt-Cl 0.1±0.157

4 Pt(NH3)4(OH)2+NaCl+ SiO2(CEDI) Pt-N 3.5±0.262

0.0025b

Pt-Cl 1.1±0.276 Note: a. coordination number was constrained at 4; b. ss was constrained at 0.0025.

Amplitude of sample 1 and sample 2 were 0.846 and 0.923 respectively. Amplitude

of sample 3 and sample 4 used the amplitude gained from sample1 and sample 2. Fourier

35

transforms of the EXAFS spectra of the samples are shown in Figure 3.9. According to

Figure 3.13 and Table 3.1., it's clear that Pt-Cl coordination for (sample#2) shows a peak

larger than Pt-N coordination (sample #1). In the sample#4 which is Pt(NH3)4(OH)2 +

NaCl, we can find a component at ~1.9 °A, which could be assigned to Pt-Cl coordination.

Figure 3.13. EXAFS: the fitting plot of (a) Pt(NH3)4(OH)2 + SiO2 (physical mixture), (b)

Na2PtCl4+SiO2 (physical mixture), (c) Pt(NH3)4(OH)2 / SiO2 (CEDI), and (d)

Pt(NH3)4(OH)2 + NaCl / SiO2 (CEDI), dashed line shows Pt-ligand bond length.

According to Figure 3.13 and Table 3.1., it's clear that Pt-Cl coordination for

(sample#2) shows a peak larger than Pt-N coordination (sample #1). In the sample#4 which

36

is Pt(NH3)4(OH)2 + NaCl, we can find a component at ~1.9 °A, which could be assigned

to Pt-Cl coordination. Apparently, ligand exchange between NH3 and Cl- occurs according

to mechanism shown in Figure 3.14. After ligand exchange the overall charge of Pt

precursor decreases from 2+ to 1+. Therefore, the interaction of precursor with support is

not that strong anymore and that gives rise to sintering and wide distribution of metal sizes.

Figure 3.14. Schematic mechanism of NH3 ligand exchange with Cl-.

CONCLUSION

XRD results shows increasing particle size for all the anions, especially Br- and Cl.

STEM images show that accompanying the larger average size is a wider particle size

distribution. This is related to different metal precursor species formed during the

impregnation and drying. This hypothesis is consistent with the evolved NH3 mass at

different temperatures during reduction which suggests multiple adsorbed Pt species in

presence of anions like Pt(NH3)3Cl or Pt(NH3)3Br.

37

CHAPTER 4

NANOPARTICLE SYNTHESIS VIA ELECTROSTATIC ADSORPTION

USING INCIPIENT WETNESS IMPREGNATION

INTRODUCTION

Supported metal catalysts for the production of fuels, commodity and specialty

chemicals, and pharmaceutical drugs, as well as for energy production and pollution

abatement, underpin the global economy and continually upgrade the world’s standard of

living. In many applications it is desirable to synthesize the smallest possible metal

particles on the catalyst support so as to maximize the number of active sites per mass of

metal. The predominant method of synthesizing metal particles on oxide and carbon

supports is incipient wetness impregnation (or pore filling or dry impregnation) in which

the pore volume of a catalyst support is just filled with a solution containing the desired

amount of metal precursor63. The thick paste is dried and pretreated in oxygen (air) and/or

hydrogen to remove the ligands and reduce the metal to its active, zero valent state. While

this method is simple and cheap, the problem with dry impregnation is that the resultant

metal particles are relatively large with wide particle size distributions55. This is due, in

most cases, to a lack of interaction between the metal precursor and the support surface,

such that metal precursors agglomerate in the liquid phase during drying 36. The resulting

metal utilization is relatively poor. A recent review of Pt nanoparticle preparation on silica,

38

aluimina, titania, and carbon supports in1500 papers from the catalysis literature the past

three years revealed that the average particle size from impregnation was 10 nm, with

average particle size distributions as large or larger than the particle size itself 55.

Based on the hypothesis that well dispersed metal nanoparticles require a well-dispersed

metal precursor, and starting from the pioneering work of Brunelle 21 and Schwarz 64,

we have developed the method of Strong Electrostatic Adsorption (SEA) 56,65,66 in which

coulombic metal precursor-support interactions are induced between a charged surface and

an oppositely charged metal precursor complex. This is achieved by controlling the final

pH of the impregnating solution relative to the point of zero charge (PZC) of the support;

values above the PZC of an acidic support like silica (PZC = 4) the surface hydroxyl groups

are deprotonated, the surface is negatively charged, and cationic complexes such as metal

ammines are strongly adsorbed. For a higher PZC support such as alumina (PZC = 8), at

acidic pHs the surface hydroxyl groups are protonated and positively charged, and anionic

precursors such as metal chloride complexes are electrostatically adsorbed. A recent

review of Pt nanoparticle preparation on silica, aluimina, titania, and carbon supports

in1500 papers from the catalysis literature the past three years revealed that the average

particle size from impregnation was 10 nm, with average particle size distributions as large

or larger than the particle size itself, while for SEA the average particle size was 1.8 nm

with standard deviation typically about 25% of the particle size55.

In the laboratory, SEA is normally employed with a great excess of solution for

experimental convenience; thin slurries minimize pH shifts and make sampling of the

liquid metal concentration easier. In large scale catalyst synthesis, however, the use of thin

slurries necessitates an additional filtration step, and any excess metal is lost in the filtrate.

39

There is no reason in principle why electrostatic attraction cannot be induced in the thick

slurries employed in dry impregnation; our preliminary work with platinum over silica,

aluimina, and carbon supports has shown that a simple modification of the dry

impregnation recipe – adding enough acid or base to charge up the support surface – results

in a drastic reduction of nanoparticle size29. We have termed this method “Charge

Enhanced Dry Impregnation” (CEDI). In the present work, we first extend CEDI to other

noble and base metals (Pt, Pd, Co, and Ni) over a silica support. We additionally refine

the method to show that the counterions (balancing salts such as chloride from platinum

tetraammine chloride, (NH3)4PtCl2) can be removed by washing. If left in the CEDI slurry,

chloride leads to larger particles 32. The removal of chloride counterions by washing

renders small particle size.

MATERIAL AND METHODS


Tetraammineplatinum(II) hydroxide (PTAOH), Tetraammineplatinum(II) chloride

(PTACl), Tetraamminepalladium(II) chloride (PdTACl), Hexamminecobalt(III) chloride

(CoHACl), and Hexaamminenickel(II) chloride (NiHACl), purchased from Sigma Aldrich,

were used as precursors without any purification. As support, SiO2 Aerosil 300 was used

as received. The PZC, BET surface area, and water accessible pore volume of the support

were 3.5, 280 m2/g, and 2.8 ml/g respectively. A spear tip pH probe was used for pH

measurements at incipient wetness.

All samples were prepared by the CEDI method with optimal initial and final pH

values as reported in previous work 35: each precursor was dissolved into NH4OH solution

40

at an initial pH of 11.5, which resulted, after 2.8 ml of the solution had been contacted with

1 g of the silica support, in a final pH of 10.0. The amount of metal precursors placed in

solution was calculated relative to one monolayer of those precursors as determined from

previous work. A monolayer of PTAOH, PTACl, PdTACl, NiHACl and CoHACl

respectively are 0.86 micromoles/m2 (4.5wt%), 0.80 micromoles/m2 (4.2 wt%)56, 0.90

micromoles/m2 (2.5 wt%), 1.4 micromoles/m2 (2.2 wt%), and 1.3 micromoles/m2 (2.1

wt%) 23. These values correspond to a close-packed layer of precursors retaining one or

two hydration sheaths55. For each metal, precursor concentrations were employed to give

metal loadings of 0.25, 0.5, 1.0, 1.5 and 2.0 monolayers.

After impregnation all samples except PTACl were dried in ambient air at 120 °C

overnight. The PTACl/silica samples were dried overnight under vacuum at room

temperature since Pt(NH3)4Cl2 precursor decomposes to a neutral Pt(NH3)2O species when

drying at 120°C or more 67. This species could be easily filtered out during washing step

which causes more than 70% Pt loss regardless of the initial metal coverage. The dried

samples were reduced for 1 h in 10% H2/He with the ramp rate of 5°C/min at the reduction

temperatures determined by temperature programmed reduction (TPR). The reduction

temperatures were based on the minimum reduction temperatures from TPR studies of

PTAOH, PTACl, PdTACl, NiHACl and CoHACl were 250 °C, 300 °C, 200 °C, 500 °C

and 450 °C, respectively23,29,56.

A set of samples was prepared with residual chloride removed. To do so, the CEDI-

derived dried, unreduced samples were re-ground to a fine powder and washed using a

dilute NH4OH solution (300 mL/g catalyst) with the pH=10.5 so that the final pH after

41

sample addition was at or close to the final pH of adsorption. The sample and wash solution

were agitated for 10 minutes by means of an orbital shaker, at a rate of 120 rpm. After the

washing, a 5cc aliquot was taken for metal loss analysis by ICP. The post-wash sample

was filtered, dried and reduced at the same condition as for the unwashed samples. An

additional series of samples was washed immediately after the impregnation, that is,

without the drying step.

4.2.2 Catalyst characterization

Powder XRD analysis was performed on a Rigaku Miniflex-II equipped with a

silicon strip detector (D/teX Ultra) and Cu Kα radiation (λ = 1.5406 Å) source, operated at

15 kV and 30 mA. Scans were made in the 2θ range of 30°−80°, with a scan rate of 2.0°

2θ/min; metal diffraction patterns were deconvoluted using Gaussian function to achieve

FWHM values. Average particle sizes were calculated using the Scherrer equation.

The weight percentages of metal for each catalyst were determined by measuring

liquid concentrations of metal in the impregnating solution and the washing solution by the

inductively coupled plasma-optical emission spectroscopy (ICP-OES) with a Perkin Elmer

Optima 2100DV.

Z contrast images were obtained using an aberration-corrected JEOL 2100F

scanning transmission electron microscope (STEM) equipped with a 200Kv field emission

gun. High angle annular dark-field (HAADF) STEM images were acquired on a Fischione

Model 3000 HAADF detector with a camera length such that the inner cut-off angle of the

detector was 50 mrad.

42


A series of silica supported Pt nanoparticles were initially synthesized by CEDI

from the Cl- free precursor, PTAOH. Powder XRD characterization of these samples is

shown in Figure 4.1. Samples were prepared at coverages of 0.25 (1.1%), 0.5 (2.3%), 1

(4.5%), 1.5 (6.8%), and 2 (9.0%) monolayers. The resulting size of Pt phase after reduction

and exposure to ambient air was estimated from the deconvoluted Pt (111) and Pt3O4 (210)

peaks. These are shown in Figure 4.2.

Figure 4.1. XRD patterns for PTAOH and PTACl samples: (a) CEDI – prepared series

of PTAOH, (b) CEDI – prepared series of PTACl, (c) DI - prepared series of PTACl,

and (d) Cl- free samples from CEDI – prepared PTACl samples after wash with solution

at pH 10.5.

43

In Figure 4.1(a) for all the loadings of Pt made by PTAOH both metal and oxide

forms of Pt can be seen (deconvolution in Figure 4.2). According an earlier work by

Banerjee et.al, oxidation of small Pt nanoparticles occurs in ambient 57. With increased

amount of platinum from 1.1 to 9.0 wt%, particle size of the metallic Pt cores increased

from 1.1 to 1.5 nm and the Pt oxide shells to a lesser extent from 1.0 to 1.3 nm. These are

in good agreement to the core and shell sizes seen earlier on silica. 48

Figure 4.2. Peak fitting for (a) 0.25 ML, (b) 0.50 ML, (c) 1.0 ML, (d) 1.5 ML, and (e) 2.0

ML Pt from Pt(NH3)4(OH)2 on SiO2 prepared by CEDI

44

In a parallel series of experiments, Pt samples were prepared by CEDI from PTACl.

XRD patterns of these samples are shown in Figure 4.1(b) and deconvolutions of them are

given in Figure 4.3. With increased wt loading of Pt, diffraction peaks became sharper.

Figure 4.3. Deconvolution of XRD peaks for (a) 0.25 ML, (b) 0.50 ML, (c) 1.0 ML, (d) 1.5 ML,

and (e) 2.0 ML Pt from Pt(NH3)4Cl2 on SiO2 prepared by CEDI.

The deconvolutions in Figure 4.3 shows that particles have bimodal size distribution for all

coverages 1.0 ML and above. To compare the effect of adjusting pH on particle size in

CEDI with no pH adjustment, a third series of PTACl samples was prepared by DI. The

XRD patterns of these reduced materials are shown in Figure 4.1 (c) and the deconvolutions

are shown in Figure 4.4. At equivalent metal loadings, the Pt nanoparticles made by DI are

45

larger than particles made by CEDI. The DI particles have a bimodal size distribution at

low Pt weight loading of 0.25ML.

Figure 4.4 XRD peak deconvolution for (a) 0.25 ML, (b) 0.5 ML, (c) 1.0 ML, (d) 1.5 ML, and

(e) 2.0 ML from Pt(NH3)4Cl2 on SiO2 prepared by DI.

.

46

Zhu et al. prepared nanoparticles at 2wt% Pt/SiO2 using PTACl by CEDI, and DI

methods. They reported particle sizes of <3 nm and 7.9 nm, respectively29. From our XRD

results, it can be estimated that at Pt weight loading around 2.1 wt%, CEDI and DI gave

particle sizes of 4.9 and 7.8 nm, respectively. The slight difference in CEDI results is from

difference in surface density. In their work surface density was around 0.3 µmole/m2 which

is lower than ours around 0.4 µmole/m2. It also can be related to sensitivity of XRD

instrument. These results show that basifying the precursor solution gives rise to

electrostatic interaction between precursor and support which results in small particles after

pretreatment. However, the addition of more metal to the solution in the form of a PTACl

precursor may also increase the concentration of counterions (e.g. Cl-) associated with the

complex35. This will increase the ionic strength of the solution for thicker slurries such as

CEDI and DI methods and increase particle size.

Therefore, it is expected that particle size can be lowered by eliminating the residual

chloride by washing the PTACl samples after impregnation. To do this, dried and

unreduced PTACl samples prepared by CEDI were washed using ammonium hydroxide

solution at pH=10.5. XRD patterns of the post washed and reduced samples revealed very

small particles (<1nm); Figure 4.1 (d) and deconvolutions are shown in Figure 4.5. The

broad Pt3O4 (201) peak appearing in the XRD patterns of these samples is again due to

spontaneous oxidation of very small Pt nanoparticles. It is notable that the XRD peaks are

even broader (the nanoparticles are even smaller) for the PTACl CEDI/washed samples in

Figure 4.1c than the corresponding PTAOH CEDI samples in Figure 4.1a.

47

Figure 4.5. X-ray diffraction pattern with deconvolution for different coverages of Pt

after washing PTACl samples for (a) 0.5 ML, (b) 1.0 ML, (c) 1.5 ML, and (d) 2 ML

To corroborate the size distribution of particles for the chloride free samples, STEM

characterization of these samples with the coverage of 1 ML was done. STEM image in

Figure 4. 6(a) shows tight size distribution for nanoparticles made from PTAOH precursor

by CEDI. PTAOH samples prepared by SEA also gives very small particles with tight size

distribution, Figure 4.6(b). Removing chloride from sample during washing step results in

small particle with the tight size distribution, shown in Figure 4.6(c). All STEM images for

these three chloride free samples support the tight size distribution which agrees reasonably

well with the XRD estimates.

48

Figure 4.6. STEM images and corresponding particle distributions of reduced Pt catalyst

samples for: (a) 1ML PTAOH prepared by CEDI, (b) 1ML PTAOH prepared by SEA, (c)

2ML post-washed samples from PTACl prepared with CEDI.

The CEDI method was extended to synthesize other metal nanoparticles such as

Pd, Co, and Ni using, PdTACl, CoHACl, and NiHACl precursors with silica as support.

Powder XRD characterization of these nanoparticles is shown in Figure 4.7. With Pd wt%

increasing from 0.7 to 5, particle size increased from 2.4 nm for 0.7% Pd to 9.8 nm for

5.0wt% Pd sample (Figure 4.7 (a)). Size was estimated from Pd (111) peak. These values

49

Figure 4.7. XRD patterns of silica supported catalysts, CEDI-prepared before wash and

after wash: (a,b) PdTACl, (c,d) CoHACl, (e,f) NiHACl. pH of solution was adjusted at

10.5.

smaller than that in the literature survey68–76. Deconvolution of the XRD patterns (Figure

4.8) suggests that the particles have a bimodal size distribution, with no evidence of PdO.

50

Figure 4.8. X-ray diffraction pattern with deconvolution for different coverages of Pd frorm

PdTACl precursor before wash (a) 0.25 ML, (b) 0.5 ML, (c) 1.0 ML, (d) 1.5 ML, and (e) 2.0 ML.

For Co catalysts, XRD results confirm a complex composition of nanoparticles

(Figure 4.7 (b)). Metallic cobalt for 0.25 (0.5wt% Co) and 0.5ML (1.1wt% Co) is detected

51

only as a fcc phase at 2θ = 44.6° with particle size of smaller than 2.5 nm. For the samples

with higher amount of Co with significantly larger particle size, one additional peak

appears at 2θ = 44.4° which may be attributed to hcp phase of metallic cobalt 77–79. With

Co wt% increasing from 2.1 to 4.2, fcc Co particles increased from 4.2 to 7.5 nm and at

the same time hcp Co particles increased from 2.2 to 2.3 nm (deconvolution in Figure 4.9).

Figure 4.9. Peak fitting of (a) 0.5 ML, (b) 1.0 ML, (c) 1.5 ML, and (d) 2 ML Co from

CoHACl precursor on silica prepared by CEDI.

The Co catalyst prepared via CEDI leads to relatively large particle size. However,

the literature shows that those catalysts prepared via IWI result in even larger particles with

the size of >20 nm, except for the case in which Co(CH3COO)2 is applied as the

precursor80–91.

52

Using CEDI with Ni yields nanoparticles at the average size of 2.1 nm for 0.57%

to 17.4 nm for 4.60% Ni (Figure 4.7 (c)). Deconvolution of the Ni (111) peak at 2θ =

4.45° (Figure 4.10) gives two different particle sizes, indicating bimodal size distribution.

Figure 4.10. X-ray diffraction pattern with deconvolution for different coverages of Ni

from NiHACl precursor before wash (a) 0.5 ML, (b) 1.0 ML, (c) 1.5 ML, and (d) 2 ML.

Ni catalysts synthesized via CEDI show smaller sizes than have been reported to date with

impregnation with the average of 25nm 92–100.

53

In another series of experiments chloride containing catalysts were washed and

filtered to remove Cl- counterion from the samples. A dilute NH4OH solution (300 mL/g

catalyst) with the pH value adjusted so that the final pH after sample addition was at or

close to the adsorption pH of 10.5 was used. The XRD profiles of the washed and then

dried and reduced samples confirm the presence of very small metal particles (<1.0 nm;

Figure 4.3 d, e, f). Jiao et al. applied SEA method to the preparation of supported metal

catalysts using the noble and base ammine complexes PdTACl, CoHACl, and NiHACl. By

their method at 2.2wt% Pd, 1.8wt%Co, and 1.6wt% Ni corresponding particle sizes were

1.6 nm, 4.3 nm, and 1.7 nm, respectively 23. This suggests that washing CEDI-prepared

samples leads to small particles similar to SEA even at higher weight loading of metal

without using a lot of solution. Summary of particle size before and after wash with the

amount of loss during washing step is shown in Figure 4.11.

ICP analysis revealed negligible metal loss for metal coverages up to 1ML after

washing. This result is consistent with the theory of strong electrostatic interaction between

the precursor and the support for the first deposited layer. For Pt and Pd metals at higher

coverages of 1.5 and 2.0 MLs, loss increases to about 15 and 30% respectively (Figure 4.11

(a,b)). Cobalt particles show 17% and 22% loss for 1.5 and 2.0 MLs correspondingly

(Figure 4.11 (c,d)). Nickel, unlike other metals during washing, shows almost no loss even

at high coverages; at present there is no explanation for the unusually strong interaction

between the Ni precursor and the support. The higher amount of metal loss after 1 ML

confirms that beyond the first layer deposition, electrostatic interaction is not strong enough

to keep the precursor attached to the support.

54

Figure 4.11. Metal particle size before and after wash with the metal loss (%) for the

silica supported catalysts, CEDI-prepared: (a) Pd(PdTACl)/silica, (b) Co(CoHACl)/silica,

(c) Ni(NiHACl)/silica.

Other experiments were performed in attempts to simplify the synthesis procedure;

these explored first, using a wash step after DI, and second, eliminating the drying

procedure between the CEDI and washing steps. As a baseline, CEDI versus DI was

performed for all four metals at 1 ML precursor deposition and XRD results are shown in

Figures 4.12 a, b, c, and d for Pt, Pd, Co and Ni respectively. From this comparison we

see that CEDI yields smaller particle size than DI. At DI conditions, the surface of the

support is negligibly charged, due to the tremendous buffering capacity of the support 33,

whereas for CEDI, the surface is strongly charged, and the precursors are deposited at the

pH of the strongest electrostatic attraction. The average particle size of Pt via

55

Figure 4.12. XRD of 1 ML prepared by DI and CEDI for before and after wash of (a,b)

PTACl, (c,d) PdTACl (e,f) CoHACl, (g,h) NiHACl. pH of solution was 10.5.

56

CEDI is about 5.9 nm which is smaller than DI with the size of 8.3 nm at the same metal

loading of 4.2% (Figure 4.12 a). The Pd catalyst (Pd% = 2.5%) prepared via CEDI has an

average particle size of around 5.1 nm, which is smaller than the size obtained by DI with

9.3 nm (Figure 4.12(b)). The Co catalyst prepared via CEDI have particle size of 4.2 nm

compared to DI with the size of 5.6 nm (Figure 4.12 (c)). Ni catalysts synthesized via CEDI

also show smaller sizes (5.6 nm) than DI (7.2 nm) (Figure 4.12 (d)).

Next, it was attempted to redisperse DI-applied precursors at 1 ML by a washing

step. When neutral pH solutions were utilized, the majority of metal was lost (not shown

in Figure 4.12 but listed in Table 4.1). However, when a high pH wash (which resulted in

a wash pH slurry of about 10.5) was applied after the DI and drying steps, metal retention

was much higher (Table 4.1), though not as high as following CEDI. XRD patterns of

high pH post washed samples prepared by CEDI and DI using PTACl, PdTACl, CoHACl,

and NiHACl are shown in Figure 4.12e, f, g, and h respectively. For all four metals the

particle sizes of the high pH washed and reduced DI samples were virtually as small as

CEDI- derived particles. Apparently, the precursors do not degrade after DI followed by

drying. When electrostatically adsorbed, dried precursors exist in an aqueous-like

environment in which electrostatic attraction can exist 101. Presumably, they maintain their

ligand and hydration sheaths even after being agglomerated during DI and drying, and the

Table 4.1. Metal loss after washing dried samples prepared by DI and CEDI methods.

Initial Coverage/wash pH Pt loss (%) Pd loss (%) Co loss (%) Ni loss (%)

1.0 ML DI, pH 6.5 85 72 87 23

1.0 ML DI, pH 10.5 12.0 9.5 9.6 0.0

1.0 ML CEDI, pH 10.5 5.0 2.0 9.0 0.0

57

presence of the high pH wash solutions allow the precursors to redissolve and adsorb over

the charged silica surface during the washing step. That is, electrostatic adsorption is

achieved following the initial DI deposition. The advantage of the initial CEDI deposition

is the somewhat higher retention of three of the four metals upon washing, notably, a 2%

loss for Pt CEDI vs 10% for DI, and a 5% loss for Pd CEDI vs. 10% for DI. At lower

metal loadings these differences would likely become smaller.

The second exploration of streamlining the synthesis was to determine the impact

of the drying step after CEDI. If electrostatic adsorption is sufficiently strong, it may be

possible to eliminate the drying step after the initial impregnation. Thus, four weight

loading or each metal precursors were deposited by CEDI, and after one hour contact were

placed in the pH 10.5 washing solutions. Metal loss in the wash solutions was measured

by ICP and results are summarized in Table 4.2.

Table 4.2. Washing samples as wet without drying.

Initial Coverage Pt loss (%) Pd loss (%) Co loss (%) Ni loss (%)

0.25 ML 0.0 2.0 0.0 0.0

0.5 ML 1.5 4.0 0.0 0.0

1.0 ML 2.0 2.0 55.0 2.3

1.5 ML 46.0 56.0 96.0 2.0

With the exception of cobalt, metal loss up to 1 ML was about what it was for the

dried samples. At 1.5 ML, however, the losses became significantly higher those with

drying which are seen in Figure 4.1 to be about 15% for all metals except Ni. Therefore,

samples with loadings higher than 1 ML require drying before washing.

58

CONCLUSION

In this chapter it has been demonstrated that charge enhanced dry impregnation of a silica

support with a wide variety of noble (Pt, Pd) and base (Co, Ni, Cu) metals from the

respective tetra- or hexa-ammine chloride precursors yields smaller particles than dry

impregnation at the same metal loading, since electrostatic interactions can be induced at

the incipient wetness condition. Metal particle size can be considerably decreased, and

approaches that of strong electrostatic adsorption, if the chloride ions can be eliminated

either by 1) starting with a hydroxide ammine complex such as Pt tetraammine hydroxide,

2) removing chloride by washing a CEDI deposition with a high pH solution (to maintain

electrostatic interactions), or even 3) washing a DI impregnation with a high pH solution

(to induce electrostatic interactions and to remove chloride). For precursor loadings greater

than 1 ML, a drying step after the CEDI deposition and before the washing step minimizes

metal loss.

It is possible to extend this method to other systems, including anionic precursor adsorption

over high PZC supports. In view of the Chapter 3 results showing that nitrate ions has a

much weaker effect on particle size growth than chloride ions, the use of metal ammine

nitrate salts may work better than metal ammine chloride salts to give small particles via

CEDI with no washing step.

59

CHAPTER 5

EVALUATION OF AG–IR/AL2O3 BIMETALLIC CATALYSTS PREPARED

BY ELECTROLESS DEPOSITION FOR HYDROGENATION OF

PROPYLENE AND HYDROGENOLYSIS OF METHYL CYCLOPENTANE

INTRODUCTION

In very recent work (submitted), Wong et al.102 have characterized Ag-modified alumina-

supported Ir catalysts prepared by electroless deposition (ED) of Ag on monometallic Ir

catalysts. For this study, they used highly dispersed, monometallic Ir particles on δ,θ-

alumina catalyst synthesized by SEA as core and Ag as shell metals. Ag-Ir catalysts were

characterized by H2 chemisorption, x-ray diffraction, scanning transmission electron

microscopy (STEM) and computational methods. After thermal treatments up to 800°C,

monometallic Ir and monometallic Ag catalysts sintered to average XRD particle sizes of

19 nm and 33 nm, respectively. The addition of a partial Ag shell on Ir cores caused a

suppression in extent of sintering for both the Ag and Ir components compared to the

monometallic analogs. H2 chemisorption results for the bimetallic catalysts treated in

flowing Ar at 400, 600, and 800°C also indicated higher Ir dispersions, thus confirming the

enhanced stability of the catalytic surface. More interestingly, following thermal annealing

at 400 and 600°C the H2 uptakes during chemisorption (at standard conditions of 40°C)

were much higher than observed at 200°C, and, in fact, were even higher than for the

monometallic Ir catalyst.

60

To investigate these unusual high chemisorption uptakes of H2 from the Ag-Ir

bimetallic particles, temperature programmed desorption (TPD) of H2 experiments were

done. For Ag-Ir bimetallic catalysts, there were high temp (~270°C) desorption peak due

to unmodified Ir surface sites and low temp peaks due to Ag-modified Ir sites at 100-170

°C. After annealing at 800°C in Ar, there was greatly reduced desorption from Ir-only sites

due to heavy sintering which confirmed that the residual H2 desorption at 100-170 °C was

associated only with Ag-modified Ir sites. This enhanced stability and unusual H2 uptake

of the Ag-Ir core shell particles were investigated using computational studies. Results

indicated that Ir atoms, or possibly very small ensembles of Ir atoms, exposed in a partial

Ag shell were able to chemisorb more than one H atom per Ir surface atom; two adsorbed

H atoms/Ir site were energetically quite favorable. Surface Ag atoms did not bind H in any

configuration or particle morphology.

Another direct method to understand how the distribution of Ag and Ir on the

surface of bimetallic Ag-Ir catalyst particles could influence H uptake is to examine a

catalytic reaction that involves adsorbed H. Such reactions include hydrogenation of simple

terminal olefins or hydrogenolysis of a structured paraffin such as methylcyclopentane

(MCP). There is extensive work regarding the catalytic activity of Ir for the hydrogenation

of propylene103–106 and hydrogenolysis of methyclcyclopentane 107–111; Ag catalysts are

reported to be inactive for both classes of reactions112–115. It has been repeatedly reported

that alkene hydrogenation is a structure- insensitive reaction. According a work done by

Boudart et al., activities for simple olefin hydrogenation on metal nano particles larger than

2 nm are independent of the catalyst particle size and morphology and therefore are

61

structure-insensitive reactions 116. Ortiz-Soto et al. showed that 2% Pt/SiO2 at reaction

condition of 40 °C, 10% C3H6, 20% H2, 70% He gives TOF= 12 sec-1 117.

Argo et al. used EXAFS spectroscopy to evaluate the Ir cluster structures and

cluster-support interactions during hydrogenation of olefin. His data indicates that however

the metal clusters remain intact and bonded to the support during reaction, but slight

rearrangements occur to accommodate reactive intermediates. Increasing concentrations of

reactive intermediates on the clusters causes cluster frame to expand, and flexed away from

the support. This shows self-inhibition of reaction occur by adsorbed hydrocarbons and

differences between hydrogenation of propene and other olefins is mainly due to different

adsorbate-adsorbate interactions118. Panjabi et al. studied supported iridium cluster

catalysts for propene Hydrogenation119. Using FTIR and EXFAS as characterization

techniques they identified Ir4 and Ir6 clusters as the catalytically active species with quite

unchanged cluster frames under the different reaction conditions which is consistent with

the results of Argo et al. 118.

In a work done by Bond et al. hydrogenation of olefins over alumina supported

platinum and iridium has been studied. They found out that (i) the behavior of platinum

and iridium is qualitatively similar which both show relatively small amounts of olefin

exchange, hydrogen exchange and isomerization (ii) with each metal the tendency to give

olefin- and hydrogen exchange is the same for ethylene, propylene and the butene, so that

the effect of molecular structure is small 120.

One of the main challenges in the chemical, petrochemical, and pharmaceutical

industries is to develop environmental friendly processes with minimum by-products. In

62

the petrochemical industry, the ring opening of methyl cyclopentane (MCP) is one of

important reactions to understand which catalyst modification could be effective towards

increasing the cetane number of gasoline/diesel feed stocks 121 with minimal unfavorable

emissions. However various noble metals of Pt, Rh, Ru, and Ir are capable of ring opening

MCP but not all of them give products with high cetane numbers122. The ring opening

products of MCP are branched including 2-methylpentane (2- MP), 3-methylpentane (3-

MP) and linear paraffins (n-hexane, C6) as desired product with higher cetane number.

Rao et al. studied the effects of structure and acid–base properties of γ-alumina and

other oxide supports on conversion and selectivity of Ir catalysts in ring opening of MCP.

Effect of potassium addition on surface properties was evaluated. They found out that

potassium is suppressing the activity and greatly enhances the selectivity to ring opening

(RO) products on γ-alumina 123.

McVicker et al. proposed a dicarbene mechanism for iridium in which cyclopentyl

rings are assumed to bond perpendicularly to the iridium surface111. Ring-opening rates

over iridium are directly proportional to the number of unsubstituted, unencumbered CH2–

CH2 bonds and markedly decreased with increasing ring substitution. Hydrogenolysis

product distributions for feed composition of H2/MCP = 5 at 275 °C on 0.9% Ir/Al2O3 were

<1% for n-C6, 70 for 2-MP, and 29% for 3-MP with the conversion of 52%.

Djeddi et al. compared the selectivity for ring opening and cracking products among

different Pt and Ir mono and bimetallic catalysts. They find out that the most active catalyst

in the conversion of MCP was Ir/-Al2O3 with the highest cracking whilst other catalysts

were inactive under the same conditions. From literatures 122,124 it is known that Ir has high

63

reactivity for naphthene conversion with high selectivity to ring opening. Interestingly,

comparison of Ir/-Al2O3 and Pt-Ir/-Al2O3 shows that Ir sites in monometallic catalyst were

very active while in bimetallic ones were inactive. In this case, the active sites of the

bimetallic Pt-Ir/-Al2O3 were similar to monometallic Pt/-Al2O3. They proposed that

opening occurs at the secondary–secondary position with dicarbene mechanism as reported

in other works110. This observation was ascribed to the possible formation of entities with

a high number of coordination sites on the catalyst surface.

Despite impressive works on Ir and Ag monometallic catalysts, to the extent of our

knowledge no studies have investigated bimetallic Ag-Ir catalysts for hydrogenation

reactions of olefin or hydrogenolysis/ hydrogenation of naphtenes like methyl

cyclopentane. Howevere, Chimentao et al. 125 studied hydrogenolysis of MCP over Ir-

Au/γ-Al2O3 bimetallic catalysts. They found that the addition of Au improved the

chemisorption and catalytic properties of Ir with products of 2-MP, 3-MP, and n-C6. In

their studies, initial rate of reaction was increased with Au content. We believe that this is

the first work that investigate bimetallic Ag–Ir catalysts for hydrogenation of propylene to

propane and hydrogenolysis of methyl cyclopentane (MCP) to isomerization and cracking

products as a probe reaction.

MATERIALS AND METHODS


SEA method was used to make monometallic Ir/Al2O3 catalyst (1% Ir; dispersion

~ 50% by H2 chemisorption) using Potassium hexachloroiridate (II) (K2IrCl6, Alfa Aesar

99.9%) as anionic precursor and γ-alumina (UOP VGL-25, BET SA- 164 m2/g) as support

64

in low pH conditions adjusted by HCl solution. Al2O3 support was calcined at 700°C for 4

hours before metal deposition. Metal concentration was determined by a Perkin-Elmer

2000DV ICP-OES. This base catalyst was then dried at 60 C in a vacuum oven for

overnight and then reduced at 400 C in 100 cm3/min (STP) of flowing H2 for 1 h before

using for next experiment. Electroless deposition of Ag on Ir/Al2O3 was performed using

aqueous solution of Potassium silver cyanide, KAg(CN)2 (Technic, Inc.) as metal

precursor, hydrazine and formaldehyde as reducing agents and NaOH to adjust high pH of

solution. Concentration of initial potassium silver cyanide in ED bath was determined

based on final coverage of Ag on Ir.

Experiments were done at room temperature (RT) with metal salt/reducing agent

molar ratio of 1:5. Stirrer bar was used to mix bath solution to prevent any mass transfer

limitations. NaOH solution carefully was added to keep the PH of bath around 10. To

specify the final concentration of Ag in bath and the deposited amount on Ir in specific

period of times small aliquots around 5 ml was taken from bath and was analyzed by

Perkin-Elmer AA 400. After completion of the Ag deposition, bath solution was filtered

and washed repeatedly with de-ionized water to remove all the remaining ligands from

sample. The filtered sample was dried at room temperature under vacuum overnight.

Therefore, various Ag-Ir bimetallic catalysts with different coverages of Ag were

synthesized.

5.2.2 H2 chemisorption studies

To determine the number of catalytically active sites, automated AutoChem II 2920

from Micromeritics was used. Samples (~ 0.1 g) were reduced in flow of 10%H2/bal. He

65

(20 sccm) at 200 C for 2 hrs followed by Ar (20 sccm) at the same temperature for 2 hrs to

remove physisorbed and chemisorbed hydrogen from particles’ surfaces. Same samples

were annealed in situ up to 400, 600, and 800 °C respectively each for 4 hrs. After each

annealing treatment, temperature was cooled down to 40 °C to run the chemisorption.

5.2.3 Temperature programmed desorption (TPD)

ASAP 2920 was used to perform TCD- H2 TPD measurements. To run the

experiment each sample was reduced in situ with the flow of 10%H2/bal. Ar at 200 C for

2 hrs followed by flowing Ar at the same temperature for 2 hrs. After sample was cooled

down to room temperature H2 was flowed over sample to populate hydrogen on the

particles surface. After saturating particles surface with hydrogen temperature ramping

started along with Ar flow. H2 desorption pattern was recorded by increasing temperature.

5.2.4 X-ray diffraction

XRD analysis was performed on a Rigaku Miniflex-II with a high sensitivity silica

strip detector (D/teX Ultra) with Cu Kα radiation (λ = 1.5406 Å), operated at 15 kV and 30

mA. Patterns were recorded in the 20°−80° 2θ range, with a scan rate of 2.0°/min, Ir and

Ag diffractions were fit using Gaussian to achieve FWHM values and particle size were

calculated by Scherrer equation.

5.2.5 Fourier transform infrared spectroscopy (FTIR)

Nicolet Nexus 4700 spectrometer equipped with a mercury–cadmium–telluride B

(MCT-B) detector was used to perform in situ FTIR spectra. At room temperature FTIR

spectra were collected for each sample with 40 mg in single beam absorbance mode with

66

a resolution of 4 cm-1. The samples were reduced in 5% H2/bal. He for 1 h at 200 C and

then cooled to room temperature. Background spectra were taken in Ar flow before Co

exposure. Sample was saturated with 1% CO and then was flushed with pure Ar for 10 min

to remove weakly bonded CO species and gas phase CO. Deconvolution of FTIR peaks

were done for all samples based on spectrum of 1% Ir/Al2O3 to obtain peak position, width,

eight, and area of overlapped peaks.

5.2.6 Catalytic evaluation: propylene hydrogenation and MCP hydrogenolysis

The monometallic Ir/Al2O3 and the series of bimetallic Ag–Ir/Al2O3 were evaluated

for hydrogenation of propylene (C3H6) to propane (C3H8). Catalysts were placed in a single

pass, 0.19” ID tubular packed bed reactor (316 stainless steel). Reactor was loaded with

0.006 g of catalyst diluted by 0.054 g of the Al2O3 supported on glass wool in the middle

of the reactor. All samples were treated in situ at 200 °C in 10%H2/bal. He for 2 hrs; then

400 C for 4 hrs and finally cool down to reaction temperature. To monitor the reaction

temperature a thermocouple was inserted into the catalyst bed. All catalysts were reduced

in situ at 200 ◦C in 10% H2/balance He for 2 h and then cooled to 100 ◦C to start the

reaction. All lines between the reactor outlet and the inlet of the GC were held at 120 °C

to prevent any condensation. Prior Using mass flow controllers for adjusting gas flows the

reaction feed stream were determined to be 10% C3H6, 50% H2, balance He at a total flow

rate of 100 SCCM. To maintain differential conversion conditions, it was tried to keep the

conversion low for all set of reactions. An automated, on-line Hewlett-Packard 5890 Series

II gas chromatograph with flame ionization detection was used for analyzing both reaction

feed and products at every 1.0 h over the full length of the run. Due to transient behavior

67

for the first several hours on line all the reaction data reported here were based on stable

catalyst performance after 15 to 20 h on line.

The kinetic measurements for methyl cyclopentane hydrogenolysis/ hydrogenation

were studied using the same reactor system. To vaporize MCP from liquid phase, a vapor

liquid equilibrium (VLE) system was used. VLE was encased in a jacketed shell with liquid

inlet and exit ports at the bottom and top of the shell, respectively, which was connected to

an ethylene glycol/H2O recirculation bath to maintain isothermal behavior to give

convenient MCP vapor pressure for reaction. Calibration for feed and all ring opening

products including 2-MP, 3-MP, C1-C4, and n-C6 were done separately.


5.3.1 SEA and ED bath development

Strong Electrostatic Adsorption (SEA) was used to form monometallic 1%Ir/Al2O3

with IrCl62- as anionic Iridium precursor over the calcined alumina supports in the acidic

pH value of 4 102. After reduction treatment, particle size was measured from XRD which

below the limit of detection (<1.5 nm).

Electroless Deposition (ED) was performed according the procedure from earlier

work in our laboratory to synthesize Ag-Ir bimetallic particles from Ir monometallic

particles. KAg(CN)2 was used as the silver precursor since it has a low standard reduction

potential E0 = -0.31 V to provide high stability in the bath. Hydrazine and formaldehyde

were selected as the reducing agents for Ag deposition since they are preferably activated

on Ir surfaces relative to the Ag metal 126.

68

5.3.2 Synthesis of Ag–Ir/Al2O3 bimetallic catalysts

Various samples of Ag-Ir catalysts with different Ag coverages as shell on Ir core

metals were synthesized by ED. A summary of samples with wt% of Ag, targeted coverage,

and experimental coverages are shown in Table 5.1. Two series of samples were prepared;

one for propylene hydrogenation analysis and another for hydrogenolysis of MC. Atomic

absorption spectroscopy (AA) was used to determine Ag wt% deposited on Ir.

Experimental coverages of Ag were obtained from H2 chemisorption. Hydrogen

chemisorption quantitatively determine the exposed Ir surface sites. Since Ag does not

chemisorb H2 at 40 °C Ag deposition on Ir should lead to a decrease in H2 uptake.

Theoretical coverages assume a 1:1 shell atom to core metal active site and only catalytic

deposition. Discrepancy between the theoretical and experimental coverages of Ag is due

to electroless deposition of Ag metal on previously deposited Ag metal. Deviation of the

experimental ED profile from the theoretical solid line for higher wt % of Ag loadings

indicates that autocatalytic deposition also occurs.

From earlier work 102, the prepared Ag-Ir bimetallic catalysts were annealed at

high temperatures of 400, 600, and 800 °C to determine their stability. The chemisorption

results of the Ag-Ir bimetallic system supported on an δ, θ alumina surface is shown in

Figure 5.1. They showed that 1.0 wt% Ir catalyst was stable up to 400°C annealing

temperatures shown by no change in the H2 uptake from chemisorption between 200 and

400°C. After the temperatures of 600C and 800 °C, the H2 uptake decreased from 12.5 to

1.2, and 1.4 µmol/g catalyst, respectively which is indicative of severe nanoparticle

sintering. This decrease in H2 uptake proves that active sites decreased as annealing

69

temperature increased. By adding Ag, the initial H2 uptake of the catalyst was decreased

because Ag is unable to chemisorb H2 at these conditions.

Table 5.1. Bimetallic Ag-Ir Catalysts for C3H6 hydrogenation, and MCP hydrogenolysis

At 400°C, Ag-Ir catalysts actually had a higher H2 uptake than initial prepared. At higher

temperatures of 600, and 800 °C these bimetallic catalysts with only a fraction of Ag added

retained a higher H2 uptake compared to the Ir monometallic catalyst at those same

temperatures. H2 uptakes for the catalysts with higher Ag coverages have reduced after

200°C. The H2 uptake of the Ag-Ir bimetallic catalysts remained higher than the Ir

monometallic catalysts after the elevated annealing temperatures of 600 and 800 °C.

Reaction Shell Ag wt % θtheo. (ML) θexp (ML)

C3H6 hydrogenation

0 0 0

0.14 0.52 0.22

0.24 0.92 0.62

0.47 1.77 0.93

MCP hydrogenolysis

0.03 0.11 n/a

0.09 0.32 n/a

0.12 0.42 0.29

0.3 1.05 0.53

0.39 1.37 0.82

0.49 1.72 0.65

70

Figure 5.1. The H2 uptake of the Ag-Ir bimetallic catalysts remained higher than the Ir

monometallic catalysts after the elevated annealing temperatures of 600 and 800°C.

XRD of the monometallic Ir and Ag catalysts showed sintering on alumina support.

Anchoring the Ag as a shell by ED prevented the sintering of Ag compared with the

monometallic Ag catalysts. This increased stability of the Ag agrees with SFE principles.

Moreover, the addition of a Ag shell to the catalysts also prevented Ir sintering. STEM was

used to confirm the bimodal distribution and stabilization of the Ag-Ir nanoparticles.

Severe sintering is prevented by the incorporation of Ag. This enhanced stability and

unusual H2 uptake of the Ag-Ir core shell Particles were investigated using computational

studies by A.C. Reber et al. It was found that Ir atoms in Ag shell bind H2 quite strongly,

but Ag binds H2 too weakly to explain low temperature H2 in TPD.

Another direct way to evaluate the Ag-Ir bimetallic catalysts is to use them in

related reactions. In here, two different hydrogenation reactions were used to evaluate

catalytic properties of the Ir@Ag catalysts, the hydrogenation of propylene (C3H6) and

hydrogenolysis of methyl cyclopentane (MCP). First hydrogenation of C3H6 was

71

performed to evaluate the annealing temperature on C3H8 formation. As shown in Figure

5.2, by increasing annealing temperature from 400 to 800 °C, TOF for C3H8 formation was

dropped which is indication of sintering at higher temperatures.

Figure 5.2. Effect of annealing temperature on TOF for C3H8 formation on (a) 1% Ir, (b)

0.14% Ag-1% Ir, (c) 0.24% Ag-1% Ir, and (d) 0.47% Ag-1% Ir.

Close look at the hydrogenation of C3H6 shown in Figure 5.2 shows very high initial

rates indicative of the large reservoir of hydrogen from Ag-Ir. However, after consumption

of hydrogen from these sites, specific activities returned to that of monometallic Ir with

TOF = 15 sec-1 indicating that only Ir sites were operative at steady state conditions. These

high Initial TOF's cannot be from exothermic reaction because the reaction bed was very

dilute and Ea values are typically only 5 - 6 kcal/mole. Most likely the reason is large

72

reservoir of adsorbed H on Ag-Ir sites. It appears that sites are not re-populated during

reaction.

Figure 5.3. Comparison of initial TOF's for Ir and Ag-Ir catalysts.

Figure 5.3 shows TOF for all samples for 1% Ir monometallic and Ag-Ir bimetallic in one

plot. It is obvious that at steady state all of the bimetallic activities approach to that of 1%

Ir. To evaluate the source of these high initial activities and to see if restructuring of Ir sites

have been occurred H2-TPD was done on samples after reaction, shown in Figure 5.4.

If we look at the plots, we can see that there is not much difference between the

TPD before reaction and the TPD after reaction. Therefore, it doesn’t appear that

restructuring that restructutring of the bimetallic surface has occurred rather it continues to

look like those particular sites that give rise to low temperature desorbed H2 while initially

73

active cannot repopulate. This is because heat of adsorption of C3H6 is about twice high as

that of H2. Thus, in a competitive adsorption environment H2 cannot be put on those sites.

Figure 5.4. H2-TPD for before and after reaction of (a,b) 1% Ir, (c,d) 0.14%Ag, (e,f)

0.24% Ag, and (g,h) 0.47% Ag.

74

In another rigorous reaction, Hydrogenolysis/hydrogenation of MCP was also used

to determine the effects of Ag on selectivity to different reaction products, specifically, the

hydrogenolysis of MCP to C1-C4 paraffins and isomerization to 2- methyl pentane (2-MP),

3- methyl pentane (3-MP) and n-hexane (n-C6). As shown in Figure 5.5. at Ag coverages,

higher than 0.53, selectivity to the isomerization products decrease while cracking to C1-

C4 hydrocarbons increases.

Figure 5.5. Effect of Ag coverage on selectivity of MCP isomerization at 10% C3H6, 50%

H2, bal He at 100 SCCM flow, 6mg cat diluted to 60 mg with δ,θ-Al2O3, Temp = 100 °C

and P = 1 atm.

Table 5.2 shows that by increasing Ag coverage, TOF is increasing from 0.16 for

1% Ir to 0.39 for θAg=0.82 with exception at Ag coverage of 0.65 with TOF of 0.18 sec-1.

However, the rate of reaction is rising by increasing the Ag coverages which is

accompanied by decrease of conversion.

Monometallic Ir/Al2O3, Ag/Al2O3 and their bimetallic catalysts annealed at 400 and

200 °C were studied by adsorption of CO at room temperature as shown in Figure 5.6. For

75

both Ir/Al2O3 and Ag–Ir/Al2O3 catalysts, CO stretching bands were observed in 1950–2150

cm-1 region, whereas no characteristic ʋCO vibrations were observed in the spectra of the

Ag/Al2O3 catalyst, in agreement with observations of 127. For the Ir/Al2O3 sample annealed

at 400 °C, the region between 1800 and 2200 cm-1 contains several overlapping features,

which can be deconvoluted into five peaks centered at 1830, 1993, 2036 and 2065 and

2080 cm-1.

Table 5.2. Selectivity to isomerization products of MCP, TOF (s-1) based on MCP reacted,

rate (µmol/g cat. Min), and Conversion (%) at reaction conditions of 300 °C and 1 atm, tot.

flow of 50 sccm.

pretreatment at 400 °C in Ar before reaction at 300 °C

wt% Ag θAg 2-mp 3-mp n-hexane C1 - C4 TOF

(sec-1) Rate

(µmol/g cat-min) %

Conv

0 0 25 17 24 34 0.16 243 24

0.03 0.11 26 17 21 36 0.22 287 27

0.30 0.53 38 22 13 27 0.25 187 18

0.49 0.65 23 13 4 60 0.18 102 10

0.39 0.82 12 7 3 79 0.39 118 11

These peaks are attributed to linearly adsorbed CO species on fully reduced Ir sites (2036,

2065 cm-1) or antisymmetric (1993 cm-1) or symmetric (2080 cm-1) or bridge (1830 cm-1)

vibrations of adsorbed dicarbonyl species on Ir, respectively. The assignment of these

peaks in this spectral region is based on our previous published work 126 and the available

literature 128. In the case of bimetallic surfaces, the intensity of FTIR spectra decreases with

increasing Ag content, indicating lower CO uptake due to electroless deposition of Ag on

Ir. The results suggest that Ag electroless deposition on Ir is not particularly favored on

any of the various Ir surface planes or other sites that are exposed, which is in agreement

with the recent literature 39,129.

76

Figure 5.6. Transmission FTIR spectra of CO adsorption on Ir/Al2O3, Ag/Al2O3 and Ag–

Ir/Al2O3 bimetallic catalysts. Arrows highlight peak intensity losses as Ag wt.% increases.

Monometallic Ir/Al2O3, and bimetallic catalysts annealed at 200 °C, shown in Figure

5.6 (b), shows much lower CO intensity than that of 400 C. For the Ir/Al2O3 sample

annealed at 200 °C, the region between 1800 and 2200 cm-1 contains several overlapping

features, which can be deconvoluted into four peaks centered at 1993, 2036 and 2065 and

2130 cm-1. These peaks are attributed to linearly adsorbed CO species on fully reduced Ir

sites (2036, 2065 cm-1) or antisymmetric (1993 cm-1) or oxide form of Ir (2130 cm-1)

species on Ir, respectively. By increasing Ag coverage, the intensity has not been changed

but there is a shift toward higher wavelength. Comparison FTIR data for two temperatures

shows by increasing annealing temperature to 400 °C, CO intensity 10 times has been

decreased. This is not consistent with CO and H2 chemisorption results. The explanation

77

for this can be that FTIR is not accurate in quantitively evaluation while it can give

information about the site species for samples.

CONCLUSION

Restructuring of the bimetallic surface has not occurred according to H2-TPD results and

those particular sites that give rise to low temperature desorbed H2 while initially were

active cannot repopulate. This is because heat of adsorption of C3H6 is about twice high

as that of H2. Thus, in a competitive adsorption environment H2 cannot be put on those

sites.

78

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