Citethis:Chem. Soc. Rev.2012 41 ,80098020 TUTORIAL … files/550__ChemSocRev_1… · 8010 Chem. Soc. Rev.,2012,41,80098020 This ournal is c The Royal Society of Chemistry 2012 is

This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8009

Cite this: Chem. Soc. Rev., 2012, 41, 8009–8020

Pd–Au bimetallic catalysts: understanding alloy effects from planar

models and (supported) nanoparticlesw

Feng Gao*aand D. Wayne Goodmanzb

Received 28th April 2012

DOI: 10.1039/c2cs35160a

Pd–Au bimetallic catalysts often display enhanced catalytic activities and selectivities compared

with Pd-alone catalysts. This enhancement is often caused by two alloy effects, i.e., ensemble and

ligand effects. The ensemble effect is a dilution of surface Pd by Au. With increasing surface

Au coverage, contiguous Pd ensembles disappear and isolated Pd ensembles form. For certain

reactions, for example vinyl acetate synthesis, this effect is responsible for reaction rate

enhancement via the formation of highly active surface sites, e.g., isolated Pd pairs. The

disappearance of contiguous Pd ensembles also switches off side reactions catalyzed by these sites.

This explains the selectivity increase of certain reactions, for example direct H2O2 synthesis. The

ligand effects are electronic perturbation of Pd by Au. Via direct charge transfer or by affecting

bond lengths, the ligand effects cause the Pd d band to be more filled, moving the d-band center

away from the Fermi level. Both changes make Pd more ‘‘atomic like’’ therefore binding reactants

and products more weakly. For certain reactions, this eliminates a so-called ‘‘self-poisoning’’

effect and enhances activity/selectivity.

1. Introduction

Alloys are a class of important heterogeneous catalysts as

they frequently exhibit much enhanced catalytic stabilities,

activities and selectivities, as compared with their single-metal

constituents.1 The Ponec and Bond definition of alloy is the

following: alloy is most conveniently defined as a metallic

system containing two or more components, irrespective of

their intimacy of mixing or, precise manner of mixing.2,3 When

alloys contain two metallic components, for example Pd–Au,

they are sometimes also called bimetallics. In this article, we

intend to call a Pd–Au catalyst ‘‘alloy’’ when Pd and Au are

intimately mixed otherwise ‘‘bimetallics’’ when Pd and Au are

segregated. Among alloy catalysts, Pd–Au has received a great

deal of attention because of its superior activity in a number of

reactions. Pd–Au catalysts are used in the industrial synthesis

of vinyl acetate (VA). In the United States alone, 4.8 million

tons of VA are produced over this catalyst annually.4 Pd–Au

catalysts also catalyze low-temperature CO oxidation,5–7

direct H2O2 synthesis from H2 and O2,8–11 hydrodechlorina-

tion of Cl-containing pollutants in underground water,12

hydrodesulfurization of S-containing organics,13 hydrogenation

of hydrocarbons,14–16 acetylene trimerization,17–20 and many

other reactions.

Alloying induces multiple changes in the physical and

chemical properties of the metallic components. Where

catalytic properties are concerned, two alloy effects are sig-

nificant: (1) ensemble effects, i.e., a finite number of atoms in a

particular geometric orientation that are required for facilitating

a particular catalytic process; and (2) ligand effects, i.e.,

electronic modifications resulting from hetero-nuclear metal–

metal bond formation. The latter could involve charge transfer

between the metals or orbital rehybridization of one or both

metallic components.1 It has to be noted that one cannot vary

the composition of the catalyst surface without affecting both

the distribution of the ensembles and changing the electronic

structure of the individual constituent atoms in the surface.21

Still, some suggest that ensemble effects play a more dominant

role than ligand effects.3,21

For the Pd–Au system in particular, an ensemble effect is

mainly a diluting effect where the catalytically more active

component (Pd) is diluted by the less active component (Au).

As the surface ratio of Au–Pd increases, sizes of contiguous Pd

ensembles decrease and eventually all Pd atoms are separated

by Au as isolated Pd monomers.1,5,6,22 With regard to the

possible ligand effects in this catalyst, one would intuitively

expect that charge is transferred from Pd to Au since Au has

higher electronegativity. This statement may be true but is

oversimplified. Among metals, Au has one of the largest

electron affinities. In bulk gold-containing intermetallic com-

pounds, Au usually gains s, p electrons, but this gain of charge

a Institute for Integrated Catalysis, Pacific Northwest NationalLaboratory, P.O. Box 999, Richland, WA 99352, USA.E-mail: [email protected]; Fax: +1 509 371 6066;Tel: +1 509-371-7164

bDepartment of Chemistry, Texas A&M University, P.O. Box 30012,College Station, TX 77842-3012, USA

w Part of the bimetallic nanocatalysts themed issue.z Dr Goodman deceased on February 27, 2012.

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8010 Chem. Soc. Rev., 2012, 41, 8009–8020 This journal is c The Royal Society of Chemistry 2012

is partially compensated by a depletion of Au 5d electrons.23

This situation also applies for the Pd–Au system. Indeed, there

appears to be general agreement that upon alloying, Au

gains s, p electrons and loses d electrons whereas Pd loses s,

p electrons but gains d electrons.1,13,14,23 Charge transfer

between Pd and Au also helps to explain why Au is able to

fully isolate Pd: notably, there exists some Coulomb Pd–Pd

repulsion in bulk Pd whereas Pd–Au attraction is realized as a

result of net charge transfer from Pd to Au. For late-transition

metals like Pd and Au, the d-character is much more impor-

tant than s, p-character in defining their chemisorption and

catalytic properties. For Pd, gaining d electrons shifts the d

band center away from the Fermi level, which leads to weaker

interaction between adsorbates and surface Pd atoms.21

Indeed, recent theoretical calculations demonstrate that the

Pd d band for Pd monomers surrounded by Au is much lower

in energy than that for Pd monolayer or bulk Pd surfaces.24

There is another reason that causes the Pd d band to narrow: a

lattice mismatch between Pd and Au, with Pd having a lattice

constant B5% smaller than Au.1 Upon alloying, however, Pd

has been found, in certain cases, to adopt the lattice constant

of Au.14,17 In this case, Pd–Pd bond length increases causing

the Fermi level within the Pd d band to rise. This also enhances

the atomic-like character of Pd atoms and correspondingly,

weakens binding toward reactants. From both charge transfer

and bond length arguments, we understand that Au is able to

weaken the binding strength of Pd by perturbing its d band.

However, this does not necessarily mean that Au weakens the

catalytic activity of Pd. In contrast, enhanced activity of Pd

within Pd–Au alloys is frequently found as compared to pure Pd.

Besides other factors, diminishing the so-called self-poisoning by

reactants/products is one reason that accounts for the activity

enhancement. Some examples are shown in the following sections.

Pd–Au catalysts are divided into two categories in this

article: (1) planar models used by surface scientists, including

single crystals and thin films; (2) nanoparticles for practical

applications. The latter include traditional high-surface-area

carrier supported metallic particles where the sizes of the

particles typically fall in the nano-dimensional range; and, in

the past two decades, the rapidly developing unsupported

nanoparticles. Each category has its advantages and dis-

advantages. Planar model catalysts are generally referred to

as ‘‘well-defined’’ catalysts as these are synthesized under well-

controlled conditions and characterized with a wide array of

surface-sensitive techniques such that they are often under-

stood at an atomic level.25,26 However these model catalysts

are typically used under non-practical ultrahigh vacuum

(UHV) conditions. To overcome the ‘‘pressure gap’’, some

researchers use coupled high-pressure reactor/UHV systems to

study catalysis on planar models at elevated pressures.27 On

the other hand, nano-particle catalysts are more practical and

can be used under various realistic reaction conditions. How-

ever, these catalysts are generally too complex to understand at

an atomic level. Indeed, as stated by Crooks and co-workers,28

unambiguous structure determination for particles in the size

range of o2 nm remains a major analytical challenge. In this

article we will cover both categories of catalysts. However to

address the alloy effects more explicitly, more attention is paid

to planar model catalysts since both ensemble and ligand

effects can be probed at an atomic level in this case.

Feng Gao

Feng Gao received under-graduate and graduate educa-tion at Tianjin University,China, in the 1990s inChemical Engineering. Hejoined the University ofWisconsin-Milwaukee in 2000as a graduate student andreceived a PhD in PhysicalChemistry in 2004 under Prof.Wilfred T. Tysoe. From 2007to 2009, he was a postdoc atTexas A&M University underProf. D. Wayne Goodman. Hehad a brief stay at WashingtonState University as a research

faculty member and is currently a staff scientist at PacificNorthwest National Laboratory (PNNL), conducting researchin basic and environmental heterogeneous catalysis. He is acoauthor of 60 publications.

D. Wayne Goodman

Wayne Goodman joined thefaculty of the ChemistryDepartment at Texas A&Min 1988 where he is currentlya Distinguished Professor andthe Robert A. Welch Chair.Previously he was the Headof the Surface ScienceDivision at Sandia NationalLaboratories. He was therecipient of the Ipatieff Awardof the American ChemicalSociety in 1983, the Colloidand Surface Chemistry Awardof the American ChemicalSociety in 1993, the Yarwood

Medal of the British Vacuum Society in 1994, a HumboldtResearch Award in 1995, a Distinguished Research Award ofTexas A&M University in 1997, the Giuseppe Parravano Awardin 2001, the Adamson Award for Distinguished Service in theAdvancement of Surface Chemistry of the American ChemicalSociety in 2002, the Gabor A. Somorjai Award of the AmericanChemical Society in 2005 and elected Fellow of the AmericanChemical Society in 2009. He is the author of over 540 publica-tions/book chapters and an active member/officer of a number ofprofessional societies. He has served as an Associate Editor forthe Journal of Catalysis, and served on the Advisory Boards ofSurface Science, Langmuir, Catalysis Letters, and The Journalof Physics: Condensed Matter.


2. Planar model catalysts

2.1. Model catalyst formation and characterization

Three types of planar model Pd–Au alloy catalysts have been

developed: (1) Pd–Au single crystals, e.g. AuPd(100)5,6,22

and Au3Pd(100) alloys;29 (2) Pd–Au thin films generated by

depositing Au on Pd single crystals,19,30–34 or depositing Pd on

Au single crystals,18,35,36 or co-depositing Au and Pd on Au37

or refractory metal substrates;1,38 and (3) Pd–Au clusters

generated by co-depositing Au and Pd on planar oxide thin

films.39,40 For the latter cases, annealing of the thin films after

deposition is required to facilitate alloying.

Since the reactions occur on surfaces of heterogeneous

catalysts, knowledge of the surface composition and structure,

preferably at an atomic level, is of vital importance. Three

important surface science techniques used to characterize

Pd–Au alloy surfaces are briefly described below. First, since

Pd and Au have different electronic density of states (DOS)

near the Fermi level and alloying does not eliminate this

difference, scanning tunneling microscopy (STM) can be used

to image a Pd–Au alloy surface which gives a contrast between

surface Pd and Au atoms. Fig. 1 presents an example of this

for a clean AuPd(100) surface.22 In this case, as shown in

Fig. 1(A), some surface atoms appear to be brighter than the

others. Following a simple data treatment to enhance the

contrast (shown in Fig. 1(B)), coupled with techniques capable

of chemical identification, one is able to construct the surface

structure shown in Fig. 1(C) (Pd atoms are displayed as darker

spots) where all Pd ensembles (isolated monomers, isolated

monomer pairs, dimers, trimers, etc.) can be identified.

A second technique that deserves mentioning is low energy

ion scattering spectroscopy (LEISS). In this technique, a beam

of noble gas ions with energy between 0.1 to 10 keV impinges

on a solid surface and are scattered. The energy of outgoing

ions is determined by the laws of energy and momentum

conservation; therefore, surface atoms with different atomic

masses are distinguished. Note that this technique is essentially

exclusively sensitive to the topmost layer of a surface since

more than 99% of the ions penetrating the first layer will be

neutralized and won’t be detected by an ion energy analyzer.

Fig. 2(A) presents a LEISS example of thin Pd–Au alloy films

deposited on Mo(110) in ultrahigh vacuum. In this experi-

ment, 5 monolayers (ML) of Au are first deposited on

Mo(110) followed by 5 ML of Pd. The thin film is then

annealed to various temperatures to allow different levels of

intermixing. A stable Pd–Au alloy, with apparent Au surface

segregation, is achieved between 600 and 1000 K. At higher

temperatures, Au first sublimes followed by Pd.1 Using the

scattering intensities of Pd and Au, corrected with a sensitivity

factor (i.e., the signal intensity ratio of pure Pd and Au), the

surface coverage of Pd and Au for any Pd–Au alloy surface

can be precisely determined. By varying the bulk Pd–Au ratio,

the composition of the stable surface layer varies accordingly

and a phase diagram is thus constructed as shown in Fig. 2(B).

Clearly, strong Au segregation to the surface occurs for all

bulk Pd–Au ratios in UHV. This is rationalized by the fact

that Au has smaller surface free energy than Pd.1 In general,

the surface segregation of an alloy component depends on the

enthalpy of mixing, the atomic sizes of the metals and the

surface free energies (which are proportional to the heats of

sublimation).23 As will be shown below, segregation can also

be adsorption/reaction induced.

Both STM and LEISS, and most other surface-sensitive

techniques require ultrahigh vacuum to operate; therefore,

in situ applications at elevated temperature and pressure are

exceedingly difficult. Fortunately, infrared reflection absorp-

tion spectroscopy (IRAS), coupled with CO titration, can be

used as a powerful tool to probe surface Pd and Au ensembles

under such conditions. Especially, when a photoelastic modu-

lator is added to the IR beam to perform polarization

modulation (PM) to eliminate gas-phase signals, the so-called

PM-IRAS technique is very useful in probing ensembles on

planar Pd–Au alloy surfaces at elevated temperature and

pressures.5,6,40 Fig. 3 presents temperature-dependent

PM-IRAS spectra of 1 � 10�3 Torr (A) and 10 Torr (B) of

CO exposed to an AuPd(100) surface initially enriched with

Au (B90%). CO vibrational features are assigned as follows:

nCO at >2100 cm�1 corresponds to atop CO on Au sites; nCOat 2060–2085 cm�1, to atop CO on isolated Pd sites; and nCObetween 1900 and 2000 cm�1, to bridging CO on contiguous

Pd sites. As is clearly displayed in Fig. 3(A), due to the strong

interaction between Pd and CO, subsurface Pd atoms are

‘‘pulled out’’ to the surface at temperatures higher than

B240 K. However the Pd segregation is insufficient to form

contiguous Pd sites at this CO pressure. Higher CO pressure is

needed to segregate a sufficient amount of Pd to the surface to

Fig. 1 (A) STM image of an AuPd(100) bulk alloy (10 nm � 10 nm, Vs = �15 mV, It = 6.3 nA). The large white features are impurities thought

to be carbon. (B) The same STM image as that in (A) excluding all data points below the cutoff height, which is set to 5 pm below the highest point

of the image. The color bar scale spans from 0 to 5 pm. The red circles denote the features designated to be Pd atoms. These red circles are set to

have an area of B0.15 nm2. (C) Schematic representation of (A) for clarity. Figure adapted with permission from ref. 22. Copyright (2007) by

American Chemical Society.


form contiguous Pd sites (Fig. 3(B)). This chemisorption

induced segregation appears to be easy to understand: as

stated by Haire et al., on thermodynamic grounds, it is to be

expected that the surface composition will adjust so that the

surface becomes enriched in the element which interacts more

strongly with the adsorbate.41 Importantly, it is found from

Fig. 3(B) that at temperatures aboveB475 K, the bridging CO

band disappears while the atop CO band is still present up to

B650 K. It is expected, however, that the binding energy of

bridging CO species is higher than atop CO. This is best

rationalized by the fact that Au starts to diffuse back to the

surface to isolate Pd as the temperature rises. This is because

the sticking probability of CO decreases at high temperatures

and the ‘‘pulling force’’ added to surface Pd therefore weakens.

This simple chemisorption experiment, nevertheless, repre-

sents a good example of dynamic catalyst restructuring at

elevated temperature and pressures. Spectroscopic methods

applicable at such conditions, for example PM-IRAS and sum

frequency generation (SFG), are highly desirable in these cases

to capture the dynamic changes. Ligand effects have also been

found to influence CO adsorption on Pd–Au alloy surfaces. It

has been well-documented that on pure Pd(111), CO occupies

threefold hollow sites at low coverage.42 Recent XPS measure-

ments on an Au/Pd(111) system has revealed that even 10%

surface Au is sufficient to switch off binding at threefold

hollow sites.43 Clearly, 10% surface Au is insufficient to

remove all Pd threefold hollow sites (ensemble effect) but

apparently does sufficiently destabilize them (ligand effect).

This example also demonstrates a synergy between ensembles

and ligand effects in affecting chemisorption.

2.2. Examples of catalytic reactions over planar models

2.2.1. Acetylene trimerization. Acetylene trimerization

(3C2H2 - C6H6) is an interesting model reaction as it occurs

both in UHV on Pd single crystal surfaces and at elevated

pressures on supported Pd catalysts. Surface science studies

have revealed that this reaction proceeds via a C4H4 metalla-

cycle intermediate; the coupling of this intermediate with

another acetylene molecule gives rise to benzene. The rate

limiting step has been found to be benzene desorption that

occurs from two states of adsorbed benzene: tilted and flat-lying.

The former state occurs at much lower temperatures than the

latter; the latter state also accompanies certain levels of product

dissociation.44 The study of acetylene trimerization over Pd–Au

alloys is interesting since (1) this reaction is sensitive to both the

structure and composition of the metal surface, and (2) pure Pd is

very active, however, pure Au is totally inert.17

Lambert and co-workers systematically studied acetylene

trimerization over Pd–Au thin films formed on Au(111)18 and

Pd(111).19 Due presumably to the mobility difference of Au in

Fig. 2 (A) LEISS spectra of 5 ML Pd/5 ML Au/Mo(110) as a

function of annealing temperature. LEISS spectra were collected at

300 K after the sample was annealed to the specified temperature. (B)

Surface concentration of various Pd–Au alloys on Mo(110) measured

by LEISS compared to the corresponding bulk concentration. The

sample was annealed at 800 K for 20 min. Figure adapted with permis-

sion from ref. 1. Copyright (2005) by American Chemical Society.

Fig. 3 (A) Temperature-dependent PM-IRAS spectra of 1 �10�3 Torr of CO on an AuPd(100) surface well-annealed at 800 K for

30 min. (B) Temperature-dependent PM-IRAS spectra of 10 Torr of

CO. Figure adapted with permission from ref. 5. Copyright (2009) by

American Chemical Society.


Pd and Pd in Au, a (O3 � O3)R301 (composition Pd2Au)

surface alloy forms following Pd deposition onto Au(111) and

annealing.18 However, after Au deposition onto Pd(111), Pd

and Au distribute quasi-randomly.19 In this case, higher

annealing temperatures induce more Au diffusion into the

Pd bulk; therefore, simply by varying the annealing tempera-

tures, surface alloys with different Pd–Au ratios are

formed.19,33 In terms of reactivity, benzene forms and desorbs

at much lower temperatures over the alloy surfaces than

Pd(111) or pure Pd overlayers on Au(111). Lambert and

co-workers gave two reasons why the alloy is more effective

than pure Pd. First, the degree of rehybridization of the

adsorbed acetylene decreases on the alloy surface. This results

in more efficient conversion of acetylene to benzene than

acetylene decomposition. Second, the strength of adsorption

of the benzene product is also decreased on the alloy surface.

Again, the weaker interaction with the alloy surface increases

the probability of product desorption without decomposition.18

Both ensemble and ligand effects seem to give satisfactory

qualitative explanations for the reduced binding of both

the reactant and product with the alloy surfaces. On the

Au/Pd(111) alloy surfaces, an AuPd6 ensemble was found to

give rise to the highest benzene yield. This suggests that a

7-atom ensemble is needed for benzene formation.19 However

the high activity of AuPd6 is obtained under highly idealized

conditions during temperature-programmed desorption

following adsorption of a saturation coverage of acetylene in

UHV. This does not necessarily mean that an AuPd6 ensemble

is highly active, or even needed under other reaction condi-

tions. The (O3 � O3)R301 (composition Pd2Au) surface alloy

formed on Au(111) is also very active yet AuPd6 ensembles

should not be present at high concentrations on this surface.

Nevertheless, the planar model catalysts can provide impor-

tant information for the acetylene trimerization reaction over

supported Pd–Au catalysts under realistic conditions. For

example, supported Pd–Au alloy catalysts show higher activity

and selectivity than supported Pd-shell–Au-core bimetallic

catalysts,17 fully consistent with the surface science studies.

2.2.2. Vinyl acetate synthesis. Acetoxylation of ethylene on

silica-supported bimetallic Pd–Au catalysts promoted with

potassium acetate is a well-established commercial route for

synthesis of vinyl acetate (VA), given by the following reac-

tion: CH3COOH+ C2H4 + 0.5O2 - CH3COOCHQCH2 +

H2O.4,35,45,46 Compared with pure Pd, the addition of Au has

been shown to significantly improve reaction rates whilst also

moderately improving the reaction selectivity.46 The side

reactions are combustion of the reactants acetic acid and

ethylene, and the product VA; ethylene combustion has been

found to be the dominant one under typical reaction condi-

tions.4 Using temperature-programmed desorption, the much

weaker binding of ethylene with Pd–Au alloys (as compared to

pure Pd) was confirmed.45 Since ethylene combustion must

involve C–C and C–H cleavage of chemisorbed ethylene, the

fact that ethylene binds much weaker on Pd–Au alloys makes

it easy to understand why VA selectivity is higher on a Pd–Au

alloy catalyst. Using silica-supported Pd (1.0 wt%) and Pd–Au

alloy (1.0 wt% Pd and 0.4 wt% Au) catalysts at reaction

conditions mimicking those used in the industrial process, VA

formation rates were found to be more than 10 times faster on

Pd–Au catalysts, consistent with findings for the industrial

process.46 This promotion was initially assigned to a ligand

effect where the weaker binding between reactants with the

alloy surfaces (for example weakly bound monodentate acetate

rather than strongly bound bidentate acetate, weakly bound

p-ethylene rather than di-s-ethylene) is expected to yield faster

coupling; and the weaker binding between the product VA

with the alloy surfaces facilitates its desorption.46

While it is clear that a ligand effect must play a role, studies

by Chen et al. on Pd/Au(100) and Pd/Au(111) model catalysts

revealed that ensemble effects play a more significant role in

affecting VA formation rates.35 Since VA formation requires

coupling between chemisorbed acetate and ethylene, a corre-

lated pair of Pd sites is needed. Considering the bond lengths

of adsorbed ethylene and acetate species, the optimized dis-

tance between two active sites is 3.3 A. Separation between

pairs of Pd monomers on Au(100) will be 4.08 A, while on

Au(111) this distance will be 4.99 A, prohibitively long for

coupling of these two reactive intermediates. The reaction

rate on Pd/Au(111) is indeed much lower than that on Pd/

Au(100) as evidenced in Fig. 4(A). The bonding and relative

distances involved between reacting species are shown

schematically in Fig. 4(B). The pair of isolated Pd sites, while

aiding in the formation of VA by providing the optimum

required spacing for coupling of the surface acetate and

ethylene species, was also proposed to suppress the formation

of reaction by-products, such as CO and CO2, thus improving

the overall selectivity.45

2.2.3. CO oxidation. Both planar models and high surface

area supported Pd are excellent catalysts for CO oxidation

(CO + 0.5O2 - CO2). Similarity in rates on both these

catalyst systems is generally regarded as strong evidence for

the structure insensitivity of this reaction over late transition

metals.26 The situation is more complex for Au. While Au

nanoparticles supported on certain oxides (especially the

reducible ones, e.g., TiO2) are specifically active for low-

temperature CO oxidation, bulk gold is inert. The reason

appears easy to understand: as a Langmuir–Hinshelwood type

of reaction, chemisorbed CO must react with a chemisorbed

active oxygen species to form CO2. In most cases this active

oxygen is atomic oxygen. The inertness of bulk gold is due to

its inability to activate di-oxygen. Indeed, if atomic oxygen is

pre-adsorbed onto planar Au, low-temperature CO oxidation

does occur facilely.47 It is therefore quite interesting to

investigate CO oxidation over bulk Pd–Au alloys. Presumably

by adding Pd (which is capable of activating di-oxygen) to Au,

one is able to form an active catalyst.

This hypothesis was tested on AuPd(100),5,6 Pd–Au alloy

thin films, and supported Pd–Au particles.40 Fig. 5(A) shows

reaction data at 10�7 Torr of CO pressure over planar Pd and

Pd–Au nanoparticles grown on a thin TiO2 layer deposited on

Mo(110).40 Clearly, while Pd alone is active, adding Au into

Pd greatly inhibits CO oxidation in vacuum. When the Au–Pd

ratio reaches 1, the alloy is totally inert. An AuPd(100) single

crystal also shows no activity under identical conditions.5,6

However, when kinetic measurements were carried out at

elevated CO pressures, the reaction data shown in Fig. 5(B)


reveal that AuPd(100) becomes orders of magnitude more

active than pure Pd at temperatures below B500 K. The

reaction kinetics shown in Fig. 5 raise three questions: (1)

what causes CO oxidation reaction to ‘‘turn on’’ at elevated

CO pressures on Pd–Au? (2) why is the alloy surface much

more active than pure Pd at elevated CO pressures and

relatively low temperatures? (3) why does CO2 formation ‘‘roll

over’’ at temperatures higher than B450 K over AuPd(100)?

The answer to the first question is twofold. (i) O2 does not

dissociate on isolated Pd sites. This is easily proven by an O2

temperature-programmed desorption experiment: on a Pd–Au

alloy surface with only isolated Pd surface sites, O2 desorption

due to recombination of chemisorbed atomic oxygen does not

occur suggesting that O2 does not dissociate during adsorption

to this surface.48 For the Pd–Au alloy model catalysts used to

acquire reaction data shown in Fig. 5(A), the lack of O2

dissociation precludes the subsequent CO2 formation reaction.

(ii) At near-atmospheric CO pressures, a sufficient amount of

Pd segregates to the surface and generates contiguous Pd sites

which, in contrast to isolated Pd sites, are capable of dissociat-

ing O2 to allow the CO2 formation reaction to proceed

(Fig. 5(B)). The driving force for this ‘‘chemisorption induced

segregation’’ is the stronger binding of CO with Pd than Au.

This phenomenon has been shown clearly by the PM-IRAS

data displayed in Fig. 3.5,6

Fig. 4 (A) Vinyl acetate (VA) formation rates (turnover frequencies,

TOFs) as a function of Pd coverage on Au(100) and Au(111). The VA

synthesis was carried out at 453 K, with acetic acid, ethylene, and O2

pressures of 4, 8, and 2 Torr, respectively. The total reaction time was

3 hours. The error bars are standard deviations, based on background

rate data. The two insets show Pd monomers and monomer pairs on

the Au(100) and Au(111) surfaces. (B) Schematic for VA synthesis

from acetic acid and ethylene. The optimized distance between the two

active centers for the coupling of surface ethylenic and acetate species

to form VA is estimated to be 3.3 A. With lateral displacement,

coupling of an ethylenic and acetate species on a Pd monomer pair

is possible on Au(100) but implausible on Au(111). Figure adapted

with permission from ref. 35. Copyright (2005) by the American

Association for the Advancement of Science.

Fig. 5 (A) CO conversion as a function of reaction temperature over

TiO2/Mo(110)-supported Pd and Pd–Au alloy particles. Reaction was

carried out at steady-state using a stoichiometric CO–O2 mixture at

PCO = 1 � 10�7 Torr. Note that kinetic data with different surfaces

are shown with different symbols. Figure adapted with permission

from ref. 40. Copyright (2010) by American Chemical Society. (B)

Arrhenius plots of the CO2 formation rate (in TOF) over AuPd(100) and

Pd(110) with 16 Torr CO and 8 Torr O2. Figure adapted with permission

from ref. 6. Copyright (2009) by American Chemical Society.


Some knowledge of the CO oxidation reaction mechanism

over Pt-group metals at near-atmospheric pressures is needed

to answer the second question. Briefly, for the majority of late

transition metals at relatively low temperatures, the metal

surfaces are covered with a full layer of chemisorbed CO

under typical reaction conditions. As such, the rate-limiting

step is CO desorption which creates empty surface sites for O2

chemisorption and dissociation. This reaction mechanism is

fully supported by (1) reaction rates displaying +1 order

dependence in O2 pressures and �1 order dependence in CO

pressures; and (2) the measured reaction activation energies

close to the CO desorption activation energies.25,26 The

reaction kinetic data shown in Fig. 5(B) indicate much weaker

binding energies for CO on a Pd–Au alloy catalyst as

compared to pure Pd. This is indeed the case proved by

calculating CO binding energies on Pd and Au sites on

AuPd(100) using the Clausius–Clapeyron relation. At zero

CO coverage, the CO heat of adsorption was found to be

69 kJ mol�1 on Au sites and 84 kJ mol�1 on Pd sites. The CO

heat of adsorption on Au single crystal surfaces is typically

very low, i.e., B50 kJ mol�1 on step sites and less than

40 kJ mol�1 on terraces. In contrast, the CO binding energy

on pure Pd is as high as 150 kJ mol�1.6 These CO binding

energies not only are consistent with reaction kinetics shown in

Fig. 5(B), but also provide strong evidence for charge transfer

from Pd to Au. Specifically, the CO binding energy of

69 kJ mol�1 on Au sites on the AuPd(100) surface is substan-

tially larger than CO binding energies on any pure Au

surfaces. This is best explained as a ligand effect where charge

transfer from Pd to Au enhances back-donation of electrons

from Au to CO, thus increasing CO binding energies.

Finally, we note that the surface composition of AuPd(100)

surfaces vary dynamically under reaction conditions where

higher temperatures cause more Au segregation to the surface

(Fig. 3). At the same time, the low binding energies of CO on

the alloy surfaces cause decreased CO residence times on the

surface at high temperatures. The combination of these two

factors explains the ‘‘rollover’’ of CO2 formation over

AuPd(100) at elevated temperatures.

3. (Supported) Nanoparticle catalysts

3.1. Synthesis

Supported Pd–Au catalysts can be easily synthesized using

traditional impregnation or deposition–precipitation of Pd

and Au salts, either concurrently or sequentially, followed

by calcination and reduction.4,7–11,13,49,50 The major drawback

is the lack of homogeneity of the formed catalysts in terms of

particle size, composition and shape. For example, it is not

uncommon to find Au alone particles, Pd alone particles and

Pd–Au alloy particles with different compositions on the same

support.7–11,13 Such catalysts may be useful for various appli-

cations but these are not ideal for fundamental catalytic

chemistry and catalyst design type of studies. In the latter

cases, metal nanoparticles with uniform composition and

monodispersion are highly desirable.51 Fortunately, during

the past two decades or so, much progress has been made in

nanomaterial synthesis such that the formation of size, shape

and orientation specific monometallic nanoparticles for use in

catalysis has become rather mature.51 The most commonly

used method is a colloid technique where nanoparticles are

synthesized by the reduction of a metal precursor with a

reducing agent in the presence of a protective agent to prevent

aggregation. Unlike the monometallic case, the formation of

bimetallic nanoparticles is more complex. By varying the

formation methods, either core–shell bimetallics or quasi-

homogeneous alloys are formed. Different techniques have

been developed in the past for bimetallic nanoparticle synth-

esis including alcohol reduction, citrate reduction, polyol

process, solvent extraction reduction, sonochemical method,

photolytic reduction, decomposition of organometallic pre-

cursors, and electrolysis of bulk metal.52 These nanoparticles

can be used directly as catalysts for certain reactions on

condition that the capping layer is porous and stable; or they

can be grafted onto high-surface-area supports, followed by a

capping layer removal step, as ‘‘regular’’ heterogeneous cata-

lysts. Two points are addressed for this process: (1) colloid

Pd–Au bimetallic nanoparticles, as these are usually synthe-

sized from Pd and Au chloride precursors, are negatively

charged due to residual Cl�. Adjusting the solution pH

to make the support surfaces positively charged helps the

grafting process.12 (2) Removal of the capping layer while

maintaining the nanoparticle dispersion, composition and size

is a great challenge. Decomposing the capping layer at the

lowest possible temperature appears to greatly inhibit sintering.53

In the following, examples are given on size and structure

control of Pd–Au nanoparticles.

Generally, for the most commonly used colloid techniques,

particle sizes are affected by multiple factors including type

and concentration of the metal precursors, reducing agents

and protective agents (soluble polymers, surfactants, and

organic ligands), formation temperature, etc. Two interesting

size control methods are briefly introduced: (1) Pd–Au can be

synthesized as dendrimer-encapsulated nanoparticles (DENs).

In this method, Pd and Au ions are first extracted from

solution into the dendrimer interior via complexation with

internal tertiary amines. Second, the metal ions are reduced

with BH4�, and the resultant atoms subsequently coalesce to

form zero-valent nanoparticles within the dendrimer templates.

In this case, the dendrimer framework not only controls the

sizes of the nanoparticles but also stabilizes them.28,54 (2)

Another interesting size managing approach is to synthesize

Pd–Au particles in reverse micelles. First, nanosized water

droplets are dispersed in a continuous oil phase. Second, metal

precursors and reductants are introduced into these water

droplets to react. The sizes of the alloy particles formed are

confined by the sizes of the water droplets.52

As to composition/structure control, the two approaches

typically used are simultaneous or sequential reduction of

appropriate precursors. In the former, weaker reducing agents,

for example polyol, lead to the formation of Au-core–Pd-shell

structures. This is because Au reduces more easily and provides

a seed for the reduction of the Pd shell. The size and thickness

of the core–shell can be controlled by the ratio of Pd–Au in the

precursor solutions. Strong reducing agents (e.g., BH4�) result

in the formation of a quasi-random distribution of Pd–Au

alloys. In sequential reduction, a monometallic core is synthesized


first and in the second step, the second metal is reduced onto

the core surface. This method is used to generate Au-core–

Pd-shell and Pd-core–Au-shell structures where the former are

more common. However, if synthesis is carried out at higher

temperatures, the enhanced mobility of atoms causes alloy

formation instead.54 Note that Au-core–Pd-shell structures are

also most often formed for supported Pd–Au bimetallics.

Besides the easier formation of Au0 as nucleation seeds, the

calcination–reduction treatments, often used during the synth-

esis of such catalysts, also promote formation of a Pd-shell.55

This is due to Pd diffusion to the surface during calcination

since it is more readily oxidized than Au. Upon reduction, Pd

can still remain enriched in the shell. It is to be noted, however,

that any stable Au-core–Pd-shell structure should be consid-

ered as kinetically stabilized because of (1) the lower surface

free energy of Au and (2) the complete miscibility of the two

metals.1

3.2. Characterization

Numerous techniques are used to characterize supported

Pd–Au bimetallic catalysts. General characterization methods,

e.g., specific surface area, pore size and distribution, metal

loading and dispersion, etc., are not discussed here. Instead,

some of those used to acquire detailed structural information

are briefly described in the following.

3.2.1 X-Ray diffraction (XRD). XRD is a routinely avail-

able technique that can be used to study Pd–Au alloys. As fcc

metals, Pd and Au have strong diffractions along the (111) and

(200) directions between 2y of 30 to 501. The diffractions of

PdxAuy alloy phases fall in between the corresponding diffrac-

tions of the pure metals. This offers straightforward identifi-

cation of alloy formation. Quantitative analysis of the XRD

patterns allows the composition of the alloy phases to be

determined. This can be done using Vegard’s law,13,15 or more

accurately, the Rietveld refinement method.7 Fig. 6 presents

XRD patterns of SiO2 supported pure Pd, Au and bimetallic

Pd–Au catalysts at various Pd–Au ratios prepared by impreg-

nation of the silica support with an alcohol solution of

colloidal dispersion of the two metal particles, followed by

air calcination.13 Several important points are worth mention-

ing: (1) the heterogeneity of the nanoparticles formed during

Pd–Au bimetallic synthesis including pure Pd phases (oxidized

to PdO during calcination), pure Au phases and alloy phases.

This is a good example demonstrating that the formation of

uniform alloy nanoparticles remains a major challenge for

supported catalysts. (2) The PdxAuy alloy phases are more

resistant to oxidation than pure Pd. This is evidenced by the

fact that during calcination in air, pure Pd oxidizes to PdO

while the alloy phases maintain metallic. This indicates that,

for catalytic reactions in heavily oxidizing environments,

Pd–Au alloys might be a good option for preventing catalyst

deactivation due to Pd oxidation. (3) The precursor Pd–Au

ratio clearly has profound effects on alloy formation where

substantially more alloy forms for Au75Pd25 compared to

Au25Pd75 precursor weight ratios.

3.2.2 X-Ray photoelectron spectroscopy (XPS). The surface-

sensitive nature of XPS allows for determination of the

near-surface composition of supported Pd–Au bimetallic

catalysts.15,7–13,49 This includes (1) near-surface Pd–Au ratios

and (2) oxidation states of Pd and Au. Also XPS analysis of

catalysts before and after catalytic reactions often yields rich

indirect information regarding changes of the catalysts during

reactions. Note that in ratioing Pd–Au XPS signals (Pd 3d3/2and Au 4f7/2 core-level features are generally used), atomic

sensitivity factors of both elements must be included.7 For

oxide supported catalysts, charging is always a significant issue

affecting accurate oxidation state determination. To circum-

vent this problem, generally binding energies (BE) are cali-

brated using internal standards (e.g., adventitious carbon C 1s

at B285.0 eV).7

In principle, the core-level BE change (as compared to BE of

pure metals) can be used to verify alloy formation since, as

discussed earlier, charge transfer does occur upon Pd–Au alloy

formation. However, for metal particles in the nano size

ranges, surface metal atoms are substantially more under-

coordinated than bulk atoms. In this case, final state effects

(e.g. screening) can be more pronounced than initial state

effects in determining binding energies in core level spectro-

scopy. Note that even when valence band spectroscopy fails to

reveal any charge transfer between the alloy components, one

can see changes in the core level BE.3 Therefore one must be

extremely careful in interpreting Pd–Au alloy formation and

charge transfer using core level XPS analysis. Finally, we note

that traditional XPS is measured at pressures lower than

B10�7 Torr; therefore, in situ applications at elevated pressures

are not possible. In recent years an ambient pressure XPS

technique (AP-XPS) has been developed. Although the highest

pressure allowable at present is only B1 Torr, this technique

has already shown the ability to monitor in situ dynamic

changes of Pd–Au alloy catalysts during CO oxidation.56

3.2.3 X-Ray absorption spectroscopy (XAS). XAS requires

synchrotron X-ray beamlines; therefore, widespread applica-

tion of the technique is limited. Still, the technique offers

unparalleled advantages for in situ applications under realistic

high temperature and pressure reaction conditions due to the

high energy and flux of synchrotron beams. XAS has two

Fig. 6 X-Ray diffractograms of monometallic and bimetallic catalysts

after calcination at 673 K. The numbers in the sample notation refer to

the relative weight percentages of the metals. Figure adapted with

permission from ref. 13. Copyright (2003) by Elsevier.


major uses: X-ray absorption near edge structure (XANES) is

used to determine local coordination geometries and metal

oxidation states; and extended X-ray absorption fine structure

(EXAFS) is used to identify neighboring atoms, interatomic

distances, and coordination numbers.

Lambert and co-workers17 were among the first to use EXAFS

and XANES to study colloidal Pd–Au nanoparticles. More work

has been done more recently by other researchers.53,54,57,58

Perhaps the best use of EXAFS in Pd–Au nanoparticles

characterization is the measurement of partial nearest-neighbor

coordination numbers (CN) nPdPd, nPdAu, nAuPd and nAuAu

obtained concurrently for data collected at Pd and Au

absorbing atom edges. The total CN for Pd is given by nPdM =

nPdPd + nPdAu, whereas the total CN for Au is nAuM =

nAuPd + nAuAu. These values are important in distinguishing

core–shell or random alloy structures. For example, if Pd

segregates to the surface while Au stays in the core of the

nanoparticles, nPdM will be smaller than nAuM. This is because

atoms at the surface have fewer neighbors than those in the

core. On the other hand, if nPdM is very close to nAuM, a quasi-

random alloy structure is likely. One might expect that

EXAFS could be a powerful technique for in situ monitoring

of dynamic composition changes of Pd–Au particles by mea-

suring the CNs described above, although not much work has

been done so far in this regard. Note also that the total

coordination numbers also provide a good estimate of particle

sizes. The other significant use of EXAFS is the determination

of Pd–Pd, Pd–Au and Au–Au bond lengths. This also yields

important information regarding the short-range environ-

ments around Pd and Au atoms. Pd–Pd bond lengths for bulk

Pd are 2.743 A and Au–Au bond lengths for bulk Au are 2.861 A.

For monometallic Pd and Au nanoparticles, these bond

lengths are slightly smaller. In Pd–Au alloys, the Pd–Au bond

lengths are rather close to Au–Au bond lengths and elongation

of Pd–Pd bond lengths also occurs. As has been discussed

earlier, this enhances the atomic-like character of Pd atoms and

weakens binding with reactants. The situation is more complex

for core–shell structures. For Pd-core–Au-shell structures,

Pd–Pd bond lengths stay rather close to pure Pd53 while for

Au-core–Pd-shell structures, the Pd shell adopts the bond

length of the Au core up to a shell thickness of B1 nm.14,54

Summarily, Pd–Pd bond lengths are more informative than

Pd–Au and Au–Au bond lengths for structure determinations.

Generally speaking, Pd–Pd bond lengths close to pure Pd

suggest Pd core structures or a high percentage of Pd-only

nanoparticles, whereas Pd–Pd bond lengths close to pure Au

suggest alloy or Pd-shell bimetallic structures.

In XANES studies, even simple comparisons of the spectra

for the nanoparticles with those for pure Pd and Au can yield

useful qualitative information regarding alloying and charge

transfer. For example, normalized derivatives of Pd K-edge

XANES spectra give rise to a broad maximum at the edge

energy, E0, attributed to weakly allowed 1s–4d transitions that

also reflect the density of unoccupied states in the Pd d band.

Comparing the spectra between pure Pd and Pd–Au colloid

samples led Lambert and co-workers to conclude that Pd

atoms go from a Pd-like environment in the core–shell struc-

ture to an alloy phase upon annealing.17 Scott and co-workers

compared XANES spectra at the Au LIII-edge between pure

Au and Pd–Au nanoparticles and observed a reduction of the

white line (the first feature after the edge jump) with increasing

Pd content. Since the intensity of the white line depends on the

number of d-holes in the Au atoms, this finding can be

rationalized by the enhanced filling of the Au d-band in

the PdAu alloys via charge transfer from the Pd s-band

(and perhaps p-band) to the Au d-band.53

3.2.4 Transmission electron microscopy (TEM). TEM is a

powerful and mature technique for studying bare and sup-

ported metallic nanoparticles. The most common low magni-

fication mode (bright field) is used to sample many particles

simultaneously to obtain particle size distributions. In the high

magnification mode (i.e., high-resolution TEM, HRTEM), single

particles can be analyzed at an atomic level to obtain information

such as faceting and lattice spacing of the nanoparticles. Com-

monly, TEM instruments are equipped with an energy dispersive

spectrometer (EDS) detector for qualitative elemental analysis.

TEM has been used extensively for studies of Pd–Au nano-

particles.4,7–11,14,15,46,53–56,60 Two examples are given below.

Fig. 7 presents scanning TEM-EDS of large particles of

Au–Pd (2.5 wt% Au–2.5 wt% Pd) catalysts supported on

carbon, TiO2 and Al2O3 calcined at 400 1C.10,55 Interestingly

for Au–Pd/C, Au and Pd maps cover identical areas and the

RGB reconstructed map is homogeneous indicating formation

of a homogeneous Pd–Au alloy. In contrast, for Au–Pd/TiO2

and Au–Pd/Al2O3, Pd maps occupy larger areas than Au maps

and the RGB maps are apparently non-uniform indicating

formation of Pd-shell–Au-core structures. In another study,

Ferrer et al. used energy-filtered TEM and scanning TEM-EDS

line-scanning techniques to image three-layer core–shell struc-

tures in Pd–Au nanoparticles.59 It is emphasized that such

structural details cannot be obtained using the other techni-

ques described above.

Other commonly used techniques to characterize Pd–Au

nanoparticles are UV-Vis spectroscopy12,14,28,52,54,59 and

Fig. 7 Montage of high-angle dark-field (HAADF) imaging (column 1),

Au map (column 2), Pd map (column 3) and RGB reconstructed overlap

map (column 4) [Au – blue: Pd – green] for calcined AuPd/C (row 1),

calcined AuPd/TiO2 (row 2) and calcined AuPd/Al2O3 (row 3). Note that

the calcined AuPd particles on TiO2 and Al2O3 supports show a Au rich-

core–Pd-rich shell morphology, whereas calcined AuPd particles on

activated C are homogeneous alloys. Figure adapted with permission

from ref. 10. Copyright (2008) by the Royal Society of Chemistry.


FTIR (especially DRIFTS).13,16,46,49 These, however, are not

covered in this article.

3.3 Examples of catalytic reactions

3.3.1. Direct H2O2 synthesis from H2 and O2. Hydrogen

peroxide is an important green oxidant that is used in many

large scale processes such as bleaching and as a disinfectant.

Current industrial synthesis of H2O2 utilizes a sequential

hydrogenation–oxidation route of an alkyl anthraquinone.

The process is only economical at large scale and at high

product concentrations. However when used, H2O2 is often

required on a much smaller scale and at lower concentrations.

Direct small scale production of H2O2 at the site where it is

used is highly desirable in these areas. Due to its superior

activity in hydrogenation, Pd was the catalyst of choice for

many initial investigations. In recent years, Hutchings and

co-workers have made major contributions in this area by

discovering that supported Pd–Au catalysts have higher

activity and selectivity for this reaction.8–11,55,60 In the follow-

ing the atomic origin of this promoting effect is addressed.

First of all, one has to realize that direct H2O2 formation

from H2 and O2 is not an oxidation reaction, but rather a

reduction reaction. This is understood by the fact that an H–H

bond breaks while an O–O bond is maintained during the

reaction. The fundamental difficulty of the direct route is that

H2O2 is unstable with respect to both hydrogenation and

decomposition while the non-selective combustion product,

H2O, is thermodynamically much more stable. This complexity

is shown schematically in Fig. 8.55 Clearly, switching off the

combustion path (that is, preventing O–O bond cleavage) is of

vital importance in enhancing H2O2 selectivity. As is well-

known, Pd is an excellent hydrogenation catalyst; unfortu-

nately for this reaction, it is also an excellent oxidation

catalyst. As such, a Pd-alone catalyst cannot achieve very

high H2O2 selectivity. The situation is drastically different for

Pd–Au catalysts. As has been discussed in detail in Section 2,

adding Au to Pd dilutes surface Pd concentrations such that

contiguous Pd sites disappear and only isolated Pd sites exist

at sufficiently high Au coverage. Significantly, isolated Pd sites

do not catalyze O2 dissociation.5,6,48 On the other hand, the

structure insensitivity for hydrogenation reactions over

Pd-based catalysts indicates that isolated Pd sites are still able

to activate H2.2,3 Therefore, by taking advantage of this

ensemble effect, one is able to tune the catalytic properties of

Pd–Au catalysts to dramatically enhance H2O2 selectivity. The

reaction results obtained by the Hutchings group are fully

consistent with this ensemble effect argument. For Pd–Au

catalysts supported on Al2O3, TiO2 and carbon, the Pd–Au/C

catalyst shows much higher selectivity than the other two. This

is because homogeneous alloys form on C while Pd-shell–

Au-core bimetallics form on Al2O3 and TiO2 (Fig. 7).10,55

Clearly, on the alloy surface Pd atoms are better isolated than

those on Pd-shell–Au-core surfaces. By using acids to treat the

Pd–Au/C catalysts, these authors found enhanced gold disper-

sion by generating smaller Pd–Au nanoparticles. Again, better

Pd isolation is fully expected upon enhanced Au dispersion.

Indeed, side reactions can be almost completely switched off

following acid treatments.60

3.3.2. Hydrodesulfurization (HDS) reaction. Noble metal

catalysts are widely used in hydrodesulfurization of petroleum

feed stocks. Pd is a good HDS catalyst although it tends to be

poisoned by the sulfur present in the feed. Venezia et al.

studied HDS of a model compound, dibenzothiophene

(DBT), over Pd/SiO2 and AuPd/SiO2 catalysts.13 The reaction

results shown in Fig. 9(A) clearly demonstrate the beneficial

effects of alloying, with Pd–Au bimetallics showing higher

Fig. 8 Schematic of the reactions and the corresponding reaction

heats during the direct synthesis of H2O2. Note that H2O2 is unstable

with respect to both hydrogenation and decomposition, and the non-

selective combustion of hydrogen is a facile competing reaction.

Figure adapted with permission from ref. 55. Copyright (2008) by

the Royal Society of Chemistry.

Fig. 9 (A) Total DBT conversion obtained at 593 K as a function of

gold content. (B) X-ray diffractograms of monometallic Pd and

bimetallic Au50Pd50 catalyst: (a) after calcination at 673 K; and (b)

after HDS of thiophene. Figure adapted with permission from ref. 13.

Copyright (2003) by Elsevier.


activity than either Pd or Au alone. Fig. 9(B) displays XRD

patterns of the catalysts after HDS. For Pd/SiO2, the metallic

phase converts to Pd4S after reaction while for Au50Pd50/SiO2,

no sulfide formation was found after reaction. The resistance

to sulfur poisoning of the alloy phase, therefore, nicely

explains the activity data. These results can again be under-

stood via an ensemble effect. First, rather large contiguous Pd

ensembles are required to form Pd4S which are lacking on the

alloy surface. Second, sulfur-containing substrate molecules

selectively adsorb on gold sites due to the strong affinity of

gold for sulfur, leaving surface Pd sites available for activating

H2 and facilitating the HDS reaction.13

Summary

Pd–Au bimetallic catalysts are technically important for vinyl

acetate synthesis. They also have great potential of being used

as catalysts for other industrial processes, including direct

H2O2 synthesis. From a basic catalytic science point of view,

the complete miscibility, small lattice mismatch, as well as

vastly different catalytic properties of Pd and Au make Pd–Au

a unique system for study. Both ensemble and ligand alloy

effects are used to describe the catalytic modification of Pd by

Au. To study these effects at an atomic scale, surface scientists

use single crystals, thin films and planar clusters as model

catalysts to study Pd–Au interactions, chemisorption and

catalytic reactions at well-defined conditions. Concurrently,

researchers in heterogeneous catalysis and materials sciences

use bare and supported Pd–Au nanoparticles to study their

catalytic properties under technically relevant conditions.

Overall, an ensemble effect is shown to be responsible for

the generation of specific surface sites that are highly active for

certain reactions. Also modification of surface ensembles

switches off side reactions and, therefore, enhances reaction

selectivities. A ligand effect is responsible for changing the d

character of Pd. This causes weaker interaction between

surface Pd and reactants/products. For certain reactions, this

also enhances activity and selectivity.

Acknowledgements

F.G. and D.W.G. gratefully acknowledge the support for this

work by the US Department of Energy, Office of Basic Energy

Sciences, Division of Chemical Sciences, Geosciences, and

Biosciences, and the Robert A. Welch foundation (A-300).

F.G. also thanks Dr C.H.F. Peden (PNNL) for fruitful

suggestions. The Pacific Northwest National Laboratory is

operated by Battelle for the U.S. Department of Energy.

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