This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

Cite this: DOI: 10.1039/c0cp01859g

Synthesis of silica supported AuCu nanoparticle catalysts and the effects

of pretreatment conditions for the CO oxidation reaction

J. Chris Bauer,aDavid Mullins,

aMeijun Li,

aZili Wu,

abE. Andrew Payzant,

b

Steven H. Overburyab

and Sheng Dai*abc

Received 18th September 2010, Accepted 10th December 2010

DOI: 10.1039/c0cp01859g

Supported gold nanoparticles have generated an immense interest in the field of catalysis due to

their extremely high reactivity and selectivity. Recently, alloy nanoparticles of gold have received

a lot of attention due to their enhanced catalytic properties. Here we report the synthesis of silica

supported AuCu nanoparticles through the conversion of supported Au nanoparticles in a

solution of Cu(C2H3O2)2 at 300 1C. The AuCu alloy structure was confirmed through powder

XRD (which indicated a weakly ordered alloy phase), XANES, and EXAFS. It was also shown

that heating the AuCu/SiO2 in an O2 atmosphere segregated the catalyst into a Au–CuOx

heterostructure between 150 1C to 240 1C. Heating the catalyst in H2 at 300 1C reduced the CuOx

back to Cu0 to reform the AuCu alloy phase. It was found that the AuCu/SiO2 catalysts were

inactive for CO oxidation. However, various pretreatment conditions were required to form a

highly active and stable Au–CuOx/SiO2 catalyst to achieve 100% CO conversion below

room-temperature. This is explained by the in situ FTIR result, which shows that CO molecules

can be chemisorbed and activated only on the Au–CuOx/SiO2 catalyst but not on the AuCu/SiO2

catalyst.

Gold has a full 5d band structure (thus no un-paired

electrons) and a high ionization potential making it a relatively

inert material. In fact, the dissociative adsorption of H2 or O2

on bulk gold does not occur1–3 and it was previously thought

to be catalytically inactive. However, in the late 1980’s Haruta

et al. demonstrated that small dispersed Au particles (2–5 nm)

on TiO2 created highly active catalysts for the oxidation of CO

at low temperatures (200 K).4 Since that major discovery there

have been a variety of methods created to synthesize highly

active gold nanoparticles on many types of solid supports. The

most traditional synthetic technique is the impregnation

method which involves immersing the metal oxide support

into an aqueous solution of HAuCl4, followed by calcination

of the dry powder, and then reduction under H2. This process

usually yields large spherical particles that are loosely attached

to the metal oxide support as well as producing Cl� ions that

can poison the catalyst. A deposition–precipitation (DP)

method was developed, to avoid the problems caused by the

impregnation process, by converting an aqueous solution

HAuCl4 to Au(OH)4� by the addition of a base and then

can be deposited as Au(OH)3 on the support.5 The product

could easily be washed in water to remove the remaining Cl�

ions and once dried could be reduced under hydrogen. Haruta

showed that Au/TiO2 synthesized using the DP method

yielded high turnover frequencies (TOF) for the CO oxidation

reaction due to the formation of hemispherical particles that

were strongly attached to the support.5 One drawback of the

DP method is that metal oxide supports with isoelectric points

below 5, such as SiO2, SiO2–Al2O3, and WO3, cannot be used

due to the highly negative surface charge in basic solution.

The ability to use SiO2 as a catalyst support is attractive

because of its high surface-area, thermal stability, mechanical

strength, and non-reducibility. Unfortunately, when silica is

used as a catalyst support for gold in CO oxidation experiments,

the catalytic activity is low.6 One possible reason is that the

Au–SiO2 interaction is weak under typical DP conditions

which can cause particle aggregation. This is because the basic

conditions required to hydrolyze HAuCl4 to Au(OH)4� forms

a highly negative silica surface due to its low isoelectric point

(B2). Though there are examples of catalysts prepared by the

DP method that yield small gold particles supported on silica

that have shown decent catalytic activities7,8 most attempts

have yielded large gold particles and low activity at temperatures

above 0 1C.9–14 Haruta synthesized small disperse gold particles

on silica by chemical vapor deposition to form highly active

CO oxidation catalysts below 0 1C.5 It was also demonstrated

that a colloidal deposition–precipitation method could routinely

deposit gold particles with a relatively uniform particle size on

a Chemical Sciences Division, Oak Ridge National Laboratory,Oak Ridge, TN 37831, USA. E-mail: dais@ornl.gov

bCenter for Nanophase Material Sciences, Oak Ridge NationalLaboratory, Oak Ridge, TN 37831, USA

cDepartment of Chemistry, University of Tennessee, Knoxville,TN 37996-1600, USA

PCCP Dynamic Article Links

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a variety of support materials (including TiO2, ZnO, Al2O3,

and ZrO2).15 Another colloidal dispersion method was used

based on the weak interactions between metal nanoparticles

and the metal oxide substrates in aprotic solvents.16 This

process created homogenously dispersed nanoparticles that

were weakly attached on the metal oxide support and when

annealed 300 1C formed strongly bound supported nano-

particle catalysts. Since Au nanoparticle samples were made

under the same conditions with the same size and structure the

effects from the supports could be studied more accurately.

Recently, Zhu et al. has modified the solution-based DP

method to produce small and disperse gold nanoparticle

catalysts supported on SiO2, using a cationic gold species

([Au(en)2]3+, en = ethylenediamine), that are highly active

toward CO oxidation below 0 1C.17 It was observed that the

catalytic activity of the catalysts was greatly affected by pH

conditions and highly stable gold particles could be formed.

Wang and co-workers reported that the CO oxidation

reaction could be improved through the synergistic effects of

an aluminosilicate supported Ag–Au alloy nanoparticles due

to the easier absorption of O2 on the surface of the catalyst.18

It was concluded that molecular oxygen and the formation of

O2� formed on the Ag sites while CO absorbed on the Au sites.

Electron transfer occurred from the Ag to the anti-bonding

orbitals of O2 contributed to weakening the OQO bond and

the reaction with CO could occur easily. On the other hand, it

was discovered that phase segregating a NiAu catalyst to

Au–NiO formed a highly active and stable catalysts for CO

oxidation.19 AuNi alloy nanoparticles (known to be unstable

at low temperatures) were formed by kinetically trapping the

alloy phase with a fast butyl lithium reduction.19 After the

alloyed AuNi nanoparticles were supported on SiO2 they were

phase separated to Au@Ni core–shell particles through high

temperature annealing in H2 and then further annealed in O2

to form Au–NiO/SiO2. This process resulted in Au nano-

particles with an average of 3.6 nm diameter and a maximized

Au–NiO interaction. This catalyst was found to be ultra-stable

and extremely active toward CO oxidation at low-temperatures.

Xie and co-workers took a similar approach by subjecting

AuSn intermetallic nanoparticles to a series of heat treatments

to yield Au/SnO2, which showed increased catalytic activity

toward CO oxidation.20 It is believed that this phase segergation

process forms a close interaction between the Au nanoparticle

and the respective oxide that stablilizes the catalyst and

prevents sintering and thus preserves the small particle size.

Although supported gold nanoparticles have proven to be

extremely active catalysts, bimetallic catalysts may have a

remarkable ability to enhance reactivity, selectivity and stability,

achieved by controlling their structure and composition.

Bimetallic nanoparticle catalysts of Au–Ag,21 Pt–Au,22

Au–Pd,23 Au–In,24 and Au–Cu25 with heteroaggregate,

core–shell, or alloy structures have shown an improvement

toward catalytic activity and selectivity for a number of

chemical reactions (including CO oxidation, alcohol oxidation,

dehydrodechlorination, and hydrogenation). In fact, a recent

review by Hutchings et al. showed that AuCu nanoalloys look

like promising catalysts for CO oxidation, propene epoxidation,

and benzyl alcohol oxidation because of their enhanced

activity and stability when compared to Au nanoparticles25a

as well as glucose oxidation.25b A synergistic effect was observed

in the case of AuCu/SBA-15 catalysts which helped provide

increased CO conversions at lower temperatures even in the

presence of rich hydrogen mixtures.26 Another study showed

that when Cu was added to Au/Al2O3 the selectivity for CO

oxidation increased and the catalyst remained stable, even

under realistic SELOX conditions.27 It seems that all studies

done on the Au–Cu system have only dealt with alloys with

varying compositions or bimetallic Au–Cu systems. To date, it

does not appear that any catalytic studies have been conducted

on ordered Au–Cu alloys, also known as intermetallic

compounds.

Intermetallic compounds have been known to show superior

catalytic properties when compared to their constituent ele-

ments as was demonstrated in a variety of chemical reactions

such as hydrogenation,28 ammonia synthesis,29 formic acid

oxidation30 and methanol oxidation.31 They have attracted an

interest in catalysis because they are atomically ordered in a

crystal structure different from their monometallic constituents

and generally have well defined compositions. Unlike disordered

alloys, intermetallic compounds are not only homogenously

mixed, but tend to have a uniform surface geometry making

them ideal for use in catalysis.

The Au–Cu system is one of the most studied alloy phases

and many synthetic methods have been developed to form

nanocrystalline alloys. Several physical methods such as

implantation32 or sputtering33 have been developed to form

nanocrystalline disordered Au–Cu alloy thin films. In another

case, Au and Cu nanoparticle composites were annealed to

form intermetallic AuCu and AuCu3 at significantly lower

temperatures due to reduced diffusion distances.34 Sol gel35

and many solution based methods with the use of organic

stabilizers have been used to form AuCu alloy nanoparticles36

and nanorods.37 Recently, Chen and co-workers used a seed-

based method where Au nanoparticles were transformed into

AuCu and AuCu3 intermetallic nanocrystals.

In this study, we modified a previously reported method

to convert supported nanoparticles into an intermetallic

nanocrystal catalyst by solution synthesis.30 Gold nanoparticles

were first supported on silica using a DP method. The silica

supported Au nanoparticles were then added to a copper

acetate solution and under the appropriate conditions Cu2+

reduced to Cu0 and diffused into the supported gold nano-

particles to form a partially ordered alloy structure. This

process offers a method to precisely control the copper

concentration as well as reduce sintering at high temperatures

necessary for intermetallic formation. This paper aims to

study the synthesis and pretreatment conditions necessary

for the activation of the intermetallic AuCu/SiO2 nanoparticle

catalyst for the oxidation of CO.

Experimental section

Materials

All chemicals were used as received and purchased from

Aldrich unless otherwise stated, copper(II) acetate, 98%;

gold(III) chloride trihydrate, ACS reagent; 1-octadecene, tech,

90%; oleylamine, approximate C18 content 80–90%; oleic

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acid technical grade, 90%; ethylenediamine, Reagentplus,

499%; and fumed silica, 99.8%.

Synthesis of Au(en)2Cl3 (en = ethylenediamine)

Au nanoparticles supported on silica were prepared by a

previously reported procedure.17 To synthesize the Au(en)2Cl3precursor, ethylenediamine (0.45 mL) was slowly added to an

aqueous solution of HAuCl4�3H2O (1.0 g in 10.0 mL of DI

H2O) to form a transparent brown solution. After stirring for

30 min. 70.0 mL of ethanol was added to induce precipitation.

The final product was centrifuged and washed in ethanol and

dried overnight.

Synthesis of the Au/SiO2 catalyst

According to a previously reported procedure17 40, 80, or

120 mg of Au(en)2Cl3 was dissolved in 100 mL of DI H2O to

make 1.6, 3.6, or 5 wt% Au loading on silica, respectively. A

1.0 M solution of NaOH was added drop wise to raise the pH

to 10.5. 1.0 g of SiO2 was added and the pH rapidly decreased.

Over the next 30 min 1.0 M NaOH solution was added to

maintain the pH at 10.5. The mixture was then transferred to a

60 1C water bath for 2 h. The final product was collected by

centrifugation, washed in H2O, dispersed by a vortexer and

centrifuged four times. The yellowish product was dried in a

vacuum oven for 5 h at 70 1C and reduced at 150 1C in 10%

H2/Ar for 1.0 h to obtain a red powder.

Synthesis of the AuCu/SiO2 catalyst

AuCu alloy nanoparticles supported on SiO2 were prepared by

first dissolving Cu(C2H3O2)2 (0.0501 mmol) into 1-octadecene

(20 mL), oleic acid (1.683 mmol), and oleylamine (1.683 mmol)

in a 100 mL 3-neck round-bottom flask. Then Au/SiO2

(180 mg, 3.6 wt% Au, 0.0387 mmol of Au) was added to the

solution and the mixture was magnetically stirred under

flowing Ar gas. The temperature was first raised to 120 1C

for 20 minutes to remove water then increased to 305 1C for

1.5 h. The heating mantle was removed and the reaction

mixture cooled to room-temperature, was diluted in ethanol,

and centrifuged at 7500 rpm for 7 min. The final product was

washed by suspending the powder in ethanol and centrifuging

four times before drying in air.

Characterization with XRD, TGA, and STEM

XRD data were collected at room temperature on a Siemens

D5005 diffractometer with Cu Ka radiation, in the range of

2y = 20–901 at a rate of 0.031 s�1. High-temperature XRD

data were collected on a PANalytical X’Pert Pro MPD

diffractometer with an Anton Paar XRK900 reaction chamber

in a 90% He–10% H2 atmosphere over the range from 20–901

2y using an X’Celerator RTMS detector. TG experiments were

conducted on a TGA 2950 instrument using a heating rate of

10 1C min�1 under N2 or air. TEM experiments were carried

out on a Hitachi HD-2000 STEM operated at 200 kV.

Catalytic experiments

To study the catalysts for CO oxidation 50 mg (unless

otherwise noted) of AuCu nanoparticles supported on SiO2

were packed into a U-shaped quartz tube (4 mm inner

diameter, i.d.) on an Altamira AMI 200 microreactor. The

catalysts were pretreated at 200–500 1C for 1 h in 10% O2 in

He, or at 400 1C in 10% O2 (He balance) followed by 300 1C in

10% H2 in Ar for 1 h. The catalyst was then cooled to room

temperature and the gas stream was switched to 1% CO

(balanced with air, H2O o 4 ppm) with a rate of 37 cm3

min�1. A portion of the product stream was extracted

periodically with an automatic sample valve and analyzed by

a dual-column gas chromatograph (GC) with a thermal

conductivity detector.

XAS/EXAFS measurements

The XAS data were recorded at the Cu k-edge (8979 eV) and

Au LIII-edge (11918 eV) at beamline X19a at the National

Synchrotron Light Source, Brookhaven National Laboratory.

Si(111) double crystal monochromator was used and detuned

by 30% at both edges to reject higher harmonics. The AuCu

nanoparticle sample was measured in a fluorescence mode

using a large area Passivated Implanted Planar Silicon (PIPS)

detector perpendicular to the upcoming beam. Ion chambers

for measuring Io and It were filled with nitrogen and a 50 : 50

mixture of N2 : Ar, respectively. The Au absorption was

measured out to k = 16. A small Zn impurity, less than

1% of the Cu concentration, limited the Cu absorption range

to k = 13.

The sample was ground to a fine powder, mixed with BN

and pressed into a 13 mm diameter pellet. The typical

absorbance of the analyte, m(x), was 0.1–0.2. The pellets weremounted in a Nashner–Adler38 reaction cell. A gas flow pipe

terminates near the sample’s surface to allow in situ studies of

temperature and gas-flow controlled experiments. A liquid

nitrogen dewar was placed on the top of the cell to enable

absorption measurements at low temperature (the lowest

temperature reached was about �110 1C) and, consequently,

minimize thermal disorder effects. The measurements were

recorded while the sample was in pure He. The sample was

heated by placing a heating element in place of the liquid

nitrogen dewar. Cu or Au foils were placed between the

sample and It during the measurements as an energy reference.

The programs ATHENA (version 0.8.056) and ARTEMIS

(version 0.8.012) were used to reduce and fit the data,

respectively.39 Data reduction consisted of pre-edge subtraction,

background determination, normalization and spectral averaging.

The k3-weighted and k1-weighted EXAFS functions, w(k),were Fourier transformed and fit in R space. The EXAFS

was fit over the first-shell neighbors, R = 1.0–3.0 A,

k = 2–13 A�1 at the Cu k-edge and R = 1.5–3.0 A,

k = 2–15 A�1 at the Au LIII-edge.

FTIR spectroscopy

An in situ FTIR study was conducted to study CO adsorption

on a diffuse reflectance cell (HC-900, Pike Technologies, cell

volume of about 6 cm3) which was used in a Nicolet Nexus

670 FTIR spectrometer with a MCT/A detector with a

spectral resolution of 4 cm�1. Approximately, 10.0 mg of

catalyst was used and after the desired pretreatments, a back-

ground spectrum was collected from the sample before CO

adsorption at room temperature using 256 scans and 4 cm�1

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resolution. Diffuse reflectance FTIR (DRIFTS) spectra were

collected at room temperature and obtained by subtracting the

background spectrum from subsequent spectra.

Results and discussion

The synthesis of a small and disperse Au/SiO2 catalyst is

difficult to achieve due to the weak interaction between gold

and silica. Therefore, a cationic gold species [Au(en)2Cl3] was

used to overcome the low isoelectric point of silica which

increased the interaction between the catalyst and support.

After the gold nanoparticles were formed from hydrogen

reduction the sample could then be used to form AuCu/SiO2

catalysts.

The synthetic procedure of ordered AuCu alloy nano-

particles supported on silica was adapted from a previous

reported method of forming a platinum-based intermetallic

catalyst by the conversion of supported platinum nano-

particles in a metal salt solution.30 In the case of AuCu,

Cu(C2H3O2)2 was dissolved in 1-octadecene, oleic acid, and

oleylamine followed by the addition of Au/SiO2 (after H2

reduction) and was heated at 305 1C. It should be noted that a

slight excess of Cu(C2H3O2)2 (1.5 molar ratio of Cu : Au) was

required to obtain a pure AuCu phase due to the possible

adsorption of some copper acetate on the support. In addition

to serving as a stabilizing agent, oleylamine also facilitated the

reduction of Cu2+ to Cu0 which could then diffuse into the

supported gold nanoparticles. In similar cases where nano-

particles are used as ‘‘seeds’’ or ‘‘reactive templates’’ it has

been shown that the reaction occurs through a solid-state

diffusion based process on the nanoscale.30,40 It is likely that

a galvanic replacement type reaction does not occur because

the reduction potential of Au0 is higher than Cu0. Previous

studies on the synthesis of AuCu nanocrystals in solution

indicate that the alloy formation begins as low as 150 1C

and weak ordering starts to occur around 250 1C.36a,41

In the X-ray powder diffraction pattern (XRD) of the

Au/SiO2 precursor is shown in Fig. 1 (bottom). It exhibits a

pattern indicative of a face-centered cubic pattern that is

referenced with simulated Au positions. The XRD pattern

exhibits short and broad peaks that confirm the formation of

nanocrystalline gold nanoparticles supported on silica after

the Au(en2)Cl3/SiO2 sample was reduced in H2. The AuCu

alloy made from the precursor is shown in Fig. 1 (top). An

XRD pattern shows a weak super lattice reflection (the 110

peak) as well as splitting reflections at the 200/002 and 220/202

peaks which indicate a tetragonal structure (P4/mmm (123)).

The 100 superlattice peak could not be observed due to the

overlap of a very broad silica peak with a maximum intensity

at B22.21 (not shown in Fig. 1). Though these features are

generally the characteristics of a AuCu intermetallic structure,

the intensities of these particular peaks are weak and may

indicate that a partially ordered alloy was formed. On the

other hand, the peak intensities increase for the 111, 200, 202,

311, and 222 peaks in the AuCu catalyst due to a more

crystalline sample from high reaction temperatures.

As seen in the bright field STEM images in Fig. 2a and b

the Au/SiO2 and AuCu/SiO2, respectively, show small and

disperse particles. Fig. 2c shows the histogram of the size

distribution of Au/SiO2 and indicates that the mean size is

approximately 4.9 � 1.4 nm. The small size corresponds to the

broad XRD diffraction peaks in Fig. 1a. Interestingly, the

mean size for AuCu/SiO2, shown in Fig. 2d, appears to have a

smaller diameter than the Au/SiO2 catalyst as well as shows a

tighter size distribution. As indicated in the histogram for

AuCu/SiO2, there is evidence that a small fraction of the

particles sinter, which accounts for particle diameters of up

to 14–15 nm, but a vast majority remain small and disperse.

The stability of the AuCu nanoparticles supported on silica

was studied by heating the catalysts in 10 1C increments for

30 minutes in an oxygen atmosphere and monitoring the

structural change by in situXRD. Fig. 3a shows XRD patterns

from the in situ experiment and it can be seen that upon

heating to 130 1C the 2y peak positions for the 111 and 200

peaks correspond to the AuCu phase at 40.491 and 45.7901,

Fig. 1 Powder XRD pattern of the intermetallic AuCu (L10 phase)

nanoparticles supported on SiO2 (top) made by reacting Au/SiO2

synthesized by the DP method after H2 reduction (bottom) with

Cu(C2H3O2)2 in a solution of 1-octadecene, oleylamine, and oleic acid.

Fig. 2 STEM image of (a) Au/SiO2 and (b) AuCu/SiO2. The corres-

ponding histograms showing the size distribution of Au/SiO2 and

AuCu/SiO2 are shown in (c) and (d), respectively.

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respectively. When the catalyst is heated above 150 1C the

peaks begin to shift toward smaller angles until they reach the

positions that correspond to pure gold at 38.185 and 44.3931

2y. It is likely that the Cu oxidized and segregated from

the alloy structure to slowly form a composite of Au and

amorphous CuOx. Bulk-scale Au–Cu alloys do not show

evidence of oxidation below 200 1C42 whereas the oxidation

at lower temperature of the nanoscale alloys can be attributed

to a large surface-area to volume ratio and short diffusion

distances. The oxidized Au–Cu nanoparticles were exposed to

a 10% hydrogen/argon atmosphere at elevated temperatures

to reduce the CuOx. The XRD patterns in Fig. 3b show that

the 111 peak (at 38.21) is stable up to 200 1C, but begins to

shift towards a higher angle (40.51) by 220 1C indicating that

Cu diffused into Au to form the alloy. By 280 1C the AuCu

alloy was fully formed and ordered into the tetragonal AuCu

intermetallic phase at 300 1C, which is consistent with previous

reports of synthesizing AuCu nanoparticles.34,36a,41,43

The phase segregation of the silica supported AuCu

nanoparticles was also investigated by XANES. The X-ray

absorption for the AuCu/SiO2 sample was measured as

inserted and after heating the sample for one hour at 150 1C

in 20% O2/He. The X-ray absorption near edge structure

(XANES) was recorded at 25 1C in pure He. Fig. 4 and 5

show the XANES at the Cu k-edge and Au LIII-edge,

respectively. The XANES of Cu foil, CuO powder and Au

foil are also shown in Fig. 4 and 5 for comparison.

The Cu k-edge XANES from the Cu foil has absorption

between ca. 8977–8982 eV. In CuO the absorption is shifted by

ca. 3 eV to higher photon energy. The as inserted AuCu

sample shows a decrease in the absorption intensity near

8981 eV compared to the Cu foil. X-Ray absorption in this

region is associated with excitation of the core electron into an

unoccupied Cu d-band.44 Therefore the Cu XANES from the

AuCu particles indicates a partial filling of the Cu d-band. A

second influence on the absorption is the possible presence

of oxidized Cu in the sample. The shape of the absorption

above 8985 eV is more consistent with Cu(I) than Cu(II).45

The presence of a small amount of oxidized Cu is indicated in

the EXAFS data (see below).

The Au LIII-edge XANES from the AuCu particles show a

small increase in intensity above the Au foil in the region from

11920 eV–11930 eV. This is where the ‘‘white line’’ absorption

occurs indicating decreased electron density in the Au d-band.

The magnitude of the change in the white line is consistent with

what has been seen in bulk AuCu alloys.46 There is also a small

shift in the edge position to higher photon energies for the as

inserted sample. Note that the photon energy has been very

carefully calibrated using an Au foil downstream of the sample.

Measurements at the two edges lead to the counterintuitive

conclusion that there is electron transfer from the Au to the Cu.

This is surprising since Au is more electronegative than

Cu. However it has been noted that the changes observed at

the Cu k-edge and Au LIII-edge primarily reflect the occupation

of the metal d-bands.44,46 The loss of electrons in the Au d-band

is compensated by a gain in the s–p bands. The net result is a

slight transfer from Cu to Au. It has also been noted that other

factors can affect the white-line intensity such as particle size and

interaction with the substrate.47,48

Fig. 3 In situ XRD patterns showing the transformation of (a) the intermetallic AuCu/SiO2 catalyst heated in air and (b) the oxidized AuCu/SiO2

catalyst heated under a 10% H2 reducing atmosphere.

Fig. 4 Cu k-edge XANES.

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When the sample is annealed in O2 at 150 1C for one hour

there appears to be no change in the Au white line intensity but

a small shift of the edge position to lower photon energy. The

nature of the shift at the Au edge is uncertain. The Cu k-edge

absorption shows a small decrease in the pre-edge absorption

intensity and a small shift in the edge position at 0.7 absorbance

to higher photon energy. These changes suggest a decrease in

the Au–Cu interaction combined with growth of oxidized Cu.

This interpretation is consistent with the EXAFS results.

The EXAFS at the Cu k-edge and the Au LIII-edge are

shown in Fig. 6 and 7, respectively. In addition to the as

inserted and oxidized AuCu nanoparticles the EXAFS for Cu

foil, CuO and Au foil are shown for comparison. Qualitatively

it can be seen that the AuCu nanoparticles differ significantly

from the reference compounds. The Cu foil has a nearest

neighbor distance of 0.255 nm which gives rise to a peak at ca.

0.22 nm in Fig. 6A. CuO has nearest Cu–O distance of 0.19 nm

and a Cu–Cu distance of 0.28 nm. The AuCu nanoparticles

have several broad peaks between 0.2 and 0.3 nm. The main

peak is at a position that is slightly greater than the peak for

Cu foil indicating a longer distance to the scattering neighbor.

There is also an indication of a peak near the Cu–O position.

While there is some indication of scattering from neighbors at

greater distances (40.3 nm) these features are weak and do

not align with the peaks from Cu foil. When the sample was

oxidized at 150 1C, the peaks between 0.2 and 0.3 nm

decreased in intensity while the Cu–O feature increased in

intensity.

The EXAFS at the Au LIII-edge (Fig. 8) also differs

significantly from the Au foil. There are two peaks evident

between 0.2 and 0.3 nm. The peak at lower R is tentatively

assigned to Au–Cu scattering while the peak at longer R is

assigned to Au–Au scattering. There is no strong evidence of

Au–O scattering. The features at lower R are attributed to

ringing in the Fourier transform. The AuCu nanoparticles

again show evidence of scattering at greater distances but the

features are weak and do not align with the Au foil. When the

sample is oxidized the Au–Cu peak decreases in intensity while

the Au–Au peak appears to slightly increase in intensity.

The first shell scattering at both edges was fit using the

ARTEMIS program to determine the identity, number and

positions of the nearest neighbors. The Cu and Au edges were

co-refined to achieve consistency in the results and they were

fit using k = 1 and k = 3 weighting to obtain good results for

low Z and high Z scattering partners. The results are shown in

Table 1 and representative fits are shown in Fig. 8 and 9. The

fits contained fifteen independent points with twelve variables

for Cu and twelve independent points with eight variables for

Au. In the as-inserted sample the numbers of Cu or Au

neighbors surrounding a given emitter are nearly identical,

i.e. the number of Cu scatterers and Au scatterers around Cu

(or Au) are nearly the same. Within error the results at the Cu

edge, indicating the scatterers around Cu, and the Au edge,

indicating the scatterers around Au, are consistent although

the total number of nearest neighbor metallic neighbors

around Au appears slightly greater. This may be due to the

presence of Cu–O around the Cu emitter that will reduce the

number of metallic neighbors. These results differ from

an earlier EXAFS study that examined bimetallic AuCu

clusters.49 In that study it was found that the nearest neighbor

shell was predominantly occupied by the same element, i.e. Cu

was mostly surrounded by Cu and Au was mostly surrounded

by Au. The interatomic distances are similar to those found in

that earlier study and are also similar to the bulk materials, i.e.

Cu foil and Au foil. The Cu–Au distance is also similar to

that found in crystalline CuAu but the Cu–Cu distance is

Fig. 5 Au LIII-edge XANES.

Fig. 6 Cu EXAFS—(A) the magnitude of the Fourier transform of

the EXAFS function and (B) the k3-weighted w(k) for CuAu nano-

particles on SiO2 and two reference materials.

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different.50 In crystalline AuCu the Cu–Cu distance is 0.28 nm.

Inclusion of a Cu–Cu shell at this distance reduced the quality

of the fits and could not be justified. Further, a Cu–Cu shell at

the nominal Cu foil distance, 0.26 nm, was necessary to obtain

an adequate fit. Therefore the EXAFS results indicate that the

AuCu nanoparticles were highly mixed, as opposed to a

core–shell or segregated structure, but the Au–Cu distances

were not consistent with crystalline AuCu.

When the sample was oxidized at 150 1C the number of

Cu–Cu neighbors decreased and the number of Cu–O neighbors

increased. There was also a slight decrease in the Au–Cu

neighbors and an increase in Au–Au suggesting that oxidation

depletes the Cu–metal interactions. As the Cu is removed from

first Au shell it is replaced by additional Au neighbors. The

nearest neighbor distances did not change at all following

oxidation. It can therefore be concluded, illustrated in

Scheme 1, that the silica supported Au catalysts do transform

into an AuCu alloy in solution and upon calcinations above

150 1C the Cu segregates and oxidizes forming an Au–CuOx

composite. The oxidized sample can then be transformed back

into the AuCu alloy under reducing conditions in H2.

The catalytic activity of intermetallic AuCu nanoparticles

supported on SiO2 was studied to observe the effects of

alloying copper with gold. The Au–Cu loading in silica is

approximately 4.6 wt%. The catalysts were subjected to a

series of heat treatments between room temperature and

500 1C in a 10% O2/He atmosphere for one hour to remove

moisture and organic residue from the surface of the catalysts.

The removal of organic residue is confirmed by thermal

gravimetric analysis (TGA), Fig. 10, which shows a large

decrease in weight (10%) between 250 1C and 400 1C. The

light off curves in Fig. 11 demonstrate how different pretreatment

conditions can influence the catalytic activity of the intermetallic

AuCu catalysts and how the activity of Au/SiO2 (calcined at

500 1C in 10% O2/He for 1 h) compares. As seen in Fig. 11a,

Au/SiO2 begins to catalyze the oxidation of CO near 0 1C,

however, after reaching its maximum conversion of 76% at

B43 1C it begins to deactivate as the temperature increases.

Previous reports of Au/SiO2 synthesized by similar methods

show similar light off temperatures that actually proceeded to

100% conversion and did not change as the temperature

increased, thus stability was not measured at higher

temperatures.51 The AuCu/SiO2 catalysts that were calcined

at 200 1C, 300 1C, 300 1C in 10% O2/He followed by annealing

10%H2/Ar at 300 1C displayed T50 conversion at approximately

220 1C, 176 1C, and 167 1C, respectively. The incomplete

removal of the organic surfactants from low temperature

calcinations may hinder access to the catalytically active sites

Fig. 7 Au EXAFS—(A) the magnitude of the Fourier transform of

the EXAFS function and (B) the k3-weighted w(k) for CuAu nano-

particles on SiO2 and Au foil.

Fig. 8 First shell Cu EXAFS fit for as inserted CuAu on SiO2—(A)

the magnitude of the Fourier transform of the EXAFS function and

the fit from 1.2–3.0 A and (B) the Fourier filtered k3-weighted w(q) dataalong with the fit.

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Table 1 Fitting results for CuAu nanoparticles on SiO2 and several reference materials

Sample Edge Path N R (0.01) s2 DE0 R factor

CuAu as inserted Cu Cu–Cu 3.5 (1.0) 2.58 0.012 2.0 0.00004Cu–Au 4.1 (1.0) 2.69 0.008 2.0Cu–O 0.9 (0.3) 1.92 0.003 5.1

Au Au–Au 4.6 (0.7) 2.86 0.006 5.1 0.0005Au–Cu 4.8 (0.7) 2.69 0.008 4.5

CuAu-oxidized Cu Cu–Cu 2.0 (1.0) 2.58 0.012 �4.38 0.0005Cu–Au 3.5 (1.0) 2.69 0.01 4.3Cu–O 1.5 (0.4) 1.92 0.003 8.7

Au Au–Au 5.5 (0.8) 2.86 0.006 6.1 0.001Au–Cu 3.8 (0.7) 2.70 0.008 5.1

Cu foil (25 1C) Cu Cu–Cu 12 2.54 0.008 3.6 0.00001CuO Cu Cu–O 4 1.95 0.002 11.9 0.001Au foil (�75 1C) Au Au–Au 12 2.87 0.005 4.6 0.00003

Fig. 9 First shell Au EXAFS fit for as inserted CuAu on SiO2—(A)

the magnitude of the Fourier transform of the EXAFS function and

the fit from 1.5–3.0 A and (B) the Fourier filtered k3-weighted w(q) dataalong with the fit.

Scheme 1 Illustration of the transformation process that silica sup-

ported gold catalysts undergo during the conversion process. The Au

nanoparticles are first transformed into weakly ordered AuCu alloy

catalysts, and then phase segregated during calcinations to Au–CuOx

heterostructures and finally can re-form the alloy under reducing

conditions.

Fig. 10 TGA curves of the AuCu/SiO2 catalyst under air (solid line)

and nitrogen (dashed line).

Fig. 11 Temperature-dependent CO oxidation over SiO2 supported

(a) Au nanoparticle catalysts pretreated at 500 1C in 10% O2/He

and intermetallic AuCu nanoparticle catalyst following different

pretreatment conditions: (b) 200 1C in 10% O2/He, (c) 300 1C

10% O2/He, (d) 300 1C in 10% O2/He for 1 h followed by 300 1C

in 10% H2/Ar for 0.5 h, (e) 400 1C 10% O2/He, and (f) 500 1C 10%

O2/He.

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and may be partly responsible for the latent CO oxidation

activity. However, when the samples are calcined at 400 1C

and 500 1C (Fig. 11e and f) the T50 conversions occur at

dramatically lower temperatures (25 1C and 6 1C) and based

on the XRD data presented in Fig. 3, the active phase is a

Au–CuO composite. In the case of the AuCu/SiO2 sample that

was calcined at 300 1C (to remove the organic surfactants)

followed by H2 to obtain the intermetallic structure it can be

observed that the catalytic activity dramatically increases at

the temperature where copper begins to segregate and oxidize

to form a Au–CuO composite structure.

Though a light off curve is useful in scanning the temperatures

as a catalyst activates, its interpretation for bimetallic catalysts

may be ambiguous if the catalyst structure changes with

temperature. This is especially true when elements, such as

copper, which can easily be oxidized under reaction conditions.

As the catalyst’s activity is probed over a range of temperatures,

as well as undergoing a variety of heat treatments prior to the

catalytic experiment, the correlation between the catalytic

activity and the phase of the Au–Cu system comes into

question. To obtain a better understanding of phase changes

on activity the weakly ordered alloy of the AuCu/SiO2 catalyst

was subjected to different pretreatment conditions and then

the CO oxidation activity was measured at room temperature

for two hours, conditions that inhibit any changes in the

crystalline phase or structure. Fig. 12 shows that when the

AuCu/SiO2 was not subjected to any pretreatment or when it

was calcined at 200 1C prior to catalysis experiment, little to no

CO conversion was observed. Interestingly, the sample heated

at 300 1C in Fig. 12c started with low conversion and gradually

approached 100% conversion within the 2 h time period.

Pretreatment of the catalyst in an O2 atmosphere at 400 1C

for 1 h yielded a Au–CuO catalyst that gave 100% CO

conversion at room temperature. It is important to note that

unlike the light-off curves in Fig. 11, the same catalyst in

Fig. 12 was sequentially subjected to all heat treatment

conditions just mentioned which may have formed a slightly

more oxidized and thus a more active catalyst Au–CuO/SiO2

at 400 1C compared to the sample in Fig. 11. The same

Au–CuO/SiO2 catalyst was then reduced in a 10% H2/Ar

atmosphere at 300 1C for 1 h to form the intermetallic AuCu

nanoparticle catalyst. It should also be safe to assume that

after heating in air at 400 1C all organic surfactants were

removed and the surface of the catalyst should be clean.

It is also certain that the intermetallic AuCu nanoparticles

supported on silica are stable up to 130 1C and by testing the

catalytic activity at room temperature the intermetallic structure

will not change. As shown in Fig. 12e, when the Au–CuO

catalyst (calcined at 400 1C) was reduced in H2 at 300 1C to

form the intermetallic AuCu/SiO2 catalyst, the activity drops

from 100% CO conversion (from the Au–CuO phase) to near

0% conversion at room temperature.

In situ FTIR spectroscopy was conducted to clarify the

catalytic behavior observed in Fig. 12. Fig. 13A–C show that

when the alloyed AuCu/SiO2 catalyst was heated in 2% O2/He

at 250 1C, 300 1C, and 350 1C, two broad absorption components

at 2117 and 2127 cm�1 were observed. These results agree

relatively well with reported FTIR spectra of CO absorbed

onto metallic Au (B2115 cm�1)52 and oxidized Cu supported

on Al2O3 (B2120).53 The broad features observed are due to

adsorbed CO that is accompanied by gas phase CO peaks that

are also observed at 2165 cm�1 and at 2115 cm�1 that underlies

the surface CO bond. Previous FTIR studies indicate that CO

absorbed on a cationic Au species exhibits an IR band at

2142 cm�1 after calcination at 400 and 500 1C.54

After the sample was oxidized at 350 1C, it was then treated

with H2 at 350 1C and then 400 1C. As shown earlier, CuOx is

reduced under these conditions and the Cu diffuses back into

the Au particles to re-form the weakly ordered intermetallic

phase. Exposure of CO at room temperature upon this

reduced AuCu/SiO2 results in no adsorbed CO. Only gas

phase CO bands are seen (Fig. 13D and E). The reduced

Fig. 12 Time-dependent CO oxidation of SiO2 supported inter-

metallic AuCu (L10 phase) after being subjected to different pretreatment

conditions: (a) as-synthesized catalyst, (b) 200 1C in O2 for 1 h, (c) 300 1C

in O2 for 1 h, (d) 400 1C in O2 for 1 h, and (e) 300 1C in H2 for 1 h.

Fig. 13 Room temperature FTIR spectra of CO adsorbed on SiO2

supported L10 intermetallic AuCu nanoparticles pretreated at (A)

250 1C in O2, (B) 300 1C in O2, (C) 350 1C in O2, (D) 350 1C in O2

followed by 350 1C in H2, (E) 350 1C in O2 followed by 400 1C in H2,

(F) 350 1C in O2, then 400 1C in H2, followed by 400 1C in O2.

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sample was then re-treated in O2 at 400 1C to form a phase

segregated Au–CuOx catalyst. As shown in Fig. 13F the bands

at 2117 and 2127 cm�1 characteristic of surface adsorbed CO

reappeared. The FTIR spectra in Fig. 13 therefore indicate

that CO will not sufficiently adsorb onto AuCu alloy nano-

particles and this fact is responsible for the low activity

observed in Fig. 12e.

Conclusion

It has been shown that preformed Au nanoparticles supported

on SiO2 can be converted into supported intermetallic AuCu

nanocrystals in an organic solution in the presence oleylamine

and oleic acid. This synthetic method allows for the formation

AuCu nanoparticles with diameters averaging around 4.0 nm

with a tight distribution in size. In situ XRD indicate that

when the silica supported AuCu intermetallic nanoparticles

are heated above 150 1C in O2 the copper oxidizes and

segregates to form a Au–CuO/SiO2 heterostructure. XANES

and EXAFS also confirmed that the AuCu nanoparticles were

homogeneously mixed and upon heating to 150 1C Cu begins

to oxidize and segregate from the intermetallic structure. CO

oxidation light-off curves indicate that the intermetallic AuCu

nanoparticles show little activity until temperatures approach

150 1C. At this point amorphous CuO begins to form and then

the catalytic activity rapidly increases to 100% CO conversion.

Catalyst samples that are treated in O2 at 400–500 1C show

high activity at room temperature. These high temperature

oxidations yield Au and amorphous CuO heterostructures are

formed with a high degree of interfacial contact and increased

thermal stability. For instance, when the weakly ordered alloy

of the AuCu/SiO2 catalyst is oxidized to Au–CuO/SiO2 at

500 1C, 100% CO conversion is reached at temperature

slightly above 0 1C and is maintained through 300 1C. It has

also been shown that even when these highly stable and

active catalysts are reduced in H2 at 300 1C to reform the

intermetallic structure the catalyst deactivates.

It has become apparent through the studies conducted

here as well as in conjunction with previous reports that

NiAu/SiO218 and AuSn19 alloy catalysts can successfully be

manipulated through a series of heat treatment conditions to

yield highly active and stable catalyst for the oxidation of CO.

The advantage of alloying a catalytically active metal with a

second metal is that one can precisely control the composition

of the alloy and therefore the amount of the oxide that is

formed to stabilize the catalyst.

Acknowledgements

The research was sponsored by the Division of Chemical

Sciences, Geosciences, and Biosciences, Office of Basic Energy

Sciences, U.S. Department of Energy, under Contract No.

DE-AC05-00OR22725 with Oak Ridge National Laboratory,

managed and operated by UT-Battelle, LLC. Use of the

National Synchrotron Light Source, Brookhaven National

Laboratory, was supported by the US Department of Energy,

Office of Science, Office of Basic Energy Sciences, under

Contract No. DE-AC02-98CH10886. A portion of this research

was conducted at the Center for Nanophase Materials

Sciences, which is sponsored at Oak Ridge National Laboratory

by the Scientific User Facilities Division, U.S. Department of

Energy.

References

1 A. G. Sault, R. J. Madix and C. T. Campbell, Surf. Sci., 1986, 169,347.

2 N. Saliba, D. H. Parker and B. E. Koel, Surf. Sci., 1998, 410, 270.3 J. Wang and B. E. Koel, J. Phys. Chem. A, 1998, 102, 8573.4 (a) M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal.,1989, 115, 301; (b) M. Haruta, S. Tsubota, T. Kobayashi,H. Kageyama, M. J. Genet and B. Delmon, J. Catal., 1993, 144,175.

5 M. Haruta, CATTECH, 2002, 6, 102.6 M. Haruta, Catal. Today, 1997, 36, 153.7 S. Galvagno and G. Parravano, J. Catal., 1978, 55, 178.8 C. Mohr, N. Hofmeister, M. Lucas and P. Claus, Chem. Eng.Technol., 2000, 23, 324.

9 Y. S. Chi, H. P. Lin and C. Y. Mou, Appl. Catal., A, 2005, 284,199.

10 C. M. Yang, M. Kalwei, F. Schuth and K. J. Chao, Appl. Catal., A,2003, 254, 289.

11 S. H. Overbury, L. Ortiz-Soto, H. G. Zhu, B. Lee, M. D. Amiridisand S. Dai, Catal. Lett., 2004, 95, 99.

12 L. Guczi, G. Peto, A. Beck, K. Frey, O. Geszti, G. Molnar andC. Daroczi, J. Am. Chem. Soc., 2003, 125, 4332.

13 G. Martra, L. Prati, C. Manfredotti, S. Biella, M. Rossi andS. Coluccia, J. Phys. Chem. B, 2003, 107, 5453.

14 M. Okumura and M. Haruta, Chem. Lett., 2000, 396.15 M. Comotti, W.-C. Li, B. Spliethoff and F. Schuth, J. Am. Chem.

Soc., 2005, 128, 917.16 N. Zheng and G. D. Stucky, J. Am. Chem. Soc., 2006, 128, 14278.17 (a) H. G. Zhu, Z. Ma, J. C. Clark, Z. W. Pan, S. H. Overbury and

S. Dai, Appl. Catal., A, 2007, 326, 89; (b) H. G. Zhu, C. D. Liang,W. F. Yan, S. H. Overbury and S. Dai, J. Phys. Chem. B, 2006,110, 10842.

18 A.-Q. Wang, J.-H. Liu, S. D. Lin, T.-S. Lin and C.-Y. Mou,J. Catal., 2005, 233, 186.

19 (a) S. Zhou, Z. Ma, H. Yin, Z. Wu, B. Eichhorn, S. H. Overburyand S. Dai, J. Phys. Chem. C, 2009, 113, 5758; (b) S. Zhou, H. Yin,V. Schwartz, Z. Wu, D. Mullins, B. Eichhorn, S. H. Overbury andS. Dai, ChemPhysChem, 2008, 9, 2475.

20 K. Yu, Z. Wu, Q. Zhao, B. Li and Y. Xie, J. Phys. Chem. C, 2008,112, 2244.

21 (a) N. K. Chaki, H. Tsunoyama, Y. Negishi, H. Sakurai andT. Tsukuda, J.Phys. Chem. C, 2007, 111, 4885; (b) M. J. Kim,K. Y. Lee, G. H. Jeong, J. K. Jang and S. W. Han, Chem. Lett.,2007, 1350; (c) J.-H. Liu, A.-Q. Wang, Y.-S. Chi, H.-P. Lin andC.-Y. Mou, J. Phys. Chem. B, 2004, 109, 40; (d) M. Tominaga,T. Shimazoe, M. Nagashima, H. Kusuda, A. Kubo, Y. Kuwaharaand I. Taniguchi, J. Electroanal. Chem., 2006, 590, 37; (e) C. Wang,H. Yin, R. Chan, S. Peng, S. Dai and S. Sun, Chem. Mater., 2009,21, 433; (f) H. G. Zhu, L. L. Bao, S. M. Mahurin, G. A. Baker,E. W. Hagaman and S. Dai, J. Mater. Chem., 2008, 18, 1079.

22 (a) H. Lang, S. Maldonado, K. J. Stevenson and B. D. Chandler,J. Am. Chem. Soc., 2004, 126, 12949; (b) S. Mandal, A. B. Mandaleand M. Sastry, J. Mater. Chem., 2004, 14, 2868; (c) S. Zhou,K. McIlwrath, G. Jackson and B. Eichhorn, J. Am. Chem. Soc.,2006, 128, 1780; (d) S. H. Zhou, G. S. Jackson and B. Eichhorn,Adv. Funct. Mater., 2007, 17, 3099.

23 (a) D. A. Cadenhead and N. G. Masse, J. Phys. Chem., 1966, 70,3558; (b) F. Ksar, L. Ramos, B. Keita, L. Nadjo, P. Beaunier andH. Remita, Chem. Mater., 2009, 21, 3677; (c) M. O. Nutt,K. N. Heck, P. Alvarez and M. S. Wong, Appl. Catal., B, 2006,69, 115; (d) M. O. Nutt, J. B. Hughes and M. S. Wong, Environ.Sci. Technol., 2005, 39, 1346; (e) D. Wang, A. Villa, F. Porta,L. Prati and D. Su, J. Phys. Chem. C, 2008, 112, 8617.

24 C. Mohr, H. Hofmeister, J. Radnik and P. Claus, J. Am. Chem.Soc., 2003, 125, 1905.

25 (a) C. L. Bracey, P. R. Ellis and G. J. Hutchings, Chem. Soc. Rev.,2009, 38, 2231; (b) M. Tominaga, Y. Taema and I. Taniguchi,J. Electroanal. Chem., 2008, 624, 1.

Dow

nloa

ded

by U

nive

rsity

of

Ten

ness

ee a

t Kno

xvill

e on

27

Janu

ary

2011

Publ

ishe

d on

18

Janu

ary

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

0CP0

1859

GView Online

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.

26 X. Liu, A. Wang, X. Wang, C.-Y. Mou and T. Zhang, Chem.Commun., 2008, 3187.

27 T. S. Mozer, D. A. Dziuba, C. T. P. Vieira and F. B. Passos,J. Power Sources, 187, 209.

28 (a) A. Bahia, I. T. Caga, J. M. Winterbottom, J. M. Brown,C. E. King and I. R. Harris, Appl. Catal., 1986, 25, 199;(b) J. Barrault, D. Duprez, A. Guilleminot, A. Percheron-Gueganand J. C. Achard, Appl. Catal., 1983, 5, 99.

29 T. Takeshita, W. E. Wallace and R. S. Craig, J. Catal., 1976, 44,236.

30 J. C. Bauer, X. Chen, Q. S. Liu, T. H. Phan and R. E. Schaak,J. Mater. Chem., 2008, 18, 275.

31 E. Casado-Rivera, D. J. Volpe, L. Alden, C. Lind, C. Downie,T. Vazquez-Alvarez, A. C. D. Angelo, F. J. DiSalvo andH. D. Abruna, J. Am. Chem. Soc., 2004, 126, 4043.

32 C. Maurizio, G. Mattei, P. Mazzoldi, S. Padovani, E. Cattaruzza,F. Gonella, F. D’Acapito and F. Zontone, Nucl. Instrum. MethodsPhys. Res., Sect. B, 2003, 200, 178.

33 (a) E. Cattaruzza, G. Battaglin, F. Gonella, R. Polloni,B. F. Scremin, G. Mattei, P. Mazzoldi and C. Sada, Appl. Surf.Sci., 2007, 254, 1017; (b) M. Twardowski and R. G. Nuzzo,Langmuir, 2002, 18, 5529.

34 (a) A. K. Sra and R. E. Schaak, J. Am. Chem. Soc., 2004, 126,6667; (b) R. E. Schaak, A. K. Sra, B. M. Leonard, R. E. Cable,J. C. Bauer, Y.-F. Han, J. Means, W. Teizer, Y. Vasquez andE. S. Funck, J. Am. Chem. Soc., 2005, 127, 3506.

35 J.-H. Gwak, S.-J. Kim and M. Lee, J. Phys. Chem. B, 1998, 102,7699.

36 (a) A. K. Sra, T. D. Ewers and R. E. Schaak, Chem. Mater., 2005,17, 758; (b) T. Del Castillo-Castro, E. Larios-Rodriguez,Z. Molina-Arenas, M. M. Castillo-Ortega and J. Tanori,Composites, Part A, 2007, 38, 107.

37 A. Henkel, A. Jakab, G. Brunklaus and C. Sonnichsen, J. Phys.Chem. C, 2009, 113, 2200.

38 M. S. Nashner, A. I. Frenkel, D. L. Adler, J. R. Shapley andR. G. Nuzzo, J. Am. Chem. Soc., 1997, 119, 7760.

39 B. Ravel and M. Newville, J. Synchrotron Radiat., 2005, 12, 537.40 N. H. Chou and R. E. Schaak, J. Am. Chem. Soc., 2007, 129, 7339.

41 W. Chen, R. Yu, L. Li, A. Wang, Q. Peng and Y. Li, Angew.Chem., Int. Ed., 2010, 49, 2917.

42 V. A. Lavrenko, L. I. Kuznetsova and A. I. Malyshevskaya,Powder Metall. Met. Ceram., 2005, 44, 377.

43 R. E. Cable and R. E. Schaak, J. Am. Chem. Soc., 2006, 128, 9588.44 M. Kuhn, T. K. Sham, J. M. Chen and K. H. Tan, Solid State

Commun., 1990, 75, 861.45 A. Tadjeddine, A. Lahrichi and G. Tourillon, J. Electroanal.

Chem., 1993, 360, 261.46 M. Kuhn and T. K. Sham, Phys. Rev. B: Condens. Matter Mater.

Phys., 1994, 49, 1647.47 V. Schwartz, D. R. Mullins, W. Yan, B. Chen, S. Dai and

S. H. Overbury, J. Phys. Chem. B, 2004, 108, 15782.48 M. Varrkamp, J. T. Miller, F. S. Modica, G. S. Lane and

D. C. Koningsberger, Proc. 10th Intern. Congr. Catal., ed.L. Guczi, F. Tetenyi, Elsevier Sci. Pub., 1993, vol. A, p. 809.

49 G. H. Via, K. F. Drake, G. Meitzner, F. W. Lytle and J. H. Sinfelt,Catal. Lett., 1990, 5, 25.

50 E. Bjerkelund, W. B. Pearson, K. Selte and A. Kjekshus, ActaChem. Scand., 1967, 21, 2900.

51 H. Yin, Z. Ma, S. H. Overbury and S. Dai, J. Phys. Chem. C, 2008,112, 8349.

52 (a) M. A. P. Dekkers, M. J. Lippits and B. E. Nieuwenhuys, Catal.Lett., 1998, 56, 195; (b) J. C. Clark, S. Dai and S. H. Overbury,Catal. Today, 2007, 126, 135; (c) J. H. Yang, J. D. Henao,M. C. Raphulu, Y. Wang, T. Caputo, A. J. Groszek,M. C. Kung, M. S. Scurrell, J. T. Miller and H. H. Kung,J. Phys. Chem. B, 2005, 109, 10319; (d) M. Mihaylov,B. C. Gates, J. C. Fierro-Gonzalez, K. Hadjiivanov andH. Knozinger, J. Phys. Chem. C, 2007, 111, 2548; (e) F. Boccuzziand A. Chiorino, J. Phys. Chem. B, 2000, 104, 5414;(f) J. D. Henao, T. Caputo, J. H. Yang, M. C. Kung andH. H. Kung, J. Phys. Chem. B, 2006, 110, 8689; (g) D. C. Meierand D. W. Goodman, J. Am. Chem. Soc, 2004, 126, 1892.

53 O. Dulaurent, X. Courtois, V. Perrichon and D. Bianchi, J. Phys.Chem. B, 2000, 104, 6001.

54 Z. Wu, S. Zhou, H. Zhu, S. Dai and S. H. Overbury, J. Phys.Chem. C, 2009, 113, 3726.

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