preparation of alumina supported gold catalysts: influence of washing procedures, mechanism of...

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Applied Catalysis A: General 298 (2006) 57–64

Preparation of alumina supported gold catalysts: Influence of washing

procedures, mechanism of particles size growth

Svetlana Ivanova, Veronique Pitchon, Yvan Zimmermann, Corinne Petit *

Laboratoire des Materiaux, Surfaces et Procedes pour la Catalyse (UMR 7515 du CNRS),

ECPM-ULP, 25 rue Becquerel, F-67087 Strasbourg Cedex 2, France

Received 14 June 2005; received in revised form 16 September 2005; accepted 22 September 2005

Available online 18 November 2005

Abstract

This publication deals with the influence upon catalytic gold particle size distribution of the washing procedures preceding calcination.

The proposed preparation method and washing procedures lead to perfectly reproducible gold catalysts with a gold particle size of less than

2 nm. An explanation for the mechanism of gold particle growth is proposed. A controlled use of ammonia as a washing agent strongly

improves the dispersion of gold metal on alumina. The possibility of gold complexes being stabilised by the presence of ammonia is also

discussed.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Gold catalysis; Supported gold; Alumina; Washing procedures; Ammonia

1. Introduction

Gold has long been believed to be inactive as a catalyst.

However, Haruta and co-workers [1,2] reported that gold

exhibits extraordinarily high catalytic activity for low

temperature CO oxidation when it is deposited on a selected

group of metal oxides as small particles. The catalyst synthesis

procedure and subsequent pre-treatment plays a crucial role in

determining the particle size and the metal–support interac-

tion. There is general agreement in the literature that the

utilisation of preparation methods such as deposition–

precipitation, coprecipitation and CVD produces catalysts

of a broad dispersion with gold particle size of less than 5 nm.

The method of ionic exchange is essentially applied to zeolites

[3], while the gold complex [Au (en)2]3+ is used for cationic

exchange [4]. The synthesis procedure employed is a direct

anionic exchange method (DAE) [5]. Control of both catalyst

preparation and reproducibility requires a profound knowl-

edge of the precursors and the nature of metal/support

interaction.

* Corresponding author. Fax: +33 3 90 24 27 68.

E-mail address: [email protected] (C. Petit).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.09.020

The low melting temperature of gold metal could provoke

gold particle sintering during the calcination and catalytic tests.

So the choice of calcination temperature is also very important.

The optimal calcination temperature reported in the literature is

300 8C and beyond this temperature certain authors have

observed an increase in the gold particle size and a parellel

diminution of catalytic activity [6,7]. However, Maciejewski

et al. [8] did not report gold particle sintering for catalysts

supported on titania and zinc oxide calcined at temperatures as

high as 600 8C. Another important parameter is the atmosphere

inwhich calcination is performed.Lee et al. [9] suggested that the

treatment of manganese supported gold catalysts in air is

preferable than the use of hydrogen or vacuum conditions.

Alternately, the presence of chloride was reported by Costello

et al. [10] and by Oh et al. [11] to be responsible for gold particle

growth during the calcination. This is why chloride removal is a

very important step in the preparationof a gold catalyst. To assure

chloride removal awashing procedure is necessary. The presence

of a basic agent will permit the substitution of chloride ions by

hydroxyl groups. Here, we have chosen ammonia as a washing

agent. This kind of treatment is similar to that proposed by Xu

et al. [12] even if the precise details are not given in this article.

The aim of this paper is to study the influence of washing

procedure on gold particle size and to explain the mechanism of

particle formation and growth.

S. Ivanova et al. / Applied Catalysis A: General 298 (2006) 57–6458

Table 1

Elemental analysis of gold and chlorine as a function of washing agent and time

Samples supported

on g-Al2O3

Washing

agent

Elemental

analysis (wt%)

Au Cl

AuW H2O, 60 min 2 0.33

Au25N 25 mol L�1 NH3 1.37 �150 ppm

Au4N5 4 mol L�1 NH3, 5 min 1.36 �150 ppm



Au4N24H 4 mol L�1 NH3, 24 h 1.57 �150 ppm

AuS NaOH, 60 min 0.53 �150 ppm

AuU Urea, 60 min 0.99 �150 ppm

2. Experimental

2.1. Catalysts preparation

We have developed a novel method of preparation based

upon the direct anionic exchange of the gold species for the

hydroxyl groups of the support [5]. Aqueous solutions of

HAuCl4 of concentration 10�4 mol L�1 (pH 3.5) were made in

order to obtain a final Au loading of 2 wt%. The support g-

Al2O3, with a BET surface of 190 m2 g�1 was sieved with the

fraction 125–250 mm being retained. The gold solution was

heated to 70 8C and the support was introduced. Then the slurry

was filtered, washed, dried in an oven at 120 8C overnight and

calcined in air at 300 8C for 4 h.

2.2. Washing procedures

The washing procedures are applied in order to remove the

chloride ligands from the gold chlorohydroxy complexes or the

chloride simply attached to the support. The washing agents

were aqueous solution of ammonia (NH3�H2O) of different

concentrations (4 and 25 mol L�1), water, urea or NaOH. Two

different washing procedures were employed in the case of the

ammonia solution, short washing: 20 min with 25 mol L�1

NH3(aq) and slow washing: 1 h with 4 mol L�1 NH3(aq). No

additional washing by water has been made after ammonia

washing procedures.

Caution/safety note: The contact of ammonia with a gold

solution could provoke the formation of gold ammonia

complexes which are explosive [13]. The use of this procedure

is not dangerous if the gold complexes are strongly attached to

the support. The assurance that there remains no gold in the

solution prior to the introduction of ammonia is a pre-requisite.

2.3. Characterisation

Chemical analysis of Au and Cl in the samples was

performed by inductively coupled plasma atom emission

spectroscopy at the CNRS Center of Chemical Analysis

(Vernaison, France). The detection limit is 150 ppm for Cl.

Chemical analysis was performed following sample calcina-

tion. The Au weight loading of the samples is expressed in

grams of Au per gram of calcined sample: wt% Au = [mAu/

(mAu + mAl2O3)] � 100.

XRD analyses were performed on a Siemens D5000 powder

X-ray diffractometer. Diffraction patterns were recorded with a

detector side Ni filtered Cu Ka radiation (1.5406 A) over a 2u

range of 15–858 and a position sensitive detector using a step

size of 0.028 and a step time of 2 s.

The catalysts were characterised by transmission electron

microscopy (TEM) Topcon EM002B for determining the

morphology of the solid and gold particle size distribution. The

dispersion was calculated using the model proposed by Polisset

[14].

Mass spectrometry measurements were carried out on a

Quattro II (Micromass) triple quadrupole mass spectrometer

fitted with an electrospray source and operated in negative

mode. The principal parameters were fixed at 80 8C for the

source temperature, sample injection flow rate 5 mL min�1, the

m/z range from 200 to 2000 was obtained in 18 s, five scans

were summed to obtain the final spectrum.

3. Results and discussion

3.1. Influence of washing agent and washing time

Elemental analysis of the samples of gold and chloride

loadings as a function of washing agent and washing time is

presented in Table 1. The theoretical gold loading per catalyst

was calculated to be 2 wt%.

As indicated, the desired quantity of gold is obtained in the

case of the water washed sample (AuW). However, the quantity

of chloride is comparatively high which means that the water

treatment is not able to remove the chloride from the catalysts.

For all the other samples the chloride level was found to be

below 150 ppm which suggests that all basic treatments prior to

calcination are able to remove the chloride from the catalysts.

The aim of this basic treatment is to replace the chloride ligands

of the gold complexes over alumina with OH� groups and to

remove, the free chloride attached to the surface of the catalyst

before the calcination. From this point of view the washing

procedures are successful as the chloride level is beneath the

detection limit of the techniques employed. For ammonia

treated samples, a constant loss of around 30% of the gold

introduced is observed. This loss was attributed to non-attached

gold complexes removal. The ammonia concentration seems

not to affect the gold loadings. We have observed the same

values for the two concentrations tested. Changing the washing

time between 5 and 60 min did not affect the gold loadings

either. In all cases, loss of Au was found to be around 30%. It

seems that in the case of ammonia washing, the replacement of

chloride by OH groups provides the greatest interest as the

concentration and the time of contact between solid and

ammonia solution do not affect the percentage of gold

deposited. However, increasing the contact time between the

solid and ammonia solution to 24 h provided an increase in gold

loading. A subsequent deposition of gold as Au(OH)3 could

perhaps be supposed. While the quantity of gold deposited is

higher the possibility of a quick precipitation of Au(OH)3remains which could produce larger undesirable gold particles.

S. Ivanova et al. / Applied Catalysis A: General 298 (2006) 57–64 59

Fig. 1. Gold loadings as a function of the number of preparation.

This increase of gold loading is thought to be a coexistence of

DAE complexes with a precipitate of gold hydroxide.

For samples washed with NaOH (AuS) and urea (AuU) in

NH3 (4 mol L�1) an equal concentration in hydroxides was

calculated. Washing with urea at 80 8C permits the decom-

position of urea and formation of OH groups. The duration of

washing is fixed at 1 h. The elemental analysis results revealed

that the use of urea and NaOH cause large losses of gold,

particularly the NaOH. It could be supposed that the effects

over the gold complexes are different from those caused by

ammonia. If the role of the ammonia solution is only to

substitute the OH groups for the chloride ligands in the

complexes, then the NaOH and urea could possibly modify the

metal/support interaction and provoke a reappearance of gold

complexes in the solution and subsequently their elimination

through washing. Alternately, it is possible in the case of urea

that increasing the duration of washing brings a larger quantity

of deposited gold. A further explanation for the higher gold

loadings, in the case were ammonia is the washing agent, is a

possible stabilisation enhanced by the substitution of chloride

by an amino-type species, as suggested by Siller et al. [15] and

Duval [16].

Fig. 2. TEM photographs of Au/Al2O3: (a) water w

3.2. Reproducibility

Repeating the synthesis seven times during 6 months

indicates the reproducibility of both the preparation method and

the washing procedures. The results are presented in Fig. 1.

Preparation of alumina supported gold catalysts by DAE is

reproducible, with an average value of 1.34 wt% actual gold

loading.

As we have already noted, washing with ammonia

completely removes the chloride from the catalyst. However,

the presence of Cl� increases the possibility of gold particle

sintering [11,12]. Gold particle size on ammonia washed

samples was smaller compared to water washed one (Fig. 2).

Ammonia washing provokes a decrease in the average gold

particule diameter, e.g. from 16 nm for non-treated sample

(AuW) to 2.4 nm for Au4N60. Particle size distribution is

narrow and depends on the concentration of the washing agent.

For Au25N, gold particles size decreases to an average of

1.9 nm though particles smaller than 1 nm could be observed.

Particle size distribution for the samples is presented in Fig. 3.

The number of measured particles is 483 for Au4N60 and 567

for Au25N. 90% of the particles for these two samples are in the

fraction between 1 and 3 nm. For the Au4N60 sample, a few

particles larger than 5 nm are observed.

The average diameters are calculated by using the equation:

d ¼P

nidiPni

where ni is the number of particles with diameter di.

Sample characterisation by TEM was also undertaken as a

function of the duration of washing with ammonia (Fig. 4). The

following average gold particles diameters were obtained:

2.6 nm for Au4N5 (for a total of 216 particles), 2.4 nm for

Au4N20 (for a total of 586 particles), 2.4 nm for Au4N60 (for a

total of 483 particles) and 3.1 nm for Au4N24H (for a total of

460 particles). The results reveal that there is an optimal time

period for washing of between 20 and 60 min. Similar particle

size and distribution are observed for the two samples, Au4N20

and Au4N60. This underlines the conclusion from elemental

ashed sample and (b) ammonia washed sample.


Fig. 3. Particle size distribution for Au/Al2O3 as a function of washing agent

concentration.

Fig. 4. Particle size distribution for Au/Al2O3 as a function of washing time

(washing agent concentration 4 mol L�1).

Fig. 5. TEM photographs of Au/Al2O3: (a) water

Table 2

Elemental analysis of gold catalysts

Washing agent Elemental analysis

(wt% Au)

Elemental analysis

(wt% Cl)

Theoritical Experimental

H2O 0.5 0.5 0.21

NH3 (4 mol L�1) 0.5 0.31 �150 ppm

H2O 1 1 0.31

NH3 (4 mol L�1) 1 0.7 �150 ppm

H2O 2 1.93 0.48

NH3 (4 mol L�1) 2 1.36 �150 ppm

H2O 5 3.7 0.81

NH3 (4 mol L�1) 5 3.1 �150 ppm

analysis that an increase in the length of time for washing up to

1 h did not affect the catalyst.

For a sample washed continuously over 24 h (Au4N24H), an

increase in the average diameter was observed. Our conclusion,

considering the elemental analysis and the higher gold loading,

is that subsequent deposition of Au(OH)3 from solution onto

the surface of the catalyst occurred. The observation of the

coexistence of a small particle size fraction with another,

greater fraction of large particles confirms the hypothesis of a

subsequent precipitation of Au(OH)3.

3.3. Influence of gold loadings

In addition to the catalysts made by anionic exchange and

having an amount of gold close to 2 wt%, a series of catalysts

with different gold loadings was prepared. The elemental

analysis for these samples compared with the theoretical one is

presented in Table 2.

The desired gold loadings were obtained in the case of all

water washed catalysts, except the most loaded one. So a

limiting value of the capacity for adsorption was observed. Our

results confirm those reported by Brunelle [17] who found the

limiting value of adsorption of 3 wt% for platinum over gamma

alumina with a specific surface of 200 m2 g�1. The quantity of

washed and (b) ammonia washed 4 mol L�1.


Fig. 6. Particle size distribution for the sample 3.1% Au/Al2O3 washed

4 mol L�1 NH3.

Fig. 8. Variation in theoretical dispersion as a function of gold particle size in

cuboctahedral model.

chloride increases in parallel with the gold loadings. This is due

to the different quantity of gold precursor used. Washing with

NH3 causes loss of around 30% of the desired gold loading in

all the cases except for the most loaded catalyst. This was not

surprising, since all of the non-attached species are lost prior to

the washing procedure.

The twomost loaded catalysts were studied by TEM (Fig. 5).

The increase in gold loading should provoke an increase in

particles size because of greater amount of gold deposited and

also in the increase in the quantity of chloride.

Large gold crystallites are obtained in the case of a water

washed catalyst. This was expected because of incapability of

this type of washing procedure to eliminate the chloride prior to

the calcination. For this sample, particles larger than 100 nm

were observed. The high degree of dispersion for the catalyst

washed with NH3 is somewhat surprising. The stabilisation of

gold nanoparticles by ammonia washing could be supposed

here. The mobility of gold nanoparticles is limited by this

treatment and species strongly anchored across the support are

obtained. Size distribution for the sample washed with

ammonia is presented in Fig. 6. The average particle size

measured over 541 particles is 1.9 nm.

High-resolution TEM photographs were taken of this sample

in order to study the morphology of the particles (Fig. 7).

Fig. 7. HR TEM of the sample 3.1% Au/Al2O3 washed with 4 mol L�1 NH3.

We observed, in accord with the literature, a non-spherical

symmetry suggesting hemispherical morphology for particles

smaller than 3 nm [18]. The inter-reticular distance was

calculated to 2.72 A. In using this data and the model for

cuboctahedral particles proposed by Polisset [14], the disper-

sion of the gold particles was calculated.

A modeling of the dispersion is presented in Fig. 8.

From the average values obtained by TEM for gold particle

diameters the corresponding dispersion could be calculated

from a theoretical curve. All of the results for gold catalysts

supported over alumina and fully characterised by TEM are

presented in Table 3.

For water washed samples, the dispersion improves with the

dilution of the initial gold solution [5]. In fact, for these samples

the model is not fully appropriate owing to the change in the

morphologyofgoldparticleswhen theaveragesize isgreater than

5 nm, above which threshold they are considered as spherical.

Washing with ammonia prior to the calcination causes a

significant decrease in gold particle size, from 16 to 2.4 nm, and

an improvement in the degree of dispersion from 9 to 54%.

Using a more concentrated washing agent (25 mol L�1 rather

than 4 mol L�1) reduces the gold particle size while augment-

ing the dispersion up to as high as 70%. The same result is

obtained for 3Au4N60 samples.

It is possible to obtain, though difficult to provide evidence

for, gold particles smaller than 1 nm. A different catalytic

Table 3

Dispersion values of gold for the series of Au/Al2O3 catalysts

Sample Average particle size (nm) Dispersion (%)

AuW 16 9

Au25N 1.9 70

Au4N5 2.6 52

Au4N20 2.4 54

Au4N60 2.4 54

Au4N24H 3.1 41

3Au4N60a 1.9 70

a 3.1 wt% Au/Al2O3 washed with ammonia 4 mol L�1.


Fig. 9. Condensation products in gold in ethyl acetate solution.

Fig. 10. Polymers identified in aqueous gold solution by MS.

performance is anticipated for these samples and studies of the

surface of the catalyst are required [19].

3.4. Mechanisms of particles growth

A number of MS experiments were undertaken in order to

study the state of the gold precursor HAuCl4, in solution [20].

Recently, the results presented by Moreau et al. [21] confirm

mainly our results presented previously for pH effect on this

salt. HAuCl4 exists in solution together with simple gold

complexes of type [Au(OH)xCl4�x]�, where x is between 0 and

4, these polymers have m/z ratios greater than 700 (Fig. 9).

These experiments show some surprising results, namely the

links in polymer chain include metallic–oxygen bond and not

the expected metallic–chloride bond.

Three species are identified at this m/z ratio:Awith principal

m/z ratio 779. The loss of two chloride atoms results in a

structural isomer B and the loss of four OH groups in isomer Cwith m/z principal ratio at 713 and 707. All of the species

identified are in ratio 1:2:1 (Fig. 10).

These observations suggest the conclusion that the same

type of complex agglomeration could be possible not only in

the solution but also at the surface. Alternately, we have

observed a change in the colour of the catalyst precursor from

pale yellow to mauve during the process of sample drying at

100 8C. This again suggests a possible reduction of the metal

during drying, always a delicate step of the preparation.

If we consider that the majority of the DAE gold complexes

are bidentate, then a model of the growth of particle size for the

catalyst of type AuW could be proposed. If four neighbouring

Fig. 11. Model of the growth gold particle si

adsorption sites on alumina are occupied by bidentate gold

complexes particle size growth is possible through formation of

Au–Cl–Au bridges. The chloride ligand adopts the role of

electron acceptor and one of the gold species being deficient in

electrons will break the Au–O–Au bond leading to a lower

oxidation state (Fig. 11).

The distance between gold atoms decreases and results in an

increase of gold particle size during the calcination. The

presence of chloride (free or as a ligand) facilitates the mobility

of DAE species on the surface which is easier for monodentate

than for bidentate species and could provoke large gold particle

formation as seen with water washed samples (Fig. 5). The

ze through Au–Cl–Au bridges formation.


Fig. 12. Stabilisation of gold particles in an ammonia washed catalyst.

lower oxidation state Au(I) is unstable and decomposes easily

with gold metal deposition [22]. This phenomenon –

autoreduction – could explain the change of catalyst colour

observed during the drying process at 100 8C.Ammonia washing replaces Cl� ligands with OH groups

in the DAE complexes. The gold complexes such as

[AuCl4]�, which are adsorbed simply, are removed from

the surface by washing procedures. The chloride is

completely removed as revealed by elemental analysis and

cannot provoke an increase in particle size during thermal

treatments. The model mentioned above could not be applied

to these catalyst precursors. However, the formation of Au–

O–Au bridges could be envisaged, though oxygen is not able

to attract and break the Al–O–Au bond. For this sample, no

colour change was observed, which is an indication that the

gold complex migration and agglomeration is not very likely.

Alternately, stabilisation of gold complexes by NHx type

ligands, which are electron donors, could occur. The model

presented in Fig. 12 describes one of the possibilities of

stabilisation and do not claim for sure the real state on the

surface.

In theory, the increasing of gold loading in percentage terms

provokes an increase of gold particle size as well as the chloride

loading on the surface. For the samples heavily charged with

gold, the saturation of alumina adsorption sites supposes that

the grafting of the species is more likely in monodentate mode.

The agglomeration of gold particles in this case is favoured for

water washed samples and particles greater than 100 nm are

observed. This is somewhat predictable because this type of

washing cannot remove the chloride and the existence of

monodentate species only facilitates the migration and for-

mation of large gold particles during the thermal treatment. The

higher degree of dispersion for the most loaded catalyst washed

with ammonia, 3Au4N60, is surprising. However, this is proof

that the agglomeration of gold particles does not occur through

Au–O–Au bridges, either as a result of of geometrical

limitations or because of dispersion stabilisation through

hydrogen bonding and as mentioned above, ammonia

stabilisation could also occur.

3.5. Catalytic activity

The catalysts were tested in the reaction of CO [5] and

hydrocarbons (C1 to C3) total oxidation [23]. The catalysts

showed high activities over a range of concentrations and

temperatures. The very positive effect of the washing

procedure is observed and consists in a gain of 95 8C in

terms of T50 (temperature defined as the temperature at which

50% conversion was obtained), i.e. decrease from 163 8Cfor the water washed catalyst to 68 8C for the ammonia

washed sample. Furthermore, a remarkable resistance

to thermal ageing at 600 8C in the absence or presence

of water was observed due to the presence of strongly

anchored nanosized gold particles during the preparation

step.

4. Conclusions

In this paper, we have shown that the desired quantity of gold

is obtained in the case of water washed catalysts supported on

alumina. The washing with ammonia of the catalyst precursor

before the calcination causes loss of up to around 30% of gold,

by weight, which is an indication of the non-attached gold

species being removed from the surface. The quantity of gold

deposited is constant and depends upon the specific surface and

number of adsorption sites on alumina. The catalysts obtained

are perfectly reproducible.

The use of other washing agents is not appropriate because

of the positive role of ammonia in the stabilisation of DAE gold

complexes.

Washing with ammonia encourages gold nanoparticle

formation with an average diameter of less than 2 nm and a

high degree of dispersion, 70%.

A model of gold particle growth is proposed through the

formation of Au–Cl–Au bridges during the thermal treatment

of the catalysts washed only with water. However, this model

could not be applied to the catalysts washed with ammonia

for which a stabilisation by the groups of amine type is more

likely.


Acknowledgements

The authors are indebted to Dr. Brooks for proof-reading of

the manuscript. We gratefully acknowledge the partial financial

support of this research within the AuTEK project by

Anglogold and Mintek (Johannesburg, South Africa). The

authors thank to H. Herschbach and E. Leize for mass

spectroscopy experiments and fruitful discussion.

References

[1] M. Haruta, Catal. Today 36 (1997) 153.

[2] S. Tsubota, D.A. Cunningham, Y. Bando, M. Haruta, in: G. Poncelet, et al.

(Eds.), Preparation of the Catalysts, vol. VI, Elsevier Science B.V., 1995,

p. 227.

[3] V. Ponec, G.C. Bond, Catalysis by Metals and Alloys, Elsevier Science

B.V., Amsterdam, 1996.

[4] G. Riahi, D. Guillemot, M. Polisset-Thfoin, A.A. Khodadadi, J. Fraissard,

Catal. Today 72 (2002) 115.

[5] S. Ivanova, C. Petit, V. Pitchon, Appl. Catal. A 267 (2004) 191.

[6] E.D. Park, J.S. Lee, J. Catal. 186 (1999) 1.

[7] F. Boccuzzi, A. Chiorino, M. Manzoli, P. Lu, T. Akita, S. Ichikawa, M.

Haruta, J. Catal. 202 (2001) 256.

[8] M. Maciejewski, P. Fabrizioli, J.-D. Grunwaldt, O.S. Becker, A. Baiker,

Phys. Chem. Chem. Phys. 3 (2001) 384.

[9] S. Lee, A. Gavriilidis, Q.A. Pankhurst, A. Kyek, F.E. Wagner, P.C.L.

Wong, K. Yeung, J. Catal. 200 (2001) 298.

[10] C.K. Costello, M.C. Kung, H.-S. Oh, Y. Wang, H.H. Kung, Appl. Catal. A

232 (2002) 159.

[11] H.-S. Oh, J.H. Yang, C.K. Costello, Y.M. Wang, S.R. Bare, H.H. Kung,

M.C. Kung, J. Catal. 210 (2002) 375.

[12] Q. Xu, K.C.C. Kharas, A.K. Datye, Catal. Lett. 85 (2003) 229.

[13] J. Fisher, Gold Bull. 36 (2003) 155.

[14] M. Polisset, Ph.D. Thesis, University of Paris VI, 1990.

[15] L. Siller, M.R.C. Hunt, J.W. Brown, J.-C. Coquel, P. Rudolf, Surf. Sci. 513

(2002) 78.

[16] C. Duval, Traite de Microanalyse Minerale Qualitative et Quantitative

(1956) 372.

[17] J.-P. Brunelle, Pure Appl. Chem. 50 (1978) 1211.

[18] M. Haruta, CATTECH 6 (2002) 102.

[19] S. Ivanova, C. Petit, V. Pitchon, Appl. Catal. A, in press.

[20] S. Ivanova, C. Petit, V. Pitchon, Catal. Today, in press.

[21] F. Moreau, G.C. Bond, A.O. Taylor, J. Catal. 231 (2005) 105.

[22] P. Braunstein, R.J.H. Clark, J. Chem. Soc., Dalton Trans. 2 (1973) 1845.

[23] S. Ivanova, C. Petit, V. Pitchon, Gold Bull., in press.

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