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Applied Catalysis B: Environmental 67 (2006) 237–245

Gold supported on ceria and ceria–alumina promoted by molybdena

for complete benzene oxidation

D. Andreeva a,*, P. Petrova a, J.W. Sobczak b, L. Ilieva a, M. Abrashev c

a Institute of Catalysis, BAS, Acad. G. Bonchev Str., bl.11, 1113 Sofia, Bulgariab Institute of Phys. Chem., PAN, Kasprzaka Str. 44/52, 01-224 Warsaw, Poland

c Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria

Received 21 March 2006; received in revised form 27 April 2006; accepted 11 May 2006

Available online 21 June 2006

Abstract

New gold–molybdena catalysts supported on ceria and ceria–alumina in reaction of complete benzene oxidation were studied. The catalysts

were characterized by means of XRD, TPR, XPS and Raman spectroscopy. High and stable catalytic activity was established in the temperature

region 200–240 8C. The presence of gold causes a modification in ceria structure leading to an increase of Ce3+ and oxygen vacancies formation.

The loading of Al3+ increases additionally the oxygen vacancies, while a tendency of decrease of Ce3+ amount was observed. The presence of

alumina results also in a larger share of active oxygen species proved by analysis of O 1s XPS spectra. The differences in the activities within the

starting temperature range (150–180 8C) and in the region of 100% conversion (200–240 8C) could be explained by supposing that in the LT region

the electron transfer between nanosized gold and ceria particles via oxygen vacancies has a crucial role. In the HT region the oxygen mobility,

provoked by the defective structure of ceria due to the presence of Al3+, becomes of prevailing importance. It was also concluded that alumina

prevents the gold and ceria agglomeration, which is the main factor to avoid deactivation under extreme reaction conditions.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Gold; Gold–molybdena; Ceria–alumina; Hydrocarbon oxidation; XRD; XPS; TPR; Raman spectroscopy

1. Introduction

Quite recently it was established in our laboratory that gold

catalysts supported on ceria–alumina promoted by vanadia

exhibit a high and stable activity in complete benzene oxidation

at low temperatures [1]. The interesting properties of ceria as a

support for the noble metals catalysts are well known to concern

mainly its function as oxygen buffer. The three-way catalysts

can catalyze simultaneously the reduction of NO and the

oxidation of CO and hydrocarbons [2]. Efforts to increase the

oxygen storage capacity by introducing of different cation

dopants, e.g. Ba, Ca, Co, Cu, Mn, Nd, Pb, Sr, Y, Zn and Zr have

been put forward [3,4]. Most of these materials have shown

promotion of both oxygen vacancy concentration and oxygen

storage capacity as well as redox activities, compared to

undoped ceria. The doping of CeO2 by the addition of metals

* Corresponding author. Tel.: +359 2 979 2268; fax: +359 2 971 2967.

E-mail address: andreev@ic.bas.bg (D. Andreeva).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.05.004

with valence state lower than 4+ aimed to increase the oxygen

vacancies content in contrast to more conventional doping of

ceria with zirconia whose purpose is to increase the thermal

stability. Using Al(3+) as a dopant to ceria should play two

roles: on one side to increase the oxygen vacancies number in

ceria and on the other side to increase the stability of both gold

and ceria particles size during catalytic operation [1]. Our

results on gold–vanadia, based on different supports: titania,

zirconia, ceria and ceria–alumina, have shown higher activity in

complete oxidation of benzene at low temperatures [1,5–8].

The role of vanadia is to activate the benzene molecules and to

enhance the transfer of electrons from benzene to oxygen via

catalyst surface, while the oxygen is activated on nanosized

gold particles, giving rise to peroxo species [8]. A strong

synergistic effect between gold and vanadia was established on

titania and zirconia [5,6,8].

The aim of the present study was to investigate the effect of

molybdena on the activity of gold catalysts supported on ceria

and ceria–alumina in complete benzene oxidation (CBO). A

relationship between structure and catalytic activity of the

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245238

samples will be discussed. The influence of the preparation

method used for synthesis of ceria–alumina on the creation of

oxygen vacancies and stability of gold catalysts in hydrocarbon

complete oxidation will be discussed as well.

2. Experimental

2.1. Sample preparation

Two series of gold–molybdenum catalysts supported on

ceria–alumina were prepared. The ceria–alumina support was

synthesized by co-precipitation using nitrates of cerium and

aluminium as initial salts in the relevant ratio with a solution of

K2CO3 (10 and 20 wt.% of alumina were added, the percentage

of alumina is shown after the symbol Al). Respectively

catalysts samples on pure ceria were also prepared. The gold

was deposited on the supports as gold hydroxide, preliminary

suspended in water, under full control of all parameters of

precipitation – pH, temperature, reactant feed flow rates, stirrer

speed, time of precipitation and aging, etc. The precipitates

were filtered and washed carefully until absence of Cl� ions in

the washing filtrate. The samples were dried under vacuum at

80 8C and calcined in air at 400 8C for 2 h. The molybdenum

was introduced by wet impregnation with (NH4)6Mo7O24

solution. The precursors were dried once again under vacuum at

80 8C and calcined in air at 400 8C for 2 h. The samples,

containing only gold on pure ceria were denoted as AuCe and

on ceria–alumina as AuCeAl10 and AuCeAl20. The samples,

containing only molybdenum, were denoted as MoCe – on

ceria, MoCeAl10 and MoCeAl20 on ceria–alumina. The

catalysts containing both gold and molybdenum were denoted

as AuMoCe, AuMoCeAl10 and AuMoCeAl20. The initial salts

used HAuCl4�3H2O, Ce(NO3)3�6H2O, (NH4)6Mo7O24�4H2O

and K2CO3 were ‘‘analytical grade’’ of purity.

2.2. Sample characterization

The BET surface area of the samples was determined on a

‘Flow Sorb II-2300’device.

The X-ray diffraction patterns were registered on a DRON-3

automatic powder diffractometer, using Cu Ka1 radiation. The

crystal size of gold, ceria and alumina particles was calculated

on the basis of the peak broadening using ‘‘Powder Cell’’

program. The program gives the possibility to approximate

XRD spectra based on the corresponding theoretical effects

structures. The instrumental broadening was taken into

consideration. The XRD profiles were approximated by Lorenz

functions.

The Raman spectra were recorded using a SPEX 1403

double spectrometer with a photomultiplier, working in the

photon counting mode. The 488 nm line of an Ar+ ion laser was

used for excitation. The laser power on the samples was 60 mW.

The samples were prevented from overheating during the

measurements by increasing the size of the focused laser spot.

The optimal conditions were chosen, checking the intensity,

position and the width of the Raman line of CeO2 at 464 cm�1.

The spectral slit width was 4 cm�1.

The TPR measurements were carried out by means of an

apparatus described elsewhere [9]. A cooling trap (�40 8C) for

removing the water formed during reduction was mounted in

the gas line prior to the thermal conductivity detector. A

hydrogen–argon mixture (10% H2), dried over a molecular

sieve 5A(�40 8C), was used to reduce the samples at a flow rate

of 24 ml min�1. The temperature was lineally raised at a rate of

15 8C min�1. The sample mass charged was 0.05 g. The

amount was selected based on the criterion proposed by Monti

and Baiker [10]. In addition TPR experiments after reoxidation

were performed. The reoxidation with purified air was carried

out at two different temperatures. In the first case of high

temperature (HT) reoxidation the H2–Ar flow was discontinued

and air was fed at the temperature just after the end of the

corresponding TPR peak of the fresh sample. The sample was

kept in air at this temperature for 15 min and the TPR spectrum

was recorded after cooling down to room temperature (RT) in

purified argon flow. In the second case after the end of the TPR

peak the sample was cooled down in purified argon flow to RT,

re-oxidized in air for 15 min and then the TPR pattern was

registered (RT reoxidation).

The X-ray photoelectron spectroscopy data were recorded

on a VG Scientific ESCALAB-210 spectrometer using Al Ka

radiation (1486.6 eV) from an X-ray source operating at 15 kV

and 20 mA. The spectra were collected with analyser pass

energy 20 eV, step 0.1 eV and an electron take off angle 908.The samples were pressed into thin wafers and degassed in a

preparation chamber before analysis. The core level spectra

were evaluated by Shirley background subtraction, followed by

fitting with Gaussian–Lorentzian product peak. The charging

effects were corrected by adjusting Ce 3d3/2 peak, usually

described as u000 peak to a position of 917.00 eV [11]. This is a

strong, individual peak and its position can be established much

more precisely than that of the commonly used C 1s peak from

adventitious carbon.

2.3. Catalytic activity measurements

The catalytic activity of the samples in complete benzene

oxidation was measured using microcatalytic continuous flow

fixed bed reactor at atmospheric pressure, connected to a

‘‘Perkin-Elmer’’ gas chromatograph, equipped with a flame

ionization detector. The following conditions were chosen:

catalyst bed volume – 0.5 cm3 (particle size 0.25–0.50 mm),

inlet benzene concentration – 4.2 g m�3 in air, space velocity –

4000 h�1, the temperature range – 150–300 8C. The catalysts

amount charged into the reactor and the catalysts particles size

were selected to be small enough to avoid both bulk and pore

diffusion retardation effects. The samples were activated ‘‘in

situ’’ by purified air at 150 8C for 1 h. The catalytic activity was

expressed as a degree of benzene conversion.

The testing of catalytic stability was carried out as follows.

After the catalytic test the temperature was increased up to

500 8C for 90 min and the catalyst was kept in the reaction

mixture for 8 h at this temperature. After that the temperature

was decreased to that of 100% conversion and the benzene

conversion was measured.

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245 239

Table 1

Chemical composition, BET surface area and MoOx surface density of the

samples

Catalysts Au content

(wt.%)

MoO3 content

(wt.%)

BET surface

area (m2 g�1)

MoOx surface

density (nm�2)

CeO2 – – 84 –

CeAl10 – – 83 –

CeAl20 – – 83 –

MoCe – 4.0 57 2.86

MoCeAl10 – 4.0 77 2.12

MoCeAl20 – 4.0 100 1.63

AuCe 3.0 – 134 –

AuCeAl10 2.8 – 103 –

AuCeAl20 2.9 – 140 –

AuMoCe 3.0 4.0 71 2.29

AuMoCeAl10 2.8 4.0 110 1.48

AuMoCeAl20 2.9 4.0 123 1.33 Fig. 2. Catalytic activity in CBO at 200 8C of the catalysts on: (1) Ce; (2)

CeAl10; (3) CeAl20.

3. Results

The chemical composition, BET surface area and MoOx

surface density, calculated on the basis of the amount of

molybdena loading and the BET surface area of the samples,

are represented in Table 1. The molybdena loading corresponds

to MoOx surface monolayer [12]. The gold content was

determined by gravimetric analysis.

3.1. Catalytic activity

The catalytic activity of the samples was estimated in the

reaction of complete oxidation of benzene. The results on the

temperature dependence of the catalytic activity of the samples

are represented in Fig. 1. Generally, the samples containing

both gold and molybdena exhibit a higher activity compared to

the activity of the catalysts, containing either gold or

molybdena. The catalysts supported on CeAl20 are more

active than the corresponding catalysts on CeAl10. In Fig. 2 are

compared the catalytic activity data of the studied catalysts at

200 8C. One can see that the activity of the catalysts on ceria is

Fig. 1. Temperature dependence of the catalytic activity in CBO of the studied

catalysts.

higher than that of the samples on ceria–alumina, but the

activity of AuMoCeAl20 is quite close to the activity of

AuMoCe catalyst. The samples containing only molybdena

show a very low activity.

The results obtained after stability test at the extreme

conditions showed that the benzene conversion was reduced to

90% for AuMoCe sample, while for the AuMoCeAl20 sample

the same treatment did not lead to a change of the activity.

3.2. Catalysts characterization

3.2.1. XRD data

The average size of gold, ceria and alumina particles was

determined on the basis of X-ray diffractograms (Fig. 3). The

analysis of the XRD data shows the diffraction lines of CeO2

typical of the cubic crystal structure of fluorite type oxides. The

average size of gold, ceria and alumina particles, calculated

using the Scherrer equation, are represented in Table 2. The

average size of gold in AuCe sample was estimated by

HRTEM. Comparing the data obtained, it is seen that the

addition of alumina leads to a decrease in the average size of

ceria and slightly increase in the average size of gold. After

Fig. 3. X-ray diffraction patterns of gold-based samples.

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245240

Table 2

Average size of gold, ceria and alumina particles determined by XRD

Catalysts Average size (nm)

Au CeO2 Al2O3 fresh

Fresh Spent Fresh Spent

AuCe <5.0a 13.0 7.0 15.0 –

AuCeAl10 4.0 – 4.0 – 8.0

AuCeAl20 6.0 7.0 3.0 4.0 5.0

a Estimated by HRTEM.

Table 3

Full width at half maximum of the CeO2 line in the Raman spectra

Samples Frequency (cm�1) FWHM (cm�1)

AuCe 459 12.0

AuCeAl10 456 39.4

AuCeAl20 456 42.7

MoCe 459 36.5

MoCeAl10 458 47.4

MoCeAl20 457 51.9

AuMoCe 457 34.2

AuMoCeAl10 455 50.0

AuMoCeAl20 456 55.1

testing the stability of the two chosen catalysts – AuMoCe and

AuMoCeAl20, it was observed that the average size of gold and

ceria in the presence of alumina practically does not change,

while for the AuMoCe sample these values increase.

3.2.2. Raman spectroscopy

The Raman spectra of the molybdena containing samples are

represented in Fig. 4. In all studied samples, a strong band in the

region 455–459 cm�1 dominates, which is characteristic of

fluorite structure ceria. A strong line at 903 cm�1 was detected

with the molybdenum containing samples as well. Based on the

literature data this band could be assigned to the octahedral

Mo O (Oh) species of Mo7O246� [13]. No bands typical of the

crystalline molydbena were detected. The presence of gold in the

catalysts makes the samples strongly absorbing and the lines

become quite weak. The full width at half maximum (FWHM) of

the main line of ceria in the Raman spectra was estimated. The

data are presented in Table 3. Depending on the amount of loaded

alumina, one can see a substantial increase in these values from

12.0 cm�1 for AuCe to 42.7 cm�1 for AuCeAl20 sample. The

loading of molybdena leads to an additional enlargement of this

value (55.1 cm�1 for AuMoCeAl20 sample). As it was shown in

our previous investigations this widening could be used as a

measure of the number of oxygen vacancies created in ceria by

the addition of Al3+ or of an increase in the ceria dispersion

[14,15]. The ceria dispersion increases after alumina addition in

view of XRD data (see Table 2). It could be accepted that this

substantial increase in FWHM is due both to a decrease in the

average size of ceria as well as in the number of oxygenvacancies

formed in the lattice of ceria.

Fig. 4. Raman spectra of the molybdena containing samples.

3.2.3. TPR measurements

Fig. 5 represents TPR spectra of gold-based catalysts (AuCe,

AuCeAl10 and AuCeAl20), the TPR spectra of the initial

supports are shown as inset. In the TPR spectrum of pure ceria

two peaks were registered: a high temperature (HT) peak at

Tmax = 855 8C, connected with the bulk reduction of ceria and a

low temperature (LT) one at Tmax = 490 8C, assigned to the

ceria surface layer reduction [16]. In the spectra of both ceria–

alumina supports only one peak (Tmax = 535 8C for the CeAl10

sample and Tmax within the interval 500–550 8C for the CeAl20

sample) was registered in the region up to 800 8C. In the case of

gold containing samples the significant lowering of the

temperature of ceria surface layers reduction was observed

as in a previous study [1]. Complex peaks, due to the bulk CeO2

reduction, were recorded in the HT interval.

The TPR spectra of molybdena containing catalysts are

shown in Fig. 6, inset the TPR profiles of samples containing no

gold are compared. The hydrogen consumption with a

maximum around 800 8C can be assigned to ceria bulk

reduction. The Tmax of the peak, assigned to the molybdena and

ceria reduction, was lowest with the MoCe sample (487 8C).

The peak with the highest Tmax (517 8C) as well as the highest

intensity was observed with the MoCeAl20 sample. The LT–

TPR peaks of gold–molybdena catalysts were located at

temperatures about 300 8C lower than those of the correspond-

ing non-containing gold samples. These peaks are obviously

Fig. 5. TPR spectra of the gold-based catalysts; inset: initial supports.

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245 241

Fig. 6. TPR spectra of gold–molybdena catalysts; inset: molybdena samples.

Fig. 7. TPR spectra after RT and HT reoxidation of: (A) gold catalysts and (B)

gold–molybdena catalysts.

complex due both to ceria surface layers and to MoOx species

reduction. The peak of AuMoCe sample is narrow with a

predominant LT part at Tmax = 120 8C and a HT shoulder at

about 180 8C, assigned to the MoOx reduction. The peaks of the

samples on mixed ceria–alumina supports are broad with Tmax

within the interval 120–180 8C. The hydrogen consumption

around 800 8C in all TPR profiles can be connected with ceria

bulk reduction.

The LT peaks are of interest with respect to catalysis and in

Table 4 the hydrogen consumption corresponding to the first

LT–TPR peak of the studied catalysts is represented. It is seen

that the hydrogen consumption of the pure supports in the

presence of alumina is significantly higher compared to that of

pure ceria. The H2 consumption of the CeAl20 sample is the

highest. This behaviour could be connected with the enhanced

reduction of deeper ceria layers in the presence of alumina.

Upon adding molybdena some supplementary amount of

hydrogen, corresponding to the reduction of MoOx species, was

consumed. The comparison between gold–molybdena contain-

ing catalysts shows that the same tendency was reproduced

(higher H2 consumption in the presence of Al3+). In the latter

case the peaks are less intensive because they are located at

significantly lower temperatures, at which the oxygen mobility

is not so high, compared to that of the catalysts without gold.

Table 4

Hydrogen consumption (HC) corresponding to the first (LT) TPR peak in the

spectra of samples studied

Sample HC–LT peak (mmol)

Ce 24.6

CeAl10 35.8

CeAl20 50.3

AuCe 23.1

AuCeAl10 26.4

AuCeAl20 28.6

MoCe 33.0

MoCeAl10 46.5

MoCeAl20 60.5

AuMoCe 27.5

AuMoCeAl10 28.5

AuMoCeAl20 36.5

The TPR profiles of gold containing samples after RT and

HT reoxidation are represented in Fig. 7 – nonpromoted

(Fig. 7A) and promoted by molybdena (Fig. 7B). The hydrogen

consumption is given in Table 5. It is seen that after RT or after

HT reoxidation, the oxygen capacity of the fresh catalysts

containing only gold cannot be reached. In the case of AuMoCe

the oxygen treatment at RT and HT is also not enough to restore

the oxygen capacity of the fresh sample. For gold–molybdena

catalysts on mixed supports the hydrogen consumption at RT

and especially at HT (the temperatures at the end of the TPR

peak of the fresh samples which are practically equal to the

reaction temperatures for the highest benzene conversion) is

very close to that of the initial samples.

Table 5

Hydrogen consumption (HC) of LT peak after RT and HT reoxidation of gold

containing catalysts

Sample HC after RT

reoxidation (mmol)

HC after HT

reoxidation (mmol)

AuCe 12.0 15.0

AuCeAl10 18.0 18.0

AuCeAl20 16.0 20.0

AuMoCe 13.0 13.5

AuMoCeAl10 25.5 27.0

AuMoCeAl20 27.5 36.0

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245242

Table 6

XPS data of gold- and gold–molybdena catalysts

Catalyst Au 4f7/2 Ce3+ [17–19] O 1s [20–22]

Peak position (eV) at.% Peak position (eV) at.% Peak position (eV) at.%

AuCe – fresh 84.89 0.70 885.36 5.14 532.01 8.24

880.25 0.54 529.78 44.34

AuCeAl10 – fresh 84.13 0.31 885.24 1.92 533.38 4.38

85.68 0.07 880.39 0.29 531.78 16.74

529.79 39.06

AuCeAl20 – fresh 84.20 0.21 885.03 1.71 533.61 5.61

86.00 0.03 880.43 0.22 532.01 27.29

530.03 30.22

AuMoCe – fresh 84.26 0.41 885.44 3.90 533.75 2.21

880.03 0.59 531.79 9.81

529.75 47.39

AuMoCe – spent 84.19 0.27 884.92 5.31 533.04 15.90

85.63 0.05 879.85 0.63 531.45 11.62

529.71 39.74

AuMoCeAl10 – fresh 84.12 0.30 886.08 2.87 533.13 9.60

879.11 0.37 531.16 21.84

529.63 28.81

AuMoCeAl10 – spent 83.99 0.25 884.96 3.49 533.28 9.84

85.65 0.02 879.90 0.45 531.82 17.12

529.87 36.31

AuMoCeAl20 – fresh 84.22 0.16 885.47 1.20 533.54 7.95

880.04 0.29 531.86 30.92

529.93 22.92

AuMoCeAl20 – spent 84.15 0.15 885.36 1.69 533.19 15.52

85.80 0.01 879.81 0.28 531.81 26.94

529.90 21.15

3.2.4. XPS measurements

The data of the XPS measurements are summarized in

Table 6. The calculated BE of Au 4f7/2 peak for a gold–ceria

sample showed that the main part of Au is present in metallic

state. For the samples supported on ceria–alumina a very small

part of this peak was centered at higher BE (85.68 eV

for AuCeAl10 (0.07 at.%) and 86.00 eV for AuCeAl20

(0.03 at.%)), which could be connected with positively charged

gold particles. For the molybdenum containing fresh catalysts

only gold in metallic state was observed, while for the spent

ones an additional small part with higher BE was registered

as well. The Mo 3d5/2 peak was centered at 232.70 eV,

corresponding to Mo(6+) state [17]. Analyzing the calculated

concentration of Ce(3+) of the sample supported on pure ceria

and the corresponding values on ceria–alumina it was found

that the at.% of Ce3+ in the AuCe sample is higher than the

corresponding values for AuCeAl10 and AuCeAl20 samples.

These values decrease with the increase of the content of Al(3+)

(see Table 6, compare AuCe with AuCeAl10 and AuCeAl20

samples). The loading of molybdena leads, respectively to a

decrease in the at.% of Ce3+. For the spent gold–molybdena

catalysts the tendency of Ce3+ amount decrease due to the

loading of alumina is the same. The XPS signals of O 1s of the

studied samples (fresh and after catalytic operation) are given in

Fig. 8. The O 1s peaks are fitted with two or three components

(Table 6). In the XPS spectra of the AuCe sample in addition to

the lattice oxygen (BE = 529.78 eV (44.34 at.%)), another peak

with higher BE is also present (BE = 532.01 (8.24 at.%)). For

all the gold–molybdena catalysts, the O 1s peak was fitted with

three components. The component of O 1s peak with BE in the

region 531.1–532.0 eV is related to the ‘‘O� ions’’ compensa-

tion of deficiencies in the subsurface of metal oxides [21]. The

relative share of this component increases with the increase of

alumina amount for gold containing samples. This observation

is valid as well as for gold–molybdena fresh and spent samples.

The third component of the O 1s peak at about 533.00 eV

(Table 6) can be assigned to the weakly adsorbed water and

OH�/CO32� [21,22].

4. Discussion

In our previous studies, it was shown that gold–vanadia

catalysts supported on ceria and ceria–alumina exhibited high

activity in the reaction of CBO [1,7]. It was found out as well

that the addition of alumina to ceria leads to a prevention of

gold and ceria agglomeration, which is the main reason for the

high stability of these catalysts under extreme working

conditions [1]. In contrast to the gold–vanadia catalysts

supported on titania and zirconia, in the case of ceria there

is no strong synergistic effect between gold and vanadia when

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245 243

Fig. 8. XPS spectra of Au–Mo-supported catalysts fresh and spent.

they are present simultaneously on ceria. In the present study

the reactivity of gold–molybdena catalysts in the same reaction

was studied and the reactivity and properties of these catalysts

were evaluated. It was also searched for the role of alumina as

stabilizing agent on the activity of the catalysts. The preparation

of ceria–alumina support by co-precipitation aims to obtain a

higher concentration of oxygen vacancies with the intention to

increase the oxidation activity of the catalysts. Comparing the

activities of the catalysts, supported on ceria–alumina with

different ratio it was found out that the activities of the samples,

based on CeAl20, were higher than those of the corresponding

samples on CeAl10, but they were lower compared to the

catalysts on pure ceria. Such behaviour was observed also for

the gold–vanadia supported catalysts [1]. This experimental

fact confirmed our supposition about the role of ceria as an

active support for this class of catalysts.

The addition of alumina up to 10 wt.% practically does not

change the average size of gold particles, while the loading of

20 wt.% leads to a slight increase of this size according to the

XRD data. On the other side, judging the XPS results, a lower

concentration of gold was found on the surface in the presence

of alumina (see at.% of gold, Table 6). The dispersion of ceria

increases after alumina loading. This is also important, having

in mind the fact that some authors consider the activity of ceria

based catalysts to be controlled by ceria dispersion [23] and our

previous results fully support this suggestion [1]. The effect of

gold in all cases of gold containing catalysts leads to a

considerable lowering of the temperature of molybdena and

ceria surface reduction. This effect has already been registered

for gold–vanadia catalysts based on ceria and ceria–alumina

supports [1,7] and this is one of the reasons for the high activity

of these catalysts in the LT region. It was noticed that the ceria

reduction is stronger, when the amount of alumina is higher.

This enhancement of ceria reduction can be connected with an

increase of the oxygen mobility in the defective structure

generated by the introduction of alumina. In the LT region a

complex TPR peak was registered due to the reduction of ceria

surface layers and MoOx species. In the presence of alumina the

LT–TPR peaks were broadened additionally. Comparing the H2

consumption of the gold containing samples in LT region it can

be seen that the highest hydrogen consumption was registered

with AuMoCeAl20 sample. This is in accordance with the

higher concentration of oxygen vacancies in the presence of

higher amount of alumina. The Raman spectroscopy data also

confirm this conclusion (see Table 3). The data from the

additional experiments of reoxidation showed that the

hydrogen consumption after RT as well as after HT reoxidation

of gold catalysts could not reach the oxygen capacity of the

fresh ones. In the case of AuMoCe the oxygen treatment at RT

and HT is also insufficient to restore the oxygen capacity of the

fresh sample. For gold–molybdena catalysts on mixed supports

the hydrogen consumption at RT and especially at HT is quite

close to that of the fresh samples. For AuMoCeAl20 (fresh and

after HT reoxidation) the same H2 consumption was estimated.

The HT reoxidation was carried out at temperatures practically

equal to the reaction temperatures of highest benzene

conversion. These results correlate with the catalytic activity

data at higher temperatures due to the higher oxygen mobility

of the gold catalysts containing alumina. The catalytic activity

data in the HT interval are close to those of the corresponding

catalysts on ceria. In the LT interval the differences in the

conversion over the samples supported on ceria and ceria–

alumina are bigger. The highest activity is exhibited by

AuMoCe sample. For the samples not containing gold on ceria–

alumina a very high H2 consumption was also registered but the

temperature interval is with about 3508 higher compared to the

corresponding gold containing samples. This correlates with

the very low activity of the catalysts without gold in the LT

region. Two factors are of great importance for the higher

oxidation activity: (i) oxygen mobility – on one side, it should

be enhanced by the oxygen vacancies in a close contact with

small gold particles and on the other by the doping of CeO2 with

cations of valences lower than +4 and (ii) electron transfer from

hydrocarbons to oxygen molecule via the catalytic surface by

activation of hydrocarbon on MoOx species being able to

enhance the redox transfer. The activation of benzene on the

VOx species was found out by some of us by FTIR with gold–

vanadia supported on titania in complete oxidation of benzene

[24]. The MoOx species play the same role like that of VOx in

gold–vanadia supported catalysts. The atomic percentage of

Ce3+ decreases upon addition of alumina (shown by XPS data)

to achieve the charge neutrality of the lattice. The analysis of

the data in the region of O 1s XPS spectra showed that in the

region 531.1–532.0 eV the XPS peaks are related to the ‘‘ions

O�’’ active oxygen, whose relative part increases with the

D. Andreeva et al. / Applied Catalysis B: Environmental 67 (2006) 237–245244

increase of alumina amount for gold containing samples. This

observation is also valid for gold–molybdena fresh and spent

samples. Some authors consider that the oxidation activity of the

doped CeO2 catalysts increases with increasing of the so called

‘‘high energy part’’ (HEO 1s) in the same BE region and this

activity depends on the nature and concentration of the dopants

[4,25]. The HEO 1s signal in the present study also increases

with the addition of Al3+ and with its concentration. This could

explain the higher activity of the catalysts on CeAl20 than that of

the samples on CeAl10. On the other hand the activity has to be

dependent on the oxygen mobility, which increases with the

increasing of the temperature. A tendency of increasing the share

of weakly bonded oxygen species at the expense of lattice

oxygen in the spent samples compared to the fresh ones is

observed. This could be an indication for the participation of the

lattice oxygen in the oxidative reaction as a result of diffusion to

the catalyst’s surface (compare O 1s signals of the samples, fresh

and spent, Fig. 8). The results obtained on the reoxidation at HT

and RT support the above suggestion. Still the catalysts on ceria

exhibit a higher activity than those on ceria–alumina. This

observation is in accordance with the observation of Zhao and

Gorte, studying hydrocarbon oxidation reactions on Sm-doped

ceria [26]. The authors concluded: ‘‘A common assumption, that

doping ceria should increase the reactivity, is not generally

correct’’. In the LT region the activity depends to a great extent

on the electron transfer between gold and Ce3+ via oxygen

vacancies. Our supposition about the active sites in hydrocarbon

oxidation reaction can be represents as a complex Au&Ce3+ (&is oxygen vacancy) and an enhanced electron transfer between

Au and Ce3+ via oxygen vacancies is the main reason for the

higher activity. As it was shown by some of us, the presence of

nanosized gold particles causes a strong modification of ceria. It

results in facile oxygen vacancies formation and an enhanced

electron transfer from the support to the small gold particles [27].

The XPS data also confirm that the concentration of Ce3+

increases in the presence of gold. The oxygen vacancies with the

participation of Al3+ are really increased, but in this case

complexes of the type Au&Al3+ are formed and the electron

transfer via vacancies ought to be retarded. This is in agreement

with our supposition of the active role of ceria as support of gold

catalysts and satisfactorily explains the higher activity of the

catalysts on ceria. Comparing the catalysts on ceria–alumina it is

obvious that the role of oxygen vacancies in the oxygen mobility

is of substantial importance. In the LT region (150 8C) the

differences in catalytic activity between the catalysts on ceria

and ceria–alumina are significantly higher than in the HT

interval (200–250 8C). A conclusion could be drawn that the

electron transfer between gold and ceria is the main reason as a

basic driving force for the higher activity at LT region, while at

higher temperatures the role of oxygen mobility becomes

prevailing. The addition of alumina plays also another important

role preventing agglomeration of gold and ceria particles giving

stability to these catalysts. It could be suggested that the main

role of alumina is to be a structural promoter for gold–ceria

catalysts, while the presence of gold influences the electron

interaction on the interface between gold and ceria increasing the

redox activity in the LT region.

5. Conclusion

High and stable activity of gold–molybdena catalysts

supported on ceria and ceria–alumina was found in the reaction

of complete benzene oxidation. The alumina loading causes a

strong modification of the ceria support by formation of oxygen

vacancies, established on the basis of Raman and TPR data. The

XPS results have shown that the atomic percentage of Ce3+

decreases in the presence of alumina. The lower average size of

ceria particles is also a result of the presence of alumina in the

ceria structure, established by XRD. About 100% conversion of

benzene over gold–molybdena catalysts, supported on ceria and

ceria–alumina was reached in the temperature interval 200–

240 8C. The following order of catalytic activities was found:

AuMoCe > AuMoCeAl20 > AuMoCeAl10. The differences

in the activities are bigger in the starting temperature range –

150–180 8C. These results could be explained by supposition

that in the LT region the electron transfer between nanosized

gold and ceria particles via oxygen vacancies is playing a

crucial role. In the HT region the oxygen mobility due to the

defective structure of ceria in the presence of Al3+ becomes of

prevailing importance. The analysis of the O 1s XPS data has

shown that in the region related to the ‘‘O� ions’’ active oxygen,

their relative part increases with the increase of alumina

amount, which is valid for fresh and spent gold–molybdena

catalysts. This is in agreement with the higher activity of the

catalysts on CeAl20 than that of the samples on CeAl10. The

doping of ceria by alumina leads not only to oxygen vacancy

creation and higher ceria dispersion but also to prevention of

gold and ceria agglomeration being a structural promoter of

these catalysts.

Acknowledgement

DA, PP and LI are gratefully acknowledged to the National

Science Fund, Ministry of Education and Sciences of Bulgaria

(project X-1502).

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