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Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47

Contents lists available at ScienceDirect

Journal of Molecular Catalysis A: Chemical

journa l homepage: www.e lsev ier .com/ locate /molcata

nfluences of CeO2 microstructures on the structure and activity ofu/CeO2/SiO2 catalysts in CO oxidation

un Qiana, Shanshan Lva, Xiaoyan Xiaob, Huaxing Suna, Jiqing Lub,engfei Luob, Weixin Huanga,∗

Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, University of Science and Technology of China,inzhai Road 96, Hefei 230026, ChinaZhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China

r t i c l e i n f o

rticle history:eceived 26 December 2008

a b s t r a c t

The influences of CeO2 microstructures on the structure and catalytic activity of supported Aunanoparticles in CO oxidation have been investigated in Au/CeO2/SiO2 catalysts. CeO2/SiO2 supports

eceived in revised form 10 February 2009ccepted 11 February 2009vailable online 21 February 2009

eywords:u/CeO2/SiO2 catalysts

with various CeO2 microstructures were prepared and used to prepare Au/CeO2/SiO2 catalysts bydeposition–precipitation using HAuCl4 as the precursor. Au(I) species and Au nanoparticles competefor the surface oxygen vacancies on CeO2 and highly dispersive CeO2 on SiO2 facilitates the formationof Au(I) species. Meanwhile, the presence of Au also facilitates the creation and stabilization of surfaceoxygen vacancies on CeO2. The Au nanoparticle–CeO2 interface plays an important role in the activity of

CO o

O oxidationtructure–activity relation

Au/CeO2/SiO2 catalysts in

. Introduction

Supported Au catalysts have been demonstrated to exhibitnique catalytic activities in several important oxidation reac-ions, such as preferential and total oxidation of CO, directionxidation of hydrogen to hydrogen peroxide, epoxidation of propy-ene, water gas shift reaction, oxidation of alcohols, and totalxidation of volatile organic compounds [1–4]. Among these reac-ions, CO oxidation catalyzed by supported Au catalysts has been

ost extensively investigated not only because this reaction hasotential applications but also because it is anticipated that thetructure–activity relation of supported Au catalysts could be elu-idated in the simple CO oxidation. The size of Au nanoparticlesnd the oxide support play key roles in the activity of supportedu catalysts in CO oxidation. The catalytic activity is higher whenhe particle size of Au nanoparticles is finer [5]. Oxide supports cane classified as inert or active according to their redox properties,

n which Al2O3, SiO2, and MgO fit with the inert supports, whileeducible transition metal oxides such as TiO2, Fe2O3, MnO2, andeO2 belong to the active supports [6]. The classification is based

n the observation that Au nanoparticles supported on inert oxidesxhibited lower intrinsic activities than those supported on activexides that could contribute to the activation and supply of oxygenor the reaction [6]. However, the nature of the active site of sup-

∗ Corresponding author. Tel.: +86 551 3600435; fax: +86 551 3600437.E-mail address: huangwx@ustc.edu.cn (W. Huang).

381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.molcata.2009.02.014

xidation.© 2009 Elsevier B.V. All rights reserved.

ported Au catalysts in CO oxidation is still a matter of discussion[7–13].

CeO2 is widely employed as the support or additive in catalystsfor oxidation reactions because of its high oxygen storage capacityand redox activity [14]. CeO2 supported Au catalysts have been pre-pared by various methods [15–22], and some of them are active inlow-temperature CO oxidation [15,19–22]. CeO2 as an additive wasreported to enhance the activity of inert oxide supported Au cata-lysts (Au/SiO2 and Au/Al2O3) in CO oxidation [23]. Pillai and Deeviassigned the high activity of Au/CeO2 catalysts for CO oxidation toAu+–OH− and highly dispersed metallic Au species strongly inter-acting with defects in the ceria surface [21]. Corma and co-workersobserved a direct correlation between the concentration of Au3+

species and catalytic activity in Au/CeO2 for CO oxidation [24] andshowed that the cationic Au species were related with the perime-ter interface between the Au particle and the support [25] and thatthe cationic Au species were stabilized during the course of cat-alytic CO oxidation [26]. Venezia et al. studied the catalytic activitiesof Au/CeO2 catalysts prepared by various methods in CO oxidationand proposed that the presence of small Au particles was not themain requisite for the achievement of the highest CO conversion,but the strong interaction between ionic Au and ceria might deter-mine the particularly high activity by enhancing the ceria surface

oxygen reducibility [20].

The structures of CeO2 supports have been found to greatly affectthe activities of Au/CeO2 catalysts in CO oxidation. Carrettin et al.reported that Au deposited on nanocrystalline particles of CeO2showed two orders of magnitude in the catalytic activity relative to

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he Au/CeO2 catalysts prepared by coprecipitation and by Au depo-ition on a regular CeO2 support [19]. Arena et al. demonstratedhat the reducibility of the active CeO2 phase affected the catalyticctivity of Au/CeO2 catalysts [27]. Recently Widmann et al. quanti-atively reported that a freshly calcined Au/CeO2 catalyst becameignificantly more active on removal of about 7% of the surface oxy-en content [28]. However, a comprehensive investigation of thenfluence of CeO2 microstructures on the structures and activitiesf supported Au particles in CO oxidation still lacks because it isifficult to systematically control the microstructures of CeO2.

In this paper, we employed silica supported CeO2 (CeO2/SiO2)ith various CeO2 microstructures as the support for Au nanopar-

icles. Because of the inertness of SiO2, the structure of Auanoparticles in the Au/CeO2/SiO2 catalysts is mainly influencedy the structure of CeO2, therefore, by investigating the structuresf various Au/CeO2/SiO2 catalysts and their catalytic activities in COxidation, we find some novel results on the relation between theicrostructure of CeO2 and the structure and catalytic activity of

u nanoparticles supported on CeO2.

. Experimental

.1. Catalyst preparation

The CeO2/SiO2 supports with a 6% CeO2/SiO2 weight ratio wererepared in two methods. The first method is incipient wetness

mpregnation. Ce(NO3)3·6H2O (Sinopharm Chemical Reagent Co.,td., ≥99.0%) was dissolved in an appropriate volume of triply dis-illed water and then added to SiO2 (20–50 mesh, Qingdao Haiyanghemicals Co.) under stirring. The powder was then dried at 60 ◦Cvernight and eventually calcinated at desired temperatures (200nd 400 ◦C) for 4 h. The resulting CeO2/SiO2 supports were denoteds IWI200 and IWI400.

In the second method, the Ce(NO3)3·6H2O aqueous solution wasdded into a three-neck bottle containing SiO2, then the pH of theolution was adjusted between 9 and 10 by adding ammonia water.he mixture was stirred at 60 ◦C for 24 h, and the solid was filterednd washed several times. The resulting powder was dried at 60 ◦Cvernight, followed by calcination at desired temperatures (200,00, and 600 ◦C) for 4 h. These CeO2/SiO2 supports were denoted asP200, DP400 and DP600.

The CeO2/SiO2 supports were then used to prepare 2%-Au/CeO2/iO2 (Au/support weight ratio) catalysts by deposition–recipitation (DP) using HAuCl4·4H2O (Sinopharm Chemicaleagent Co., Ltd., Au content ≥47.8%) as the Au precursor. TheAuCl4·4H2O aqueous solution and ammonia water were slowlyo-added into a three-neck bottle containing CeO2/SiO2, whose pHas controlled between 9 and 10. The mixture was stirred at 60 ◦C

or 24 h. Then the solid was filtered and washed several times. Theesulting powder was dried at 60 ◦C overnight, followed by calci-ation at 200 ◦C for 4 h. The supported Au catalysts were denotedy “C + support name”, for example, the catalyst using IWI200 ashe support was denoted as CIWI200. The Au/SiO2 catalyst waslso prepared by the same DP method for the comparison purpose.

.2. Catalyst characterization

The elemental composition of supports and catalysts were ana-yzed by inductively coupled plasma atomic emission spectrometerICP-AES). Two methods were employed to dissolve the catalysts.ne was the dissolution of catalyst in aqua regia and the other was

he dissolution of catalysts in the mixture of H2O2 and aqua regia.he ICP-AES working curves for Au and Ce were plotted employingAuCl4·4H2O and Ce(NO3)3·6H2O standard solutions, respectively.ET surface areas were acquired on a Beckman Coulter SA3100 sur-

ace area analyzer, in which the sample was degassed at 120 ◦C for

sis A: Chemical 306 (2009) 40–47 41

30 min in the nitrogen atmosphere before the measurement. XRDmeasurements were performed on a Philips Xpert PRO SUPER X-ray diffractometer with a Ni-filtered Cu K� X-ray source operatingat 40 kV and 50 mA. High resolution X-ray photoelectron spec-troscopy (XPS) measurements were performed on an ESCALAB 250high performance electron spectrometer using monochromatizedAl K� excitation source (h� = 1486.6 eV)). The binding energy of Si2p in SiO2, which was assumed to be 103.3 eV, was employed asthe reference to correct the likely charging effect during the XPSmeasurements. The symmetric Si 2p XPS spectrum excludes theexistence of differential charge on the samples. Transmission elec-tron microscopy (TEM) experiments were preformed on a JEOL2010 high resolution transmission electron microscope with anenergy dispersive spectrum (EDS) analysis facility. Temperature-programmed reduction (H2-TPR) experiments were carried outusing a 5% H2–N2 mixture (40 mL/min flow) at a heating rateof 10 ◦C/min. 50 mg catalyst was used. The catalyst was heatedat 200 ◦C for 0.5 h and then cooled to room temperature in Ar(30 mL/min) prior to the TPR experiment. The consumption of H2during the TPR experiment was measured by a thermal conductivitydetector (TCD).

2.3. Catalytic activity measurement

The catalytic activity was evaluated on a fixed-bed flow reactor.The used catalyst weight was 100 mg and the reaction gas consist-ing of 1% CO and 99% dry air was fed at a rate of 20 mL/min. Thesteady-state composition of the effluent gas was analyzed with anonline GC-14C gas chromatograph equipped with a TDX-01 column(T = 80 ◦C, H2 as the carrier gas at 30 mL/min) after the desired reac-tion temperature had been kept for 30 min. The conversion of COwas calculated from the change in CO concentrations in the inletand outlet gases.

3. Results

3.1. Macroproperties of catalysts

The BET surface areas and the compositions of catalysts are sum-marized in Table 1. The BET surface area of bare SiO2 is 390 m2/g.DP200, IWI200, and IWI400 are with similar BET surface areasbut DP400 and DP600 are with smaller BET surface areas. AllAu/CeO2/SiO2 catalysts are with similar BET surface areas.

By dissolving the catalysts in aqua regia, we measured the com-positions of catalysts by ICP-AES. Au loadings in all Au/CeO2/SiO2catalysts are similar, much higher than the Au loading in Au/SiO2[29] prepared by the same procedure. This indicates that the DPefficiency of Au is higher on CeO2/SiO2 than on bare SiO2. Similarresults have also been observed in Au/CoOx/SiO2 and Au/ZnO/SiO2catalysts [30,31]. During the course of DP, the pH value of HAuCl4solution was adjusted to 9–10, forming the surface-reactive anionicgold hydroxide species AuCl(OH)3

− and Au(OH)4− [32] that would

efficiently deposit onto a positively charged oxide surface. Accord-ingly, oxides with a higher isoelectric point (IEP) will facilitate thedeposition and precipitation of the Au precursor. The IEP value ofCeO2 is much higher than that of SiO2. Therefore, employing thesame DP procedure, the loading of Au is higher in Au/CeO2/SiO2than in Au/SiO2.

The Ce concentrations were also measured but differed verymuch in various catalysts. The weight ratio of CeO2 in DP200 andDP400 are 5.75% and 5.42%, respectively, whereas that in DP600,

IWI200, IWI400 decreased to as low as 1.61%, 0.31% and 0.18%,respectively. The calculated weight ratios of CeO2 are 5.66% and5.55% in CeO2/SiO2 and Au/CeO2/SiO2, respectively. However, whendissolving the catalysts in the mixture of aqua regia and H2O2, theobtained ICP-AES results give similar weight ratios of CeO2 in the

42 K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47

Table 1Macroproperties of catalysts.

Catalyst Weight ratios in the catalysts measured by ICP-AES Average crystalline size dXRD (nm) BET surface area (m2/g)

Aua CeO2a CeO2

b Au CeO2

DP200 N.A. 5.75% 5.52% N.A N.D. 366DP400 N.A. 5.42% N.M. N.A N.D. 278DP600 N.A. 1.61% 5.47% N.A N.D. 293IWI200 N.A. 0.31% 5.68% N.A 5.6 355IWI400 N.A. 0.18% N.M. N.A 7.3 345CDP200 2.2% 5.61% N.M. 15 N.D. 267CDP400 2.2% 5.77% N.M. 14 N.D. 278CDP600 2.2% 2.20% N.M. 11 2.3 259CIWI200 1.9% 0.28% N.M. 10 5.6 261CIWI400 2.1% 0.17% N.M. 13 6.8 291Au/SiO2

c 1.0% N.A. N.A. 12 N.A. 316

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.A.: not applicable; N.M.: not measured; N.D.: not detectable.a Dissolution in aqua regia.b Dissolution in H2O2 and aqua regia.c From Ref. [29].

atalysts approaching the calculated values, proving the successfuloading of Ce on SiO2. Therefore, the different solubility of CeO2upported on SiO2 in aqua regia should arise from their differenticrostructures. It has been reported that Ce(IV) could be reduced

o Ce(III) by H2O2 in the lower pH range, which could enhanceshe dissolution rate of CeO2 in aqua regia [33]. Therefore, the dis-olution rate of supported CeO2 in aqua regia could be taken as anndication of the oxidation state of Ce. The faster the dissolutionate, the larger the fraction of Ce(III) and thus the concentrationf oxygen vacancy in CeO2. Since the Ce(III) fraction and the oxy-en vacancy concentration in CeO2 play an important role in COxidation catalyzed by CeO2, their characterizations are impor-ant. With this respect, our results provide a fast and convenient

ethod.Fig. 1 shows XRD patterns of various catalysts. It could be seen

hat the preparation method greatly affects the dispersion of CeO2n SiO2. Strong diffraction peaks arising from the cubic fluorideeO2 are present in the XRD patterns of IWI200 and IWI400. How-ver, no diffraction peaks associated with CeO2 appears in the XRDatterns of DP200 and DP400, and very weak and diffuse diffractioneaks corresponding to CeO2 appear in the XRD pattern of DP600ue to the elevated calcination temperature. The XRD patterns ofeO2 in Au/CeO2/SiO2 catalysts are similar to those in the corre-ponding CeO2/SiO2 supports. The XRD peaks arising from metallicu are clearly visible in the XRD patterns of all Au/CeO2/SiO2 cat-lysts. The average crystalline sizes of Au and CeO2 in variousatalysts were calculated based on the Scherrer equation, whoseesults are summarized in Table 1. The average crystalline size ofu nanoparticles in Au/CeO2/SiO2 varies between 10 and 15 nm,imilar to that of Au in Au/SiO2 prepared by the same procedure.

.2. Catalytic activity of catalysts in CO oxidation

Fig. 2 displays the CO conversion as a function of the reactionemperature over various catalysts. The CeO2/SiO2 catalysts exhibitery poor catalytic performances. DP200, DP400, and DP600 do nothow any activity in CO oxidation under the investigated temper-tures. IWI400 and IWI200 become active above 300 and 270 ◦C,espectively.

The Au/CeO2/SiO2 catalysts exhibit better catalytic perfor-ances than the Au/SiO2 catalyst, and it could be seen that theore active the CeO2/SiO2, the more active the corresponding

u/CeO2/SiO2. This demonstrates that addition of CeO2 to Au/SiO2enefits CO oxidation. However, none of our Au/CeO2/SiO2 cata-

ysts is as active in low-temperature CO oxidation as previouslyeported Au/CeO2 catalysts [15,19–22]. CIWI200 with the best cat-lytic performance achieves a 50% CO conversion at 110 ◦C (T50%)

Fig. 1. XRD patterns of (A) CeO2/SiO2 and (B) Au/CeO2/SiO2. The peaks marked bythe solid and dash lines arise from CeO2 and Au, respectively.

K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47 43

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Fig. 2. Catalytic performances

nd a complete CO conversion at 210 ◦C, and other catalysts achievecomplete CO oxidation at 270 ◦C or above. Dekkers et al. [23] pre-iously reported the promotion effect of CeO2 in both the reductionf NO with H2 and the oxidation of CO with O2 catalyzed by Au/SiO2nd Au/Al2O3 catalysts, and the T50% of their Au/CeO2/SiO2 catalystas 115 ◦C. This value is close to the T50% of our CIWI200 catalyst

T50% = 110 ◦C).

.3. Structures of catalysts

Fig. 3 shows Ce 3d XPS spectra of various catalysts. The intensityf the Ce 3d XPS peak in IWI and CIWI catalysts is much weaker thanhat in DP and CDP catalysts. XPS is a surface sensitive technique.t a fixed total volume of particles, particles with a higher disper-ion are of a larger surface-to-bulk atomic ratio and thus exhibit atronger peak in the XPS spectrum. Therefore, the Ce 3d XPS resultsmply that CeO2 in DP and CDP catalysts is much more dispersivehan that in IWI and CIWI catalysts, in consistence with XRD results.he Ce 3d XPS spectra show multiple states arising from different

e 4f level occupancies in the final state [34]. We performed peak-tting of the Ce 3d XPS spectra in DP and CDP (Fig. 3A). The labels inig. 3A follow the convention established by Burroughs et al. [35], inhich V and U refer to the 3d5/2 and 3d3/2 spin-orbital components,

espectively. The V/U, V′′/U′′, and V′′′/U′′′ components are features of

Fig. 3. Ce 3d XPS spectra of DP200, DP400, DP600, CDP200, CDP400, and CDP60

ious catalysts in CO oxidation.

Ce(IV) and the V′/U′ component is the feature of Ce(III) [36]. TheCe(III)/Ce(IV) atomic ratio in various catalysts that indicates theconcentration of surface oxygen vacancies was also calculated bythe peak area of Ce(III)/the total area of Ce(IV) peaks. These resultsare summarized in Table 2. The Ce(III)/Ce(IV) decreases in the DPcatalysts with the increasing calcination temperature, implying thatcalcination at high temperatures in air annihilates surface oxygenvacancies in CeO2 supported on SiO2. This is in consistence withthe ICP-AES results. Interestingly, the Ce(III)/Ce(IV) atomic ratio inCDP catalysts is nearly as twice as that in the corresponding DP cat-alysts. Although the low intensity of Ce 3d XPS peaks of IWI andCIWI catalysts does not allow the peak-fitting, it could be also seenin Fig. 3B that the peak V′ in CIWI catalysts is stronger than thatin the corresponding IWI catalysts, indicating a larger Ce(III)/Ce(IV)atomic ratio in CIWI catalysts.

The Au 4f XPS spectra are displayed in Fig. 4. The Au 4f XPS spec-tra in all catalysts could be well fitted with two components withthe Au 4f7/2 binding energy at ∼83.8 and ∼85.6 eV, which could beassigned to the metallic Au and Au(I) species. The Au(I)/Au atomic

ratio in the catalysts was calculated by the peak area of Au(I)/thepeak area of metallic Au. The peak-fitting results are summarizedin Table 2. An interesting observation is that the Au(I)/Au ratiovaries proportionally with the Ce(III)/Ce(IV) atomic ratio in the CDP-series Au/CeO2/SiO2 catalysts, indicating that the formation of Au(I)

0 catalysts (A) and IWI200, IWI400, CIWI200, and CIWI400 catalysts (B).

44 K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47

Table 2Peak-fitting results of Ce 3d and Au 4f XPS spectra in various catalysts.

Catalyst Ce 3d5/2 (eV) Ce(III)/Ce(IV) atomic ratio Au 4f7/2 (eV) Au(I)/atomic ratio

Ce(IV) Ce(III) Metallic Au Au(I)

U U′′ U′′′ U′

DP200 883.4 888.5 898.8 886.3 0.16 N.A. N.A. N.A.DP400 883.5 888.9 898.9 886.5 0.11 N.A. N.A. N.A.DP600 883.5 888.8 898.9 886.4 0.05 N.A. N.A. N.A.CDP200 883.2 888.8 899.1 886.3 0.31 83.9 85.7 0.30CDP400 883.4 888.8 899.0 886.4 0.21 83.8 85.6 0.23CDP600 883.4 888.7 899.0 886.4 0.10CIWI200 N.A. N.A. N.A. N.A. N.A.CIWI400 N.A. N.A. N.A. N.A. N.A.

N.A.: not applicable.

Fig. 4. Au 4f XPS spectra of Au/CeO2/SiO2 catalysts.

Fig. 5. TEM images of CIWI200 (A and B), CIW

83.7 85.5 0.1483.8 85.5 0.1483.8 85.5 0.12

species is associated with the density of surface oxygen vacanciesin CeO2.

Above results demonstrate that the Au loading, the averageAu crystalline size, and the Au 4f binding energy are similar inCIWI200, CIWI400, and CDP600, but CIWI200 shows a much higheractivity in CO oxidation than CIWI400 and CDP600. We thus investi-gated the morphology of CIWI200, CIWI400, and CDP600 in detailby TEM. Fig. 5 displays the representative TEM images. The TEMresults (not shown) show that large CeO2 aggregates are presentin CIWI200 and CIWI400, but absent in CDP600. This agrees withthe XRD results that CeO2 in CDP600 are more dispersive thanthat in CIWI200 and CIWI400. We analyzed the size distribution ofobserved Au nanoparticles in TEM images (Fig. 6). The average sizeof Au nanoparticles is 9.7, 11.4 and 14.3 nm in CIWI200, CDP600andCIWI400, respectively. It can also be seen that among these catalystsCIWI200 has the highest density of Au nanoparticles with 5–7 nmsizes.

Interestingly, the distribution of Au nanoparticles on CeO2/SiO2differs much in CIWI200, CIWI400, and CDP600. We analyzed the

composition of the areas indicated by the circle in the TEM imagesby EDS, whose results is summarized in Table 3. In CIWI200, bothTEM results and EDS results demonstrate that Au nanoparticlesare dispersed on the CeO2 aggregates (area a in Fig. 5A) and the

I400 (C and D), and CDP600 (E and F).

K. Qian et al. / Journal of Molecular Catalysis A: Chemical 306 (2009) 40–47 45

Fig. 6. Size distribution of Au nanoparticles in CIWI200, CIW

Table 3The elemental compositions in atomic ratio of each area calculated from EDS results.

Element CIWI200 CIWI400 CDP600

Fig. 5A Fig. 5B Fig. 5C Fig. 5D Fig. 5E Fig. 5F

Area a Area b

O 66.94 68.45 67.44 59.73 61.30 59.93 60.74SCA

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are SiO2 surface (area b in Fig. 5A). However, in CIWI400, few Auanoparticles are observed on the CeO2 aggregates by TEM (Fig. 5Cnd D), the weak Au signal on the CeO2 aggregate marked in Fig. 5Cetected by EDS might arise from the Au(I) species. There is noeO2 aggregate in CDP600, but few Au nanoparticles are observedn the area (Fig. 5E) with a high Ce concentration whereas the Ceoncentration in the area (Fig. 5F) with abundant Au nanoparticless low. These results indicate that the density of Au nanoparticlesupported on CeO2 follows: CIWI200 > CDP600 > CIWI400.

Fig. 7 shows the TPR results of selective catalysts. CeO in IWI200

2s much more facile to be reduced than that in DP600. The additionf gold significantly improves the reducibility of CeO2 supportedn SiO2, in consistence with previous observations in Au/CeO2 cat-lysts [21]. Meanwhile, the reducibility of catalysts correlates well

Fig. 7. H2-TPR profiles of indicated catalysts.

I400, and CDP600 estimated from the TEM results.

with their activity in CO oxidation, indicating that the reduction ofCeO2 is involved in the catalytic CO oxidation.

4. Discussion

4.1. Microstructures of CeO2

Due to the different preparation methods, CeO2 in variousCeO2/SiO2 exhibits different microstructures, such as the parti-cle size and the surface oxygen vacancy concentration. CeO2 inDP catalysts is finer in the average particle size than that inIWI catalysts, and the particle size of CeO2 increases with theincreasing calcination temperature. Meanwhile, the surface oxygenvacancy concentration of CeO2 in DP catalysts, as indicated by theCe(III)/Ce(IV) atomic ratio, decreases with the increasing calcina-tion temperature. CeO2 in IWI200 is with a better crystallinity and alarger average particle size than that in DP600, but CeO2 in IWI200is much more facile to be reduced than that in DP600. All theseresults imply that the CeO2–SiO2 interaction is stronger in DP cata-lysts than in IWI catalysts. This could be attributed to the differentcerium species prior to the calcination, which is cerium hydroxideand Ce(NO3)3 in DP and IWI catalysts, respectively. Calcination ofcerium hydroxide supported on SiO2 leads to the formation of ultra-fine CeO2 particles whereas calcination of Ce(NO3)3 supported onSiO2 leads to the formation of large CeO2 aggregates.

Very interestingly we observed the different solubility of CeO2 invarious catalysts in aqua regia, which could be associated with themicrostructures of supported CeO2. Due to the disordered structure,CeO2 with a poor crystallinity might dissolve facilely in aqua regia.A dramatic change of the solubility of CeO2 was observed betweenCDP400 and CDP600 although CeO2 in CDP600 only displays veryweak and diffuse diffraction peaks in the XRD spectrum. CeO2 inDP200 and DP400 dissolves in aqua regia much faster than CeO2 inDP600, IWI200 and IWI400. The complete dissolution of catalysts inH2O2 and aqua regia shows that Ce contents in catalysts prepared bydifferent methods are almost the same. These phenomena togetherwith XPS results indicate more Ce(III) and oxygen vacancies existin DP200 and DP400 than other catalysts since H2O2 enhances thedissolution of CeO2 by reducing Ce(IV) to Ce(III).

Various CeO2/SiO2 catalysts exhibit different catalytic per-formances in CO oxidation. It is generally accepted that COoxidation under stationary conditions over ceria follows the Mars-van Krevelen-type mechanism, where reaction involves alternatereduction and oxidation of the oxide surface with the formationof surface oxygen vacancies (as the key step) and their replen-ishment by gas-phase oxygen [14]. We believe that the strongCeO2–SiO2 interaction in DP-series CeO2/SiO2 catalysts blocks thereduction–oxidation cycle on CeO2 so that DP-series CeO2/SiO2 cat-

alysts do not show any activity in CO oxidation although they exhibitlarge densities of surface oxygen vacancies. It is most likely thatthe replenishment of surface oxygen vacancies by gas-phase oxy-gen is blocked because the surface oxygen vacancies in DP-seriesCeO2/SiO2 are stabilized by the strong CeO2–SiO2 interaction. With

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he reduced CeO2–SiO2 interaction, CeO2 in IWI200 and IWI400xhibits certain activities in CO oxidation, and IWI200 is more activehat IWI400 due to the finer particle size of CeO2.

.2. Au–CeO2 interaction in Au/CeO2/SiO2 catalysts

An interesting observation is that the Ce(III)/Ce(IV) atomic ration supported CeO2 increases greatly when CeO2/SiO2 catalysts areoaded with Au, which is independent of the microstructures ofeO2. Since CeO2/SiO2 was calcined prior to the loading of Au,his observation clearly demonstrates that the loading of gold oneO2/SiO2 facilitates the formation of surface oxygen vacancies ineO2, which could then be stabilized. TPR results also demonstratehat the addition of gold significantly improves the reducibility ofeO2 supported on SiO2. Fu et al. firstly reported that the addi-ion of gold improve the reducibility and the OSC of cerium oxide16]. Pillai and Deevi also observed a similar result [21]. Shapovalovnd Metiu performed a theoretical study of CO oxidation catalyzedy AuxCe1−xO2 and found that vacancy formation in AuxCe1−xO2s exothermic, which suggests the oxygen atoms nearest and next-earest the Au dopant are chemically active [37].

Our results evidence that the formation of Au(I) species islosely associated with Ce(III) sites, i.e., surface oxygen vacancyites on CeO2 in CeO2 supported gold catalysts. The Au(I)/Au atomicatio varies proportionally with the Ce(III)/Ce(IV) atomic ratio inu/CeO2/SiO2 whereas no Au(I) species was observed by XPS onu/SiO2 prepared by the similar method [29]. Furthermore, the TEMnd EDS results show that there is still considerable gold signals oneO2 areas without Au nanoparticles, which could only arise fromu(I) species. These results suggest that surface oxygen vacanciesnd Au(I) species in Au/CeO2/SiO2 interact so as to stabilize eachther.

The microstructure of CeO2 in CeO2/SiO2 affects the distributionf Au nanoparticles. A surprising result is that Au nanoparticles sup-orted on CeO2 are evident only in CIWI200, but not in CIWI400 andDP600. This indicates that both Au(I) species and Au nanoparticlesre formed on CeO2 in CIWI200 whereas Au(I) species dominatesn CeO2 in CIWI400 and CDP600. The Ce(III)/Ce(IV) atomic ratio inP catalysts demonstrates that increasing the calcination temper-ture reduces the surface oxygen vacancy concentrations on CeO2,ut the XPS results show that the Au(I)/Au atomic ratios are almosthe same in CIWI200 and CIWI400. Therefore, the surface oxygenacancies on CeO2 in IWI400 are mostly occupied by Au(I) species,nd subsequently few Au nanoparticles are formed. We proposehat Au(I) species and Au nanoparticles compete as products fromhe Au precursor on CeO2. During the course of calcination, some ofuCl(OH)3

− and Au(OH)4− is transformed into Au(I) stabilized on

eO2, other forms Au clusters which migrate and aggregate on theurface. Au(I) species is stabilized by the surface oxygen vacancy siten CeO2 and Au nanoparticles also nucleate on the surface oxygenacancy site on CeO2. The available surface oxygen vacancy sitesnd their distribution on CeO2, the surface oxygen vacanc–Au(I)nteraction, and the surface oxygen vacancy–Au cluster interaction

ight cooperatively determine resulted Au species in the catalysts.nferred from the results of the catalysts prepared by DP method,ighly dispersed CeO2 in CeO2/SiO2 facilitates the formation of Au(I)pecies.

.3. Structure–activity relation of Au/CeO2/SiO2 catalysts in COxidation

No matter where Au nanoparticles distribute in the catalyst, allu/CeO2/SiO2 catalysts exhibit better catalytic performances in COxidation than Au/SiO2 with similar Au particle size. The promo-ion effect of CeO2 could be attributed to the contribution of CeO2o the activation and supply of oxygen for the reaction, which has

[

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sis A: Chemical 306 (2009) 40–47

been previously proposed [6] and spectroscopically observed [24].The microstructure of Au/CeO2/SiO2 catalysts strongly affects theircatalytic activities, which offers an opportunity to understand theirstructure–activity relation in CO oxidation. CIWI200 with both Au(I)species and Au nanoparticles on CeO2 is much more active in CO oxi-dation than CIWI400 and CDP600 with Au(I) species dominatingthe CeO2 surface. This indicates that Au(I) species on CeO2 aloneis not active in CO oxidation and the presence of Au nanoparti-cles on CeO2 is important, i.e., the Au nanoparticles–CeO2 interfaceplays an important role in the activity of Au/CeO2/SiO2 catalystsin CO oxidation. Meanwhile, the activity of Au/CeO2/SiO2 catalystin CO oxidation could be correlated well the reducibility of CeO2 inAu/CeO2/SiO2 and with the activity of the corresponding CeO2/SiO2catalyst in CO oxidation, indicating the important role of the abil-ity of CeO2 to activate and supply oxygen. However, CIWI200 withthe best performance does not show any activity in CO oxida-tion at room temperature, indicating the lack of the active sitesfor low-temperature CO oxidation. The ensemble in Au/CeO2 cat-alysts active for low-temperature CO oxidation has been proposedto consist of coexisting highly dispersed Au particles (2–4 nm) andAu(I)–OH species in intimate contact with the ceria surface defects[19–21], which lacks in CIWI200 because Au(I) species exists onCeO2 but 2–4 nm Au nanoparticles do not. Meanwhile, comparingunsupported CeO2 catalysts, the ability of CeO2 supported on SiO2in CIWI200 for the activation and supply of oxygen is poor due tothe CeO2–SiO2 interaction as reflected by the catalytic performanceof IWI200 in CO oxidation. It is under investigation in our lab how toengineer the microstructure of supported CeO2 for the fabricationof Au/CeO2/SiO2 catalysts active in low-temperature CO oxidation.

5. Conclusions

The influences of CeO2 microstructures on the structure andcatalytic activity of supported Au nanoparticles in CO oxidationhave been manifested in Au/CeO2/SiO2 catalysts with various CeO2microstructures. Au(I) species and Au nanoparticles compete forthe surface oxygen vacancy sites on CeO2. Highly dispersive CeO2on SiO2 facilitates the formation of Au(I) species, and the presenceof Au species also facilitates the creation and stabilization of sur-face oxygen vacancies on CeO2. Au(I) species on CeO2 alone is notactive in CO oxidation and Au nanoparticles in contact with CeO2in Au/CeO2/SiO2 catalysts are important in catalyzing CO oxidationreaction.

Acknowledgements

This work was financially supported by National Natural Sci-ence Foundation of China (grant 20773113), the “Hundred TalentProgram” of Chinese Academy of Sciences, the MOE program forPCSIRT (IRT0756), and the MPG-CAS partner group program.

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