DOI: 10.1002/cctc.201300709

Morphology Effect of CeO2 Support in the Preparation,Metal–Support Interaction, and Catalytic Performance ofPt/CeO2 CatalystsYuxian Gao, Wendong Wang, Sujie Chang, and Weixin Huang*[a]

Introduction

Oxides are widely employed as catalyst supports. The surfacecomposition and surface structure of the oxide support greatlyaffects the structure and catalytic performance of oxide-sup-ported catalysts. Crystal planes exposed on the surface of anoxide particle determine both the surface composition and thesurface structure. According to Wulff’s rule,[1] crystal planes ex-posed on the surface of a crystalline material are determinedby its morphology. Thus the morphology of an oxide supportis an important macroscopic structural factor that affects thecatalyst structure and catalytic performance of oxide-support-ed catalysts. However, oxide particles in an oxide-supportedpowder catalyst usually exhibit uneven and irregular morphol-ogies and expose various types of crystal planes. Such mor-phological complexity and inhomogeneity of oxide particlesmake it difficult to investigate and understand the effect of themorphology of the oxide support in oxide-supported catalysts.In the last decade, the controlled synthesis of oxide nanocrys-

tals with uniform and well-defined morphologies has advancedsignificantly, consequently, the effect of the morphology ofoxide nanocrystals on the structure and catalytic performanceof oxide-involved catalysts has been adequately demonstratedand the morphology-controlled strategy has become a newstrategy to tune the catalytic performance of oxide-involvedcatalysts.[2–13]

Ceria (CeO2) is among the oxides nanocrystals of which havebeen successfully synthesized with various uniform and well-defined morphologies.[14–17] Meanwhile, CeO2 is one of themost important oxides in heterogeneous catalysis,[18] therefore,the catalytic performance of CeO2 nanocrystals with variousmorphologies has been explored extensively and morphology-dependent catalytic performance has been observed ina series of catalytic reactions.[19–30] CeO2-supported catalyststhat employ CeO2 nanocrystals have also been prepared, andthe effect of the morphology of the CeO2 support was also ob-served on the active component–CeO2 interaction, the struc-ture, and catalytic performance.[31–42] Among CeO2 rods, cubes,and polyhedra, CeO2 nanorods enclosed by {11 0} and {1 0 0}crystal planes are most active for gold stabilization/activation,and thus a Au supported on CeO2 nanorods catalyst is mostactive in catalyzing the water–gas–shift (WGS) reaction.[31] AAu/CeO2-rods catalyst was also reported to be more active in

Pt/CeO2 catalysts with various Pt loadings were prepared bya conventional incipient wetness impregnation method thatemployed CeO2 cubes (c-CeO2), rods (r-CeO2), and octahedra(o-CeO2) as the support and Pt(NH3)4(NO3)2 as the metal precur-sor. Their structures and catalytic activities in CO oxidation inexcess O2 and the preferential oxidation of CO in a H2-rich gas(CO-PROX) were studied, and strong morphology effects wereobserved. The impregnated Pt precursor interacts more strong-ly with CeO2 rods and cubes than with CeO2 octahedra, andthe reduction/decomposition of the Pt precursor impregnatedon CeO2 octahedra is easier than that on CeO2 rods and cubes.With the same Pt loading, the Pt/o-CeO2 catalyst contains thelargest fraction of metallic Pt, whereas the Pt/c-CeO2 catalystcontains the largest fraction of Pt2 + species. The reducibility ofpure CeO2 and CeO2 in the Pt/CeO2 catalysts follows the orderr-CeO2>c-CeO2>o-CeO2, and the reducibility of CeO2 dependson the Pt loading for the Pt/c-CeO2 catalysts but not much forthe Pt/r-CeO2 and Pt/o-CeO2 catalysts. The catalytic perfor-

mance of Pt/CeO2 catalysts in both CO oxidation and the CO-PROX reaction follows the order Pt/r-CeO2>Pt/c-CeO2>

Pt/o-CeO2. The Pt0-CeO2 ensemble is more active than the Pt2+

-CeO2 ensemble in the catalysis of CO oxidation in excess O2.H2-assisted CO oxidation catalyzed by the Pt/CeO2 catalystswas observed in the CO-PROX reaction, and the Pt2 + speciesand CeO2 with a large concentration of oxygen vacancies con-stitute the active structure of the Pt/CeO2 catalyst for the CO-PROX reaction. The effect of the morphology of the CeO2 sup-port in the preparation, metal–support interaction, and catalyt-ic performance of Pt/CeO2 catalysts can be correlated the ex-posed crystal planes and surface composition/structure of theCeO2 support with different morphologies. These results notonly demonstrate that the structure and catalytic performanceof oxide-supported catalysts can be tuned by controlling themorphology of the oxide support but also deepens the funda-mental understanding of CO oxidation reactions catalyzed byPt/CeO2 catalysts.

[a] Y. Gao, Prof. Dr. W. Wang, S. Chang, Prof. Dr. W. HuangHefei National Laboratory for Physical Sciences at the MicroscaleCAS Key Laboratory of Materials for Energy ConversionDepartment of Chemical PhysicsUniversity of Science and Technology of ChinaJinzhai Road 96, Hefei 230026 (China)E-mail : huangwx@ustc.edu.cn

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3610

CHEMCATCHEMFULL PAPERS

catalyzing the preferential oxidation of CO in a H2-rich gas (CO-PROX) than Au/CeO2-polyhedra and Au/CeO2-cubes.[32, 33] We re-cently proposed a morphology-dependent interplay betweenoxygen vacancies and Ag-CeO2 in Ag/CeO2 catalysts that con-trols their structure and catalytic performance.[40]

Pt/CeO2 catalysts are promising catalysts for low-tempera-ture CO-PROX and WGS reactions and thus have attractedgreat interest.[43–48] Feng et al. deposited Pt clusters on CeO2

octahedra and rods by employing electron-beam evaporationand observed the morphology-dependent structure of the de-posited Pt clusters and catalytic performance in CO oxidationand dehydrogenation reactions.[34] As far as we know, no studyof Pt/CeO2 catalysts prepared by conventional methods forpowder catalyst preparation, such as deposition–precipitationand incipient wetness impregnation that employs CeO2 nano-crystals as the support, has been reported. Meanwhile, theeffect of metal loading on the structure and catalytic perfor-mance of metal/CeO2 catalysts that employ CeO2 nanocrystalsas the support has been seldom addressed in previous studies.In this study, we prepared Pt/CeO2 catalysts with various Ptloadings by a conventional incipient wetness impregnationmethod that employed CeO2 cubes (c-CeO2), rods (r-CeO2), andoctahedra (o-CeO2) as the support. Their structures and catalyt-ic activities in CO oxidation in excess O2 and the CO-PROX re-action were studied in detail. Strong morphology effects of theCeO2 support were observed in the preparation, metal–supportinteraction, and catalytic performance of the Pt/CeO2 catalystsand could be correlated the exposed crystal planes and surfacecomposition/structure of the CeO2 nanocrystals. The differentcatalytic behaviors of Pt0-CeO2 and Pt2+-CeO2 ensembles in thecatalysis of CO oxidation and the CO-PROX reaction were alsoobserved.

Results and Discussion

TEM and high-resolution TEM (HRTEM) images of r-CeO2,c-CeO2, and o-CeO2 are shown in Figure 1, and their morpholo-gies are all quite uniform. c-CeO2 has edge lengths mostly be-tween 20 and 40 nm, r-CeO2 has a diameter distribution of11�4 nm and a length distribution between 30 and 200 nm,and o-CeO2 has edge lengths mostly between 100 and300 nm. As reported previously[14, 15] and revealed by the

HRTEM images, c-CeO2 has selectively exposed {1 0 0} crystalplanes, r-CeO2 has exposed {1 0 0} and {11 0} crystal planes, ando-CeO2 has selectively exposed {111} crystal planes. TheCeO2(1 0 0) surface is a polar plane composed of CeIV layers andO layers and its surface is terminated by the O layer. TheCeO2(11 0) surface is a nonpolar plane formed by a stack ofstoichiometric CeO2 layers and its surface has both exposedCeIV and O atoms. The CeO2(111) surface is also a nonpolar sur-face formed by a stacking sequence of O�Ce�O trilayers andits surface has both exposed CeIV and O atoms.[49] The XRD pat-terns of c-CeO2, r-CeO2, and o-CeO2 are shown in Figure 2 andthey all display a typical cubic fluoride CeO2 crystal phase(JCPDS card No. 34-0394). The diffraction peaks of o-CeO2 arenarrower than those of r-CeO2 and c-CeO2. This agrees withthe microscopic results that o-CeO2 has the largest size. TheBET specific surface areas of c-CeO2, r-CeO2, and o-CeO2 weremeasured to be 23, 69, and 4 m2 g�1, respectively.

Figure 1. TEM and HRTEM images of CeO2 rods (a, b), cubes (c, d), and octa-hedra (e, f). The insets schematically illustrate the crystal planes exposed onthe CeO2 rods, cubes, and octahedra.

Figure 2. XRD patterns of A) c-CeO2 and the Pt/c-CeO2 catalysts, B) r-CeO2 and the Pt/r-CeO2 catalysts, and C) o-CeO2 and the Pt/o-CeO2 catalysts.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3611

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

For each type of CeO2 nano-crystal, three Pt/CeO2 catalystswith calculated Pt loadings of0.2, 0.5, and 1 % were preparedby a conventional incipient wet-ness impregnation method thatemployed Pt(NH3)4(NO3)2 as theprecursor followed by subse-quent H2 reduction at 200 8C for3 h. If we take the Pt/CeO2 cata-lyst with a calculated Pt loadingof 0.2 % that employs c-CeO2 asthe support as an example, thecatalyst is denoted as 0.2 %-Pt/c-CeO2. The H2 reduction treat-ment of the catalyst precursorswas monitored by MS, and theresults (not shown) demonstratethe complete reduction of NO3

�

to gaseous NH3. It was reportedpreviously that the reductiontemperature of NO3

� can be low-ered significantly from 460 to220 8C by Rh.[50] Thus no N-con-taining species exist in our Pt/CeO2 catalysts. The actual Ptloading in Pt/CeO2 catalysts wasanalyzed by using inductivelycoupled plasma atomic emissionspectroscopy (ICP-AES), and theresults are summarized inTable 1. The Pt loadings in Pt/c-CeO2, Pt/r-CeO2, and Pt/o-CeO2

are similar for low Pt loadingsbut the Pt loading in 1 %-Pt/o-CeO2 is a little smaller than thatin 1 %-Pt/c-CeO2 and 1 %-Pt/r-CeO2. The XRD patterns of thePt/CeO2 catalysts are shown inFigure 2. No diffraction patternthat arises from Pt could be observed in all of the Pt/CeO2 cat-alysts except 1 %-Pt/o-CeO2, which exhibits a weak Pt(111) dif-fraction peak at 39.88 in its XRD pattern. The morphologiesand structures of Pt/CeO2 were examined by TEM. Pt nanopar-ticles were only clearly identified in 1 %-Pt/r-CeO2, 1 %-Pt/c-CeO2, and 1 %-Pt/o-CeO2, the TEM and HRTEM images of whichare shown in Figure 3. The original morphology of the CeO2

supports is well preserved after the loading of Pt. By countingmore than 100 Pt nanoparticles for each catalyst, we acquiredthe size distributions of the supported Pt nanoparticles in 1 %-Pt/r-CeO2, 1 %-Pt/c-CeO2 and 1 %-Pt/o-CeO2. The mean sizes ofthe Pt nanoparticles in 1 %-Pt/r-CeO2, 1 %-Pt/c-CeO2, and 1 %-Pt/o-CeO2 do not differ much and are 2.0, 1.5, and 1.7 nm, re-spectively. As shown below, the Pt2 +/Pt0 ratios in 1 %-Pt/r-CeO2, 1 %-Pt/c-CeO2, and 1 %-Pt/o-CeO2 differ significantly,which will affect the sizes of the supported Pt nanoparticles.Almost all of the counted Pt nanoparticles in the catalysts are

supported on the faces of CeO2 nanocrystals rather than ontheir edges and truncated corners.

The surface compositions of Pt/CeO2 catalysts were studiedby X-ray photoelectron spectroscopy (XPS). No N signal wasdetected, which also demonstrates the absence of N-contain-ing species in the catalysts. PO4

3� was employed in the synthe-sis of o-CeO2 nanocrystals, and our XPS analysis results demon-strate that the P/Ce ratio is �0.06 in o-CeO2 and the Pt/o-CeO2

catalysts. The Pt 4f XPS spectra of various Pt/CeO2 catalysts areshown in Figure 4. The Pt 4f XPS spectra can be adequatelyfitted with two components with the Pt 4f7/2 binding energy at�72.6 and �71.5 eV. The former can be assigned to Pt2+ spe-cies and the latter can be assigned to Pt0 (Pt nanoparticles).[47]

It can be seen that the Pt species in the Pt/CeO2 catalystsdepend on the Pt loading and the morphology of the CeO2

support. The Pt0/Pt2 + ratios in the Pt/CeO2 catalysts estimatedfrom the Pt 4f XPS results are summarized in Table 1. For the

Table 1. Pt loading, Pt0/Pt2 + ratio, and specific reaction rate of various Pt/CeO2 catalysts.

Catalyst Pt loading Pt0/Pt2 + Specific reaction rate of Specific reaction rate ofratio CO oxidation (40 8C) the PROX reaction (60 8C)

[wt %] [mmolCO molPt�1 s�1] [mmolCO molPt

�1 s�1]

0.2 %-Pt/r-CeO2 0.12 0 68 380.5 %-Pt/r-CeO2 0.33 0 118 951 %-Pt/r-CeO2 0.87 0.53 147 320.2 %-Pt/c-CeO2 0.14 0 very low 90.5 %-Pt/c-CeO2 0.39 0 23 191 %-Pt/c-CeO2 0.88 0.22 40 130.2 %-Pt/o-CeO2 0.14 0.35 very low 30.5 %-Pt/o-CeO2 0.32 0.4 18 71 %-Pt/o-CeO2 0.72 1.58 8 very low

Figure 3. Representative TEM and HRTEM images and the particle size distribution of 1 %-Pt/r-CeO2 (A1, A2, A3),1 %-Pt/c-CeO2 (B1, B2, B3), and 1 %-Pt/o-CeO2 (C1, C2, C3).

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3612

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

same CeO2 support, the Pt0/Pt2 + ratio increases with the Ptloading. For the same Pt loading, the Pt0/Pt2 + ratio follows theorder Pt/o-CeO2>Pt/r-CeO2>Pt/c-CeO2. However, only Pt2+ ispresent in Pt/r-CeO2 and Pt/c-CeO2 with low Pt loadings, andPt0 appears in 1 %-Pt/r-CeO2 and 1 %-Pt/c-CeO2. As the catalystswere prepared by H2 reduction of the catalyst precursor at200 8C for 3 h, these results indicate a morphology-dependentPt2 +–CeO2 interaction in the catalyst precursor. Pt2+ , which hasa stronger Pt2+–CeO2 interaction, is more resistant to reduc-tion. Thus the Pt2+–CeO2 interaction follows the order Pt2+–c-CeO2> Pt2 +–r-CeO2> Pt2+–o-CeO2.

The H2 temperature-programmed reduction (TPR) results ofvarious catalyst precursors (Figure 5) also demonstrate that thePt2 +–CeO2 interaction follows the order of Pt2 +–c-CeO2> Pt2 +

–r-CeO2> Pt2 +–o-CeO2. The H2-TPR of the catalyst precursorsincludes the reduction/decomposition of impregnated Pt-(NH3)4(NO3)2 (Pt2+ and NO3

�) on CeO2 and the reduction ofCeO2. The reduction of the bare CeO2 supports starts aroundor above 300 8C (Figure 6), thus the signals observed below300 8C are initiated by the reduction/decomposition of impreg-nated Pt(NH3)4(NO3)2. The precursors of Pt/o-CeO2 undergo thereduction/decomposition process far below 200 8C (Figure 5 C),consistent with the observation of Pt nanoparticles in Pt/o-

CeO2 catalysts prepared by the reduction of catalyst precursorsat 200 8C for 3 h. The multiple peaks in the TPR spectra weretentatively attributed to the reduction/decomposition process-es at different stages or of precursors with different environ-ments. Compared with those of the precursors of Pt/o-CeO2,the profiles of the precursors of Pt/c-CeO2 and Pt/r-CeO2 (Fig-ure 5 A and B) are simple but their reduction/decompositiontemperatures depend on the Pt loading. The reduction/decom-position process of Pt precursors in 0.2 %-Pt/c-CeO2 and 0.2 %-Pt/r-CeO2 initiates at �215 8C. This agrees with the observationof only Pt2 + species in both catalysts prepared by the reduc-tion of the catalyst precursors at 200 8C for 3 h. With increasedPt loading, the reduction/decomposition process shifts to�200 8C. However, as a result of the strong Pt2+–c-CeO2 andPt2 +–r-CeO2 interactions, only Pt2+ species form in 0.5 %-Pt2 +

/c-CeO2 and 0.5 %-Pt2+/r-CeO2, and Pt2 + and Pt0 species coexistin 1 %-Pt2+/c-CeO2 and 1 %-Pt2+/r-CeO2. The reduction/decom-position rate of the precursor of 1 %-Pt/r-CeO2 is much fasterthan that of 1 %-Pt/c-CeO2. This might explain the larger Pt0/Pt2 + ratio in 1 %-Pt/r-CeO2 than in 1 %-Pt/c-CeO2. We foundthat the H2-TPR profiles of the precursors of Pt/r-CeO2 and Pt/c-CeO2 are similar at low Pt loadings but differ at the 1% Ptloading. As c-CeO2 has exposed {1 0 0} crystal planes and

Figure 4. Pt 4f XPS spectra of A) Pt/c-CeO2, B) Pt/r-CeO2, and C) Pt/o-CeO2.

Figure 5. H2-TPR profiles of the catalyst precursors of A) Pt/c-CeO2, B) Pt/r-CeO2, and C) Pt/o-CeO2 catalysts.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3613

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

r-CeO2 has exposed {1 0 0} and {11 0} crystal planes, it is likelythat the Pt precursor preferentially interacts with the {1 0 0}crystal planes of r-CeO2.

The XRD, TEM, XPS, and precursor-TPR results show clearlythat the interaction and reduction/decomposition of the Ptprecursor with the CeO2 support vary with the morphologyand exposed crystal planes of the CeO2 support. DFT calcula-tions show that the surface energy of low-indexed CeO2 surfa-ces follows the order (111)< (11 0)< (1 0 0).[49] Thus it is reason-able that the Pt2 + precursor preferentially interacts with theCeO2(1 0 0) surface, and the Pt2+–CeO2 interaction followsthe order Pt2 +–c-CeO2 (CeO2(1 0 0))> Pt2 +–r-CeO2

(CeO2(1 0 0)+(11 0))>Pt2 +–o-CeO2 (CeO2(111)), as we observedexperimentally. If we compare Pt/r-CeO2 and Pt/c-CeO2, thepoor dispersion of Pt nanoparticles in Pt/o-CeO2 results fromboth the very low specific surface area of o-CeO2 and the weakPt2 +–o-CeO2 (CeO2(111)) interaction. The transformation of thecatalyst precursor into an active supported catalyst is verycomplicated but plays a key role in the catalyst compositionand structure. Our results reveal a strong effect of the mor-phology of the oxide support in such a process and shed lighton the fundamental understanding of the preparation ofoxide-supported catalysts.

The reduction behavior of CeO2 in various Pt/CeO2 catalystswas also studied by H2-TPR (Figure 6). The Pt/CeO2 catalystswere oxidized at 300 8C in 20 % O2/Ar for 2 h and then cooledto room temperature in Ar prior to the measurements. Thestarting surface reduction temperature of bare CeO2 nanocrys-tals follows the order r-CeO2 (�290 8C)<c-CeO2 (�330 8C)<o-CeO2 (�400 8C). This indicates that the surface reducibilityfollows the order r-CeO2<c-CeO2<o-CeO2. As the reduction ofthe surface leads to the formation of surface oxygen vacancies,the formation energy of surface oxygen vacancies should playan important role in the reducibility of the CeO2 surface. DFTcalculations show that the oxygen vacancy formation energyof low-indexed CeO2 surfaces follows the order (11 0)< (1 0 0)<(111),[51–54] which supports our experimental results. The sur-face reduction of CeO2 supports is promoted significantly in

Pt/CeO2 catalysts. Interestingly, the promotion effect does notdepend much on the Pt loading and Pt species. For the sameCeO2 support, all Pt/CeO2 catalysts exhibit similar H2-TPR pro-files. These observations suggest that a small amount of Pt,either as Pt2 + or Pt0, is enough to activate the surface Ce�Obond and promote the surface reduction of CeO2 to the maxi-mum extent. The only exception is the 0.2 %-Pt/c-CeO2 catalyst,the low-temperature surface reduction peak of which is muchstronger than that of 0.5 %-Pt/c-CeO2 and 1 %-Pt/c-CeO2. Thisresult is reproducible but we do not yet understand the origin.However, the promotion effect of Pt on the surface reductionof CeO2 in Pt/CeO2 catalysts depends on the morphology ofthe CeO2 support. The surface reduction of CeO2 in the Pt/o-CeO2 catalysts starts at �100 8C and evolves into a main peakat �306 8C with a shoulder at �438 8C, whereas a low-temper-ature surface reduction feature that starts at �40 8C occurs forCeO2 in the Pt/r-CeO2 and Pt/c-CeO2 catalysts. This low-temper-ature surface reduction feature evolves into a broad peak be-tween �40 and �285 8C for the Pt/r-CeO2 catalysts but intoa main peak at �90 8C with a shoulder at �65 8C and a longtail extending to �250 8C for the Pt/c-CeO2 catalysts. The ab-sence of the low-temperature surface reduction peak in the Pt/o-CeO2 catalysts could be attributed the weak Pt–o-CeO2 inter-action and the stable CeO2(111) surface. Notably, r-CeO2 andPt/r-CeO2 exhibit the largest surface reduction peaks. Thiscould be associated with their large specific surface areas.These H2-TPR results clearly demonstrate the morphology-de-pendent surface reduction of CeO2 nanocrystals and the mor-phology-dependent promotion effect of Pt on the surface re-duction of CeO2 nanocrystals in the Pt/CeO2 catalysts. The dis-tinctly different surface reduction behaviors of the CeO2(111)surface in the Pt/o-CeO2 catalysts and the CeO2(11 0) and (1 0 0)surfaces in Pt/r-CeO2 and Pt/c-CeO2 catalysts provide an alter-native explanation for the TPR profile of the oxide support inoxide-supported powder catalysts in which various reductionpeaks in the TPR profile of the catalyst might result from thereduction of the different crystal planes exposed on the oxidesupport.

Figure 6. H2-TPR profiles of A) c-CeO2 and as-prepared Pt/c-CeO2, B) r-CeO2 and as-prepared Pt/r-CeO2, and C) o-CeO2 and as-prepared Pt/o-CeO2 catalysts sub-jected to reoxidation treatment at 300 8C in 20 % O2-Ar for 2 h.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3614

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

The concentrations of the oxygen vacancies of various CeO2

and Pt/CeO2 catalysts were further studied by visible and UVRaman spectroscopy (Figure 7 and 8). As a result of the stron-ger absorption of CeO2 in the UV region than in the visibleregion,[40] UV Raman spectroscopy is more surface sensitive inthis case than visible Raman spectroscopy. Two peaks were ob-served in the Raman spectra, a strong peak at �465 cm�1 anda weak peak at �598 cm�1, which correspond to the F2g anddefect-induced (D) modes of the cubic CeO2 fluoride phase, re-spectively.[55] Peak-fitting analysis was performed for the ac-quired Raman spectra and the ID(�598 cm�1)/IF2g

(�465 cm�1)ratio was calculated to approximate the defect concentration(Figures 7 and 8). For bare CeO2 nanocrystals, the ID/IF2g

ratio es-timated from the visible Raman spectra follows the order r-CeO2>c-CeO2>o-CeO2, which agrees with the order of theirreducibility; however, the ID/IF2g

ratio estimated from the UVRaman spectra follows the order c-CeO2> r-CeO2>o-CeO2,which indicates that defects in the surface region are morestable in c-CeO2 than in r-CeO2. The Pt loading affects the ID/IF2g

ratios of c-CeO2, r-CeO2, and o-CeO2 in different ways. For thePt/o-CeO2 catalysts, the ID/IF2g

ratio in the visible Raman spectra

does not vary much, whereas that in the UV Raman spectra in-creases slowly with the Pt loading. This could be associatedwith both the weak Pt–o-CeO2 interaction and the stability ofthe CeO2(111) surface. The ID/IF2g

ratio in the UV Raman spectraincreases from 0.39 for 0.2 %-Pt/c-CeO2 to 0.64 for 0.5 %-Pt/c-CeO2, whereas that in the visible Raman spectra does notchange. Meanwhile, the ID/IF2g

ratio in the visible Raman spectraincreases from 0.04 for 0.2 %-Pt/r-CeO2 to 0.18 for 0.5 %-Pt/r-CeO2, whereas that in the UV Raman spectra does not increasemuch. These observations again indicate that defects in thesurface region are more stable in c-CeO2 than in r-CeO2. Withthe increase of Pt loading to 1 %, the ID/IF2g

ratios in both theUV and visible Raman spectra increase significantly for Pt/c-CeO2, which implies that the Pt nanoparticles are stronger tocreate oxygen vacancies within c-CeO2 than the Pt2 + species;however, the ID/IF2g

ratio in the UV Raman spectra does notchange much for Pt/r-CeO2 but that in the visible Raman spec-tra decreases to 0.07 from 0.18 for 0.5 %-Pt/r-CeO2, which sug-gests a decrease of the concentration of oxygen vacancies. Asdiscussed above, in the 0.2 %-Pt/r-CeO2 and 0.5 %-Pt/r-CeO2

catalysts, Pt mainly interacts with the (1 0 0) surface exposed

Figure 7. Visible Raman spectra of various CeO2 and Pt/CeO2 catalysts.

Figure 8. UV Raman spectra of various CeO2 and Pt/CeO2 catalysts.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3615

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

on r-CeO2 and exists as Pt2+ ; in 1 %-Pt/r-CeO2, Pt interacts withboth the (1 0 0) and (11 0) surfaces exposed on r-CeO2 andexists as both Pt2 + species and Pt nanoparticles. Therefore, thedecrease of the concentration of oxygen vacancies in Pt/r-CeO2

with the increase of Pt loading from 0.5 to 1 % could be associ-ated with the presence of Pt nanoparticle–CeO2(11 0) interac-tions in 1 %-Pt/r-CeO2.

The catalytic performances of CeO2 and the Pt/CeO2 catalystswere examined for CO oxidation in excess O2 (Figure 9) andthe CO-PROX reaction (Figure 10). The bare CeO2 supports donot show any catalytic activity in either of the reactions underthe investigated experimental conditions. For the Pt/CeO2 cata-lysts with the same Pt loading, the catalytic performance fol-lows the order Pt/r-CeO2>Pt/c-CeO2>Pt/o-CeO2 in CO oxida-tion, and the same order holds in the CO-PROX reaction at re-

Figure 9. Catalytic performance of Pt/CeO2 catalysts in CO oxidation. The reactants consist of 1 % CO balanced with dry air.

Figure 10. CO conversion (A1–C1) and O2 conversion and selectivity (A2–C2) of the CO-PROX reaction catalyzed by Pt/CeO2 catalysts. The reactants consist of1 % CO, 1 % O2, and 50 % H2 balanced with N2.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3616

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

action temperatures below 80 8C. This order is consistent withthe order of reducibility of the various CeO2 supports. In COoxidation reactions catalyzed by CeO2-involved catalysts, thesurface lattice oxygen of CeO2 is an active oxygen species.[18]

Thus, the easier the reduction of the employed CeO2 support,the weaker the surface Ce�O bond, and the more active thecatalyst. Therefore, among the CeO2 nanocrystals investigatedherein, as a result of its strong reducibility and large specificsurface area, r-CeO2 is the best support to prepare efficient Pt/CeO2 catalysts for CO oxidation and the CO-PROX reaction. Athigh reaction temperatures in the CO-PROX reaction at whichthe O2 conversion approaches 100 %, the CO conversion andO2 selectivity of Pt/CeO2 catalysts do not depend much on thePt loading and CeO2 morphology and decrease with the reac-tion temperature. This is because high reaction temperaturesincrease the surface coverage of H(a) (chemisorbed H adatoms)on Pt surfaces at the expense of the surface coverage of CO(a)(chemisorbed CO) on Pt surfaces.[48]

Interestingly, Pt/CeO2 catalysts with the same type of CeO2

support exhibit a morphology-dependent catalytic behavior onthe Pt loading in CO oxidation and the CO-PROX reaction atlow temperatures. In CO oxidation, the CO conversions of thePt/c-CeO2 and Pt/r-CeO2 catalysts increase with the Pt loading,whereas those of the Pt/o-CeO2 catalysts follow the order0.5 %-Pt/o-CeO2>1 %-Pt/o-CeO2>0.2 %-Pt/o-CeO2. In the CO-PROX reaction, the CO and O2 conversion of the Pt/c-CeO2 andPt/o-CeO2 catalysts follow the same orders as in CO oxidationbut those of Pt/r-CeO2 do not. The CO conversion in CO oxida-tion follows the order 1 %-Pt/r-CeO2>0.5 %-Pt/r-CeO2>0.2 %-Pt/r-CeO2 but the CO and O2 conversion in the CO-PROX reac-tion follow the order 0.5 %-Pt/r-CeO2>1 %-Pt/r-CeO2>0.2 %-Pt/r-CeO2. We further compared the specific reaction rates of vari-ous Pt/CeO2 catalysts normalized to the Pt loading in CO oxi-dation (40 8C) and the CO-PROX reaction (60 8C) at low temper-atures, and the results are summarized in Table 1. The activestructure of Pt/CeO2 catalysts in CO oxidation reactions is gen-erally believed to be the Pt–CeO2 interface, but it is still arguedabout whether Pt0 or Pt2 + is more active.[44–49] 0.2 %-Pt/c-CeO2

(0.2 %-Pt/r-CeO2) and 0.5 %-Pt/c-CeO2 (0.5 %-Pt/r-CeO2) onlycontain Pt2 + species, whereas 1 %-Pt/c-CeO2 (1 %-Pt/r-CeO2)contains both Pt2+ species and Pt nanoparticles. In CO oxida-tion, the specific reaction rate of 1 %-Pt/c-CeO2 (1 %-Pt/r-CeO2)is higher than those of 0.2 %-Pt/c-CeO2 (0.2 %-Pt/r-CeO2) and0.5 %-Pt/c-CeO2 (0.5 %-Pt/r-CeO2). This suggests that the Pt0-CeO2 ensemble is more active than the Pt2 +-CeO2 ensemble inthe catalysis of CO oxidation in excess O2. In Pt/o-CeO2 cata-lysts, which contains both Pt2 + and Pt0 species, the reducedcatalytic activity of 1 %-Pt/o-CeO2 than 0.5 %-Pt/o-CeO2 can beattributed to the aggregation of supported Pt nanoparticles, asevidenced by both XRD and TEM results. This reduces thenumber of active Pt-CeO2 ensemble sites and thus the catalyticactivity.

In the CO-PROX reaction, the specific reaction rate of 0.5 %-Pt/c-CeO2 (0.5 %-Pt/r-CeO2) is higher than that of 1 %-Pt/c-CeO2

(1 %-Pt/r-CeO2), which is in contrast to the case in CO oxida-tion. This suggests that CO oxidation in the CO-PROX reactiondoes not follow the same reaction mechanism as CO oxidation

in excess O2. It has been established that H2-assisted CO oxida-tion occurs in the CO-PROX reaction catalyzed by Pt/CeO2 cata-lysts.[44–46] In the presence of H2, H(a) formed on Pt surfacespills over to the CeO2 support to form oxygen vacancies inCeO2 as well as to hydroxylate and hydrate the CeO2 surface.The presence of oxygen vacancies in CeO2 promotes its abilityto activate O2, meanwhile, the hydroxylated and hydratedCeO2 surface supplies additional oxidizing species to oxidizeCO to CO2. It was also found that the presence of a significantdensity of oxygen vacancies in CeO2 is beneficial for the H2-as-sisted CO oxidation mechanism. Recently model catalyst stud-ies have also revealed that hydroxyl groups on an oxide sur-face can react with CO(a) on the neighboring Pt surface toform CO2 at �300 K and this reaction is favored by the pres-ence of oxygen vacancies in oxides if a large amount of H(a)coexists with CO(a) on a Pt surface.[56–58] Therefore, both reac-tion mechanisms of CO oxidation in excess O2 and H2-assistedCO oxidation should coexist in the CO-PROX reaction catalyzedby Pt/CeO2 catalysts. Moreover, as the Pt0-CeO2 ensemble ismore active than the Pt2 +-CeO2 ensemble in the catalysis ofCO oxidation following the CO oxidation mechanism in excessO2, the higher specific reaction rate of 0.5 %-Pt/c-CeO2 (0.5 %-Pt/r-CeO2) over 1 %-Pt/c-CeO2 (1 %-Pt/r-CeO2) in the CO-PROXreaction indicates that the Pt2+-CeO2 ensemble should bemore active than the Pt0-CeO2 ensemble in the catalysis of COoxidation following the H2-assisted CO oxidation mechanism.Notably, the CO conversion of 0.5 %-Pt/r-CeO2 with a lower Ptloading is higher that of 1 %-Pt/r-CeO2. This cannot be merelyattributed to the Pt2 + species because 1 %-Pt/r-CeO2 containsboth Pt2 + and Pt0, but can be reasonably associated with thelarge concentration of oxygen vacancies in CeO2 of 0.5 %-Pt/r-CeO2 that benefits the H2-assisted CO oxidation. The ID/IF2g

ratioin the visible Raman spectra is 0.18 for 0.5 %-Pt/r-CeO2 and0.07 for 1 %-Pt/r-CeO2. Thus the Pt2 + species and CeO2 withlarge concentration of oxygen vacancies constitute the activestructure of the Pt/CeO2 catalyst for the CO-PROX reaction.

The above results provide solid experimental evidence forthe effect of the morphology of the CeO2 support in Pt/CeO2

catalysts from the catalyst preparation process to the catalyticperformance. The effect of the morphology of the CeO2 sup-port can be reasonably correlated with different exposed crys-tal planes on the CeO2 supports with different morphologiesand their surface composition/structure. c-CeO2 has selectivelyexposed {1 0 0} crystal planes, r-CeO2 has exposed {1 0 0} and{11 0} crystal planes, and o-CeO2 has selectively exposed {111}crystal planes. DFT calculations show that the surface energy oflow-indexed CeO2 surfaces follows the order (111)< (11 0)<(1 0 0)[49] but the oxygen vacancy formation energy follows theorder (11 0)< (1 0 0)< (111) because of the restructuring of the(11 0) surface.[51–54] During the Pt/CeO2 catalyst preparation pro-cess, the least stable CeO2(1 0 0) surface (c-CeO2) interacts moststrongly with the Pt2+ precursor, followed by CeO2(11 0)(r-CeO2) and CeO2(111) (o-CeO2). Reduced at the same temper-ature, Pt/o-CeO2 catalysts with the weakest Pt2 + precursor–CeO2 interaction contain the largest fraction of metallic Pt,whereas Pt/c-CeO2 catalysts with the strongest Pt2 + precursor–CeO2 interaction contain the largest fraction of Pt2 + species.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3617

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

The reducibility and concentration of oxygen vacancies ofCeO2 in the Pt/CeO2 catalysts vary with the CeO2 morphologyand the Pt–CeO2 interaction. Thus, prepared by the samemethod, Pt/CeO2 catalysts with the same Pt loading thatemploy CeO2 cubes, rods, and octahedra as the supports havedifferent structures and exhibit different catalytic activities. Onone hand, the effect of the morphology of the CeO2 support inthe Pt/CeO2 catalysts adequately reveals the structural com-plexity of oxide-supported catalysts. Different crystal planes ex-posed on the catalyst nanoparticle surface exhibit different sur-face reactivities and catalytic activities, thus their contributionsto the observed overall catalytic activity must be unequal. Ithas been reported that the coupling between adjacent crystalplanes with different catalytic activities can occur by the migra-tion of surface species during the catalytic reaction.[13, 59, 60] Thestructural complexity at the level of the crystal plane and mor-phology is a major hindrance for the correlation between thestructure and catalytic activity of powder catalysts. On theother hand, the effect of the morphology of the CeO2 supportin the Pt/CeO2 catalysts proves that the structure and catalyticperformance of Pt/CeO2 catalysts for CO oxidation and the CO-PROX reaction can be tuned by controlling the morphology ofthe CeO2 support. Among the various morphologies of theCeO2 support investigated in this study, CeO2 rods are the bestsupport for the preparation of active Pt/CeO2 catalysts. There-fore, the morphology-controlled strategy of oxides will contrib-ute significantly not only to the fundamental understanding ofheterogeneous catalysis by oxide-involved catalysts but also tothe innovation of efficient oxide-involved catalysts.[5]

Conclusions

We have comprehensively studied the structures and catalyticperformances of Pt/CeO2 catalysts with different Pt loadingsand different morphologies of the CeO2 support (cubes, rods,and octahedra) in CO oxidation and the preferential oxidationof CO in a H2-rich gas (CO-PROX) reaction. The effect of themorphology of the CeO2 support exists in Pt/CeO2 catalystsfrom the catalyst preparation process to the catalytic perfor-mance. The impregnated Pt precursor interacts more stronglywith CeO2 rods and cubes than with CeO2 octahedra and its re-duction/decomposition is easier on CeO2 octahedra than onCeO2 rods and cubes. The Pt/CeO2-octahedra catalyst containsthe largest fraction of metallic Pt, whereas the Pt/CeO2-cubescatalyst contains the largest fraction of Pt2 + species. The re-ducibility of pure CeO2 and CeO2 in the Pt/CeO2 catalysts fol-lows the order CeO2-rods>CeO2-cubes>CeO2-octahedra, andthe promotion effect of Pt on the reducibility of CeO2 is stron-ger in Pt/CeO2-rods and Pt/CeO2-cubes than in Pt/CeO2-octahe-dra. The concentration of oxygen vacancies of CeO2 in Pt/CeO2

catalysts varies with the CeO2 morphology and the Pt–CeO2 in-teraction. The catalytic performance of the Pt/CeO2 catalysts inboth CO oxidation and the CO-PROX reaction follows the orderPt/CeO2-rods>Pt/CeO2-cubes>Pt/CeO2-octahedra. The Pt0-CeO2 ensemble is more active than the Pt2 +-CeO2 ensemble inthe catalysis of CO oxidation in excess O2. H2-assisted CO oxi-dation catalyzed by the Pt/CeO2 catalysts was observed in the

CO-PROX reaction, and the Pt2 + species and CeO2 with a largeconcentration of oxygen vacancies constitute the active struc-ture of the Pt/CeO2 catalyst for this reaction. These results notonly demonstrate that the structure and catalytic performanceof oxide-supported catalysts can be tuned by controlling themorphology of the oxide support but also greatly deepens thefundamental understanding of CO oxidation reactions cata-lyzed by Pt/CeO2 catalysts.

Experimental Section

All the chemicals were analytical grade reagents and were pur-chased from Sinopharm Chemical Reagent Co., Ltd. and used as re-ceived in our experiments. The synthesis of CeO2 cubes and rodsfollowed the procedure of Mai et al.[14] Typically, Ce(NO3)3·6 H2O(1.96 g) was dissolved in ultrapure water (40 mL; resistance>18 MW), and NaOH (16.88 g) was dissolved in ultrapure water(30 mL). The NaOH solution was added dropwise into the Ce(NO3)3

solution under stirring at RT. The solution was stirred for an addi-tional 30 min at RT and then transferred into a 100 mL Teflonbottle. The Teflon bottle was tightly sealed and hydrothermallytreated in a stainless-steel autoclave at 180 8C for 24 h. After cool-ing, the obtained white precipitate was collected, washed with ul-trapure water, and dried in vacuo at 80 8C for 16 h. Then the ac-quired yellow powder was calcined in a muffle oven at 500 8C for4 h to synthesize CeO2 cubes. The synthesis procedure for CeO2

rods was the same as that for CeO2 cubes except that the hydro-thermal treatment temperature was 100 8C. The synthesis of CeO2

octahedra followed the procedure of Yan et al.[15] Typically, Ce-(NO3)3·6 H2O (2 mmol) was dissolved in ultrapure water (79 mL),and then Na3PO4 (1 mL, 0.02 m) was added. The mixed solutionwas stirred for 1 h at RT and then transferred into a 100 mL Teflonbottle. The Teflon bottle was tightly sealed and hydrothermallytreated in a stainless-steel autoclave at 170 8C for 10 h. After cool-ing, the obtained white precipitate was collected, washed with ul-trapure water and ethanol several times, and dried in vacuo at80 8C for 16 h. Then, the acquired white powder was calcined ina muffle oven at 500 8C for 4 h to synthesize CeO2 octahedra.

The Pt/CeO2 catalysts were prepared by a conventional incipientwetness impregnation method. Typically, the desired amount of Pt-(NH3)4(NO3)2 solution was added dropwise to CeO2 nanocrystals(0.3 g) and ultrasonicated for 10 min. After the impregnation, thesample was kept at RT for 24 h and then dried at 80 8C for 12 h toprepare the catalyst precursor. The catalyst precursors were re-duced in pure H2 at 200 8C for 3 h to prepare the Pt/CeO2 catalysts.Three Pt/CeO2 catalysts with calculated Pt loadings of 0.2, 0.5, and1 % (Pt/CeO2 weight ratio) were prepared.

The loading of Pt in the Pt/CeO2 catalysts was determined by usingan Optima 7300 DV inductively coupled plasma atomic emissionspectrometer (ICP-AES). BET specific surface areas were acquired byusing a Beckman Coulter SA3100 surface area analyzer, and thesample was degassed at 300 8C for 5 h in a N2 atmosphere beforethe measurement. XRD patterns were recorded by using a PhilipsX’Pert PRO diffractometer using a Ni-filtered CuKa (wavelength =0.15418 nm) radiation source with the operation voltage and oper-ation current of 40 kV and 40 mA, respectively. XPS measurementswere performed by using an ESCALAB 250 high-performance elec-tron spectrometer using monochromatized AlKa (hn= 1486.7 eV) asthe excitation source. The likely charging of samples was correctedby setting the binding energy of the adventitious carbon (C 1s) to284.8 eV. Laser Raman spectra were obtained in back-scattering

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3618

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

configuration by using a LABRAM-HR Confocal Laser Raman spec-trometer. Ar+ (514.5 nm) and He�Cd (325 nm) lasers were em-ployed as the excitation source to obtain visible Raman and UVRaman spectra, respectively. TEM and HRTEM images were ac-quired by using Jeol-2010 and Jeol-2100F high-resolution transmis-sion electron microscopes with the electron acceleration energy of200 kV.

H2-TPR experiments were performed by using a MicromeriticsChemiSorb 2750 instrument, in which the sample (50 mg) washeated at a rate of 10 8C min�1 in a 5 % H2-Ar mixture with a flowrate of 20 mL min�1. During the TPR measurements of the Pt/CeO2

catalysts, the catalyst precursors were reduced at 200 8C with pureH2 for 3 h, reoxidized at 300 8C in 20 % O2-Ar for 2 h, and thencooled to RT in Ar prior to the measurements.

The catalytic activity of the Pt/CeO2 catalysts in CO oxidation andthe CO-PROX reaction were evaluated by using a fixed-bed flow re-actor. In CO oxidation, Pt/CeO2 (25 mg) diluted with SiO2 (50 mg)was used, and the reaction gas that consisted of 1 % CO balancedwith dry air was fed at a rate of 30 mL min�1. In the CO-PROX reac-tion, Pt/CeO2 (100 mg) diluted with SiO2 (100 mg) was used, andthe feed gas stream that consisted of 1 % CO, 1 % O2, and 50 % H2

balanced with N2 was fed at a total flow rate of 100 mL min�1. Thecatalyst was heated to the desired reaction temperature at a rateof 2 8C min�1 and then kept for 35 min until the catalytic reactionreached the steady state. Then the composition of the effluent gaswas analyzed by using an online GC-14 gas chromatograph. Theconversion of CO and O2 were calculated from the change in theirconcentrations of the inlet and outlet gases. O2 selectivity to CO2

was as calculated according to Equation (1):

SO2¼ XCO

2 X O2

� 100 % ð1Þ

in which X and S are the conversion and selectivity, respectively.

Acknowledgements

This work was financially supported by the National Basic Re-search Program of China (2013CB933104, 2010CB923301), theNational Natural Science Foundation of China (21173204), andthe Fundamental Research Funds for the Central Universities.

Keywords: cerium · metal-support interaction · oxidation ·platinum · supported catalysts · structure–activity relationships

[1] G. Wulff, Z. Kristallogr. Mineral. 1901, 34, 449 – 530.[2] J. A. van Bokhoven, ChemCatChem 2009, 1, 363 – 364.[3] K. B. Zhou, Y. D. Li, Angew. Chem. 2012, 124, 622 – 635; Angew. Chem.

Int. Ed. 2012, 51, 602 – 613.[4] Y. Li, W. J. Shen, Sci. China Chem. 2012, 55, 2485 – 2496.[5] W. X. Huang, Top. Catal. 2013, DOI : 10.1007/s11244-013-0139-6.[6] B. M. Choudary, R. S. Mulukutla, K. J. Klabunde, J. Am. Chem. Soc. 2003,

125, 2020 – 2021.[7] L. H. Hu, Q. Peng, Y. D. Li, J. Am. Chem. Soc. 2008, 130, 16136 – 16137.[8] X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta, W. J. Shen, Nature 2009, 458, 746 –

749.[9] H. Z. Bao, W. H. Zhang, D. L. Shang, Q. Hua, Y. S. Ma, Z. Q. Jiang, J. L.

Yang, W. X. Huang, J. Phys. Chem. C 2010, 114, 6676 – 6680.[10] Q. Hua, D. L. Shang, W. H. Zhang, K. Chen, S. J. Chang, Y. S. Ma, Z. Q.

Jiang, J. L. Yang, W. X. Huang, Langmuir 2011, 27, 665 – 671.

[11] H. Z. Bao, W. H. Zhang, Q. Hua, Z. Q. Jiang, J. L. Yang, W. X. Huang,Angew. Chem. 2011, 123, 12502 – 12506; Angew. Chem. Int. Ed. 2011, 50,12294 – 12298.

[12] Q. Hua, K. Chen, S. J. Chang, Y. S. Ma, W. X. Huang, J. Phys. Chem. C2011, 115, 20618 – 20627.

[13] Q. Hua, K. Chen, S. J. Chang, H. Z. Bao, Y. S. Ma, Z. Q. Jiang, W. X. Huang,RSC Adv. 2011, 1, 1200 – 1203.

[14] H. X. Mai, L. D. Sun, Y. W. Zhang, R. Si, W. Feng, H. P. Zhang, H. C. Liu,C. H. Yan, J. Phys. Chem. B 2005, 109, 24380 – 24385.

[15] L. Yan, R. Yu, J. Chen, X. Xing, Cryst. Growth Des. 2008, 8, 1474 – 1477.[16] D. Zhang, X. Du, L. Shi, R. Gao, Dalton Trans. 2012, 41, 14455 – 14475.[17] C. Sun, H. Li, L. Chen, Energy Environ. Sci. 2012, 5, 8475 – 8505.[18] A. Trovarelli, Catal. Rev. Sci. Eng. 1996, 38, 439 – 520.[19] K. Zhou, X. Wang, X. Sun, Q. Peng, Y. Li, J. Catal. 2005, 229, 206 – 212.[20] W. Q. Han, L. J. Wu, Y. M. Zhu, J. Am. Chem. Soc. 2005, 127, 12814 –

12815.[21] K. B. Zhou, Z. Q. Yang, S. Yang, Chem. Mater. 2007, 19, 1215 – 1217.[22] Tana, M. L. Zhang, J. Li, H. J. Li, Y. Li, W. J. Shen, Catal. Today 2009, 148,

179 – 183.[23] Y. Guan, E. J. M. Hensen, Phys. Chem. Chem. Phys. 2009, 11, 9578.[24] X. Liu, K. Zhou, L. Wang, B. Wang, Y. Li, J. Am. Chem. Soc. 2009, 131,

3140 – 3141.[25] J. Lv, Y. Shen, L. Peng, X. Guo, W. Ding, Chem. Commun. 2010, 46,

5909 – 5911.[26] Z. Wu, M. Li, J. Howe, H. M. Meyer, S. H. Overbury, Langmuir 2010, 26,

16595 – 16606.[27] Z. Wu, M. Li, S. H. Overbury, J. Catal. 2012, 285, 61 – 73.[28] S. Agarwal, L. Lefferts, B. L. Mojet, ChemCatChem 2013, 5, 479 – 489.[29] L. Torrente-Murciano, A. Gilbank, B. Puertolas, T. Garcia, B. Solsona, D.

Chadwick, Appl. Catal. B 2013, 132 – 133, 116 – 122.[30] T. D�saunay, G. Bonura, V. Chiodo, S. Freni, J. P. Couzini�, J. Bourgon, A.

Ringued�, F. Labat, C. Adamo, M. Cassir, J. Catal. 2013, 297, 193 – 201.[31] R. Si, M. Flytzani-Stephanopoulos, Angew. Chem. 2008, 120, 2926 – 2929;

Angew. Chem. Int. Ed. 2008, 47, 2884 – 2887.[32] G. Yi, N. Xu, G. Guo, K.-i. Tanaka, Y. Yuan, Chem. Phys. Lett. 2009, 479,

128 – 132.[33] G. Yi, H. Yang, B. Li, H. Lin, K.-i. Tanaka, Y. Yuan, Catal. Today 2010, 157,

83 – 88.[34] L. Feng, D. T. Hoang, C. K. Tsung, W. Y. Huang, S. H. Y. Lo, J. B. Wood,

H. T. Wang, J. Y. Tang, P. D. Yang, Nano Res. 2011, 4, 61 – 71.[35] Y. Guan, D. Ligthart, �. Pirgon-Galin, J. Pieterse, R. van Santen, E.

Hensen, Top. Catal. 2011, 54, 424 – 438.[36] X. Han, H. J. Kim, S. Yoon, H. Lee, J. Mol. Catal. A 2011, 335, 82 – 88.[37] L. Liu, Z. Yao, Y. Deng, F. Gao, B. Liu, L. Dong, ChemCatChem 2011, 3,

978 – 989.[38] X. Du, D. Zhang, L. Shi, R. Gao, J. Zhang, J. Phys. Chem. C 2012, 116,

10009 – 10016.[39] Z. Wu, M. Li, S. H. Overbury, ChemCatChem 2012, 4, 1653 – 1661.[40] S. Chang, M. Li, Q. Hua, L. Zhang, Y. Ma, B. Ye, W. Huang, J. Catal. 2012,

293, 195 – 204.[41] Z. Wu, V. Schwartz, M. Li, A. J. Rondinone, S. H. Overbury, J. Phys. Chem.

Lett. 2012, 3, 1517 – 1522.[42] D. Gamarra, A. L. C�mara, M. Monte, S. B. Rasmussen, L. E. Chinchilla,

A. B. Hungr�a, G. Munuera, N. Gyorffy, Z. Schay, V. C. Corber�n, J. C.Conesa, A. Mart�nez-Arias, Appl. Catal. B 2013, 130 – 131, 224 – 238.

[43] C. Ratnasamy, J. P. Wagner, Catal. Rev. 2009, 51, 325 – 440.[44] O. Pozdnyakova, D. Teschner, A. Wootsch, J. Krçhnert, B. Steinhauer, H.

Sauer, L. Toth, F. Jentoft, A. Knop-Gericke, Z. Pa�l, R. Schlçgl, J. Catal.2006, 237, 1 – 16.

[45] O. Pozdnyakova-Tellinger, D. Teschner, J. Krçhnert, F. C. Jentoft, A. Knop-Gericke, R. Schlçgl, A. Wootsch, J. Phys. Chem. C 2007, 111, 5426 – 5431.

[46] D. Teschner, A. Wootsch, O. Pozdnyakovatellinger, J. Krçhnert, E. Vass,M. H�vecker, S. Zafeiratos, P. Schnçrch, P. Jentoft, A. Knop-Gericke, R.Schlçgl, J. Catal. 2007, 249, 318 – 327.

[47] Y. Zhai, D. Pierre, R. Si, W. Deng, P. Ferrin, A. U. Nilekar, G. Peng, J. A.Herron, D. C. Bell, H. Saltsburg, M. Mavrikakis, M. Flytzani-Stephanopou-los, Science 2010, 329, 1633 – 1636.

[48] C. S. Polster, R. Zhang, M. T. Cyb, J. T. Miller, C. D. Baertsch, J. Catal.2010, 273, 50 – 58.

[49] M. Nolan, S. C. Parker, G. W. Watson, Surf. Sci. 2005, 576, 217 – 229.[50] J. Barbier Jr. , F. Marsollier, D. Duprez, Appl. Catal. A 1992, 90, 11 – 23.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3619

CHEMCATCHEMFULL PAPERS www.chemcatchem.org

[51] M. Nolan, S. C. Parker, G. W. Watson, Surf. Sci. 2005, 595, 223 – 232.[52] M. Nolan, Chem. Phys. Lett. 2010, 499, 126 – 130.[53] D. O. Scanlon, N. M. Galea, B. J. Morgan, G. W. Watson, J. Phys. Chem. C

2009, 113, 11095 – 11103.[54] M. V. Ganduglia-Pirovano, J. L. F. Da Silva, J. Sauer, Phys. Rev. Lett. 2009,

102, 026101.[55] W. H. Weber, K. C. Hass, J. R. McBride, Phys. Rev. B 1993, 48, 178 – 185.[56] L. Xu, Y. Ma, Y. Zhang, Z. Jiang, W. Huang, J. Am. Chem. Soc. 2009, 131,

16366 – 16367.[57] L. Xu, Z. Wu, Y. Zhang, B. Chen, Z. Jiang, Y. Ma, W. Huang, J. Phys. Chem.

C 2011, 115, 14290 – 14299.

[58] L. Xu, Z. Wu, Y. Jin, Y. Ma, W. Huang, Phys. Chem. Chem. Phys. 2013, 15,12068 – 12074.

[59] V. Gorodetskii, J. Lauterbach, H. H. Rotermund, J. H. Block, G. Ertl, Nature1994, 370, 276 – 279.

[60] W. X. Huang, R. S. Zhai, X. H. Bao, Langmuir 2001, 17, 3629 – 3634.

Received: August 25, 2013

Published online on September 23, 2013

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3610 – 3620 3620

CHEMCATCHEMFULL PAPERS www.chemcatchem.org