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Synthesis of Different CeO2 Structures on Mesoporous Silica

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Published: February 22, 2011r2011 American Chemical Society4114 dx.doi.org/10.1021/jp1105746|J. Phys. Chem. C 2011, 115, 4114–4126ARTICLEpubs.acs.org/JPCCSynthesis of Different CeO2Structures on Mesoporous Silica andCharacterization of Their Reduction PropertiesJennifer Strunk, William C. Vining, and Alexis T. Bell*Chemical Engineering Department, University of California, Berkeley, and Lawrence Berkeley Nationa l Laboratory, Berkeley, California94720, United StatesABSTRACT: An investigation was performed to establish the effects ofprecursor composition on the structure of ceria deposited onto meso-porous silicas, MCM-41 and SBA-15. The structure of the deposited ceriawas characterized by XRD, TEM, Raman and UV-visible spectroscopy,XANES, and EXAFS. Using Ce(acac)3as the precursor resulted information of 3 nm diameter ceria particles located predominantly inthe mouths of the support pores. Amorphous agglomerates, but withdomains of comparable size to those obtained using Ce(acac)3, wereobtained using Ce(OiPr)4. Much smaller domains and a high degree ofdispersion were obtained using Ce(OtBu)4as the precursor. Thestructure of the deposited ceria affects its reduction-reoxidation properties. Reduction and reoxidation are almost fully reversiblefor the nanoparticles. Only surface reduction is observ ed up to 723 K, whereas partial bulk reduction takes place at higher reductiontemperatures. In case of the well-dispersed, layerlike Ce species on the silica surface, an extent of reduction similar to that ofthe nanoparticles is achievable, but upon heating the samples in 10% H2/He to 923 K the reduction becomes partially irreversible.The latter effect might be explained by a formation of cerium silicates that prevent the complete reoxidation of the ceriumspecies.’ INTRODUCTIONDispersion of a transition metal oxide on a high surface areasupport such as Al2O3or SiO2makes it possible to combine thecatalytic properties of the transition metal oxide with the highsurface area and thermal and mechanical stability of the support.1Good illustrations of the manner in which the transition metaloxide is introduced onto the support and the physi cal propertiesof the deposited oxide are given for the cases of TiOx,1-9VOx,5,9-14or MoOx10,11,15-21dispersed on high surface areasupports. A notable consequence of this approach is the forma-tion of catalytically active sites which are not present on either thesupport or the bulk transition metal oxide. For example, neitherTiO2nor SiO2is active for the liquid-phase epoxidation ofalkenes, but TiO2/SiO2exhibits high activity and selectivity forthis reaction.6,22-24The appearance of such novel catalyticallyactive sites has been attributed to the formation of a two-dimensional oxide overlayer whose physical properties differfrom those of the corresponding bulk oxide. Dispersed transitionmetal oxides have also been shown to stabilize the dispersion ofsmall metal particles and to enhance the activity of supportedmetal oxo species. For example, Au particles have been shown tobe much more stable on TiOx/SiO2than on SiO225and isolatedvanadate species dispersed on TiOx/SiO2exhibit turnoverfrequencies that are up to 60-fold higher than when dispersedon SiO2.7CeO2is a highly reducible oxide exhibiting oxygen storage andrelease properties that are impor tant for the operation of three-way, automotive exhaust catalysts.26It can also stabilize thedispersion of noble metal particles,27and CeO2-supportedvanadia is much more active for methanol synthesis than vanadiadispersed on other supports.10,28Since it is hard to prepare bulkCeO2with a high surface area, interest has arisen in dispersingceria onto high surface area supports. While there have beennumerous attempts to disperse ceria on SiO2or Al2O329-49or toincorporate cerium atoms into the walls of mesoporoussilica,50-52these efforts have largely led to the formation ofsmall crystallites of CeO2because ceria has a strong tendency tosinter at elevated temperatures.53-55While there have beensome reports of noncrystalline CeO2species grafted onSiO2,39,45,55-58the depos ited material has not been well char-acterized. What is known, though, is that the structure of CeO2deposited on inert supports depends on the composition of thecerium precursor and the deposition conditions. In general,alkoxide precursors and cerium acetylacetonate lead to theformation of smaller CeO2particles on SiO2than depositsproduced using cerium nitrate or cerium ammonium nitrate asthe precursor.32,41,42,45,55The aim of the present work was to establish the influenc e ofprecursor composition and support structu re on the depositionof ceria on silica. Mesoporous silicas—MCM-41 and SBA-15—were used as the supports. Previous work has shown that highdispersion of V,59Pt,60and Ti61species can be deposited by usingReceived: November 4, 2010Revised: January 18, 20114115 dx.doi.org/10.1021/jp1105746 |J. Phys. Chem. C 2011, 115, 4114–4126The Journal of Physical Chemistry CARTICLEorganometallic complexes with spatially demanding ligands. Onthe basis of these fi ndings, cerium precursor molecules with bulkyligands were selected for the deposition of ceria layers on silica.Due to the generally superior performance of alkoxide precursorsfor grafting transition metals on silica, cerium isopropoxide andcerium t-butoxide were investigated in this study. The use ofcerium tert-butoxide for the grafting of ceria on silica has not beenreported previously. Cerium acetylacetonate was also usedbecause it grafts differently to the surface than the alkoxideprecursors. It will be shown that the use of these three precursorsallows the controlled synthesis of layerlike structures, uniformceria nanoparticles, and partially amorphous agglomerates onMCM-41 and SBA-15. The different structu res were identifiedand characterized by XRD, TEM, Raman and UV-visiblespectroscopy, XANES, and EXAFS. The reduction and reoxida-tion properties of the samples were also characterized andcompared.’ EXPERIMENTAL SECTIONMCM-41 was prepared following previously reportedprocedures.62,63The resulting material had a surface area ofalmost 1100 m2/g and an average pore diameter of 18-20 Å.SBA-15 was synthesized following a modification of the methoddescribed in ref 64, using Pluronic P123 (ÆMWæ = 5800) as thestructural templ ate. The template was suspended in water andstirred for 1.5 h before it was fully dissolved by adding 120 mL of2 M HCl and stirred for another 2 h. TEOS was


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