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The reduction and reoxidation of submonolayer coverages of TiO2 deposited onto MCM-48 were investigated. The deposited TiO2 was characterized by Raman and UV−visible spectroscopy. Raman spectra show that Ti atoms are bonded to the silica support by Ti−O−Si bonds and that crystalline TiO2 is not formed. The results of the Raman and UV−visible spectroscopy suggest that the dispersed TiO2 is present as two-dimensional oligomeric structures. Reduction in H2 at 923 K produces Ti3+ cations observable by EPR (g = 1.932), suggesting the formation of oxygen vacancies. The fraction of Ti that could be reduced increased with TiO2 surface concentration. This observation is attributed to the ease with which O atoms can be removed from the TiO2 overlayer as the size of the titania patches increases. The amount of oxygen removed during reduction was quantified by pulsed reoxidation. It was observed that the temperature required for complete reoxidation decreased with increasing surface coverage of the silica support by TiO2. This trend is explained with a proposed model of the reoxidation process, in which the rate limiting step is the migration of peroxide species through or between the deposited TiO2 patches. A linear correlation was established between the intensity of the EPR signal for Ti3+ and the amount of oxygen removed from TiO2/SiO2. This relationship was then used to determine the oxygen vacancy concentration present on the surface of TiO2/SiO2 after temperature-programmed oxidation of methanol.

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An investigation was performed to establish the effects of precursor composition on the structure of ceria deposited onto mesoporous silicas, MCM-41 and SBA-15. The structure of the deposited ceria was characterized by XRD, TEM, Raman and UV-visible spectroscopy, XANES, and EXAFS. Using Ce(acac) 3 as the precursor resulted in formation of 3 nm diameter ceria particles located predominantly in the mouths of the support pores. Amorphous agglomerates, but with domains of comparable size to those obtained using Ce(acac) 3 , were obtained using Ce(O i Pr) 4 . Much smaller domains and a high degree of dispersion were obtained using Ce(O t Bu) 4 as the precursor. The structure of the deposited ceria affects its reduction-reoxidation properties. Reduction and reoxidation are almost fully reversible for the nanoparticles. Only surface reduction is observed up to 723 K, whereas partial bulk reduction takes place at higher reduction temperatures. In case of the well-dispersed, layerlike Ce species on the silica surface, an extent of reduction similar to that of the nanoparticles is achievable, but upon heating the samples in 10% H 2 /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 cerium species. ' INTRODUCTIONDispersion of a transition metal oxide on a high surface area support such as Al 2 O 3 or SiO 2 makes it possible to combine the catalytic properties of the transition metal oxide with the high surface area and thermal and mechanical stability of the support. 1 Good illustrations of the manner in which the transition metal oxide is introduced onto the support and the physical properties of the deposited oxide are given for the cases of TiO x , 1-9 VO x , 5,[9][10][11][12][13][14] or MoO x 10,11,15-21 dispersed on high surface area supports. A notable consequence of this approach is the formation of catalytically active sites which are not present on either the support or the bulk transition metal oxide. For example, neither TiO 2 nor SiO 2 is active for the liquid-phase epoxidation of alkenes, but TiO 2 /SiO 2 exhibits high activity and selectivity for this reaction. 6,[22][23][24] The appearance of such novel catalytically active sites has been attributed to the formation of a twodimensional oxide overlayer whose physical properties differ from those of the corresponding bulk oxide. Dispersed transition metal oxides have also been shown to stabilize the dispersion of small metal particles and to enhance the activity of supported metal oxo species. For example, Au particles have been shown to be much more stable on TiO x /SiO 2 than on SiO 2 25 and isolated vanadate species dispersed on TiO x /SiO 2 exhibit turnover frequencies that are up to 60-fold higher than when dispersed on SiO 2 . 7 CeO 2 is a highly reducible oxide exhibiting oxygen storage and release properties that are important for the operation of threeway, automotive exhaust catalysts. 26 It can also stabilize the dispersion of n...