Synthesis of different CeO₂ structures on mesoporous silica and characterization of their reduction properties

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(OⁱPr)₄. Much smaller domains and a high degree of dispersion were obtained using Ce(O^tBu)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% 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 cerium species.