Characterization of Ceria Films in SBA-15

Ceria films were deposited on SBA-15 by Atomic Layer Deposition (ALD) to loadings of 40-wt %, and the materials were then examined as possible supports for Pd in the CO oxidation and water-gas shift (WGS) reactions. N-2 adsorption isotherms and transmission electron microscopy confirmed that CeO2 was uniformly deposited in the pores. X-ray Diffraction and X-ray Absorption Near Edge Structure (XANES) results suggested that the Ce was present as small, 2.4 nm CeO2 crystallites following oxidation at 1073 K, but heating in H-2 at 1073 K resulted in the complete reduction of the Ce cations from +4 to +3. Raman spectra of the reduced samples suggested the formation of a cerium silicate. Coulometric titration measurements showed that equilibrium P(O-2) for ceria films on SBA-15 was shifted to values between 10(12) and 10(15) higher than those of ceria films on Al2O3 at 1073 K, implying ceria films in SBA-15 are much more easily reduced. However, the CO oxidation and WGS rates for Pd catalysts supported on CeO2/SBA-15 were lower than rates on CeO2/Al2O3, implying that higher reducibility may not always lead to higher reaction rates.