Fundamental Understanding of Transition Metal Oxide Catalysts for Selective Oxidations
High surface area VOx/CeO2 model catalysts were developed to understand the effects of support on VOx catalysts in selective oxidation reactions. The VOx/CeO2 model catalysts were prepared by depositing highly dispersed VOx species on CeO2 nanoparticles including nanorods, nanocubes and nanopolyhedra with dominating low index (100), (110) and (111) facets. To understand and eliminate the possible interference of sodium residuals from CeO2 synthesis, we first investigated effects of Na on the surface structure and catalytic properties of VOx/CeO2 catalysts using oxidative dehydrogenation (ODH) of methanol as a probe reaction. Experimental results indicate that the effects of sodium on VOx/CeO2 are highly dependent on the Na/V ratio. At low Na/V ratios (Na/V<0.25), sodium addition has minimal effect on its activity and selectivity to formaldehyde formation. At high Na/V ratios (Na/V>0.25), sodium alters the nature of the active sites via V-O-Ce bond cleavage and V-O-Na bond formation, leading to significantly reduced activity of VOx/CeO2 catalysts. The selectivity to formaldehyde also decreases with increasing Na/V ratio. With safe exclusion of sodium interference, VOx catalysts supported on CeO2 nanorods, nanocubes and nanopolyhedra were employed to investigate the effects of support facets on VOx catalysts in ODH of methanol. In the presence of mixed facets, surface vanadia species preferentially deposit on CeO2 (100) facets, presumably because of its higher surface energy. At the same surface vanadium densities, VOx species on (100) facets show better dispersion, followed by (110) and (111) facets. The VOx species on CeO2 nanorods with (110) and (100) facets display higher activity and lower apparent activation energies compared to that on CeO2 nanopolyhedra with dominating (111) facets and CeO2 nanocubes with dominating (100) facets. The higher activity of VOx/CeO2(110) might be related to the more abundant oxygen defects present on the (110) facets. Further Study on VOx/CeO2 (100) model catalysts was conducted. Experimental results show that the higher oxygen defect densities of smaller CeO2 nanocubes supported VOx catalysts effectively promote the redox property and lower the activation energy for methoxyl decomposition. These results further confirm the important role of oxygen vacancies in promoting the activity of VOx species in methanol oxidation.