Identification and roles of the different active sites in supported vanadia catalysts by in situ techniques

Abstract

The roles of the different sites and structures of supported vanadium oxide catalysts have been studied by in situ spectroscopic techniques (UV-Vis-DRS and Raman) as well as by the use of isotopic oxygen labeling, in order to propose a molecular understanding of the roles of the different oxygen sites. The remarkable effect of the different oxide supports on the activity per vanadium site (TOF) underlines the relevance of the bridging V-O-support bond as the active site. The reducibility of the support appears to correlated with the activity per vanadium site. In situ Raman studies employing isotopic oxygen labeling rule out the terminal V=O bond as the critical site involved in the rate determining step for the oxidation of ethane. The bridging V-O-V bonds appear to have a moderate participation and the bridging V-O-Support bonds appear to be the critical site for ethane oxidation. In situ UV-Vis-DRS and in situ Raman spectroscopy are complementary for the determination of the polymerization degree of the surface vanadium oxide species. Under reducing conditions, polymeric surface vanadia species are more reducible than isolated species and vanadia supported on reducible oxides (titania, zirconia, ceria) is more reducible than vanadia supported on less reducible oxides (silica, alumina). However, the oxidation state under reaction conditions does not depend on the reducibility of the surface vanadia species, but on the average oxidation state under steady-state oxidation of ethane.