CH3OH oxidation over well-defined supported V2O5/Al2O3 catalysts:: Influence of vanadium oxide loading and surface vanadium-oxygen functionalities

A series of supported V2O5/Al2O3 catalysts were synthesized by incipient wetness impregnation with vanadium isopropoxide in isopropanol solutions and subsequent calcination. The vanadium surface density was varied from 0.3 to 11.4 V atoms/nm(2) spanning the sub-monolayer and above-monolayer regions. The resulting supported vanadium oxide catalysts were physically characterized with in situ Raman spectroscopy and chemically probed by CH3OH TPSR and steady-state methanol oxidation. The Raman characterization under dehydrated conditions revealed that the supported vanadium oxide catalysts contained only surface vanadia species below monolayer coverage (100% dispersed) and both surface vanadia species and crystalline V2O5 nanoparticles (NPs) above monolayer coverage. The CH3OH oxidative dehydrogenation kinetic parameters were found to be independent of the surface bridging V-O-V bond concentration (polymerization extent), surface V=O bond length/strength, vanadium surface density on the alumina support, surface acidity, and surface vanadia reduction characteristics during H-2-TPR. The crystalline V2O5 NPs above monolayer coverage were relatively inactive and served only to decrease the number of exposed catalytic active surface vanadia sites by covering them. (C) 2008 Elsevier Inc. All rights reserved.