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/nm² 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{single bond}O{single bond}V bond concentration (polymerization extent), surface V{double bond, long}O bond length/strength, vanadium surface density on the alumina support, surface acidity, and surface vanadia reduction characteristics during H₂-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.