Mechanism and Site Requirements for Ethanol Oxidation on Vanadium Oxide Domains

The mechanism and structural requirements for ethanol oxidation to acetaldehyde were examined on VOx domains supported on gamma-Al2O3 at surface densities of 1.7-11.8 VOx/nm(2). Raman and UV-visible spectra showed that VOx species evolve from monovanadate to polyvanadate structures with increasing surface density with only traces of crystalline V2O5. Oxidative dehydrogenation (ODH) of ethanol to acetaldehyde occurs at low temperatures (473-523 K) with high primary selectivities of CH3CHO (similar to 80%) on a catalyst with one theoretical polyvanadate monolayer. ODH turnover rates (per V-atom) increased with increasing VOx surface density for surface densities up to 7.2 V/nm(2), indicating that polyvanadate domain surfaces are more reactive than monovanadate structures. Similar trends were evident for alkane ODH reactions that also involve kinetically relevant H-abstraction steps within reduction-oxidation catalytic sequences. Turnover rates ultimately decreased at higher surface densities because of the incipient formation of three-dimensional structures. VOx domains of intermediate size therefore provide a compromise between site reactivity and accessibility during ethanol ODH. The effects of O-2 and C2H5OH pressures on ethanol ODH rates and the kinetic isotope effects for C2H5OD and C2D5OD confirmed the kinetic relevance of H-abstraction from ethoxide species formed in quasiequilibrated ethanol dissociation steps; taken together with in situ infrared spectra, these data also show that ethoxide species are present at near saturation coverages on fully oxidized VOx domains that undergo reduction-oxidation cycles during each ethanol oxidation turnover.