Mechanism and Site Requirements for Ethanol Oxidation on Vanadium Oxide Domains

The mechanism and structural requirements for ethanol oxidation to acetaldehyde were examined on VO x domains supported on γ-Al 2 O 3 at surface densities of 1.7−11.8 VO x /nm 2 . Raman and UV−visible spectra showed that VO x species evolve from monovanadate to polyvanadate structures with increasing surface density with only traces of crystalline V 2 O 5 . Oxidative dehydrogenation (ODH) of ethanol to acetaldehyde occurs at low temperatures (473−523 K) with high primary selectivities of CH 3 CHO (∼80%) on a catalyst with one theoretical polyvanadate monolayer. ODH turnover rates (per V-atom) increased with increasing VO x 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. VO x domains of intermediate size therefore provide a compromise between site reactivity and accessibility during ethanol ODH. The effects of O 2 and C 2 H 5 OH pressures on ethanol ODH rates and the kinetic isotope effects for C 2 H 5 OD and C 2 D 5 OD 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 VO x domains that undergo reduction−oxidation cycles during each ethanol oxidation turnover.