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The mechanism for the reoxidation step in the Mars−van Krevelen mechanism for ethanol partial oxidation over vanadia anchored on titanium oxide is examined. Kinetic parameters such as ethanol heat of adsorption, the activation energy for the rate-limiting step (α-hydrogen abstraction on the adsorbed ethoxide) were obtained while the energetics of the catalyst reoxidation step were explored. A comparison of the parameters obtained from kinetic analysis and the apparent activation energies reported in the literature indicated that a kinetic model that incorporates a catalyst reoxidation step, where molecular oxygen adsorbs into a titania vacancy, accurately predicted the kinetic parameters. In contrast, a model where molecular oxygen directly adsorbs on the reduced vanadia resulted in an underestimation of the ethanol heat of adsorption and activation energy for the α-hydrogen abstraction step. A computational analysis was implemented to elucidate a mechanistic pathway for reduced vanadia that incorporates oxygen adsorption on a titania vacancy. The results indicated that the vanadia reoxidation step involves surface oxygen migration from the titania surface to the reduced vanadia center. The quantification of oxygen uptake by the reduced catalyst validates the premise of this assumption: titania vacancies are created during ethanol partial oxidation and are active sites for oxygen adsorption.

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The catalytic activity for ethanol partial oxidation of vanadium oxide (VOx) anchored on titanium oxide was correlated to their electronic structure. In situ Raman spectroscopy and temperature‐programmed desorption (TPD) experiments indicate that the presence of catalytically active VOx moieties is very sensitive to vanadia loading: highly dispersed VOx predominantly exists at low VOx contents whereas larger vanadia clusters coexist at higher VOx loadings. In situ UV/Vis spectroscopy revealed that a significant fraction of these larger clusters remain reduced during catalysis, and thus do not fully participate in catalytic turnovers. The electronic structures of model VOx nanoclusters of different sizes (monomer, dimer, trimer, and one‐dimensional polymers) were investigated by using periodic density functional theoretical calculations. Results indicate that their electronic structures are significantly affected by their size. Our analysis also revealed that the formation of reduced VOx species (V4+) during catalysis is concomitant to the reduction of adjacent Ti cations (Ti3+). Theoretically calculated optical absorption spectra matched the experimental spectroscopic results obtained under in situ reaction conditions. Furthermore, the determination of defect formation enthalpies reported previously as the main descriptor for catalytic activity of vanadia nanoclusters, predicted that isolated monomeric VOx clusters predominantly take part in catalytic turnovers.

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