Ethanol Partial Oxidation over VOX/TiO2 Catalysts: The Role of Titania Surface Oxygen on Vanadia Reoxidation in the Mars-van Krevelen Mechanism

The mechanism for the reoxidation step in the Marsvan 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 (alpha-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 a-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.