Oxygen Sites at Molybdena and Vanadia Surfaces: Energetics of the Re-Oxidation Process

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Abstract
In oxidation reactions proceeding in accordance with the Mars-van Krevelen mechanism lattice oxygen plays the role of an oxidizing agent. Surface vacancies created by incorporation of lattice oxygen into reacting molecules are filled in a subsequent step by gaseous oxygen or, if not enough oxygen is present in the reaction environment, by oxygen diffusion from the bulk. During this process, a very active, electrophilic surface oxygen species may be formed. In effect, total combustion takes place decreasing the selectivity for partial oxidation products. The thermodynamic aspect of this effect (neglecting reaction barriers) is demonstrated for molybdenum trioxide and vanadium pentoxide. On the catalytically most interesting surfaces, MoO3(010) and V2O5(010), three structurally different types of oxygen sites are present which exhibit different properties with respect to vacancy creation and annihilation. Re-oxidation of the catalyst by gaseous oxygen leads to oxygen molecules adsorbed in vacancies, preferably in an orientation parallel to the surface. Adsorption of the oxygen molecule in the vacancy leads to its activation followed by easy release of a neutral oxygen atom, which can be identified as the electrophilic species responsible for total combustion.