CO oxidation by rutile TiO2(110) doped with V, W, Cr, Mo, and Mn

Abstract

We used density functional theory to study CO oxidation catalyzed by TiO2(110), in which some Ti atoms on the surface are replaced with V, Cr, Mo, W, or Mn. We find that in the presence of 0, V, Cr, Mo, and W dopants at the surface bind an oxygen atom so that the dopant has formula MO (M = V, Cr, Mo, W). Rutile doped with Mn does not take an oxygen atom from the gas phase. We find that these materials oxidize CO by a Mars-van Krevelen mechanism in which the role of the dopant is to facilitate the formation of oxygen vacancies. The energy of CO reaction with an oxygen atom from the surface layer decays linearly with the energy of vacancy formation Delta E-v, whereas the energy of adsorption of O-2 at a vacancy is a linear function of Delta E-v. These are the only two reactions in the mechanism whose energy varies from one doped oxide to another. Because they both depend on the energy of oxygen vacancy formation, the latter quantity is a good descriptor of catalytic activity. In deciding which intermediate reactions are most likely from an energetic point of view, we impose a "spin conservation" rule: a reaction that requires "flipping a spin" is too slow for catalysis. Because of this, we only consider reactions that conserve spin. We find that all the dopants studied here lower the energy of vacancy formation; therefore, the doped oxides are better oxidants than the undoped ones.