No abstract

GGA+U calculation were performed for oxides of Ti, V, Mo, and Ce with the objective of establishing the best value of the parameter Ueff to use in order to match the calculated reduction and oxidation energies of each oxide with experimental values. In each case, the reaction involved the hydrogen reduction of an oxide to its next lower oxide and the formation of water. Our calculations show that the optimal value of Ueff required to match calculated and experimental values of the reaction energy are significantly different from those reported in the literature based on matching lattice parameters or electronic properties and that the use of these values of Ueff can result in errors in the calculated redox energies of over 100 kJ/mol. We also found that, when an element exhibits more than two oxidation states, the energy of redox reactions between different pairs of these states are described by slightly different values of Ueff.

We used density functional theory to study CO oxidation catalyzed by TiO 2 (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 O, 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 ∆E v , whereas the energy of adsorption of O 2 at a vacancy is a linear function of ∆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.