(356a) O2-Coverage Dependent O2 Adsorption And Co Oxidation On Reduced TiO2(110): A First Principles Study

The interaction of TiO2 with molecular oxygen is an important factor in determining many fundamental reactions that take place on TiO2-based materials. While adsorption of O2 molecules is known to be mainly mediated by oxygen vacancies, the atomic structure and reactivity of adsorbed O2 molecules on the reduced surface are still uncertain, particularly in the saturation coverage. Results from recent temperature-programmed desorption (TPD) measurements suggested that: the full O2 coverage on TiO2(110) at low temperature ( 120 K) would approximately be three times the surface vacancy population; and adsorbed O2 molecules may exist in either weakly or strongly bound states associated with TPD features below 200 K and above 400 K, respectively. In addition, earlier experiments identified the existence of two chemisorptions states for molecular oxygen on TiO2(110); that is, one can photo-oxidize coadsorbed CO and the other only undergoes fast photodesorption. Most of recent theoretical studies using density functional theory (DFT) have focused on understanding the nature of O2 interaction with the reduced surface, by examining the adsorption and dissociation of a single oxygen molecule per vacancy or more. In fact, based on unrestricted Hartree Fock calculations a structural model was proposed for adsorption of three O2 molecules per vacancy, in which one O2 molecule is located at the vacancy site while the others are at the sites atop adjacent Ti fivefold-coordinated atoms, all in a perpendicular fashion. In this talk, we present a new adsorption model for molecular oxygen on reduced TiO2(110), based on extensive first principles density functional calculations of the structure, bonding, and energetics of adsorbed oxygen species by changing the number of adsorbed O2 molecules per vacancy. For the first time, our calculations predict formation of tetraoxygen (O4) anchored at the vacancy site, which in turn allows adsorption of three O2 molecules per vacancy in saturation coverage. The O4 complex turns out to be substantially more stable than two separately adsorbed O2 molecules. We have also determined that thermally-activated O2 desorption would take place via two channels that require overcoming barriers of 0.41 eV and 1.25 eV, respectively. In addition, Our study clearly demonstrates that there is a strong O2 coverage dependence of the activation energy for CO oxidation on TiO2(110). Our findings associated with tetraoxygen complexes are consistent with existing experimental results. The detailed understanding will greatly assist in understanding and predicting a variety of chemical and photochemical processes occurring on TiO2 surfaces.