Kinetics and selectivity of methane oxidation on an IrO2(110) film.

Undercoordinated, bridging O-atoms (Obr) are highly active as H-acceptors in alkane dehydrogenation on IrO2(110) surfaces but transform to HObrgroups that are inactive toward hydrocarbons. The low C-H activity and high stability of the HObrgroups cause the kinetics and product selectivity during CH4oxidation on IrO2(110) to depend sensitively on the availability of Obratoms prior to the onset of product desorption. From temperature programmed reaction spectroscopy (TPRS) and kinetic simulations, we identified two Obr-coverage regimes that distinguish the kinetics and product formation during CH4oxidation on IrO2(110). Under excess Obrconditions, when the initial Obrcoverage is greater than that needed to oxidize all the CH4to CO2and HObrgroups, complete CH4oxidation is dominant and produces CO2in a single TPRS peak between 450 and 500 K. However, under Obr-limited conditions, nearly all the initial Obratoms are deactivated by conversion to HObror abstracted after only a fraction of the initially adsorbed CH4oxidizes to CO2and CO below 500 K. Thereafter, some of the excess CHxgroups abstract H and desorb as CH4above ∼500 K while the remainder oxidize to CO2and CO at a rate that is controlled by the rate at which Obratoms are regenerated from HObrduring the formation of CH4and H2O products. We also show that chemisorbed O-atoms ('on-top O') on IrO2(110) enhance CO2production below 500 K by efficiently abstracting H from Obratoms and thereby increasing the coverage of Obratoms available to completely oxidize CHxgroups at low temperature. Our results provide new insights for understanding factors which govern the kinetics and selectivity during CH4oxidation on IrO2(110) surfaces.

Redox-mediated transformation of a Tb2O3(111) thin film from the cubic fluorite to bixbyite structure.

We used temperature programmed desorption (TPD) and low energy electron diffraction (LEED) to investigate the isomeric structural transformation of a Tb2O3 thin film grown on Pt(111). We find that repeated oxidation and thermal reduction to 1000 K transforms an oxygen-deficient, cubic fluorite (CF) Tb2O3(111) thin film to the well-defined bixbyite, or c-Tb2O3(111) structure, whereas annealing the CF-Tb2O3(111) film in UHV is ineffective in causing this structural transformation. We estimate that the final stabilized film consists of about ten layers of c-Tb2O3(111) in the surface region plus about eight layers of CF-Tb2O3(111) located between the c-Tb2O3(111) and the Pt(111) substrate. Our measurements reveal the development of two distinct O2 TPD peaks during the CF to bixbyite transformation that arise from oxidation of c-Tb2O3 domains to the stoichiometrically-invariant ι-Tb7O12 and δ-Tb11O20 phases and demonstrate that the c-Tb2O3 phase oxidizes more facilely than CF-Tb2O3. We present evidence that nucleation and growth of c-Tb2O3 domains occurs at the buried TbOx/CF-Tb2O3 interface, and that conversion of the interfacial CF-Tb2O3 to bixbyite takes place mainly during thermal reduction of TbOx above ∼900 K and causes newly-formed c-Tb2O3 to advance deeper into the film. The avoidance of low Tb oxidation states may facilitate the CF to bixbyite transformation via this redox mechanism.