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

Undercoordinated, bridging O-atoms (O) are highly active as H-acceptors in alkane dehydrogenation on IrO(110) surfaces but transform to HOgroups that are inactive toward hydrocarbons. The low C-H activity and high stability of the HOgroups cause the kinetics and product selectivity during CHoxidation on IrO(110) to depend sensitively on the availability of Oatoms prior to the onset of product desorption. From temperature programmed reaction spectroscopy (TPRS) and kinetic simulations, we identified two O-coverage regimes that distinguish the kinetics and product formation during CHoxidation on IrO(110). Under excess Oconditions, when the initial Ocoverage is greater than that needed to oxidize all the CHto COand HOgroups, complete CHoxidation is dominant and produces COin a single TPRS peak between 450 and 500 K. However, under O-limited conditions, nearly all the initial Oatoms are deactivated by conversion to HOor abstracted after only a fraction of the initially adsorbed CHoxidizes to COand CO below 500 K. Thereafter, some of the excess CHgroups abstract H and desorb as CHabove ∼500 K while the remainder oxidize to COand CO at a rate that is controlled by the rate at which Oatoms are regenerated from HOduring the formation of CHand HO products. We also show that chemisorbed O-atoms ('on-top O') on IrO(110) enhance COproduction below 500 K by efficiently abstracting H from Oatoms and thereby increasing the coverage of Oatoms available to completely oxidize CHgroups at low temperature. Our results provide new insights for understanding factors which govern the kinetics and selectivity during CHoxidation on IrO(110) surfaces.