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

Undercoordinated, bridging O-atoms (O-br) are highly active as H-acceptors in alkane dehydrogenation on IrO2(110) surfaces but transform to HObr groups that are inactive toward hydrocarbons. The low C-H activity and high stability of the HObr groups cause the kinetics and product selectivity during CH4 oxidation on IrO2(110) to depend sensitively on the availability of O-br atoms prior to the onset of product desorption. From temperature programmed reaction spectroscopy (TPRS) and kinetic simulations, we identified two O-br-coverage regimes that distinguish the kinetics and product formation during CH4 oxidation on IrO2(110). Under excess O-br conditions, when the initial O-br coverage is greater than that needed to oxidize all the CH4 to CO2 and HObr groups, complete CH4 oxidation is dominant and produces CO2 in a single TPRS peak between 450 and 500 K. However, under O-br-limited conditions, nearly all the initial O-br atoms are deactivated by conversion to HObr or abstracted after only a fraction of the initially adsorbed CH4 oxidizes to CO2 and CO below 500 K. Thereafter, some of the excess CH x groups abstract H and desorb as CH4 above similar to 500 K while the remainder oxidize to CO2 and CO at a rate that is controlled by the rate at which O-br atoms are regenerated from HObr during the formation of CH4 and H2O products. We also show that chemisorbed O-atoms ('on-top O') on IrO2(110) enhance CO2 production below 500 K by efficiently abstracting H from O-br atoms and thereby increasing the coverage of O-br atoms available to completely oxidize CH x groups at low temperature. Our results provide new insights for understanding factors which govern the kinetics and selectivity during CH4 oxidation on IrO2(110) surfaces.