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

Abstract

Undercoordinated, bridging O-atoms (O_br) are highly active as H-acceptors in alkane dehydrogenation on IrO₂(110) surfaces but transform to HO_brgroups that are inactive toward hydrocarbons. The low C-H activity and high stability of the HO_brgroups cause the kinetics and product selectivity during CH₄oxidation on IrO₂(110) to depend sensitively on the availability of O_bratoms 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 CH₄oxidation on IrO₂(110). Under excess O_brconditions, when the initial O_brcoverage is greater than that needed to oxidize all the CH₄to CO₂and HO_brgroups, complete CH₄oxidation is dominant and produces CO₂in a single TPRS peak between 450 and 500 K. However, under O_br-limited conditions, nearly all the initial O_bratoms are deactivated by conversion to HO_bror abstracted after only a fraction of the initially adsorbed CH₄oxidizes to CO₂and CO below 500 K. Thereafter, some of the excess CH_xgroups 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 O_bratoms are regenerated from HO_brduring 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 O_bratoms and thereby increasing the coverage of O_bratoms available to completely oxidize CH_xgroups at low temperature. Our results provide new insights for understanding factors which govern the kinetics and selectivity during CH₄oxidation on IrO₂(110) surfaces.

Keywords: IrO2; TPRS; iridium; kinetic simulations; methane oxidation.