Adsorption and Oxidation of CH4 on Oxygen-Rich IrO2(110)

We investigated the oxidation of CH4 on oxygen-pre-covered IrO2(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT). Our TPRS results show that on-top oxygen (O-ot) species hinder CH4 adsorption, providing evidence that CH4 adsorbs on coordinatively unsaturated Ir atoms. We also find that the fractional yield of adsorbed CH4 that reacts during TPRS remains constant at similar to 70% as the O-ot-coverage increases to about 0.5 monolayer for saturation CH4 coverage, demonstrating that O-rich IrO2(110) surfaces are highly active in promoting CH4 C-H bond cleavage. Our results show that O-ot atoms promote CH4 oxidation to CO2 as well as H2O formation while suppressing CO and recombinative CH4 desorption, as evidenced by an increase in the fractional yield of CO2 produced during TPRS and a downshift of CO2 and H2O TPRS peak maxima with increasing O-ot-coverage. DFT predicts that initial CH4 bond cleavage is highly facile on both stoichiometric and O-rich IrO2(110) and can occur by either H-transfer to an O-ot or a bridging O-atom of the surface. Our calculations also predict that oxidation of the CHx species that result from CH4 activation is more facile on O-rich compared with stoichiometric IrO2(110), and that complete oxidation is strongly favored on the O-rich surface, in good agreement with our experimental findings. According to the calculations, key steps in the CH4 oxidation pathway have significantly lower-energy barriers on O-rich vs stoichiometric IrO2(110) because these steps involve reaction with O-ot atoms initially present on the surface rather than the abstraction of more strongly bound O-br species. High coverages of O-atoms also enable adsorbed intermediates to oxidize extensively on O-rich IrO2(110), without the intermediates needing to overcome diffusion barriers to access reactive O-atoms. Our results provide insights for understanding CH4 oxidation on IrO2(110) surfaces under reaction conditions at which O-ot atoms and adsorbed CH4 can co-exist.