Methanol oxidation on stoichiometric and oxygen-rich RuO₂(110)

We used temperature-programmed reaction spectroscopy (TPRS) to investigate the adsorption and oxidation of methanol on stoichiometric and O-rich RuO₂(110) surfaces. We find that the complete oxidation of CH₃OH is strongly preferred on stoichiometric RuO₂(110) during TPRS for initial CH₃OH coverages below ∼0.33 ML (monolayer), and that partial oxidation to mainly CH₂O becomes increasingly favored with increasing CH₃OH coverage from 0.33 to 1.0 ML. We present evidence that an adsorbed CH₂O₂ species serves as the key intermediate to complete oxidation and that CH₂O₂ formation is intrinsically facile but becomes limited by the availability of bridging O-atoms on stoichiometric RuO₂(110) at initial CH₃OH coverages above 0.33 ML. We show that methanol molecules adsorbed in excess of 0.33 ML dehydrogenate to mainly CH₂O and desorb during TPRS, with adsorbed CH₃O groups mediating the evolution of both CH₂O and CH₃OH. We find that O-rich RuO₂(110) surfaces are also highly active toward methanol oxidation and that selectivity toward the complete oxidation of methanol increases markedly with increasing coverage of on-top O-atoms (O_ot) on RuO₂(110). Our results demonstrate that CH₃OH species adsorbed within O_ot-rich domains react efficiently during TPRS, in parallel with reaction of CH₃OH adsorbed initially on cus-Ru sites. The data suggests that the facile hydrogenation of O_ot atoms and the resulting desorption of H₂O at low-temperature (<∼400 K) provides an efficient pathway for restoring reactive O-atoms and thereby promoting complete oxidation of methanol on the O-rich RuO₂(110) surface.