Dynamic Equilibrium of Reversible Reactions and Migration of Hydrogen Atoms Mediated by Diffusive Methanol on Rutile TiO2 (110)-(1 × 1) Surface

We report our investigation of the reversible reaction of methanol and the migration of hydrogen adatom (H ad ) on TiO 2 (110)-(1 × 1) surface at various temperatures and methanol coverages using scanning tunneling microscopy joint with density functional theory (DFT) calculations. At a relatively low coverage measured at room temperature, the methanol species adsorbed at the oxygen vacancy (O V ) sites are immobile and appear only as a dissociative form, and the observed H ad migration events are very few. However, when the O V sites are fully filled by methanol in the methanol-overdosed sample, the methanol species at the O V sites keep immobile but frequently switch between molecular and dissociative forms, accompanied by dramatically enhanced H ad migration. Meanwhile, an established equilibrium shows a concentration ratio of 1:3 between the molecular and dissociative methanol. At 235 K, we directly visualized and confirmed that the reversible reactions of methanol and the enhanced H ad migration are mediated by the diffusive methanol adsorbed at the 5-fold coordinated Ti sites. Our DFT calculations well elucidate the experimental results using the modeled configurations by considering the exchange processes of H atoms, reaching a clear atomistic picture for the dynamic equilibrium of the reversible reactions and the H ad migration.