Near-Surface Hydrogen Species Tune the Selectivity of Chemical Reactions on Metal Oxide Surfaces

Upon interaction with H₂, metal oxides can be reduced. Such a reduction may consequently alter their interactions as well as the evolution of hydrogen species on metal oxide surfaces. However, this has not been thoroughly and precisely studied on the atomic scale. Accordingly, systematic density functional theory (DFT) calculations were performed on a representative metal-oxide catalyst surface, namely β-Ga₂O₃(100). It was found that oxygen vacancy clusters can readily form upon reduction and that such vacancy clusters enable the infiltration of surface hydrogen species into the near-surface region. The calculated relative population of near-surface hydrogen species reaches approximately 5% under typical experimental conditions for hydrogenation and dehydrogenation reactions on Ga₂O₃ surfaces, and thus, these species cannot be ignored. To study the effect of near-surface hydrogen species on the surface chemistry of metal oxides, two important reactions were considered. The first is H₂ dissociation, which is an important process in catalytic hydrogenation (and dehydrogenation) reactions. The second is CO₂ hydrogenation, which is a representative hydrogenation reaction. It was found that the presence of near-surface hydrogen species results in modulation of the surface electronic and chemical properties of β-Ga₂O₃(100), leading to a change in the preferred pathway for these surface chemical reactions. These findings emphasize the crucial role of near-surface hydrogen species in tuning the selectivity of chemical reactions on metal oxide surfaces. This perspective has not been identified thus far, to the best of our knowledge, but is consistent with previously reported experimental results in many aspects.