Role of Heterolytic H ₂ Splitting in CO ₂ Hydrogenation on Oxides

The concentration and reactivity of H atoms on oxide surfaces govern the rate and selectivity of many hydrogenation reactions, such as CO2 conversion to methanol. On some oxides, H2 molecules may dissociate in a heterolytic fashion and populate the surface with chemically distinct proton H+ and hydride H− species. In this study, we demonstrate through systematic density functional theory (DFT) simulations that H2 dissociates heterolytically on MgO(110), CaO(110), SrO(110), anatase TiO2(101), ZrO2(101), HfO2(001), ZnO (101̅0), α-Al2O3(110), γ-Al2O3(110), and β-Ga2O3(100) surfaces. These oxides do not form two H+ species upon H2 dissociation because their high band gaps are found to hinder the injection of released electrons into the conduction band. In contrast, reduced oxide surfaces such as Ti2O3(012), Ce2O3(011̅1), and FeO(100) preferentially stabilize H− over H+ upon adsorption of a single H atom. The oxygen vacancy formation energy was found to serve as a general descriptor of the preference for H+ + H− over H+ + H+ as the H2 dissociation product across all considered oxides. Moreover, we show that the surface H+ species preferentially react with O during CO2 hydrogenation to form COOH intermediates, whereas surface H− species preferentially attack C atoms, forming HCOO intermediates. Thus, tuning the band gap of oxide surfaces is suggested as a strategy to control the charge of surface H atoms, which is shown to direct the hydrogenation pathway and the selectivity of oxide catalysts.