A DFT study on the active surfaces of the ZnGa2O4 spinel catalyst for CO2 hydrogenation to methanol

Abstract

In this work, we constructed the low Miller index surfaces of (100), (001), (111), and (110) of the ZnGa₂O₄ spinel catalyst. Through extensive DFT calculations, we found that the oxygen vacancy (V_O) formation energies on the (100) and (001) surfaces were <1.0 eV, whereas those on the (111) and (110) surfaces were >3.0 eV. We further examined the dissociation of molecular hydrogen (H₂) on these surfaces, and found that H₂ homolytic dissociation tended to occur on surfaces with lower V_O formation energies, while H₂ heterolytic dissociation were favored on those with higher V_O formation energies. CO₂ adsorption and activation on the different surfaces of the ZnGa₂O₄ spinel catalyst were investigated, and on surfaces with lower V_O formation energies, CO₂ adsorbs in a linear configuration (ln-CO₂*), while on surfaces with higher V_O formation energies, CO₂ adopts in a bent adsorption geometry (bt-CO₂*) and is easier to dissociate. For HCOO formation from ln-CO₂* + H*, the (110) surface has the lowest energy barrier, so it is likely the most active surface for CO₂ hydrogenation to methanol (CH₃OH). Thus, we compared the CH₃OH formation pathway on the ZnGa₂O₄(110) surface with the pathways for direct and indirect CO₂ dissociation, and predicted it to favor CH₃OH production. Our calculations reveal the active surfaces of the ZnGa₂O₄ spinel catalyst for CO₂ hydrogenation to CH₃OH, and provide insights into the experimentally observed high CH₃OH selectivity, which should be important for the rational design of Zn-based spinel catalysts for this reaction.