Theoretical study of methanol synthesis from CO_2 hydrogenation on PdCu_3(111) surface

摘要

Carbon dioxide (CO2) hydrogenation to methanol is a promising method for the activation and conversion of CO2. Density functional theory (DFT) calculations were carried out to explore the reaction mechanisms on PdCu3(111) surface. The stable adsorption structures of all possible intermediates were evaluated, showing that the interaction between saturated species and PdCu3(111) surface are much weaker than that of unsaturated species. Three possible pathways of forming formate (HCOO) and hydrocarboxy (COOH) and reverse water gas shift (RWGS) followed by CO hydrogenation have been considered. The H2COO formation is kinetically and energetically prohibited with the highest activation barrier and reaction energy of 1.84 and 1.26 eV, indicating HCOO pathway is highly unlikely to occur. In addition, the byproducts of HCOOH and HCHO can be formed in this route. The rate-limiting steps of HCOH/hydrogenation in COOH route and cis-COOH/dissociation in RWGS + CO-Hydro route need to overcome almost the same barriers of similar to 1.40 eV. However, the byproduct of HCOOH/exists in the latter route. Furthermore, the effect of H2O which is a reaction product of methanol synthesis has been also discussed. It is shown that H2O not only enhances the adsorption of intermediates involved in rate-limiting steps but also reduces the kinetics of rate-limiting steps, especially in COOH pathway. The COOH pathway of CO2 - trans-COOH - t, t-COHOH - t, c-COHOH - c, c-COHOH - COH - HCOH - H2COH - CH3OH is found to be the most favorable. The calculated results provide a potential candidate for methanol synthesis and the present insights are helpful for CO2 conversion and utilization with Pd-Cu bimetallic catalysts. (C) 2018 Elsevier B.V. All rights reserved.