Catalytic hydrogenation of CO₂ over Pt- and Ni-doped graphene: A comparative DFT study

Today, the global greenhouse effect of carbon dioxide (CO₂) is a serious environmental problem. Therefore, developing efficient methods for CO₂ capturing and conversion to valuable chemicals is a great challenge. The aim of the present study is to investigate the catalytic activity of Pt- or Ni-doped graphene for the hydrogenation of CO₂ by a hydrogen molecule. To gain a deeper insight into the catalytic mechanism of this reaction, the reliable density functional theory calculations are performed. The adsorption energies, geometric parameters, reaction barriers, and thermodynamic properties are calculated using the M06-2X density functional. Two reaction mechanisms are proposed for the hydrogenation of CO₂. In the bimolecular mechanism, the reaction proceeds in two steps, initiating by the co-adsorption of CO₂ and H₂ molecules over the surface, followed by the formation of a OCOH intermediate by the transfer of H atom of H₂ toward O atom of CO₂. In the next step, formic acid is produced as a favorable product with the formation of CH bond. In our proposed termolecular mechanism, however, H₂ molecule is directly activated by the two pre-adsorbed CO₂ molecules. The predicted activation energy for the formation of the OCOH intermediate in the bimolecular mechanism is 20.8 and 47.9kcalmol^-1 over Pt- and Ni-doped graphene, respectively. On the contrary, the formation of the first formic acid in the termolecular mechanism is found as the rate-determining step over these surfaces, with an activation energy of 28.8 and 45.5kcal/mol. Our findings demonstrate that compared to the Ni-doped graphene, the Pt-doped surface has a relatively higher catalytic activity towards the CO₂ reduction. These theoretical results could be useful in practical applications for removal and transformation of CO₂ to value-added chemical products.