Facet effect on CO₂ adsorption, dissociation and hydrogenation over Fe catalysts: Insight from DFT

Periodic density functional theory (DFT) calculations were performed to investigate the facet effect on CO₂ adsorption, dissociation and hydrogenation over Fe catalysts. The energetically most stable configurations of CO₂ and H₂ adsorption over different Fe facets were identified from which we observed that CO₂ adsorption on Fe(211) and Fe(111) is much stronger than other facets, indicating more sufficient activation of CO₂ on these two surfaces. CO₂ adsorption stability was found to be impacted by the surface coverage of H∗on these Fe facets, showing that when surface H∗coverage exceeds to certain percentage, CO₂ adsorption is largely weakened whilst the electrons transfer from the Fe surface to CO₂ becomes decreased. These results suggest that an appropriate H₂-CO₂ co-adsorption equilibrium is important for effective activation of reactants. Based on the examination of CO₂ dissociation and hydrogenation on these Fe facets, the Fe(111) is potentially the most active facet for CO₂ conversion due to a lower barrier for HCOO∗formation via CO₂ hydrogenation while this facet is also catalytically more active for activating CO₂. Fe(110) and Fe(100) exhibit more facile ability to dissociate CO₂ to CO∗while kinetically competitive formation of CO∗and HCOO∗was observed over Fe(211). The present work demonstrates that the facet of Fe catalysts can impact the molecular adsorption, activation and conversion path in CO₂ hydrogenation and thus can alter the product selectivity.