Comparative computational study of CO ₂ dissociation and hydrogenation over Fe-M (M = Pd, Ni, Co) bimetallic catalysts: The effect of surface metal content

Density functional theory (DFT) calculations were performed to study CO ₂ adsorption, dissociation and hydrogenation over Fe-M (M = Pd, Ni, Co) bimetallic catalysts, with a focus on probing the effect of surface content of the added transition metals to Fe. Various Fe-M bimetallic surfaces were constructed with varied surface atomic ratios of Fe/(Fe + M), based on which CO ₂ and atomic H∗adsorptions were systematically examined. The H∗was found to be energetically favorable adsorbed at the 4-fold hollow site of Fe-M catalysts and the adsorption stability was slightly impacted by the surface content of the introduced transition metal. For CO ₂ adsorption, stable bent structures adsorbed on the 4-fold hollow sites were identified on Fe-Ni and Fe-Co surfaces, no matter at which Fe-M formulations. However, on Fe-Pd surfaces, CO ₂ adsorption configurations were found to be sensitive to surface Pd content, resulting in large distinctions in adsorption stabilities of CO ₂ as compared to Fe-Co and Fe-Ni surfaces. CO ₂ dissociation and initial hydrogenation were comparatively investigated on Fe-M bimetallic surfaces, and the calculation results demonstrated that CO ₂ conversion properties are similar over Fe-Ni and Fe-Co catalysts, with CO∗and HCOO∗as the preferred intermediates but the barriers are still above 0.8 eV. While on Fe-Pd bimetallic surfaces, CO ₂ reactions exhibit significant distinctions with varying the surface Pd content, showing a dramatic preference (E _act around 0.3∼0.4 eV) towards HCOO∗and CO∗formation at surface Pd/(Pd + Fe) atomic ratios of 4/9 and 5/9. The superior catalytic activities of Fe-Pd catalysts are attributed to the particular surface structures and electronic features at specific bimetallic formulations which result in unique adsorption configurations of CO ₂ and facilitate the stabilization of transition states in CO∗and HCOO∗formation pathways in CO ₂ conversion.