Comparative computational study of CO2 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 CO2 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 CO2 and atomic H-star adsorptions were systematically examined. The H-star 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 CO2 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, CO2 adsorption configurations were found to be sensitive to surface Pd content, resulting in large distinctions in adsorption stabilities of CO2 as compared to Fe-Co and Fe-Ni surfaces. CO2 dissociation and initial hydrogenation were comparatively investigated on Fe-M bimetallic surfaces, and the calculation results demonstrated that CO2 conversion properties are similar over Fe-Ni and Fe-Co catalysts, with CO star and HCOO star as the preferred intermediates but the barriers are still above 0.8 eV. While on Fe-Pd bimetallic surfaces, CO2 reactions exhibit significant distinctions with varying the surface Pd content, showing a dramatic preference (E-act around 0.3 similar to 0.4 eV) towards HCOO star and CO star 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 CO2 and facilitate the stabilization of transition states in CO star and HCOO star formation pathways in CO2 conversion.