CO2 adsorption and activation over γ-Al2O3-supported transition metal dimers: A density functional study

Catalytic conversion of CO2 to liquid fuels has the benefit of reducing CO2 emission. Adsorption and activation of CO2 on the catalyst surface are key steps of the conversion. Herein, we used density functional theory (DFT) slab calculations to study CO2 adsorption and activation over the γ-Al2O3-supported 3d transition metal dimers (M2/γ-Al2O3, M = Sc–Cu). CO2 was found to adsorb on M2/γ-Al2O3 negatively charged and in a bent configuration, indicating partial activation of CO2. Our results showed that both the metal dimer and the γ-Al2O3 support contribute to the activation of the adsorbed CO2. The presence of a metal dimer enhances the interaction of CO2 with the substrate. Consequently, the adsorption energy of CO2 on M2/γ-Al2O3 is significantly higher than that on the γ-Al2O3 surface without the metal dimer. The decreasing binding strength of CO2 on M2/γ-Al2O3 as M2 changes from Sc2 to Cu2 was attributed to decreasing electron-donation by the supported metal dimers. Hydroxylation of the support surface reduces the amount of charge transferred to CO2 for the same metal dimer and weakens the CO2 chemisorption bonds. Highly dispersed metal particles maintained at a small size are expected to exhibit good activity toward CO2 adsorption and activation.