A new and different insight into the promotion mechanisms of Ga for the hydrogenation of carbon dioxide to methanol over a Ga-doped Ni(211) bimetallic catalyst
The hydrogenation of CO2 to CH3OH is one of the most promising technologies for the utilization of captured CO2 in the future. Nano Ni–Ga bimetallic catalysts have been proven to be excellent catalysts in the hydrogenation of CO2 to CH3OH. To investigate the promotion mechanisms of Ga for the hydrogenation of CO2 to CH3OH over Ga-doped Ni catalysts and the wide application of these promotion mechanisms in other catalysts and reactions, herein, density functional theory (DFT) was employed. The reaction mechanisms and the properties of Ni(211) and Ga–Ni(211) surfaces were comparatively studied. The results show that the Ni sites on both the Ni(211) and the Ga–Ni(211) surfaces are active sites, and the most stable structures of the intermediates are similar. Moreover, the Ga–Ni(211) surface is more favorable for the hydrogenation of CO2, whereas Ni(211) is more favorable for the dissociation of CO2. The activation barrier of the rate-limiting step in the CH3OH formation pathway on Ni(211) is 0.54 eV higher than that on Ga–Ni(211). According to the analyses of the projected density of states (PDOS) and Hirshfeld charge transfer, the addition of Ga atoms demonstrates the reactivity of the Ga-doped Ni(211) surfaces. Most importantly, the replacement of some secondary active sites of Ni atoms with the non-active Ga atoms may lower the activities of the secondary active sites and strengthen the activities of the active sites at the step edge. These results provide a new perspective for the reaction mechanism of the hydrogenation of CO2 to CH3OH over the state-of-the-art Ga-doped catalysts.