Thermodynamic and Kinetic Study on Carbon Dioxide Hydrogenation to Methanol over a Ga3Ni5(111) Surface: The Effects of Step Edge

Density functional theory (DFT) was used to study the mechanisms of carbon dioxide (CO2) hydrogenation to methanol (CH3OH) on a stepped Ga3Ni5(111) surface. Surface properties, adsorption energies of reactants, and potential intermediates and products, as well as thermodynamic and kinetic parameters of elementary steps, were calculated. It is found that a stepped Ga3Ni5(111) surface with low surface energy not only can highly activate CO2 but also is beneficial to dissociative H2 adsorption. Moreover, the reactants, intermediates, and products on the Ga3Ni5(111) surface prefer to adsorb to Ni sites at step edges. Accoring to calculated thermodynamic and kinetic parameters of all the elementary steps, CO2 is hydrogenated to CH3OH via trans-COOH, COHOH, COH, HCOH, and CH2OH intermediates because this pathway has the lowest activation barriers and highest rate constants. Meanwhile, water (H2O) formation is the rate-limiting step. On the basis of microkinetic modeling, Ga3Ni5(111) shows higher selectivity to CH3OH than CH4. In all, the stepped Ga3Ni5(111) surface is beneficial in facilitating CO2 hydrogenation to CH3OH, and the presence of steps and the existence of Ga on those steps instead of step edge are required for the high activity of the Ga3Ni5 catalyst.