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.