CO ₂ hydrogenation to formic acid on Ni(111)

Periodic, self-consistent, density functional theory (DFT) calculations are employed to study CO ₂ hydrogenation on Ni(111). CO ₂ hydrogenation with H adsorbed on the surface and with H absorbed in the subsurface is investigated systematically, and the respective microscopic reaction mechanisms are elucidated. We show that on Ni(111) CO ₂ hydrogenation to formate intermediate is more favorable than to carboxyl intermediate. The hydrogenation to formate goes through the unidentate structure that rapidly transforms into the bidentate structure. Further hydrogenation from formate to formic acid is energetically more difficult than formate formation. Formation of adsorbed formic acid from adsorbed CO ₂ and surface hydrogen is an endothermic reaction. Because subsurface H in Ni(111) is substantially less stable compared to surface H, its reaction with adsorbed CO ₂ to adsorbed formic acid is an exothermic one. Our results may have significant implications for the synthesis of liquid fuels from CO ₂ and for catalytic hydrogenation reactions in general.