Reducing CO₂ to CO and H₂O on Ni(110): The Influence of Subsurface Hydrogen

Abstract

Subsurface hydrogen atoms are relatively high-energy species located below metal surfaces that are readily produced when H, H+, or high-pressure H2 interacts with the surface under the right conditions. These species can get involved in catalytic processes associated with hydrogenation reactions, either as spectator species, in which they modify the energetics of reactions occurring on the surface, or directly as a reactant for hydrogenation reactions. In this study, we investigate two mechanisms, associative or redox, in CO2 conversion to CO through the reverse water–gas shift (RWGS) reaction on Ni(110) in the presence of a high density of subsurface hydrogen using spin-polarized periodic density functional theory. The results indicate that subsurface H atoms play an important role for this reaction, both as spectators which lower the energies of barriers and intermediates and as reactants for hydrogenation reactions. The most favorable RWGS pathway involves CO2 dissociation to CO + O followed by hydrogenation to give CO + H2O. Direct associative hydrogenation of CO2 to give COOH and then CO + H2O can also occur, and this is found to be a more important reaction than hydrogenation of CO2 to give HCOO, and then formic acid as the latter product has a high desorption energy that can result in decomposition.