Mechanism of Methanol Synthesis from CO2 Hydrogenation over Cu/γ-Al2O3 Interface: Influences of Surface Hydroxylation

Abstract

The adsorption and hydrogenation of carbon dioxide on γ-Al2O3(110) surface-supported copper clusters of different sizes are investigated using density functional theory calculations. Our results show that the activation of CO2 is most obvious at the Cu/γ-Al2O3 interface containing the size-selected Cu4 cluster. It is interesting that the CO2 activation is more pronounced at the partially hydroxyl-covered interface. The catalytic mechanisms of CO2 conversion to methanol at the dry and hydroxylated Cu4/γ-Al2O3 interfaces via the formate route and the pathway initiated through the hydrogenation of carbon monoxide produced by the reverse water–gas shift reaction are further explored. On both interfaces, the formate pathway is identified as the preferred reaction pathway, in which the hydrogenation of HCOO to H2COO is the rate-limiting step (RLS). However, since the surface OH group can act as a hydrogen source in some elementary reactions, unlike the dry surface, the production of H2COOH species along the formate pathway is found at the hydroxylated interface. In addition, the introduction of OH at the interface leads to an increase in the kinetic barrier of the RLS, indicating that surface hydroxylation has a negative effect on the catalytic activity of CO2 conversion to CH3OH at the Cu/γ-Al2O3 interface.