Unveiling the role of adsorbed hydrogen in tuning the catalytic activity of CO2 conversion to methanol at Cu/TiC surfaces

First-principles calculations were performed to explore the detailed reaction mechanisms of CO2 conversion to methanol over two size-selected copper clusters supported on the TiC(001) surface, in which three potential routes including the formate, the reverse water-gas-shift (RWGS) + CO-hydrogenation, and the C-O bond cleavage pathways were considered. Our findings show that the adsorption and migration of hydrogen atoms have obvious impact on the catalytic activity for CO2 conversion. The limited size of the active site of small Cu cluster with a planar configuration results in that the formate route is difficult to occur because the creation of H2COOH* intermediate requires the spillover of H atoms from the substrate to the active center by overcoming a high kinetic barrier. On the contrary, the polyhedron structure in the large Cu cluster can act as a reservoir for the hydrogen adsorption, making it possible to produce methanol via the formate pathway. Although the RWGS + CO-hydrogenation pathway is identified as the preferred reaction pathway on both surfaces, the relatively strong binding of hydrogen on the large copper cluster causes difficulty in the migration of H toward the reaction intermediate. The results of microkinetic simulations indicate that the rate-limiting steps are sensitive to cluster size, and small Cu cluster exhibits better catalytic activity for the conversion of CO2 to CH3OH.