CO2 Activation and Hydrogenation: A Comparative DFT Study of Ru10/TiO2 and Cu10/TiO2 Model Catalysts

Using DFT+U calculations with inclusion of van-der-Waals (vdW) forces, we studied CO2 activation and the initial steps of CO2 hydrogenation over Cu10 and Ru10 clusters supported on the TiO2 anatase (101) surface. CO2 is readily adsorbed and activated on the Ru cluster where direct CO2 dissociation proceeds with a barrier of 0.8 eV. When H atoms are co-adsorbed on the Ru cluster, H-addition to CO2 becomes preferred, as the best Ru sites for CO2 dissociation are blocked. A H atom is added to the CO2 molecule with formation of a formate [HCOO] species and an activation barrier of 1.2 eV. On Cu10/TiO2, only weak adsorption modes of the CO2 molecule are found, whereas H2 readily adsorbs on the Cu cluster. A reduction of the titania support does not significantly change this picture. Therefore, the only viable pathway for the CO2 hydrogenation over Cu10/TiO2 is the addition of a pre-adsorbed H atom to CO2 coming from the gas phase. This corresponds to an Eley–Rideal mechanism for the H-association to CO2. The work shows the importance to consider the hydrogen coverage on the metal cluster as an important variable in modeling the CO2 hydrogenation reaction. Graphical Abstract: [Figure not available: see fulltext.].