Hydrogen spillover in Ga2O3-Pd/SiO2 catalysts for methanol synthesis from CO2/H2

The hydrogenation of CO2 was investigated on Ga2O3-promoted Pd/SiO2 catalyts and mechanical mixtures of Ga2O3/SiO2 and Pd/SiO2 catalysts (H-2/CO2 = 3; P = 3.0 MPa; T = 523 K). By means of the latter it was possible to demonstrate that atomic hydrogen, H-s, can be generated by Pd-0 far from Ga2O3, and move (spill-over) there to reach the other reactive species (formates) and complete the reaction cycle. The reaction results indicate that (as also evidenced by in situ FTIR) the Ga2O3-Pd/SiO2 catalyst works as a true bi-functional system. The metal-promoter intimacy is not decisive in terms of the catalytic chemistry of the system, but the closeness between the Pd crystallites and the Ga2O3 surface patches boost the activity, owing to a minimized effort in the H-s supply to the latter.