CO2 hydrogenation to methanol on Ga2O3-Pd/SiO2 catalysts: Dual oxide-metal sites or (bi)metallic surface sites?

A series of palladium (2 wt.%) catalysts supported on silica (301 m(2)/g) and loaded with increasing amount of gallium - ratio of Ga/Pd = 2, 4 and 8 atom/atom - were investigated for CO2 hydrogenation to methanol. The turnover frequency to methanol (H-2/CO2 = 3; 523 K, 3 MPa), based on surface palladium, showed a 200-fold enhancement as compared to the monometallic Pd/SiO2 catalyst. Additionally, the apparent activation energy for methanol synthesis decreased from 60 kJ/mol on Pd/SiO2 to similar to 40 kJ/mol on the supported Ga-Pd catalysts. Characterization of the Pd-Ga catalyst series by X-ray absorption spectroscopy and high resolution transmission electron microscopy indicates the formation of Pd2Ga bimetallic nanoparticles partially covered by a thin layer of Ga2O3 on the silica surface. In situ infrared spectroscopy was employed to examine the reaction mechanism during the CO2 adsorption and hydrogenation at 0.7 MPa. It is proposed a bifunctional pathway where the carbonaceous species bound to the gallium oxide surface are hydrogenated, stepwise, to formate and methoxy groups by atomic hydrogen, which spillovers from the Pd-Ga bimetallic nanoparticles.