CO2 hydrogenation to methanol on Pd-Cu bimetallic catalysts: H2/CO2 ratio dependence and surface species

Abstract

Our laboratory has observed a strong bimetallic promoting effect on SiO2-supported Pd-Cu catalysts for CH3OH formation from CO2 hydrogenation when the Pd/(Pd + Cu) atomic ratio lied within 0.25–0.34. In the present study, the activities of bimetallic and monometallic catalysts were comparatively evaluated at H2/CO2 = 1, and the origin for the bimetallic promoting effect was sought by a kinetic study and in situ DRIFTS analysis. The bimetallic promoting effect was retained at H2/CO2 = 1, although both the activity and CH3OH selectivity decreased, indicating the significant role of hydrogen partial pressure in the CO2 hydrogenation. Kinetic study provided insights into the strong dependence of CH3OH synthesis on H2/CO2 ratios and roles of Pd-Cu alloys in the observed CH3OH promotion, wherein the alloys could tune the surface sites balance of adsorbed species, and enabled the reduction of activation barrier for CH3OH synthesis. Temperature-programmed reduction results corroborated the strong interaction between Pd and Cu and its impact on the alloy structuring and reducibility. In situ DRIFTS analysis identified formate and carbonyl species were dominant on the surface during the reaction. The surface coverage of formate species was dependent on the catalyst composition, and appeared to correlate to the methanol promotion, implying its key role in the CH3OH synthesis on Pd-Cu catalysts.