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

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.