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 SiO 2 -supported Pd-Cu catalysts for CH 3 OH formation from CO 2 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 H 2 /CO 2  = 1, and the origin for the bimetallic promoting effect was sought by a kinetic study and in situ FT-IR analysis. The bimetallic promoting effect was retained in at H 2 /CO 2  = 1, although both the activity and CH 3 OH selectivity decreased, indicating the significant role of hydrogen partial pressure in the CO 2 hydrogenation. Kinetic study provided insights into the strong dependence of CH 3 OH synthesis on H 2 /CO 2 ratios and roles of Pd-Cu alloys in the observed CH 3 OH promotion, wherein the alloys could tune the surface sites balance of adsorbed species, and enabled the reduction of activation barrier for CH 3 OH 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 compostion, and appeared to correlate to the methanol promotion, implying its key role in the CH 3 OH synthesis on Pd-Cu catalysts.