Origin of Pd-Cu bimetallic effect for synergetic promotion of methanol formation from CO2 hydrogenation

A strong synergetic effect was observed in our previous work on Pd-Cu bimetallic 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 importance of Pd-Cu alloy in selective CH3OH promotion was evidenced and correlated with alloy contents quantitatively through X-ray diffraction (XRD), scanning transmission electron spectroscopy with energy-dispersive X-ray spectroscopy (STEM/EDS), and H2-O2 titration and N2O titration. The surface chemical properties of Pd-Cu combinations were characterized by H2-/CO2-temperature-programmed desorption (TPD), diffuse reflectance infrared FT spectroscopy (DRIFTS), and density functional theory (DFT), and experimentally evaluated along with monometallic counterparts. Detailed characterization results reveal a unique shift in adsorption towards weakly-bonded H2 and CO2 on Pd-Cu bimetallic surface which appear to correlate to the CH3OH promotion. DFT calculations on adsorption properties of H2 and CO2 show good agreement with the observation from TPD experiments. DFT study also provides insights into the impact of Pd-Cu combination on the activation and initial hydrogenation of CO2 to formate (HCOO∗) and hydrocarboxyl (COOH∗) intermediates. HCOO∗ formation was found to be kinetically more favored than COOH∗ on monometallic Cu and Pd-Cu surfaces. The lowest barrier for HCOO∗ formation was observed at Pd/(Pd + Cu) atomic ratio of 0.33, around which a good CO2 conversion and high methanol selectivity were achieved experimentally.