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.