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 H-2-O-2 titration and N2O titration. The surface chemical properties of Pd-Cu combinations were characterized by H-2-/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 H-2 and CO2 on Pd-Cu bimetallic surface which appear to correlate to the CH3OH promotion. DFT calculations on adsorption properties of H-2 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. (C) 2018 Published by Elsevier Inc.