Bimetallic CuPd Nanoparticles Supported on ZnO or Graphene for CO2 and CO Conversion to Methane and Methanol

Abstract

Carbon dioxide (CO2) and carbon monoxide (CO) hydrogenation to methane (CH4) or methanol (CH3OH) is a promising pathway to reduce CO2 emissions and to migitate dependence on rapidly depleting fossil fuels. Along these lines, a series of catalysts comprising copper (Cu) or palladium (Pd) nanoparticles (NPs) supported on zinc oxide (ZnO), as well as bimetallic CuPd NPs supported on ZnO or graphene were synthesized via various methodologies. The prepared catalysts underwent comprehensive characterization via high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) mapping, electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), hydrogen temperature-programmed reduction and desorption (H2-TPR, -TPD), and deuterium temperature-programmed desorption (D2O-TPD). In the CO2 hydrogenation process carried out at 20 bar and elevated temperatures (300 to 500°C), Cu, Pd, and CuPd NPs (<5wt.% loading) supported on ZnO or graphene predominantly yielded CH4 as primary product, with CO generated as a byproduct via the reverse water gas shift (RWGS) reaction. For CO hydrogenation between 400 and 500°C , the CO conversion was at least 40% higher than that of CO2 conversion, with CH4 and CO2 identified as main products, the latter from water gas shift. Employing 90wt.% Cu on ZnO led to an enhanced CO conversion of 14%, with the CH3OH yield reaching 10% and the CO2 yield reaching 4.3% at 230°C. Overall, the results demonstrate that lower Cu/Pd loading (<5wt.%) supported on ZnO/graphene favored CH4 production, while higher Cu content (90wt.%) promoted CH3OH production, both for CO2 and CO hydrogenation at high pressure.