Highly selective synthesis of methanol from methane over carbon materials supported Pd-Au nanoparticles under mild conditions

As one of the Holy Grail reactions in C1 chemistry, direct selective oxidation of methane to methanol under mild and non-harsh conditions remains a big challenge. Hydroperoxide (H2O2), as a primary oxidant, applied widely in the low-temperature direct selective conversion of methane to methanol. Moreover, the hydrogen and oxygen mixture gas achieves better reaction activity and higher methanol selectivity than H2O2 when using palladium-gold (Pd-Au) bimetallic nanoparticles as the catalyst. In this paper, we studied the key roles of the physical and chemical characteristics of Pd-Au nanoparticles in this direct selective oxidation reaction with hydrogen and oxygen as the oxidant at 50 degrees C. Au metal shows an indispensable role in direct methane to methanol process in this study. High methanol productivity and selectivity are achieved when Au of 0.5-2.5% is added into the carbon nanotubes (CNTs) supported Pd catalyst. Moreover, the loading amount of Pd-Au nanoparticles also affect the methane activation ability and the methanol selectivity obviously. Much more active and stable Pd-Au nanoparticles are generated in CNTs supported 2.5% Pd-2.5% Au nanoparticles catalyst than those in lower metal content catalysts. In addition, the Pd bivalent state is proved to be the most active content in the Pd-Au nanoparticles catalyst, achieving much higher methanol selectivity and productivity when compared to Pd metallic state. For further clarifying the physical and chemical characteristics of catalysts, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Hydrogen-Temperature-programmed reduction (H-2-TPR), and CO pulse adsorption measurement (CO-PULSE) analysis methods are also measured.