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

Abstract

Abstract 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 °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 (H2-TPR), and CO pulse adsorption measurement (CO-PULSE) analysis methods are also measured.