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Abstract:
Methane oxidation using O2 over transition metal oxides often requires severe conditions ( >500 °C) to achieve detectable conversion. In this study, NO was used to transfer oxygen atoms from O2, through the facile gas-phase formation of NO2 at moderate conditions (0.1 MPa and 300–400 °C), to oxidize methane over silica-supported transition metal oxides (VOx, CrOx, MnOx, NbOx, MoOx, and WOx). In situ infrared spectroscopy measurements indicated that the reaction likely proceeded by the formation of surface monodentate nitrate intermediates. These nitrate species were formed by the interaction between adsorbed NO2 and the supported metal oxides. During the reaction, the oxides of vanadium, molybdenum, and tungsten formed formaldehyde and CO2, whereas the oxides of chromium, manganese, and niobium produced only CO2. These results are consistent with the known hydrocarbon oxidation chemistry of the metal oxides. Contact time measurements on VOx/SiO2 indicated that formaldehyde was a primary product and CO2 was the final product; conversely, analogous measurements on MnOx/SiO2 showed that CO2 was the sole product. The formaldehyde production rate on VOx/SiO2, MoOx/SiO2, and WOx/SiO2, based on surface sites measured by high temperature oxygen chemisorption, compared favorably to oxygenate production rates for stronger oxidants (N2O and H2O2) reported in the literature.
