Methane selective oxidation on metal oxide catalysts at low temperatures with O2 using an NO/NO2 oxygen atom shuttle

Methane oxidation using O-2 over transition metal oxides often requires severe conditions (>500 degrees C) to achieve detectable conversion. In this study, NO was used to transfer oxygen atoms from O-2, through the facile gas-phase formation of NO2 at moderate conditions (0.1 MPa and 300-400 degrees 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. (c) 2021 Elsevier Inc. All rights reserved.

Automated summary

This study achieves methane oxidation on transition metal oxides at low temperatures (0.1 MPa and 300-400°C) using NO as a mediator. In situ infrared spectroscopy measurements reveal that the reaction mainly occurs on surface-formed nitrate intermediates. Different metal oxides result in different oxidation products of methane, with vanadium, molybdenum, and tungsten oxides forming formaldehyde and CO2, while chromium, manganese, and niobium oxides only producing CO2. The formaldehyde production rate on VOx/SiO2, MoOx/SiO2, and WOx/SiO2 is comparable to oxygenate production rates of known oxidants N2O and H2O2.