A proposed reaction mechanism for the selective oxidation of methane with nitrous oxide over Co-ZSM-5 catalyst forming synthesis gas (CO + H2)

Abstract

Probable reaction intermediates and mechanistic steps involved for the selective oxidation of methane (CH4) with nitrous oxide (N2O) forming synthesis gas (CO + H2) over Co-ZSM-5 was examined. To assess the reaction kinetics reactions were conducted within a temperature range of 300 °C–550 °C and at atmospheric pressure, and the composition of reactant feed (N2O/CH4) was varied between three fixed ratios (5, 3, and 1). TPD, XRD, TEM and FT-IR techniques were used for catalyst characterization and mechanistic studies. It was found, that H2 selectivity is higher for feeds with lower N2O concentration. Furthermore, CO formation commences at lower temperatures in comparison to H2 gas which is a result of the formation of methanol (CH3OH) and carbon oxides (CO and CO2) at low CH4 conversions.Based on the reaction conditions, the literature proposes direct and an indirect mechanistic route for synthesis gas formation. Our catalytic study suggests a direct route for the synthesis gas formation; i.e. H2 gas is formed by the decomposition of intermediate species (methoxy and CH2O). This is supported by spectroscopic investigation. Under favourable reaction conditions and over the Co-ZSM-5 catalyst, N2O first oxidizes CH4 to CH3OH, an active reaction intermediate, which then subsequently is converted into (formaldehyde) CH2O and H2. CH2O is highly reactive and decomposes into CO and H2.