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

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.