Effects of N2O and Water on Activity and Selectivity in the Oxidative Coupling of Methane over Mn–Na2WO4/SiO2: Role of Oxygen Species

The understanding of the reaction mechanism of product formation in the oxidative coupling of methane (OCM) is the prerequisite for designing catalysts with improved selectivity to the desired products, i.e., C2H6 and C2H4 (C2 hydrocarbons). One step in this direction is to understand the kind of oxygen species involved in the selective and nonselective reactions. Against this background, a series of kinetic and mechanistic tests of the OCM reaction with N2O (N2O–OCM) were carried out over the Mn–Na2WO4/SiO2 system in the absence and the presence of cofed water. The usage of N2O instead of O2 was proven to suppress the direct oxidation of CH4 to CO2 in favor of the formation of C2 hydrocarbons. This inhibition was explained by a lower concentration of surface biatomic oxygen species participating in the formation of CO2. The formation of such species from O2 was supported by oxygen isotopic exchange and electron paramagnetic resonance (EPR) tests. As proven by in situ UV–vis tests under various reaction conditions at 750 °C, N2O–OCM and O2–OCM mainly proceed through a Mars van Krevelen sequence. As MnOx is reduced faster than Na2WO4, it should be primarily involved in oxidant activation. In comparison with O2, N2O was proven to reoxidize reduced catalysts significantly slower. This results in reducing surface density (spatial separation) of lattice oxygen species in N2O–OCM that is unfavorable for the undesired oxidation reactions leading to carbon oxides. As a result, the selectivity to C2 hydrocarbons increases. This knowledge is essential for catalyst design through controlling the rates of generation and consumption of oxygen species. A further aspect of this work is the positive effect of H2O on the rate of methane conversion and the selectivity to C2 hydrocarbons in N2O–OCM. The strength of this effect on the activity is lower than in O2–OCM. The enhancing effect was related to H2O-mediated transformation of surface biatomic oxygen species into monatomic ones.