Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

A series of pure, nanostructured magnesium oxides prepared by different synthesis techniques that show different initial, but similar steady-state activity in the oxidative coupling of methane (OCM) (Schwach et al., submitted for publication) has been studied by infrared and photoluminescence spectroscopy in the dehydroxylated state before the reaction and after catalysis. The abundance of structural defects, in particular mono-atomic steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146 cm(-1) and Lewis acid/base pairs probed by co-adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation. Infrared spectroscopy evidences strong polarization of C-H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high number of mono-atomic steps. It is postulated that these sites effectively promote intermolecular charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissociation of one C-H bond in the methane molecule and simultaneous formation of a superoxide species. Heterolytic splitting of C-H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temperature has been proven by the appearance of an EPR signal associated with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono-atomic steps and the reconstruction of the MgO termination under formation of polar and faceted surfaces. (C) 2015 Elsevier Inc. All rights reserved.