Mechanism of NO-SCR by methane over Co,H-ZSM-5 and Co,H-mordenite catalysts

•The NO-SCR by methane over Co,H-zeolites proceeds via bifunctional mechanism.•Cobalt-oxide clusters promote the NO oxidation to NO2 intermediate (NO-COX).•Co2+/[Co-OH]+ sites are responsible for the N2-forming step of the NO-SCR reaction.•Accelerated NO-COX speeds up surface NO3− formation, methane activation and NO-SCR.•Below ∼700 K the NO-SCR rate must be controlled by the desorption rate of product water.

Results of X-ray photoelectron spectroscopic (XPS) examination and temperature-programmed reduction measurements by H2 (H2-TPR) showed that the Co-zeolite catalysts, which were found most active in the selective catalytic reduction of NO by methane to N2 in the presence of excess O2 (NO-SCR), contain both Co2+/[Co-OH]+/H+ exchange cations, Co-oxo species and cobalt oxide clusters. Using operando Diffuse Reflectance Infrared Fourier Transform Spectroscopic method (DRIFTS method) the NO-SCR reaction was shown to proceed in consecutive steps via bifunctional mechanism over active sites (i) promoting the oxidation of NO by O2 to NO2 (NO-COX reaction), and sites (ii) whereon disproportionation and charge separation of 2NO2 generates activated surface intermediate NO3−/NO+ ion pair. Latter process was found to require Co2+ zeolite cations. The NO-COX reaction was shown to proceed over Co-oxo species and cobalt oxide, if present, and also over Brønsted acid sites but at a significantly lower rate. In the reaction of methane and the NO3−/NO+ ion pair CO2, H2O, and N2 was formed and the active Co2+ sites were recovered (CH4/NO-SCR reaction). The surface concentration of the NO3−/NO+ ion pair must have been controlled by the relative magnitude of the apparent rate constants of the consecutive NO-COX and CH4/NO-SCR reactions. Below about 700 K reaction temperature latter reaction governed the rate of the consecutive NO reduction process. Above about 700 K combustion became the main reaction of methane. Because of the low equilibrium NO2 concentration at these high temperatures the NO-COX reaction took over the control over the rate of the NO-SCR process. Under steady state reaction conditions a temperature-dependent fraction of the Co2+ active sites was always poisoned by adsorbed H2O formed in the CH4 oxidation reaction.

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