FTIR in situ mechanistic study of the NH3NO/NO2 “Fast SCR” reaction over a commercial Fe-ZSM-5 catalyst

New insight in the mechanism of the so-called “Fast SCR” reaction over a commercial Fe zeolite catalyst was gained by an in situ FTIR investigation of the evolution of the surface intermediates during step response runs addressing the adsorption of NO2, as well as the reactivity of surface NOx with NH3 and with NO. The collected data, combined with the analysis of companion transient flow reactor experiments demonstrating the corresponding dynamics of gaseous reactants and products, confirm the key role of ferric nitrites/nitrates in the reaction pathway, and emphasize the cooperative interaction between the zeolite acidic sites, where ammonia is stored, and the Fe red-ox sites of the catalyst, which are involved in the critical reduction of surface nitrates by NO.

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► We study by in situ FTIR the individual steps of the Fast SCR mechanism over Fe-ZSM-5.
► Ferric nitrates were the prevailing and stable terminal products of NO2 adsorption.
► Preadsorbed NH3 reacted completely with NO2 to form nitrates and nitrogen.
► Though undetected, ferric nitrites are likely key intermediates in the NO2 reactivity.
► NO reduced preadsorbed ferric nitrates already at very low temperature.