The chemistry of the NO/NO2–NH3 “fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis

We present a systematic study of the chemical steps in the NO/NO2–NH3 fast SCR reaction 2NH3 + NO + NO2 → 2N2 + 3H2O over a commercial Fe-ZSM5 catalyst. The study is based on transient reaction experiments at realistic conditions for removal of NOx from mobile diesel exhausts. Its goal is to assess and critically evaluate the current ideas on the SCR mechanism, and also to establish to what extent the mechanistic pathways demonstrated for V-based catalysts also apply to Fe-promoted zeolites. Results show that the fast SCR reaction proceeds at low temperature via a global sequence involving NH4NO3 or related surface species as intermediates, 2NO2 + 2NH3 → N2 + NH4NO3 + H2O, NO + NH4NO3 → NO2 + N2 + 2H2O. Such a sequential scheme is the same as that proposed previously for the fast SCR chemistry over V-based catalysts and other zeolite catalysts and thus is considered a general mechanism. It explains all of the available observations for stoichiometry (e.g., optimum NO/NO2 unit molar ratio), selectivity (e.g., N2O from NH4NO3 decomposition), and kinetics (e.g., rate of fast SCR = rate of nitrate reduction by NO). We further show that the redox reaction between NO and nitrates is the rate-controlling step and is inhibited by ammonia. Remarkably, the same strongly enhanced deNOx activity observed in the fast SCR reaction also was observed in the absence of gaseous NO2 but in the presence of surface nitrates. We accordingly propose a general summary of the fast SCR chemistry over V-based and zeolite catalysts that emphasizes the key role of surface nitrates.