Identification of nitrites/HONO as primary products of NO oxidation over Fe-ZSM-5 and their role in the Standard SCR mechanism: A chemical trapping study

We present direct evidence for the formation of nitrites/HONO in the oxidative activation of NO over Fe-zeolites. Exposure of a Fe-ZSM-5 + BaO/Al2O3 physical mixture to NO + O2 at 120 °C resulted in forming nitrites on Fe-sites and trapping them on BaO via gas-phase equilibrium with HONO. Identification of nitrites was confirmed by: (i) their thermal decomposition to an equimolar mix of NO and NO2 during TPD, (ii) N2 formation during their reaction with NH3 at 120 °C, (iii) ex situ IR analysis of the BaO powder after discharge and separation of the mechanical mixture. The reactivity data suggest that nitrites/HONO are intermediates both in the NO oxidation to NO2 and in the Standard SCR reactions.