An in situ IR study of the NO_x adsorption/reduction mechanism on modified Y zeolites

The surface species formed during adsorption of NO, NO + O2 and NO2 on sodium and barium exchanged Y zeolites have been investigated by in situ IR spectroscopy. Ionically bound nitrates and nitrites on the exchanged metal cation sites are the main species formed during adsorption of NO and NO2. Extra framework alumina was identified as additional sorption site forming small concentrations of bridging, chelating and monodentate nitrates. N2O4 and NO+ were found to be reaction intermediates during the NOx adsorption process. The direct oxidation of NO2 with reactive oxygen from the zeolite surface is facilitated by the formation of nitrates via the disproportionation reaction of N2O4 to NO+ and NO3−. NO+ was found to act as precursor for the creation of nitrites. Decomposition of the nitrate species occurs between 150 and 450 °C. During the temperature increase less stable nitrite/nitrate species are transformed into Ba-nitrates showing the highest thermal stability. The stability of surface nitrates/nitrites was found to be lower, if NO instead of NO2 is present in the feed during temperature increase. For the interaction of surface NOx species with propene two pathways are proposed. At low temperatures, NO+ was identified as the active NOx surface species reacting with propene to nitriles. At higher temperatures the reduction of surface nitrates/nitrites occurred via organic nitro/nitrito species, carboxylic species and isocyanates.