An in situIR study of the NOxadsorptionreduction mechanism on modified Y zeolites

The surface species formed during adsorption of NO, NO  O2and NO2on sodium and barium exchanged Y zeolites have been investigated by in situIR 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. N2O4and NOwere found to be reaction intermediates during the NOxadsorption process. The direct oxidation of NO2with reactive oxygen from the zeolite surface is facilitated by the formation of nitrates viathe disproportionation reaction of N2O4to NOand NO3−. NOwas 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 nitritenitrate species are transformed into Ba-nitrates showing the highest thermal stability. The stability of surface nitratesnitrites was found to be lower, if NO instead of NO2is present in the feed during temperature increase. For the interaction of surface NOxspecies with propene two pathways are proposed. At low temperatures, NOwas identified as the active NOxsurface species reacting with propene to nitriles. At higher temperatures the reduction of surface nitratesnitrites occurred viaorganic nitronitrito species, carboxylic species and isocyanates.