In situ FTIR investigation of the role of surface isocyanates in the reduction of NOX by CO and C3H6 over model Pt/BaO/Al2O3 and Rh/BaO/Al2O3 NOX storage and reduction (NSR) catalysts

Monometallic Pt- and Rh-containing model NOX storage and reduction (NSR) catalysts were investigated by in situ FTIR spectroscopy in order to determine the type of species present on the surface under simulated lean exhaust conditions and to identify potential reaction intermediates during reduction by CO and propylene. The effect of precious metal selection, temperature, and water presence on the NOX storage and reduction chemistry was considered. The formation of surface isocyanate (NCO) species was observed during all cycling experiments, even in the presence of H2O, which prompted additional investigation on the role of these species in the NOX reduction mechanism. Stability and reactivity experiments with BaO/Al2O3, Pt/BaO/Al2O3 and Rh/BaO/Al2O3 confirmed that the reactions of NCO with NO and O2 – producing N2 – are metal catalyzed pathways, while the reaction of NCO with H2O – producing NH3 – is not. The contribution of NCO-related pathways to the overall N2 production mechanism could be significant, since quantification of the FTIR results suggests that up to 30% of the total N2 produced could be directly related to the reaction of surface isocyanates with O2.

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► Formation of surface isocyanate (NCO) species was observed during reduction of stored NOX with CO.
► Surface NCO species showed similar reactivity towards NO, O2 and H2O.
► Reaction of surface NCO with NO and O2 is a metal catalyzed pathway.
► A significant amount of N2 can be produced through an NCO pathway.