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 O-2 - producing N-2 - are metal catalyzed pathways, while the reaction of NCO with H2O - producing NH3 - is not. The contribution of NCO-related pathways to the overall N-2 production mechanism could be significant, since quantification of the FTIR results suggests that up to 30% of the total N-2 produced could be directly related to the reaction of surface isocyanates with O-2. (C) 2011 Elsevier B.V. All rights reserved.