Rh-exchanged Al-MCM-41 is studied for selective catalytic reduction (SCR) of NO by C3H6 in the presence of excess oxygen. It shows a high activity in converting NO to N-2 and N2O at low temperatures. In situ FT-IR studies indicate that Rh-NO+ species (1910-1898 cm(-1)) is formed on the Rh-A1-MCM-41 catalyst in flowing NO/He, NO + O-2/He and NO + C3H6 + O-2/He at 100-350 degrees C. This species is quite active in reacting with propylene and/or propylene adspecies (e.g., pi-C3H5, polyene, etc.) at 250 degrees C in the presence/absence of oxygen, leading to the formation of the isocyanate species (Rh-NCO, at 2174 cm(-1)), CO, and CO2. Rh-NCO is also detected under reaction conditions. A possible reaction pathway for reduction of NO by C3H6 is proposed. In the SCR reaction, Rh-NO+ and propylene adspecies react to generate the Rh-NCO species, then Rh-NCO reacts with O-2, NO, and NO2 to produce N-2, N2O, and CO2. Rh-NO+ and Rh-NCO species are two main intermediates for the SCR reaction on Rh-Al-MCM-41 catalyst.