In situ FT-IR study of Rh-Al-MCM-41 catalyst for the selective catalytic reduction of nitric oxide with propylene in the presence of excess oxygen

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 N2 and N2O at low temperatures. In situ FT-IR studies indicate that Rh-NO+ species (1910-1898 cm-1) is formed on the Rh-Al-MCM-41 catalyst in flowing NO/He, NO+O2/He and NO+C3H6+O2/He at 100-350 °C. This species is quite active in reacting with propylene and/or propylene adspecies (e.g., π-C3H5, polyene, etc.) at 250 °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 O2, NO, and NO2 to produce N2, N2O, and CO2. Rh-NO+ and Rh-NCO species are two main intermediates for the SCR reaction on Rh-Al-MCM-41 catalyst.