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 C 3 H 6 in the presence of excess oxygen. It shows a high activity in converting NO to N 2 and N 2 O 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 + O 2 /He and NO + C 3 H 6 + O 2 /He at 100−350 °C. This species is quite active in reacting with propylene and/or propylene adspecies (e.g., π-C 3 H 5, 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 CO 2 . Rh−NCO is also detected under reaction conditions. A possible reaction pathway for reduction of NO by C 3 H 6 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 NO 2 to produce N 2, N 2 O, and CO 2 . Rh−NO + and Rh−NCO species are two main intermediates for the SCR reaction on Rh−Al−MCM-41 catalyst.