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.