Book Description

In the last annual reports, we reported Cu-exchanged pillared clays as superior selective catalytic reduction (SCR) catalysts. During the past year we explored the possibilities with MCM-41, a new class of molecular sieve. In this report, Rh exchanged Al-MCM-41 is studied for the SCR of NO by C[sub 3]H[sub 6] in the presence of excess oxygen. It shows a high activity in converting NO to N[sub 2] and N[sub 2]O at low temperatures. In situ FT-IR studies indicate that Rh-NO[sup+] species (1910-1898 cm[sup[minus]1]) is formed on the Rh-Al-MCM-41 catalyst in flowing NO/He, NO+O[sub 2]/He and NO+C[sub 3]H[sub 6]+O[sub 2]/He at 100-350 C. This species is quite active in reacting with propylene and/or propylene adspecies (e.g., [pi]-C[sub 3]H[sub 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[sup[minus]1]), CO and CO[sub 2]. Rh-NCO is also detected under reaction conditions. A possible reaction pathway for reduction of NO by C[sub 3]H[sub 6] is proposed. In the SCR reaction, Rh-NO[sup+] and propylene adspecies react to generate the Rh-NCO species, then Rh-NCO reacts with O[sub 2], NO and NO[sub 2] to produce N[sub 2], N[sub 2]O and CO[sub 2]. Rh-NO[sup+] and Rh-NCO species are two main intermediates for the SCR reaction on Rh-Al-MCM-41 catalyst.