Temperature-programmed desorption/surface reaction (TPD/TPSR) study of Fe-exchanged ZSM-5 for selective catalytic reduction of nitric oxide by ammonia

Temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) were employed to study Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia. TPD profiles of ammonia and NOx showed that both NOx and NH3 adsorbed on Fe-ZSM-5. Physisorbed NOx and NH3 were not affected significantly by iron content. With increasing iron content, chemisorbed NO, (mainly NO2 bonded to iron sites) increased while chemisorbed NH3 (mainly NH4+ on Bronsted acid sites) decreased due to substitution of protons by iron ions. The TPSR results indicated that ammonia adsorbed species were quite active in reacting with NO, O-2, NO + O-2, and NO2 (producing H2O, N-2 and/or N2O), following the reactivity rank order NO2 NO + O-2 > NO > O-2. NO, adsorbed species were also reactive to NH3 at high temperatures. With NH3 and NOx coadsorbed on Fe-ZSM-5, TPSR with gaseous He, NO, and NO2 showed two kinds of reactions for N-2 formation. One reaction near 55 degreesC originated from decomposition of ammonium nitrite, which was not affected by Fe3+ content. The other reaction at higher temperatures (170-245 degreesC) was due to an adsorbed complex, probably [NH4+](2)NO2, reacting with NO or NO2. A possible reaction path was proposed for NO reduction involving NO2 and [NH4+](2)NO2 as intermediates. Since the reactivity of [NH4+](2)NO2 to NO (producing only N-2 at 170 degreesC) was higher than that to NO2 (producing both N-2 and N2O at 200 degreesC), it is reasonable to deduce that [NH4+](2)NO2 prefers to react with NO and not NO2, both of which are present in the SCR reaction. This may be the reason for N-2 being the only product for SCR on Fe-ZSM-5. (C) 2001 Academic Press.