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 NOx (mainly NO2 bonded to iron sites) increased while chemisorbed NH3 (mainly NH+4 on Brønsted 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, O2, NO+O2, and NO2 (producing H2O, N2 and/or N2O), following the reactivity rank order NO2∼NO+O2>NO>O2. NOx 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 N2 formation. One reaction near 55°C originated from decomposition of ammonium nitrite, which was not affected by Fe3+ content. The other reaction at higher temperatures (170–245°C) was due to an adsorbed complex, probably [NH+4]2NO2, reacting with NO or NO2. A possible reaction path was proposed for NO reduction involving NO2 and [NH+4]2NO2 as intermediates. Since the reactivity of [NH+4]2NO2 to NO (producing only N2 at 170°C) was higher than that to NO2 (producing both N2 and N2O at 200°C), it is reasonable to deduce that [NH+4]2NO2 prefers to react with NO and not NO2, both of which are present in the SCR reaction. This may be the reason for N2 being the only product for SCR on Fe–ZSM-5.