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 cat alytic 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 conte nt, chemisorbed NO, (mainly NO2 bonded to iron sites) increased while chemi sorbed NH3 (mainly NH4+ on Bronsted acid sites) decreased due to substituti on of protons by iron ions. The TPSR results indicated that ammonia adsorbe d species were quite active in reacting with NO, O-2, NO + O-2, and NO2 (pr oducing 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 temp eratures. 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 nea r 55 degreesC originated from decomposition of ammonium nitrite, which was not affected by Fe3+ content. The other reaction at higher temperatures (17 0-245 degreesC) was due to an adsorbed complex, probably [NH4+](2)NO2, reac ting 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 d educe 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.