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 NO_x showed that both NO_x and NH₃ adsorbed on Fe-ZSM-5. Physisorbed NO_x and NH₃ were not affected significantly by iron content. With increasing iron content, chemisorbed NO_x (mainly NO₂ bonded to iron C sites) increased while chemisorbed NH₃ (mainly NH₄⁺ 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 > O_2· NO_x adsorbed species were also reactive to NH₃ at high temperatures. With NH₃ and NO_x 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 [NH₄⁺]₂NO₂, reacting with NO or NO₂. A possible reaction path was proposed for NO reduction involving NO₂ and [NH₄⁺]₂NO₂ as intermediates. Since the reactivity of [NH₄⁺]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 [NH₄⁺]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.