Characterization of Fe-ZSM-5 catalyst for selective catalytic reduction of nitric oxide by ammonia

XPS, H₂-TPR, ESR, and FTIR spectroscopy were used to characterize Fe-exchanged ZSM-5 (Fe/Al = 0.193), the most active catalyst known for the SCR of NO with ammonia. Fe cations were present as Fe³⁺ ions with tetrahedral coordination, along with a small amount of Fe²⁺ and aggregated Fe³⁺ ions. The Fe³⁺ ions could be partially reduced to Fe²⁺ ions by H₂ at 300°-600°C, but the oxidation was reversible when O₂ was introduced into the reduced catalyst at 500°C. NO molecules could be oxidized by O₂ to adsorbed N₂O₃, NO₂, and nitrate. The NO_x adspecies were not stable at > 300°C in helium, but the adsorbed NO and NO₂ could be observed in flowing NO + O₂/He. Ammonia molecules were adsorbed on Broensted acid and Lewis acid sites of Fe-ZSM-5 to generate NH₄⁺ ions (majority) and coordinated NH₃ (minority), respectively. The pretreatment by SO₂ + O₂ at 400°C increased the Broensted acidity of the Fe-ZSM-5 due to formation of surface sulfate species of iron, resulting in an enhancement of SCR activities by the presence of H₂O + SO₂ at > 350°C. NH₄⁺ ions, NO, and NO₂ adspecies may all play an important role in the SCR reaction on the Fe-ZSM-5 catalyst.