Characterization of Fe-ZSM-5 Catalyst for Selective Catalytic Reduction of Nitric Oxide by Ammonia

Fe-exchanged ZSM-5 (Fe/Al=0.193), the most active catalyst known for the selective catalytic reduction (SCR) of NO with ammonia, was characterized by X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), electron spin resonance (ESR), and FT-IR spectroscopy. XPS and ESR results indicated that iron cations were present mainly 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. FT-IR spectra showed that NO molecules could be oxidized by O2 to adsorbed N2O3, NO2, and nitrate. The NOx adspecies were not stable at above 300°C in He, but the adsorbed NO and NO2 could be observed in flowing NO+O2/He. NH3 molecules were adsorbed on Brønsted acid and Lewis acid sites of Fe-ZSM-5 to generate, respectively, NH4+ ions (majority) and coordinated NH3 (minority). The NH4+ ions with three hydrogen atoms (3H structure) bonded to AlO4 tetrahedra of ZSM-5 were more stable at high temperatures (e.g., 300–400°C) than those with two bonds and the coordinated NH3. The pretreatment by SO2+O2 at 400°C increased the Brønsted acidity of the Fe-ZSM-5 due to formation of surface sulfate species of iron. This resulted in an enhancement of SCR activities by the presence of H2O+SO2 at high temperatures (>350°C). At 300°C, the NH4+ ions with 3H structure were active in reacting with NO and NO+O2, but the reaction rate with NO+O2 was much higher than that with NO. The results indicate that NH4+ ions with 3H structure as well as NO and NO2 adspecies play an important role in the SCR reaction on the Fe-ZSM-5 catalyst. The role of Fe3+ is to oxidize NO to NO2. Overexchange decreased the activity; the optimal Fe/Al ratio was 0.19–0.43.