Adsorption complexes of O_2 on Fe/MFIF and their role in the catalytic reduction of NO_x

摘要

Exposure to oxygen of Fe/MFI catalysts, prepared by sublimation, results in an adsorbed oxygen species capable of reacting with nitrogen oxide or carbon monoxide at room temperature. The IR bands in the perturbed T-O-T vibration region indicate oxidation of Fe~(2+) at room temperature. The FTIR data also show that more nitro groups and nitrate inos are formed from NO + O_2, if Fe/MFI is exposed to oxygen first, than if nitrogen oxide is dosed first. Formation of these NO_y groups is faster on re-reduced than on pre-oxidized catalysts, which suggests that diatomic oxygen is adsorbed on sites which, in the pre-oxidized catalysts, are occupied by oxide ions. Over pre-reduced catalysts, carbon monoxide oxidation with ~18O_2 yields significant amounts of C_18O_16O and a small amount of C_18O_2, even at room temperature.It is uncertain whether the adsorbed diatomic oxygen is presen as a superoxide ion, as previously detected by ESR on Fe/MFI, Co/MFI and V/MFI at 77K, or as a peroxo groups bridging over two iron ions, as identified in DFT analysis. formation of nitrate ions is visualized by interaction of nitrogen oxide with a superoxide ion, possibly via a peroxynitrite ion.