Adsorption complexes Of O-2 on Fe/MFI and their role in the catalytic reduction of NOx

Exposure to oxygen of Fe/MFI catalysts, prepared by sublimation, results in an adsorbed oxygen species capable of reacting with nitrogen oxide or carb on monoxide at room temperature. The IR bands in the perturbed T-O-T vibrat ion region indicate oxidation of Fe2+ at room temperature. The FTIR data al so show that more nitro groups and nitrate ions are formed from NO + O-2, i f Fe/MFI is exposed to oxygen first, than if nitrogen oxide is dosed first. Formation of these NOy groups is faster on pre-reduced than on pre-oxidize d catalysts, which suggests that diatomic oxygen is adsorbed on sites which , in the pre-oxidized catalysts, are occupied by oxide ions. Over pre-reduc ed catalysts, carbon monoxide oxidation with O-18(2) yields significant amo unts of (COO)-O-18-O-16 and a small amount of (CO2)-O-18, even at room temp erature. It is uncertain whether the adsorbed diatomic oxygen is present as a superoxide ion, as previously detected by ESR on Fe/MFI, Co/MFI and V/MF I at 77 K, 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. (C ) 2001 Elsevier Science B.V. All rights reserved.