Active Sites in Fe/MFI Catalysts for NOx Reduction and Oscillating N2O Decomposition

Fe/MFI catalysts prepared either by sublimation of FeCl3 onto H-MFI or by solid state ion exchange were characterized by XRD, FTIR, and ESR spectroscopy. Ligated Fe3+ ions, such as (Fe(OH)2)+ or (Fe=O)+ in distorted tetrahedral coordination, multinuclear oxocations, such as (HO-Fe-O-Fe-OH)2+, and Fe2O3 oxide particles have been identified. Their relative abundances depend on the Si/Al ratio and the Fe loading. Nitro and nitrate groups, identified by their FTIR bands, are formed upon exposing Fe/MFI with high Fe loading to N2O. These adsorption complexes are reduced with benzene at 200°C but not at room temperature. Their reduction with CO occurs at a lower temperature than that of the bridging oxygen. Exposure to N2O at 320°C and 0.5 kPa pumps 30% more active oxygen into Fe/MFI than calcination in O2 of 100 kPa at 500°C. Thermal decomposition releases NO and O2 at the same temperature. The catalytic decomposition rate of N2O to N2+12O2 in diluted feed streams over Fe/MFI is first order in N2O. The rate constant is very low for samples with low Fe loading, but jumps to a much higher value for high Fe loadings, indicating the important role of multinuclear Fe clusters in accordance with recent EXAFS results. Stable isothermal oscillations are observed with overexchanged Fe/MFI (Fe/Al=1; Al/Si=1/14), in the presence of H2O vapor, showing that two different states of catalytic activity exist. No oscillation is observed with a dry feed or over Fe/MFI with low loading.