NO oxidation kinetics on iron zeolites:: influence of framework type and iron speciation

Zeolites having MFI, FER and *BEA topology were loaded with iron using solid state cation exchange method. The Fe:Al atomic ratio was 1:4. The zeolites were characterized using nitrogen adsorption, FTIR and DR UV-Vis-NIR spectroscopy. The catalytic activity in NO oxidation and the occurrence of NO, adsorption was determined in a fixed-bed mini reactor using gas mixtures containing oxygen and water in addition to NO and NO2 and temperatures of 200-350degreesC. Under these reaction conditions, the NOx adsorption capacity of these iron zeolites was negligible. The kinetic data could be fitted with a LHHW rate expression assuming a surface reaction between adsorbed NO and adsorbed O-2. The kinetic analysis revealed the occurrence of strong reaction inhibition by adsorbed NO2. FER and MFI zeolites were more active than *BEA type zeolite. MFI zeolite is most active but suffers most from NO, inhibition of the reaction rate. FTIR and UV-Vis spectra suggest that isolated Fe3+ cations and binuclear Fe3+ complexes are active NO oxidation sites. Compared to the isolated Fe3+ species, the binuclear complexes abundantly present in the MFI zeolite seem to be most sensitive to poisoning by NO2.