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

Abstract

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 _x 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 200–350 °C. Under these reaction conditions, the NO _x 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 O₂. 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 NO₂ 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.