Influence of iron state and acidity of zeolites on the catalytic activity of FeHBEA, FeHZSM-5 and FeHMOR in SCR of NO with NH3 and N2O decomposition

Abstract

FeHBEA, FeHZSM-5 and FeHMOR with 1wt% of Fe were prepared by combination of ion exchange and impregnation procedures. These zeolites have been characterized by X-ray diffraction, DR UV–vis, 57Fe Mössbauer, XPS, NH3-TPD and FTIR. The similar XRD diffractograms, obtained for all the samples, have showed that introduction of iron ions into the zeolite supports did not induce significant changes in their structure. The FeHBEA zeolite possessed a pseudo-tetrahedral Fe(III) species as evidenced by combined used of DR UV–vis, 57Fe Mössbauer and XPS studies. In contrast, in FeHZSM-5 and FeHMOR zeolites, the pseudo-tetrahedral and octahedral Fe(III) were identified. FTIR studies for the samples preadsorbed with NO proved the presence of Fe3+OH groups that were reduced at room temperature by NO to Fe2+NO and NO+. The catalytic activity of FeHBEA, FeHZSM-5 and FeHMOR in SCR of NO with ammonia and N2O decomposition strongly depended on the speciation of iron introduced into different zeolite structure and their acidity. The highest catalytic activity in SCR of NO with NH3 was obtained for the FeHBEA containing iron as pseudo-tetrahedral Fe(III) and characterized by relatively high acidity. The smaller activity of FeHZSM-5 and FeHMOR in this process is probably related to the side reaction of direct ammonia oxidation by oxygen on a polynuclear octahedral Fe(III) present in both catalysts. In contrast, FeHZSM-5 and FeHMOR possess the higher catalytic activity than FeHBEA in N2O decomposition because of the presence not only well dispersed pseudo-tetrahedral Fe(III) but also much stronger acidic sites than that identified in FeHBEA.