To elucidate the interaction of iron and the supports in Fe/zeolite catalysts, XPS was conducted to characterize iron MK5108 in the surface region of the samples. In Fig. 11, the Fe 2p3/2 spectra for the four Fe/zeolites catalysts are shown. Deconvolution of the Fe 2p3/2 peak from each sample was performed by fitting a Gaussian–Lorentzian (GL) function with a Shirley background . Peaks located near 711.5 and 710 eV can be assigned to Fe3+ and Fe2+  and , respectively, indicating that the iron in our samples is present both as Fe3+ and Fe2+. The results from deconvolution (i.e., the areas under Gaussian component peaks, which correspond to Fe2+ and Fe3+ species) are presented in Table 2. High abundance of trivalent iron is found in Fe/MOR, Fe/FER, and Fe/ZSM-5 samples. However, divalent iron is the prevailing iron species in the Fe/Beta catalyst. The Fe2+/Fe3+ ratio in the surface layer of Fe/zeolites decreased in the order of Fe/Beta > Fe/FER > Fe/MOR > Fe/ZSM-5. Both Fe2+ and Fe3+ are active sites of Fe-based catalysis for HC–SCR . Lobree et al.  illustrated a possible mechanism for the reduction of NO by C3H8 over Fe/ZSM-5 in the presence of O2. Fe2+ reacted with O2 to form Fe3+(O2−). The reaction of Fe3+(O2−) with NO resulted in the formation of either Fe3+(O−)(NO2) or Fe3+(NO3−). Upon the reaction of these NO2/NO3 species with C3H8, CN and NCO species were formed. CN and NCO could subsequently react with NO2 or O2 to form nitrogen- and carbon-containing combustion products, whereas the reduction of Fe3+ to Fe2+ was assumed to occur via the reduction of O− with CN or NCO. Our results reveal that NO reduction is dependent on the Fe2+/Fe3+ ratio, especially at low temperatures. An optimum ratio of Fe2+/Fe3+ exists for the HC–SCR reaction because Fe/MOR shows the highest activity among the Fe/zeolite samples.