Identification of iron species in FeSiBEA by DR UV–vis, XPS and Mössbauer spectroscopy: Influence of Fe content

Abstract

The nature of iron in FeSiBEA zeolite was identified using DR UV–vis, XPS and Mössbauer spectroscopy. Various FexSiBEA zeolites (x=3.8, 8.2 and 15wt.%) were prepared by a two-step post synthesis method, which consisted in the creation of vacant T-atom sites by dealumination of tetraethylammonium BEA zeolite with nitric acid and then impregnation of resulting SiBEA with an aqueous solution of Fe(NO3)3. The incorporation of iron into the framework of SiBEA was evidenced by XRD. In Fe3.8SiBEA with low Fe content mainly framework tetrahedral Fe(III) species is present, in contrast, with high Fe content additional octahedral FeOx oligomers (Fe8.2SiBEA) and iron oxides (Fe15SiBEA) occur. The presence of iron in its 3+ oxidation state as isolated tetrahedral Fe(III) species for low metal content was demonstrated by DR UV–vis, XPS and Mössbauer spectroscopy. Moreover, the presence of Fe(III) species in two different tetrahedral environments was evidenced by Mössbauer results. For high iron content, diffuse reflectance UV–vis and Mössbauer spectra revealed additional presence of extra-framework FeOx oligomers. After calcination and rehydration significant amounts of oxide phases are formed, which were evidenced by DR UV–vis, Mössbauer spectroscopy and XPS.