Coordination and oxidation changes undergone by iron species in Fe-MCM-22 upon template removal, activation and red-ox treatments: an in situ IR, EXAFS and XANES study

We report about the characterization of an isomorphously substituted Fe-MCM-22 sample with high iron content (Si/Fe = 18) using FTIR, XANES and EXAFS spectroscopies. Template burning and subsequent activation in vacuo caused the migration of a fraction of framework Fe3+ species to extraframework positions, accompanied by the reduction of a fraction of Fe3+ to Fe2+. A fraction of extraframework Fe2+ sites was able to adsorb NO, forming Fe2+(NO)n complexes (n=1,2,3), which indicates a high coordinative unsaturation of such sites. The parallel experiment monitored by X-ray absorption spectroscopies testified that this fraction is however small, as both EXAFS and XANES spectra were almost unperturbed by NO adsorption. The corresponding FTIR bands are highly broad and asymmetric, which does not allow to detect the presence of Fe3+(NO) complexes. The broad character of the nitrosyl bands suggests the presence of small oxidic clusters, in agreement with XANES and EXAFS evidences. The nature and distribution of extraframework Fe species are influenced by water preadsorption, which causes the increase of the amount of the most coordinatively unsaturated Fe2+ sites, able to form Fe2+(NO)3 complexes. The effect of red-ox treatments with O2 and H2 was also investigated. Upon oxidation, adsorbed oxygen is formed, which efficiently shields the Fe centers and does not rapidly nor efficiently react with NO. Upon reduction, the intensity of nitrosyl complexes increases, indicating the reduction of extraframework Fe3+ (likely present on the surface of small oxidic clusters) to Fe2+. Comparison with the most investigated Fe-MFI system is made.