Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe²⁺ homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe²⁺ grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe²⁺ presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N₂O decomposition on isolated sites, whose properties are more similar to those of the (FeO)²⁺ in cationic complexes (included the [(H₂O)₅FeO]²⁺ “brown ring” complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

The NH₃-mediated selective catalytic reduction of NOx (NH₃-SCR) over Cu-CHA catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH₃-SCR, oxygen only reacts with Cu^I ions, which are present as mobile Cu^I diamine complexes [Cu^I(NH₃)²]⁺. To determine the structure and reactivity of the species formed by oxidation of these Cu^I diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by X-ray absorption spectroscopies (XANES and EXAFS) and Diffuse Reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu₂(NH₃)₄O₂]²⁺ mobile complex with a side-on μ-η2,η2-peroxo diamino dicopper (II) structure, accounting for 80-90% of the total Cu content. These [Cu₂(NH₃)₄O₂]²⁺ are completely reduced to [Cu^I(NH₃)²]⁺ at 200 °C in a mixture of NO and NH₃. Some N₂ is formed as well, which suggest the role of the dimeric complexes in the low-temperature NH3-SCR reaction. The reaction of [Cu₂(NH₃)₄O₂]²⁺ complexes with NH₃ leads to a partial reduction of the Cu without any formation of N₂. The reaction with NO results in an almost complete reduction to Cu^I, under the formation of N₂. This indicates that the low temperature NH₃-SCR reaction proceeds via a reaction of these complexes with NO.

The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl₂/γ-Al₂O₃ systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu⁺N₂, Cu⁺(CO)₃, Cu⁺(NH₃)(CO) and Cu⁺(NO)₂) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.