Reduced iron sites in Fe–BEA and Fe–ZSM-5 zeolites: FTIR study of CO adsorption and 12C16O–13C18O co-adsorption Original Research Article

Accessible iron in oxidized Fe–ZSM-5 and Fe–BEA zeolites is mainly in the form of Fe3+ species that do not interact with CO. Some exchanged cations are stabilized as Fe2+ species and form carbonyls detected in the 2205–2190 cm−1 region. Evacuation of the samples at 673 K provokes autoreduction of part of Fe3+ and creation of more Fe2+ sites detectable by CO. Reduction of the samples with CO at 673 K leads to a significant increase of the number of Fe2+ sites and creation of new reduced iron sites (supposed to be Fe+ cations). With CO the latter forms tri- and tetra-carbonyl complexes. In order to clarify the structure of these species, adsorption of CO isotopic mixtures is studied. It is demonstrated that the use of 12C16O–13C18O mixture provided a better resolution of the carbonyl bands as compared to the conventional 12C16O–13C16O mixture.