Reduced iron sites in Fe–BEA and Fe–ZSM-5 zeolites: FTIR study of CO adsorption and 12C 16O– 13C 18O co-adsorption

Abstract

Accessible iron in oxidized Fe–ZSM-5 and Fe–BEA zeolites is mainly in the form of Fe 3+ species that do not interact with CO. Some exchanged cations are stabilized as Fe 2+ species and form carbonyls detected in the 2205–2190 cm −1 region. Evacuation of the samples at 673 K provokes autoreduction of part of Fe 3+ and creation of more Fe 2+ sites detectable by CO. Reduction of the samples with CO at 673 K leads to a significant increase of the number of Fe 2+ 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 12C 16O– 13C 18O mixture provided a better resolution of the carbonyl bands as compared to the conventional 12C 16O– 13C 16O mixture.