Remarkable N2 affinity of a steam-activated FeZSM-5 catalyst: a 57Fe Mössbauer study

57Fe Mössbauer spectroscopy was used to investigate the redox behavior of iron species in steam-treated FeZSM-5, a catalyst material with outstanding properties in benzene-to-phenol oxidation as well as nitrous oxide decomposition. It was found that Fe3+ ions in isolated extraframework sites and in extremely small oxo–iron clusters are easily converted into Fe2+ species using a thermal treatment at 623 K. Fe3+ ions in larger iron oxide particles of 2 nm in diameter located at the outer surface of the zeolite are less prone to reduction. Some of the iron species in the FeZSM-5 catalyst show an unusual affinity for N2 leading to redox changes (Fe3+→Fe2+) at temperatures as low as 300 K. In addition, it has been observed that some of the Fe2+ species, which are present in the catalyst after thermal reduction, reversibly adsorb N2.