Nature and Redox Properties of Iron Sites in Zeolites Revealed by Mössbauer Spectroscopy

Iron-containing zeolite-based catalysts play a pivotal role in environmental processes aimed at mitigating the release of harmful greenhouse gases, such as nitrous oxide (N₂O) and methane (CH₄). Despite the rich iron chemistry in zeolites, only a fraction of iron species that exhibit an open coordination sphere and possess the ability for electron transfer are responsible for activating reagents. In addition, the splitting of molecular oxygen is facilitated by bare iron cations embedded in zeolitic matrices. Mössbauer spectroscopy is the ideal tool for investigating the valency and geometry of iron species in zeolites because it leaves no iron forms silent and provides insights into in-situ processes. This review is dedicated to the utilization of Mössbauer spectroscopy to elucidate the nature of the extra-framework iron centers in ferrierite (FER), beta-structured (*BEA), and ZSM-5 zeolite (MFI) zeolites, which are active in N₂O decomposition and CH₄ oxidation through using the active oxygen derived from N₂O and O₂. In this work, a structured summary of the Mössbauer parameters established over the last two decades is presented, characterizing the specific iron active centers and intermediates formed upon iron's interaction with N₂O/O₂ and CH₄. Additionally, the impact of preparation methods, iron loading, and the long-term stability on iron speciation and its redox behavior under reaction conditions is discussed.