N₂O Decomposition over Liquid Ion-Exchanged Fe-BEA Catalysts: Correlation Between Activity and the IR Intensity of Adsorbed NO at 1874 cm⁻¹

Abstract

Various Fe-BEA catalysts have been synthesized by liquid ion-exchange, varying the iron precursor (nitrate, sulfate or chloride), the iron content (0.2–1.5 wt% Fe), and the conditions of activation, i.e. atmosphere (air or inert) and heating rate (5 or 20 K min⁻¹). The catalysts were tested in direct N₂O decomposition in the temperature range of 625–825 K. An optimal iron loading of 0.8 wt% Fe was found, with no significant influence of the used Fe-precursor. Activation of the ion-exchanged catalysts in inert gas yields significantly better performance than activation in air. NO adsorption combined with infrared analysis was used to characterize the various Fe²⁺ species present in the differently prepared Fe-BEA zeolites. A correlation exists between the absorption intensity of NO at 1874 cm⁻¹ and the activity of the catalysts in N₂O decomposition. This relationship, which can be used as a reliable and fast assessment of catalyst performance, suggests an important role of ferrous ions in the activity of these catalysts. Based on these results and previous mechanistic studies using transient techniques, the NO absorption band at 1874 cm⁻¹ is tentatively assigned to oligonuclear oxocations in the zeolite channels, with general formula Fe _x O _y .