NO Oxidation and Storage Properties of a Ruddlesden-Popper-Type Sr₃Fe₂O_7-δ-Layered Perovskite Catalyst

The development of NO-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NO oxidation and storage reactions proceed over Pt-free Sr₃Fe₂O_7-δ with a Ruddlesden-Popper-type layered perovskite structure. Two types of Sr-Fe perovskite with oxygen storage capacity, namely, SrFeO_3-δ and Sr₃Fe₂O_7-δ, are studied as NO-trapping catalysts. Sr₃Fe₂O_7-δ shows higher NO storage capacity than SrFeO_3-δ; its activity is comparable to that of Pt/Ba/Al₂O₃ calcined at 1273 K. NO temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NO-trapping ability of Sr₃Fe₂O_7-δ over SrFeO_3-δ. In addition, NO temperature-programmed reactions and O₂ temperature-programmed desorption experiments reveal that these catalysts operate through a novel NO oxidation mechanism involving the consumption of their lattice oxygens and topotactic structural changes at a temperature of around 350-400 K. The reduction performance of trapped NO on Pd-modified Sr-Fe perovskites is investigated by lean-rich cycle experiments using H₂ as the reductant. Pd/Sr₃Fe₂O_7-δ shows significantly high NO removal efficiency over the entirety of each lean-rich period. Modifying Sr₃Fe₂O_7-δ with Pd is also effective for NO storage in the presence of H₂O and CO₂ and the regeneration of the catalyst following SO sorption. Sr₃Fe₂O_7-δ, with both NO adsorption and NO oxidation capabilities, acts as a Pt-free NO-trapping catalyst, exhibiting both high NO storage capacity and high thermal tolerance.