Abstract
The development of NOx-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NOx oxidation and storage reactions proceed over Pt-free Sr3Fe2O7-δ with a Ruddlesden-Popper-type layered perovskite structure. Two types of Sr-Fe perovskite with oxygen storage capacity, namely, SrFeO3-δ and Sr3Fe2O7-δ, are studied as NOx-trapping catalysts. Sr3Fe2O7-δ shows higher NOx storage capacity than SrFeO3-δ; its activity is comparable to that of Pt/Ba/Al2O3 calcined at 1273 K. NOx temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NOx-trapping ability of Sr3Fe2O7-δ over SrFeO3-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 NOx on Pd-modified Sr-Fe perovskites is investigated by lean-rich cycle experiments using H2 as the reductant. Pd/Sr3Fe2O7-δ shows significantly high NOx removal efficiency over the entirety of each lean-rich period. Modifying Sr3Fe2O7-δ with Pd is also effective for NOx storage in the presence of H2O and CO2 and the regeneration of the catalyst following SOx sorption. Sr3Fe2O7-δ, with both NOx adsorption and NO oxidation capabilities, acts as a Pt-free NOx-trapping catalyst, exhibiting both high NOx storage capacity and high thermal tolerance.
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