Synthesis of Pd0.01FexCe(1−x)/2Zr(1−x)/2Oy catalysts and their catalytic performance for ammonia production by passive SCR reaction

Abstract

The passive ammonia SCR system has been considered as a novel after-treatment method to reduce the emission of NO_x and produce ammonia for downstream SCR. A series of Pd_0.01Fe_xCe_(1−x)/2Zr_(1−x)/2O_y catalysts (x = 0, 0.1, 0.2, and 0.4) was synthesized through a solution combustion synthesis (SHS) method. Their catalytic activities were investigated by XRD, BET, Raman, H₂-TPR, XPS, and in situ DRIFTS. The relationship between the reactivity and species composition, acidic properties, surface oxygen vacancies, and redox behavior of the Pd_0.01Fe_xCe_(1−x)/2Zr_(1−x)/2O_y catalysts was established. Results demonstrated that a moderate amount of Fe doping can markedly enhance the specific surface area, improve the surface oxygen vacancies, increase the surface acidic sites, and tune the valence distribution of surface elements. Pd_0.01Fe_0.1Ce_0.45Zr_0.45O_x showed the highest ammonia yield with a wide reaction-temperature window (250–400 °C) and >95% NO conversion and complete conversion from nitric oxide to ammonia at 300–400 °C. This may be due to the phenomenon that highly dispersed Fe₂O₃ enhanced the interaction among PdO, Fe₂O₃ and the Ce_0.5Zr_0.5O₂ support, improved the displacement of active species on the catalyst surface, and increased the surface oxygen vacancies, thus improving the catalytic activity.