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Study on the Structure, Property and Storage Mechanism of the Rare-Earth Perovskite-type La1-xSrxCo1-yFeyO3 NOx Storage-Reduction Catalyst

Author: ZouHongZuo
Tutor: MengMing;LiXinGang
School: Tianjin University
Course: Industrial Catalysis
Keywords: La1-xSrxCo1-yFeyO3 perovskite NO_x storage sulfur resistance
CLC: O643.36
Type: Master's thesis
Year: 2009
Downloads: 25
Quote: 2
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Abstract


A series of the LaCo(1-x)FexO3 (x=0, 0.15, 0.3, 0.45) catalysts was prepared by sol-gel method using citrate and EDTA as the complexing agent. The catalysts obtained in this way possessed big BET surface area (13-15 m2/g), as well as complete perovskite structure. When the B site element Co was partially substituted by Fe, the perovskite lattice expansion appeared. After the NOx storage capacity (NSC)tests on the catalysts, it was found that the LaCoO3 sample possessed the biggest NOx storage capacity (137μmol/g) among these catalysts, and the NSC showed the tendency to decrease when the ratio of the Fe doping increased.To enhance the NOx storage capacity of the catalysts, a series of the La1-xSrxCoO3 catalysts had been prepared with the same preparation method. The XRD patterns of the catalysts illustrated that the addition of Sr led to the generation of the SrCO3 phase as detected in the samples when the Sr doping ratio was above 0.1. From the results of the NSC, it indicates that the best reacting temperature for NOx storage is 300 oC. Generally, this series of catalysts showed good NOx storage ability, among them, La0.7Sr0.3CoO3 with the biggest BET surface area (19.1 m2/g), appeared to be the most excellent one for NOx storage. When the NOx storage process reached balance, the NSC of La0.7Sr0.3CoO3 was 2595μmol/g, with the ratio that NO converted to NO2 reaching 73 % at the same time. However, after the SO2 pretreatment, the NSC of this series of catalysts decreased sharply, revealing their poor sulfur resistance. The mechanism of the NOx storage and the sulfur poisoning on the perovskite catalysts was investigated by the characterizations of H2-TPR, O2-TPD, FT-IR, EXAFS, and XPS under different experimental conditions. NOx mainly stored on the oxygen vacancies around A site Sr in the form of N-bounded nitrate produced by the interaction between NOx and the perovskite structure of the catalyst. The main function of the SrCO3 phase existed in the catalysts was to promote the dispersion of the surface active sites, which led to the improvement of the oxidative capabilities of the catalysts. Meanwhile, the SrCO3 phase also participated in the process of the NOx storage. In the course of the NOx storage, NO was firstly oxidized to NO2 by the Co ions with high valence in the perovskite structure, and then was stored on the oxygen vacancies around Sr in the form of N-bounded nitrates, or was stored on the SrCO3 nearby in the form of bulk nitrates. The decrease of the catalysts’NSC after SO2 pretreatment was caused by the formation of bulk sulfates produced by the combination of SO2 and SrCO3 on the surface of the catalyst. On one hand, the sulfate covered the catalyst surface, which meant great obstacle to the NOx storage on the perovskite phase. On the other hand, the S atom in SO2 might occupy the oxygen vacancies around Sr in the perovskite phase, and partly destruct the surface perovskite structure. Therefore, the oxidative activities and the NOx storage capabilities of the catalysts decreased greatly.To improve the sulfur resistance capabilities of the above perovskite catalysts, Fe was utilized to substitute the B site Co of La0.7Sr0.3CoO3 with different ratios. The amount of SrCO3 in the catalysts decreased gradually as the ratio of Fe doping increased, as well as the NSC of the catalysts. Nevertheless, the sulfur resistance capabilities of the catalysts had been improved to some extent. Of all the catalysts, the La0.7Sr0.3Co0.4Fe0.6O3 sample performed best on the aspect of sulfur resistance, and the NSC of La0.7Sr0.3Co0.4Fe0.6O3 gained obvious recovery after H2 reduction, which demonstrated that it had good regeneration ability.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalyst
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