| NOx is one of the main pollutants in the diesel exhaust and the selective catalytic reduction of NOx with NH3 is the most effective method for the removal of NOx, where the research and development of catalysts is the key technique. Cu-zeolites have attracted extensive attention and are considered as NH3-SCR catalysts presently due to their excellent catalytic performances. In this work, a lower cost and environment friendly Cu-SSZ-13 was synthesized and modified by transition metal according to the limitation of catalytic activity and anti-poisoning properties of Cu-SSZ-13. Therefore, the catalytic performances of the above catalysts for NH3-SCR were systematically investigated.Firstly, Cu-SSZ-13 zeolite was prepared by one-pot hydrothermal method with Cu-TEPA as a template. The catalyst showed more than 90% NO conversion berween 200? and 400? under GHSV of 100,000h-1. The results of XRD and BET showed that the catalyst exhibited SSZ-13 characteristic peaks and high crystallinity, and the larger specific surface area and pore volume, which was in accordance with micropore structure characteristics, facilated gases adsorption. The catalyst showed the regular hexagonal morphology in SEM images. EPR and H2-TPR results indicated that the catalyst had good redox property and isolated Cu2+ was the active species.Secondly, in order to widen the reaction temperature window to response to various exhaust composition and temperature from diesel vehicle, Cu-SSZ-13 was modified by transition metal Fe and Ce. Furthermore, the effect of preparation method of Fe/Cu-SSZ-13 on NO conversion was also studied. The research found that the catalysts prepared by liquid ion exchange method exhibited the best catalytic performance. The XRD and SEM results indicated that all Fe/Cu-SSZ-13, Ce/Cu-SSZ-13 and Cu-SSZ-13 exhibited identical characteristic peaks and morphology, which suggested that the loaded metal ion could not change the crystal structure and surface morphology of catalysts. However, the results of H2-TPR, XPS and UV-Vis DRS suggested that more iron and cerium active species appeared after ion exchange. The proportion of metal ions with low oxidation valence and the oxygen vacancies increased obviously, which promoted oxidation of NO to NO2 and accelerated "fast SCR reaction" efficiency. More reduction peaks of exchanged metal species appeared as well, which enhanced the redox abilities of Cu-SSZ-13 after undergoing ion exchange. The interaction mechanism between NO+O2 and NH3 was investigated via In-situ DRFIT. The results indicated that in the low-temperature NH3-SCR process, NH3 adsorbed onto Br(?)nsted and Lewis acid sites. NO+O2 adsorbed on the catalysts surface and gave rise to the key intermediates, such as cis-N2O22-, NO+ and NO3-. Then NH3 was mainly reduced to N2.In this work, the deNOx activity, N2 selectivity, resistance to high space velocity, anti carbon deposition, resistance to SO2, hydrothermal stability performances and the effect of feed gases composition on NO conversion were analyzed completely. Fe/Cu-SSZ-13 showed more than 85% NO conversion above 400? under GHSV of 150,000h-1, indicating that the incorporation of Fe improved the high temperature activity. However, the addition of Ce improved the low temperature (<200?) activity. N2 selectivity approached 100% in the test temperature range over modified catalysts. The hydrothermal stability was also further improved. Moreover, NO conversion reached more than 90% in the condition of 300?,100mg/LSO2,5%H2O,5%C2H4 and 5%CO2, which suggested that modified catalysts had good anti-coke, resistance to H2O and SO2 properties. The addition of transition metal inhibited the formation of NH4HSO4 and (NH4)2SO4, but formed small amounts of metal sulphate, leading to uncomplete elimination of sulfur poisoning. |
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