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Journal of Zhejiang University SCIENCE A
ISSN 1673-565X(Print), 1862-1775(Online), Monthly
2016 Vol.17 No.4 P.325-334
Impact of diesel emission fluid soaking on the performance of Cu-zeolite catalysts for diesel NH3-SCR systems
Abstract: Diesel emission fluid (DEF) soaking and urea deposits on selective catalytic reduction (SCR) catalysts are critical issues for real diesel engine NH3-SCR systems. To investigate the impact of DEF soaking and urea deposits on SCR catalyst performance, fresh Cu-zeolite catalyst samples were drilled from a full-size SCR catalyst. Those samples were impregnated with DEF solutions and subsequently hydrothermally treated to simulate DEF soaking and urea deposits on real SCR catalysts during diesel engine operations. Their SCR performance was then evaluated in a flow reactor with a four-step test protocol. Test results show that the DEF soaking leached some Cu from the SCR catalysts and slightly reduced their Cu loadings. The loss of Cu and associated metal sites on the catalysts weakened their catalytic oxidation abilities and caused lower NO/NH3 oxidation and lower high-temperature N2O selectivity. Lower Cu loading also made the catalysts less active to the decomposition of surface ammonium nitrates and decreased low-temperature N2O selectivity. Cu loss during DEF impregnation released more acid sites on the surface of the catalysts and increased their acidities, and more NH3 was able to be adsorbed and involved in SCR reactions at medium and high temperatures. Due to lower NH3 oxidation and higher NH3 storage, the DEF-impregnated SCR catalyst samples showed higher NOx conversion above 400 °C compared with the non-soaked one. The negative impact of urea deposits during DEF impregnation was not clearly observed, because the high-temperature hydrothermal treatment helped to remove the urea deposits.
Key words: Diesel engine emission control, Selective catalytic reduction (SCR), NOx reduction, Cu-zeolite catalyst, Diesel emission fluid (DEF) soaking
Chinese Summary <28> 尿素水溶液浸渍对柴油机NH3-SCR后处理铜基分子筛催化剂性能的影响
创新点：1. 通过小样DEF浸泡和水热处理模拟柴油机NH3-SCR系统在实际使用中催化剂表面被DEF浸渍、尿素结晶生成及演变的情况；2. 采用四步法在流动反应器上对被DEF浸泡和水热处理后的催化剂小样进行性能评估和分析。
方法：1. 对全尺寸铜基分子筛SCR催化剂取小样并进行700 °C和4小时的水热预处理使其性能稳定；2. 通过1~2小时DEF浸泡和高达550 °C的水热处理，模拟柴油机NH3-SCR系统在实际使用中催化剂表面DEF浸渍、尿素结晶生成与演变情况；3. 采用四步法在流动反应器上对被DEF浸泡和水热处理后的催化剂小样进行NOx转化率、N2O选择性、NO和NH3氧化性以及NH3存储等性能评估；4. 以新鲜催化剂小样为参照，分析DEF浸渍和尿素结晶对铜基分子筛SCR催化剂性能的影响。
结论：1. DEF浸渍会轻微减小铜基分子筛SCR催化剂的铜载量；2. 铜载量的降低削弱了催化剂的氧化性，导致较低的NO/NH3氧化性和高温N2O选择性；3. 铜载量的降低减缓了催化剂表面硝酸氨的分解，导致较低的低温N2O选择性；4. DEF浸渍引起铜载量变化的同时增强了催化剂表面酸性，使更多NH3可以被吸附和参与SCR反应；5. DEF浸渍过的催化剂小样由于具有较低的NH3氧化性和较高的NH3存储能力，所以在400 °C以上具有更好的NOx转化率；6. 尿素结晶对催化剂性能影响不明显，主要是因为DEF浸泡后的高温水热处理已经去除了催化剂表面大部分的尿素结晶。
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DOI:
10.1631/jzus.A1500215
CLC number:
TK427
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On-line Access:
2016-04-05
Received:
2015-07-30
Revision Accepted:
2016-01-09
Crosschecked:
2016-03-08
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