高效双功能ASC催化剂的涂层结构设计、作用机理及应用性能研究

【作者】 常仕英；

【导师】 易健宏；

【作者基本信息】 昆明理工大学 ， 材料学， 2020， 博士

【摘要】 氨气（NH₃）作为典型的有害工业废气,对人体健康和大气环境具有严重危害。国六以上柴油车超低排放标准为实现更高的NOx处理效率,需提高SCR系统的尿素水溶液喷射量,使得NH₃泄露问题进一步加剧,对城市大气环境造成严重污染,成为大气污染的新排放源。NH₃泄露催化剂（ASC）可将NH₃氧化为N₂,成为超低排放柴油车的必备后处理催化技术。纯贵金属催化剂（PGM）因N₂选择性差,严重限制其应用。常规双层结构的双功能催化剂通过耦合SCR催化剂和PGM催化剂,可显著提高N₂选择性,但无法突破低温NH₃氧化效率不足关键问题。本文基于SCR催化剂和PGM催化剂的不同耦合方式,设计了多种涂层结构的双功能ASC催化剂,并围绕单一SCR催化剂、单一PGM催化剂及双功能催化剂的NH₃-SCO反应、反应路径、热老化机制及整机应用性能进行了系统研究。得到如下主要研究结果:1、制备并开展了单一催化剂和双功能催化剂的新鲜态和热老化态NH₃-SCO性能研究,明确了不同涂层类型、不同涂层结构催化剂的NH₃氧化效率、N₂选择性和副产物生成规律,获得了具有优异NH₃-SCO性能和抗热老化性并具备复合-双层结构的ASC-MD催化剂,新鲜态的平均NH₃氧化效率和N₂选择性分别达85.66%和89.25%,热老化后NH₃氧化效率和N₂选择性的平均劣化量仅为1.59%和7.43%。研究表明,双功能ASC催化剂通过高效耦合PGM催化剂和SCR催化剂,可协同实现高的NH₃氧化效率和高的N₂选择性,但耦合方式对双功能催化剂的NH₃氧化能力和抗热老化性具有决定性的影响。2、建立了“五工况”评价测试方法,采用该方法研究了各催化剂在不同气氛、不同温区的反应路径,提出了单一催化剂和双功能催化剂的NH₃-SCO反应路径,揭示各催化剂在不同温区的NH₃-SCO反应路径。低温区,NH₃-SCO_PGM反应和NH₃-SCO_Inter反应是决定双功能催化剂NH₃氧化效率的关键反应,也是NOx副产物的主要贡献源;高温区,NH₃-SCO反应效率主要取决于SCR_SCR、NH₃-SCO_SCR、NH₃-SCO_PGM反应,其中NOx主要来源于NH₃-SCO_PGM。但双功能催化剂的NOx副产物高低取决于催化剂中SCR反应强度和涂层界面类型。N₂O副产物则主要来源于PGM催化剂相关的反应。3、采用XPS、BET、NH₃-TPD、SEM、EPMA等技术研究了单一催化剂的热老化机理,明确了热老化对单一催化剂的NH₃-SCO反应路径的影响规律。结合双功能催化剂的涂层结构特点,探明了双功能催化剂的热老化机制。研究表明,非活性Cu O生成和酸性位减少是导致SCR-Cu催化剂热老化态性能衰减的关键原因。而PGM-Pt催化剂在热老化过程中会出现Pt颗粒长大和表面迁移富集,从而增强NH₃氧化能力,使得NH₃氧化效率不降反升。双功能催化剂中,涂层中的SCR催化剂性能衰减是导致热老态NH₃氧化效率下降和副产物增加的重要原因。4、按照《重型柴油车污染物排放限值及测量方法》（第六阶段）（GB17691-2018）规定的测试方法和设备,开展了ASC-MD催化剂和商用ASC-P催化剂的整机排放性能研究。结果显示,无论催化剂是新鲜态还是老化态,装配两种催化剂的发动机的NH₃泄露量均小于1ppm,完全满足国六排放要求的10ppm限值,其中ASC-MD催化剂在新鲜态和老化后均显示出比商用ASC-P催化剂更高的NH₃氧化效率和N₂选择性,具有很强的应用前景。更多还原

【Abstract】 Ammonia（NH₃）,as a typical harmful industrial waste gas,has a serious harm to human health and atmospheric environment.The ultra-low emission standard of diesel vehicles within/above the China VI emission legislation requires improving the injection amount of urea aqueous solution in SCR system,so as to achieve higher NOx purification efficiency and meet the ultra-low NOx emission requirement.However,increasing the injection of urea aqueous solution will lead to higher NH₃ slip,which will cause serious air pollution and become a new source of pollution.NH₃ oxidation catalyst（ASC）,which can oxidize NH₃ to N₂,has become a necessary technology for diesel vehicle after-treatment within/above China VI legislation.Due to the poor selectivity of N₂,the application of pure precious metal catalysts（PGM）has been limited.The dual layer structure,coupled SCR and PGM catalyst,can significantly improve the selectivity of N₂.However,it still cannot solve the problem of NH₃ oxidation efficiency under low temperatures.Therefore,based on the different coupling modes of SCR and PGM catalyst,this paper developed various bi-functional ASC catalysts with different coating structures,and systematically studied the NH₃-SCO reaction performance,NH₃-SCO mechanism,thermal aging mechanism and its application on engine dyno.The main results are as follows:1.Pure SCR,pure PGM catalyst and bi-functional catalyst with different coating structure were designed and prepared.The NH₃-SCO performance of fresh and thermal aged catalysts were studied.The oxidation efficiency of NH₃,N₂ selectivity and by-products of catalysts with different coating types and coating structures were determined.ASC-MD catalyst with excellent NH₃-SCO and thermal aged performance that composite-layered structure was developed.The average NH₃ oxidation efficiency and N₂ selectivity of fresh catalyst were 85.66%and 89.25%,respectively.The average degradation of NH₃ oxidation efficiency and N₂ selectivity after thermal aging was only1.59%and 7.43%.The results showed that the high NH₃ oxidation efficiency of PGM catalyst and the high N₂ selectivity of SCR catalyst could be achieved by effective coupling together,but the coupling mode had a decisive influence on its NH₃-SCO performance and thermal aging resistance.2.The"5 mode test cycles"evaluation method was established to study the reaction path of catalysts under different atmosphere and temperatures,and to propose the NH₃-SCO reaction models of catalysts.It is found that the reaction mechanism of ASC catalyst is different under different temperatures.In the low temperature region,NH₃-SCO_PGM and NH₃-SCO_Inter are the key reactions to determine the oxidation efficiency of bi-functionnal catalysts,which is the main contribution of NOx.But the SCR reaction and interface type determine the concentration of NOx by-product of bi-functionnal catalysts.The N₂O mainly comes from PGM related reactions.3.The thermal aging mechanism of pure SCR and pure PGM catalyst were studied by means of XPS,BET,NH₃-TPD,SEM,EPMA,etc,and the effect of thermal aging on the NH₃-SCO reaction pathway was be clarifed.The thermal aging mechanism of bi-functional catalyst was explored based on the analysis of coating structures characteristics.The results showed that the thermal aging resulted in the formation of inactive Cu O and the decrease of acid sites,which lead to the significant degradation of SCR-Cu performance.The growth and surface migration of Pt particles in thermal aging process also enhanced the NH₃ oxidation efficiency of PGM-Pt catalyst.In the bi-functionnal catalyst,the performance degradation of SCR catalyst is an important factor for the decrease of oxidation efficiency and the increase of by-products.4.According to the test methods and equipment specified in the standards of《limits and measurement methods for emissions from diesel fueled heavy-duty vehicles》（China VI）（GB17691-2018）,the emission performance of ASC-MD and commercial ASC-P catalysts were tested.The results showed that the NH₃ leakage of the engine equipped with the two catalysts was less than 1ppm whether the catalyst was fresh or aged,which fully meet the China VI emission standard of 10ppm.ASC-MD catalyst showed higher NH₃ oxidation efficiency and N₂ selectivity than commercial ASC-P catalyst in both fresh and aged state,which has a strong application prospect.更多还原