【作者】 张业新；

【导师】 高希彦； 张昭良； 苏庆运；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2012， 博士

【摘要】 由于良好的动力性和经济性,柴油车正赢得越来越多的轻型车市场份额。但是,其排放的四种污染物—包括碳烟(PM),氮氧化物(NOx),一氧化碳(CO)和未燃碳氢化合物(HC)—正损害着自然环境和人类健康,而且不能满足日益严格的排放法规。利用催化后处理技术可以控制柴油车污染物的排放,其中同时控制四种污染物排放的技术称为四效催化技术,所采用的催化剂为四效催化剂。基于Toyota公司提出的DPNR (Diesel Particulate NOx Reduction system)概念,耦合催化碳烟燃烧和NOx存储-还原两类技术研究和发展了一种新型柴油车四效催化剂。该催化剂以Mg-A1水滑石复合氧化物(MgAlO)为载体,以碱金属K和贵金属Pd为活性组分。其中,K催化碳烟燃烧和NOx存储,贵金属催化NOx还原以及CO和HC氧化。在柴油车排放的模拟实验以及台架试验中发现,该催化剂可以实现四种污染物的去除。同时对该催化剂及催化反应进行了详细表征,探讨了碳烟燃烧和NOx存储-还原的机理。本论文的主要工作和发现有：(1)将K负载到Mg-A1水滑石复合氧化物(MgAlO)上,得到K/MgAlO催化剂。结构表征发现,K增加了MgAlO的碱性和表面晶格氧的浓度。当K负载量≤8wt.%时,K高度分散在MgAlO表面,并与载体作用形成新的Lewis碱位,包括：①K取代MgAlO弱碱位OH上的质子形成弱碱性的Mg(Al)-O-K;②K与强碱位表面02-结合形成强碱性的Mg-O-K；③当K负载量≥5wt.%时,存在与载体作用较弱并具有更强碱性的准自由KOx。K增强MgAlO碱性的本质是K的电荷转移到了表面氧负离子上(2)测试了O2气氛中K/MgAlO催化碳烟燃烧的活性,发现K可以显著降低碳烟的起燃温度,并减弱催化剂对碳烟接触方式的依赖。其中,K的最佳负载量低于8wt.%。用离线红外技术发现了碳烟燃烧反应过程中出现的碳氧络合物—烯酮物种。结合其他表征手段,确定了K催化碳烟燃烧的“氧溢流”机理。K位上的表面活性氧溢流到碳烟表面的自由碳位上,形成烯酮物种。烯酮物种与另外的表面活性氧结合生成CO2。在碳烟被消耗的同时,大量的自由碳位被暴露。副产物CO则源于自由碳位与气相O2之间的反应。事实上,溢流的活性氧延展了催化剂的作用范围,弱化了催化剂对碳烟接触方式的依赖。此外,K/MgAlO在催化碳烟燃烧过程中具有相对较高的可重复性,这是因为载体中A1与K之间的作用可以稳定K的存在。(3)测试了在NO+02气氛中K/MgAlO催化碳烟燃烧的活性,发现K同时促进了碳烟燃烧和NOx还原。另外,NOx的引入有利于低温的碳烟燃烧(<300℃),而对高温的碳烟燃烧(>300℃)有所抑制,但伴随有NOx还原。从原位红外实验中发现,反应中存在的两个中间物—烯酮物种和异氰酸根物种,确认除“氧溢流”机理外,碳烟燃烧还存在另外2个反应路径：①NO反应路径：NO与K位上表面活性氧结合形成亚硝酸盐,亚硝酸盐与自由碳位作用形成烯酮物种,烯酮物种进一步被临近的亚硝酸盐氧化为CO2,并生成NO(低温)或N2(高温)；②NO2反应路径：NO2与自由碳位直接作用,生成烯酮物种或异氰酸根物种,后者进一步被O2或NO2氧化为CO2和N2。由于NO,与碳烟的反应受限于自由碳位的数量,而只有碳烟与O2在高温的反应才会产生大量的自由碳位。因此,NOx对低温碳烟燃烧的促进作用有限。此外,NOx吸附K位上形成硝酸盐,对K的催化作用有所钝化。(4)将Pd和K同时负载到MgAlO上得到了Pd-K/MgAlO催化剂。结构表征发现,K+与Pd紧密接触,覆盖了部分Pd位,降低了Pd的分散度。两者相互作用形成了Pd-O-K物种,减弱了Pd的氧化还原特性。尽管如此,这种相互作用使得表面的准自由KOx颗粒分散成了小粒径的颗粒,提高了K的分散度。活性测试发现,Pd-K/MgAlO保持了K/MgAlO的活性,可以同时促进碳烟燃烧和NOx去除。另外,在Pd和K协同作用下,NOx的最大去除效率达45%左右,高于Pd/MgAlO和K/MgAlO的去除效率。这是因为除了部分NOx被碳烟还原外,还有部分NOx发生了分解反应。(5)测试了Pd-K/MgAlO的Nq存储-还原性能。Pd-K/MgAlO表现出优异的NOx存储能力,在300℃时的存储量为890.4μmol/g,高于Pd/MgAlO和K/MgAlO.这是因为Pd和K间的作用提高了K的分散度,提供了更多的K存储位。原位红外表征发现,NOx在Pd-K/MgAlO上的存储存在三个路径：①亚硝酸盐路径：NO吸附在表面K物种上形成了亚硝酸盐；②亚硝酸盐-硝酸盐路径：靠近Pd的K位(即Pd-O-K位)上的亚硝酸盐逐渐被氧化为硝酸盐；③硝酸盐路径：NO2吸附在K位上直接形成了硝酸盐。在Pd的催化作用下,存储的NOx可被H2还原,起始还原温度为250℃。(6)将Pd-K/MgAlO催化剂涂覆到壁流式蜂窝陶瓷上,得到四效催化剂产品。将该产品封装后匹配到柴油发动机上,进行了台架实验。稳态工况下,碳烟去除率在90%以上；排气温度在400℃以上时,可部分去除NO,去除率为6%；排气温度在300℃以上时,CO去除效果明显,去除率最高可达95%左右,但易受碳烟影响；HC去除效果不佳,去除率在10%以内。基于NOx存储-还原机理,在稀燃/浓燃瞬变工况下可以实现对NOx的部分去除,浓燃阶段去除率在20%左右,整体去除率为7.0%,同时CO和HC的去除率分别为81.9%和36.9%。更多还原

【Abstract】 In recent years, the diesel vehicles have achieved a growing share of the light-duty vehicle market due to their high efficiency and low operating costs. However, the emission of their pollutants have caused severe environmental and health problems and cannot meet the demands of the more and more stringent legislation. The four main pollutants from diesel vehicles, including soot particulates, nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbonates (HC), can be simultaneously controlled using aftertreatment catalytic technologies, i.e. four-way catalysis (FWC).On the base of the concept of DPNR (Diesel Particulate NOx Reduction system) proposed by Toyota Corporation, a new four-way catalyst for diesel exhaust was investigated and developed, in which both technologies of catalytic soot combustion and NOx storage and reduction (NSR) were coupled. For the catalyst, alkaline metal K and noble metal Pd were employed as the catalytic components, which were supported by the Mg-Al hydrotalcite-based mixed oxide (MgAlO). The K plays the roles of catalyzing soot combustion and storing NOx, while the noble metal acts as the roles of catalyzing reduction of NOx and oxidation of CO and HC. In the experiments of simulated diesel emission and the bench test of diesel engine, the four pollutants can be reduced under the roles of the catalyst. Furthermore, the catalyst and the catalytic reaction were characterized in details, and thus the reaction mechanism of soot combustion and NOx storage and reduction were expolored.The main works and findings are as following.(1) The K supported MgAlO (K/MgAlO) were obtained and characterized by several techniques. The results show that K was highly dispersed on the surface of MgAlO when the loading amount is below8wt.%. New Lewis basic sites were formed through the interaction between K and MgAlO. Amongst, Mg(Al)-O-K species with the weak basicity were converted from Mg(Al)-OH by the substitution of the proton. While the Mg-O-K species were obtained by the combination with the strongly basic O2-sites. When the loading of K was between5wt.%and8wt.%, the quasi free KOx species, which interact weakly with the support and show stronger basicity than Mg-O-K species, were formed. The increase in the basicity for K/MgAlO can be attributed to the charge transfer from K to the surface oxygen anions, which increased the negative charge of the strongly basic sites.(2) The catalytic activity of K/MgAlO for soot combustion with O2was tested. It was found that presence of K significantly improved soot combustion and depressed the sensitivity to the contact between soot and catalyst. The optimum amount of potassium was below8 wt.%of the supporting amount. Furthermore, a carbon-oxygen complex, ketene group, was observed as a reaction intermediate of soot combustion using ex situ IR. Combined with other characterization, an oxygen spillover mechanism for soot combustion with O2on K supported samples was determined. First, the surface-activated oxygen on K sites spill over to free carbon sites on soot to form the ketene group, which combined with another active oxygen species to give out CO2. Thus, more amount of free carbon sites were exposed, resulting in the depletion of soot. The byproduction CO came from the direct reaction of free carbon sites and gas phase O2. The spillover oxygen may have acted as the spreading of catalysts, which ameliorated loose contact activity. Additionally, the high repeated activity of K/MgAlO was found. This is because the stability of K is greatly improved through the interaction with Al.(3) The catalytic activity of K/MgAlO for soot combustion with NO+O2were also tested. The presence of K improved both soot combustion and NOX reduction. The presence of NOx in O2favors the soot combustion at lower temperature (<300℃). However, a little suppression was observed at higher temperature (>300℃), which was accompanied by a substantial NOx reduction. The reaction intermediates, the ketene group and the isocyanate ions, were observed using the in situ IR technique and thus the reaction mechanism was determined. In the combustion with NO+O2, in addition to the oxygen spillover mechanism mentioned above, two other pathways exist. i) The nitrite route:the NO first combines with surface oxygen on K sites forming nitrites. Then, the nitrites interact with the free carbon sites on soot to produce the ketene group. Finally, the ketene group is further oxidized to CO2by adjacent nitrites, regenerating NO at lower temperatures and/or producing N2at higher temperatures. ii) The NO2route:the NO2forming from NO oxidation directly reacts with the free carbon sites producing the ketene group and isocyanate ion. The latter is further oxidized into N2and CO2by O2or NO2. However, the reactions of NOx with soot are limited by the amount of free carbon sites, which can be provide by the oxidation of soot by O2at higher temperature. Additionally, the formation of nitrates from NOx adsorption might poison the active K sites to a certain extent.(4) The Pd and K co-supported Mg-A1hydrotalcite oxides (Pd-K/MgAlO) were prepared by impregnation method. The results of structure characterization shows that there is a intimated contact between Pd and K+, resulting in that the Pd particles were partly covered by K+and thus the Pd dispersion decreased. A strong chemical interaction exists between K+and Pd, which leads to the formation of Pd-O-K species and suppresses the redox properties of Pd. Furthermore, the interaction disperses the quasi free KOx species into the smaller particle, improving the dispersion of K. The results of activity tests for soot combustion with NO+O2show that Pd-K/MgAlO kept the activity of K/MgAlO for simultaneous catalytic removal of soot and NOx. Due to the existence of the synergism between Pd and K, furthermore, the maximum conversion of NOx reached to45%, which is superior to Pd/MgAlO or K/MgAlO. The higher conversion of NOx during soot combustion is ascribed not only to the reduction of NOx with soot but also to the decomposition of NOx.(5) The activities of Pd-K/MgAlO were tested for NOx storage and reduction. It was found that Pd-K/MgAlO behaves superior capacity for NOx storage, which was evaluated as890.4μmol/g, which is higher than those of both Pd/MgAlO and K/MgAlO. This is because the improvement on K dispersion due to the interaction between Pd and K provides more available K sites to NO storage. Accordingly to results of in situ IR, three pathways were distinguished for NOx storage. ⅰ) The nitrite route:NO is stored on surface K species in the form of nitrites, ⅱ) The nitrite-nitrate route:NO is adsorbed on surface Pd-O-K sites in the form of nitrites, which are progressively transformed into nitrates. ⅲ) The nitrate route:the thermodynamically produced NO2is directly adsorbed to form nitrates. Under the catalytic riles of Pd, the stored NOx on Pd-K/MgAlO can be reduced by H2, in which the initial reduction temperature is250℃.(6) The Pd-K/MgAlO catalyst was coated onto the ceramic honeycomb wall-flow filter and then was packed into a converter. Then, the converter was connected to a diesel engine, and the bench tests were performed to test the activity of the catalyst. Under the static condition, the four pollutants can be eliminated in different degree:ⅰ) the soot was converted by more than90%; ⅱ) the NO was partly converted by about6%when the exhaust temperature was above400℃; ⅲ) the elimination of CO occurred when the exhaust temperature was beyond300℃and the maximum conversion can be reached to95%, but the elimination tent to be suppressed by the presence of soot; iv) the elimination of HC was not significant and the conversion was within10%. In the term of NSR principle, furthermore, the lean/rich transient condition was operated. During the rich stage, the conversion of NO was about20%. In the whole process, the conversion of NO was7.0%while the conversion of CO and HC were81.9%and36.9%, respectively.更多还原