稀燃柴油机尾气净化Pt基催化剂研究

【作者】 桓源峰；

【导师】 孙加林； 胡劲；

【作者基本信息】 昆明理工大学 ， 材料学， 2013， 博士

【摘要】 由于柴油车辆具有良好的燃油经济性、优异的动力性和较低的CO2排放,在全球范围内得到广泛推广和应用。柴油车排放的污染物主要包括固态颗粒物碳烟(Soot)以及气态的CO、HC和NOx等。柴油发动机在稀燃条件下运行,传统的三效催化剂已不能有效发挥催化净化作用；而且柴油机内Soot和NOx的控制存在trade-off(此消彼长)的关系,一般而言柴油机后处理要采用多种技术集成系统,NOx选择性催化还原(SCR)和颗粒捕集(DPF)技术成为重型柴油车尾气净化研究和应用的焦点。相比之下,对污染物主要为Soot和HC化合物的轻型柴油车的技术研究较少。论文针对轻型柴油车尾气排放控制技术的关键问题,如何提高Soot燃烧的低温活性和HC的低温吸附性能,分别以稀土和钛等为主的复合氧化物为载体的Soot燃烧Pt催化剂以及HC吸附分子筛催化剂开展研究,从催化剂的制备、反应性能和结构特征、反应机理及应用等方面进行了深入研究和探讨。论文的主要研究内容如下：论文重点首先针对传统的Soot消除贵金属催化剂成本高易中毒失活的特点,系统研究开发了以稀土(CeO2)、过渡金属(TiO2)和少量贵金属(Pt)活性成分为主的新型soot消除催化剂。(1)通过改变合成过程中沉淀顺序的不同,制备一系列的Y2O3-CeO2-TiO2复合氧化物载体(成分比例固定),并与机械混合制备的载体对比,研究其对应催化剂在NO气氛下对碳烟的消除性能。发现沉积顺序不同,催化剂中Ce02及Ti02的衍射强度差别较大,前者衍射峰强度稍小于后者时,催化剂的热稳定性较好,而前者远小于后者时催化剂的热稳定性较差。三种复合氧化物Y2O3/CeO2/TiO2间过弱及过强的相互作用对催化剂的热稳定性及活性均是不利的。影响催化剂活性的参数按重要度排列为：Ce02后沉积>Y与Ce02组合>催化剂的比表面积。O2-TPD及XPS结果表明催化剂中化学吸附的表面活性氧物种(O2-,0-)的量越多,催化剂在NO气氛下对碳烟的消除能力越强。In Situ DRIFT表征证明双齿硝酸盐、吸附NO2和桥式硝酸盐是反应所需的活性中间物种。(2)共沉淀法制备不同Ce02基的载体CeO2-LnOx(Ln=La,Y,Pr,Nd:LnOx掺杂量为10wt%),研究其在NO下的碳烟消除性能,结果表明：1)稀土氧化物LnOx (Ln=La, Y,Pr, Nd)的添加能抑制Ce02晶粒长大、大幅提高催化剂的比表而积和提高催化剂表而活性氧物种(O2-,O-)的量,但对催化剂中Pt粒子大小的影响很小;2)催化剂的比表面积越大、孔径分布越规则、Ce02晶面越小、表面活性氧物种(O2,O-)的量越多,催化剂在NO气氛下对碳烟的消除能力越强；3)In Situ DRIFT表征和活性结果证明NO储存的中间物种对碳烟消除的活性顺序为：螯合双齿硝酸盐>桥式双齿硝酸盐>单齿硝酸盐,并且中间物种的量越多,催化剂的活性越强。(3)通过引入高比表面积的Al2O3,制备高比表面积的CeO2-TiO2-Al2O3载体。研究Ce、Ti、Al的三者前驱体的沉淀行为,发现载体制备时的沉淀顺序对热稳定性至关重要。进一步研究了铝和铈钛沉淀pH值、晶化温度及模板剂种类等参数的影响,结果显示如合成后的载体XRD谱图只出现单一、较强的TiO2衍射峰,就意味着载体的热稳定性较差,而Ce02和Ti02同时出现较强的衍射峰,则意味着载体的热稳定性较好；另一方面,利用优化合成的高比表面积复合氧化物作为载体制备Pt/CeO2-TiO2-Al2O3催化剂,调控过程合成中的pH研究催化剂体系在NO气氛下对碳烟的消除能力。结果表明催化剂合成过程中的pH值越低,催化剂中贵金属粒子越大、同时催化剂中化学吸附的表面活性氧物种(02-,O-)的量越多,催化剂对碳烟的消除能力越强。In-situ DRIFTs表征证明双齿硝酸盐和吸附N02是反应所需活性中间物种。优化制备的催化剂P3具有优异的抗H2O及抗SO2中毒的性能,具备较好的应用前景。论文其次针对稀燃柴油车工作温度低,氧化型Pt催化剂对HC的氧化性能的不足的特点,进一步开展柴油机冷启动条件下HC吸附材料的应用研究。(1)通过对比研究HY, HZSM-5和Hp三种分子筛的抗热及抗水热性能,发现Hβ抗热及水热老化性能相对好,并对Hβ进行Ce改性研究。优化工艺参数为Ce3+的离子浓度为0.025mol/L、离子交换时的pH为6、离子交换溶液的温度为50℃和交换二次。在此基础上对Ce改性的分子筛材料进行负载贵金属了研究,发现pH调节对催化剂的甲烷催化氧化活性的影响至关重要,而pH值为6制备的催化剂性能最优。进一步对不同分子筛负载贵金属的条件制备的催化剂的碳烟消除性能进行了研究,结果表明,Ce改性样品的性能要远高于未改性样品的性能,并且也是以过程pH调变为6的样品催化剂性能为最佳。催化剂性能提升的原因是由于催化剂中Ce-Pt之间及分子筛Hp与Pt之间的强相互作用使得催化剂中p活性氧物种的量发生改变,氧物种量越多,活性越强。(2)研究改性分子筛、未改性及传统的A1203等三种材料负载贵金属Pt的催化剂的台架活性,发现材料的性质对催化剂的催化活性有较大的影响。由改性的分子筛材料Ce-Hp制备的催化剂的活性最高,空燃比窗口最大,储氧能力也最大。这与贵金属活性组分Pt在外表面的分散度以及氧化还原特性有关,其对CO和THC的起燃温度比由未经改性的Hβ所制备的催化剂的均降低了23℃;相比传统的A1203负载贵金属Pt制备的催化剂的CO和HC的T50分别下降了44℃和42℃。论文最后利用前面研究工作的成果,结合我们在柴油车尾气治理催化剂方面的应用经验,设计制备新型催化剂,并与传统催化剂和国外厂家催化剂在柴油车发动机上进行对比试验。发现传统的催化剂的整体性能不如国外厂家生产的催化剂性能,而新研发的催化剂不仅在CO、HC的氧化方面强于同类型的催化剂,在PM消除方面亦优于国外催化剂。更多还原

【Abstract】 Diesel engine has been widely promotion and application on a global scale because of its good fuel economy, excellent performance and lower CO2emissions. Solid particles (Soot) and CO, HC, NOx are the main pollutant in diesel engine emissions. Because diesel engine is operated in the lean-burn condition, the traditional three-way catalysts (TWCs) could not effectively play a catalytic purification, and the emission control between Soot and NOx has a trade-off relationship in diesel engine, so in general, the after-treatment system of diesel engine need to adopt a variety of technology. NOx selective catalytic reduction (SCR) and particle capture (DPF) become the research and application focus for heavy duty diesel exhaust purification. By contrast, the research about eliminating the light-duty diesel exhaust which the main pollutants are soot and HC compounds is less. For the key problem of light diesel exhaust emission control technology, namely, how to improve the low temperature activity of Soot combustion and HC adsorption.The Pt-based catalysts that rare earth and titanium are respectively the main composition in the carrier for soot combustion and HC adsorption of molecular sieve catalyst were studied in this dissertation. The preparation of catalysts, reaction performance, structure characteristics, reaction mechanism and application were also studied and discussed. The main research content is as follows:Firstly, because the traditional soot removing precious metal catalyst has the characteristics of expensive and easy to poisoning, new soot eliminate catalysts were developed and the active component is mainly rare earth (CeO2), transition metal (TiO2) and small amounts of precious metals (Pt)The aspects of soot eliminate aspects, firstly, the ratio among ternary complex oxides Y2O3-CeO2-TiO2was fixed, through changing the precipitation sequence in the synthesis process to prepare a series of supports to study the soot eliminate performance under NO atmosphere, and the mechanical mixing of supports were also prepared to compare. It is found that the diffraction intensity of CeO2and TiO2vary greatly with different precipitation sequence. The catalysts possess good thermal stability when the intensity of the former slightly less than the latter, while the former is far less than the latter, the thermal stability of catalysts is poor.The too weak or strong interaction among Y2O3-CeO2-TiO2is badly for thermal stability and activity of catalysts. Parameters that affect the catalytic activity arranged by importance is CeO2deposited later> the combination between Y and CeO2> the SBET of catalysts. According to the results of O2-TPD, XPS and activity, It is deduced that the content of chemisorbed surface active oxygen species (O2-, O") is more, the soot eliminate performance under NO atmosphere is better. The results of In-situ DRIFT show that the bidentate nitrates, adsorbed NO2and bridging nitrates are the active intermediate species.Secondly, a series of CeO2-LnOx(Ln=La, Y, Pr, Nd:the loading of LnOx is10wt%) were prepared by co-precipitation, and the soot eliminate performance under NO atmosphere was studied carefully. The results could be summarized:(1) The doping of LnOx could inhibit the grain growth of CeO2, greatly increase specific surface area and the content of chemisorbed surface active oxygen species (O2-,O-), but it has little effect on the Pt dispersion in catalysts;(ii) The greater the surface area is, the more orderly the pore size distribution is, the smaller the grains of CeO2are, and the more surface active oxygen species the catalyst has, then the better eliminating performance under NO atmosphere is;(iii) Combination the results of In-situ DRIFTs and activity, the active intermediate NOx species by importance order is chelating bidentate nitrates>bridging bidentate nitrates> mondenate nitrates, in addition, the more intermediate species the catalyst has, the better the activity is.The synthesis of high specific surface area support CeO2-TiO2-Al2O3has also been studied. Through study sedimentation behavior of the three precursor (Ce, Ti and Al), it is found that the thermal stability of support is depended on the order of precipitation. Furthermore, the effect of pH of Al and Ce-Ti, crystallization temperature, and the type of template are also been studied. The results show that the supports possess poor thermal stability when the strong intensity of TiO2appears only in XRD spectra, however the strong intensity of both TiO2and CeO2implies that the supports possess good thermal stability. Further, a series of catalysts Pt/CeO2-TiO2-Al2O3were prepared using the supports which prepared by optimized method. By changing the pH of Pt loading process, the soot eliminate performance under NO atmosphere was also studied carefully. The results show that the lower pH of Pt loading process, the catalysts possess bigger Pt particle size, larger amount of chemisorbed surface active oxygen species (O2-, O-) and better activity. Combination the results of In-situ DRIFT and catalytic activity, it is concluded that bidentate nitrates and adsorbed NO2are the active intermediate species. Different concentrations of H2O and SO2resistant test were carried out on the P3catalyst, it is found that the performance changed little before and after poisoning, which implies that the stability of catalyst is good and possesses prospect for industrial applications.As the temperature of lean-burn diesel engine was low and the HC performance for oxide supported Pt based catalyst was not good, the further research work about HC adsorption material which used under the cold start condition is an important part of this paper. The conclusion are following:Comparison of the surface area of the three commercial zeolites HY, HZSM-5and Hβ after heat treatment and hydrothermal treatment, it found that the heat resistance and hydrothermal performance of HP are relatively quite good and may play unique advantages in gas purification. The experiments of Ce modified Hβ were also been studied. Combined with the industrial production practice, it ultimately determined that the optimal parameters for Ce3+ion concentration is0.025mol/L, the pH of ion exchange is6, the temperature of ion exchange solution is50℃and the exchange time is2. In addition, after determining the appropriate conditions for the modification of Hβ with Ce, the precious metal load conditions on modified molecular sieve material were studied. It found that the PH adjustment is crucial to the catalytic oxidation activity of methane in the process of loading, of which the pH6is the best. Furthermore, the elimination performance of the catalyst which prepared using different molecular sieve loaded precious metal on soot were been studied, the results show that the performance of Ce modified samples is much better than the performance of the unmodified sample, and the catalyst performance of samples which PH6is the best. The improvement of catalyst performance is due to the strong interaction between the Ce-Pt and zeolite Hp with Pt that makes the amount of β reactive oxygen species in the catalyst changed, the more the oxygen species are, the stronger the activity is.The catalytic activity of three catalysts Pt/modified zeolite, Pt/unmodified zeolite and Pt/Al2O3were evaluated on the engine bench, it found that the nature of materials has a great effect on the catalytic activity. The catalyst prepared by Ce modified Hβ has the greatest activity, the largest air-fuel ratio window and oxygen storage capacity. It is relative with the outer surface dispersion of Pt and the redox performance. Compared with the catalysts prepared by the unmodified Hβ, the ignition temperature of CO and THC of the catalyst prepared by modified zeolite reduces by23℃; moreover, compared with Pt/Al2O3, the T50of CO and HC were reduced by44℃and42℃respectively.In the last parts of study, the monolithic catalysts have been prepared, combined the conclusion of previous research work and the experience in the application of the diesel exhaust catalyst.Compared with the catalyst from foreign manufacturers and the traditional active coating material catalysts which prepared under the same condition, it is found that the overall performances of traditional catalyst are not as good as the catalyst produced by foreign manufacturers, however, new development catalysts not only have better conversion efficiency for CO, HC than the same type catalyst, but also in PM eliminate. Therefore, the company’s technical level had been improved and the company’s brand image had been maintained through the applied base research.更多还原