汽车服务工程专业毕业设计外文翻译-氮氧化物催化技术在汽车尾气中的最新研究

1、 外文翻译专 业 汽 车 服 务 工 程 学 生 姓 名 班 级 B汽车081 学 号 6 指 导 教 师 外文资料名称：氮氧化物催化技术在汽车尾气中的最新研究 外文资料出处： Catalysis Surveys from Japan 3(1999)139-146 附 件： 1.外文资料翻译译文 附 件： 2.外文原文 指导教师评语：签名： 年 月 日 氮氧化物催化技术在汽车尾气中的应用日本尼桑汽车有限公司材料研究中心横须贺237-8523。电子邮件:a-hiroshimail。nissan。co。jp;ken-matsushitamail。nissan。co。jp 季阳译目前我们的开发工作正

2、致力于氮氧化物催化剂在汽油和柴油发动机中的应用，以进一步提高燃油经济性。那时废气温度会降低，同时废气中会有少量烃（碳氢化合物），因此，有必要提高氮氧化物催化剂的活性，使其无论在低温还是高温下都能实现氮氧化物-碳氢化物的反应，利用贵金属催化剂是实现这一目标的有效途径，在低温下碳氢吸附和碳氢催化剂技术的提升是提高催化剂效能的关键技术。关键词：氮氧化物催化剂，沸石催化剂，铂，碳氢吸附，碳氢催化技术的提升1. 导论现在人们强烈要求提高汽车的燃油经济性以减少二氧化碳的排放，为此出现了快速扩散燃烧和直喷式汽油发动机。在直喷式柴油发动机已成为主流情况下，可以采用高压燃油喷射技术进一步提高燃油经济性。图1显示

3、了欧洲和日本现有和建议的汽油和柴油轿车的氮氧化物排放标准。这些标准将逐步加强人们对环境的重视，正因为如此，迫切需要能大量应用在稀薄燃烧发动机的高转换效率的催化剂。 由于标准十分严格，催化剂的研究有很多的障碍。现在有些氮氧化物分解技术和氮氧化物催化技术已应用于一些稀薄燃烧发动机。参照汽油引擎1,2,汽油发动机的废气中含有很多不饱和碳氢化物，但柴油发动机没有有效的控制技术来实现这一目标，它全靠较高的空燃比条件。 汽油发动机有很多不饱和碳氢化物，他们有利于降低氮氧化物的排放，另一方面，柴油发动机排气含有较多的重碳氢化物，随着二氧化硫引起的催化剂失效，他们难以用作还原剂。这些条件阻碍了催化剂在实际中的

4、应用。要求降低排气温度以达到低水平碳氢化物剩余，然而这种要求在未来汽油和柴油发动机的高效率燃烧中是很难实现的。因此，如果开发出一种高效氮氧化物催化剂，它可以适用于这种情况下，废气的清洁将获得进一步的提升，同时保证了燃油经济性。本文介绍催化剂技术在实际生产中必须解决的问题和发展状况。图1.目前和建议的汽油和柴油轿车的氮氧化物排放标准2 排放特性图2显示了欧洲经委会（欧州经济委员会）和EUDC(市区外驾驶循环)热模式在直喷式柴油发动机的碳氢化物和氮氧化物的排放排气温度的研究结果。发动机是四缸2.5升直喷式内联涡轮增压。排气温度在涡轮增压器出口测量。大部分氮氧化物时在加速过程中产生的，此时碳氢含量不

5、高，这意味着碳氢化物与氮氧化物之比较低的条件下该催化剂活性很高。 图2表1为柴油机排气的特点总结，目前柴油发动机和汽油之间差异相当明显。然而，如果将来汽油发动机的燃烧效率再提高那么他们的排气特性会越来越像柴油机的。众所周知，排气温度和环境碳氢化物和碳氢化物之比时影响氮氧化物催化剂的最重要的因素。图3显示了在不完全燃烧汽油发动机台架测试氮氧化物和碳氢化物之比对催化剂的影响。这个实验是测试起燃催化剂的特性。图中显示的是催化剂在排气温度350至400摄氏度，碳氢化物和氮氧化物之比小于2，此时氮氧化物转化率很低。 图3 为碳氢化物和氮氧化物之比和氮氧化物转换率间的关系。汽车发动机（燃）测试：2升，直6

6、缸发动机，空燃比=1922 : 1，频率=约30000次每小时。要未来的引擎实现较高的燃烧效率就要求排气温度较低，这需要在排气管中碳氢含量较低的情况下制定高效的碳氢氮氧化物催化剂。3. 催化剂的类型选择图4显示了催化剂在最大活性温度下的氮氧化物转化水平，这些数据是在过去几年日本催化学会（CATSJ）和日本化学学会（电子）的年度会议上编订的。 图5.氮氧化物转换的三种催化剂。柴油引擎（起燃）测试：2升柴油，平均碳氢和硝之比等于1，频率为35000次每小时。 实验的每次实验条件不一样因此很难准确的进行。然而，它可能催化剂有三组：贵金属（铂类特殊催化剂），金属离子交换沸石型（铜离子交换分子筛）和碱金

7、属氧化物型。图5显示了在柴油发动机台架上进行的从三个类别和评价氮氧化物催化剂性能。虽然在入口气体中平均碳氢和硝之比较低，其大小大约为1，铂催化剂和沸石催化剂的转换率相对较高。这两种技术是发展高效氮氧化物催化剂的关键。碱金属氧化物型要求在超过450摄氏度的高温下才可以实现高效转化，但实际应用中由于汽车尾气排放温度要求越来越低，所以这类催化剂不可能。表2总结了这三种类型催化剂的特点。贵金属催化剂，尤其是其是铂催化剂在低温区域具有较好的氮氧化物转化性能。不过，正如图6所示，众所周知铂催化剂产生的一氧化二氮是一种典型的温室气体。所以在使用铂催化剂时必须采用一些手段抑制来二氧化亚氮形成。4. 氮氧化物催

8、化剂的发展战略 表3显示了我们催化剂开发的战略。通过提高二氧化氮和碳氢化合物的反应以满足排气温度的要求，提高碳氢化物的利用率还有增大催化剂表面的碳氢和硝之比以降低废气中碳氢化物含量。 岩本等人提出了一种在中间添加还原剂的方法，在保证二氧化氮含量和碳氢化物的利用条件下提高催化剂的效能。这方法是在较低温度有效利用贵金属的改进催化剂的活性。有效利用碳氢吸附和碳氢催化技术的提升是改善催化剂的氮氧化物碳氢反应的关键因素。我们以前报告说，使用沸石碳氢吸附剂可以有效的增加在催化剂表面碳氢和氮氧化物之比36 , 37，其他研究人员 38 , 39 的方法都直接或间接要求适合还原剂反应的废气流来增加碳氢和氮氧化

9、物的比例，但这些方法都牺牲了燃油经济性。5总结 进一步提高燃烧的效率会导致在未来的排气系统中碳氢含量很少，因此，要加强催化剂在较低温度下的活性，以更有效地利用碳氧化合物作为氮氧化物还原剂。利用贵金属是提高低温下的催化活性区有效的方法，但是，使用贵金属（铂）催化剂时一定要抑制一氧化二氮形成。在这项工作中，氮氧化物催化剂可以用一种烃类物质来代替，它可以增加碳氢化物对氮氧化物的还原作用。为了氮氧化物催化剂可以在实际中加以应用出了要进一步改进催化剂系统描述的那些还要提高催化剂的活性和耐久性。Catalysis Surveys from Japan 3 (1999) 139146Recent lean

10、NOx catalyst technologies for automobile exhaust control Hiroshi Akama and Kenjiro MatsushitaMaterials Research Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka 237-8523, Japan,E-mail:a-hiroshimail.nissan.co.jp;ken-matsushitamail.nissan.co.jpThe current status

11、of our development work on lean NOx catalysts for application to future gasoline and diesel engines is described. As a result of further improvements in fuel economy, the temperature of exhaust gas will be lower and there will be smaller quantities of hydrocarbons (HCs) in the exhaust of future engi

12、nes. Therefore, it is necessary to improve the activity of lean NOx catalysts at lower temperatures and achieve higher selectivity of the NOxHC reaction. Utilizing precious metal catalysts is one effective way of improving catalyst activity at lower temperatures, and HC adsorption and reforming are

13、two key technologies for improving the catalyst selectivity for the NOxHC reaction.Keywords: lean NOx catalyst, zeolite, platinum catalyst, HC adsorption, HC reforming1. IntroductionImproving the fuel economy of automobile engines is strongly demanded today for reducing CO2 emissions. For this reaso

14、n, there has been a rapid diffusion of lean-burn and direct-injection gasoline engines. Among diesel engines as well, the direct-injection type has become the mainstream,and further improvement of fuel economy has been achieved by applying high-pressure fuel injection technology. Figure 1 shows the

15、current and proposed NOx emission standards for gasoline and diesel passenger cars in Europe and Japan. These emission standards will be progressively tightened with an eye toward resolving environmental concerns. Due to this regulatory trend, there has been a tendencyto expand the lean-burn operati

16、ng region of engines and higher conversion efficiency is being demanded of catalysts. Tighter standards pose a particularly high hurdle for diesel engines. NOx trap and lean NOx catalyst technologies has been applied to some lean-burn and direct-injection Figure 1. Current and proposed NOx emission

17、standards for gasoline and diesel passenger cars. gasoline engines 1,2, but no effective NOx control technology has been implemented yet for diesel engines that operate under a full lean condition with higher A/F ratios. The exhaust temperature of a diesel engine is lower, andthere are more little H

18、Cs remaining in the exhaust compared with a gasoline engine.There are many C3 to C5 nonsaturated HCs which are advantageous for reducing NOx emissions in gasoline engine exhaust. On the other hand, diesel engine exhaust contains heavy HCs which are harder to use as a NOx reducing agent, along with S

19、O2 that causes catalytic deactivation. These conditions have thwarted practical application of HC-SCR technology to diesel engines.However, the tendency toward lower exhaust gas temperatures and a lower level of residual HCs will result in the same typical exhaust gas conditions for future gasoline

20、and diesel engines that achieve high fuel combustion efficiency. Therefore, if high-performance lean NOx catalysts are developed which can be applied to such conditions, cleaner exhaust will be obtained, leading to further improvements in fuel economyThis paper describes the current development stat

21、us of our HC-SCR technologies and discusses the problems that must be addressed to apply these technologies to production vehicles. 2. Exhaust characteristicsFigure 2 shows profiles of the HC and NOx emissions and exhaust temperature of the direct-injection diesel engine used in this work under the

22、ECE (Economic Commission for Europe)+EUDC (Extra-Urban Driving Cycle) hot mode. The engine was a 2.5-liter inline 4-cylinder directinjection turbodiesel. The exhaust temperature was measured at the turbocharger outlet. A high level of NOx was produced during acceleration, although the HC level was n

23、ot so high. This means that the ambient HC/NOx ratio is low under the conditions where the NOx reduction activity of the catalyst is most needed.The characteristics of diesel exhaust are summarized in table 1. The differences in exhaust characteristics between gasoline and diesel engines are quite s

24、triking at present. However, if the fuel combustion efficiency of gasoline engines is further improved in the future, their exhaust characteristics will steadily come to resemble those of diesel engines.It is well known that the exhaust temperature and ambient HC/NOx ratio are the most important fac

25、tors affecting the performance of lean NOx catalysts. Figure 3 shows the relationship between the ambient HC/NOx ratio and Nox conversion performance of the Cu-MFI catalyst, a typical lean NOx catalyst, in a lean-burn gasoline engine bench test. This test was conducted to investigate the light-off c

26、haracteristics of the catalyst. The plots in the figure are for catalyst inlet exhaust temperatures of 350 and 400 C. Under an ambient HC/NOx ratio of less than 2, an extremely low level of NOx conversion was obtained.In view of the lower exhaust temperature expected for future engines that achieve

27、high fuel combustion efficiency it will be necessary to develop high-performance lean NOx catalysts compatible with the low HC level in the exhaust. 3. Selection of catalyst type Figure 4 shows a map of the NOx conversion level of lean NOx catalysts as a function of the maximum activity temperature.

28、 These data were compiled from reports presented at the annual conferences of the Catalysis Society of Japan (CATSJ) and the Chemical Society of Japan (CSJ) inthe past several years 334.The experimental conditions of each catalyst in the map differ, making it difficult to compare their activities ac

29、curately. However, it is possible to classify the catalysts into three groups: the precious metal type (supported platinum catalyst in particular), the metal ion exchange zeolite type (copper ion exchange MFI zeolite in particular), and the base metal oxide type.The NOx reduction performance of a ty

30、pical catalyst selected from each of the three categories and evaluated in diesel engine bench tests is shown in figure 5. Although the average HC/NOx ratio of the catalyst inlet gas in the activ ity test was at a low level of approximately 1.0, the supported platinum catalyst and the Cu-zeolite cat

31、alyst showed comparatively high conversion rates.The use of these two kinds of catalysts is included among the key technologies for developing high-performance lean NOx catalysts. The base metal oxide type shows the highest NOx conversion at high temperatures over 450 C, but it is assumed that this

32、type of catalyst will not likely find practical application for automobile exhaust control due to the trend toward lower exhaust gas temperatures.The characteristics of these three types of catalysts are summarized in table 2. The precious metal catalyst, es pecially the platinum catalyst, shows bet

33、ter NOx conversion performance in low temperature regions. However, as shown in figure 6, it is well known that the platinum catalyst produces N2O, which is a typical greenhouse gas. Some means of suppressing N2O formation is necessary when the platinum catalyst is used. 4. Development strategy for

34、lean NOx catalystsTable 3 shows our strategy for catalyst development. Effective ways of coping with the trend toward a lower exhaust temperature are to utilize the high reactivity of NO2 and to improve the reactivity of HCs. Improving the rate of HC utilization and increasing the HC/NOx ratio on th

35、e catalyst surface are effective measures for dealing with the lower HC content in the exhaust gas.Iwamoto et al. proposed the intermediate addition of reductant (IAR) method 35 as a way of using the high reactivity of NO2 and improving the HC utilization rate. This method is effective in utilizing

36、the precious metal to improve catalyst activity at lower temperatures. Effective use of the HC adsorption/reforming function is a key factor in improving the selectivity of catalysts for the NOxHC reaction. We have previously reported that the use of zeolite as an HC adsorbent is effective in increa

37、sing the HC/NOx ratio on the catalyst surface 36,37. Other researchers 38,39 have suggested approaches involving the direct or indirect supply of a suitable reductant to the exhaust stream so as to increase the ambient HC/NOx ratio, but these methods sacrifice fuel economy.5. SummaryFurther improvem

38、ent of fuel efficiency will result in a smaller quantity of HCs in the exhaust of future engines. Therefore, catalyst activity at lower temperatures will have to be improved to make more effective use of HCs as Nox reductants. Utilization of precious metals is effective in improving catalytic activity in the lower temperature region, but it is necessary to suppress N2O formation when a precious metal (platinum) catalyst is used. In this work, lean NOx catalyst performance was improved markedly by using a hydrocarbon-reforming catalyst, which increased the rate of HC utilization as No