Desempenho Motor

Topics in Catalysis Vol. 28, Nos. 1–4, April 2004 (
2004)

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Lean-burn catalysts from the perspective of a car manufacturer. Early work at Volkswagen Research Axel Ko¨niga,*, Wolfgang Heldb, and Thomas Richterc a

Galgenkamp 13, D-38448 Wolfsburg, Germany Wilhelmstrase 1, D-38448 Wolfsburg, Germany c Advanced Gasoline Engines, Letter Box 1685, Volkswagen AG, D-38436 Wolfsburg, Germany b

Lean burn engines offer significantly better fuel economy and also lower engine out emissions of gaseous pollutants than engines tuned to stoichiometric air-fuel ratio. However, the high oxygen excess in the exhaust prohibits the use of the 3-waycatalyst technology for further NOx reduction. New catalytic processes and catalyst materials had to be developed for NOx aftertreatment, to ensure that diesel engines and lean burn DI gasoline engines can comply with very stringent exhaust emissions standards. The paper describes the start and some aspects of the history of that development, from the viewpoint of a car company. KEY WORDS: nitrogen oxides; NOx catalysis; zeolite catalysts; Cu-ZSM5 catalyst; exhaust after treatment; learn burn engines

1. Background For a car company, exhaust aftertreatment is not an end in itself, because it normally does not add directly additional benefit to its customers. The research for lean NOx aftertreatment processes, beginning in the early eighties was targeted at an indirect benefit for car customers and society: the synthesis of low tail pipe emissions and exceptionally good fuel economy. At that time, the world economy had suffered from two oil crises, but crude oil was again available in unlimited quantities and oil prices had come down again. Smog formation and forest damages by acid rain and ozone were high on the environmental agenda, so the main emphasis in future powerplant development was on exhaust emissions. Nevertheless, people were still aware of the vulnerability of oil supply, and a few people already saw that global warming and CO2 emissions would become a main challenge in the foreseeable future. In the United States and Japan gasoline engines with port fuel injection, stoichiometric engine setting and exhaust aftertreatment with three-way-catalyst systems were standard for passenger cars. In Europe, the vast majority of passenger cars had carburated gasoline engines, tuned to the most fuel efficient setting, that means slightly lean. Discussions on the introduction of catalytic exhaust aftertreatment with three way catalysts in Europe, too, had started, but there was still a strong opposition because of the resulting increase in fuel consumption, and lead free gasoline was not available yet. Passenger cars with diesel engines nearly exclusively * To whom correspondence should be addressed. E-mail: [email protected]

used indirect injection prechamber or swirl chamber engines. Injection systems were mechanical, with very limited variability of injection parameters. First direct injection engines for passenger cars were in development, but had hardly entered the market. The truck market was dominated by DI-diesel propulsion, as is the case also today.

2. The project At Volkswagen Research, where a programme on NOx catalysis under lean exhaust conditions was initiated in 1984, several potential applications had been envisaged: • The most important issue was NOx reduction for diesel engines, because these engines became more and more important for the Volkswagen Group, due to its favourable fuel economy. Swirl chamber engines were still able to meet the existing standards for gaseous exhaust components all over the world without additional exhaust treatment. DI diesel engines fulfilled the NOx requirements with the help of EGR. Further reductions of NOx limits, however, could mean a threat to the future of the diesel engine. • Extension of the lean operating range of three way catalyst aftertreatment systems, to enable lean part load operation of conventional port fuel injection gasoline engines, thus improving their part load efficiency. • NOx reduction for a lean operating DI gasoline engine, which was in preparation as powerplant for the Volkswagen research vehicle ‘‘Futura’’ [1]. 1022-5528/04/0400–0099/0
2004 Plenum Publishing Corporation

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The project started with a literature search and a look to other processes and other industries where NOx was formed and had to be aftertreated. Existing NOx treatment systems were analyzed with respect to their applicability for lean NOx catalysis in vehicle exhaust, especially under the boundary conditions of diesel exhaust. It became obvious that a nonselective catalytic process with noble metal catalysts, like the three way process, did not have any potential for a diesel engine, due to the high oxygen excess even at full load. Selective noncatalytic processes with ammonia, urea or other Ncontaining reducing agents, which were used for NOx control during combustion in furnaces could not be applied because of their high operating temperature and low reaction rate. Therefore, the research work was concentrated from the very beginning on selective catalytic processes, with different reducing agents. The first attempts to find partners from industry for catalyst development and supply and from academia for scientific evaluation of the problem and for catalyst screening on laboratory level did not show much prospect. Suppliers of standard exhaust catalysts, although interested in the topic, were rather sceptical about the success of the project, and the only research institute that claimed to be able to reduce NOx under oxygen excess could do this only on a single platinum crystal under ultrahigh vacuum. In the end, a working group was formed, together with the research division of Bayer AG and the Institute of Fuel Chemistry and Chemical Technology at RWTH Aachen, Prof. Hammer.

3. First results Catalysts for the project were initially taken from three groups, which had already shown potential for SCR with ammonia: titania vanadia catalysts, zeolites

and activated carbon. The highest priority for reducing agents of course was given to the hydrocarbon fuel on board the vehicle or components which could be easily produced from fuel. However, there were no absolute restrictions on reducing agents in the beginning. First tests with titania vanadia catalysts revealed that these catalysts did not show any prospect for SCR with hydrocarbons. Their potential for SCR with ammonia was well known, but ammonia was not seen as a realistic feature on an automobile, because of its high safety risks. Activated carbon, in particular when doped with some base metal catalysts showed some NOx reduction during SCR with hydrocarbons. This was, however, more of interest for mechanistic studies and not so much for a practical application, because of the risk of uncontrolled combustion in a lean exhaust environment. Moreover, pellets of activated carbon were rather brittle, and the application of activated carbon onto monoliths, which are much better suited for use in an automobile, seemed to be rather difficult. The investigations were therefore more and more concentrated on zeolite catalysts. In autumn 1985, after a couple of months of experimental work with different catalysts and reducing agents showing only low NOx conversion rates with hydrocarbons, a breakthrough was achieved. When a copper exchanged mordenite catalyst was heated in a gas stream, containing nitrogen oxide, a hydrocarbon (methane, ethene, propene, or butane), oxygen, and nitrogen as balance, a distinct reduction of nitrogen oxide concentration was detected in a temperature range between 200 and 400 C, accompanied by the formation of CO and a decline of HC concentration. At higher temperatures, both the rate of NO reduction and the concentration of CO in the product gas went down again and carbon dioxide was the only product of hydrocarbon oxidation (figure 1). The NO reduction in

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the respective temperature interval was obviously attributed to a kind of incomplete HC oxidation, and the first idea was that the intermediately produced CO was the reducing agent for NO. However, the addition of CO to the reactant feed in a subsequent experiment did not lead to increased NO reduction. So it became clear that other intermediates of HC oxidation were responsible for the remarkable effect. The first sets of experiments were conducted with zeolite catalysts of the types X, Y and mordenite. Cations exchanged into the catalysts included copper, chromium, iron, manganese, vanadium, cobalt, nickel and silver. The highest NO conversion rates of 66% were achieved with copper exchanged mordenite, in a temperature range of 150–230 C, which seemed to be very favourable for the application on a diesel engine. However, these first experiments were carried out under laboratory conditions that differed considerably from conditions in real exhaust gas. The most severe shortcoming was the absence of water vapor in the reactant feed. When water was added in subsequent experiments, a severe decline of the NO conversion rate on Cu-mordenite was observed. Therefore, the next set of catalysts produced by Bayer for evaluation at RWTH Aachen were of a more hydrophobic nature. Copper exchanged ZSM-5 catalysts showed distinct overall advantages in wet gas mixtures compared to Cumordenite, although the maximum NO conversion rate was somewhat lower and was achieved in a higher temperature interval around 350 C. Figure 2 shows a comparison of the catalyst performance of Cu-mordenite and Cu-ZSM-5 at 350C and different water content in the reactant. It is obvious that water vapor inhibits the NO conversion on Cu-mordenite to a much higher degree, especially in the concentration range around 10% water vapor, which is most relevant for exhaust aftertreatment of lean burn engines. Further tests using ZSM-5 catalysts exchanged with other transition metals (nickel, chromium, cobalt, manganese and iron) gave inferior results in comparison to copper exchanged catalysts. Catalysts exchanged with

noble metals (platinum, rhodium, iridium) showed only low NOx conversion rates in a rather narrow temperature range, accompanied by a distinct formation of NO2. The best results in this set of experiments were achieved on iridium containing catalysts, however, iridium catalysts were not seen as a realistic alternative for automotive application at that time, due to its limited availability, high cost and the comparably high vapor pressure at elevated temperature. With Cu-ZSM-5 catalysts first tests under real engine conditions were carried out. For these experiments the active catalyst material was coated onto conventional cordierite exhaust gas monoliths. For the experiments with a lean burn gasoline engine (1.8l displacement) on an engine test bench, two catalyst converters (4¢¢ diameter, 6¢¢ length) were arranged in parallel in the exhaust system. Engine speed, load and air/fuel ratio were selected to obtain a catalyst temperature of around 400 C and a space velocity of approximately 15,000/h in each converter. The results showed that NO conversion was strongly dependent on the ratio of nitrogen oxide and hydrocarbon concentration in the exhaust (figure 3). For HC concentrations higher than NO concentrations, conversion rates of more than 45% could be achieved. Vehicle tests on a roller test bench with the same engine and a conventional exhaust system with only one Cu–ZSM-5 catalyst brick , using the FTP test procedure gave the following results: For the cold start and warmup phase of the test (bag 1), a NOx conversion rate of only 15% could be achieved, due to the rather long period of time that the catalyst needed to achieve its optimum operating temperature of around 350 C, and the higher space velocity, compared to the tests on the engine test bench. During the hot start phase (bag 3), an overall NOx conversion rate of 30% was found. A general drawback of the principle of lean NOx conversion with hydrocarbons on the engine in use resulted from the emission characteristics of that engine. During acceleration periods, where NOx concentrations in the exhaust went up, hydrocarbon emissions went down, and

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of course exhaust volume and respectively, space velocity on the catalyst went up. This effect limited the potential of the process during the most critical phases of the test. For use on a diesel engine, the low concentration of hydrocarbons in the exhaust means a general limitation. Moreover, due to the normally unthrottled operation of the diesel engine, exhaust gas volume is generally higher than on gasoline engines, and at part load oxygen concentration is high and exhaust gas temperature is low. All these boundary conditions restrict the efficient use of Cu–ZSM-5 catalysts on diesel engines with hydrocarbons as the reductant. Initial tests on an engine test bench with a diesel engine only showed significant NOx conversions when additional hydrocarbons were injected into the exhaust system, and even under these conditions not more than 20% conversion could be achieved. For later tests a diesel engine was modified to obtain higher exhaust gas temperature and higher hydrocarbon concentrations in the raw exhaust gas, at the expense of fuel efficiency. This engine was used in a vehicle for tests on a roller test bench. However, NOx conversion rates above 20% could not be achieved. The same set of catalysts that was used for engine tests, was also investigated in respect to their hydrothermal stability. Monolithic catalyst samples were heated in a furnace for 4 h to 600, 700, 800 and 850 C, respectively, in an oxidizing, wet atmosphere (2% of O2 and 10% water vapor in nitrogen). Subsequently the NOx conversion behaviour was measured in a laboratory device at 350 and 400 C at different air– fuel ratios, characteristic for a lean burn gasoline engine. It turned out that aging at 600 C already increased the optimum operating temperature of the catalyst by 50 to 400 C. Aging at 700 C reduced the maximum obtainable NOx conversion rate to about 25%, and to 10% after aging at 800 C. The catalyst sample aged at 850 C showed no activity at all. Structural analysis of the spent catalysts revealed that the crystallite structure of the catalyst had not been altered during aging, therefore the reason for the thermal deactivitation should result from the status of the copper ion. In the following period of time the work was concentrated on improvements of the catalyst material

to overcome the main shortcomings of temperature window of optimum operation, the comparably low activity and the insufficient aging resistance. The main influencing factors under investigation were Si/Al-module, catalyst loading, and different procedures of catalyst preparation. Moreover, catalysts with an additional content of rare earths (cerium, lanthanium) were investigated on catalyst conversion efficiency and hydrothermal stability. Unfortunately, only limited progress was achieved and it was not possible to exploit the remarkable scientific findings for a product of technical significance. With this in mind it was decided to publish some of the results and, later on, to bring the acquired knowledge into a joint European research project, in order to broaden the work basis. A first presentation of results was given on the occasion of a conference on ‘‘The future of the diesel engine’’ in late 1988, at Wolfsburg, Germany [2] and a second one at the SAE conference in March 1990 at Detroit [3]. At the same time it become obvious that a group at Hokkaido University, led by Prof. Iwamoto, had found similar results at around the same time [4]. A (friendly, but not too serious) dispute arose, about who really had found for the first time that Cu-exchanged catalysts were able to selectively reduce nitrogen oxides under net oxidizing gas conditions. This ‘‘dispute’’ was never really decided.

4. Subsequent work In late 1991 Volkswagen Research started to initiate a project called ‘‘Nitrogen oxide removal from diesel and lean otto engine exhaust (Lea NOx) in the framework of the European Brite EuRam program, which finally started officially in November 1992. Seven European car manufacturers and seven research institutes were involved in the project, and six partners from the catalyst industry were associated to the project for catalyst supply. The project included a broad range of basic investigations, material development and application oriented test work. It must be stated, however, that the outcome from that project and also from subsequent

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similar projects added more to basic knowledge and understanding than to real exploitable results on a technical level. The proprietary work at Volkswagen Research on zeolite catalysts was shifted away from lean NOx conversion with hydrocarbons to two other topics: • Lean NOx conversion on diesel engines with urea, which showed from the very beginning more potential for higher conversion rates under the given boundary conditions, of course at the expense of additional technical and logistic effort during practical use. • Application of zeolite based catalysts for particulate reduction [5]. Later on, when NOx storage catalysts came up [6], the work was focused on these catalysts for application on the newly developed DI gasoline engines with flexible air–fuel ratio, and on diesel engines. The process of direct NOx reduction with hydrocarbons as reducing agent has been applied only to a very limited extent to DI gasoline engines, using iridiumcontaining catalysts [7]. Selective catalytic reduction with urea on titania vanadia catalysts has been developed for serial application, mainly for the use on trucks [8]. The technical potential of zeolite-based catalysts for this purpose still remains to be evaluated. Zeolite components in noble metal oxidation catalysts for particulate conversion are a standard feature today. Their main purpose is the improvement of catalyst cold start behaviour. The progress on injection technology and, above all, on engine and injection control, together with its inherent advantages concerning overall conversion effi-

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ciency gave a strong push to the application of NOx storage catalysts. These catalysts are now in serial use on DI gasoline engines, and they could also be used for diesel engines, provided that low sulfur diesel fuel would be available [8]. The technical effort and the cost for exhaust aftertreatment with NOx storage catalysts is, however, considerable. Therefore, the search for catalyst materials and processes for NOx dissociation or a simple direct NOx reduction with hydrocarbons remains an important challenge.

References [1] K.D. Emmenthal, H.J. Grabe, W. Oppermann and H. Scha¨perto¨ns, Engine with gasoline direct injection and evaporation cooling for the VW research vehicle IRVW-Futura (in german). Motortechnische Zeitschrift 50 (1989), Heft 9. [2] A. Ko¨nig, W. Held, T. Richter and L. Puppe, Catalytic reduction of nitrogen oxides for diesel engines (in german). VDI Berichte, 714, 1988. [3] W. Held, A. Ko¨nig, T. Richter and L. Puppe, Catalytic NOx reduction in net oxidizing exhaust gas SAE 900496. [4] M. Iwamoto, Metal ion-exchanged zeolites as highly active materials for removal of nitrogen monoxide by catalytic decomposition, selective catalytic reduction or adsorption. Proc. of Meeting of Catalytic Technology for Removal of Nitrogen Monoxide, (Tokyo 1990). [5] Volkswagen, A.G. and Bayer, A.G., German Patent DE 4105534 (1991). [6] N. Miyoshi, T. Tanizawa, S. Takeshima, N. Takahashi and K. Kasahara, Development of NOx storage-reduction 3-way catalyst for lean-burn engines Toyota Technical Review, Vol. 44 (1995) [7] Mitsubishi Heavy Ind. Ltd, Japan Patent JP 07060067 (1995). [8] E. Pott, G. Splisteser, R. Bosse, A. Ko¨nig, F.-J. Quissek and I. Kutschera, Potential of NOx-trap catalyst application for DIdiesel engines (in german). 20, Wiener Motorensymposium, (Wien 1999).