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Technical Paper

Hydrocarbon DeNOx Catalysis - System Development for Diesel Passenger Cars and Trucks

1 In recent years Diesel DeNOx catalysts using additional hydrocarbons as reducing agents have been the focus of exhaust aftertreatment. The NOx reduction potential was often limited to 20 - 30 % in the European MVEG-A or the US FTP cycle by just adding a DeNOx catalyst on a vehicle. This result is explained by the fact that the catalyst was treated as a separate item and that the emission reduction strategy was not developed in a system approach. This paper summarizes results regarding the potential of state of the art Diesel DeNOx catalysts fitted to passenger cars and trucks when the exhaust gas system is optimized as a whole. The easiest way for a system approach is the combination of DeNOx catalysts with different working temperatures for NOx reduction. This has been demonstrated by the usage of several base metal catalysts for heavy duty applications. For passenger cars Platinum containing catalysts are strongly favored.
Technical Paper

Durability Aspects of NOx Storage Catalysts for Direct Injection Gasoline Vehicles

The introduction of gasoline direct injection technology into the European market will depend mainly on the availability of an effective and durable aftertreatment system, in order to reach future stringent European emission standards. NOx storage technology provides a reasonable chance of fulfilling future emission goals, but durability problems such as thermal degradation and sulfur poisoning have yet to be overcome. The present paper is dedicated to these problems, and demonstrates the progress achieved so far. The influence of different aging modes and aging severity on the NOx conversion efficiency of an advanced generation of NOx storage catalysts, is described in detail. It was found that the severity of aging at comparable catalyst bed temperatures, increases in the following order: hydrothermal aging in N2/H2O < engine aging w/o fuel cut at λ-1 < furnace aging in air < engine aging with fuel cut at λ-1.
Technical Paper

The Impact of High Cell Density Ceramic Substrates and Washcoat Properties on the Catalytic Activity of Three Way Catalysts

The present paper describes the results of a joint development program focussing on a system approach to meet the EURO IV emission standards for an upper class passenger car equipped with a newly developed high displacement gasoline engine. Based on the well known catalyst systems of recent V6- and V8-engines for the EURO III emission standards with a combination of close coupled catalysts and underfloor catalysts, the specific boundary conditions of an engine with an even larger engine displacement had to be considered. These boundary conditions consist of the space requirements in the engine compartment, the power/torque requirements and the cost requirements for the complete aftertreatment system. Theoretical studies and computer modeling showed essential improvements in catalyst performance by introducing thin wall substrates with low thermal inertia as well as high cell densities with increased geometric surface area.
Technical Paper

Development and Application of a Computer Aided Engineering Tool for Hydrocarbon Adsorber Catalysts

To support the application and design of exhaust gas aftertreatment systems for gasoline fueled passenger cars based on hydrocarbon adsorber catalysts, a computer model was developed. This model is based on simplified, lumped kinetics for the adsorption and desorption of hydrocarbons and for the oxidation of CO and hydrocarbons. Also included in the model are convective transport of heat and mass in the gas phase, mass and heat transfer to the washcoat layer, and diffusion with reaction in the washcoat layer. The continuity equations for this model with the appropriate boundary conditions were solved for a single channel assuming adiabatic behavior. After validation of the prediction on experimental results, this model was used to perform a simple parametric study on the influence of inlet temperature,CO concentration, washcoat loading, adsorber content, and cell density on the HC emission.
Technical Paper

Emission Control Systems for Two Stroke Engines - A Challenge for Catalysis=

The exhaust emissions of two stroke vehicles like motorbikes and scooters contribute to the pollution in urban areas of developing countries in South East Asia and India to a major extent. But also in Japan and selected European countries exhaust gas limitations become effective from 10/1998 and 06/1999 for these vehicles. To control this emissions catalytic aftertreatment by Hot Tubes® and/or monolith type catalysts are applied. Due to the constant rich operation of the two-stroke engines, common design criteria for three-way catalysts fail. Extremely high exhaust gas hydrocarbon concentrations lead to high exotherms during oxidation which increases the exhaust gas temperature to a range between 800 and 900 °C. Furthermore the lack of oxygen limits the CO and HC oxidation under certain engine operation conditions. Therefore, water-gas shift and steam reforming reactions play an important part in catalytic aftertreatment of two-stroke exhausts.
Technical Paper

New Low Cost and High Performance Catalyst-Single Layer Pd/Rh Catalyst Development

In order to meet recent and future stringent hydrocarbon emission regulations of passenger cars, the use of Pd-containing catalysts is of growing interest. This is especially true for Pd/Rh and Pt/Pd/Rh catalysts. To optimize the function of the individual precious metals, most high-performance catalysts have a double layer configuration. This double layer avoids undesired interactions between Pd and Rh after reacting with exhaust gas at a high temperature level. Of course, these double layer technologies lead to a more complex capacity utilization coating process during the manufacture of the catalyst. The present work summarizes the results of a research program targeting the development of a high-performance single layer Pd/Rh catalyst technology. The starting point was the functional improvement of Pd and Rh only catalysts then subsequently combining the best of these technologies.
Technical Paper

Development of Close-Coupled Catalyst Systems for European Driving Conditions

The present paper describes the results of a joint development program focussing on a system approach to meet the proposed EURO III and IV emission standards for a passenger car equipped with a 3.2 liter, 18 valve gasoline engine. Starting with the in-production configuration of a EURO II certified vehicle (model year 1997) the following improvement points were investigated in detail. By the introduction of a close-coupled catalyst in combination with engine measures to improve the catalyst light-off the proposed EURO III limits were met. The proposed EURO IV hurdle could be overcome by further using secondary air injection during cold-start in combination with an increased precious metal loading for the close-coupled catalyst.
Technical Paper

Diesel Particulate Emissions of Passenger Cars - New Insights into Structural Changes During the Process of Exhaust Aftertreatment Using Diesel Oxidation Catalysts

Diesel particulate mass emissions and their corresponding size distributions have been investigated on a diesel passenger car at steady state conditions using standard filters and a cascade impactor. These tests have been carried out at two different engine operating conditions (2100 rpm, 2.7 and 13.3 kW, respectively) corresponding to low and high exhaust gas temperatures. Two diesel fuels differing in their sulfur content (150 ppm and 2500 ppm S) have been used for these investigations. The particulate size distribution after diesel oxidation catalyst was found to be affected by the sulfur content of the diesel fuel and by the exhaust gas temperature. Interpretations of these results on a mechanistic basis are given. The diesel particulate emission studies have been extended to dynamic vehicle tests.
Technical Paper

Development of Oxidation and de-NOx Catalyst for High Temperature Exhaust Diesel Trucks

SOF and de-NOx catalysts are applied to heavy-duty diesel trucks which are regulated by European 13 mode or Japanese 13 mode cycles. Precious metal free catalysts can reduce SOF at low temperatures without increasing sulfates up to 670C. This catalyst shows little deterioration after 400 hours of high temperature engine aging. 32% PM and 47% SOF reduction is observed under 13 mode tests when the exhaust gas temperature exceeds 700C (ECE-13 mode). This precious metal free catalyst is suitable for diesel trucks, especially trucks with natural aspirating engine whose exhaust gas temperature is very high. De-NOx catalysts with a 300-500C NOx reduction temperature window are applied to the Japanese heavy-duty test cycle (Japan 13 mode). When secondary diesel fuel is added under modes 8 to 12, (secondary fuel addition only when catalyst inlet temperature is more than 300C), 19-25% NOx can be reduced with 2-4% fuel penalty.
Technical Paper

The Role of Zirconium in Novel Three-Way Catalysts

Zirconium dioxide (zirconia) is a well-known material often being a major component in the washcoat systems of three-way catalysts (TWC) and diesel oxidation catalysts. One important characteristic of zirconia containing washcoats is an improved aging stability which is required to meet the more and more stringent emission standards. In the last few years the utilization of zirconia became even more important - especially for high sophisticated three-way washcoat systems. This was due to the development of high temperature stable oxygen storage components, containing cerium dioxide (ceria) in combination with different other oxides - one very promising candidate being zirconia. In the present work the results of a research program are discussed, focusing on the influence of zirconia in combination with ceria and additional rare earth promoters on the stability of the oxygen storage characteristics.
Technical Paper

Evaluation of NOx Storage Catalysts for Lean Burn Gasoline Fueled Passenger Cars

Engine and laboratory tests were carried out to examine the performance of NOx adsorption catalysts for gasoline lean burn engines in fresh and aged condition. The results show that fresh NOx adsorption catalysts have the potential to meet EURO III emission standards. However, to accomplish these the fuel must contain a low sulfur concentration and the engine must be tuned to optimize the efficiency of the catalyst. After engine or furnace aging upto 750°C the catalyst shows some loss of NOx adsorption efficiency. This deterioration can be offset somewhat by increasing the frequency of lean/rich switching of the engine. Temperatures higher than 750°C may cause an irreversible destruction of the NOx, storage features while the three-way activity of the catalyst remains intact or even may improve. With reference to several physicochemical investigations it is believed that the detrimental effect of catalyst aging is attributed to two different deactivation modes.
Technical Paper

New Developments in Lean NOx Catalysis for Gasoline Fueled Passenger Cars in Europe

There is an increasing interest in running gasoline fueled passenger cars lean of stoichiometric air to fuel (A/F) ratio to improve fuel economy. These types of engines will operate at lean A/F ratios during cruising at partial load and return to stoichiometric or even rich conditions when more power is required. The challenge for the engine and catalyst manufacturer is to develop a system which will combine the high activity rates of a state-of-the-art three way catalyst (TWC) with the ability to reduce nitrogen oxides (NOx) under excess of oxygen. The target is to achieve the future legislation limits (EURO III/IV) in the European Union. Recent developments in automotive pollution control catalysis have shown that the utilization of NOx adsorption materials is a suitable way for reduction of NOx emissions of gasoline fueled lean burn engines.
Technical Paper

Advanced Catalyst Studies of Diesel NOx Reduction for On-Highway Trucks

To date, several non-SCR catalysts and catalytic systems have been suggested for NOX reduction under oxygen rich (lean) conditions, such as those which exist in diesel engine exhaust gas. However, the performance of such catalysts and catalyst systems is not clear when used on actual diesel engines. This paper reports on experimental results obtained when lean NOx catalysts are applied to diesel engine exhaust. Particularly, the catalysts' NOx performance is examined when secondary hydrocarbons are added as reducing agents directly in the exhaust gas stream. In addition, the effect of different catalyst formulations and secondary hydrocarbon addition on particulate emissions is monitored. Finally, partial system optimization is performed and the applicability of such catalysts and systems to engines operating under the US Heavy Duty Transient Cycle is examined.
Technical Paper

Catalytic Reduction of Nox with Hydrocarbons Under Lean Diesel Exhaust Gas Conditions

This paper reports first results of research and development work to achieve Nox reduction under lean diesel exhaust gas conditions by using a special coated, zeolite based monolith catalyst. Much attention is paid to the optimization of the activated zeolite system and the influence of group Ib and VIII elements of the periodic system. A major part of the paper deals with the influence of hydrocarbons, carbon monoxide, sulfur dioxide and water on the activity of the catalyst. Another aspect discussed is the influence of the residence time of the exhaust gas components. The thermal stability and the influence of poisoning elements on the catalyst performance is demonstrated by model gas reactor tests on oven and engine aged samples. Finally, first results on the performance of the catalyst system in a vehicle dynometer test are given.
Technical Paper

A New Generation of Diesel Oxidation Catalysts

An overview is given on the state of the art of a new catalytic exhaust gas aftertreatment device for diesel engines. The function of a precious metal based, flow-through type diesel oxidation catalyst is explained. Much attention is paid to the durability of the diesel oxidation catalyst and especially to the influence of poisoning elements on the catalytic activity. Detailed data on the interaction of poisoning elements such as sulfur, zinc and phosphorus with the catalytic active sites are given. Finally it is demonstrated that it is possible to meet the stringent emission standards for diesel passenger cars in Europe with a new catalyst generation over 80.000 km AMA aging.
Technical Paper

Development of Scavenger-Free Three-Way Automotive Emission Control Catalysts with Reduced Hydrogen Sulfide Formation

Fundamental research work was undertaken to elucidate the mechanism of hydrogen sulfide formation on three-way automotive exhaust catalysts during the lean to rich engine operation sequence and to identify the role of the different catalyst components in this phenomenon. Based upon this knowledge, new catalysts were developed with reduced ability to form hydrogen sulfide by minimizing the storage of sulfur oxides. Engine dynamometer tests confirmed that the suppression of the hydrogen sulfide formation was obtained without loss of catalyst activity or aging stability. The role of the catalyst's age in the hydrogen sulfide formation is discussed. The development presented shows that it is possible to avoid “scavengers” to minimize the emission of hydrogen sulfide from three-way emission control catalysts.