Material Aspects and Catalysis in Automotive Catalytic Converters

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Automotive Catalytic Converters

Edited by Hans Bode

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B. Cornils, W. A. Herrmann, R. Schlägl, C.-H. Wong (Eds.) Catalysis from A-Z

ISBN 3-527-29855-X S. Hagen, S. Hawkins Industrial Catalysis ISBN 3-527-29528-3 S. M. Thomas, W. J. Thomas

Principles and Practice of Heterogenous Catalysis ISBN 0-471-29239-X

G. Ertl, H. Knözinger, S. Weitkamp Handbook of Heterogenous Catalysis ISBN 0-471-29212-8

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Deutsche Gesellschaft für Materialkunde e.V.

Material Aspects in Automotive Catalytic Converters

Edited by Hans Bode

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FG Werkstofftechnik Gaußstr. 20 D-42097 Wuppertal Germany

International Congress „Material Aspects in Automotive Catalytic Converters“, held from 03–04 October 2001 in Munich, Germany

Organizer:

DGM · Deutsche Gesellschaft für Materialkunde e.V.

This book was carefully produced. Nevertheless, authors, editor and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. applied for.

A catalogue record for this book is available from the British Library Deutsche Bibliothek Cataloguing-in-Publication Data:

A catalogue record for this book is available from Die Deutsche Bibliothek ISBN 3-527-30491-6

All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

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Bindung: J. Schäffer GmbH + Co. KG, Grünstadt Printed in the Federal Republic of Germany

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Based on increased ecological demands, the car and car-supplying industries strive to meet the challenging requirements for higher performance and extended service life of future vehicle generations. Maintaining good performance is mandatory particularly in the view of thinner supports, higher cell densities and higher temperatures. Performance and service life predictions, based on tests or on modelling and simulation techniques, will depend on reliable materials data.

Only very close cooperation between researchers and producers will help to meet these requirements.

It was the aim of MACC, the second international conference on Materials Aspects in Automotive Catalytic Converters, to foster this cooperation. It refered to papers from both industry and research institutes which concentrate on the high-temperature mechanical and oxidation behaviour of both metal-supported and ceramic supported automotive catalysts. The metal- supported catalyst is based on a ferritic steel with 5–8% aluminum, 17–22% chromium and small additions of reactive elements. More than 11,000,000 units were produced in the year 2000. The ceramic-supported catalytic converter is based on corderite. The production rate is much higher. Both materials have specific advantages and disadvantages which determine the application for a given car model.

In addition to these two basic groups of catalytic carriers, coating and canning aspects were also addressed by the conference programme. Especially the influence of coating thickness and composition is becoming more and more important when going to thinner supports and higher cell densities.

I am very obliged to the authors for their valuable contribution to a comprehensive programme that covers the whole chain of product development and application, beginning with the melting process and ending with recycling aspects.

Munich, October 2001 Prof. Dr.-Ing. Hans Bode Conference Chairman

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Contribution of Automotive Catalytic Converters

R. Searles, Association for Emissions Control by Catalyst, Brussels (B) ...3

II Metals

Development Status of Metal Substrate Catalysts

R. Brück, Emitec GmbH, Lohmar ...19 Materials Issues Relevant to the Development of Future Metal Foil Automotive Catalytic Converters

J. Nicholls, Cranfield University, Cranfield (GB); W. Quadakkers, Forschungszentrum Juelich (D) ...31 High Temperature Corrosion of FeCrAlY / Aluchrom YHf in Environments Relevant to Exhaust Gas Systems

A. Kolb-Telieps, Krupp VDM GmbH, Altena (D); R. Newton, Cranfield University, SIMS, Cranfield (GB); G. Strehl, TU Clausthal, Institut für Allgemeine Metallurgie Clausthal-Zellerfeld (D); D. Naumenko, W. Quadakkers, Forschungszentrum Jülich,

IWV-2, Jülich (D) ...49 Improved High Temperature Oxidation Resistance of REM Added Fe-20%Cr-5%Al Alloy by Pre-Annealing Treatment

K. Fukuda, K. Takao, T. Hoshi, O. Furukimi, Technical Research Laboratories,

Kawasaki Steel Corp., Chiba (Japan) ...59 Oxidation Induced Length Change of Thin Gauge Fe-Cr-Al Alloys

C. Chang, L. Chen, B. Jha, Engineered Materials Solutions, Inc., Attleboro (USA) ...69 Improvement in the Oxidation Resistance of Al-deposited Fe-Cr-Al Foil by Pre-oxidation S. Taniguchi, T. Shibata, Department of Materials Science and Processing,

Graduate School of Engineering, Osaka University, Osaka (J); A. Andoh, Steel and

Technology Development Laboratories, Nisshin Steel, Osaka (J) ...83 Factors Affecting Oxide Growth Rates and Lifetime of FeCrAl Alloys

W. Quadakkers, L. Singheiser, D. Naumenko, Forschungszentrum Jülich (D);

J. Nicholls, J. Wilber, Cranfield University, School of Industrial and Manufacturing

Science, Cranfield (GB) ...93 On Deviations from Parabolic Growth Kinetics in High Temperature Oxidation

G. Borchardt, G. Strehl, Institut für Metallurgie, TU Clausthal (D) ...106

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Effect of Reactive Elements and of Increased Aluminum Contents on the Oxide Scale Formation on Fe-Cr-Al Alloys

V. Kolarik, M. del Mar Juez-Lorenzo, H. Fietzek, Fraunhofer-Institut für Chemische Technologie, Pfinztal (D); A. Kolb-Telieps, H. Hattendorf, R. Hojda, Krupp

VDM GmbH, Werdohl (D) ...117 High Temperature Strength of Metal Foil Materials

M. Cedergren, K. Göransson, R&D, AB Sandvik Steel, Sandviken (S) ...126 Lifetime Predictions of Uncoated Metal-Supported Catalysts via Modeling and Simulation, based on Reliable Material Data

H. Bode, University of Wuppertal (D); C. Guist, BMW AG, Munich (D) ...134 Elastic-Plastic Thermal Stress Analysis for Metal Substrates for Catalytic Converters

S. Konya, A. Kikuchi, Nippon Steel Corporation, Futtsu (J) ...144 A New Type of Metallic Substrate

R. Lylykangas, H. Tuomola, Kemira Metalkat Oy (SF) ...152

III Ceramics

Development Status of Ceramic Supported Catalyst

C. Vogt, E. Ohara, NGK Europe GmbH; M. Makino, NGK Insulators Ltd ...173 Evaluation of In-Service Properties and Life Time of Automotive Catalyst

Support Materials

U. Tröger, M. Lang, Zeuna Stärker GmbH & Co. KG, Augsburg (D) ...186 Loads, Design and Durability Evaluation of Mount Systems for Ceramic Monoliths

G. Wirth, J. Eberspächer GmbH & Co., Esslingen (D) ...191 High Performance Packaging Materials

M. Vermoehlen, D. Merry, S. Schmid, Corning GmbH, Wiesbaden (D) ...202

IV Catalysts

Three-way Catalyst Deactivation Associated With Oil-Derived Poisons

J. Kubsh, Engelhard Corporation, Environmental Technologies Group Iselin (USA) ...217 Catalytic Reduction of NOx in Oxygen-rich Gas Streams, Deactivation of NOx Storage- Raduction Catalysts by Sulfur

C. Sedlmair, K. Sehan, Technische Universität München, Institut für Technische Chemie II, Garching (D); J. Lercher, A. Jentys, University of Twente, Faculty of

Chemical Technology, Enschede (NL) ...223

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Catalytic Reduction of NOx in Oxygen-rich Gas Streams: Progress and Challenges in Catalyst Development

W. Grünert, Lehrstuhl Technische Chemie, Ruhr-Universität Bochum (D) ...229 Atomic Structure of Low-Index CeO2 Surfaces

H. Nörenberg, University of Oxford, Department of Materials, Oxford (GB); J. Harding, University College London, Department of Physics and Astronomy, London (GB);

S. Parker, University of Bath, Department of Chemistry, Bath (GB) ...237 Nanostructured Ceria-Zirconia as an Oxygen Storage Component in 3-way Catalytic

Converters-Thermal Stability

B. Djuricic, Austrian Research Centers, Seibersdorf (A), S. Pickering, Institute for

Advanced Materials, Petten (NL) ...241

V Recycling

Recycling Technology for Metallic Substrates: a Closed Cycle

C. Hensel, Demet Deutsche Edelmetall Recycling AG & Co. KG, Alzenau (D) ...251

VI Miscelleanous

Hot-Corrosion of Metal and Ceramic Honeycombs by Alkaline Metals for NOx

Adsorption

M. Yamanaka, Nippon Steel Technoresearch, Futtsu (J); Y. Okazaki, Nippon Steel,

Toukai, (J) ...263 The Effect of Trace Amounts of Mg in FeCrAl Alloys on the Microstructure of

the Protective Alumina Surface Scales

P. Untoro, M. Dani, National Nuclear Energy Agency, Kawasan PUSPIPTEK, Serpong (Ind); H. Klaar, J. Mayer, Gemeinschaftslabor für Elektronenmikroskopie, RWTH Aachen (D); D. Naumenko, J. Kuo, W. Quadakkers, Institut für Werkstoffe und

Verfahren der Energietechnik (IWV-2), Forschungszentrum Jülich (D) ...271

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*The page numbers refer to the first page of the article A

Adsorption, NOx 263 Al deposition 83 Alkaline metals 263 Alloys 271

Aluchrom YHf 49 Alumina surface 271 Aluminum content 117 Annealing 59

Atomic structure 237

Automotive catalysts 3, 31, 186 C

Catalyst 134 - ceramic 173 - three-way 217, 241 - deactivation 217 - development 19, 229 - support materials 186

Catalytic converters 3, 31, 144, 241 Catalytic reduction 223, 229 CeO2 surfaces 237

Ceramic components 241 Ceramic honeycomb 263 Ceramic monoliths 191 Ceramic supported catalyst 173 Ceria-Zirconia 241

Corrosion 49, 263 D

Data, reliable 134 Deactivation 217, 223 Development 173 - catalysts 19, 229 - converters 31

Durability evaluation 191 E

Elastic thermal stress 144

Elements, reactive 117 Emission limits 152, 202, 251 Environmental protection 251 Exhaust gas systems 49, 152 F

Fe-Cr-Al alloy 59, 69, 83, 93, 117, 271 FeCrAlY/Aluchrom YHf 49

Foil 31, 83, 126 G

Gas flow 152 Gas streams 223, 229 Gas systems 49 Growth kinetics 106 H

High temperature - corrosion 49 - oxidation 59, 106 - strength 126 Honeycomb 263 Hot-corrosion 263 I

Increased aluminum content 117 In-service properties 186 K

Kinetics 106 L

Length change 69 Lifetime 93, 186, 251 Lifetime predictions 134 Loads 191

Low-index CeO2 surfaces 237

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M

Mat, vermiculite 202 Material

- data, reliable 134 - issues 31 - packaging 202 - strength 126 Metal foil 31, 126 Metal honeycomb 263 Metallic substrate 152, 251 Metals, alkaline 263

Metal substrate catalysts 19, 144 Metal-supported catalysts 134 Mg 271

Microstructure 271 Mixed gas flow 152 Modeling 134 Monoliths, ceramic 191 Mount systems 191 N

Nanostructure 241 NOx

- adsorption 263 - reduction 223, 229 - storage 223 O

Oil-derived poisons 217 Oxidation 69, 106 Oxidation resistance 59, 83 Oxide growth rates 93 Oxide scale formation 117 Oxygen-rich gas streams 223, 229 Oxygen storage 241

P

Packaging materials 202

Parabolic growth kinetics 106 Plastic thermal stress 144 Poisons, oil-derived 217 Pre-annealing treatment 59 Pre-oxidation 83

Protective surface 271 R

Reactive elements 117 Recycling technology 251 Reliable material data 134 REM 59

S

Separation process 251 Simulation 134

Storage-reduction catalysts 223 Stress analysis 144

Structure, atomic 237 Substrate 19, 144, 152 Sulfur, NOx reduction 223 Surfaces 237, 271 T

Thermal stability 241 Thermal stress analysis 144 Thin alloys 69

Three-way catalyst 217, 241 Trace amounts, alkaline 271 U

Uncoated metal-supported catalysts 134 V

Vermiculite mat 202

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Robert A Searles

Association for Emissions Control by Catalyst, Brussels, Belgium

1 Abstract

Catalyst-equipped cars were first introduced in the USA in 1974 but only appeared on Euro- pean roads from 1985. In 1993 the European Union set new car emission standards that effec- tively mandated the installation of emission control catalysts on gasoline-fuelled cars. Now more than 300 million of the world’s over 500 million cars and over 85% of all new cars produced worldwide are equipped with autocatalysts. Catalytic converters are also increasingly fitted on heavy-duty vehicles, motorcycles and off-road engines and vehicles. The paper will review the technologies available to meet the exhaust emission regulations for cars, light-duty and heavy-duty vehicles and motorcycles adopted by the European Union for implementation during the new century. This includes low light-off catalysts, more thermally durable catalysts, improved substrate technology, hydrocarbon adsorbers, electrically heated catalysts, DeNOx catalysts and adsorbers, selective catalytic reduction and diesel particulate traps. The challenge is to abate the remaining pollutants emitted while enabling fuel-efficient engine technologies to flourish. This is paramount to the achievement of air quality and greenhouse gas targets given the large increase in the number of vehicles on European roads since 1970 and the projections for further increases in vehicle numbers and greater distances driven each year in future.

2 Introduction

AECC is an international association of European companies making the technologies for automobile exhaust emissions control: autocatalysts, ceramic and metallic substrates, specialty materials incorporated into the catalytic converter and catalyst, adsorber and filter based systems for the control of gaseous and particulate emissions from diesel and other lean burn engines.

2.1 European Emission and Fuel Legislation

The European Union (EU) emission limits for passenger cars set from 1993 have already been lowered from 1996 and again from 2000. For passenger cars and light commercial vehicles the emission standards and fuel composition, including sulfur levels, have been agreed for 2000 and 2005. [1]

New test cycles (ESC and ETC) and tougher emission standards for heavy-duty diesel vehicles have been finalized for 2000 and 2005. The limits for Enhanced Environmentally.Friendly Vehicles (EEV) are set and can serve as a basis for fiscal incentives by EU Member States. A

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further reduction in limit values for nitrogen oxides (NOx) in 2008 is subject to a review by the European Commission in 2002 on technical progress. [2]

The Working Party on Pollution and Energy (GRPE), an expert group of the World Forum for Harmonization of Vehicle Regulations (WP.29) at UN-ECE in Geneva, is developing a Worldwide Harmonized Heavy Duty Certification procedure and is looking in new measurement protocols in order to ensure that ultra fine particles are controlled by future emission legislation to minimize the health effects of diesel particle emissions.

A proposal by the European Commission to set tougher, catalyst-requiring emission limits for motorcycles is being ratified by the European Parliament and Council. Tighter emission limits from 2003 for new types of motorcycles are agreed and correspond to a reduction of 60% for hydrocarbons and carbon monoxide for four-stroke motorcycles, and 70% for hydrocarbons and 30% for carbon monoxide for two-stroke motorcycles. A second stage with new mandatory emission limits for 2006 are expected to be based on the new World Motorcycle Test Cycle (WMTC) which is also being developed by the UN-ECE in Geneva. In the final report of the European Auto Oil II Programme [3], it was concluded that some air quality problems, such as atmospheric levels of particulate matter and ozone, are not yet solved. The challenge is to abate the remaining pollutants emitted while enabling the development of fuel- efficient engine technologies. This is paramount to the achievement of air quality and greenhouse gas targets given the large increase in the number of vehicles on European roads and the projections for further increases in vehicle numbers and greater distances driven each year in future.

2.2 Exhaust Emissions from Internal Combustion Engines Exhaust emissions can be lowered by:

· Reducing engine-out emissions by improving the combustion process and fuel management, or by changes to the type of fuel or its composition

· Retrofitting catalytic converters and associated engine and fuel management systems if they are not original equipment

· Decreasing the time required for the catalytic converter to reach its full efficiency

· Increasing the conversion efficiency of catalysts

· Storing pollutants during the cold start for release when the catalyst is working

· Using catalysts and adsorbers to destroy nitrogen oxides under lean operation

· Using particulate filters with efficient regeneration technology

· Increasing the operating life of autocatalysts and supporting systems.

This paper reviews all the above opportunities, except the first, from the standpoint of material requirements and will also look back into the history of the materials developed for catalytic converters with these requirements in mind.

3 A Brief History of Automotive Catalysts

The first reference to a catalytic converter known to AECC is a patent [4] published to a French chemist, Michel Frenkel, in 1909. The device uses a kaolin (china clay) “honeycomb”

with 30 grams of platinum as the active catalytic material. (Figure 1) The patent describes

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“deodorizing” the exhaust using air blown in by a fan. As far as is known the device was not put into commercial production at the time. No doubt the high loadings of platinum were a deterrent and there was no air pollution concerns in those early, carefree days of motoring.

Figure 1: 1909 Catalytic Converter invented by Michel Frenkel

Figure 2: Eugene Houdry in 1953 with a small prototype catalytic converter

The next report [5] of the concept of catalytic converters was in the 1920s. Another Euro- pean invention, this time German, was taken to General Motors in the US and was described as a collection of wires and beads, again coated with platinum. The tests were at first a success with the device glowing red, but within seconds the catalyst had failed. This was because tetra- ethyl lead had been recently introduced in the US as an octane booster, but was at that time unknown in Europe. Lead poisons catalytic converters.

The French engineer Eugene Houdry can be considered as the father of the modern catalytic converter. Born in France he moved to the US and invented a revolutionary method for cracking low-grade crude to high-octane gasoline – the “cat cracking” process. After the 1939–1945 war he set up the Oxy-Catalyst company and turned his attention to the health risks from the increasing volumes of automobile and industrial exhausts. In 1962, the year of his death, he patented the first modern catalytic converter (Figure 2).

The modern history of the catalytic converter started with the developments that lead to the 1970 US Clean Air Act and the rate of invention has accelerated greatly. Excellent histories of the industry [6, 7, 8] have been published so only a summary will be covered here.

The modern catalytic converter, based on platinum group metals deposited on a ceramic honeycomb base or monolith, was first patented in 1965 [9]. However the industrial use of catalysts was then dominated by catalysts deposited on pellet or bead supports. In the first years after 1974 when catalytic converters were used in the US and Japanese markets, both pellet (Figure 3) and monolithic converters (Figure 4) were used. The loss of catalyst material by attrition in pellet converters was largely overcome by reactor design.

Early prototypes of ceramic honeycombs were made by two approaches:

1. Dipping paper in ceramic slurries, corrugating them and laying up a unitary structure, firing the composite and shaping

2. Calendaring a plastic material containing ceramic powders between grooved and plain rollers, rolling up into a unitary structure, firing the composite and shaping.

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Figure 3: Schematic of pelletized catalytic converter

Figure 4: “Cutaway” ceramic monolith catalytic converter Both of these developments were ultimately replaced by extruded honeycomb substrates.

These are based on cordierite (2MgO .2Al2O3 .5SiO2) and made from natural raw materials and a plastic material that is extruded to form a unitary structure with parallel fine channels and then fired to the final shape. These materials have high thermal shock resistance and high melting and softening points with higher attrition resistance and lower pressure losses than pellet converters, which they ultimately replaced.

In the 1970s new ferritic steels became available that could be made into ultra thin foils, corrugated and then laid up to form a honeycomb structure. One such steel was developed at the Atomic Energy Research Establishment in Harwell, UK for “canning” Uranium 235 and was called Fecralloy. This name reflects the components of the alloy - Iron (Fe), Chromium (Cr), Aluminum (Al) and Yttrium (Y). The formation of a self-healing protective “skin” of alumina (Al₂O₃) allows the ultra-thin steels to withstand the high temperatures and corrosive conditions in auto exhausts. These materials also have high thermal shock resistance and high melting and softening points and facilitated the development of high cell densities with very low pressure losses.

a) b)

Figure 5: a: Metallic substrate converter, b: Ceramic substrate converter

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Further development of metallic and ceramic substrates (Figure 5a & 5b) is described in the next section.

4 Current Catalyst Technology for Emissions Control Autocatalysts Oxidation catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO2) and water and decrease the mass of diesel particulate emissions but have little effect on nitrogen oxides (NOx).

Three-way catalysts (TWC) operate in a closed loop system including a lambda- or oxygen sensor to regulate the air-fuel ratio. The catalyst can then simultaneously oxidize CO and HC to CO2and water while reducing NOx to nitrogen. These simultaneous oxidizing and reducing reactions have the highest efficiency in the small air-to-fuel ratio window around the stoichiometric value, when air and fuel are in chemical balance.

4.1 Fast light off catalysts

The catalytic converter needs to work as fast as possible by decreasing the exhaust temperature required for operation so that untreated exhaust is curtailed at the start of the legislated emissions tests and on short journeys in the real world. Changes to the type and composition of the precious metal catalyst (Figure 6) and to the thermal capacity of substrates (figure 7) have together effected big reductions in the required operating temperature and light off times have been reduced from one to two minutes down to less than 20 seconds. [10]

Figure 6: Effect of catalyst technology on light off temperature

Figure 7: Effect of substrate cell density on light off time

The introduction of the new generation platinum/rhodium (Pt/Rh) technology for current and future emission standards is a technically and commercially attractive alternative for current palladium (Pd) based technologies for high demanding applications in close-coupled and under floor positions using different cold start strategies. [11]

4.2 More thermally durable catalysts

Increased stability at high temperature allows the catalytic converter to be mounted closer to the engine and increases the life of the converter, particularly during demanding driving. Pre-

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cious metal catalysts with stabilized crystallites and washcoat materials that maintain high surface area at temperatures around 1000°C are needed. Improved oxygen storage components stabilize the surface area of the washcoat, maximize the air-fuel “window” for three-way operation and indicate the “health” of the catalytic converter for On Board.Diagnostic (OBD) systems. Figure 8 shows the progress made with mixed cerium and zirconium oxides. [12]

Figure 8: Improvements to thermal stability and oxygen storage capacity (OSC)

4.3 Substrate Technology

The technology of the substrates, on which the active catalyst is supported, has seen great progress. In 1974 ceramic substrates had a density of 200 cells per square inch of cross section (31 cells/square cm.) and a wall thickness of 0.012 inch or 12 mil (0.305 mm). By the end of the 1970’s the cell density had increased through 300 to 400 cpsi and wall thickness had been reduced by 50% to 6 mil.

Now 400, 600, 900 and 1200 cpsi substrates are available and wall thickness can be reduced to 2 mil - almost 0.05 mm (Figure 9). [13, 14, 15, 16, 17]

In the late 1970's substrates derived from ultra thin foils of corrosion resistant steels came onto the market. In the beginning the foils could be made from material only 0.05 mm thick allowing high cell densities to be achieved. Complex internal structures can be developed and today wall thickness is down to 0.025 mm and cell densities of 800, 1000 and 1200 cpsi are available (Figure 10). [18, 19]

This progress in ceramic and metal substrate technology has major benefits. A larger catalyst surface area can be incorporated into a given converter volume and this allows better conversion efficiency and durability. The thin walls reduce thermal capacity and avoid the penalty of increased pressure losses. Alternatively the same performance can be incorporated into a smaller converter volume, making the catalyst easier to fit close to the engine, as cars get more compact.

These improvements in substrate technology are now being applied in conjunction with heavy-duty diesel engines with catalysts placed as close as possible to the engine in order to.reduce the time to light off. To improve conversion behavior, catalysts are placed close to

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the exhaust port before the turbocharger (Figure 11) and close-coupled catalysts using hybrid substrates are fitted (Figure 12). [20]

Figure 9: Progress in ceramic substrate design Figure 10: Progress in metallic substrate design

Figure 11: Pre-turbo catalyst Figure 12: Close-coupled hybrid catalyst

4.4 New Technology for Emissions Control Stoichiometric combustion

Conventional three-way catalysts are continually developed to improve high temperature stability and light off performance and to meet the demands of both the most challenging emission legislation in the world and new applications including motorcycles. Their performance can be further extended by the following additional technologies.

4.4.1 Hydrocarbon adsorbers

Hydrocarbon adsorber systems incorporate special materials, such as zeolites, into or upstream of the catalyst. Hydrocarbon emissions are collected when exhaust temperatures are too low for effective catalyst operation. The hydrocarbons are then desorbed at higher temperatures when the catalyst has reached its operating temperature and is ready to receive and destroy the hydrocarbons. This technology has the potential to reduce hydrocarbons to less than half the levels emitted from a three-way catalytic converter (Figure 13). [21]

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Figure 13: Influence of improved three-way catalyst and hydrocarbon adsorber on emissions (European cycle).

4.4.2 Electrically heated catalyst systems

This uses a small catalyst ahead of the main catalyst. A metallic substrate, onto which the catalyst is deposited, allows an electric current to pass so it will heat up quickly. This brings the catalyst to its full operating temperature in a few seconds. [22]

4.5 Lean Combustion

With the development of lean burn direct injection gasoline engines and increased use of diesel engines, lean combustion is the challenge for automotive catalysis but is essential to reduce fuel consumption and limit CO2 emissions. New diesel technologies with electronic management and direct injection can achieve further fuel consumption improvements.

Conventional three-way catalyst technology used on gasoline engines needs a richer environment with lower air-fuel ratios to reduce NOx so a radical new approach is required. De- NOx catalysts and NOx traps hold out the prospect of substantially reduced emissions of oxides of nitrogen. NOx conversion rates depend on exhaust temperature and availability of reducing agents. There are four systems under evaluation by industry:

1. Passive DeNOx Catalysts using reducing agents available in the exhaust stream 2. Active DeNOx Catalysts using added hydrocarbons as reducing agents 3. NOx traps or adsorbers used in conjunction with a three-way catalyst

4. Selective Catalytic Reduction using a selective reductant, such as ammonia from urea.

Each of these systems offers different possibilities in the level of NOx control possible and the complexity of the system. Fuel parameters such as sulfur content can affect catalyst performance.

4.5.1 DeNOx (or Lean NOx) Catalysts

DeNOx catalysts use advanced structural properties in the catalytic coating to create a rich

"microclimate" where hydrocarbons from the exhaust can reduce the nitrogen oxides to nitrogen, while the overall exhaust remains lean. Further developments focus on increasing the operating temperature range and conversion efficiency.

4.5.2 NOx Adsorbers (or Lean NOx Traps)

NOx traps are a promising development as results show that NOx adsorber systems are less constrained by operational temperatures than DeNOx catalysts. NOx traps adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2) using an oxidation or three-way catalyst mounted close to the

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engine so that NO2can be rapidly stored as nitrate. The function of the NOx storage element can be fulfilled by materials that are able to form sufficiently stable nitrates within the temperature range determined by lean operating points of a direct injection gasoline engine. Thus especially alkaline, alkaline earths and to a certain extent also rare-earth compounds can be used.

When this storage media nears capacity it must be regenerated. This is accomplished in a NOx regeneration step. Unfortunately, alkaline and alkaline earth compounds have a strong affinity for sulfation. As a consequence alkaline and alkaline earth compounds are almost irreversibly poisoned by the sulfur contained in the fuel during the NOx storage operation mode, leading to a decrease in NOx adsorption efficiency during operation.

The stored NOx is released by creating a rich atmosphere with injection of a small amount of fuel. The rich running portion is of short duration and can be accomplished in a number of.ways, but usually includes some combination of intake air throttling, exhaust gas recircula- tion, late ignition timing and post combustion fuel injection.

The released NOx is quickly reduced to N2by reaction with CO (the same reaction that occurs in the three-way catalyst for spark-ignited engines) on a rhodium catalyst site or another precious metal that is also incorporated into this unique single catalyst layer (Figure 14).

Figure 14: NOx adsorber working principle

Under oxygen rich conditions, the thermal dissociation of the alkaline and alkaline earth sulfates would require temperatures above 1000 °C. Such temperatures cannot be achieved under realistic driving conditions. However, it has been demonstrated in various publications [23, 24, 25] that it is in principle possible to decompose the corresponding alkaline earth sulfates under reducing exhaust gas conditions at elevated temperatures. In this way the NOx storage capacity can be restored. The heating of the catalyst, for example by late ignition timing, does however result in a considerable increase in fuel consumption, which is dependent upon the sulfur content. Therefore, reducing the sulfur concentration in the fuel must be regarded as the most effective way of using the full potential of modern direct injection gasoline engines with respect to fuel economy and CO2reduction.

One of the demands for a desulfation strategy must be to avoid any H2S emissions above the odor threshold during desulfation. [26, 27] Developments and optimization of NOx adsorber systems have been and are currently underway for diesel and gasoline engines. These technologies have demonstrated NOx conversion efficiencies ranging from 50 to in excess of 90 percent depending on the operating temperatures and system responsiveness, as well as fuel sulfur content. [28, 29] The system is in production with direct injection gasoline engines.

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4.5.3 Selective Catalytic Reduction (SCR)

SCR is a widespread technology to reduce nitrogen oxide emissions from coal, oil and gas fired power stations, marine vessels and stationary diesel engine applications. SCR technology has been used successfully for more than two decades. SCR technology for heavy-duty diesel vehicles has been developed to the commercialization stage and will be available as an option in the series production of several European truck-manufacturing companies in 2001.

SCR technology permits the NOx reduction reaction to take place in an oxidizing atmosphere. It is called “selective” because the catalytic reduction of NOx with ammonia (NH3) as a reductant occurs preferentially to the oxidation of NH3 with oxygen. Several types of catalyst are used, the choice of which is determined by the temperature of the exhaust environment. For mobile source applications the reductant source is usually urea, which can be rapidly hydro- lyzed to produce ammonia in the exhaust stream.

SCR for heavy-duty vehicles reduces NOx emissions by circa 80%, HC emissions by circa 90% and PM emissions by circa 40% in the EU test cycles, using current diesel fuel (<350 ppm sulfur). [30, 31] Fleet tests with SCR technology show excellent NOx reduction performance over more than 500,000 km of truck operation, and the experience is based on over six million kilometers of accumulated commercial fleet operation. [32, 33, 34]

Though real world durability has been proven, the real challenge for using SCR systems to reduce NOx with heavy-duty diesel vehicles and buses is the development of an infrastructure for the delivery of the preferred reductant; a urea/water solution.

The combination of SCR with a pre-oxidation catalyst, a hydrolysis catalyst and an oxidation catalyst enables higher NOx reduction under low loads and low temperatures. For combination technologies using oxidation catalysts, catalyzed filters or any catalyst formulations including precious metals the use of diesel fuel with sulfur lower than 10 ppm is necessary to keep sulfate particulate formation below future legislated limits. [35, 36, 37]

4.5.4 Diesel particulate filters (DPF)

DPF systems consist of a filter material positioned in the exhaust designed to collect solid and liquid particulate matter (PM) emissions while allowing the exhaust gases to pass through the system. One type of filter material based on a highly porous cordierite monolith with channels blocked at alternate ends is shown in Figure 15.

Figure 15: Wall flow diesel particulate filter Figure 16: Flow-through diesel particulate trap

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Figure 17: Performance of wall flow DPF on PM size and number

A number of filter materials are used, including ceramic monoliths, woven silica fiber coils, ceramic foam, wire mesh and sintered or shaped metals. A new flow-through particulate trap has recently been developed using metal foils (Figure 16). [38]

Collection efficiencies of these various filters range from 30 percent to over 90 percent, but most DPFs achieve over 99% when expressed as numbers of ultra fine particles (Figure 17).

This is very important since health experts believe that it is the fine particulate that is carried deep into the lungs and which is thought to be the most dangerous size of PM. Since the wall flow filter would readily become plugged with particulate material in a short time, it is necessary to “regenerate” the filtration properties of the filter by regularly burning off the collected PM. The most successful methods to initiate and sustain regeneration include:

1. Incorporating a catalytic coating on the DPF to lower the temperature at which particulate matter burns. [39]

2. Using very small quantities of fuel-borne catalyst, such as cerium oxide. The catalyst, when collected on the DPF as an intimate mixture with the particulate, allows the particulate to burn at normal exhaust temperatures to form carbon dioxide and water, while the solid resi- dues of the catalyst are retained on the DPF. [40]

3. Incorporating an oxidation catalyst upstream of the DPF that, as well as operating as a conventional oxidation catalyst, also increases the ratio of NO2to NO in the exhaust. Trapped particulates burn off at normal exhaust temperatures using the powerful oxidative properties of NO2. [41]

4. Electrical heating of the DPF either on or off the vehicle, which would allow simple regeneration but imposing a fuel penalty.

5. DPF systems and intelligent engine-management allow efficient regeneration under all operating conditions. Diesel passenger cars equipped with a DPF in conjunction with fuel- borne catalyst and an oxidation catalyst are now in series production. Recent evaluations indicate a good and durable performance of the system. [42, 43] Continuously regenerating DPFs are very successful in retrofit applications of older heavy-duty diesel vehicles and buses in various regions over the world. Real world durability of these systems is proven every day in major cities in Europe and the US. [44, 45, 39]

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4.5.5 Combined emission control systems

The stringent emission limit values (NOx and PM) for heavy-duty diesel (HDD) engines in 2005 and 2008, the request by the transport sector for minimum fuel consumption of the engine, requiring an optimized combustion process and therefore minimum CO2 and engine-out particulate matter, and the political, public and health concerns on the emission of ultra fine particles into the atmosphere make a combined emission control system an attractive proposi- tion. Urea-based SCR or NOx adsorber systems, in combination with DPFs and an appropriate regeneration strategy will, it is anticipated, be used on the new generation HDD engines and show significant reduction of both NOx and particulate matter. [46, 47, 48]

5 Conclusions

Gaseous and particulate emission controls using catalytic, adsorption and trapping technologies with advanced materials have been developed. The automotive emissions control industry working with its partners in the automotive industry will meet the challenge of future emission regulations. Advanced catalyst and trap systems, with optimized engine management, will aid the achievement of the future low emission standards deemed necessary to meet air quality goals. Retrofitting of catalyst systems and particulate traps is increasing in response to fiscal incentive schemes introduced by governments and to meet the requirements of environmental zones - particularly in cities.

6 References

[1] Official Journal of the European Communities, L 350, Vol. 41, 28 December 1998;

Directives 98/69/EC (emissions) and 98/70/EC (fuels) [2] Directive 1999/96/EC

[3] COM (2000) 626

[4] French Patent FR 402173 and British Patent GB 9364/1909 [5] Scientific American

[6] R. M. Heck and R. J. Farrauto, “Catalytic Air Pollution Control – Commercial Technol- ogy”, Van Nostrand Reinhold, 1995.

[7] M. L. Church, B. J. Cooper and P. L. Wilson, “Catalyst Formulations 1960 to Present”, SAE 890815.

[8] J. R. Mondt, “Cleaner Cars – The History and Technology of Emission Control Since the 1960s”, SAE, 2000.

[9] Engelhard Industries, US 3441381.

[10] R. J. Brisley et al, "The Use of Palladium in Advanced Catalysts", SAE 950259.

[11] J. Schmidt et al, “Utilization of advanced Pt/Rh TWC technologies for advanced gasoline applications with different cold start strategies”, SAE 2001-01-0927, March 2001 [12] J-P. Cuif et al, “(Ce, Zr)O2 Solid Solutions for Three Way Catalysts”, SAE 970463.

[13] S. T. Gulati, "Thin Wall Ceramic Catalyst Supports", SAE 1999-01-0269

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[14] J. Schmitt et al, "The Impact of High Cell Density Ceramic Substrates and Washcoat Properties on the Catalytic Activity of Three Way Catalysts", SAE 1999-01-0272 [15] S. Kikuchi et al, “Technology for Reducing Exhaust Gas Emissions in Zero Emission

Level Vehicles”, SAE 1999-01-0772

[16] S. T. Gulati, "Design Considerations for Advanced Ceramic Catalyst Supports", SAE 2000-01-0493

[17] K. Nishizawa et al, “New Technologies Targeting Zero Emissions for Gasoline En- gines”, SAE 2000-01-0890

[18] R. Brück et al, "The Necessity of Optimizing the Interactions of Advanced Post- Treatment Components in Order to Obtain Compliance with SULEV-Legislation", SAE 1999-01-0770

[19] W. Maus, R. Brück and G. Holy, “Zukünftige Abgasnachbehandlungstechnologien für Otto-Motoren; Die nächste Generation Niedrigstemissionsfahrzeuge”, AVL Congress, Graz, Sept. 1999

[20] M. Reizig, R. Brück, R. Konieczny, P. Treiber, “New approaches to Catalyst Substrate Application for Diesel Engines”, SAE 2001-01-0189, March 2001

[21] N. Noda, A. Takahashi, Y. Shibagaki and H Mizuno, “In-line Hydrocarbon Adsorber for Cold Start Emissions – Part II, SAE 980423, Feb 1998

[22] F. J. Hanel, E. Otto, R. Brück, T. Nagel and N. Bergaul, “Practical Experience with the EHC System in the BMW ALPINA B12”, SAE 970263

[23] W. Strehlau, J. Leyrer, E.S. Lox, T. Kreuzer, M. Hori and M. Hoffmann: “New Devel- opments in Lean NOx Catalysis for Gasoline Fuelled Passenger Cars in Europe”, SAE 962047

[24] M.S. Brogan, R.J. Brisley, A.P. Walker, D.E. Webster, W. Boegner, N.P. Fekete, M.

Kraemer, B. Krutzsch and D. Voigtlaender, “Evaluation of NOx Storage Catalysts as an Effective System for NOx Removal from the Exhaust Gas of Lean burn Gasoline En- gines”, SAE 952490

[25] U. Göbel, T. Kreuzer and E. S. Lox, “Moderne NOx-Adsorber-Technologien, Grundla- gen, Voraussetzungen, Erfahrungen”, Proceedings of the VDA-conference, Frankfurt (1999)

[26] K.-H. Glück, U. Göbel, H. Hahn, J. Höhne, R. Krebs, T. Kreuzer and E. Pott: “Die Ab- gasreinigung der FSI-Motoren von Volkswagen”, MTZ Motortechnische Zeitschrift, 6, 402 (2000)

[27] U. Göbel, J. Höhne, E.S. Lox, W. Müller, A. Okumura, W. Strehlau and M. Hori, “Dura- bility Aspects of NOx-Storage Catalysts for Direct Injection Gasoline Vehicles”, SAE 99FL-103

[28] H. Lüders, P. Stommel and S. Geckler, “Diesel Exhaust Treatment – New Approaches to Ultra Low Emission Diesel Vehicles”, SAE 1999-01-0108, Mar 1999.29 N. Ruzicka and T. Liebscher, “Possible Aftertreatment Concepts for Passenger Car Diesel Engines with Sulphur-free Fuel”, SAE1999-01-1328, Mar 1999

[29] S. Fischer, L. Hofmann and W. Mathes, “The development of the SINOx system for commercial vehicles for serial applications”, 20 th Vienna Motor Symposium, May 6-4 1999, VDI Fortschrittsberichte Reihe 12, Nr. 376, 267–282

[30] B. Amon, S. Fischer, L. Hofmann and J. Zuerbig, “The SINOx system for trucks to fulfil the future emission regulations”, CAPoC 5 Brussels, April 14–16 2000

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[31] Siemens AG and TÜV Automotive, “Investigation on long-term stability of diesel De- NOx catalyst exhaust gas aftertreatment systems on 3 MAN and 10 DaimlerChrysler trucks – results of the 2nd Bavarian Road Test” – Final Report

[32] N. Fritz, W. Mathes, J. Zürbig and R. Mueller, “On-road demonstration of NOx emission control for diesel trucks with SINOx urea SCR system”, SAE paper 1999-01-0111 [33] W. Miller, J. Klein, R. Mueller, W. Doelling and J. Zürbig, “The development of urea-

SCR technology for US Heavy-Duty Trucks”, SAE paper 2000-01-0190

[34] E. Jacob, G. Emmerling, A. Döring, U. Graf, M. Harris, J. van den Tillart and B. Hup- feld, “Reduction of NOx from HD diesel Engines with urea SCR compact systems”, 19th Vienna Motor Symposium, May 3-5 1998, VDI Fortschrittsberichte Reihe 12, Nr. 348, 366–386

[35] E. Jacob and A. Döring, “GD-Kat: Exhaust Treatment System for Simultaneous Carbon Particle Oxidation and NOx Reduction for Euro 4/5 Diesel HD Engines”, 21st Vienna Motor Symposium, May 4–5 2000

[36] J. Gieshoff, A. Schäfer-Sindlinger, P.C. Spurk, J.A.A. van den Tillaart and G. Garr,

“Improved SCR Systems for Heavy Duty Applications”, SAE 2000-01-0189

[37] R. Brück, P. Hirth, M. Reizig, P. Treiber and J. Breuer, “Metal Supported Flow-Through Particulate Trap; a Non-Blocking Solution”, SAE 2001-01-1950.

[38] K. Voss et al, “Engelhard’s DPX catalysed soot filter technology for emissions reduction from Heavy-Duty Diesel engines with passive regeneration”, presentation given by R.

Kakwani, SAE TOPTEC, Gothenburg, September 2000

[39] P. Zelenka et al, "Towards Securing the Particulate Trap Regeneration: A System Com- bining a Sintered Metal Filter and Cerium Addition", SAE 982598

[40] P. N. Hawker et al, "Effect of a Continuously Regenerating Diesel Particulate Filter on Non-Regulated Emissions and Particle Size Distribution", SAE 980189

[41] O. Salvat, P. Marez, G. Belot, “Passenger car serial application of a particulate filter system on a common rail direct injection diesel engine”, SAE 2000-01-0473, March 2000

[42] ADAC and UBA Press Release, 28 August 2001.

[43] R. Allansson, B. J. Cooper, J. E. Thoss, A. Uusimäki , A. P. Walker and J. P. Warren,

“European experience of high mileage durability of continuously regenerating diesel particulate filter technology”, SAE 2000-01-0480, March 2000

[44] T. Lanni, S. Chatterjee, R. Conway, H Windawi, et al, “Performance and durability evaluation of continuously regenerating particulate filters on diesel powered urban buses at NY city transit”, SAE 2001-01- 0511, March 2001

[45] M. Khair, J. Lemaire and S. Fischer, “Achieving Heavy-Duty Diesel NOx/PM levels below the EPA 2002 standards – an integrated solution”, SAE 2000-01-0187

[46] M. Khair, J. Lemaire and S. Fischer, “Integration of EGR, SCR, DPF and fuel-borne catalyst for NOx/PM reduction”, SAE 2000-01-1933

[47] G.R. Chandler, B.J. Cooper, J.P. Harris, J.E. Thoss, A. Uusimäki, A.P. Walker and J.P.

Warren, “An Integrated SCR and Continuously Regenerating Trap System to Meet Fu- ture NOx and PM Legislation”, SAE 2000-01-0188

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Rolf Brück Emitec GmbH, Lohmar

1 Introduction

The need for mobility will increase in future, particularly in the fast-developing nations. The event of this will permit division of labour, i.e. increased productivity, which in turn will bring the necessary increase in national income. (see figure 1).

Figure 1: The relationship between vehicle volume and gross national product

Growing world prosperity will also increase the volume of cars and motorcycles. A dispro- portionately large increase in mobility is therefore to be expected as compared to population growth. With a prospective population growth of 15 percent by the year 2010, this could lead to a 45 percent increase in car numbers and even a 50 percent increase in the volume of two and three wheel vehicles [1]. But this economically necessary development should not come at the expense of the environment. For this reason, emissions limits are being introduced around the world and continually being made stricter. The rapid further development in exhaust gas treatment systems caused by this is supported by new engine technologies, with reduced fuel consumption, low untreated emissions, and better engine control systems. Due to its geo- graphical position and the associated climactic conditions of the Los Angeles valley, which have lead to high concentrations of exhaust contaminants, California has assumed a leading role in legislating for vehicle emissions. The remaining US states, Europe, Japan and other nations are following suit with similar emissions limits.

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Figure 2 shows the development of the Californian emissions legislation for required hydrocarbon (HC) conversion rates. The conversion rates were based on untreated emissions of 2.0 g/mile.

Figure 2: The required conversion rates for the whole FTP test depending on untreated emissions and emissions limits

Figure 3: Current and future propulsion systems

The increase in conversion rates from 96.25 percent for low emission vehicles to 99.5 percent for super ultra low emission vehicles (with reference to the untreated emission mentioned

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above of 2.0 g/mile) only amounts to around 3 percent in absolute terms, but compared to the residual emissions means a 7.5- fold reduction. The aim of all these efforts is to make the car so „clean“ in terms of its limited, contaminant emissions, that compared to environmental immissions, it emits equally low or even lower concentrations. This means that is not polluting the environment, but can even clean it.

As well as improving traditional engines (Otto and Diesel) and catalyst technology, new propulsion concepts such as hybrid vehicles and fuel cell systems are also being discussed in order to reach these goals. However, when one compares the states of development and above all the costs of these systems, it is clear that at least over the next 15–20 years, the „normal“

combustion engine will continue to be the leading propulsion concept.

2 Metal Substrate Catalyst Technologies for Otto Engines

Besides ceramic catalysts, Metallit metal substrate catalysts have also been increasingly used over the last 15 years for exhaust gas treatment in mass-produced vehicles. The substrates comprise thin (0.05 mm–0.02 mm), smooth and corrugated metal foils that are connected to the monolith in a high-temperature brazing process. Figure 4 shows an example of a close-coupled catalyst.

Figure 4: Metal substrate catalyst in a close-coupled arrangement; metal microstructure with catalytic active coating

A large catalytic surface area is required for optimal catalytic effectiveness. Metal substrates are produced today in cell densities of up to 1600 cells/inch 2 (cpsi) and provide a specific geometric surface area (GSA) of up 6.8 m 2 /l (figure 5).

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Figure 5: Geometric surface area (GSA) of uncoated Metalit substrates; influence of cell density

The catalytic reaction of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) only occurs at temperatures above 250 °C, so that for a cold start, before the catalyst has been heated by the exhaust gas, emissions are not converted. To be able to meet the required total conversion rates shown in figure 2, the cold start phase must be reduced to a few seconds.

The heat capacity of the catalyst as an inert mass is the most important factor here. With con- sistent further development of the foil material, it is possible to reduce the foil thickness to 0.02 mm. And it is thus possible to produce catalyst substrates with 1600 cpsi, which have heat capacities 20 % lower than traditional 400 cpsi substrates (figure 6). Clearly improved cold start behaviour can thus be attained.

Figure 6: Heat capacity of coated Metalit substrate; Influence of foil thickness and cell density

The advantages of a high cell density catalyst with respect to HC results for a medium range car are shown in figure 7. The 1600 cpsi catalyst shows an improvement of 50 percent compared to a 600 cpsi catalyst. Uniform flow distribution is necessary to guarantee optimal utilisation of the catalyst volume. Often it is not possible to achieve a good flow in the car, par-

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ticularly in the case of the close- coupled arrangement because of the spatial conditions. Due to the design freedom afforded by metal substrates, it is possible to produce cone-shaped catalysts (figure 8). The cone-shaped channels also guide the flow to the peripheral areas of the second substrate installed behind the „ConiCat^®“, allowing better flow distribution and therefore a cost-optimised use of the catalyst system.

Figure 7: Influence of the substrate‘s cell density on HC emissions in the FTP emission test cycle;

Æ 118 × 150 mm

Figure 8: Cone-shaped catalyst, „ConiCat^®“; improved flow distribution and better space utilisation

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3 Metal Substrate Catalyst Technologies for Diesel Engines

Following the experience of tailor fitted solutions for metal substrates for gasoline engines, research in automotive catalysts was carried out to develop components, which substantially improve emissions control of Diesel engines. As a result of the very efficient combustion process, Diesel engine exhaust gas temperatures are relatively low, especially under real driving conditions at partial load and speed. While for gasoline engines the main emissions control problem is how to reach light-off temperature in the catalyst as quickly as possible after start- ing the engine, Diesel engines have an additional problem. They tend to fall back below light- off temperature during deceleration and low idling modes e.g. in the European driving cycle (figure 9).

Figure 9: Temperature comparison of a Diesel and a spark ignition engine in the European driving cycle Figure 10 shows an engine layout with possible locations for installing catalysts. The closest position to the engine is with the catalyst in front of the turbo charger. This location offers the advantage of higher exhaust temperature compared to fitting it after turbine because this device works as a heat sink during cold start and takes away energy during operation. Further possibilities are the close-coupled position after the exhaust turbocharger and the standard underfloor position.

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Figure 10: Possible catalytic converter positions

3.1 The pre-turbo catalyst

The most important criterion for the design of this type of catalyst is the very small space which is available in the connection of an exhaust manifold to the turbocharger. Most of the pre-turbo catalysts have substrate volumes of distinctly less than 100 cm³. Figure 11 shows the pre-turbo charger catalyst in its fitted position. To show the mode of operation of the pre-turbo charger catalyst, emission tests were carried out on a 3.0 l 6-cylinder Diesel engine in the European test cycle. Figure 12 shows the advantages in the HC conversion rates.

Figure 11: Pre-turbo catalyst

It can be seen that the conversion rates were clearly improved over the whole test cycle. An average improvement of 20 percent was achieved in the first 500 phases of the test. Similar results were also obtained for CO.

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Figure 12: Influence of the pre-turbo catalyst on HC conversion efficiency; 200 cpsi; length: 30 mm

3.2 The hybrid catalyst

The idea is to influence the light-off behaviour of the catalyst by different substrate thermal masses. Using metal foils as the basis for a substrate, it is easy to vary the foil thickness (and thus the thermal mass) for optimisation of the catalyst.

Since the exhaust temperature for Diesel engines always hovers around the light-off temperature (figure 9), it is advantageous to have a low heat capacity in the front part of the catalyst (foil thickness 0.03 mm) to allow quick heating when the gas temperature rises.

By using a thicker foil (0.08 mm) a heat accumulator is fitted in the rear part, so that even when the gas temperature falls below the light-off temperature this part of the catalyst remains catalytically active because of the energy stored in it (figure 13).

Figure 13: Hybrid catalyst

Measurement results – A modern 3 litre 6-cylinder in-line engine with common-rail injection and inter-cooled turbo charging (turbine with variable geometry) on a test bench was used

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for laboratory testing. During all experiments the engine setting was unchanged. Two quick- response analysing systems were installed for modal analysis of the gaseous concentrations in the untreated exhaust and behind the catalytic converter. The exhaust system was original equipment from a medium-sized passenger car. The sequence of engine speed and torque was set according to measured values during EU III test runs on a chassis dynamometer. The hybrid catalysts were in the close-coupled position of the real car instead of the original ceramic cats. The insulation of the exhaust system was also taken from the car. The exhaust mass flow was calculated by measuring the fuel consumption and the mass flow of the combustion air on a modal basis. Two systems with identical coating were compared: the original ceramic close- coupled catalyst (oval shape, diameters 90 mm × 185 mm, length 114 mm, volume 1492 cm 3 , 62 cells/cm 2 , wall thickness 165 m) and the hybrid catalyst (diameter 127 mm, 62 cells/cm², 1. brick length 31 mm with 30 m foil thickness, 2. brick length 70 mm with 80 m foil thickness, volume of the hybrid catalyst 1393 cm³) plus the original underfloor catalysts (volume 762 cm³each, 62 cells/cm ², foil thickness 40 m).

Figure 14 shows the HC conversion rates for the hybrid and the mass-produced catalyst system in the European test cycle.

Figure 14: EU III test results of a hybrid catalyst compared to the standard production system

The hybrid concept shows clear advantages over the standard system, with HC results reduced by 50 percent from 0.16 g/km to 0.08 g/km. CO emissions were even reduced by 65 %.

3.3 Flow through particulate trap

„Wall flow“ particle traps consisting of ceramic substrates with alternately sealed channels have been available for a number of years and have already been installed in mass produced vehicles.

Such a filter can achieve an efficiency of more than 95 % over the total range of particle sizes. In addition to chemical interactions with additives and special coatings, the safe regeneration, i.e. combustion of soot in all sorts of vehicle operation, still cause problems. With excessive amounts of deposited soot the exhaust gas back-pressure increases and during the soot incineration process, temperature peaks develop in the filter leading to mechanical dam-

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age. Therefore, silicon carbide is used in modern applications in view of its high temperature stability.

In order to circumvent the disadvantage of discontinuous regeneration, continuously regenerating filter systems CRT were developed. In such a system particles are incinerated at temperatures above 200 °C via oxidation by NO2. The NO2 is often generated in oxidation catalysts upstream of the particle trap. The exhaust gas temperatures of a Diesel engine, especially in low-load operation, are so low, however, that only an insufficient amount of NO is oxidised to NO2. The oxidation behaviour can be improved by means of pre-turbo charger (3.1) and hybrid catalyst (3.2).

In addition, such a trap would have to be installed close to the engine to guarantee the highest possible exhaust gas temperature. Upstream of the filter an oxidation catalyst which oxi- dises CO and HC and subsequently NO into NO2 has to be installed.

It is well known that soot is deposited at the gas inlet front face of the catalyst in the Diesel exhaust gas pipe which partially reacts with NO2.

It was the task of the flow through trap development to ensure that the filtering efficiency of a catalytic substrate was increased and that deposited particles would not be blown out again by a sudden increase of the mass flow and the resulting aerodynamic forces. The development is based on a mixing catalyst support originally used for the distribution of Urea in SCR systems (Figure 15). The vanes of the mixing section pass part of the exhaust gas of each cell into a neighbouring channel. The construction of a mixing honeycomb substrate is used but the perforated flat foils are replaced by sintermetal foils so that gases can pass from one channel into the neighbouring channel. If a porous flat foil of wire mesh, fibre material or stretch material is used, some of the particles which are in the exhaust gas can be trapped by passing the porous foil.

Figure 15: Flow through particulate trap

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The following four constant load points were driven in order to see the trapping efficiency.

The results are shown in figure 16. A trapping efficiency between 12 % and 31 % can be seen.

Higher efficiencies should be possible by increasing the size of the trap. The results are shown in figure 16.

Figure 16: Particle emissions in different constant load points with and without flow through trap

Table 1: Torque and engine speed of tested constant load points

Engine Speed [rpm] 1200 2 000 3 000 4 000

Torque [Nm] 150 195 194 155

4 Mechanical and thermal loads on catalysts

Metal substrates are used today for a great variety of applications. Figure 17 shows the resulting mechanical and thermal loads.

Using FEM calculations it is possible to test the construction of the catalyst substrate with respect to application-dependent loads. Figure 18 shows the calculated and measured self- resonance of a metal substrate. It can be seen that both the vibration characteristics and the frequency match well. Values calculated in this way can be compared to stimuli from the vehicle.

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Figure 17: Thermal / mechanical load distribution of various applications

Figure 18: FF Analysis of metallic substrates; modelling and results of vibration analysis

5 Conclusions

Driven by stricter emissions limits, the effectiveness of catalyst systems must be increased continually. Metal catalyst substrates offer a variety of solutions for all combustion engine applications:

· Metal substrate technologies are available for both spark ignition and Diesel engines

· All emissions (HC, CO, NOx, PM) can be significantly reduced

· New developments like the cone-shaped catalyst and the open particulate traps help to meet future emissions legislation

· New, high cell density, ultra thin foil substrates further increase catalyst efficiency

· Stress and durability of metal substrates can be precalculated using detailed FEM models before component or engine tests

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Automotive Catalytic Converters

J. R. Nicholls* and W. J. Quadakkers

*Cranfield University, Cranfield, Bedford, UK Forschungszentrum Juelich, Juelich, Germany

1 Abstract

Metal supported automotive catalytic converter bodies are based on ferritic steels, with 5–8wt%Al, 17–22wt%Cr plus small additions of reactive elements. To improve the catalyst performance there is a continued drive towards higher operating temperatures, thinner components and alternative geometries offering large surface area to volume ratios. Maintaining ac- ceptable component lives is mandatory, even when thinner support geometries and higher operating temperatures are envisaged. This has led to a number of materials development strategies, including alternative substrate geometries, modifications to the alloy composition, both through the addition of multiple reactive elements and through the close control of trace element additions, and the development of surface treatments to increase the available aluminium reservoir. Each of these strategies will be reviewed.

Service life predictions, not only relies on suitable materials, but also on the existence of adequate models and simulation techniques, supported by reliable data. In the present paper, the latest thoughts on modelling the oxidative failure of the FeCrAlRE based materials will be presented.

2 Introduction

Following the Kyoto agreement, there has been a concerted and intensified push to lower the worldwide emissions from industrialised power plants [1]. Motor vehicles and the “mobile society” are significant contributors; within the industrialised countries the norm is 400–600 cars per 1000 population, USA is the highest at 800 cars per 1000 population, whilst the less developed countries are much lower (50–100 cars per 1000 population) but aspire towards the industrialised position [2]. Following this scenario, it is likely that there will be 1,500 million vehicles in use worldwide in 2010, thus low emission vehicles are perceived as a major factor in achieving the Kyoto emissions targets. Lowering of hydrocarbon emission from motor vehicles can be achieved in one of two ways, firstly, making the engines more fuel efficient and secondly, cleaning up the hydrocarbon emissions from the exhaust gas. It can be seen therefore that catalytic converters must play a significant role and will be an indispensable part of future '‘clean engine' design.

As a result of this worldwide emissions awareness a demand for new, innovative, efficient catalytic converter technologies has arisen. Ceramic and metal substrate technologies have

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been developed, together with catalytic coatings to provide efficient conversion [3]. Metallic substrates offer improved efficiencies as cell densities can be increased and they produce a lower pressure drop per unit length of converter in the exhaust gas system. This leads to an increased volume converter and therefore further reduction in emissions. Figure 1 illustrates this reduction in total hydrocarbon emissions with both cell density and catalytic converter volume increase. The future direction is obvious, to higher capacity converters with increased cell densities (up to or in excess of 1200 cpsi), whilst maintaining or reducing the exhaust gas pressure drop. This has significant implications on the materials used to design the catalytic converter, particularly the support substrate. Furthermore, the metallic substrate converter has a lower heat capacity and therefore can more rapidly rise to the operating temperature from a cold start. A significant fraction of emissions is produced in this ‘cold start’ phase, particularly for vehicles undertaking short duration journeys.

This paper addresses material issues relevant to the development of future metal foil automotive catalytic converters.

Figure 1: Converter emissions efficiency depends on the converter surface area and cell density [reproduced from reference 2 – hydrocarbon emissions measured in bag 1 of an FTP test after a ‘cold start’ operating condi- tion].