Platinum Metals Rev., 2002, 46, (1), 27
Exhaust Emission Catalyst Technology
New Challenges and Opportunities in Europe
- By Dirk Bosteels
- Robert A. Searles
- Association for Emissions Control by Catalyst, Av. de Tervueren 100, B-1040 Brussels, Belgium, www.aecc.be
New technologies, incorporating the platinum group metals, are available to meet the exhaust emission regulations for cars, light-duty and heavy-duty vehicles and motorcycles being adopted by the European Union for implementation during the new century. These technologies include 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 filters. This large range of technologies will allow exhaust emissions from all engines, both on- and non-road, to be lowered to unprecedented levels. This paper examines the state of emission control technologies currently available for all types of engine.
Catalyst-equipped cars were first introduced in the U.S.A. in 1974 (1) but only appeared on European roads in 1985. Indeed, it was not until 1993 that the European Union (EU) set new car emission standards that effectively mandated the installation of emission control catalysts on gasoline-fuelled cars. Worldwide, now more than 275 million of the world's 500 million cars and over 85 per cent of all new cars produced are equipped with autocatalysts.
In the EU directives are being adopted to legislate the maximum exhaust emissions permitted from a wide range of vehicles and equipment powered by the internal combustion engine. exhaust emission catalyst technology is increasingly being fitted on heavy-duty vehicles, buses, motorcycles and on non-road engines and vehicles. The 13 candidate Countries currently negotiating entry to the EU (including Poland, Hungary and the Czech Republic) will, as a result of adopting the limit values of the EU, increase the application of the technologies developed to control emissions in the EU.
European Union exhaust Emission and Fuel Legislation
The final report of the European auto Oil II programme (2) concluded that some air quality problems, such as atmospheric levels of particulate matter (PM) and ozone, are not yet solved. The challenge is to abate the remaining pollutants emitted while enabling use of fuel-efficient engine technologies. This is paramount for achieving good air quality and the targets for greenhouse gas reductions given the large increase in the number of vehicles on European roads, the projections for further increases in vehicle numbers and the greater distances driven each year.
Emissions from On-Road Vehicles
The EU emission limits for passenger cars that came into effect from 1993 were lowered in 1996 and again in 2000. For passenger cars (Tables I and II) and light commercial vehicles the emission standards have been agreed for 2000 and 2005 (3). Light commercial vehicle limit values are adjusted to compensate for the higher vehicle weights.
|Year||CO||HC||NOx||HC + NOx|
|1996/97(*)||2.2 (2.7)||-(0.341)||- (0.252)||0.5|
|Year||CO||HC + NOx||NOx||PM|
Heavy-duty diesel (HDD) vehicles have new test cycles and tougher emission standards finalised for 2000 and 200.5. The limit values for enhanced environmentally friendly vehicles (EEVs) are set to serve as a basis for fiscal incentives by EU Member States. The 200.5 (Euro IV) emission standards set limit values for carbon monoxide (CO) (1.5 g/kWh), hydrocarbons (HCs) (0.46 g/kWh) and nitrogen oxides (NOx) (3.5 g/kWh) and also PM limit values intended to force the use of particulate traps/diesel particulate filters (DPFs). The PM limits are 0.02 g/kWh on the steady state cycle (ESC) and 0.03 g/kWh on the transient cycle (ETC). The new limit values are a 30 per cent reduction in CO, HC and NOx and an 80 per cent reduction in PM from Euro ΠΙ limit values of October 2000.
In 2008 (Euro V), the NOx limit of 2.0 g/kWh reflects the need for DeNOx or Selective Catalytic Reduction (SCR) catalysts. The limit is subject to a European Commission study, which will report by the end of 2002, on technical progress of the required emission control technologies (4).
The Working Party on pollution and Energy (GRPE), an expert group of the World Forum for Harmonization of Vehicle regulations’ (WP.29) at the United Nations Economic Commission for Europe (UNECE) in Geneva, is developing a worldwide heavy-duty certification procedure (WHDC) and is looking at new measurement protocols in order to ensure that ultra fine particles are controlled by future emission legislation to minimise the health effects of diesel particle emissions.
Another current action is a proposal by the European Commission to set tougher, catalyst-requiring emission limits for motorcycles. This 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 per cent for HCs and CO for four-stroke motorcycles, and 70 per cent for HCs and 30 per cent for CO for two-stroke motorcycles. A second stage with new mandatory emission limits for 2006 is proposed, to be based on the new worldwide motorcycle emission test cycle (WMTC), which is also being developed by the UNECE in Geneva.
Emissions from Engines Used in Non-Road applications
The process of bringing emission limit values and fuel quality in non-road applications in line with those of on-road vehicles has begun. Technology adopted and proven in on-road applications will in time be transferred to non-road applications. The first European legislation to regulate emissions from non-road mobile equipment is being implemented in two stages (5):
Stage I implemented in 1999
Stage II implemented from 2001 to 2004, depending on the engine power output.
The equipment covered includes mobile construction machinery, forklift trucks, road maintenance equipment, ground support equipment in airports, aerial lifts and mobile cranes. Agricultural and forestry tractors have the same emission standards but with different implementation dates (6). Engines used in ships, railway locomotives, aircraft, and generating sets, not yet covered by the directive, are being considered by a Commission working group and will be included in the future.
Stages I and II are based on a steady-state 8mode test procedure, and emission limit values are shown in Table III.
Values for the Stage III limits using a new transient test procedure (NRTC) are under discussion in a European Commission working group. The intention is to develop global solutions and to develop the legislation in close cooperation with the U.S.A. and Japan, using the global agreement under UNECE in Geneva as the basis for legislation.
A European Commission Task Force has been set up to improve the fuel quality for non-road applications. Reducing sulfur content in non-road fuels in line with those being introduced in the road sector would allow engine makers to use the advanced emission control technologies of catalysts and traps to reduce gaseous and particulate emissions. A Commission proposal was issued at the end of 2000 (7) and established the first emission limits for small spark-ignition engines below 19 kW used in lawn mowers, chain saws, bush cutters, trimmers and snow removal equipment. The proposal, which is now being considered by the European Parliament and Council, has been developed in cooperation with the U.S. Environmental Protection Agency in a move toward worldwide harmonisation.
The proposal includes two stages of limit values: the first to be met 18 months after the directive comes into force and the second one between 2004 and 2010 depending on the category of the engine. The second stage will lower emissions from handheld engines by about 80 to 85 per cent.
Gasoline and diesel engines installed in recreational crafts and personal watercrafts are already subject to some emission and noise requirements and limits (8). The European Commission proposed amendments in October 2000 to the current directive, further reducing the exhaust pollutants.
The mandatory standards for fuel sulfur levels for on-road fuels were set in 1998 (9). However after a full technical study the European Commission adopted a proposal in May 2001 to require the introduction of sulfur-free (< 10 ppm) gasoline and diesel by each EU Member State from 1 January 200.5. Under the proposal, still to be ratified, gasoline and diesel fuel with ‘a sulfur content’ would be banned from the EU market from 2011. These fuels will speed the introduction of the latest fuel-efficient technologies in cars and other vehicles, significantly reducing the emissions of carbon dioxide (CO2).
Exhaust Emissions from Internal Combustion Engines
There are a number of ways in which exhaust emissions can be lowered:
Reducing engine-out emissions by improving the Combustion process and fuel management, or by changes to the type of fuel or its composition
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 NOx under lean operation
Using particulate filters with efficient regeneration technology
Increasing the operating life of autocatalysts and supporting systems
Retrofitting catalytic converters, particulate traps and associated engine and fuel management systems if they are not original equipment.
All, except the first, of the above opportunities, will now be examined, looking at established and new technologies.
Established exhaust Emission Catalyst Technology
Oxidation catalysts convert CO and HCs to CO2 and water and decrease the mass of diesel particulate emissions but have little effect on NOx. Three-way catalysts (TWCs) operate in a closed loop system which contains a lambda- (or oxygen) sensor to regulate the air/fuel ratio. CO and HC are simultaneously oxidised by the catalyst to CO2 and water While NOx is reduced to nitrogen. These simultaneous oxidising 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. The active components of these catalysts are the platinum group metals (pgms) Platinum (Pt), Palladium (Pd) and rhodium (Rh) in various combinations.
Diesel Oxidation Catalysts (DOCs)
An oxidation catalyst will remove the soluble organic fraction (SOF) of diesel particulate by up to 90 per cent (10). Destruction of SOF is important because this portion of the particulate contains numerous chemicals of concern to health experts. DOCs can thus reduce total particulate emissions by 25 to 50 per cent, depending on the constituents that make up the total particulate. They also reduce diesel smoke, eliminate the pungent diesel exhaust odour and make significant reductions in CO and HCs. However the number of particles is unchanged and issues associated with the effects of ultra-fine particulates are unresolved. DOC technology has been successfully used on all diesel cars sold in Europe since 1996; not many heavy-duty vehicles are equipped with a catalyst (11).
DOCs are also used in conjunction with NOx adsorbers, DeNOx catalysts, DPFs or selective catalytic reduction (SCR) to increase NO2 levels or to ‘clean-up’ any by-pass of injected reductant used for NOx reduction (HCs or ammonia). The preferred pgm for DOCs is Pt.
Fast Light-off Catalysts
The catalytic converter has to work soon after the engine is switched on, and to achieve this the exhaust temperature at which it starts to function needs reducing so that untreated exhaust emitted at the start of the legislated emissions test and on short journeys in the real world is minimised. Reductions in the thermal capacity of substrates and improvements to the type and composition of the active pgm catalyst have reduced light-off times from as long as one to two minutes down to less than 20 seconds (12). In Figure 1, the effect of different catalyst compositions as they affect the light-off temperature are shown.
The introduction of a new generation of Pt/Rh technologies for current and future emission standards is a technically and commercially attractive alternative to Pd-based technologies for high demanding applications in close-coupled and underfloor positions using different cold start strategies (13).
More Thermally Durable Catalysts
Increased stability of the catalytic converter at high temperature allows it to be mounted closer to the engine and increases its life, particularly during demanding driving. Catalysts with stabilised pgm crystallites and washcoat materials which maintain high surface area at temperatures around 1000°C are necessary. The surface area of the washcoat is stabilised by improved oxygen storage components that also maximise the air-fuel ‘window’ for three-way operation and indicate the ‘health’ of the catalytic converter for onboard diagnostic (OBD) systems. Figure 2 shows the progress made with mixed cerium and zirconium oxides (14).
Great progress has been made in the technology of the substrates on which the active catalyst is supported. In 1974 ceramic substrates based on cordierite (2MgO.2Al2O3.5SiO2) had a density of 200 Cells per square inch (cpsi) of cross-section (31 Cells cm-2) and a wall thickness of 0.012 inch or 12 mil (0.30.5 mm). By the end of the 1970s the Cell density had increased through 300 to 400 cpsi and wall thickness had been reduced by 50 per cent to 6 mil. Now 400, 600, 900 and 1200 cpsi substrates are available and wall thickness can be reduced to 2 mil - almost 0.0.5 mm (15–19). Further increases in Cell density and reductions in wall thickness are in development.
Substrates derived from ultra thin foils of corrosion-resistant steels came onto the market in the late 1970s. In the beginning the foils were made from material 0.0.5 mm thick, allowing high Cell densities to be achieved and complex internal structures could be developed. Today, wall thickness is down to 0.025 mm and Cell densities of 800, 1000 and 1200 cpsi substrates are available (20, 21).
Progress in ceramic and metal substrate technology has given 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, which, as cars become more compact, makes the catalyst easier to fit close to the engine.
New Technologies for exhaust Emissions Control
Special materials, such as zeolites, are incorporated upstream of or into the catalyst in HC adsorber systems. Hydrocarbon emissions are collected when exhaust temperatures are too low for effective catalyst operation. The HCs are then desorbed at higher temperatures when the catalyst has reached its operating temperature and is ready to receive and destroy the HCs. This technology has the potential to reduce HCs to less than half the levels emitted from a TWC converter (22), see Figure 3.
Electrically Heated Catalyst Systems
A small metallic substrate ahead of the main catalyst, and 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 (23).
The development of lean bum direct injection gasoline engines and increased use of diesel engines makes lean Combustion a challenge for automotive catalysis. Lean Combustion is essential to limit CO2 emissions and reduce fuel consumption. New diesel technologies with electronic management and direct injection can achieve further improvements in fuel consumption. Conventional TWC technology (used on gasoline engines) needs a richer environment with lower air/fuel ratios to reduce NOx, so a radical new approach is required.
The use of DeNOx catalysts and NOx traps brings the prospect of substantially reduced emissions of oxides of nitrogen. NOx conversion rates depend on exhaust temperature and the availability of reducing agents. There are currently four NOx-reducing systems under evaluation and development by industry:
Passive DeNOx catalysts using reducing agents available in the exhaust stream
Active DeNOx catalysts using added HCs as reducing agents
NOx traps or adsorbers used in conjunction with a TWC
Selective catalytic reduction using a selective reductant, such as ammonia from urea.
Each of these systems offers different possibilities in the amount of NOx control achievable and in the complexity of the system. Fuel parameters, such as sulfur content, can affect catalyst performance.
DeNOx (or Lean NOx) Catalysts
Advanced structural properties in the catalytic coating of DeNOx catalysts are used to create a rich ‘microclimate’ where HCs from the exhaust can reduce the NOx to nitrogen, While the overall exhaust remains lean. Further developments focus on increasing the operating temperature range and the conversion efficiency.
NOx Adsorbers (or Lean NOx Traps)
NOx traps or adsorbers are a more 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 a pgm-containing oxidation catalyst or TWC mounted close to the engine so that NO2 can 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 the lean operation regime 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 accompushed 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 accompushed in a number of ways, but usually includes some combination of intake air throttling, exhaust gas recirculation, late ignition timing and post Combustion fuel injection. The released NOx is quickly reduced to N2 by reaction with CO (the same reaction that occurs in a TWC for spark-ignited engines) on a Rh catalyst site or on another pgm catalyst that is also incorporated into this unique single catalyst layer (Figure 4).
Thermal dissociation of the alkaline and alkaline earth sulfates, under oxygen-rich conditions, would require temperatures above 10000C Such temperatures cannot be achieved under realistic driving conditions. However, it has been demonstrated that it is possible to decompose the corresponding alkaline earth sulfates under reducing exhaust and elevated temperature conditions to restore the NOx storage capacity (24–26). The necessary catalyst heating, for example by late ignition timing, results in a considerable increase in fuel consumption, dependent upon the sulfur content. Reducing fuel surfur levels is the best way of using the full fuel economy and CO2 reduction potential of modern direct injection gasoline engines.
Developments and optimisation of NOx adsorber systems are currently underway for diesel and gasoline engines. These technologies have demonstrated NOx conversion efficiencies ranging from 50 to in excess of 90 per cent, depending on the operating temperatures and system responsiveness, as well as fuel sulfur content (27, 28). The system is in production with direct injection gasoline engines.
Selective Catalytic Reduction (SCR)
SCR technology has been used successfully for more than two decades to reduce NOx emissions from power stations fired by coal, oil and gas, from marine vessels and stationary diesel engines. SCR technology for HDD vehicles has been developed to the commercialisation stage and was 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 oxidising atmosphere. It is called ‘selective’ because the catatytic reduction of NOx with ammonia as a reductant occurs preferentially to the oxidation of ammonia with oxygen. Several types of catalyst are used and the temperature of the exhaust environment determines the choice. For mobile source applications the preferred reductant source is aqueous urea, which rapidly hydrolyses to produce ammonia in the exhaust stream.
SCR for heavy-duty vehicles reduces NOx emissions by around 80 per cent, HC emissions by around 90 per cent and PM emissions by around 40 per cent in the EU test cycles, using current diesel fuel (< 350 ppm sulfur) (29, 30). Fleet tests with SCR technology show excellent NOx reduction performance for more than 500,000 km of truck operation. This experience is based on over 6,000,000 km of accumulated commercial fleet operation (31-33).
Diesel-Particulate Filters (DPFs)
A diesel particulate filter is positioned in the exhaust and is designed to collect Solid and liquid PM emissions while allowing the exhaust gases to pass through the system. Figure 5 shows one type of filter material.
However, a number of filter materials are used, including ceramic monoliths, woven silica fibre coils, ceramic foam, wire mesh and sintered or shaped metals. Collection efficiencies of these various filters range from 30 to over 90 per cent, but most DPFs achieve over 99 per cent when expressed as numbers of ultra fine particles. This is very important since health experts believe that it is the fine particulate carried deep into the lungs which could be the most dangerous size of PM to health.
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:
Incorporating a pgm-based oxidation catalyst upstream of the DPF. This operates as a conventional oxidation catalyst but also increases the ratio of NO2 to NO in the exhaust. Trapped particulates bum off at normal exhaust temperatures using the powerful oxidative properties of NO2 (37).
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 bum at normal exhaust temperatures to form CO2 and water, While the Solid residues of the catalyst are retained on the DPF (37).
Incorporating a catalytic coating, typically a combination of pgm and base metals, on the DPF to lower the temperature at which PM bums.
Electrical heating of the DPF either on or off the vehicle, which would allow simple regeneration, but would impose a fuel penalty.
A combination of the above mentioned systems and intelligent engine-management allows efficient regeneration under all operating conditions. The first diesel passenger car equipped with a combined filter system was commercialised in 2000 and evaluations indicate a good and durable performance of the system. The system has now been extended to other models (39, 40).
Continuously regenerating DPFs are very successful in retrofit applications of older HDD 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 U.S.A. (41–43). With any catalyst/DPF combination which includes pgms the use of diesel fuel having a sulfur content lower than 10 ppm is necessary to keep the formation of surfate particulates within future legislated limits (44).
Combined Emission Control Systems (DPF + SCR)
A combined emission control system is an attractive proposition; this is because of the stringent emission limit values for NOx and PM set for HDD engines in 200.5 and 2008, the demands by the transport sector for minimum fuel consumption of the engine and the political public and health concerns over the emission of ultra fine particles into the atmosphere. Urea-based SCR systems and DPFs with an appropriate regeneration strategy are anticipated to be used on the next generation HDD engines and to show significant reductions of both NOx and PM (45–47).
There is thus a wide range of gaseous and particulate emission control options based on the use of catalytic, adsorption and trapping technologies, with the pgms playing an essential role. Retrofitting of catalyst systems and particulate traps is increasing in response both to fiscal incentive schemes introduced by governments and to meet the requirements of environmental zones, particularly in cities. Europe has a strong automotive emissions control industry well placed to help meet the challenge of future emission regulations by working with its partners in the automotive industry. Advanced catalyst and trap systems, together with optimised engine management and emission controls can aid the achievement of the future low emission standards deemed necessary to meet air quality goals.
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Dirk Bosteels is Technical Manager of AECC and his professional background is in emissions testing and vehicle safety homologation.
Rob Searles is Executive Director of AECC and has been involved with catalytic emissions control for more than 30 years.