Article - Issue 9, August 2001
Catalysts: key to the quest for clean air
Robert Evans MSc
Catalytic emissions control systems are employed on all new cars sold in Europe and more than 90% of the world’s new car production. These systems are a speciality of Johnson Matthey, recognised by the Royal Academy of Engineering through two MacRobert Awards. The first was given in 1980 for autocatalysts for passenger cars and the second, in 2000, for the Continuously Regenerating Trap for the control of diesel pollution on trucks and buses. Johnson Matthey is also active in a third area of catalytic systems for an alternative clean vehicle technology – the fuel cell. All of these systems are under continuous development to help realise the next generation of clean cars, motorcycles, trucks and buses. Despite being the elder statesman amongst the catalytic systems, the autocatalyst continues to develop and the story of what has been achieved and what is still possible might just surprise you.
In 1977, there were over 121 smog alert days in Los Angeles, or the equivalent of four months of bad air. During ‘first-stage episode’ alert days, adults with lung or heart conditions were advised to stay indoors and everyone was to refrain from vigorous exercise when outdoors. In the twenty years that followed, the number of alert days has fallen steadily and in 1999, for the first time, there were no ‘first-stage’ alert days. This was achieved despite the fact that twice as many miles are driven today as in 1977. The South Coast Air Quality Management District, which monitors air quality in the Los Angeles Basin, attributes this fall to the switch from cars without catalytic converters to those with them. The outcome is that Los Angeles is no longer the most polluted city in America.
However, the smog alerts that were once a feature of just a few cities like Los Angeles are now a more universal problem, particularly in the densely populated cities of the developing world. By affecting both health and the environment, poor air quality further impacts on sustainable development measures like economic growth and quality of life.
Just as the United States sought to tackle poor air quality using regulation to leverage technology, so too are other governments around the world. The regulatory process has focused technical progress, encouraging improved combustion, better exhaust after-treatment and the uptake of cleaner fuels. Catalytic systems are a central part of this approach, which demonstrably works as the Los Angeles effect shows.
The catalytic systems
Catalysts are materials which boost the rate of a reaction without being consumed or otherwise undergoing any permanent change. In the reactions that produce emissions control the catalyst helps oxidise hydrocarbon (HC) species from unburnt and partially burnt fuel into carbon dioxide (CO2) and water (H2O) and helps oxidise carbon monoxide (CO) into carbon dioxide (CO2). It can also reduce oxides of nitrogen (NOx) to nitrogen (N2).
The key properties of catalysts are ‘reactivity’, to enhance reaction rates at lower energy levels so as to save energy (and assist low-temperature activity), ‘selectivity’ to direct reactants to specific products with high conversion efficiencies, and ‘durability’ to ensure the catalytic effect is maintained. Platinum-group metals catalysts are particularly effective in the oxidation and reduction reactions that form the basis of emissions control. They are also durable in a way that base metal catalysts would not be in the same operating conditions.
The catalysts for transport applications, commonly called autocatalysts, comprise a ceramic or metallic substrate on to which a catalytic coating is applied, containing platinum group metals, rare earth and some base metals. Support materials like alumina, silica and zeolites help provide a large surface area over which the catalytically active materials are dispersed. The different components act either to promote reactions, inhibit other reactions, or stabilise the catalyst’s physical characteristics under adverse operating conditions. The autocatalyst itself is wrapped and packaged into a stainless steel exhaust to make a catalytic converter.
The first catalytic systems were employed on cars for the United States and Japanese markets from 1975. By the late 1980s some cars in Europe were already catalyst equipped but it was not until 1993 that all petrol cars sold in the region employed catalysts. Progress in emissions control continues apace and has been at least as exceptional as the other more visible areas of automotive engineering, including noise, vibration and harshness (NVH), safety, in-car entertainment and comfort. Taking Europe as an example, since their 1993 introduction, the emissions limits that promoted the switch to catalytic systems have tightened for a second and third time. Furthermore, a number of popular cars being sold in the UK today already meet a fourth stage of limits, not due to be introduced on all new cars until 2005. This tightening of regulations mirrors that in the United States, as illustrated in the graph.
The application of catalyst technology used to be called ‘exhaust after-treatment’. However, because of the very close working relationship between automotive engineers and catalyst scientists, those working in the area today reject the term after-treatment in favour of ‘emissions control’. They argue that emissions control is no longer an afterthought but integral to the engineering of the vehicle’s power train.
This partnership engineering approach is illustrated by a review of the progress made in tackling cold-start emissions. Autocatalysts require energy input to activate the reactions of emissions control and this energy is delivered as heat from the engine exhaust. Once at operating temperature, total conversion of pollutant emissions is possible, a remarkable achievement when one considers that exhaust gases travel through the autocatalyst in under one-tenth of a second. However, when the car starts from cold, emissons cannot be controlled until the autocatalyst is hot. On the first generation of autocatalysts this could take a minute or two but this has now been cut to less than 20 seconds through a combination of advanced autocatalysts and engine management.
To tackle cold-start emissions, catalyst scientists have developed more thermally durable catalyst formulations. These formulations allow autocatalysts to be placed close to the engine’s exhaust manifold, leading to rapid warm-up as heat losses are minimised and heat uptake maximised. The thermal durability requirement is critical because these close-coupled catalysts can experience temperatures greater than 1000°C when the engine is operating at high speeds or high loads. Close-coupled catalysts tend to use the newest substrates, with lower thermal mass and high geometrical surface area, which heat up and cool down more rapidly. Further catalyst developments include more active formulations that commence operation at much lower temperatures.
While advanced catalyst formulations were being developed, automotive engineers were making in-cylinder modifications to promote better combustion. They were further developing the control of fuelling through better fuel injection, and perfecting the overall system control through better engine management.
The traditional cold-start problem is that of stalling rather than emissions. Stalling is an unwanted consequence of the poor mixing of fuel and air, aggravated by the condensation of fuel on cold surfaces in the inlet manifold and combustion chamber. To overcome this ‘wall wetting’ the car is normally tuned to run fuel-rich at start-up. However, rich starting causes higher HC and CO emissions as there is insufficient air in the combustion chamber to burn all the fuel. To cut these emissions, some emissions control systems have employed secondary air injection to add air downstream of the combustion chamber, which turns a rich exhaust into a lean one, thereby promoting oxidation reactions.
Today, fuel injection enables the more precise mixing of fuel and air, enabling lean operation at start-up, helping to cut HC and CO emissions without the need for secondary air injection. A typical modern calibration sees the ignition timing retarded at start-up. Instead of the spark igniting the fuel and air mix when the cylinder is atop dead centre, it sparks at a later point. This causes a loss of power but by combusting the air and fuel mix when it is in the exhaust stroke, burning continues as the mix passes out of the chamber and into the exhaust, thereby increasing exhaust gas temperatures to warm the catalyst quicker. As the vehicle pulls away more power is needed so spark injection timing is revised.
The key to achieving high efficiencies for the simultaneous conversion of all three pollutants (HC, CO and NOx) is operation close to the ideal stiochiometric air–fuel ratio (to ë = 1) where complete combustion is theoretically possible. To minimise cold-start emissions, a fast star-tup lambda sensor is employed ahead of the catalyst to measure the air-to-fuel ratio of the exhaust accurately and provide signals to the electronic engine management system, which in turn modifies the fuel injection. The overall objective of the engine management system is to balance the emissions control needs (operation close to ë = 1) with the need for good driving characteristics (power on demand).
Systems are already under development to meet Californian regulations for Super Ultra Low Emission Vehicles (SULEV), and a further category called Partial Zero Emission Vehicles (PZEV) that require pollutant conversion from the moment the key is turned. One of the approaches tested for these systems is to use catalysts that are able to trap pollutants when cold and convert them on their release when the exhaust gas is hot. For these systems Johnson Matthey has patented a catalysed hydrocarbon trap (CHT™) which combines both functions on a single substrate.
Diesel emissions control
The emissions control systems of petrol cars take advantage of the flexibility of spark ignition engines to operate at a variety of air-to-fuel ratios. However, for diesels, compression ignition combustion is always lean and the exhaust gases are more complex than for a petrol exhaust, containing solids and liquids in addition to the three gaseous pollutants. The solids are predominantly carbon particles, or particulates (also known as soot), while the liquids are unburnt diesel and lubricating oils, referred to as soluble organic fraction (SOF). Catalysts that can help cut SOF, HC and CO emissions are now employed on all diesel passenger cars in Europe for these reasons. However, catalytic systems also offer unique approaches for the control of NOx and particulates and these systems are being developed with both light- and heavy-duty diesels in mind.
Johnson Matthey’s Continuously Regenerating Trap, or CRT™, is already proven on heavy-duty diesel trucks and buses. The CRT™ comprises a special catalyst placed in front of a wall flow filter. The filter traps the particulates, whilst the catalyst converts nitrogen monoxide from the engine exhaust into nitrogen dioxide. This second species then burns off the carbon soot trapped in the filter. The continuous trapping and burning of soot prevents the filter from blocking. Using the CRT™ it is possible to remove 99% of carbon particles from the exhaust. The main alternative to the CRT™ is to add a soot-burning catalyst into the diesel fuel.
Beyond PM control, additional approaches are needed to tackle NOx emissions. The key to these is the introduction of a reductant into the exhaust gas stream. The options are to use either diesel fuel, ammonia, or a chemical like urea which will decompose to ammonia. By injecting precise amounts of ammonia into the exhaust gases it is possible to use Selective Catalytic Reduction (SCR) catalysts to reduce NOx to nitrogen. Again, a sophisticated engine management system is needed to control the rate of injection of the reductant. Siemens has taken the lead in the testing of SCR systems on trucks, whilst Johnson Matthey is testing SCRT™, the combination of Selective Catalytic Reduction (SCR) with CRT™. Using SCRT™ it is possible to reduce HC, CO, PM and NOx emissions by more then 90%.
Such are the emissions control capabilities of CRT and SCRT systems that they have the potential to be employed on all new light- and heavy-duty diesels.
Modern emissions control systems combine advanced catalyst formulations with sophisticated engine management to deliver near-total control of exhaust emissions. The results are exceptional and the beneficial effects for air quality management are being experienced across the globe as the technology is applied.
the smog alerts that were once a feature of just a few cities like Los Angeles are now a more universal problem, particularly in the densely populated cities of the developing world
close-coupled catalysts can experience temperatures greater than 1000°C when the engine is operating at high speeds or high loads
Robert Evans MSc
Public Affairs Manager, Johnson Matthey Catalytic Systems Division
Robert Evans is Public Affairs Manager for Johnson Matthey Catalytic Systems Division.