Critical Topics in Exhaust Gas Aftertreatment
Critical Topics in Exhaust Gas Aftertreatment
No combustion process is completely efficient. The exhaust gas from an internal combustion engine, besides containing water (H2O) and carbon dioxide (CO2) – the main products of combustion – also contains some unburned and various partially-oxidised hydrocarbons (HCs), and carbon monoxide (CO). During combustion the flame front temperature is so high (1600°C or even higher) that the endothermic equilibrium between nitrogen (N2) and oxygen (O2) to form nitric oxide (NO) is established. Subsequent rapid cooling and ejection of combusted gas into the exhaust system freezes this equilibrium, so exhaust gas also contains significant levels of NO.
Vehicle exhaust emissions are undesirable and can cause serious pollution via a series of complex photo-initiated free-radical reactions. In bright sunlight reactions between NO, HC and O2 result in low but significant levels of ozone (O3). This is a bronchial irritant, and the main oxidising species in the photochemical smog that has plagued many major cities. Other undesirable chemicals, including the powerful lachrymator peroxyacetyl nitrate, can also be formed. CO can participate as a reductant in related photochemical reactions, but these reactions are much less important than those involving HCs. Therefore, controlling HC and nitrogen oxides (NOx) emissions from vehicles can have a direct impact on restricting the formation of urban photochemical smog.
The key step in controlling car exhaust emissions was the introduction of ‘autocatalysts’ some twenty-five years ago. Initially a platinum-based (Pt) oxidation catalyst was used with an air pump which provided excess air in the exhaust gas to oxidise HC and CO to less harmful CO2 and H2O. A few years later, legislation demanded lower NOx levels. Initially, NOx was reduced to N2 by HC or CO, by passing O2-deficient (‘rich’) exhaust gas over a Pt/Rh (rhodium) catalyst before the addition of air. After air addition, a Pt-based catalyst oxidised the remaining HC and CO. Thus, with two catalysts operating under different conditions, all three legislated tail-pipe emissions were controlled.
By the early 1980s it had been discovered that CO and HC could be oxidised and NOx reduced simultaneously over a single ‘three-way catalyst’ (TWC) containing Pt and Rh. The exhaust gas had to contain chemically equivalent (stoichiometric) amounts of reductant (HC and CO) and O2. This condition was maintained through a combination of technologies: ‘electronic fuel injectors’ operated by a microprocessor fed with signals from an exhaust gas O2 sensor (a solid stabilised zirconia electrolyte with Pt electrodes). Since that time combustion engineering, electronic engine management and catalyst technology have advanced greatly, and now extremely low emissions can be achieved from gasoline engines equipped with TWCs. Indeed, ambient air may now contain higher levels of some pollutants than tail-pipe exhaust gas! When catalyst is designed for a particular vehicle, the required performance is that it meets the legal emissions when aged. In U.S. legislation, this is now 150,000 miles for the most demanding PZEV (partial zero emission vehicle) limits.
Three-Way Catalysts (TWCs)
The continuous measurement of O2 levels in exhaust gas is a key part of the emissions control system on gasoline engines. Once the operating temperature is reached, a modern TWC is remarkably efficient in removing pollutants: over 99 per cent conversion efficiencies are possible. In recent years, effort has gone into minimising the time taken for a TWC to reach operating temperature after engine start-up. As a result of engine management techniques, such as ‘spark retard’ (causing combusting gas to burn in the exhaust manifold) the time before the TWC operates can now be only a few seconds, rather than the several minutes of a few years ago. Placing the catalyst close to the exhaust manifold can further reduce the time, but it may experience very high temperatures. Johnson Matthey developed special heat-durable Pd/Rh and Pt/Rh formulations for this duty. These have very stable components, including some that store and release O2 as the exhaust gas composition oscillates from slightly lean to slightly rich, respectively. Catalyst formulations, that trap HCs formed during the initial low temperature engine cranking and release them when the temperature is high enough for their oxidation, have also been developed.
Lean-Burn Gasoline NOx-Traps
Increasing fuel prices and a desire to reduce CO2 emissions are two reasons to improve fuel efficiency. Improvements toward this can be obtained with spark ignition gasoline engines having direct injection of fuel into the cylinder under overall lean conditions. This technology is gradually being introduced. However, when running lean, NOx removal is difficult since the only practical process involves reduction to N2. This problem has been solved by ‘NOx-traps’ which contain Pt and Rh to trap NOx as nitrate (NO3-) when the engine is running lean. This stored NO3- is reduced to N2 by periodic short rich excursions. Thus the overall process involves a Pt-based catalyst oxidising NO to NO2 which is subsequently converted to NO3-. During the rich excursions the storage process is reversed and released NOx is reduced to N2 over a Rh-based catalyst.
Poisoning by sulfur is a concern here as sulfur dioxide (SO2) present in the exhaust gas is oxidised to sulfur trioxide (SO3) and then to sulfate (SO42-) in the NOx-trap. SO42- is more stable than NO3- and therefore the NOx capacity of the trap gradually deteriorates. To restore the original NOx capacity, SO42- can be removed from the trap by high temperature (600-700°C) treatment under reducing conditions.
Diesel Engine Exhaust
The traditional characteristic component of diesel engine exhaust is soot, although modern diesel engines produce very much less soot or particulate matter (PM) than even a few years ago. In a diesel engine, the compression-ignition process involves combustion of fuel droplets rather than ignition of the almost-homogeneous gas mixture characteristic of a gasoline engine. Diesel PM emissions can be controlled by trapping them in several different kinds of diesel particulate filter (DPF), and the practical problem is then to prevent PM build-up in the filter as this causes high back-pressure which could prevent proper functioning of the engine.
The exhaust gas temperature of a diesel engine is usually insufficient to burn PM in a DPF, and burners and electrical heaters have been used to increase the temperature to initiate PM combustion. However, once PM in a full DPF starts to burn, at around 550°C in air, excessively high temperatures may result that could cause major problems, such as melting the filter. Catalyst (e.g. Pt) within the DPF can mediate combustion, but the most effective method of removing PM from a DPF involves oxidation at lower temperature, with nitrogen dioxide produced by the catalytic oxidation of NO over a Pt catalyst upstream of the DPF*. On large diesel vehicles (trucks or buses) this reaction takes place most of the time, so this system is called a continuously regenerating trap (CRT)**.
Carbon Monoxide and Hydrocarbons
With diesel engines, Pt oxidation catalysts are used for removal of HC, CO and partially-oxidised species, such as aldehydes, which cause the characteristic odour of diesel exhaust. Sulfur compounds present in the fuel burn to SO2 in the engine. Modern oxidation catalysts are efficient, but SO2 is a catalyst poison that can significantly impair performance. Johnson Matthey have developed diesel oxidation catalysts that are more tolerant to these effects, and therefore have improved performance.
At higher temperatures SO2 is oxidised to SO3 over the Pt catalyst, and the SO3 is hydrated to sulfuric acid mist, which contributes to the measured amount of tail-pipe PM. This problem is not as significant as it once was because fuel sulfur levels have been markedly reduced. When diesel fuel contained high sulfur levels, the catalyst selectivity was tailored by modifying the formulation to control the amount of SO3 produced at higher temperatures.
Removal of NOx
In contrast to the high efficiency of TWCs operating around stoichiometric conditions, NOx removal from lean diesel engine exhaust gas is difficult. Under ideal conditions, almost 50 per cent NOx reduction is possible over a Pt catalyst, by small amounts of added HC, with overall exhaust gas staying lean. In practice this is not a solution, as it is only effective over a restricted temperature range. Fuel combustion over a catalyst increases its temperature, and this must also be noted to stay operating in the optimum temperature window.
Two other ways of dealing with the ‘lean-NOx’ problem are in development. The first is selective catalytic reduction (SCR) with ammonia (NH3). NH3 SCR is now used to control NOx from large power and chemical plants, and from some stationary diesel engines. NH3 can be conveniently obtained in situ from aqueous urea solutions ((NH2)2CO), but for small vehicles there are practical problems associated with SCR, such as maintaining adequate temperature and preventing NH3 tail-pipe emissions.
The second approach involves Pt/Rh NOx-traps similar to those used on lean-burn gasoline engines to store NOx as NO3- under lean conditions. NO3- is periodically reduced to N2 by having the engine run rich. There are technical problems to be overcome before NOx-traps can be widely used on diesel cars. As with SCR, low temperatures are a problem. Another concern is sulfur accumulation in a NOx-trap. This progressively lowers the NOx storage capacity, but can be overcome by sporadic rich high temperature treatment. Providing rich exhaust gas to reduce stored NO3- and achieving higher temperatures for desulfation are adverse to fuel economy. It may, however, be expected that efficient NOx removal systems for diesel cars will become available.
Monitoring and Control Systems
In-use monitoring of catalyst performance is now a legal requirement, and is referred to as onboard diagnostics (OBD). This covers emissions and a dozen other distinct parameters. Catalyst malfunction is defined in terms of HC conversion efficiency, and should this fall below a threshold value (which depends on the model year) a malfunction indicator lamp (MIL) alerts the driver that it is necessary to have the car checked.
The most widely reported diagnostic method is the determination of the catalyst O2-storage capacity. This can be measured by applying perturbations to the air/fuel ratio and finding the time taken by the catalyst to return to its normal operating condition, using O2 sensors before and after the catalyst. The dual O2 sensor method is used because reliable and more direct approaches are not yet available. NOx sensors will be used in applications, such as lean-burn gasoline and diesel engines equipped with NOx-traps, and NH3 sensors in SCR systems. It is clear that sensors will be increasingly important in the future for control and monitoring of emissions systems.
Platinum group metals-based catalysts are critical in controlling exhaust emissions, and their range is widening. As well as TWCs and oxidation catalysts, NOx-traps are used on gasoline lean-burn engines and may be used on diesel engines. Platinum group metals can enable control of diesel PM emissions, and they are also used in sensors that provide signals for overall computer control. It is therefore clear that the important use of platinum group metals in the vehicle emissions area will continue well into the future.
P. Eastwood, “Critical Topics in Exhaust Gas Aftertreatment”, Research Studies Press Ltd ., Baldock, England, 2000
R M. Heck and R. J. Farrauto, “Catalytic Air Pollution Control: Commercial Technology”, Van Nostrand Reinhold, New York, 1995
“21st Century Emissions Technology”, IMechE Conf. Trans. 2000-2, Professional Engineering Publishing Ltd., Bury St Edmonds, England, 2001
P. Degobert, “Automobiles and Pollution”, Editions Technip, Paris, 1995
Martyn Twigg is the European Technical Director of Johnson Matthey Catalytic Systems Division and is based in Royston, near Cambridge.