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 (SO₄^2-) in the NOx-trap. SO₄^2- is more stable than NO₃^- and therefore the NOx capacity of the trap gradually deteriorates. To restore the original NOx capacity, SO₄^2- can be removed from the trap by high temperature (600-700°C) treatment under reducing conditions.

Particulate Matter

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 NO_x

In contrast to the high efficiency of TWCs operating around stoichiometric conditions, NO_x removal from lean diesel engine exhaust gas is difficult. Under ideal conditions, almost 50 per cent NO_x 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-NO_x’ problem are in development. The first is selective catalytic reduction (SCR) with ammonia (NH3). NH₃ SCR is now used to control NO_x 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 NO_x-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 NO_x-traps can be widely used on diesel cars. As with SCR, low temperatures are a problem. Another concern is sulfur accumulation in a NO_x-trap. This progressively lowers the NO_x 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 NO_x 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. NO_x sensors will be used in applications, such as lean-burn gasoline and diesel engines equipped with NO_x-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.

Conclusions

Platinum group metals-based catalysts are critical in controlling exhaust emissions, and their range is widening. As well as TWCs and oxidation catalysts, NO_x-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.