Driving down emissions

The three-way catalytic converter is a wonderful example of what technology can achieve in the face of new legislation, says Rosslyn Nicholson. But there’s no room for complacency.

There is a contradiction present in modern-day society - the desire for things to be bigger, better and faster flies in the face of our good intentions to limit the impact that we are having on our planet. Legislation is increasingly responsible for dictating the direction of research centred on cutting emissions and dampening the destruction caused by our gas-guzzling toys. Current catalytic converter technology has evolved from such legislative steering and it is from this pressure that the development of the three-way converter (TWC) has emerged.

Pressure on car manufacturers to reduce exhaust emissions has risen unabated since the 1960s. Before 1966 the average car belched out 10.6g of hydrocarbons, 84g of CO and 4.1g of NO _x per mile driven. It would have been hard to believe, almost 40 years ago that today’s Euro III regulations would only permit emissions of up to 0.32g of hydrocarbons, 3.68g of CO and 0.24g of NO_x per mile driven. And legislation around the world is fuelling the demand for even higher standards.

The US state of California has long been ahead of the field. Spurred on by a pall of photochemical smog obscuring their skyline, in 1960 the state’s beleaguered residents created the Motor Vehicle Pollution Control Board and, in 1967, a US Federal Act allowed California to set its own standards for vehicle emissions. At first the motor industry protested loudly, complaining that there just wasn’t any technology available to eliminate the noxious gases. But the lawmakers prevailed and car manufacturers eventually came up with the goods.

The first vehicles with two-way catalytic converters were sold in 1975. Oxidation catalysts were coated onto pellets and placed in a metal canister in the car’s exhaust pipe. The catalysts worked by removing two of the harmful gases from the exhaust stream: unburned hydrocarbons and CO. Both gases are thermodynamically unstable and should theoretically be first to react with any available oxygen - forming water and CO₂. In practice the activation energies of these reactions are so high that they don’t proceed without the help of a catalyst. Lots of metals are capable of catalysing the oxidation of a mixture of unburned hydrocarbons and CO to the more benign H₂O and CO₂, but few can keep up with the process in cars travelling at motorway speeds. A mixture of platinum and palladium provided the optimum conditions, and is still used today. However, the traditional pelletised supports introduced in the 1970s that rattled and shook with every bump in the road were soon abandoned in favour of a solid, and quieter, honeycomb structure.

In the US, two-way converters helped to reduce accidental deaths related to CO emissions from cars by 21 per cent between 1975 and 1996. There was just one problem: they did nothing to reduce NO _x emissions. Following the success of the two-way catalysts at reducing hydrocarbon and CO emissions, pressure mounted to improve further the environment by eliminating NO _x emissions too.

Catalysts in series were tried, with a reduction catalyst (to deal with the NO) followed by an air intake then an oxidation catalyst (to deal with unburned hydrocarbons and CO). But it seems that many metals capable of catalysing NO reduction end up forming quantities of ammonia in the first process, which is subsequently oxidised back to NO _x in the second catalyst.

Several power plants have solved this problem by removing NO using a technology called Selective Catalytic Reduction (SCR). A reductant, such as urea (an ammonia source) or ammonia itself, is added to the effluent and the NO is essentially titrated to N ₂ and water. Any ammonia that slips through the system is then removed downstream. But while SCR may prove useful with very large diesel vehicles, there are obvious difficulties with carrying ammonia or urea around in a car.

However, the situation is not as bad as it may seem. Given roughly equivalent levels of oxidising and reducing species in the exhaust, it is possible to oxidise unburned hydrocarbons and reduce NO _x at the same time. To achieve this, the engine must be carefully calibrated to ensure that exactly the right amount of fuel is delivered in each ’injection’ event. If the gas feed is too oxidising, then CO and unburned hydrocarbons are duly oxidised, but NO _x cannot be reduced. If the gas feed is too reducing, significant NO _x reduction still occurs, but there won’t be enough oxygen to allow complete oxidation of CO and unburned hydrocarbons. Optimum conversion of all three pollutants only occurs when the air:fuel ratio is around 15:1.

The challenge back in the 1970s was to find a catalyst that could cope with the high throughput and specific temperature inherent in car exhausts, but that only produced N ₂. Unfortunately, while the mixture of platinum and palladium used in two-way converters is effective at catalysing the oxidation of unburned hydrocarbons and CO, the same mixture is ineffective at simultaneously reducing NO _x. This meant that a different combination of metals had to be found. Credit for finding the critical mix goes to G Meguerian, E Hirschberg and F Rakowsky of the then Amoco Corporation, who filed a patent in the 1970s on their new catalytic mixture of metals that this time contained rhodium.

Cerium slows down the aggregation - and associated loss of surface area - of the noble metals in the catalyst, and stabilises the g-form of the alumina support by stopping a potential change to the higher density a-form. The loss of surface area reduces the activity of the catalyst, so a major benefit of using ceria is the significant improvement in the catalyst’s durability. Oxides of zirconium are used for equally good reasons: their addition may help to prevent any thermal aggregation of the ceria particles, and might assist the transport of oxygen atoms in and out of the bulk material.

This is a difficult problem to solve because catalytic systems are so difficult to study. No-one has yet found a way to see what the molecules in the TWC are doing as the car speeds along, and the only thing to do is study a model system and hope that the results will extrapolate to the real catalyst. Much of the data available are from infrared spectroscopy studies, and assigning reaction mechanisms on the basis of such data is fraught with pitfalls. However, model studies have yielded some significant data.

It has become a widely held belief that the mechanism of NO reduction in a TWC starts with the pairing of two NO molecules on the metal surface, followed by sequential loss of the oxygen atoms. To facilitate the initial pairing, the metal has to release some electron density to the NO and electronic studies suggest that rhodium is in a better position to do this than platinum or palladium. Other studies focused on platinum’s lack of activity for NO reduction by CO. When researchers studied a mixture of NO and CO over finely dispersed platinum by IR spectroscopy, no peaks due to adsorbed NO were seen. Perhaps platinum has such a high affinity for CO that NO cannot compete for sites on the metal surface.

Alternatively, other base metals, especially molybdenum and tungsten, can catalyse the reduction of NO by CO but far too slowly to be of use. Attempts have been made to pair these metals with a more active metal such as platinum or palladium. The problem with this is that molybdenum and tungsten are easily deactivated (or poisoned) by sulphur in fuel, and form volatile oxyhydrides in the exhaust - creating a pollution problem as bad as the one researchers were trying to solve.

Palladium is a more promising candidate for replacing rhodium. Lead and sulfur easily poison it, but this problem is not insurmountable as lead has now almost been legislated out of petrol in Europe and the US, and low-sulfur fuels are beginning to catch on. Palladium-only TWCs have been reported that use two to five times more of the catalyst metal than traditional TWCs. But why is palladium so promising? Under TWC operating conditions palladium can undergo a redox cycle and so might have the ability to act as an oxygen storage medium, even after thermal sintering has reduced the effectiveness of the ceria. Unfortunately, as a replacement for rhodium, palladium is still not quite ideal because, although palladium can deal with unburned hydrocarbons and CO as fast as rhodium, it is not so active for reducing NO.

The TWC is a wonderful example of the advances in technology that can be achieved under pressure, but there is no room for complacency. Traffic pollution is still a huge problem, so the lawmakers are one step ahead and real challenges remain. One thing is certain: anything that reduces pollution without restricting travel is a sure vote winner. Improvements and developments to TWCs can be fully expected well into the coming decades.

Acknowledgements

Rosslyn Nicholson

Further Reading

• M. Shelef and G. W. Graham, Catal. Rev-Sci. Eng., 1994, 36, 433. 
• M. Shelef and R. W. McCabe, Catal. Today, 2000, 62, 35.
• H. Muraki and G. Zhang, Catal. Today, 2000, 63, 337.
• W. A. Brown and D. A. King, J. Phys. Chem. B, 2000, 104, 2578.      

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