Award winning technology for controlling diesel vehicle pollution
Heavy duty diesel vehicles, such as trucks and buses, are vital for transportation, especially in urban areas, but produce harmful pollutants as a result of combusting diesel fuel. As well as producing hydrocarbons, carbon monoxide (CO) and oxides of nitrogen (NOx), similar to those from gasoline powered engines, diesels also produce ultrafine particulate matter made from soot – the characteristic dirty black smoke from diesel exhausts. These soot particles pose a potential risk to our health and there are concerns that their effects may be linked to increased asthma, respiratory and cardiovascular complaints.
But could chemistry and catalysis provide a solution for soot? Armed with our expertise in controlling gaseous emissions from vehicles, Johnson Matthey set out to tackle the dirt in diesel.
It was known that particulate filters (traps) could capture the soot but the real challenge was to find a means of burning off this soot, thereby regenerating the trapping function of the filter. In the late 1980s and early 1990s many systems were tested however none were effective, reliable and durable in real world operations.
Thinking outside the box
About the same time, a team from Johnson Matthey decided to take an unconventional look at the chemistry of soot combustion and made a crucial discovery – they found that nitrogen dioxide (NO
2) could be employed to burn soot at low temperatures – as low as 275oC, compared with around 600oC when burned with oxygen. This lower temperature is more typical of diesel exhaust and thereby offered the prospect of continuous soot combustion.
The team discovered that platinum was converting a fraction of nitric oxide (NO) in the diesel exhaust to NO
2 and that placing a platinum catalyst in the exhaust gave a reduction in pressure, indicating combustion of soot from the filter. However, by injecting NO2 with no catalyst present, a similar reduction in pressure was still observed. This indicated that it was the NO2 that was burning the soot and that the platinum catalyst was helping to convert NO into NO2. Work then began to develop this important discovery into a workable and reliable piece of equipment.
The team developed optimised platinum catalysts for converting NO to NO
2 and then started to look at the best way of employing the newly discovered soot burning effect. Work began on developing a catalytically active filter but trials showed this did not meet the operating standards required. The team then made a simple but highly effective innovation – instead of locating the catalyst on the filter, they kept the catalyst as a separate unit and placed it in front of the filter instead.
As well as cutting harmful hydrocarbon and carbon monoxide emissions, this catalyst converts NO from the diesel exhaust into NO
2, the active species for soot conversion. And with the catalyst in front of the filter, the soot travels through the catalyst substrate into the bare filter and becomes trapped. The catalyst generates NO2 which travels with the exhaust gas into the filter where it combusts the trapped soot. Tests showed that using this system, the filter was being continuously cleaned or 'regenerated'.
This was a highly effective invention but it could not be immediately employed because the sulphur content of diesel fuel being used at the time was too high. Despite much effort in search of a sulphur tolerant catalyst and further development work on the system, it was clear that, as had been the case with gasoline autocatalysts, low sulphur fuel was required if we were to bring this invention – called the Continuously Regenerating Trap, or CRT® technology – to commercial reality.
Interest was growing into the use of cleaner fuels and this focused attention on the possibility of reformulating diesel fuel. Public transport authorities drove the implementation of cleaner diesel fuels and this led to the introduction of a regulated standard of a 'sulphur free' (<10 ppm sulphur) fuel.
The scene was now set for the technology to be trialled in a live operating environment, allowing us to optimise its performance further. Following its commercial launch in 1995 CRT® systems were first retrofitted to buses in Sweden and then buses in London. Germany, the USA and Japan soon followed. Using CRT® technology, soot particles are cut by more than 90% whilst hydrocarbon and carbon monoxide emissions are virtually eliminated. This pioneering work earned the Johnson Matthey team the Royal Academy of Engineering MacRobert Award 2000, in recognition of an outstanding innovation to benefit society.
Today, legislation has spread to include new vehicles too and Johnson Matthey's pioneering work in developing CRT® technology and its success in the retrofit market has stood the company in good stead. We have continued to develop our 'toolbox' of catalyst solutions and are leaders in the supply of these advanced emission control technologies to heavy duty diesel engines around the world.