Harmful nitrogen oxides are produced by diesel engines when the fuel is burned. To reduce these emissions, the automotive industry developed a process that involves adding gaseous ammonia to the exhaust gas. Stimulated by a catalytic converter, the ammonia reacts with the nitrogen oxides to form harmless nitrogen and water.
According to the Paul Scherrer Institute (PSI), this process can reduce nitrogen oxide emissions by up to 90 per cent, but there is one problem: it does not function properly at low temperatures, for example during a cold start. For the same reason, it also does not work as well on cold winter days.
PSI researchers have now discovered a way to overcome this problem. They initially studied the chemical process in a copper-zeolite compound and illuminated the catalyzer material with highly concentrated x-rays using the Swiss Light Source (SLS), a third-generation synchrotron light source at PSI.
“This allows us to see what happens at the level of the copper atoms and the molecules bound to them during the reactions,” said PSI researcher Maarten Nachtegaal.
The researchers were able to capture not only snapshots of the reaction, but also the course of the entire reaction. “Our time resolution here at SLS is almost unique the world over,” emphasized Nachtegaal.
Gaining a precise understanding of the temporal changes in the process is important because both the exhaust gas temperature and the quantities of ammonia and nitrogen oxide supplied to the catalytic converter change constantly while the car is driving.
The most important finding of the researchers is that ammonia reduces the efficiency of the copper in the catalytic converter at low temperatures. Based on this discovery, they tested the current process used by the automotive industry in the laboratory under different temperature conditions and with different amounts of ammonia.
“This allows us to say precisely when and how much ammonia should be added to the catalytic converter in order to keep the nitrogen oxide in the exhaust gas as low as possible at all times,” said Nachtegaal.