Swiss research: Clean diesel exhaust gases
EXHAUST GAS CLEANING Scientists at the Paul Scherrer Institute PSI have for the first time understood at the molecular level why the SCR process for cleaning diesel exhaust gases does not work optimally at low temperatures and what can be done to remedy the situation. Harmful nitrogen oxides (NOx) are produced in diesel engines when the fuel is burned. The vehicle industry has therefore developed a process that reduces emissions: The exhaust gas is [...]
Harmful nitrogen oxides (NOx) are produced in diesel engines when the fuel is burned. The vehicle industry has therefore developed a process that reduces emissions: Gaseous ammonia is added to the exhaust gas which, stimulated by a catalytic converter, reacts with the nitrogen oxides to form harmless nitrogen and water (so-called SCR process for selective catalytic reduction. Editor's note). However, this process does not yet function optimally at low temperatures. Scientists at the Paul Scherrer Institute PSI have now understood for the first time at the molecular level what remedies this in the engine.
SCR reduces nitrogen oxide emissions from diesel engines by up to 90 percent. This is achieved by means of an additive known under the brand name AdBlue. The additive is injected into the exhaust gas, where it breaks down into ammonia. With the help of a catalytic converter, the ammonia converts the harmful nitrogen oxides into harmless nitrogen and water. However, SCR only delivers satisfactory results from an exhaust gas temperature of well over 200 degrees Celsius. During a cold start, it therefore takes several minutes for the nitrogen oxides to be optimally broken down. For the same reason, the SCR also has a reduced performance on cold winter days.
To find out why the reaction processes proceed so differently under different conditions, the PSI researchers used a time-resolved spectroscopy method. The most important finding: this ammonia of all things reduces the performance of the copper in the catalyst at low temperatures. Depending on the temperature and operating conditions, different amounts of ammonia are therefore required to optimally break down the nitrogen oxides. Following experiments, the researchers can even say exactly when and how much ammonia should be added to the catalytic converter in order to keep the nitrogen oxides in the exhaust gas as low as possible at all times. The PSI researchers in Switzerland are thus showing the automotive industry a way to significantly improve air quality in cities in the medium term.
