Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (16), 2934 - 2937
Removal of Nitrogen
from Motor Exhaust
Who doesn't want to save fuel? Vehicles with lean-burning engines are therefore gaining ground. These engines work with excess air and can run on noticeably less fuel. This spares the wallet of the frequent driver, and also releases less carbon dioxide into the atmosphere. However, these exhaust gases are not a pure joy for the environment, they too must be carefully, catalytically scrubbed. The main problem with the exhaust from lean-burning motors is the considerable increase in the nitrogen oxides (NOx) that are generated on combustion. Various new concepts for the reduction of NOx are now in development. The temporary storage of the nitrogen oxides on an adsorbent surface plays a key role in this. The snag: combustion of sulfur-containing fuels - nearly all fuels contain sulfur - produces sulfur compounds that "poison" the adsorbent.
An international team of chemists working with Johan A. Martens in Leuven, Belgium has now developed a new approach to removing nitrogen from the exhaust of lean-burning motors. Their storage medium is a zeolite that is not sensitive to the sulfur-containing combustion products. The zeolite consists of a scaffolding with many little "cages" that absorb the nitrogen oxides - NO and NO2 - from the exhaust. At an operating temperature above 200°C a zeolite cage will hold one NO and one NO2 molecule, which react to form a molecule of N2O3 that is then stored. If the exhaust contains more NO2 than NO, the excess reacts to form N2O4, which is also adsorbed. At regular intervals humid, outside air is then made to pass over the zeolite. The water forces N2O3 and N2O4 out of the cages, and they revert to NO and NO2. In the following interval of the motor's operation, these nitrogen oxides can be introduced into the carburetor, where they are reduced to environmentally friendly dinitrogen by the rich combustion gases.
"Through tests with sulfur dioxide containing exhaust gases, we have been able to demonstrate that our zeolite has a high resistance to sulfur. It is high enough that we can use the zeolite in the removal of nitrogen from auto exhaust," Martens says optimistically. Another advantage of this method is the easy desorption of the nitrogen oxides by humid air. Heating is not necessary - unlike in many other such processes.