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Removal of Nitrogen from Motor Exhaust Sulfur-resistant process

August 16, 2000

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 thewallet 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

WILEY-VCH Verlag GmbH