Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1998, 37 (13/14), 1901
Silver Knocks Nitrogen Oxides
New catalyst converts harmful NOx from car exhausts
into ordinary nitrogen
The use of a new catalyst, developed by Johan A. Martens' research group at the Belgische Centrum voor Oppervlaktechemie (Belgian Centre for Surface Chemistry), was able to reduce the nitrogen-oxide content of car exhaust fumes drastically. In a trial run, their system could eliminate up to 85% of the NOx component nitrogen dioxide. Nitrogen oxides (NOx) are partially responsible for the "summer smog".
The majority of the global nitrogen-oxide emission is caused by road traffic - a normal car emits about a quarter of a gram of NOx per kilometer in urban traffic. These gases are inevitably produced during high-temperature combustion, and cannot be avoided effectively even by the most efficient lean-burn excess-air engines. Naturally there has been no shortage of attempts at making effective "DeNOx" catalytic converters for removing nitogen oxides from exhaust gases, but all previous systems have foundered for three reasons: they are not particularly efficient at the job, they are not durable enough, and they can only operate reliably in a very narrow temperature range. Such factors have made their technical application very difficult. The Martens catalytic converter promises to be a much more practical solution. It consists of a porous inorganic material with silver and hydrogen ions on its surface. These ions are responsible for the NOx removal: they interfere with the exhaust-gas chemistry in three separate places and effectively control a complex sequence of reactions whereby nitrogen dioxide is converted into harmless nitrogen, water, acetic acid and - as a necessary evil - carbon monoxide. The Belgians' trump card above all is the use of silver, which is not affected either by water vapor or the presence of too much oxygen or sulfur dioxide in the exhaust mix. These gases have proved to be the Achilles' heel of many forerunners: they ruin the beneficial catalytic activity of other metals that have been tried, such as copper and cobalt.
Nevertheless, the new system does not work quite so simply as the conventional models: in order to remove the nitrogen dioxide another gas - in the experiment it was propene - must be added to the exhaust stream. A second drawback is that nitrogen dioxide is only one part of the NOx mixture; for operation in a car the other main component, nitric monoxide, would have to be previously converted into nitrogen dioxide. But this should present only a minor technical hurdle for the researchers to overcome.