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David Bradley ISSUE #70
November 2007
Marcelo Ackermann
Cats Don't Work Like That

The three-way catalytic converter in your car does not, it turns out, work the way chemists thought it did. One of the key functions of a "cat" is to convert toxic carbon monoxide into carbon dioxide. However, chemists in The Netherlands have found that this occurs not in a single reaction step, but in at least two different steps.

Catalytic converter

Marcelo Ackermann, a surface scientist from Leiden University, and colleagues have discovered a second reaction path in one of the fundamental operations of a vehicle's catalytic converter of which chemists were previously entirely unaware. His results call into question our understanding of cats. However, they could also open up the possibility of design improvements for the next generation of these devices essential to cutting harmful vehicle pollutants in our towns and cities.

Until now, deficiencies in catalytic converter technology have been explained away by studies pointing to the formation of thin layers of oxide materials on the catalyst surface. However, Ackermann's work suggests that an oxide layer, rather than being detrimental to operation, is essential.

Ackermann examined the elementary steps in the catalytic oxidation of carbon monoxide (CO) on platinum and palladium under the conditions of high temperature and high gas pressure that simulate an active vehicle cat. He used X-ray diffraction to get an atom-by-atom picture of the surface.

The study revealed that an atom-thin layer of platinum or palladium oxide forms with which carbon monoxide from the exhaust gases come into contact. At this point they are immediately oxidized to carbon dioxide in a second step. This process takes place only if the pressure of oxygen is relatively high in relation to the pressure of carbon monoxide.

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At low temperature and pressure, the oxide layer never forms and so surface studies carried out by chemists, such as this year's Nobel prize winner Gerhard Ertl, have always assumed that any oxide layer formed in a working cat was a detrimental byproduct.