In the catalysts of cars poisonous carbon monoxide reacts with oxygen to form carbon dioxide before the exhaust is emitted. This reaction takes place on a surface of platinum, for instance. Ertl has shown that the reaction is considerably more complicated than could be expected and in this way demonstrated the strength of his own methodology in surface chemistry. The speed of the reaction varies depending on how much of the surface is covered by carbon monoxide or by oxygen. Sometimes this gives rise to a chaotic process. The circular patterns in the background plate show fields that alternate between being rich in oxygen (dark) and short of oxygen (pale). During the course of the reaction these change constantly.