The coating of a catalytic converter can be considered as a chemical plant in miniature, where toxic gases are converted into non toxic ones.
There are two distinct parts to the coating.
- The first of these is known as the ‘washcoat’, the purpose of which is to provide the maximum possible surface area for reactions to take place. To extend the analogy of the chemical plant, the washcoat would be the factory floor. If examined under a very powerful microscope, a good washcoat appears extremely rough, a bit like sandpaper. The surface area of a washcoat is a good indicator of how effective a catalytic converter will be, and a high quality modern washcoat can provide amounts of surface area which are difficult to comprehend for a non-chemist. For example, a washcoat providing less than 100 square metres of surface area per gram of weight would not meet the grade for most OEM catalytic converters. Most washcoats consist primarily of alumina, however ceria is also a common addition as are various chemicals known as ‘rare earths’. Although, as mentioned above, the primary purpose of the washcoat is to provide a large surface area, the ingredients of the washcoat also play an important part in the chemical reactions which take place on its surface.
- The second stage of the coating is the application of precious metals to the washcoat, usually from the platinum group. These metals play a critical role in the oxidation and reduction reactions which the catalytic converter is designed to promote. Oxidation reactions are normally promoted by platinum, palladium or a mixture of the two, whilst rhodium is used to promote reduction reactions. It is common to specify the precious metals content of a catalytic converter as a ratio followed by a density, for example Pt:Rh 5:1 @ 25g/ft3 means that there are five parts of platinum for every one part of rhodium, and a total of 25 grams of precious metals are used for every cubic foot of substrate. A specification in this form, although common, is not actually a very good way of predicting the performance of a catalytic converter, as an expert chemist could achieve better results with a small quantity of precious metals than an average chemist could achieve with ten times the amount !