An important attribute of a catalytic converter is that it must provide a large surface area so that a substantial volume of exhaust gas can come into contact with the catalytic material, and also remain in contact long enough for the desired reactions to occur.
At the heart of every catalytic converter is a ‘substrate’, a block containing numerous small passageways through which the exhaust gas must pass. This is the starting point in providing the required surface area.
The substrate is then coated with a ‘washcoat’ which, after it has dried, leaves a surface which is very uneven (at a microscopic level), the effect of this being to increase the surface area many times.
On top of the washcoat, small particles of catalytic material are deposited, which are usually metals of the platinum group.