The job of the coating in a catalytic converter is to provide a surface where molecules of gases are broken down so that the atoms can be recombined in different ways.
There are two parts to the coating: the washcoat and the catalytic materials.
The main purpose of the washcoat is to provide a large surface area. The surface area of a washcoat is far greater than what is visible to the naked eye – it is the roughness and porosity which is apparent under extreme magnification which counts. The main ingredient of most washcoats is gamma alumina, which usually has a surface area of over 100 square metres per gram. It’s worth stopping to consider that figure for a moment – 100 m2 is bigger than the average house in the UK, and that’s just a gram of the stuff ! Additional ingredients may include ceria, which serves a useful purpose by storing oxygen, and also elements known as rare earths which stabilise the washcoat.
The particle size of the materials in the washcoat makes a big difference to its efficiency. Traditional washcoats start off as suspensions (often referred to as slurries in the industry), which means that they are a dispersion of solid particles in a liquid, where the particle sizes are over a micron in diameter. Despite a micron being only a thousandth of a millimetre, these particles are still heavy enough to settle out of the liquid in which they are dispersed within a few minutes. However, the main problems arising from their relatively large particle size are as follows:
- The particles in the washcoat are bigger than the pores in the substrate, so they block them up. This reduces the surface area of the substrate, which is detrimental since surface area is the most important factor determining the efficiency of a catalytic coating.
- As the washcoat normally includes several different elements, it works best if these are finely mixed, however it is obviously more difficult to finely mix large particles than small ones, especially if they have different densities.
A more advanced type of washcoat incorporates particles which are less than a micron in diameter and are therefore measured in nanometres (there are 1000 nanometres in a micron). These are known as ‘nanoparticles’, and when they are dispersed in a liquid, this is known as a ‘colloid’. Nanoparticles behave differently compared to larger particles, and these properties have given rise to a field of science known as nanotechnology. One important distinction is that the effect of gravity on nanoparticles is negligible, and therefore they can remain dispersed in a liquid for months if not years. Whilst they are dispersed, they can be mixed together so that particles of different elements are very close to one another, which facilitates a coating that enables the different elements to interact with each other effectively. The fact that they are so small also means that they can penetrate porous surfaces to make the most of the available surface area. The process of turning a colloid into a coating relies on a technique known as ‘sol-gel’, in which, as the liquid is removed, a gel is formed which subsequently dries to form a ceramic coating with a well-defined three-dimensional structure.
Once the washcoat has been applied, the final stage is to add the catalytic material. In the case of oxidation catalytic converters this normally consists of platinum and/or palladium. These are used as acidic solutions, which are soaked up by the washcoat in a stage known as ‘impregnation’. Platinum, and especially palladium, are very expensive, therefore it is important not to use any more of them than absolutely necessary, which is why it is worth paying careful attention to the washcoat. A washcoat incorporating large particles might be quick to make and apply, but it is sub-optimal at promoting reactions at the atomic scale.
Applying a washcoat which relies on nanotechnology requires a more advanced preparation and coating process but enables substantial savings to be made in terms of precious metals.
