Precious Metals in Catalytic Converters

So why have precious metals in Catalytic Converters?

They help to transform toxic emissions into non-hazardous ones:

Catalytic Converters containing precious metals are like tools with which one can fuse or separate chemical compounds.
Without these metals we would need more power to achieve this separation or chemical fusion, and hence more energy. In addition to this it would take much longer to fuse or separate chemicals without these precious metals.

One Converter only contains a few grams of precious metals. In numbers: 1.8-5.3 g per one cubic litre of overall Converter volume. Combination and quantity of each precious metal in a given Converter are specified by the individual use for each category of Converter. Since Rhodium is a very active element, its quantity in a three-way Converter will be kept very low (ca. 1/5 of the platinum quantity used).

For Converters in automobiles this means:
The transformation of toxins will be faster and conducted at lower temperatures, due to the contained precious metals.

Currently Palladium, Platinum and Rhodium are widely used for this process. A thin layer of a combination of these, is used as a coating in the Washcoat component of the Converter which in turn is mounted to the carrier of the Converter, the monolith.

Platinum and Palladium are used as tools to bind Oxygen. For example: Carbon-monoxide will turn into Carbon-dioxide. Since oxygen is added, this is referred to as oxidation.

Rhodium will work conversely: it will separate oxygen from nitrogen oxides, also referred to as Reduction.
Nitrogen oxides will thus be transformed into nitrogen - 80% of the air we breathe is composed of it.
This only works when all oxygen is used up by the nitrogen oxides and is added to carbon monoxide during the oxidation process.

One Exception; the Diesel Burning Engine:

Due to the excess of oxygen in emissions of diesel burning engines the diesel Converter does not contain Rhodium, it solely consists of Platinum in its Washcoat and is hence a purely oxidation based Converter.