I run one of the largest salvage yard in the state and process cats. They contain platinum, palladium and rhodium. When the fuel in your engine burns it has to combine with oxygen. Oxygen naturally comes as O2 with two of them bonded together. Inside the cat there are tiny spikes of these metals that cause the O2 to break in two.

When the oxygen is floating around as single atoms it causes the combustion temperature to drop substantially causing more complete burning of the exhaust products and a cleaner exhaust.