The metal catalysts used in catalytic converters are usually platinum, palladium, and rhodium. These metals are part of the platinum group metals (PGMs), which are known for their catalytic properties and are extremely rare. The catalysts are in the form of nanoparticles, coated on a substrate or "brick". The amount of each metal used in a catalytic converter can vary depending on the specific design and application, but platinum is typically used the most. The precious metal content of catalytic converters is what makes them valuable and attractive to thieves. Scientists are currently researching ways to reduce the amount of precious metals needed in catalytic converters while still maximizing their effectiveness.