Rhodium

Rhodium, one of the platinum-group precious metals, is applied as a thin coating on the honeycomb structure inside catalytic converters. Because it is essentially the only practical catalyst metal capable of efficiently reducing nitrogen oxides in exhaust gas into nitrogen and oxygen under real-world driving conditions, it is considered the most specialized and valuable component of a catalytic converter. Although used in tiny quantities by weight, its market price can at times exceed that of gold, making converters containing it an attractive target for theft and recycling.