Transition metals are used directly as catalysts in the anti–pollution catalytic converters in car exhausts. For example, expensive transition metals such as Platinum and rhodium are used in the catalytic converters in car exhausts to reduce the emission of carbon monoxide and nitrogen monoxide, which are converted to the non–polluting gases nitrogen and carbon dioxide. 2NO (g) + 2CO(g) ⇒ (g) +2 (g)