The heart of automotive catalyst systems is the auto-catalysts or so-called catalytic converter which plays a vital role in the modern motor vehicles. The catalytic converter is a part of the exhaust system containing noble metals (rare-earth metals), which convert hydrocarbon and carbon monoxide emissions to un-polluting water vapor, carbon dioxide and nitrogen before expulsion into the atmosphere. In general, the conversion efficiency of the catalysts are about 90%.

In the catalytic converters, an extruded ceramic cordierite honeycomb structure is coated with a complex ϒ-alumina system, forming a wash-coat. The cordierite honeycomb structure directs the exhaust gas through a large number of parallel channels or tubes, about 400 per square inch, producing a large surface area required for very rapid chemical reactions. The reactive surface area is increased by the nature of the alumina coating system, with the rare-earth catalyst metal particles having an average diameter of 10 nm. It is estimated that the final reactive area is equivalent to that of three football pitches. The cordierite honeycomb is finally enclosed in a stainless steel can, with an intumescent interlayer of ceramic mat or stainless steel mesh, and is attached to the vehicle's exhaust pipe.

The wash-coat contains stabilizing compounds and agents that promote catalytic activity. There are three types of noble metals that are normally deposited on the surface of the wash-coat, which are platinum (Pt), palladium (Pd) and rhodium (Rh), stabilized with nickel (Ni), barium (Ba), cerium (Ce), lanthanum (La), neodymium (Nd), strontium (Sr), iron (Fe), zirconium (Zr), and/or silicon (Si), to promote the catalytic action, retard the sintering of the noble metals and maintain the alumina in its gamma form. In principle, the Pt and Pd are sufficient to enable oxidation reactions to occur. However, the quantity of Pt/Pd is determined by a compromise of various criteria such as effectiveness at low temperature, resistance to sintering and poisoning, etc. The Rh is used to ensure the reduction of NO_x to N₂. The Ba and Sr also serve as stabilizers and can store NO_x if the engine is running lean, though that is not their primary purpose. A small amount of Ni suppresses the formation of hydrogen sulfide from the relative high amount of sulfur that is still present in gasoline. These noble catalyst metals must operate at the normal operating range of a gasoline engine (from low initial temperatures to operating temperatures between 350 ºC and 650 ºC), but they are durable to over 1000 ºC. In all cases, the total mass of noble metals is very low: on average, small quantities of 1 to 2 grams for each vehicle. Note that the assembly of the catalytic converter is a very-complex operation.

Catalytic converters have been widely used since 1975. Between 1972 and 1982 in Japan, air pollutants declined by a third even though the number of cars increased three-fold during the same period. The source of this decline was the mandatory catalytic converter, combined with strictly enforced speed limits. Catalytic converters perform under extreme conditions of chemical, thermal and rate constraints as well as requiring significant mechanical properties. The standard catalytic converter on gasoline-powered vehicles uses a three-way catalyst engineered over many years to be extremely durable and meet emissions standards for ~120,000 miles. However, the catalytic converter performance degrades over tens of thousands of miles. Note that the car still can run if you get rid of the catalytic converter in your car.