Three-way reactors are used in gasoline engines, including the oxidation of hydrocarbons to water and carbon dioxide, as well as the reduction of nitrogen oxides. Diesel vehicles use oxidation reactors to oxidize carbon monoxide molecules and hydrocarbons. Catalysts are made from a honeycomb ceramic monolith or metal (made from a perforated strip). The entire catalyst is usually in the form of a long metal tube of various diameters and shapes. The catalytic converter, commonly known as the catalyst, is the most important element of the exhaust system responsible for exhaust gas cleaning, together with the DPF filter, it is the exhaust gas cleaning system in automobiles.