Richer catalyst technologies for emissions control-Oxidation Catalysts(DOC)

Oxidation catalysts are the original type of autocatalysts and were used from the mid-1970's for petrol-engined cars until superseded by three-way catalysts. They look much the same as three-way catalysts and their construction and composition is similar but slightly less complex. Oxidation catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO₂) and water but have little effect on nitrogen oxides (NOx). They are now rarely used on gasoline cars in Europe because of the advantages of three-way catalysts, but they are still used in some parts of the world where emissions legislation is less stringent. They may also be used on some buses running on Compressed Natural Gas (CNG), motorcycles and for applications such as small petrol engines for hand-held equipment such as strimmers and recreational boats.

Diesel Oxidation Catalysts (DOC) remain a key technology for diesel engines where the high oxygen content of the exhaust precludes the use of three-way catalysts. These Diesel Oxidation Catalysts (DOC) convert CO and HC but also decrease the mass of diesel particulate emissions by oxidizing some of the hydrocarbons that are adsorbed onto the carbon particles (19). All new diesel engines mounted in passenger cars, light-duty and heavy-duty trucks and busses are now equipped with DOCs.

DOCs may also be used in conjunction with NOx adsorbers, DPFs or SCR catalysts to increase the NO₂:NOx ratio or to minimize any residual injected reductant used for NOx reduction (hydrocarbons or ammonia).