Diesel Oxidation Catalyst (DOC)
Since 1991, diesel oxidation catalysts have been used in passenger cars in Europe, and for a certain time with commercial vehicles in the USA. They reduce the amount of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas by oxidizing them to carbon dioxide and water at high temperature (>100°C).
2 CO + O2 → 2 CO2
CxHy + (x+y/4) O2 → x CO2 + (y/2) H2O
In addition, they contribute to the reduction of the particulate mass, by the oxidation of long-chain hydrocarbons which are adsorbed on these particles. These long-chain hydrocarbons result from unburned fuel and lubricating oil and are generally considered as soluble organic fraction - SOF. DOCs have little effect on the reduction of nitrogen oxides (NOx). However, they are able to oxidize NO to NO2. The oxidation of NO to NO2 is helpful in aftertreatment systems of DOC and SCR in enabling fast reduction of NOx by NH3 on SCR technologies.
2 NO + O2 → 2 NO2