The emission reductions in the Diesel Oxidation Catalyst occur through chemical oxidation of pollutants occurring over the active catalytic sites. Hydrocarbons are oxidized to form carbon dioxide and water vapor. Since carbon dioxide and water vapor are considered harmless, the action brings an obvious emission benefit. However, an oxidation catalyst will promote oxidation of all compounds of a reducing character; some of the oxidation reactions can produce undesirable products and, in effect, be counterproductive to the catalyst purpose. Oxidation of sulfur dioxide to sulfur trioxide with the subsequent formation of sulfuric acid is perhaps the most important of these processes.
When the exhaust gases are discharged from the tailpipe and mixed with air, either in the environment or in the dilution tunnel which is used for particulate matter sampling, their temperature decreases. Under such conditions the gaseous sulfuric acid combines with water molecules and nucleates forming liquid particles composed of hydrated sulfuric acid. This material, called sulfate particulates, contributes to the total particulate matter emissions from the engine. Catalytic formation of sulfates, especially in conjunction with high sulfur content diesel fuel, can significantly increase the total particulate matter emissions and, thus, become prohibitive for the catalyst application.
A Diesel oxidation catalyst works similar to the way that the catalytic converter on a car functions. As exhaust gas passes over the catalyst material, a chemical exchange occurs and the emissions constituents are oxidized to CO2 and water. An oxidation catalyst is often referred to as a passive technology because it requires no external supplemental energy to be added. It needs only the natural temperatures within the diesel exhaust to catalytically convert through chemical.
Diesel oxidation catalysts typically reduce up to 85 percent of carbon monoxide, up to 60 percent of hydrocarbons and up to 20 percent of particulate matter from the exhaust stream. An oxidation catalyst is ideal for equipment that works in indoor applications, confined spaces or highly populated areas. Unlike some after-treatment options, the oxidation catalyst can be used regardless of the level of sulfur in the fuel, although they are most effective with fuels that have sulfur content of 500ppm or less.