Diesel oxidation catalysts (DOCs) are the most common aftertreatment emissions
control technology found in current diesel vehicles. DOCs are very similar to the earliest catalysts used for gasoline engines. Oxidation catalysts work by oxidizing CO, HC and the soluble organic fraction of the PM to CO2 and H2O in the oxygen rich exhaust stream of the diesel engine.
When sulfur is present in the fuel, DOCs also increase the oxidation rate of SO2, leading to dramatic increases in sulfate nanoparticle emissions. Sulfate conversion depends on overall catalyst efficiency, with more efficient catalysts capable of converting nearly 100% of the SO2 in the exhaust to sulfate. Generally, one should restrict the use of DOCs to areas which have fuel sulfur levels of 500 PPM or below. With low sulfur fuel, a DOC can reduce PM emissions by 25 to 30%.