Nonthermal plasma discharges in combination with catalysts are being developed for diesel aftertreatment. NOx conversion has been shown over several different catalyst materials. Particulate removal has also been demonstrated.
The gas phase chemistry of the plasma discharge is described. The plasma is oxidative. NO is converted to NO2, CH3ONO2 and HNO3. Hydrocarbons are partially oxidized resulting in aldehydes and CO along with various organic species. Soot will oxidize if it is held in the plasma. When HC is present, SO2 is not converted to sulfates.
Suitable plasma-catalysts can achieve NOx conversion over 70%, with a wider effective temperature range than non-plasma catalysts. NOx conversion requires HC and O2. Electrical power consumption and required exhaust HC levels increase fuel consumption by several percent.
A plasma catalyst system has demonstrated over 90% particulate removal in vehicle exhaust.
The plasma catalyst system is the subject of active research and knowledge is growing rapidly. Significant questions remain about the mechanisms of plasma catalysis both for NOx and particulates. The development is very early and rapid progress is being made.