The role of NO Selective Catalysts in the Plasma Enhanced Removal of NOx and PM from Diesel Exhausts 2001-01-3568

Non-thermal plasma (NTP) systems are attracting increasing interest as candidates for future generation diesel exhaust aftertreatment technologies. Strategies are being investigated for lean NOx reduction, particulate (PM) oxidation and for combined NOx and PM aftertreatment. One approach for plasma-catalyst NOx reduction is to convert NO to NO₂ with a plasma and then reduce the NO₂ over a suitable catalyst. However, it has been recently suggested that particulate in the diesel exhaust may inhibit the NTP conversion of NO into NO₂ in a plasma regenerating diesel particulate filter (DPF). In addition some NO₂ selective catalysts are demonstrating high levels of N₂O production rather than being selective to N₂. It has been suggested that combined NOx and PM removal could be achieved by employing an NO selective reduction catalyst in combination with a plasma DPF.

Silver-doped alumina (Ag-Al₂O₃) is shown to be a particularly effective catalyst for NO reduction. Combination of this catalyst with a plasma resulted in the NO_x reduction being enhanced. This effect is especially pronounced at lower exhaust temperatures characteristic of passenger car exhausts. It was found that the plasma catalyst enhancement was effective with both propene and diesel fuel acting as a reductant.

Studies appear to show a link between plasma enhanced NO reduction and the generation of active species such as formaldehyde (CH₂O). It appears that the composition of the reductant mixture in the exhaust stream plays a fundamental role in the plasma catalyst mechanism that enhances NO reduction. The combination of this catalyst and plasma would support the proposed PM / NO_x removal strategy.