On the Development of Low PGM Content Direct Soot Combustion Catalysts for Diesel Particulate Filters 2010-01-0558
A study of the application of base-metal- and precious-metal-doped base metal direct soot oxidation materials to the remediation of diesel particulate matter has been performed. The study has addressed the impact of reactive gas environment on the de-coupling of catalyst to soot contact, the control of secondary emissions of CO, HC and NOx and the impact of "dilution" of the washcoat by the substrate mass. These studies have confirmed previous findings regarding the toxic effect of NO₂ generation at the catalyst-soot interface but also demonstrate that under ideal circumstances propagation of the exotherm related to CO and (especially) HC light-off can result in an "exotherm cascade" capable of providing soot ignition at T ≺ 250°C. Moreover it is shown that the presence and type of substrate (Cordierite versus SiC) has a direct impact on this reaction propagation phenomenon and thus affects both the effectiveness of the direct soot combustion catalyst as well the extent of secondary emissions of CO and HC. The study is completed by an examination of the implementation of a secondary emissions control function which is derived from the addition of low Precious Group Metal (PGM) contents to the catalyst formulation. It is shown that addition of low levels of Pd completely addresses CO and HC, while the addition of low concentrations of Rh both addresses CO and HC but also affects the reduction of NOx and N₂O to N₂ under the locally fuel-rich conditions present during the soot combustion event.