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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.

A study of the impact of the exhaust environment and catalyst composition on the performance of direct soot oxidation catalysts has been performed. In agreement with previous studies it is noted that contact efficiency between soot and catalyst dictates soot oxidation rate, with tight contact required for low temperature soot oxidation. However it is shown that contact efficiency is not a static feature and under application conditions a ‘De-Coupling’ of intimately mixed catalyst and soot can occur in-situ, resulting in loss of contact efficiency and activity. The mechanism of ‘De-coupling’ has been investigated and demonstrated to arise from the ubiquitous NO2-mediated soot oxidation mechanism. It is proposed that incomplete filter regeneration by NO2 dictates the requirement for active filter regeneration due to retention of residual soot, physically ‘distant’ from the washcoat.