An Investigation into the NO2-Decoupling of Catalyst to Soot Contact and Its Implications for Catalysed DPF Performance

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. Conversely it is shown enhanced, and in some cases complete, low temperature catalysed soot combustion is possible by application of appropriate washcoat formulations which overcome the deleterious impact of NO₂-decoupling. The benefits of this new generation of formulations are demonstrated in engine dynamometer soot loading and regeneration trials.