Impact of Different Forms of Sulfur Poisoning on Diesel Oxidation Catalyst Performance

Despite drastic reduction of sulfur content in diesel fuel in the recent years, especially with the introduction of Ultra-Low Sulfur Diesel (ULSD), sulfur poisoning remains one of the most significant factors impacting performance of various catalysts in diesel aftertreatment systems. This is because even with ULSD, cumulative exposure of a catalyst over its lifetime in a heavy-duty diesel system may amount to kilograms of sulfur. In this study, we have found that the impact of sulfur poisoning on the performance of various diesel oxidation catalysts (DOC) strongly depends on the catalyst's operation history. For example, exposing a DOC to limited amounts of freshly deposited sulfur in bench reactor testing was shown to have a substantial detrimental effect. On the other hand, several samples which returned from vehicle or test-cell aging with high sulfur loading, have shown no signs of poisoning. We were able to reconcile this apparent contradiction and simulate both types of behavior in a single sample. All the experimental data suggest that precious metal sites are being selectively poisoned during the initial sulfur deposition, presumably due to SO₂ oxidation to SO₃ on these sites. Evidently, SO₃ can subsequently redistribute to the support material and away from the precious metal sites, thus drastically reducing sulfur impact on catalyst performance.