A Systematic Evaluation of Sulfur Poisoning and Desulfation Behavior for HD Diesel Oxidation Catalysts

To enable better matching of Diesel Oxidation Catalyst (DOC) properties to aftertreatment system and application requirements, a systematic evaluation of the effects of sulfur poisoning and desulfation was undertaken on a number of Heavy Duty DOC formulations at representative Platinum Group Metal (PGM) loadings. Uniformly coated DOCs having PGM ratios from 1/0 Pt/Pd to 0/1 Pt/Pd with commercial HDD DOC washcoats were evaluated on a Tier 3 Non-Road engine. In addition, a new DOC formulation intended for reduced sulfur sensitivity, a DOC containing zeolite for hydrocarbon (HC) adsorption, and a layered DOC containing both high and low Pt/Pd ratio layers were compared. Two levels of PGM loading were included for three of the uniform sample formulations. The primary evaluation was through a series of light - off tests under which the NO₂ make by oxidation of NO, and the HC and CO oxidation efficiency were observed, quantifying the expected increased sensitivity to poisoning as the Pt/Pd ratio decreases. The catalysts were evaluated as hydrothermally aged to a full useful life reference aging, after being sulfur poisoned, and as desulfated at successively higher steady state temperatures. This allowed a steady state desulfation temperature to be identified for each catalyst, but additional testing was also done to observe the desulfation effect of transient cycles. Sulfur poisoning was accomplished by the addition of 40 ppm of SO₂ gas to the engine exhaust to a level of ~3 g/L of DOC elemental sulfur exposure, and the decline of the NO, CO, HC oxidation were observed over the period. Additional poisonings of one example were made at lower SO₂ concentrations showing that the lower poisoning acceleration rate resulted in less severe poisoning than the equivalent sulfur mass exposure at the higher rate.