The Effect of Pt: Pd Ratio on Heavy-Duty Diesel Oxidation Catalyst Performance: An Experimental and Modeling Study

A combined experimental and modeling study was carried out to investigate the effects of Pt: Pd ratio on the performance of diesel oxidation catalysts (DOC) for heavy-duty applications(1) (PGM< 50 g/ft(3)). In the first part of this work, transient light-off and steady-state experiments were performed over a series of hydrothermally aged DOCs with different Pt: Pd ratios and PGM loadings. It was found that n-decane and NO oxidation activities increased monotonically as the Pt: Pd ratio was increased while the oxidation of unsaturated hydrocarbons (HC) (C-3 H-6 and C-7 H-8) first increased with an increase in Pt: Pd ratio and then plateaued at higher Pt content. In contrast, the CO oxidation exhibited opposite trend, with the catalyst containing low Pt (high Pd) level being more active. The presence of HC lowered the outlet NO2 /NOx ratio by reducing the NO2 generated via NO oxidation back to NO. The negative effect of HCs on NO2 /NOx ratio increased in the order: C-3 H-6 < C-7 H-8 < C-10 H-22. The effect of oxidative and reductive pre-treatments on "inverse hysteresis"phenomena observed during NO oxidation was also studied for selected degreened catalysts. The results show a strong dependence of the NO oxidation on the catalysts' oxidation state for different Pt-Pd DOCs. In the second part, a comprehensive kinetic model was developed to capture the effect of Pt: Pd ratio and PGM loading on key DOC reactions. The model was then validated against engine data. A case study was also carried out to demonstrate the capabilities of the model in designing an appropriate DOC for a given application.