Improved CO, hydrocarbon and NO oxidation performance using zone-coated Pt-based catalysts

Abstract

The effect of an axial Pt distribution along a diesel oxidation catalyst (DOC) was investigated by comparing a standard catalyst, with a homogeneously distributed Pt amount along the length, with a non-homogeneously distributed catalyst (zoned). The zoned catalyst had more Pt located at the upstream portion, and less downstream, while maintaining the same total amount of Pt as the standard case. The effects of flow rate on NO, CO or C3H6 oxidation, and during oxidation of NO and C3H6 as a mixture, were used for the comparison. The reaction details along the catalyst were also resolved using spatially resolved, capillary inlet mass spectrometry (Spaci-MS). Results showed that the performance of the two catalysts were similar at low flow rate and with a single reacting gas, while the zoned sample worked better for CO and C3H6 oxidation as the flow rate increased, and better for NO oxidation in the NO/C3H6 gas mixture. With CO or C3H6 oxidation, the superior performance of the zoned sample was due to a larger, localized exotherm and a decreased self-poisoning effect. The exothermic heat produced in the front part of the zoned catalyst allowed it to reach higher temperature at the front faster than the homogeneous/standard sample and it also lowered the effect of self-poisoning by converting most of the reactants in the front part. NO oxidation, being kinetically more challenging, occurred along the entire length of catalyst at low temperature, not achieving near 100% conversion in these tests. Spatially resolved experiments during C3H6 and NO oxidation, as a mixture, showed that NO oxidation started after C3H6 was consumed. Therefore, for the zoned sample, C3H6 was oxidized closer to the inlet portion of the catalyst, where a higher density of Pt was located, leaving the rest of the catalyst available for NO oxidation. However, with the standard sample, C3H6 oxidation utilized a larger part of the catalyst leaving a smaller portion of the monolith for NO oxidation.