Hysteresis Phenomena on Pt- and Pd-Diesel Oxidation Catalysts: Experimental Observations

In Diesel Oxidation Catalysts (DOC) platinum (Pt), palladium (Pd), or a combination of both (alloyed or multilayered) are the typical active components. During lightoff/lightout experiments, hysteresis phenomena of CO, NO, and HC conversion have repeatedly been reported and attributed to thermal effects, to surface coverage effects and/or to noble metal oxidation. A detailed understanding of the causes of these hysteresis phenomena will be crucial, especially for modeling purposes. The present contribution provides detailed experimental evidence on single component conversion as well as on the mutual influence of CO, NO, and HC on the respective conversion for close to commercial Pt- and Pd-only catalysts under isothermal conditions. It will be shown, that hysteresis effects on Pd-only catalysts mostly vanish after one lightoff. Contrarily, on Pt-only catalysts, a cyclic, stationary conversion hysteresis during lightoff/lightout experiments is observed for CO and NO. C₃H₆-only however does not show this phenomenon whereas small hysteresis in conversion can be observed in combination with CO and/or NO.