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The suitability of newly developed non-promoted and ceria-promoted palladium−rhodium automotive catalysts for the exhaust gas control of a hybrid drive system has been tested by light-off experiments with steady and cycling feed stoichiometry, and pulsed-flow operation. The dynamic behavior of the honeycomb-type catalysts has been compared to the performance of standard honeycomb platinum−rhodium catalysts. Light-off tests carried out in the range 150−500 °C indicated significant differences in the conversion of NO x , CO, and hydrocarbons during warm-up, depending on the catalyst composition and whether λ-cycling was applied or not. Appropriate λ-cycling substantially improved the behavior of all catalysts in the lower light-off region. At higher temperatures cycling afforded lower conversions of all target components. Under pulsed-flow operation with an air pulse preceding the exhaust pulse (filling of the cylinder with air), asymmetric λ-cycling with longer rich half-cycles resulted in CO and HC conversions as well as N2 yields similar to or higher than those without an air pulse for the ceria-promoted catalysts. The catalytic tests suggest that ceria-promoted palladium is competitive to Pt−Rh−Ce in a hybrid vehicle application.

Alloy I Catalysis I Glasses I Materials Properties I MetalsPalladiudzirconia catalysts highly active for the oxidation of CO can be prepared by exposing amorphous Pd-Zr alloys to CO oxidation conditions at 280°C. The bulk chemical and structural changes occuring under these conditions have been studied using thermoanalytical methods (TG, DTA) combined with mass spectrometry and in-situ powder XRD. Amorphous PdZr2 and PdZr3 alloys exhibit virtually no activity when exposed to CO oxidation conditions, mainly due to their low specific surface area (-0.01 m'lg). The activity develops with time on stream, passes through a maximum and reaches a stable state only after several hours. The maximum in the activity is observed when about 50-70% of the amount of oxygen necessary for complete oxidation of the precursor to PdO and Zr02 has been consumed. The oxidation of the amorphous Pd-Zr alloys, which results in a drastic increase of the specific surface area of the samples, starts at significantly lower temperature than the crystallization temperatures of the alloys. The stable catalysts contain poorly crystalline monoclinic and tetragonal ZrOz, metallic palladium and PdO as bulk phases. The concentration of these phases is influenced by a series of simultaneously occurring reactions, including: the oxidation of the alloy constituents by O2 which results in PdO and Zr02, the oxidation by COz resulting in Pd and Zr02, and the reduction of the PdO formed by CO and by metallic Zr present in the unreacted part of the alloy. The solid state reduction 2 PdO + Zr -Pd + ZrOz contributes significantly to the reduction of the PdO as long as metallic Zr is abundant in the alloys.

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