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Palladium catalysts are known to show higher methane oxidation performance than platinum and/or rhodium catalysts. In this paper, the higher oxidative dehydrogenation activity on palladium is proposed as a reason for the superior methane oxidation. When other oxidation reactions are considered, higher affinity of palladium to oxygen has also been suggested[1]. In this study, oxygen chemical potential on platinum and palladium catalyst surfaces under oxidation conditions was measured using a specially designed electrochemical sensor. The oxygen chemical potential was calculated from the sensor potential by the Nernst equation. As a result, oxygen potential on palladium during the methane oxidation reaction was found to be much higher than that of platinum, correlating with affinity to oxygen and higher methane oxidation performance. The rate of oxygen adsorption and desorption on platinum and palladium was evaluated in an engine experiment using a dual lambda-sensor procedure.

Sulfate generation by diesel oxidation catalysts (DOC) is still a problem although sulfur concentration in the diesel fuel will be reduced in future. Two approaches were attempted to reduce the sulfate generation without inhibiting the HC and CO oxidation performance. One was to use an optimized support material that adsorbs less SO2 and has sufficient specific surface area for HC/CO oxidation. Another approach was to apply a layer on the catalyst, which prevents SO2 adsorption. Sulfate generation was successfully reduced while maintaining high HC/CO oxidation performance.

Combustion behavior of the SOF (Soluble organic fraction) fraction of diesel particulate by flow-thru type diesel oxidation catalysts (DOC) was studied. A two brick DOC system with an air gap showed higher SOF performance than a single brick DOC of the same total volume. Collision frequency of the TPM (total particulate matter) to the catalyst layer was studied by calculation of the turbulence energy in the gas flow channel. No large difference in collision frequency was observed between one brick and two bricks. The front face effect was calculated from the geometric surface and it was confirmed that such an effect was small in the two brick DOC case. The SOF performance advantage for the two brick DOC system separated by an air gap was due to a thermo-mass effect created by reducing the DOC volume.

The transient HC performance of diesel oxidation catalysts is known to be greatly improved by addition of Zeolite material. The authors already reported how to estimate the effective washcoat thickness in our previous study [1]. To understand in more detail the effective catalyst layer thickness, a precise gas diffusion model and parameters of HC adsorption and desorption rate were determined in this study. The random pore model was used for a gas diffusion calculation to simulate the macro porosity of the catalyst layer and micro porosity of the Zeolite material. HC adsorption capacity as a function of temperature and HC concentration was measured by Temperature Programmed Desorption (TPD). HC desorption rate was evaluated by changing the TPD ramping rate. HC reaction rate was evaluated by using a model gas reactor. Calculated catalyst performance correlated to the experimental results, thus validating the model.

Palladium is well known to catalyze methane (CH4) oxidation more efficiently than platinum (Pt) and/or rhodium (Rh) catalysts. The mechanism for methane oxidation on palladium is hypothesized to proceed via a radical intermediate. Direct identification of a radical species was not detected by Electron Spin Resonance Spectroscopy (ESR). However, indirect evidence for a radical intermediate was found by identification of ethane (C2H6), the methyl radical(CH3 ˙ ) coupling product, by Mass spectroscopy analysis under CH4/O2 conditions.

Diesel oxidation catalysts (DOCs) combining the functions of Pd and Pt-Pd alloys have been used in practice to satisfy the strict exhaust emission regulations that have been specified for passenger cars in recent years. Pd is an indispensable component in DOCs because it exhibits superior oxidation activity for CO and HC. To reduce the amount of precious metal used and to improve robustness, it is important to control the electronic state and gas adsorption characteristics of Pd and PdOx during catalytic reactions.In this study, by investigating the CO adsorption behavior of Pd, it was observed that Pd supported on a CeO2/ZrO2 mixed-oxide material (CZ) showed a preferable CO adsorption state and better CO light-off performance. Pd in Pd/CZ became metallic with increasing reaction time, and the CO oxidation performance of Pd/CZ decreased. This change in activity was correlated with CO adsorption on Pd changing from linear-type to bridge-type adsorption.