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Technical Paper

Modeling of OSC Function over Three-way Catalyst

Three way catalysts (TWC) contain Oxygen Storage Component (OSC) materials to enhance HC, CO oxidation and NOx reduction performance under standard operating conditions where there is rapid perturbation of the air-to-fuel ratio (A/F). The OSC function is required to storage and to release oxygen, however the optimum storage capacity and release rate to maximize HC, CO and NOx conversion varies as a function of engine operating conditions, such as A/F perturbation frequency, amplitude and temperature. At the same time, it is necessary for the vehicle on board diagnostics (OBD) systems to monitor that the catalyst OSC is functioning correctly. Detailed understanding of how OSC characteristics can simultaneously match gas performance and OBD functionality are not well known. In this study, modeling of the OSC function was attempted by considering chemical functions to be analogous to that in an equivalent electrical circuit, having components of resistance and capacitance.
Technical Paper

Oxygen Partial Pressure over Precious Metals and Its Effect on HC Oxidation Performance

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.
Technical Paper

Effective Catalyst Layer Study by Computer Calculation

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.
Technical Paper

Development of Methane Oxidation Catalyst and Its Mechanism

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.
Technical Paper

SOF Combustion Behavior in Flow-thru Diesel Oxidation Catalysts

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.
Technical Paper

Low Sulfate Generation Diesel Oxidation Catalyst

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.