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. The palladium catalyst showed higher oxygen storage characteristics and higher HC oxidation performance under a transient rich and lean cycle. As a result of this study, the higher methane (and HC) oxidation performance of palladium is now considered to be due to its higher oxygen affinity, higher oxygen adsorption and desorption rate as well as its higher dehydrogenation performance.