Catalytic oxidation of graphite by iridium and rhodium

Abstract

Controlled atmosphere electron microscopy has been used to investigate the catalytic influence of iridium and rhodium on the graphite-oxygen reaction. Both systems were found to exhibit a similar behavioral pattern. As the temperature was raised, the initial catalytic action was followed by a short period of inactivity before a second, more intense catalytic attack occurred at temperatures >1000 °C. It is probable that the two activity regions correspond to the existence of oxides, IrO 2 and Rh 2O 3 at the lower temperatures and the respective metals at temperatures >1000 °C. This pattern is quite different than that found for platinum and palladium, which are present in the metallic state throughout the gasification sequence and as a consequence exhibit only a single continuous catalytic action on the graphite-oxygen reaction. Another aspect revealed from these experiments is the manner by which iridium and rhodium particles supported on graphite sinter in an oxidizing environment. Atomic migration accounts for the growth of inactive particles below the Tammann temperature for the metal (1110 °C for iridium; 910 °C for rhodium); above this temperature the overriding mode of growth is via particle migration. Particle mobility on graphite can occur below the Tammann temperature as the catalytic channeling action induces motion into active particles.