INSITU ELECTRON-MICROSCOPY INVESTIGATION OF THE BEHAVIOR OF SUPPORTED COBALT PARTICLES

In situ electron microscopy studies have shown that cobalt particles supported on graphite exhibit some very unexpected reactivity patterns during gasification reactions. When the reaction was performed in the presence of oxygen the cobalt particles were observed to initially wet and spread along graphite edges and accelerate the removal of carbon atoms from these locations by the edge recession mode. This type of action persisted up to temperatures of around 880°C at which point particles reformed and the mode of action changed to that of channeling. Quantitative kinetic analysis of these events showed the existence of three distinct activity regions, which corresponded to the formation of three oxidation states of cobalt, Coo (475-640°C), Co304 (665-880°C), and Co (>880°C), as determined from in situ electron diffraction analysis. It was significant that postreaction electron diffraction examination failed to show the presence of the metallic phase in specimens which had been cooled to room temperature. When colbalt/graphite specimens were treated in hydrogen, initial attack took place by the edge recession mode and was replaced by a channeling action at 650°C which coincided with the formation of particles at edge sites. This form of attack continued up to temperatures in excess of 1000°C, and the catalyst particles did not appear to show any tendency to deactivate. The results obtained in this study are compared with those found for iron and nickel from previous investigations. © 1991.