Effects of Selected Catalysts on Kinetic Parameters of the Carbon-Steam Reaction

A comparison has been made between the catalytic effects of two groups of elements on the steam gasification of carbon: the alkaline earths and noble transition metals of Group VIII. Spectroscopically pure graphite was selected as a model carbon because it consists of well-defined crystals with a large number of atoms located in basal planes, which, without a catalyst, are of low reactivity. Moreover, foreign atoms which can enhance or inhibit catalysis are eliminated. Local overheating is prevented by the endothermic reaction with steam. It is not possible to distinguish unambiguously reaction-rate increases associated with a lowering of the activation energy and those caused by an increase of the reaction-site density. Thus conclusions about the mode of catalyst action are by necessity tentative. However, evidence from electron micrographs, in connection with elemental maps and theoretical calculations indicates that catalysis in the case of alkaline earths can be explained essentially by an increase of the reaction-site density. A close contact between these catalysts and the carbon surface is preserved during the reaction. In contrast, metals of Group VIII have a tendency to coalesce and to become isolated from the graphite surface. Here a lowering of the activation energy is a distinct possibility due to intercalation of these metals and weakening of the C-C bonds which are to be severed in the carbon gasification.