AN X-RAY STUDY OF PARTICIPATION OF BULK PHASE OF COBALT OXIDE IN OXIDATION CATALYSIS

Although catalysis is ordinarily associated with surface reactions, certain metal oxides, such as cobalt oxide, exhibit profound changes in bulk-phase composition during catalytic oxidation. We have studied the bulk-phase transformations accompanying the oxidation of hydrocarbons using in situ X-ray diffraction, a method which allows measurement under reaction conditions, i.e., high temperature and a reactive gas atmosphere. This technique permits us to perform successive measurements on the same solid sample at different conditions, and results in enormous time-saving over conventional X-ray diffraction. The data reveal a stable coexistence of Co3O4 and CoO in the presence of a hydrocarbon-air mixture. The proportion of these two phases is shown to vary in a regular fashion with temperature, hydrocarbon-to-air ratio, reaction initiation procedure, and precalcination temperature of the sample. This variation allows a computation of the activation energy for reorientation of the catalyst and indicates a very rapid ionic diffusion within the solid. From the data, it can be demonstrated that within a range of conditions determined by the prior treatment of the catalyst, a constancy exists in the order of the reaction between the solid and the gas phase.