STRUCTURAL EVOLUTION OF A CO MG OXIDE CATALYST IN HYDROGEN AND CARBON-MONOXIDE ATMOSPHERES - EFFECT OF THE COBALT PARTICLE-SIZE ON THE STRUCTURE OF CARBON FORMED IN THE REACTION OF CO DISPROPORTIONATION

The behavior of solid solutions of Co and Mg oxides in hydrogen and ca rbon monoxide upon changing the treatment temperature from ambient to 750 or 500 degrees C, respectively, was examined by thermogravimetry, X-ray diffraction analysis, and electron microscopy. The crystal struc ture, microtexture, and morphology of the samples were analyzed depend ing on the initial composition, preparation procedure, and subsequent treatment. It was found that Co2+ incorporated into a solid solution w as reduced to the metal state in an Hz atmosphere to form the beta-Co phase at 360-600 degrees C. Cobalt metal obtained from the solid solut ions Co1-xMgxO was much more finely dispersed than that prepared by re duction of individual CoO. Upon treatment of the reduced samples with carbon monoxide at temperatures above 280 degrees C, the reaction of C O disproportionation proceeded with the formation of two types of grap hite-like carbon: shells up to 100 Angstrom in thickness on Co particl es with a diameter larger than 250 Angstrom and nanotubes with an exte rnal diameter of 100-200 Angstrom and an internal diameter of 30-50 An gstrom, which were associated with parent Co particles 100-200 Angstro m in diameter. The structure of large Co particles was found to be hig hly defective upon CO treatment (the existence of extended regular def ects). At the same time, the structure of the particles of size simila r to 100 Angstrom remained defect-free after this treatment.