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 carbon 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 structure, microtexture, and morphology of the samples were analyzed depending on the initial composition, preparation procedure, and subsequent treatment. It was found that Co2+ incorporated into a solid solution was 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 solutions Co1-xMgxO was much more finely dispersed than that prepared by reduction of individual CoO. Upon treatment of the reduced samples with carbon monoxide at temperatures above 280 degrees C, the reaction of CO disproportionation proceeded with the formation of two types of graphite-like carbon: shells up to 100 Angstrom in thickness on Co particles with a diameter larger than 250 Angstrom and nanotubes with an external diameter of 100-200 Angstrom and an internal diameter of 30-50 Angstrom, which were associated with parent Co particles 100-200 Angstrom in diameter. The structure of large Co particles was found to be highly defective upon CO treatment (the existence of extended regular defects). At the same time, the structure of the particles of size similar to 100 Angstrom remained defect-free after this treatment.