PYROLYZED COBALT PHTHALOCYANINE AS ELECTROCATALYST FOR OXYGEN REDUCTION

Cobalt phthalocyanine (CoPc) adsorbed on carbon black (Vulcan XC-72) and heat-treated at temperatures ranging from 300 to 1150-degrees-C display catalytic activity toward the electroreduction of oxygen in acidic medium (H2SO4, pH 0.5). The best catalysts are obtained for pyrolysis temperatures ranging from 700 to 950-degrees-C. X-ray diffraction performed on CoPc/XC-72 pyrolyzed between 700 and 1150-degrees-C reveals the presence of beta-Co particles whose average size varies from 9 nm at 700-degrees-C to 44 nm at 1150-degrees-C. Co and N bulk elemental analyses have been performed on CoPc/XC-72 heat-treated from 20 to 1150-degrees-C. These show that: (i) there is no loss of Co even after pyrolysis ai 1150-degrees-C when the loading is at 2 weight percent (w/o) Co; (ii) the bulk N content decreases as the pyrolysis temperatures are increased and the N content reaches the detection limit (0.5 w/o) at pyrolysis temperatures greater-than-or-equal-to 1000-degrees-C. Our x-ray photoelectron spectroscopy (XPS) study shows that at 600-degrees-C there is a sudden-three-fold increase in the surface concentration of Co and N at the surface of the carbon black support. A sublimation-redistribution of the CoPc is proposed. The effect appears to limit the Co loading to approximately 2 w/o (At loadings of 4 and 8 w/o Co, is most of the Co is lost due to the sublimation.) The XPS study also shows that metallic Co particles begin to be formed at 600-degrees-C, and that the formation and growth of Co particles occurs as the pyrolysis temperature increases to 1050-degrees-C. The chemical stability of the pyrolyzed catalysts was evaluated in concentrated H2SO4, HCl and HNO3 for time periods ranging from 1 to 30 min. Bulk Co analysis, after immersion in acid, indicate that up to 40% of the Co can be lost in the process, and that this induces a decrease in the catalyst activity. All of the Co leaching was found to occur during the first minute of immersion. As both Co and Co oxides readily dissolve in these acidic conditions, it is proposed that the metallic Co particles detected by x-ray diffraction and XPS are protected by a corrosion resistant material whose structure and composition still remains to be established.