Transition metal macrocycles supported on high area carbon: Pyrolysis-mass spectrometry studies

A number of cobalt and iron porphyrins heat treated at temperatures as low as 400°C exhibit substantial electrocatalytic activity for O₂ reduction and reasonable stability in alkaline electrolytes. The effects of this heat treatment on the structure and overall properties of these materials, however, are not well understood. Differential thermogravimetry analyses have shown that in the case of the Co and Fe-μ-oxo forms of tetra methoxyphenyl porphyrin (TMPP) and the metal-free form as well, the onset for partial decomposition occurs at temperatures of about 400-500°C for the macrocycles either as crystals or dispersed on high area Vulcan XC-72. The results obtained with several pyrolysis-mass spectrometric techniques have indicated that the fraction of volatile nitrogen to non-nitrogen containing species generated during the heat treatment is much higher for the metal-free than for the iron-μ-oxo or CoTMPP. Microanalysis also confirms that with the Fe-μ-oxo and CoTMPP part of the nitrogen is retained. Possible models for the nature of the active sites are discussed.