Recently investigations into the suitability of organic substances as catalysts for the cathodic reduction of oxygen in acid electrolytes have been started because sufficiently active inorganic substances have not yet been found. Organic metal chelates play an important role in nature as active groups in enzyme catalysts. Cytochromes are mentioned as an example. They serve as redox systems or for the activation of oxygen and have derivatives of porphyrin chelates as active groups. We therefore investigated as catalyst candidates the following metal chelates: tetraaryl porphyrin, phthalocyanine, tetraazaannulene and Pfeiffer complexes. The catalytic activity of phthalocyanines for the cathodic reduction of oxygen is known. Jasinsky 1 used cobalt-phthalocyanine in alkaline electrolyte; Jahnke 2'3 discovered the catalytic activity of phthalocyanine-carbon support combinations for the oxygen reduction in acid electrolyte.
In order to determine the catalytic activity of the catalyst powder we used the method of measuring in suspension (suspension electrode) after Podwjaskin and Shiygin 4 , Held and Gerischer s . In this method the catalyst is suspended by vigorous stirring and tossed against a gold gauze. The electrochemical reaction by which the adsorbed oxygen is reduced takes place during the time of contact. Afterwards the catalyst particles again adsorb oxygen while whirling about in the electrolyte through which oxygen is bubbled.
We synthesized derivatives of tetraphenyl porphyrin (Fig. ) by condensation of the corresponding derivatives of benzaldehyde with pyrrole 6 . The porphyrins were dissolved in dioxane and precipitated onto the carbon support. The active carbon (Norit BRX) we used has a fairly high electric conductivity and a large surface area. Figure shows a current-voltage curve for the oxygen reduction obtained with 5 mg chelate. It was measured potentiodynamically with low-voltage speed. From this kind of curves we can derive information on the activity of the catalyst. It is however, not possible to draw quantitative conclusions regarding the stationary current-voltage curve with solid porous electrodes. At these electrodes potentials higher by about 100 to 200 mV are obtained than with the suspended catalysts as we know from measurements with the pure BRX carbon. Fig. also shows a current-voltage curve obtained with iron-phthalocyanine on BRX carbon. In contrast to the monomeric porphyrin, the iron-phthalocyanine is only appreciably active in the polymeric form. The polymeric iron-phthalocyanine was synthesized using pyromeUithic dianhydride and urea ~ . It was precipitated onto the support from a solution in concentrated sulfuric acid.