Mechanism of reductive oxygen adsorption on active carbons with various surface chemistry

With the use of a semi-empirical quantum chemical method AM1, it is shown that the availability of 4-6% oxygen heteroatoms of furan- and pyrone-type, and/or also 2-3% N of pyrrol-type in a carbon matrix should provide greatest electron-donor ability to carbons. The synthetic carbons HPSC and SCN, containing various states and amounts of O- and N-heteroatoms, were prepared and analyzed to confirm this prediction experimentally. Utilizing an electrochemical cell with divided electrode spaces, the reduction processes of a series of strong oxidizers and 02 (on a spongy platinum electrode) in synthetic carbons with various surface chemical compositions were investigated directly by measuring a current in an external circuit. A good correlation between calculated data and electrochemical measurements was obtained. Two mechanisms of chemisorption of oxygen on evacuated carbons are suggested: homolytic (free radical) at small degrees of filling of a surface by oxygen, and heterolytic (at large degrees of filling) causing the fixation of oxygen on a surface in the form of a superoxide-ion O*(2-). (C) 2003 Published by Elsevier B.V.