TRANSITION METAL MACROCYCLES AS CATALYSTS FOR THE ELECTROCHEMICAL REDUCTION OF OXYGEN-2 (CALCULATION, PHTHALOCYANINE, MOLECULAR ORBITAL THEORY, IN-SITU, MOESSBAUER).

摘要

Several transition metal macrocycles have been used as catalysts for the electrochemical reduction of O(,2). In an effort to obtain theoretical insight a molecular orbital study was performed using the semiempirical atom superposition electron delocalization (ASED-MO) theory. The calculations have been carried out starting from a primitive fragment of the Me-N(,4) type-catalyst and building up larger ring molecules containing various substituents. The energy difference between the d(,xz), d(,yz) orbitals of the metal and the electron acceptor orbital of the ligand, (DELTA)E, is defined and appears to be an important parameter related to the activation of O(,2) for a variety of Me-N(,4) type-catalysts such as metal phthalocyanines and porphyrins. Significant changes in the value of (DELTA)E are observed when the resonant structures of the different macrocycles are compared.; The calculations have been complemented by the rotating ring-disk electrode studies of O(,2) electroreduction and the result found to be in agreement with the theoretical predictions. In addition cyclic voltammetry, complemented with Mossbauer spectroscopy was done on iron phthalocyanine in adsorbed and bulk forms in carbon electrodes. From in-situ Mossbauer measurements it was found that one of the doublets believed to be associated with adsorbed iron phthalocyanine (FePc) was in fact caused by FeOOH generated following the adsorpton procedure as a result of heat treatment in preparation of the porous carbon electrodes. The research also demonstrates for the first time the use of Mossbauer spectroscopy in an operating fuel cell.; Several groups have reported considerable gain in stability without loss of the catalytic activity for the overall O(,2) reduction in acid and alkaline electrolyte upon heat treatment of metal porphyrins preadsorbed on high surface area carbon. The conclusion of this research on heat treated catalysts include the following: (a) after thermal treatment at 800(DEGREES)C no evidence of Co-N(,4) center was found and (b) the catalytic activity requires the presence of the transition metal during heat treatment for high activity.