Fe- and Co Macrocycles After the

In Sect. 2.1 it was shown that three molecular structures were proposed during the 1980s by van Veen, Yeager, Wiesener, and their respective collaborators, as potential catalytic sites for the reduction of oxygen in acid medium. In this section, how these three molecular models for the catalytic site evolved since their first appearances in the literature is described. 

During the 1990s, van Veen and collaborators mainly studied the electrochemical kinetics of oxygen reduction. Their results are presented in Sect. 3. These mechanistic studies were, however, always based on the model in which the C0-N4 or Fe-N4 moieties of the respective macrocycles were retained intact at all pyrolysis temperatures. Their last contribution to the molecular structure of the catalytic site was a study in 2002 of catalysts obtained by adsorption of iron tetramethoxyphenyl porphyrin chloride (ClFeTMPP) on Vulcan XC-72, heat treated between 325 and 800°C in inert atmosphere, and characterized by EXAFS and Mossbauer spectroscopy, as well as by cyclic voltammetry . The loading of these catalysts was 7 wt% chelate ( 0.5 wt% Fe). 

Publications from several new groups, including ours, appeared during the 1990s and the idea of two catalytic sites, one at low pyrolysis temperatures and another one at high pyrolysis temperatures, was put forward. The most important players during these years (at least by the number of their publications) were Savy, Savinell, and their respective collaborators, as well as our group. 

Further studies by the same group involved mainly CoTAA and also CoTMPP. These molecules were adsorbed on Norit SX Ultra and were usually pyrolyzed between 600 and 800°C. The optimum CoTAA loading was 13 wt% chelate ( 2.3 wt% Co). At the lowest heat-treatment temperature (600°C), the 

Lower nitrogen oxidation states such as NH3OH+ or N2H5+ are highly favorable to a protonation reaction such as  

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