Transition metal macrocycle catalysts heat treated

This class of PC catalysts has also been extensively studied as a potential substitute for Pt as they are low cost, methanol tolerant and have reasonably good activity and remarkable selectivity toward ORR [194]. They normally catalyze a direct 4e reduction of O2 to )deld water. The major drawback of this kind of catalyst is of low stability in acidic media [195]. However, when the catalyst is heat treated, the activity and stability of transition metal macrocycle complex (TMMC) are improved significantly [194]. The molecules of TMMC have a square planar structure with the metal ion symmetrically surrounded by four nitrogen atoms these nitrogen atoms are from each member of the ring systems which, in turn, are connected by carbon atoms (porphyrins) or nitrogen atoms (phthalocyanines). 

Heat treatment of macrocyclic compounds was shown to dramatically increase their ORR catalytic properties. The important factors in the heat treatment of catalysts will be discussed in this section together with their effect on the ORR catalysis. Also discussed will be the catalyst structure. The majority of research in this field has focused on the macrocyclic complexes of transition metals, heat-treated in the presence of various carbons. That class of ORR catalysts will be addressed below. 

In 1964, Jasinski discovered that certain macrocyclic compounds of transition metals (Fe, Co, etc.), N4 compounds such as phthalocyanins and tetra-azaannulenes, are very active catalysts for oxygen reduction in acidic solutions. However, their stability during prolonged work in acidic media was found to be very low. Sometime later it could be shown (Bagotsky et al., 1977-1978 that after thermal treatment at 700 to 800°C these compounds, despite their partial decomposition, not only retain their catalytic activity but acquire the chemical stability required for long-term operation. Sometimes their activity even increases. Heat-treated compounds of this type then became the subject of numerous studies. 

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