Heat-treated macrocyclic compounds structure

In spite of a very significant progress achieved with heat-treated macrocyclic compounds as ORR catalysts since the early 1970s, the activity and durability of that family of catalysts are stiU insufficient for replacing platinum at the fuel cell cathode and in other applications. Furthermore, the complex structure of macrocyclic compounds makes their synthesis expensive and potentially noncompetitive with precious-metal-based catalysts also from the materials cost point of view. For those reasons, much effort has been invested by the electrocatalysis research community in recent years into finding less expensive and catalytically more active non-precious metal ORR catalysts that would not rely on macrocylic compounds as either catalysts or catalyst precursors. In the past decade, there has been a significant improvement both in the activity and of non-macrocyclic catalysts, expected to be manufactured at a fraction of the cost of their macrocyclic counterparts. In this section, we review the precursors, synthesis routes, and applications of this relatively new family of catalysts. 

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. 

The real structure, or structures, of heat-treated ORR catalysts is yet to be revealed. The catalyst makeup is likely to strongly depend on the precursors used, with the structure of catalysts derived from different macrocycle compounds differing from one another due to such factors as the type of a macrocycle, substituent, carbon support, and the metal center. This is an intriguing subject of future research that promises to produce non-precious metal ORR catalysts with much improved activity and performance durability. 

History. Braun and Tschemak [23] obtained phthalocyanine for the first time in 1907 as a byproduct of the preparation of o-cyanobenzamide from phthalimide and acetic anhydride. However, this discovery was of no special interest at the time. In 1927, de Diesbach and von der Weid prepared CuPc in 23 % yield by treating o-dibromobenzene with copper cyanide in pyridine [24], Instead of the colorless dinitriles, they obtained deep blue CuPc and observed the exceptional stability of their product to sulfuric acid, alkalis, and heat. The third observation of a phthalocyanine was made at Scottish Dyes, in 1929 [25], During the preparation of phthalimide from phthalic anhydride and ammonia in an enamel vessel, a greenish blue impurity appeared. Dunsworth and Drescher carried out a preliminary examination of the compound, which was analyzed as an iron complex. It was formed in a chipped region of the enamel with iron from the vessel. Further experiments yielded FePc, CuPc, and NiPc. It was soon realized that these products could be used as pigments or textile colorants. Linstead et al. at the University of London discovered the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines from 1929 to 1934 [1-11]. The important CuPc could not be protected by a patent, because it had been described earlier in the literature [23], Based on Linstead s work the structure of phthalocyanines was confirmed by several physicochemical measurements [26-32], Methods such as X-ray diffraction or electron microscopy verified the planarity of this macrocyclic system. Properties such as polymorphism, absorption spectra, magnetic and catalytic characteristics, oxidation and reduc-... 

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