Metal-N4 chelates

Since the 1970s, it has been reported that the heat treatment of metal N4 chelates at high temperature could significantly increase their catalytic activity and stability towards the electroreduction reaction of oxygen [13, 26]. Since then, this pioneering work has led to worldwide research interest. Figure 15.2 shows the effect of heat treatment on CoTAA catalytic activity. Maximum ORR catalytic activity can be achieved at a heat-treatment temperature of 600 °C, and optimal stability at 800-900 °C. 

Low molecular N4-chelates may react to polymers where the metal atom is a part of a polymer chain. Generally, ihm w( s for construction of such a polymer are possible (Scheme 5) ... 

A new kind of interesting polymer with an acetylene dianion between the metal atoms of N4-chelates was synttesized Polymeric chelates [Si(C)2Pc] (74) were prepared by reacting Si(Cl)2Pc (63) in THF with bis-bromomagnesiumacetylenes or Si(C2MgBr)2Pc (75) (Eq. 33). 

A new concept for metallic conductivity in staggered N4-chelates was introducted by Hanack and Seelig After calculating MO, a tetraazaporphyrine macrocycle with iron as central metal and acetylide ion ( 2 ) as bridging ligand leads to a change in energy band structure. 

Polymer N4-Chelates with the Metal in the Main Chain... 

A third school, led by Wiesener, proposed that the Co or Fe ions of the adsorbed N4 chelates promote the decomposition of the chelate upon thermal treatment followed by the formation, at high temperature, of a special form of carbon that would be the true catalyst. In this scenario the metal is only an intermediate and has no active role in the electroreduction of oxygen. In a later publication, they concluded that nitrogen is involved in the electrocatalytic active group on carbon. 

As far as the kinetics and mechanistic aspects of oxygen reduction on these non-noble metal electrocatalysts are concerned, it has been shown that these catalysts may reduce O2 to water with an apparent number of electrons transferred, n, that may reach values very close to 4. This is especially true for Fe-based electrocatalysts made either from Fe-N4 chelates or from cheaper Fe salt precursors. It seems also that the Fe-N2/C catalytic site, which is the most active site in catalysts obtained after a pyrolysis temperature > 800°C, is characterized by a low release of peroxide. Co-based catalysts release, on average, more peroxide than the corresponding Fe-based materials. Studies that were undertaken to decouple the direct 4-electron reduction of oxygen to water from the successive 2 X 2-electron reduction indicate that the direct 4-electron reduction path may be important for these catalysts. This result is in agreement with the quantum theoretical approach of Anderson and Sidik about a model of the pyrolyzed... 

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