Metallomacrocyclics, catalytic activities

Phosphoric acid is one of the electrolytes used in fuel cells and very few reports have focused their attention on the activity of metallomacrocyclics in this electrolyte (without heat treatment). Vasudevan et alP investigated the electro-catalytic activity of cobalt phthalocyanine monomers and polymers with imido and carboxylic group ends. The complexes were mixed with carbon powder and polyethylene powder. The activity of the monomeric compounds was found to be higher than that of polymeric compounds. 

Since the interaction of the oxygen with the active site involves a partial oxidation of the metal in the complex or at least a decrease of electron density in the metal upon interacting with O2, it is interesting to compare the catalytic activity of metallomacrocyclic with their M(111)/(II) formal potential. Since the formal potentials are sensitive to the pH of the electrolyte, it should be measured in the same media in which the catalytic activity is examined [14, 20, 98]. Further, they should also be measured with the complex adsorbed on the electrode and not in solution phase. When comparing phthalocyanines, the Co and Fe derivatives show the highest activity for the reduction of O2 but they behave differently. As pointed out above, Co complexes exhibit Co(lll)/(II) transition that is far more positive than the onset potential for the reduction of O2, whereas for Fe complexes the onset potential for the catalytic reduction of O2 is very close to the Fe(III)/(II) transition [14, 82,97,99]. For both types of complexes, there is in situ spectroscopic evidence... 

In a recent study [111], it has been discussed that the changes in the formal potential of the catalyst could explain the high catalytic activity of ORR that has been obtained by heat treatment of metallomacrocyclics and other starting materials. For example, when comparing data obtained with heat-treated catalysts prepared by very different techniques and starting materials, not necessarily involving metalloma-crocyclic complexes [112], a correlation of log i (as currents normalized per mass of catalysts) vs. the formal potential of the catalyst gives what could be considered a linear correlation (see Fig. 7.21). 

Zagal JH, Paez MA, Silva JF (2006) Fundamental Aspects on the Catalytic Activity of Metallomacrocyclics for the Electrochemical Reduction of O2. In Zagal JH, Bedioui F, Dodelet JP (eds) N4-Macrocyclic Metal Complexes. Springer, New York, pp 41-82... 

Other interesting systems that show catalytic activity is a variety of molecules that contain thiol groups [39 1]. Studies of the electrooxidation of thiols to give disulfides have shown that the catalytic activity of the molecular electrodes of gold. In this case, the metallomacrocycles of Co and Fe have exhibited the high activity. A very interesting case corresponds to the oxidation of L-cysteine and... 

The development of N4 metallomacrocyclic compounds was inspired by biological catalysts such as cytochrome c and hemoglobin, and has received considerable attention in the last three decades. The attraction of these complexes is due to their lower costs, compared to nohle metals, and the ability to modulate their catalytic activity by changing the stmcture of the macrocyclic ligand. Figure 2 illustrates typical N4 macrocyclic stractures. 

In this chapter, we discuss the catalytic properties of metallomacrocyclics (MN4) on the electrochemical oxidation of hydrazine molecules in aqueous media. The different factors affecting the catalytic activity are discussed, such as the influence of the central metal and the ligand on hydrazine electrooxidation. The practical performance of the MN4 macrocydie-modified electrode is also discussed. 

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