Macrocycles, transition metal, oxygen reduction

Oxygen Reduction on Macrocyclic Transition Metal Complexes on Graphite and Carbon Surfaces... 

Transition Metal Macrocycles for the Oxygen Reduction Reaction... 

Iliev I., Gamburzev S., Kaisheva A., Gas-diffusion electrodes with transition metal macrocyclic catalysts for electrochemical reduction of oxygen, Proceedings of the 31 ISE Meeting 1980 Sept. 22 - 26, Venice, Italy, Vol. I, p. 286-288. 

Transition metal compounds, such as organic macrocycles, are known to be good electrocatalysts for oxygen reduction. Furthermore, they are inactive for alcohol oxidation. Different phthalocyanines and porphyrins of iron and cobalt were thus dispersed in an electron-conducting polymer (polyaniline, polypyrrole) acting as a conducting matrix, either in the form of a tetrasulfonated counter anion or linked to... 

Mossbauer spectroscopy may be important and useful when applied to electrodes which contain ferromagnetic components. It is basically an in situ tool which provides valuable information on possible orientation and oxidation states of ferromagnetic species in the electrodes as a function of the electrochemical process and the potential applied. For example, electrodes for oxygen reduction may be highly catalytic when containing macrocycles with transition metal cations such as Fez+, Niz+, Coz+ [89,90], A typical apparatus for this technique is described in Ref. 91. 

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

For molecular electrocatalysts otherwise, and especially transition metal macrocycles, the electrocatalytic activity is often modified by subtle structural and electronic factors spanning the entire mechanistic spectrum, that is, from strict four-electron reduction, as for the much publicized cofacial di-cobalt porphyrin, in which the distance between the Co centers was set at about 4 A [12], to strict two-electron reduction, as in the monomeric (single ring) Co(II) 4,4, 4",4" -tetrasulfophthalo-cyanine (CoTsPc) [20] and Co(II) 5,10,15,20-tetraphenyl porphyrin (CoTPP) [21]. Not surprisingly, nature has evolved highly specific enzymes for oxygen transport, oxygen reduction to water, superoxide dismutation and peroxide decomposition. 

Systematic studies of the role of such factors as the nature of the metal center and the detailed structure of the chelating ring, particularly its peripheral functionalization, can afford valuable information toward unveiling structure-activity relationships for macrocycles as electrocatalysts for oxygen reduction. The following sub-sections describe some of the most salient aspects of a selected number of transition metal phthalocyanines and porphyrins, including the effects of redox and non-redox active substituents on the properties of Co porphyrins. 

Transition Metal Phthalocyanines Water-soluble tetrasulfonated phthalocyanines incorporating transition metals of the first row, particularly, FeTsPc and CoTsPc, may be regarded as among the first adsorbed macrocycles for which the reaction mechanisms for oxygen reduction in aqueous electrolytes have been studied in depth using RDE and RRDE techniques [60, 90]. 

In alkaline and neutral solutions silver and carbon are also used as catalysts. In acid electrolytes carbon is not effective for O2 reduction. New ways for oxygen reduction catalysis have been offered via the interaction of O2 with transition metal complexes, as demonstrated for the face-to-face Co-Co-4 porphyrin and a number of transition metal macrocycles on earbon, graphite, or metal substrates. Heat treatment at 700-1200 K of macroeyeles such as cobalt tetramethoxyphenyl porphyrin (Co-TMPP) and Fe-(TMPP) improve the activity in alkaline and acid media, respectively. 

Gupta, S., C. Fierro and E.Yeager (1991). The effects of cyanide on the electrochemical properties of transition metal macrocycles for oxygen reduction in alkaline solutions. J. Electroanal. Chem. 306, 239-250. 

Shi Z, Zhang J. Density functional theory study of transitional metal macrocyclic complexes dioxygen-binding abilities and their catalytic activities toward oxygen reduction reaction. J Phys Chem C 2007 111 7084-90. 

This chapter focuses on the theoretical modeling studies of ORR catalysts for PEMFC. Theoretical methods, such as density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulation, are presented. Current understanding of ORR mechanism in acidic medium is briefly discussed. Recent theoretical investigations on oxygen reduction electrocatalysts, such as Pt-based catalysts, non-Pt metal catalysts (Pd, Ir, CuCl), and non-precious metal catalysts (transitional metal macrocyclic complexes, conductive polymer materials, and carbon-based materials), are reviewed. The oxygen reduction mechanisms catalyzed by these catalysts are discussed based on the results. 

He H, Lei Y, Xiao C, Chu D, Chen R, Wang G (2012) Molecular and electronic structures of transition-metal macrocyclic complexes as related to catalyzing oxygen reduction reactions a density functional theory study. J Phys Chem C 116(30) 16038-16046... 

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