Electrocatalysts transition metal macrocycles

In Situ Spectroscopic Studies of Oxygen Electrocatalysts Involving Transition Metal Macrocycles... 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Transition Metal Macrocycles as Electrocatalysts for Dioxygen Reduction... 

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. 

As mentioned above, the pretreatment of the carbon electrode surface can significantly affect its ORR activity, and even alter the ORR mechanism. Actually, the carbon surface can also be modified by some monolayer substances such as anthraquinones (AQ), and transition metal macrocycle complexes. Due to the conjugate structures of these molecules, they can irreversibly adsorb on the carbon surface to form monolayers that could serve as an electrocatalyst for ORR. This kind of electrode structure can be used to evaluate the electrocatalyst s ORR activity, particularly in acidic solution. 

Scherson DA, Palencsar A, Tohnachev Y, Stefan I (2008) Transition metal macrocycles as electrocatalysts for dioxygen reduction. In Alkire RC, Kolb DM, Lipkowski J, Ross PN (eds) Electrochemical surface modification thin films, functionalization and characterization. Wiley-VCH Verlag GmbH Co. KGaA, Weinheim, Germany... 

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. 

Non-precious metal catalyst research covers a broad range of materials. The most promising catalysts investigated thus far are carbon-supported M-N /C materials (M = Co, Fe, Ni, Mn, etc.) formed by pyrolysis of a variety of metal, nitrogen, and carbon precursor materials [106]. Other non-precious metal electrocatalyst materials investigated include non-pyrolyzed transition metal macrocycles [107-122], coti-ductive polymer-based complexes (pyrolyzed and non-pyrolyzed) [123-140], transition metal chalcogenides [141-148], metal oxide/carbide/nitride materials [149-166], as well as carbon-based materials [167-179]. The advances of these types of materials can be found in Chaps. 7-10 and 12-15 of this book. 

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