Transition metal macrocycle complex

Estiu, G., A. Jubert, J. Molina, J. Costamagna, J. Canales, and J. Vargas (1995). Semiempirical SCF/CI interpretation of the origin of the catalytic activity of transition metal-macrocycle complexes. J. Mol. Struct Teochem. 330(1-3), 201-210. 

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. 

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. 

Lamh, J.D. Smith, R.D. Anderson, R.C. Mortensen, M.K.J. Anion separations on columns based on transition metal-macrocycle complex exchange sites. J. Chromatogr. 1994, 671, 55-62. 

One of the most important factors affecting the activity of non-heat-treated transition metal macrocyclic complexes is their structure, more specifically, the ligand, the substituents, and the metal center [28]. The role of those factors closely ties to the mechanism by which the complexes are activated and bind oxygen. 

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. 

Their calculations demonstrated that dioxygen-binding abilities of the transition metal macrocyclic complexes are determined by central metal, ligand, and substituents. For cobalt phthalocyanine systems, electron-donating substituents increase... 

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... 

The first-principles DFT calculations of ORR on various M-N4 macrocyclic complexes have been carried out by several research groups [93, 168-173]. Through comparative study of O2 dissociation on different metalloporphyrins (MnP, FeP, CoP, NiP), the trends of the activation barriers for the O2 dissociation with respect to LUMO-HOMO characters of these metalloporphyrins have been discussed by Tsuda et al. [172]. FeP is demonstrated to be the best one due to the large d electrons contribution to the LUMO-HOMO level of the FeP and the stable Fe-O bond [172]. Shi and Zhang performed the DFT calculation on the O2 adsorption on various iron and cobalt porphyrins and phthalocyanines [171]. The catalytic activities of the transition metal macrocyclic complexes were positively related with... 

Liu K, Lei Y, Wang G (2013) Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes. J Chem Phys 139 204306... 

ORR Mechanisms Catalyzed by Transition Metal Macrocyclic Complexes... 

Theoretical approaches have in addition predicted that the substituents on the macrocyclic rings can also affect ORR catalytic activity. Co-phthalocyanine complexes with electron donating substituents should show improved ORR catalytic activity because the substituents can increase flie binding energy between O2 and the metal center(s) [55]. Calculation indicates fliat the catalytic activity of the transition metal macrocyclic complexes is due to flie partial electron transition between the filled d, dy, and empty r/ 2 orbitals of the transition metals, and the... 

Normally, transition metal macrocyclic complexes do not have long-term stability... 

Thermal treatment might lead to phase ehange in the transition metal macrocyclic complexes. Using XRD measurements, Baranton [58] found that received iron phthalocyanine was under a phase and that after being heat-treated at 450 °C it was under p phase. 

Three types of ORR catalyst are presently used transition metals and alloys non-noble metals and metal oxides and transition-metal macrocyclic complexes. For transition metal catalysts, two-electron reduction is reported for less active metals such as Au and Hg. For the most active catalyst, Pt, four-electron reduction is generally believed to occur, although its pathways and mechanisms are still elusive. For the ORR on Pt, two Tafel regions have been observed in both acid and alkaline solutions. At low current densities, a Tafel slope of-60 mV/dec was noted and at high current densities, a slope of-120 mV/dec [26]. The difference in Tafel slopes is attributed to partial coverage of the Pt surfaee by intermediates, especially by O. It is reported that at a potential > 0.8 V (NHE), Pt eoverage by O could reach... 

This class of PC catalysts has also been extensively studied as a potential substitute for Pt as they are low cost, methanol tolerant and have reasonably good activity and remarkable selectivity toward ORR [194]. They normally catalyze a direct 4e reduction of O2 to )deld water. The major drawback of this kind of catalyst is of low stability in acidic media [195]. However, when the catalyst is heat treated, the activity and stability of transition metal macrocycle complex (TMMC) are improved significantly [194]. The molecules of TMMC have a square planar structure with the metal ion symmetrically surrounded by four nitrogen atoms these nitrogen atoms are from each member of the ring systems which, in turn, are connected by carbon atoms (porphyrins) or nitrogen atoms (phthalocyanines). 

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