Transition metal macrocycle catalysts

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. 

Two types of non-Pt catalysts are currently being explored non-Pt metals and N-containing transition metal macrocycle catalysts. 

The first In situ MBS Investigation of molecules adsorbed on electrode surfaces was aimed primarily at assessing the feasibility of such measurements In systems of Interest to electrocatalysis (18). Iron phthalocyanlne, FePc, was chosen as a model system because of the availability of previous situ Mossbauer studies and Its Importance as a catalyst for O2 reduction. The results obtained have provided considerable Insight Into some of the factors which control the activity of FePc and perhaps other transition metal macrocycles for O2 reduction. These can be summarized as follows ... 

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. 

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. 

Chapter 13 and 14 summarize the development of transitional metal oxides and transition metal chalcogenides for ORR, respectively. Chapter 15 is the only chapter in this book dedicated to the ORR catalysis of alkaline fuel cells. Electrocatalytic properties of various non-Pt catalysts including Ag, Pd, transition metal macrocycles, metal oxides, and multifunctional materials are presented. Fundamental issues related to the design of low-cost, high-performance electrocatalysts for alkaline fuel cells are discussed. Chapter 16 and 17 review the recent advances on the study of ORR on Au and Pd-based catalysts, respectively. 

Normally, the kinetics of ORR and OER occurring at the cathode of fuel cells, including direct methanol fuel cells (DMFCs) is very slow. In order to speed up the ORR kinetics to reach a practical usable level in a fuel cell, ORR catalyst is needed at the air cathode. Platinum (Pt)-based materials are the most practical catalysts used in PEM technology. These Pt-based catalysts are too expensive to make fuel cells commercially viable, and hence extensive research over the past several decades has been focused on development of alternative catalysts. These alternative electrocatalysts include noble metals and allo37S, carbon materials, quinone and its derivatives, transition metal macrocyclic compounds, transition metal chalcogenides, transition metal carbides and transition metal oxides. In this chapter, we focus on both noble and nonnoble electrocatalysts being used in air cathodes and the kinetics and mechanisms O2 reduction/oxidation reaction (both ORR and OER), catal37zed by them. 

Electrocatalytic ORR carries out in three pathways the 1-electron transfer pathway, producing superoxide ion the 2-electron transfer pathway, producing hydrogen peroxide and the 4-electron transfer pathway, producing water. In a non-aqueous aprotic solvent system, a room-temperature ionic liquid system, and on specific transition-metal, macrocyclic-compounds-coated graphite electrodes in alkaline solutions, 1-electron reduction can be observed. Carbon materials, quinone and derivatives, mono-nuclear cobalt macrocyclic compounds, and some chalcogenides can only catalyze 2-electron ORR. Noble metal, noble metal alloy materials, iron-macrocyclic complexes, di-nuclear cobalt macrocyclic complexes, some chalcogenides, and transition-metal carbide-promoted Pt catalysts can catalyze 4-electron reduction. 

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... 

In 1964, Jasinski reported his pioneering work on using cobalt phthalocyanine, adsorbed on carbon and nickel eleetrodes, as a promising catalyst for the reduction of oxygen [8]. Following Jasinski s work, many other transition metal macrocyclic N4-complexes, including porphyrins, phthalocyanines, and tetraazannulenes, were also explored. The transition metals evaluated inelude Mn, Ru, Pd, Pt, Ir, Cr, Ni, Cu, Zn, Mo, Al, Sn, Sb, Ga, Na, Ag, vanadyl ion, as well as uranyl ion. All of these compounds show a certain level of eleetroeatalytie aetivity towards the ORR [6, 9]. 

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