Electrocatalysts phthalocyanines

Lalande G, Cote R, Tamizhmani G, Guay D, Dodelet JP. 1995. Physical, chemical and electrochemical characterization of heat-treated tetracarboxylic cobalt phthalocyanine adsorbed on carbon black as electrocatalyst for oxygen reduction in polymer electrolyte fuel cells. Electrochim Acta 40 2635-2646. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

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... 

Transition-metal -phthalocyanines as catalysts in acid medium. To prevent carbonate formation by the carbon dioxide in the air or that produced by oxidation of carbonaceous fuels, an acid electrolyte is necessary hence it is important to find electrocatalysts for an acid medium. Independently of Jasinski, we were soon able to show 3>4> that under certain conditions the reduction of oxygen in dilute sulfuric acid proceeded better with phthalocyanines on suitable substrates than with platinum metal. The purified phthalocyanines were dissolved in concentrated sulfuric acid and precipitated on to the carbon substrate by addition of water. This coated powder was made into porous electrodes bound with polyethylene and having a geometrical surface of 5 cm2 (cf. Section 2.2.2.1.). The results obtained with compact electrodes of this type are shown in Fig. 6. 

Beck 23,37,38) carried out voltammetric investigations of chelates in solution (phthalocyanine and tetra-aza-annulene in 85—90% H2SO4). The particularly active electrocatalysts (FePc and CoTAA) showed very positive redox potentials and a pronounced positivization of the polarographic oxygen step in 85 % sulfuric acid. On the basis of these results, Beck proposed the following mechanism... 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with Fe phthalocyanines or phenylporphyrins. For a comprehensive discussion of the subject, the reader is referred to refs. (76, 95, and 132-136). 

A typical approach is to utilise a substrate which when hydrolysed by the enzyme gives rise to a product which can be easily detected elect-rochemically. Thiocholine can be easily detected using screen-printed carbon electrodes doped with cobalt phthalocyanine (CoPC) [18,19], which acts as an electrocatalyst for the oxidation of thiocholine at a lowered working potential of approximately +100 mV (vs. Ag/AgCl) [18,19], thereby minimising interference from other electroactive compounds ... 

Electrochemical NO sensors based on platinized or electrocatalyst-modified electrodes often in combination with a permselective and charged membrane for interference elimination were proposed. Although the catalytic mechanism is still unknown, it can be assumed that NO is co or dinative ly bound to the metal center of porphyrin or phthalocyanine moieties immobilized at the electrode surface. The coordinative binding obviously stabilizes the transition state for NO oxidation under formation of NO+. Typically, sub-pM concentrations of NO can be quantified using NO sensors enabling the detection of NO release from individual cells. 

This section describes various strategies for the immobilization of macrocycles on electrode surfaces and their characterization by both electrochemical and in situ spectroscopic techniques in solutions devoid of dioxygen. It also provides theoretical foundations involved in the analysis of the mechanisms of oxygen reduction at such interfaces based on measurements performed under forced convection. Studies involving a number of carefully selected phthalocyanines, and porphyrins, will be presented and discussed, which in our view best illustrate the nuances of the rich behavior this class of adsorbed electrocatalysts can exhibit. These examples serve to... 

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. 

The catalytic activity of phthalocyanine organometallic complexes in hydrocarbon oxidations 281) led to testing such compounds as fuel cell electrocatalysts (282). Phthalocyanine complexes have the structure of Fig. 22 with a multivalent metal, Fe, Co, Ni, or Cu, surrounded by four symmetric nitrogen atoms. These ligands (L) activate the 0-0 bond by forming an adduct with oxygen and thus promote reaction with hydrocarbons... 

Other metal ligands that can bind oxygen have also been tested for oxygen reduction, such as porphyrins, thiospinels, and diamine (Pfeiffer) complexes (284, 285). Porphyrin complexes (Fig. 22) were the most active, even better than phthalocyanines, with activity decreasing in the order Co(II) > Fe(ni) > Ni(II) Cu(II). Figure 23 shows potential-current curves for some phthalocyanine and porphyrin electrocatalysts. The higher the current at a given potential, the more active is the catalyst. 

Fig. 23. Potential-current curves for some metal ligand electrocatalysts (284) solid lines, porphyrins dashed line, polymeric phthalocyanine.

Polymeric phthalocyanine films prepared from 51a have been investigated as electrocatalysts, in biological applications and as amperometric biosensors [157]. The electropolymerized films exhibited better activities than analogous low molecular weight phthalocyanine films. Some examples are given ... 

Vasudevan, R, S.N. Mann, and S. Tyagi (1990). Transition-metal complexes of porphyrins and phthalocyanines as electrocatalysts for dioxygen reduction. Transition... 

In the acid medium of PEM fuel cells that may also contain some levels of fluoride derived from membrane degradation, Pt cannot be replaced with a non-noble metal or a metallic oxide, as both will corrode in such environment. This chapter describes some of the efforts that have been made over 40 years to obtain other non-precious metal electrocatalysts for ORR in acidic medium. They aU started with the discovery in 1964 that C0N4 phthalocyanine was capable of oxygen reduction in an alkaline solution. ... 

Lalande, G., G. Tamizmani, R. Cote, L. Dignard-Bailey, M.L. Trudeau, R. Schulz, D. Guay, and J.P. Dodelet (1995). Influence of loading on the activity and stability of heat-treated carbon-supported cobalt phthalocyanine electrocatalysts in solid polymer electrolyte fuel cells. J. Electwchem. Soc. 142,1162-1168. 

Another selective electrocatalyst Co phthalocyanines-poly-4-vinylpiridine (PVP) has been used to modified electrodes in this case the method of... 

Vasudevan, R, N. Phougat, and A.K. Shukla (1996). Metal phthalocyanines as electrocatalysts for redox reactions. AppZ. Organometal. Chem. 10(8), 591-604. 

Wang, J., T. Golden, and R. Li (1988). Cobalt phthalocyanine/cellulose acetate chemically modified electrodes for electrochemical detection in flowing streams Multifunctional operation based upon the coupling of electrocatalysts and permselectivity. AnoZ. Chem. 60(15), 1642-1645. 

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