Metal ligand electrocatalyst

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

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. 

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. 

Numerous metal complexes have been proven to be active electrocatalysts for C02 reduction.1,66-68 These catalysts can be conveniently grouped into three main families metal complexes with polypyridyl ligands, metal complexes with macrocyclic ligands, and metal complexes with phosphorus ligands. 

In this work we have studied the preparation of electrocatalysts on the graphite matrix using tri-nuclear complexes of 3d-metals with aminoalcohol ligands. Tri-nuclear complexes, 2[Co(Etm)3] Me(N03)2, where Etm = ethanolamine, Me = Zn2+, Cu2+, Ni2+, Co2+, were investigated. 

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... 

An important frontier in cluster chemistry is the effect of electron count on reactivity. Are clusters merely passive electron reservoirs or does the number of electrons have a critical influence on reactions other than electron transfer Conversely, how does the number of metal atoms, the relative ratio of heterometals, and the specific ligand set relate to the ability of clusters to enter into redox reactions These questions are of fundamental interest, but the answers also may have practical consequences, for example, in the development of metal nanoparticles or colloids to act as electrocatalysts. In a final group of articles Longoni and coworkers review the ability of homoleptic carbonyl clusters to act as electron-sinks , Zanello and Fab-rizi de Biani examine the effect of heterometallic interactions on cluster redox aptitude, and Ignaczak and Gomes report on modelling of electrode interactions with metal clusters. 

One or more of the CO ligands in these complexes can be substituted by more electron-donating ligands such as phosphines, bipyridine, or N-heterocyclic carbenes (Fig. 11c). These electron-rich complexes tend also to be electrocatalysts, but as a consequence of the higher electron density at the metals, they require substantial overpotential for catalysis [56]. 

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