Four-Electron Reduction Catalysts

Following the line of thought that cofacial structures would promote the four-electron reduction of O2 some authors have suggested that it is possible that when depositing cobalt complexes, they could adopt configurations where two cobalt centers are separated by an optimal distance to promote the splitting of the 0-0 bond. An example of this is the work of Biloul et using heat-treated CoNPc-deposited carbon black. These authors found evidence for a four-electron reduction process as they determined the number of electrons to be ca. 3.5. 

Anson et found evidence for a four-electron reduction mechanism on adsorbed multilayers of cobalt tetra-4-pyridylporphyrin on a graphite electrode but considerable amounts of peroxide were produced in the reaction. In a series of articles Anson and coworkers have shown that by plac- 

Anson et have proposed the following mechanism for the four-electron reduction reaction catalyzed by ruthenated Co porphyrins  

This reaction scheme involves the cobalt porphyrin confined on a graphite surface. The active Co(II) species is generated in step (2.15) at potentials more positive than those where Ru(ni) and coordinated O2 are reduced. The O2 adduct is formed in step (2.16) that is reduced more easily than unbound O2. The equilibrium constant of step (2.16) is small but the reaction rate for adduct formation 

Imaoka and Yamamoto reported that porphyrins possessing four ionic substituents of (mCTO-tetrakis(Af-methyl-4-pyridiniumyl)porphyrinatoRu(II) (RuTMPyP) and (me.y(9-tetrakis(4-sulfonatophenyl)porphyrinato)cobalt(II) (CoTPPS) associate spontaneously to form a dinuclear complex. Formation of CoTPPS-RuTMPyP was confirmed by UV-vis titration and TOF-mass spectra. The dinuclear complex exhibits an acceleration of intrinsic FT when it is present in a Nation film deposited on glassy carbon. The CoTPPS-RuTMPyP-Nation system catalyzes the four-electron reduction of O2 with an efficiency of 95%. 

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It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Three new criteria were proposed [Collman et al., 2003a] to establish that the four-electron reduction of O2 by a catalytic film represents the selectivity of an individual molecule of the catalyst (i) independence of the i /i ratio on the catalyst surface coverage (ii) much higher stabUity of the catalyst in the ORR compared with reduction of H2O2 under conditions that reproduce the concentration of H2O2 in the film that would be generated if ORR proceeded by a step-wise mechanism and (iii) the nature of the turnover-determining step. 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

In a subsequent study of this type (Durand, Bencosme, Collman Anson, 1983), dimers of type (143) were investigated as potential redox catalysts for the four-electron reduction of dioxygen to water (via peroxide). The Co(ii)/Co(ii) dimer is an effective catalyst for the electrochemical reduction of dioxygen once again the dioxygen binds... 

On an actual Ti02 microparticle, depending upon wnether there is a depositee catalyst, there can also be varying contributions of these latter three reactions. As discussed later, some photocatalytic reaction pathways can involve 02", and therefore it appears to be a desirable product as such, metallic catalysts may in fact be counterproductive in this respect, f they promote the two or four- electron reduction reactions. 

However, Nature remains the best catalyst designer for the four-electron reduction of 02 via cytochrome-c oxidase (+0.3 V vs. SCE). Even with this four-metal-centered protein the reduction potential is 0.3 V less positive than the thermodynamic limit (Table 9.3). 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

The reactions described in Equation (20) are, however, only known for heterogeneous catalysts. For example, formaldehyde was formed selectively by a four-electron reduction of CO2 by an aluminum amalgam in the presence of titanium trichloride, even at room temperature and atmospheric pressure. The reaction is catalytic wiih respect to titanium [117,118]. 

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