The four-electron reduction of oxygen to water on a planar cobalt chelate

The four-electron reduction of oxygen to water on a planar

cobalt chelate

Citation for published version (APA):

Putten, van der, A. M. T. P., Elzing, A., Visscher, W., & Barendrecht, E. (1986). The four-electron reduction of oxygen to water on a planar cobalt chelate. Journal of the Chemical Society, Chemical Communications, (6), 477-479. https://doi.org/10.1039/c39860000477

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10.1039/c39860000477 Document status and date: Published: 01/01/1986 Document Version:

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J . CHEM. S O C . , CHEM. COMMUN.,

1986

The Four-electron Reduction of Oxygen to Water on a Planar Cobalt Chelate

A. van der Putten, A. Elzing, W. Visscher, and E. Barendrecht

Laboratory for Electrochemistry, Department of Chemical Technology, Eindhoven University of Technology,

P.O. Box 513, 5600 MB Eindhoven, The Netherlands

In the reduction of oxygen on the planar binuclear cobalt chelate bis-(3,5-di-2-pyridyl-l,2,4-triazole)dicobaIt

dichloride, adsorbed on pyrolytic graphite, reduction to water occurred in alkaline solution, whereas in acid solution only hydrogen peroxide was formed; the results are compared with those for other binuclear cobalt chelates already reported.

For almost two decades, 3d transition metal chelates have been studied as electrocatalysts for the cathodic reduction of oxygen. The best activities were obtained with the N4-chelates of Fe and Co.1 It was also shown that mononuclear Co chelates reduce oxygen to hydrogen peroxide; only with Fe as a central metal ion was reduction of oxygen to water observed. In 1979 Collman, Anson, and collaborators,2 however, reported the reduction of oxygen to water on the cofacial dicobaltporphyrin (1). The two porphyrin rings are so spaced that bridging absorption of O2 is possible. Clearly, this bridging absorption, requiring the presence of two Co centres at the appropriate distance, is a crucial condition for the occurrence of 4e- reduction. In principle, this condition can also be fulfilled by planar chelates, containing two Co-ions. Indeed, 4e- reduction of oxygen in the planar cobalt complex (2) was reported recently by Yeager and Sarangapani.3

We now report our results with a different planar binuclear Co complex, namely bis-(3,5-di-2-pyridyl-l,2,4-triazole)-

dicobalt dichloride (3) [C02(dpt)~Cl~], kindly provided by Dr. R. Prins, Leiden State University. The electrochemical measurements were performed with a rotating ring (Pt)- pyrolytic graphite disc electrode (S = 0.5 cm2; N = 0.27), in a standard three-compartment electrochemical cell. All poten- tials are given with respect to the reversible hydrogen electrode (R.H.E.). The ring was slightly platinized to ensure quantitative H 2 0 2 detection; the ring potential was set at 1.2 V vs. R.H.E. Before each experiment, the ring was activated by periodic evolution of hydrogen and oxygen for 1 min. The catalyst was applied to the disc (previously polished with 0.3

pm A1203, Buehler) via irreversible adsorption from a 5 X

1 0 - 3 ~ solution of Co2(dpt)2C12 in warm (-40 "C) dimethyl sulphoxide. Since the adsorption appeared to be slow, an adsorption time of 2 h was used. The reduction of 0 2 was

measured in both alkaline and acidic 02-saturated solutions (1 M KOH and 0.5 M H2SO4, respectively), by scanning the disc potential from 1.0 to 0.2 V vs. R.H.E. at 50 mV s-l. Moreover, an attempt was made to characterize the disc electrode in 02-free 1 M KOH. The results for the 0 2

reduction in alkaline solution are given in Figure 1, the curves for which were taken after ca. 5 scans at 64 s-l. During the first scans, qualitatively the same results were obtained; however, somewhat less H202 was produced. Since the diffusion-limited current for O2 into H 2 0 conversion at 64 s-l

is 3 mA in this electrolyte, it is clear from both disc and ring current that O2 is reduced to water in two waves; in the first wave, the main product is H202. During the second wave, the limiting current for the 4e- reduction of water is virtually reached.

The results in acid solution are presented in Figure 2. Since the complex is unstable in acid solution, only the first scans at 16 s-1 are given. The diffusion-limited current for the

reduction of O2 to H 2 0 at 16 s-1 is 2 mA under these

conditions; it is therefore clear that virtually only H202 is

formed. Owing to the lack of stability in acid solution the characterization of the disc electrode was only performed in 02-free 1 M KOH. In this case the disc potential was varied from 1 to 0 V vs. R.H.E. at 100 mV s-1, and vice versa. The results (Figure 3) show that although the cyclic voltammogram

478 J. CHEM. SOC., CHEM. COMMUN.,

1986

I

!

+

14+

of the modified disc electrode significantly differs from that of the unmodified electrode background, no distinct redox couples can be detected, as was also reported by Sarangapani3 for complex (2). If nevertheless the observed humps are considered to correspond to one-electron redox processes, one can conclude from Figure 2 that the complex is adsorbed on a monolayer level. Summarizing, the conclusion of this study is that C ~ ~ ( d p t ) ~ C l , is able to reduce O2 to H 2 0 in alkaline solution.

If we now compare the properties of the binuclear cobalt chelates reported so far, two groups can be distinguished: planar complexes, reducing O2 to H 2 0 in alkaline solution, and 'sandwich' complexes, either amide,2 complex ( l ) , or anthracene4 bridged, complex (4), giving reduction to H 2 0 in acid solution. Both groups allow the formation of p-peroxo adducts so this cannot explain the reversed selectivity. In our view this phenomenon is related to the formal redox potentials of the Co centres; 4e- reduction seems only to occur if these potentials have values in the range 0.6-0.7 V vs. R.H.E. The

- 3000

Figure 1. Oxygen reduction in 1 M KOH at Co,(dpt),CI,, (3),

adsorbed on pyrolytic graphite. Rotation frequencies 4 (i), 16 (ii), 36 (iii), and 64 s-1 (iv).

? O O l 1 00

Figure 2. Oxygen reduction in 0 . 5 ~ H2S04 at Co,(dpt),CI, (3),

adsorbed on pyrolytic graphite. Results of the first scans at a rotation frequency of 16 s-1. i, unmodified disc electrode; ii, disc electrode modified with Co,(dpt),CI,, scan 1, iii, scan 4; iv, scan 10.

sandwich complexes fulfill this condition in acid solution. If it is assumed that the CoILCoIII redox processes are pH- independent, the potentials with respect to R.H.E. in the same solution can shift 60 mV in the anodic direction per unit increase in pH. As a consequence, the potentials of the sandwich complexes will be too high in alkaline solution, leading to H202 production. Unfortunately, neither we nor Sarangapani and Yeager3 were able to detect distinct redox peaks at the planar complexes in alkaline solution, but the experimental results indicate that these potentials have the appropriate values in this electrolyte. Likewise, these poten- tials will be too low in acid solution, leading, again, to H 2 0 2 as end product. If this model is correct, it also implies that it is impossible to develop cobalt-containing catalysts that give 4e- reduction both in acid and in alkaline solution since the reduction of O2 and H 2 0 can only occur in a limited pH range. The observation that the sandwich complexes ( l ) , (4), and

( 5 ) produce virtually no H 2 0 2 , while the planar complexes (2) and (3) yield considerable amounts, can be explained by the fact that the activity of the corresponding monomeric adducts differ in acid and alkaline solution. If 0 2 is attached to one Co

atom, the most probable rate-determining step is the forma- tion of superoxide: O2

+

J. CHEM. SOC., CHEM. COMMUN.,

1986

479

Figure 3. Characterization in 0,-free 1 M KOH of C ~ , ( d p t ) ~ C l ~ , (3), adsorbed o n pyrolytic graphite. Scan rate 100 mV s-1. 1, unmodified

electrode; ii, electrode modified with C~,(dpt)~Cl,.

solution because its rate-determining step is pH-independent. At high pH the monomeric and dimeric pathways proceed at comparable rates, leading to mixed production of H 2 0 and H202. The dimeric pathway, however, seems to be pH- dependent. The E4 of this pathway with respect to R.H.E. remains therefore unchanged at different pH values. The 154 of the monomeric pathway is pH-independent and this EJ vs. R.H.E. shifts 60 mV in the anodic direction per unit increase in pH. In acid solution, the dimeric pathway is therefore much more favourable, so at low overpotential, there will be no competition between the two pathways, and only H20 is formed. At potentials where the monomers also start to reduce oxygen, H 2 0 2 can be formed. This indeed is observed experimentally. 2b

Finally, we discuss a striking result of Liu et al. ,5 namely the

fact that the anthracene-linked diporphyrin ( 5 ) , containing only one cobalt atom, was also able to reduce O2 to H 2 0 , but at lower rates than the corresponding dicobalt complex. Although the second porphyrin ring does not contain a Co ion,

the potential of the remaining Co has shifted to 0.6 V vs. R.H.E.5 Nevertheless, the formation of an intramolecular adduct seems unlikely since the molecule contains only one catalytic centre. Perhaps in this case intermolecular pperoxo adducts are formed. The lower activity of the monocobalt diporphryin (5) is easily explained by a lower number of active sites, since only relatively few dimeric species will be present on the surface. Such a decrease in the number of active sites shifts EJ in the cathodic direction, but does not prevent the attainment of the limiting current.6 This hypothesis could be checked by repeating the experiment on ‘stress annealed’ pyrolytic graphite.’ This approximates to a perfectly smooth surface. Assuming that the porphyrin rings of adsorbed molecules lie parallel to the surface, intermolecular Co-O-0- Co binding will be absent on this substrate, and consequently no 4e- reduction should occur.

Received, 12th November 1985; Com. 1596

References

1 H. Jahnke, M. Schonborn, and G. Zimrnermann, Top. Curr. Chem., 1976, 61, 133.

2 J. P. Collman, M. Marocco, P. Denisevich, C. Koval, and F. C. Anson, J . Efectroanal. Chem., 1979, 101, 117; J. P. Collman, P. Denisevich, Y. Konai, M. Marocco, C. Koval, and F. C. Anson, J. A m . Chem. S O C . , 1980, 102, 6027; R . R. Durand, Jr., C. S.

Bencosme, J . P. Collman, and F. C. Anson, ibid., 1983,105,2710. 3 S. Sarangapani, Ph.D. Thesis, Case Western Reserve University, Cleveland, 1983; E. Yeager, Electrochim. Acta, 1984, 29, 1527. 4 C. K. Chang, H. Y. Liu, and I . Abdalmuhdi, J. A m . Chem. S O C . ,

1984, 106, 2725.

5 H. Y. Liu, I. Abdalmuhdi, C. K. Chang, and F. C. Anson, J . Phys.

Chem., 1985, 89, 665.

6 A. Elzing, A. van der Putten, W. Visscher, and E . Barendrecht, J . Electroanal. Chem., accepted for publication.

7 J. H. Zagal-Moya, Ph.D. Thesis, Case Western Reserve Univer- sity, Cleveland, 1978.