Electrocatalytic reduction of molecular oxygen by mononuclear and binuclear cobalt phthalocyanines

Abstract

Investigations of the electrocatalytic activity of mononuclear and binuclear cobalt phthalocyanines for the reduction of molecularoxygen established that the latter compounds are more effective catalysts than the corresponding mononuclear species.A correlation between the half-bandwidths for the Q band of these species and their catalytic efficiency, expressed as a kinetically-limited current, suggests that the increase in activity may be attributed to electronic coupling between the phthalocyanine rings.Mrcnner R. Her*apstpnp, A. B. P. Leven et Cllppotto C. LtzNopr. Can. J. Chem. 65,2671Chem. 65, (1987 (22)(23)(24)(25)(26)(27)(28) and Chang and co-workers (29)(30)(31)(32)(33)(34)(35) has revealed that certain binuclear porphyrins are much more effective. Similarly, mononuclear phthalocyanines of iron and cobalt are effective (36-50), but some ill-characterised "dimeric" and polymeric phthalocyanines appear more effective (51)(52)(53)(54)(55)(56)(57)(58)(59)(60)(61)(62). Savy and co-workers (54-56) performed ESCA, esr, Mdssbauer and optical spectroscopic sfudies to establish correlations between the spectroscopic properties of polymeric and monomeric iron phthalocyanines and the eiectrocatalytic activity of these species towards oxygen reduction. We consider here a correlation between the optical spectra of a series of well-characterized mono-and binuciear cobalt phthalocyanines and their electrocatalytic activity. for about an hour before scanning the working electrode at a rate of 100 mV/s over the range 0.00 to *1.00 V versus the silver wire.Adsorbed layers on OPG or SAPG electrodes were obtained by cycling until the anodic and cathodic peak currents associated with the adsorbed species remained constant over 5 min ofcontinuous scanning. When this condition was satisfied, the adsorbed layer was assumed to have reached a steady coverage. Scans at various rates were then recorded to determine the surface coverage (vide infra).The catalytic efficiency of the various phthalocyanine compounds was investigated with the adsorbed species on the OPG electrode. This modified electrode was removed from the phthalocyanine solution, washed with ethanol, then with water, and finally placed in the cell for the aqueous studies. The 0.l0 M NaOH solution in this cell had been purged with oxygen for several hours to ensure saturation. The electrode was then scanned over the range 0.00 to -1.00 V versus sce at a rate of l0 mV/s for rotation rates ranging from 400 to l0 0O0 rpm.After each scan. the electrode was held at 0.00 V for I min to oermit the rotation rate to be changed for the next scan. The theory of intramolecular coupling in these binuclear phthalocyanines has been presented elsewhere (66). In solution, these binuclear species are expected to exist in a dynamic equilibrium between various conformations, dependent upon the nature of the bridge since, in some cases, steric effects may provide barriers to free rotation. Therefore, the electronic absorption spectrum of a solution of one of these binuclear species wo...

Synthesis and electrochemistry of linear and cofacial conjugated binuclear phthalocyanines covalently linked by alkyne and alkene bridges

“…Complexes 8 and 11 were also deposited as monolayer films on highly pyrolytic graphite rotating disk electrodes following methods described earlier (23). These modified electrodes electrocatalyse the reduction of molecular oxygen in a twoelectron process to hydrogen peroxide.…”

A career in phthalocyanine chemistry – the Linstead Award 2002

“…Nevertheless, the pH dependence and [76] potential for oxygen reduction is largely the same (see Fig. 12) for most MPc surfaces [31,[102][103][104]112] being observed at ca. -0.2 to -0.5 V vs SCE and pH independent above about pH 4.…”

“…3b,c,f) [19][20][21][22][23][24][25][26][27][28][29][30] in an effort to devise an electrocatalyst in which both metal nuclei would act, in concert, in the multi-electron reduction of oxygen to water. Some advantages were observed in using a dinuclear rather than a mononuclear electrocatalyst but the results of the effects were disappointingly small [31].…”

“…It is also evident that the detailed mechanisms vary with the MPc surface itself. Thus evidence exists [31,[101][102][103][104][105][106][107][108][109][110][111][112][113][114][115][116][117][118][119] for oxygen reduction being catalyzed by both Co II Pc and Co I Pc depending on the surface. Nevertheless, the pH dependence and [76] potential for oxygen reduction is largely the same (see Fig.…”

Ir and Uv-Visible Spectra of Iron(ii) Phthalocyanine Complexes With Phosphine or Phosphite

“…These in turn led to the study of semi-conductivity, photoconductivity, photochemical reactions, photosynthetic and electrocatalytical activities, etc. [3][4][5][6][7]. An important theme has been to determine the influence of axial or equatorial ligands on the physico-chemical properties of the complexes [8][9][10][11].…”