van der Amtp Putten

Oxygen reduction on pyrolysed carbon-supported transition metal chelates

Abstract A review is given of the pyrolysis behaviour of carbon-supported transition metal chelates and the developed theories explaining the increased activity and stability towards oxygen reduction. Additional measurements which a new carbon-modified rotating ring-disc electrode show that the central metal ionN 4 unit remains the active site after pyrolysis. The selectivity is increased after pyrolysis since relatively less H 2 O 2 is detected at the ring. The number of available active sites is an important parameter for the resulting activity. During the heat treatment, different processes take place: migration of the chelates over the carbon support, increasing the number of availableactive sites; reaction of the outer fringes of the organic skeleton retaining the metal ionN 4 unit; and degration of the catalyst with loss of activity. The condition of the pyrolysed catalyst and its corresponding activity are determined by the relative rate of these processes at the given pyrolysis temperature.

Redox potential and electrocatalysis of O2 reduction on transition metal chelates

Abstract The redox potentials of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and cobalt tetraazaannulene (CoTAA), irreversibly adsorbed on pyrolytic graphite, were investigated as a function of the pH by cyclic voltammetry. The reduction of oxygen was studied under the same conditions with the rotating disc electrode technique. At high pH, where the reduction to H2O2 on the cobalt-containing chelates is more reversible, the value of the redox potential is less important. At low pH, where this reduction is irreversible, there is a clear correlation between the redox potential of the central metal ion and the O2 reduction behaviour. In acid media, a 470 mV more positive redox potential of CoTAA compared to that of CoPc, resulted in a half-wave potential, E 1 2 , for the O2 reduction being 400 mV more positive than the E 1 2 of CoPc.

The cathodic reduction of oxygen at cobalt phthalocyanine: Influence of electrode preparation on electrocatalysis

Oxygen reduction has been investigated in acid and alkaline solutions on electrodes with the same catalyst (cobalt phthalocyanine or cobalt tetrasulfonato-phthalocyanine), but prepared by different methods. It is concluded that the number of active sites can differ widely from the expected number of active sites as derived from the total amount of catalyst present on the electrode surface. In acid solution, where the reduction is irreversible, the half-wave potential shifts with respect to the number of active sites, according to a theoretical description. For the determination of the “intrinsic catalytic activity” (turnover number), electrodes with an irreversibly absorbed (sub)monolayer are recommended.

The mechanism of oxygen reduction at iron tetrasulfonato-phthalocyanine incorporated in polypyrrole

Abstract The reduction of oxygen at iron tetrasulfonato-phthalocyanine (FeTSPc) incorporated in polypyrrole is studied. Compared with the results of FeTSPc adsorbed on pyrolytic graphite (Cp), a remarkable shift in the reduction onset potential in 0.05 M H 2 SO 4 is observed, but unfortunately thin polypyrrole/FeTSPc films show high instability. Thicker films (equivalent to 10 mC or more) are more stable and lead to an increased four-electron reduction of oxygen. It is proposed that dimeric FeTSPc species are present in polypyrrole and that they are responsible for the marked shift in the reduction onset potential.

The cathodic reduction of oxygen at metal tetrasulfonato-phthalocyanines : influence of adsorption conditions on electrocatalysis

Some conditions which promote the adsorption of tetrasulfonato-phthalocyanines on pyrolytic graphite are formulated. As a consequence of one of these, the ionic strength, aggregation in the solution is postulated to occur in order to explain some observations made by UV-vis spectroscopy and the formation of a deposit in 1 M KOH solutions. A model based on “surface diffusion” is presented to explain the observed “kinetic limitation” for electrodes with a low coverage of cobalt tetrasulfonato-phthalocyanine (CoTSPc), without the introduction of a rate-determining chemical step. When applied to FeTSPc, the same model leads to the conclusion that dimers of FeTSPc are present on the graphite surface. The difference in behaviour between FeTSPc and FePc (iron phthalocyanine) is discussed.

Oxygen reduction on vacuum-deposited and adsorbed transition-metal phthalocyanine films

Abstract The reduction of oxygen was studied on vacuum-deposited iron and cobalt phthalocyanine (Pc) films, both in acid and alkaline solutions. CoPc reduces oxygen to H 2 O 2 only; FePc produces a mixture of H 2 O 2 and H 2 O: in acid solution, considerable amounts of H 2 O 2 are formed: in alkaline solution, virtually only H 2 O is produced. It was shown that the reaction takes place at the substrate/phthalocyanine film interface. A comparison with electrodes onto which a monolayer was irreversible adsorbed showed that only a very small fraction of the catalyst molecules in the vacuum-deposited film is electrochemically active. The electrochemical behavior of these films is to a large extent determined by the physical properties of the films, such as the conductivity and the permeability towards oxygen. The difference between Fe and Co as the central metal atom is discussed.

The electrodeposition and dissolution of zinc and amalgamated zinc in alkaline solutions

Abstract The reaction mechanism of zinc and amalgamated zinc was investigated with the galvanostatic transient technique in the concentration range 1.5–10 M KOH. The Tafel slopes of the zinc electrode were 40 mV anodically and 120 mV cathodically. The cathodic reaction order in zincate was found to be +1. From the Tafel slopes and the dependence of the exchange current density on the activity of the KOH, the reaction orders in OH − were calculated, yielding values of 2.3± 0.8 in the anodic and −0.8 ± 0.2 in the cathodic direction. These results are consistent with the suggested mechanism of Bockris et al .[16] for the zinc electrode. The Tafel slope in cathodic direction of the amalgamated zinc electrode was a function of the KOH concentration (120 mV at KOH concentrations up to 3 M; about 60 mV in 10 M KOH); the anodic Tafel slope was 30 mV over the whole concentration range. These results and measurements at constant ionic strength suggest a mechanism which involves the participation of water. The difference in behaviour of the zinc electrode and the amalgamated zinc electrode is probably caused by changes in the adsorption characteristics due to amalgamation.

Increased valence theory and the four electron reduction of O2 to H2O

Abstract The four electron reduction of oxygen to water, without the intermediate formation of hydrogen peroxide has been reported on dicofacial dicobalt porphyrins and planar dicobaltchelates, containing two CoN4 units in the plane of the molecules. These two classes of compounds display “reversed selectivity” with respect to the solution pH. Dicofacial dicobalt porphyrins directly reduce O2 to H2O in acid solution, and to hydrogen peroxide in alkaline solution. Planar dicobaltchelates show four electron reduction Of O2 in alkaline solution, but yield exclusively hydrogen peroxide at low pH. By using the increased valence theory, an attempt is made to provide a valence bond representation for the electronic reorganization that could be associated with each of these phenomena. For the results obtained in acid solution, this theory indeed offers an excellent description. In alkaline solution, however, application of the increased valence theory alone does not fully account for the experimental observations, unless the following condition is also satisfied. Besides bridge adsorption of oxygen, appropriate values for the redox potentials of the Co centres also seem to be a prerequisite for the occurrence of direct reduction of oxygen to water.

Spectroscopic measurements on metal tetrasulphonato-phthalocyanines

Abstract An optically transparent thin layer cell was used to obtain light absorption spectra of iron tetrasulphonato-phthalocyanine (FeTSPc) dissolved in both acidic and alkaline solutions, with the iron ion in the first, second and third oxidation state, respectively. Further, electroreflectance (ER) spectra for the complex adsorbed on the basal plane of stress-annealed pyrolytic graphite (BP of SA-Cp) were recorded. The ER spectrum due to the couple Fe(I)/Fe(II)TSPc has, as predicted theoretically, a shape similar to that of the difference spectrum constructed by subtracting the absorption spectra observed for the components of the couple from each other. For the other couple (Fe(II)/Fe(III)TSPc) also, a similarity exists between the electroreflectance spectrum and the constructed difference spectrum in acidic electrolyte. However, there is hardly any resemblance in alkaline solution, indicating that in this electrolyte the surface has a greater influence on the adsorbed complex than in acidic solution. The importance of these observations with respect to the reduction of dioxygen is discussed.

A NEW METHOD OF PREPARING A ROTATING RING-DISC ELECTRODE FOR THE STUDY OF CARBON SUPPORTED CATALYSTS

Abstract A new preparation method is described for the study of carbon supported electrodes, which are currently under investigation for fuel cell applications. The carbon is applied to the disc of a rotating ring—disc electrode (RRDE) through incorporation of the carbon particles ina polypyrrole film. Electron transport to the carbon is possible because the polypyrrole is electronically conducting; the high porosity of the film enables the diffusion of reactants into the pores of the catalyst. This new technique offers an easy and quick way of measuring both the activity and selectivity of carbonsupported catalysts.