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Homogeneous redox catalysis of electrochemical reactions

1980; Elsevier BV; Volume: 113; Issue: 1 Linguagem: Inglês

10.1016/s0022-0728(80)80508-0

2590-2954

C.P. Andrieux, C. Blocman, J.M. Dumas-Bouchiat, F. M'Halla, Jean‐Michel Savéant,

Advanced battery technologies research

Cyclic voltammetry appears as a more convenient tool for studying homogeneous redox catalysis than polarography, due to the recording of the current potential being much faster and the control of the diffusion process more accurate and involving a much more extended range of rates. The theory of homogeneous redox catalysis in the context of cyclic voltammetry is presented for the EC reaction scheme, i.e. the simplest of the mechanisms in which the initial electron-transfer process is followed by chemical reaction. The treatment involves a stationary-state assumption regarding the initial product of the electron transfer The dimensionless voltammograms then depend upon two kinetic parameters and of the ratio of the substrate and catalyst concentrations. Two limiting situations are found for the kinetic control of the catalytic process according to the relative magnitude of the, follow-up chemical reaction and the backward electron-transfer rates. In one of these the kinetic control is by the forward homogeneous electron transfer, while in the other it is by the follow-up chemical reactions. In these two cases the system depends upon only two parameters, allowing a detailed discussion of the shape and characteristics of the voltammograms to be presented, as well a set of working curves giving the relative catalytic increase of the peak current as a function of the pertinent kinetic parameter and of the substrate—catalyst concentration ratio. A numerical procedure is presented for the resolution of the general case involving a mixed kinetic control by the two reactions. Diagnostic criteria for the recognition of the two limiting cases and the transition to mixed kinetic control are given, as well as procedures for determining the characteristic rate constants. The theory is then extended to the more frequently encountered two-electron processes which involves only a slight modification of the results pertaining to the EC scheme. Experimental illustrations of the main theoretical results are given in the context of the reduction of aromatic halides in aprotic media. The results obtained in this study have as their main application the determination, through the redox catalytic method, of the rate and equilibrium parameters for irreversible systems that would be far beyond the possibilities offered by the standard use of electrochemical techniques.