The electrochemical oxidation of hydrogen peroxide on platinum electrodes at phosphate buffer solutions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand

Abstract

The kinetics and mechanism for the electrochemical oxidation of H2O2 on platinum electrodes in phosphate buffers were studied. A mechanistic model for this reaction was developed that involves binding sites, on the surface of the electrode, that are thought to be based on some form of hydrous platinum oxide, initially identified as Pt(OH)2. Hydrogen peroxide adsorbs onto the binding sites to form the complex Pt(OH)2·H2O2. The complex then undergoes internal electron transfer to form a reduced platinum site, Pt, with the release of the products water and oxygen. The binding sites regenerate electrochemically to give rise to an amperometric signal together with the release of protons. Two side reactions were proposed, the first involved a competitive inhibition of the binding sites by oxygen to form the species Pt(OH)2·O2. The second involved a non-competitive inhibition of the complex Pt(OH)2·H2O2 by protons. A rate equation was derived to account for all electrode sites involved in the proposed mechanism, with kinetic parameters for electrode reactions, and was validated over a range of bulk [H2O2], rotation rates, potentials, temperatures, buffer concentrations and pH. The kinetic and equilibrium constants for the model were optimised using a SIMPLEX procedure. The equilibrium constants were found to be potential- and temperature-invariant (K1 = 6.38 x 10-3 m3 mol-1, K4 = 0.128 m3 mol-1 and K5 = 0.053 m3 mol-1). The diffusion coefficient for H2O2 was found to be in the range 0.55 - 0.66 x 10-9 m2 s-1. These values were lower than those reported in the literature. The rate constants, k2N and k3N, were found to vary with potential and temperature, and pseudo-activation energies for k2 were found to range from 70 - 40 kJ mol-1 (dependent on the potential). The model was further developed to account for the formation of the platinum binding sites (labelled as Pt BS) from precursor sites, Pt PS. A series of experiments employing phosphate buffers with a range of concentration and pH were performed. It was found that steady-state responses for the oxidation of H2O2 increased with increasing phosphate concentration. In the absence of phosphate, an alternative binding mechanism was evident. A maximum response was found at pH 6.8 and decreased markedly at more basic or acidic conditions. This pH-dependence suggested that H2PO4- was the species involved in the formation of the binding sites. The decrease in response at pH > 6.8 being caused by the decrease in [H2PO4-], whilst an inhibition of the precursor site by protons was proposed to account for the depression in electrode response at pH < 6.8. The influence of chloride upon the kinetics of H2O2 oxidation was examined and described qualitatively in terms of the new model. It was found that the rate of oxidation was decreased markedly in the presence of chloride. Two possible inhibition modes for chloride were identified and it was established that a non-competitive inhibition of the precursor sites was likely to be the dominant cause for the chloride inhibition. The work described in this thesis has not only identified a new and comprehensive mechanism for the oxidation of H2O2 at platinum electrodes, but also provides information that may prove useful when designing sensors that rely upon this reaction. In particular the important role of hydrodynamic conditions, buffer composition and concentration are clearly identified. Publications arising from this work i) Electrochemical oxidation of hydrogen peroxide at platinum electrodes. An adsorption-controlled mechanism. S. B. Hall, E. A. Khudaish and A. L. Hart, Electrochim. Acta, 1998 (43) 579. ii) Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Effect of potential. S. B. Hall, E. A. Khudaish and A. L. Hart, Electrochim. Acta, 1998 (43) 2015. iii) Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Effect of temperature. S. B. Hall, E. A. Khudaish and A. L. Hart, Electrochim. Acta, 1999 (44) 2455. iv) Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Phosphate buffer dependence. S. B. Hall, E. A. Khudaish and A. L. Hart, Electrochim. Acta, 1999 (44) 4573. v) Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Inhibitory effect by chloride ions. S. B. Hall, E. A. Khudaish and A. L. Hart, Electrochim. Acta, submitted for publication.