An Electrochemical Study Of H2O2 Decomposition on Single-Phase α-Fe2O3 Films

Abstract

The electrochemical kinetics of hydrogen peroxide on α-Fe₂O₃ films were studied over the pH range of 9.2 to 12.6 and the H₂O₂ concentration range of 10^-4 to 10^-2 mol⋅dm^-3. The Tafel slopes for H₂O₂ reduction obtained from polarization measurements are 124 ± 6 independent of pH and the concentration of H₂O₂. H₂O₂ oxidation reactions are adsorption behaviours, currents slightly increasing with potentials at lower overpotentials, and rapidly increasing with Tafel behaviours at higher overpotentials. Both the reduction and oxidation of H₂O₂ on α-Fe₂O₃ have a first-order dependence on the concentration of molecular H₂O₂. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH^-. For both cases the electroactive species is the molecular H₂O₂, not its conjugate base form, HO₂^-. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HO•OH^- and HO• for the reduction, and HO₂•H⁺, HO₂•, and •O₂^- for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH^- and H⁺ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H₂O₂ to the corresponding intermediates, a reaction step which involves the use of Fe^III/Fe^II sites in the α-Fe₂O₃ surface as an electron donor-acceptor relay. The rate constant for the H₂O₂ decomposition on α-Fe₂O₃ determined from the oxidation and reduction Tafel lines is very low, indicating that the α-Fe₂O₃ surface is a poor catalyst for H₂O₂ decomposition.