An electrochemical study of H2O2 decomposition on single-phase α-Fe2O3 films

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 FeIII/FeII 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. (author)