An electrochemical study of H 2O 2 decomposition on single-phase γ-FeOOH films

Abstract

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2–12.6 and the H 2O 2 concentration range of 10 −4 to 10 −2 mol dm −3. The Tafel slopes for H 2O 2 reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec −1, respectively, independent of pH and the concentration of H 2O 2. Both the reduction and oxidation of H 2O 2 on γ-FeOOH have a first-order dependence on the concentration of molecular H 2O 2. 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 H 2O 2, not its base form, HO 2 −. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HO OH − and HO for the reduction, and HO 2 H +, HO 2 , and O 2 − 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 H 2O 2 to the corresponding intermediates, a reaction step which involves the use of Fe III/Fe II sites in the γ-FeOOH surface as an electron donor–acceptor relay. The rate constant for the H 2O 2 decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H 2O 2 decomposition.