Reduction of hypochlorite is the most important side reaction in the sodium chlorate reactor leading to high energy losses. Today chromate is added to the reactor solution to minimize the hypochlorite reduction but a replacement is necessary due to health and environmental risks with chromate. In order to understand the effect of different substrates on the hypochlorite reduction, α-FeOOH, γ-FeOOH, Cr 2 O 3 and CrOH 3 were electrodeposited on titanium and subjected to electrochemical investigations. These substances are commonly found on cathodes in the chlorate process and can serve as model substances for the experimental investigation. The mechanism of hypochlorite reduction was also studied using DFT calculations in which the reaction at Fe(III) and Cr(III) surface sites were considered in order to single out the electrocatalytic properties. The experimental results clearly demonstrated that the chromium films completely block the reduction of hypochlorite, while for the iron oxyhydroxides the process can readily occur. Since the electrocatalytic properties per se were shown by the DFT calculations to be very similar for Fe(III) and Cr(III) sites in the oxide matrix, other explanations for the blocking ability of chromium films are addressed and discussed in the context of surface charging, reduction of anions and conduction in the deposited films. The main conclusion is that the combined effect of electronic properties and reduction of negatively charged ions can explain the reduction kinetics of hypochlorite and the effect of chromate in the chlorate process.
Water reduction on corroded iron surfaces is technologically and fundamentally important. Here, the technological interest originates from the chlorate process where water reduction is the main cathodic process. Fundamentally, water reduction on oxide surfaces raises questions on the stability of the oxide and the nature of electrocatalytic surface sites. Two iron oxyhydroxides, αand γ-FeOOH, were electrodeposited on titanium substrate and their reduction processes were followed in detail with in-situ Raman spectroscopy, using low incident laser power to avoid sample damaging. Polarization to negative potentials show two reduction peaks for γ-FeOOH and one peak for α-FeOOH prior to hydrogen evolution. The characteristic Raman peaks gradually disappear as the potential is made more negative but no new peaks can be observed. δ-FeOOH was detected as an intermediate phase upon oxidation of the reduced surface layer. This indicates that Fe(OH) 2 is formed during cathodic polarization and initially re-oxidized to the isostructural δ-FeOOH. Characteristic Raman signals of the original phases appear upon further oxidation in air.
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