Study of Hypochlorite Reduction Related to the Sodium Chlorate Process

Abstract

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.

Electrochemical Investigation of the Hydrogen Evolution Reaction on Electrodeposited Films of Cr(OH)3 and Cr2O3 in Mild Alkaline Solutions

“…The electrochemical and chemical reactions yielding chlorate have been well described in the literature [4][5][6][7][8][9][10][11]. The main half reactions are the cathodic reduction of water (Eq.…”

“…As an alternative to soluble molybdate, MoO 2 nanoparticles have been immobilized onto Ti and Fe electrodes in a Cr 2 O 3 matrix, yielding a surface very selective towards water reduction in the presence of hypochlorite [28]. However, it was shown elsewhere that electrodeposited Cr 2 O 3 completely prevents hypochlorite reduction on Ti electrodes [11] and, hence, the effects of MoO 2 particles as presented in [28] cannot be precisely evaluated.…”

“…In this context, a recent paper focusing on Cr(OH) 3 and Cr 2 O 3 surfaces has shown that both are capable of blocking hypochlorite reduction, probably due to the p-type semi-conducting properties of Cr(III) oxide-like surfaces [11]. However, studies regarding water reduction on such films were not deeply investigated.…”

Current efficiency of individual electrodes in the sodium chlorate process: a pilot plant study

“…3b. The addition of sodium dichromate provides selectivity towards water reduction by forming a chromium (III) oxide or hydroxide that effectively blocks the reduction of hypochlorite and chlorate [16]. In addition to current efficiency, there are also a change in physical appearance as a result of the formation of the chromium (III) film, i.e., the observable corrosion product and chromium (III) layer thickness.…”

“…The current efficiency has been studied in some publications, but the most common approach is to study the cell or electrode potential and oxygen production [9,10,[12][13][14][15][16][17]. It has also been reported that the time it takes to reach stable conditions in a chlorate cell or at the surface of a chlorate cathode is significantly long [8].…”