Currently the dominating bleaching technology for kraft pulp is the usage of chlorine dioxide. This is formed on site from sodium chlorate, dominantly produced by the electrolysis of concentrated brine solutions. This latter process is very energy intensive and, as large amounts are produced, the annual electricity needs worldwide add up to about 20 TWh, which is comparable to the electricity consumption of a smaller European country or the State of Idaho. The losses caused by parasitic cathodic reactions in the electrochemical process are minimized by the addition of sodium dichromate to the electrolyte. Due to health concerns, the European Commission urge the replacement of this compound, and it is therefore vital to find alternative solutions for the sustainable production of sodium chlorate.

The chlorate synthesis is performed in undivided electrochemical cells, where reduction of the hypochlorite intermediate and the chlorate product lead to significant efficiency losses in the absence of dichromate. In this work we present the effect of several transition metal compound additives on the electrochemical reduction of hypochlorite using model electrode surfaces. Most importantly in sodium permanganate containing solutions the hypochlorite reduction is effectively suppressed. This is related to the continuous build-up of an amorphous manganese oxide layer during the electrolysis. At the same time, the hydrogen evolution reaction (HER) proceeds on this electrode without significantly altered kinetics. The effect of the sodium permanganate additive was quantified by mass spectrometric measurements, where a significantly increased Faraday efficiency was observed compared to the addition free case (Figure 1).

Figure 1. H₂ production rate measured during electrolysis with platinum counter and working electrodes at j = 100, 200, 300 mA cm⁻² in an 80 mM NaOCl + 2 M NaCl solution, pH = 12 and T = 80°C. The horizontal lines show the maximum H₂ production rates at the different current densities, assuming 100 % current efficiency for H₂ production.

To circumvent the unlimited growth of the manganese oxide film, ex situ prepared electrodes of similar composition were also tested in the process. Using MnO₂/Ti electrodes, formed by thermal decomposition, a high selectivity towards HER is achieved in hypochlorite containing solutions. The HER selectivity depends on the composition, morphology and thickness of these electrodes. The effect of these parameters is addressed in our experiments.

Another very important sub-step of the chlorate production process is the disproportion of the hypochlorite intermediate to chlorate. As a parallel reaction, oxygen formation from the hypochlorite decomposition also occurs, which is catalyzed by different metal ions. To fully explore the effect of the promising solution additives, we also studied the effect of these compounds on the selectivity between chlorate and oxygen formation from the decomposition of hypochlorite.