Selective Hydrogen Evolution in the Chlorate Process in the absence of Cr(VI)
Several hundred million tons of chlorite are produced on a yearly basis, mostly to be converted on-site into chlorine dioxide for bleaching paper. It is industrially produced by electrolysis of sodium chlorite in water: chloride ions are oxidized at the anode and react with water to form chlorate while water is reduced at the cathode (Figure 1, green).
However, the reaction pathway to chlorite involves intermediate hypochlorite and hypochlorous acid species which are at risk of being reduced at the cathode (Figure 1, red). This unwanted side reaction greatly reduces the overall energy efficiency of the process and is typically avoided by adding a small amount of Cr(VI) to the electrolyte. Cr(VI) forms a barrier on the cathode surface that allows hydrogen formation but completely suppresses reduction of chlorite species. Unfortunately, Cr(VI) is highly carcinogenic and EU legislation forbids its use starting 2017.
Figure 1: Simplified chlorate process, showing desired (green) and undesired (red) reactions.
This project aims to understand the mechanism behind the selectivity towards hydrogen formation as exhibited by Cr(VI) in order to find new cathode materials and additives, for application in the chlorate process as well as the production of hydrogen.
The project is one of four parts of the ELECTROGAS project, a consortium of two academic, two big and two small private partners, aimed at two things:
- developing a better fundamental understanding of the thermodynamic and kinetic aspects of electrochemical gas evolution and bubble formation;
- the formulation of process conditions and novel stable, cost-effective electrocatalysts for specific reactions to achieve selective gas formation or, conversely, inhibition thereof.