On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions

Abstract

The goal of this study was to gain a better understanding of the feasibility of replacing Cr(VI) in the chlorate process by Mo(VI), focusing on the cathode reaction selectivity for hydrogen evolution on steel and titanium in a hypochlorite containing electrolyte. To evaluate the ability of Cr(VI) and Mo(VI) additions to hinder hypochlorite reduction, potential sweep experiments on rotating disc electrodes and cathodic current efficiency (CE) measurements on stationary electrodes were performed. Formed electrode films were investigated with scanning electron microscopy and energy-dispersive X-ray spectroscopy. Cathodic hypochlorite reduction is hindered by the Mo-containing films formed on the cathode surface after Mo(VI) addition to the electrolyte, but much less efficient compared to Cr(VI) addition. Very low levels of Cr(VI), in the mM range, can efficiently suppress hypochlorite reduction on polished titanium and steel. Phosphate does not negatively influence the CE in the presence of Cr(VI) or Mo(VI) but the Mo-containing cathode films become thinner if the electrolyte during the film build-up also contains phosphate. For a RuO2-TiO2 anode polarized in electrolyte with 40 mM Mo(VI), the anode potential increased and increased molybdenum levels were detected on the electrode surface.<p>QC 20130109</p

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

“…The validation tests were run with four cells for 19 days with one power shutdown before the evaluation of the performance. The cathodic current efficiency was determined to be 97.2 ± 0.4% which is well in line with the work of Gustavsson et al [20] as well as Spasojevi´c et al [7]. The cell voltage was 3.135 ± 0.015 V at a current density of 2 kA m -2 .…”

“…Stability tests and validation trials were made in the test pilot unit with titanium cathodes since they have proven to be stable and give high faradaic yields [20,22]. The validation tests were run with four cells for 19 days with one power shutdown before the evaluation of the performance.…”

“…Several studies on the cathodic current efficiency, as related to the chlorate process, clearly show that the performance is time dependent. Directly after startup, a quick and immediate increase in current efficiency is observed, followed by a slow increase in current efficiency until a final steady state far away from the timeline of most experiments is reached [8,20,21]. Hence, to achieve results that are relevant to full-scale conditions, it is of utmost importance to allow the system to reach the steady state in the study of the parameters.…”

Understanding Selectivity in the Chlorate Process: A Step towards Efficient Hydrogen Production

“…As demonstrated in the present paper, a complete coverage of the substrate is required to fully block the hypochlorite reduction. This is also supported by results obtained for polished titanium and steel substrates, where it was shown that μM levels of chromate efficiently suppress hypochlorite reduction . However, in the commercial chlorate process, concentrations between 2–8 gL −1[48–49] are required to obtain 90–95% cathodic current efficiency .…”

Study of Hypochlorite Reduction Related to the Sodium Chlorate Process

“…However, sodium dichromate is listed in the Annex XIV under REACH [18], which means that without authorization it is forbidden to use after the sunset date in late 2017. Efforts have been made to exchange chromium with molybdenum, yttrium and other rare earth metals but so far nothing has proven good or long-term stable enough to replace chromate [19][20][21][22][23]. Recently, it was shown that adding trivalent chromium to the electrolyte will be sufficient to in situ form the desired species [24].…”