Identifying the sources of oxygen in the chlorate process is challenging due to the complex set of chemical and electrochemical reactions involved. Here, two types of electrodes have been investigatedTi0.7Ru0.3O x , and electrodes with aimed composition Ti0.34Ru0.3Sn0.3Sb0.06O x , both compared with platinum anodes. The cell oxygen off-gas was analyzed employing mass spectrometry together with ex situ UV–vis spectroscopy to quantify the kinetic rate constants. Noteworthy is that the respective rates of oxygen formation from anodic and chemical reactions in the presence of hypochlorite are of the same magnitude. The addition of Sn and Sb doubled the surface area of the electrodes and decreased oxygen production when electrodes were used for the first time. However, rate constants for total oxygen production with reused electrodes follow the trend: homogeneous hypochlorite decomposition < TiRu < TiRuSnSb to the highest value obtained by Pt. The same trend is noticed for rate constants concerning the hypochlorite decomposition.
BACKGROUND: Sodium chlorate (NaClO 3 ) is extensively used in the paper industry, but its production uses strictly regulated highly toxic Na 2 Cr 2 O 7 to reach high hydrogen evolution reaction (HER) Faradaic efficiencies. It is therefore important to find alternatives either to replace Na 2 Cr 2 O 7 or reduce its concentration. RESULTS: The Na 2 Cr 2 O 7 concentration can be significantly reduced by using Na 2 MoO 4 as an electrolyte co-additive. Na 2 MoO 4 in the millimolar range shifts the platinum cathode potential to less negative values due to an activating effect of cathodically deposited Mo species. It also acts as a stabilizer of the electrodeposited chromium hydroxide but has a minor effect on the HER Faradaic efficiency. X-ray photoelectron spectroscopy (XPS) results show cathodic deposition of molybdenum of different oxidation states, depending on deposition conditions. Once Na 2 Cr 2 O 7 was present, molybdenum was not detected by XPS, as it is likely that only trace levels were deposited. Using electrochemical measurements and mass spectrometry we quantitatively monitored H 2 and O 2 production rates. The results indicate that 3 ∼mol L −1 Na 2 Cr 2 O 7 (contrary to current industrial 10-30 mmol L −1 ) is sufficient to enhance the HER Faradaic efficiency on platinum by 15%, and by co-adding 10 mmol L −1 Na 2 MoO 4 the cathode is activated while avoiding detrimental O 2 generation from chemical and electrochemical reactions. Higher concentrations of Na 2 MoO 4 led to increased oxygen production.CONCLUSION: Careful tuning of the molybdate concentration can enhance performance of the chlorate process using chromate in the micromolar range. These insights could be also exploited in the efficient hydrogen generation by photocatalytic water splitting and in the remediation of industrial wastewater.
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