Abstract
The hydrogen evolution reaction (HER) from water reduction is the main cathodic reaction in the sodium chlorate process. The reaction typically takes place on electrodes covered with a Cr(III) oxide-like film formed in situ by reduction of sodium dichromate in order to avoid reduction of hypochlorite and thereby increase the selectivity for the HER. However, the chemical structure of the Cr(III) oxide-like film is still under debate. In the present work, the kinetics of the HER were studied using titanium electrodes covered with electrodeposited Cr(OH)3 or Cr2O3, which were characterized by means of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. A clear difference in the morphology of the deposited surfaces was obtained, and the structure could be revealed with Raman spectroscopy. The kinetics for the HER were investigated using potentiodynamic and potentiostatic techniques. The results show that the first electron transfer is rate limiting and that the activity decreases in the order Cr2O3@Ti > bare Ti > Cr(OH)3@Ti. The low activity obtained for Cr(OH)3@Ti is discussed in terms of the involvement of structural water in the HER and the slow ligand exchange rate for water in Cr(III) complexes, while the high activity obtained for Cr2O3@Ti is rationalized by a surface area effect in combination with reduction of surface water and water in solution.Graphical ᅟ
Highlights
The electrosynthesis of sodium chlorate is one of the most important industrial processes, especially for the pulp and paper industry [1]
For a long time it was believed that the reduction of chromate yields a hydrated Cr(OH)3 film on platinum and gold [15] electrodes, but recently Hatch and Gewirth [20] determined with in situ Raman spectroscopy and atomic force microscopy (AFM) that the structure of chromate electrochemically reduced on gold is a Cr2O3 thin film
Even though energydispersive x-ray spectroscopy (EDX) analyses have shown the expected Cr/O ratio for both the Cr(OH)3@Ti and Cr2O3@Ti phases (Table 2), this is not sufficient to characterize the molecular structure of the films, which we have investigated by Raman spectroscopy
Summary
The electrosynthesis of sodium chlorate is one of the most important industrial processes, especially for the pulp and paper industry [1]. With the aim to fill this gap, the present paper provides a comprehensive investigation of the mechanism and kinetics of water reduction on Cr(OH)3- and Cr2O3-electrodeposited films.
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