Study on the Electrochemical Reduction Reaction Process of Cr(Ⅲ) in ChCl - EG Deep Eutectic Solvent System

Abstract

In order to explore the electrochemical reduction reaction process of trivalent chromium ions, ultraviolet visible spectrophotometry (UV - Vis), linear sweep voltammetry (LSV), cyclic voltammetry (CV) and other methods were used to study the existing forms of the coordination compounds at the initial reduction stage of Cr (Ⅲ) ions in choline chloride - ethylene glycol (ChCl - EG) deep eutectic solvent system, the electrochemical reduction reaction process of Cr (Ⅲ) ions, the apparent activation energy of the reduction process of Cr (Ⅲ) ions in the electrolyte and the control steps of the reduction process. Results showed that at the initial reduction stage of Cr (Ⅲ) in deep eutectic solvent, the coordination compounds of [Cr(H2O)3Cl3] and [Cr(H2O)2Cl4 ]- would be preferentially formed, and the [Cr(H2O)2Cl4]- was the main coordination compound. With the increase of the concentration of CrCl3·6H2O, the form of coordination compound had a tendency to partially convert from [Cr(H2O)2Cl4]- to [Cr(H2O)3Cl3], and through an analysis taking into account both this tendency and the dissociation energy, it could be found that the transformation of the coordination compound made the reduction of Cr3+→Cr2+ easier. The reduction of trivalent chromium ions in ChCl - EG deep eutectic solvent was a two - step process: Cr3+→Cr2+→Cr. In the first step, the apparent activation energy was in the range of 12～16 kJ/mol and controlled by the diffusion step. In the second step, the apparent activation energy was in the range of 16～40 kJ/mol and controlled by both electrochemical and diffusion steps.

Keywords

• trivalent chromium|coordination compound|reduction process|chcl - eg deep eutectic solvent|activation energy|control steps