ChCl-EG低共熔溶剂体系中Cr(Ⅲ)的电化学还原反应过程研究

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

为探究三价铬离子的电化学还原反应过程,使用紫外-可见分光光度法(Ultraviolet visible spectropho-tometry,UV-Vis)、线性扫描伏安法(Linear sweep voltammetry,LSV)、循环伏安法(Cyclic voltammetry curve,CV)等方法,分别对Cr(Ⅲ)离子在氯化胆碱-乙二醇(ChCl-EG)低共熔溶剂体系中的还原初期配合物的存在形式、Cr(Ⅲ)离子电化学还原反应过程、Cr(Ⅲ)在电解液中还原过程表观活化能以及还原过程控制步骤进行研究。结果表明,在低共熔溶剂(Deep eutectic solvent,DES)中Cr(Ⅲ)还原初期会优先形成的配位化合物为[Cr(H_(2)O)_(3) Cl_(3)]、[Cr(H_(2)O)_(2)Cl_(4)]^(-)2种,其中[Cr(H_(2)O)_(2) Cl_(4)]^(-)为主要配合物。随着CrCl_(3)·6H_(2)O浓度的增加,配位化合物形式有从[Cr(H_(2)O)_(2) Cl_(4)]^(-)部分转化为[Cr(H_(2)O)_(3)Cl_(3)]的趋势,并结合键解离能得知,配合物的转变使Cr^(3+)→Cr^(2+)的还原更容易。Cr(Ⅲ)离子在ChCl-EG低共熔溶剂中为两步还原过程,即Cr^(3+)→Cr^(2+)→Cr。其中,第1步还原(Cr^(3+)→Cr^(2+))的表观活化能在12~16 kJ/mol范围内,受扩散步骤控制,第2步还原(Cr^(2+)→Cr)的表观活化能在16~40 kJ/mol范围内,受电化学步骤和扩散步骤共同控制。

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(Ch Cl-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(H_(2)O)_(3) Cl_(3)]and[Cr(H_(2)O)_(2)Cl_(4)]^(-) would be preferentially formed,and the[Cr(H_(2)O)_(2) Cl_(4)]^(-) was the main coordination compound.With the increase of the concentration of Cr Cl_(3)·6H_(2)O,the form of coordination compound had a tendency to partially convert from[Cr(H_(2)O)_(2) Cl_(4)]^(-)to[Cr(H_(2)O)_(3)Cl_(3)],and through an analysis taking into account both this tendency and the dissocia-tion energy,it could be found that the transformation of the coordination compound made the reduction of Cr^(3+)→Cr^(2+) easier.The reduction of trivalent chromium ions in Ch Cl-EG deep eutectic solvent was a two-step process:Cr^(3+)→Cr^(2+)→Cr.In the first step,the apparent activation energy was in the range of 12~16 k J/mol and controlled by the diffusion step.In the second step,the apparent activation energy was in the range of 16~40 k J/mol and controlled by both electrochemical and diffusion steps.