Oxidation reactions in chromium(III) formate electrolytes at platinum and at a catalytic mixed metal oxide coating of iridium oxide and tantalum oxide

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• 作者：J.H.O.J. Wijenberg^a ; ^{jacques.wijenberg@tatasteel.com" class="auth_mail" title="E-mail the corresponding author} ; A.C.A. de Vooys^a ; R. Kortlever^b ; M.T.M. Koper^b
• 关键词：trivalent chromium electrolytes ; formate ; anodes ; oxidation reactions
• 刊名：Electrochimica Acta
• 出版年：2016
• 出版时间：20 September 2016
• 年：2016
• 卷：213
• 期：Complete
• 页码：194-200
• 全文大小：1470 K

文摘

In trivalent chromium plating processes, the breakdown of useful electrolyte components or the formation of hazardous products, in particular Cr(VI), at the anode is obviously undesired. In this study, the oxidation reactions in trivalent chromium electrolytes with formate as complexing agent are investigated at platinum and at a catalytic mixed metal oxide (MMO) coating comprising iridium oxide (IrO₂) and tantalum oxide (Ta₂O₅) using cyclic voltammetry combined with On-Line Electrochemical Mass Spectrometry (OLEMS) measurements.At platinum the following sequence of reaction steps is proposed (all potentials vs. SCE):

1. Oxidative adsorption of [Cr(HCOO)(H₂O)₅]²⁺, reversible at −0.4 V;

2. CO₂ evolution from the Cr(III)-formate complex above +0.4 V;

3. Further oxidation to $\text{HCr}{\text{O}}_4^{-}$ with simultaneous CO₂ and O₂ evolution above +1.2 V.

Cr(VI) formation also occurs in the absence of formic acid, i.e. when only $\mathrm{C}\mathrm{r}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6^{3+}$ is present in the electrolyte.

At a catalytic MMO coating the adsorption step is absent, as well as any subsequent oxidation of the Cr(III)-formate complex. This suggests that adsorption of the Cr(III) complex is required for Cr(VI) formation.

MMO electrodes allow a safe usage of the electrolyte for industrial applications, whereas platinum electrodes do not.