Electrodeposition Behavior of Extracted Platinum Complex in Ionic Liquids Evaluated By Electrochemical Quartz Crystal Microbalance

Abstract

1. Introduction From the perspective of process simplification and secondary waste reduction, the solvent extraction-electrodeposition method is effective in which the ionic liquid (IL) phase containing the extracted complexes is used as an electrodeposition bath and the extracted complexes are electrodeposition to recover the metals efficiently. We have applied the solvent extraction-electrodeposition method to Ru [1], In [2] and Nd [3] in the IL system based on bis(trifluoromethyl)sulfonyl amide (NTf2 -). In addition, the complexation states between [NTf2 -] and the metal have been clarified from a computational chemical point of view [4] based on Raman spectroscopy and density functional theory. The electrochemical quartz crystal microbalance method (EQCM) was used to evaluate the electrodeposition behavior of the extracted platinum complex in the IL phase after the extraction by solvent extraction. The purpose of this study is to analyze the electrodeposition behavior of extracted platinum complex, the weight change of the electrode interface, and the alternation of viscosity or density of the IL at the electrode interface. 2. Experimental The resonance frequency and resistance of an AT-cut Pt-coated quartz oscillator [9 MHz, φ=5.0 mm, Seiko EG&G, QA-A9M-PT(P)] were determined using an electrochemical quartz crystal microbalance (EQCM) system (Seiko EG&G, QCA922) with a well-type cell. The EQCM system was heated with a heating mantle controlled by a thermostat with a PID controller. The applicability of the EQCM technique at elevated temperature was demonstrated in our previous study [5]. Voltammetric measurements were carried out using an electrochemical analyzer (ALS-440A, BAS Inc.) with a Pt-coated quartz oscillator in the EQCM system as a working electrode. The surface area of the QCM crystal was 1.9635×10-5 m2. Two Pt wires with inner diameters of 0.5 mm were used as the counter and quasi-reference electrodes (QRE). The counter electrode was surrounded by a Vycor glass filter at the bottom to prevent diffusion of the decomposition components from the anode into the electrolyte. The Pt QRE showed high stability and good reproducibility of the potential at elevated temperature. The potential was compensated against a Fc/Fc+ redox couple. The electrochemical behavior of the extracted platinum complex was investigated at 323 K with a sweep rate of 2.0 mV s-1. 3. Results and discussion The result of the CV/EQCM analysis of [R3NH]2[PtCl6]IL in Alamine336/[P2225][NTf2] was shown in Fig. 1. Preliminary, in the case of Alamine336/[P2225][NTf2] system, which did not contain the Pt(IV) extraction complex and consisted of the extractant and the diluent, there were no main peaks other than the solvent decomposition during this potential region. It indicated that Alamine336 extractant did not undergo the electrochemical decomposition. On the other hand, a slight reduction peak was observed at -0.53 V for the extracted Pt(IV) complex, which showed little change with respect to Δm, suggesting that it was corresponded to the charge transfer reaction of Pt(IV)/Pt(II). The main reduction peak was observed at -1.65 V, and simultaneously an increase in Δm and a decrease in Δηρ were also observed in this study. The molecular weight of the electrodeposited species: M app was corresponded to be 193.7 calculated from Δm, suggesting that the main reduction reaction was the electrodeposition of Pt(II)/Pt(0); the decrease in Δηρ represented a localized viscous change in the IL phase near the electrode interface, indicating the consumption of Pt(II) by the electrodeposition process. The change in Δηρ during the electrodeposition process was consistent with the EQCM analysis of Ag(I)/Ag(0) [6].