Temperature-dependency of back-extraction behavior of inert platinum group metals (PGMs) like Ru(III) and Rh(III) has been investigated in a biphasic system consisting of H2C2O4(aq) and betainium bis(trifluoromethylsulfonyl)amide ([Hbet][Tf2N]) after extracting these PGMs as well as that of non-inert Pd(II). At 298 K, Pd(II) in [Hbet][Tf2N] phase was stripped high efficiency (78.8%) by 0.1 M H2C2O4 within 2 min. In contrast, back-extraction of Rh(III) and Ru(III) did not proceed at all in practice under the same condition, demonstrating the inertness of these PGMs. When the sample temperature was risen up to 353 K, the stripping of Rh(III) was promoted, and completed within 300 min at 353 K. The stripping efficiency of Ru(III) was still as low as 24.2% at 300 min and 353 K, and took 48 h in use of 1 M H2C2O4(aq) for completion. Further mechanistic studies by means of UV-vis absorption spectroscopy clarified that the complexation of these PGMs and oxalate ion (C2O42-) is rate-determining in their stripping, and is successfully accelerated and enhanced by elevating the temperature and increasing the concentration of the stripping agent.
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