99Tc and Re Incorporated into Metal Oxide Polyoxometalates: Oxidation State Stability Elucidated by Electrochemistry and Theory

摘要

The radioactive element technetium-99 (99Tc, half-life =n2.1 × 105 years, β− of 253 keV), is a major byproduct of 235U fission innthe nuclear fuel cycle. 99Tc is also found in radioactive waste tanks and innthe environment at National Lab sites and fuel reprocessing centers.nSeparation and storage of the long-lived 99Tc in an appropriate andnstable waste-form is an important issue that needs to be addressed.nConsidering metal oxide solid-state materials as potential storagenmatrixes for Tc, we are examining the redox speciation of Tc on thenmolecular level using polyoxometalates (POMs) as models. In this studynwe investigate the electrochemistry of Tc complexes of the monovacantnWells−Dawson isomers, α1-P2W17O61n10− (α1) and α2-P2W17O61n10− (α2)nto identify features of metal oxide materials that can stabilize thenimmobile Tc(IV) oxidation state accessed from the synthesized Tc(V)Onspecies and to interrogate other possible oxidation states available to Tcnwithin these materials. The experimental results are consistent with density functional theory (DFT) calculations.nElectrochemistry of K7−nHn[TcVO(α1-P2W17O61)] (TcVO-α1), K7−nHn[TcVO(α2-P2W17O61)] (TcVO-α2) and their rheniumnanalogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues.nBoth Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate thatnthe highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation statenfor Tc is VI. The MV→ MIV reduction processes for TcVO-α1 are not pH dependent or only slightly pH dependent suggestingnthat protonation does not accompany reduction of this species unlike the MVO-α2 (M = 99Tc, Re) and ReVO-α1 where MV/IVnreduction process must occur hand in hand with protonation of the terminal MO to make the π*(MO) orbitals accessiblento the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-raynabsorption near edge structure (XANES) data that reveal that the TcV is “pulled” into the -α1 framework and that may facilitatenthe reduction of TcVO-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, andntheir impact on the chemical properties of the Tc substituted in these positions.