Photoreduction of {sup 99}Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

Technetium-99 ({sup 99}Tc) ({beta}{sup -}{sub max}: 293.7 keV; t{sub 1/2}: 2.1 x 10{sup 5} years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral- basic environment, the pertechnetate anion (TcO{sub 4}{sup -}) is stable. TcO{sub 4}{sup -} is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils or sediments. This study moves forward a new strategy for the reduction of TcO4- and chemical incorporation of the reduced Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), {alpha}{sub 2}-[P{sub 2}W{sub 17}O{sub 61}]{sup 10-}, that can be photoactivated in the presence of 2-propanol to transfer electrons to TcO{sub 4}{sup -}, and incorporate the reduced Tc covalently into the {alpha}2- framework to form the Tc{sup V}O species, Tc{sup V}O({alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}){sup 7-}. This occurs via the formation of an intermediate species that slowly converts to Tc{sup V}O({alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}){sup 7-}. EXAFS and XANES analysis and preliminary EPR analysis, suggests that the intermediate consists of a Tc(IV) {alpha}2- species where the Tc is likely bound to only 2 of the 4 W-O oxygen atoms in the {alpha}{sub 2}-[P{sub 2}W{sub 17}O{sub 61}]{sup 10-} defect. This intermediate then oxidizes and converts to the Tc{sup V}O({alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}){sup 7-} product. The reduction and incorporation of TcO{sub 4}{sup -} was accomplished in a "one pot" reaction using both sunlight and UV irradiation, and monitored as a function of time using multinuclear NMR and radio TLC. The process was further probed by the "step-wise" generation of reduced {alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}{sup 12-} through bulk electrolysis followed by the addition of TcO{sub 4}{sup -}. The reduction and incorporation of ReO{sub 4}{sup -}, as a non-radioactive surrogate for {sup 99}Tc, does not proceed through the intermediate species, and Re{sup V}O is incorporated quickly into the {alpha}{sub 2}-[P{sub 2}W{sub 17}O{sub 61}]{sup 10-} defect. These observations are consistent with the periodic trends of Tc and Re. Specifically, Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and mechanisms of the reduction and incorporation of Tc into a material

Biomolecules Electrochemical Sensing Properties of a PMo11V@N-Doped Few Layer Graphene Nanocomposite

A novel hybrid nanocomposite, PMo11V@N-doped few layer graphene, was prepared by a one-step protocol through direct immobilization of the tetrabutylammonium salt of a vanadium-substituted phosphomolybdate (PMo11V) onto N-doped few layer graphene (N-FLG). The nanocomposite characterization by FTIR and XPS confirmed its successful synthesis. Glassy carbon modified electrodes with PMo11V and PMo11V@N-FLG showed cyclic voltammograms consistent with surface-confined redox processes attributed to Mo-centred reductions (MoVI→MoV) and a vanadium reduction (VV→VIV). Furthermore, PMo11V@N-FLG modified electrodes showed good stability and well-resolved redox peaks with high current intensities. The observed enhancement of PMo11V electrochemical properties is a consequence of a strong electronic communication between the POM and the N-doped few layer graphene. Additionally, the electro-catalytic and sensing properties towards acetaminophen (AC) and theophylline (TP) were evaluated by voltammetric techniques using a glassy carbon electrode modified with PMo11V@N-FLG. Under the conditions used, the square wave voltammetric peak current increased linearly with AC concentration in the presence of TP, but showing two linear ranges: 1.2 × 10−6 to 1.2 × 10−4 and 1.2 × 10−4 to 4.8 × 10−4 mol dm−3, with different AC sensitivity values, 0.022 A/mol dm−3 and 0.035 A/mol dm−3, respectively (detection limit, DL = 7.5 × 10−7 mol dm−3)

Electrochemical behavior and electrocatalytic properties towards hydrogen peroxide, dioxygen and nitrate of the polyanions [(Ni(II)OH₂)₂(Fe(III))₂(X₂W₁₅O₅₆)₂]¹⁴¯ (X = P(V) or As(V)): A comparative study

In this study, the electrochemical behavior and the electrocatalytic properties towards the reduction of hydrogen peroxide, dioxygen and nitrate are compared for the two mixed sandwich-type complexes Ni₂Fe₂P₄ and Ni₂Fe₂As₄. In media of pH 3, Ni₂Fe₂P₄ displayed two waves corresponding to the reduction of the two Fe(III) centers within the complex followed by four W(VI) based-waves. Compared to Ni₂Fe₂P₄, Ni₂Fe₂As₄ displayed only one Fe-based wave and three W-waves. In other words, the two Fe(III) waves and the first two W(VI) waves observed for Ni₂Fe₂P₄ merge into one Fe-based wave and one W-wave in Ni₂Fe₂As₄. This is probably related to a slight difference in acidity between the two complexes. In other words, Ni₂Fe₂P₄ is slightly more basic than Ni₂Fe₂As₄. This difference in acidity is also reflected in the position of the potentials. Compared to Ni₂Fe₂P₄, the peak potentials of Ni₂Fe₂As₄ are slightly shifted towards positive values, and the shift is more pronounced for the W-based waves than for the Fe-waves. The remarkable stability of the two complexes (roughly from pH 0 to 7) permitted to evaluate their catalytic behavior towards H₂O₂, O₂ and NO₃. For the electrocatalytic reduction of H₂O₂, it is noticed that Ni₂Fe₂As₄ is more efficient than Ni₂Fe₂P₄. However, for O₂ and NO¯₃ it is observed that Ni₂Fe₂P₄ is more efficient than Ni₂Fe₂As₄.status: publishe

Synthesis, structure elucidation and redox properties of 99Tc complexes of lacunary Wells Dawson polyoxometalates: insights into molecular 99Tc - metal oxide interactions

The isotope 99Tc (beta max: 250 keV, half-life: 2 x 105 year) is an abundant product of uranium-235 fission in nuclear reactors and is present throughout the radioactive waste stored in underground tanks at Hanford and Savannah River. Understanding and controlling the extensive redox chemistry of 99Tc is important to identify tunable strategies to separate 99Tc from spent fuel and from waste tanks and once separated, to identify and develop an appropriately stable waste-form for 99Tc. Polyoxometalates (POMs), nanometer sized models for metal oxide solid-state materials, are used in this study to provide a molecular level understanding of the speciation and redox chemistry of incorporated 99Tc. In this study, 99Tc complexes of the (alpha 2-P2W17O61)10- and (alpha 1-P2W17O61)10- isomers were prepared. Ethylene glycol was used as a"transfer ligand" to minimize the formation of TcO2 cdot xH2O. The solution structures, formulations, and purity of TcVO(alpha 1/alpha 2-P2W17O61)7- were determined by multinuclear NMR. X-ray Absorption Spectroscopy of the complexes are in agreement with the formulation and structures determined from 31P and 183W NMR. Preliminary electrochemistry results are consistent with the EXAFS results, showing a facile reduction of the TcVO(alpha 1-P2W17O61)7- species compared to the TcVO(alpha 2-P2W17O61)7- analog. The alpha1- defect is unique in that a basic oxygen atom is positioned toward the alpha1- site and the TcVO center appears to form a dative metal-metal bond with a framework W site. These attributes may lead to the assistance of protonation events that facilitate reduction. Electrochemistry comparison shows that the ReV analogs are about 200 mV more difficult to reduce in accordance with periodic trends

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

The radioactive element technetium-99 (⁹⁹Tc, half-life = 2.1 × 10⁵ years, β^– of 253 keV), is a major byproduct of ²³⁵U fission in the nuclear fuel cycle. ⁹⁹Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived ⁹⁹Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells–Dawson isomers, α₁-P₂W₁₇O₆₁^10– (<b>α1</b>) and α₂-P₂W₁₇O₆₁^10– (<b>α2</b>) to identify features of metal oxide materials that can stabilize the immobile Tc­(IV) oxidation state accessed from the synthesized Tc­(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K_7–<i>n</i>H_<i>n</i>[Tc^VO­(α₁-P₂W₁₇O₆₁)] (<b>Tc</b>^<b>V</b><b>O-α1</b>), K_7–<i>n</i>H_<i>n</i>[Tc^VO­(α₂-P₂W₁₇O₆₁)] (<b>Tc</b>^<b>V</b><b>O-α2</b>) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary <b>α1</b> site as compared to the <b>α2</b> site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The M^V→ M^IV reduction processes for <b>Tc</b>^<b>V</b><b>O-α1</b> are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the <b>M</b>^<b>V</b><b>O-α2</b> (M = ⁹⁹Tc, Re) and <b>Re</b>^<b>V</b><b>O-α1</b> where M^V/IV reduction process must occur hand in hand with protonation of the terminal MO to make the π*­(MO) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the Tc^V is “pulled” into the <b>-α1</b> framework and that may facilitate the reduction of <b>Tc</b>^<b>V</b><b>O-α1</b> and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions

Photoreduction of 99

Technetium-99 ({sup 99}Tc) ({beta}{sup -}{sub max}: 293.7 keV; t{sub 1/2}: 2.1 x 10{sup 5} years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral- basic environment, the pertechnetate anion (TcO{sub 4}{sup -}) is stable. TcO{sub 4}{sup -} is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils or sediments. This study moves forward a new strategy for the reduction of TcO4- and chemical incorporation of the reduced Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), {alpha}{sub 2}-[P{sub 2}W{sub 17}O{sub 61}]{sup 10-}, that can be photoactivated in the presence of 2-propanol to transfer electrons to TcO{sub 4}{sup -}, and incorporate the reduced Tc covalently into the {alpha}2- framework to form the Tc{sup V}O species, Tc{sup V}O({alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}){sup 7-}. This occurs via the formation of an intermediate species that slowly converts to Tc{sup V}O({alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}){sup 7-}. EXAFS and XANES analysis and preliminary EPR analysis, suggests that the intermediate consists of a Tc(IV) {alpha}2- species where the Tc is likely bound to only 2 of the 4 W-O oxygen atoms in the {alpha}{sub 2}-[P{sub 2}W{sub 17}O{sub 61}]{sup 10-} defect. This intermediate then oxidizes and converts to the Tc{sup V}O({alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}){sup 7-} product. The reduction and incorporation of TcO{sub 4}{sup -} was accomplished in a &quot;one pot&quot; reaction using both sunlight and UV irradiation, and monitored as a function of time using multinuclear NMR and radio TLC. The process was further probed by the &quot;step-wise&quot; generation of reduced {alpha}{sub 2}-P{sub 2}W{sub 17}O{sub 61}{sup 12-} through bulk electrolysis followed by the addition of TcO{sub 4}{sup -}. The reduction and incorporation of ReO{sub 4}{sup -}, as a non-radioactive surrogate for {sup 99}Tc, does not proceed through the intermediate species, and Re{sup V}O is incorporated quickly into the {alpha}{sub 2}-[P{sub 2}W{sub 17}O{sub 61}]{sup 10-} defect. These observations are consistent with the periodic trends of Tc and Re. Specifically, Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and mechanisms of the reduction and incorporation of Tc into a material

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

The radioactive element technetium-99 (99Tc, half-life = 2.1 × 105 years, β– of 253 keV), is a major byproduct of 235U fission in the nuclear fuel cycle. 99Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived 99Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells–Dawson isomers, α1-P2W17O6110– (α1) and α2-P2W17O6110– (α2) to identify features of metal oxide materials that can stabilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K7–nHn[TcVO(α1-P2W17O61)] (TcVO-α1), K7–nHn[TcVO(α2-P2W17O61)] (TcVO-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The MV→ MIV reduction processes for TcVO-α1 are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the MVO-α2 (M = 99Tc, Re) and ReVO-α1 where MV/IV reduction process must occur hand in hand with protonation of the terminal M═O to make the π*(M═O) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the TcV is “pulled” into the -α1 framework and that may facilitate the reduction of TcVO-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions