Radiochemistry, Vol. 47, No. 1, 2005, pp. 54 !57. Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 53!56. Original Russian Text Copyright + 2005 by Kostyuk.

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Anodic Dissolution of U3Si and UMo in Nonaqueous Media N. N. Kostyuk Sevchenko Institute of Applied Physical Problems, Belarussian State University, Minsk, Belarus Received March 5, 2004

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Abstract Anodic dissolution of Mo, U3Si, and UMo in dry acetonitrile containing 0.1 M (C2H5)4NCl and in anhydrous and 96% ethanol containing 0.1 M LiCl was studied under potentiostatic conditions. The oxidation potentials of the U and Mo alloys were determined. Anodic dissolution of UMo in the presence of b-diketones yields uranium and molybdenum chelates, and anodic dissolution of U3Si yields U(IV) chelates in an inert medium. The conditions for electrochemical preparation of uranyl b-diketonates involving oxidation of U(IV) with atmospheric oxygen in the course of electrolysis were determined.

Direct electrochemical synthesis of uranium b-diketonates was reported in [133]. The electrochemical synthesis of chelates of f elements has a number of apparent advantages. Anodic dissolution of a metal in organic solvents in the presence of a ligand [4] excludes formation of water and eliminates problems with hydration and internal hydrolysis of the target product [5]. This is particularly important in synthesis of volatile compounds of f elements [5, 6]. The process is one-pot, and the required equipment is relatively simple [6]. However, electrolysis processes involving uranium metal are complicated by the radioactivity of uranium and its susceptibility to oxidation with atmospheric oxygen. Because of formation of uranium dioxide on the anode surface, the anode should be cleaned before each synthesis run. This operation involves appearance of radioactive dust. Therefore, it seems appropriate to look for other materials suitable for electrochemical synthesis of uranium coordination compounds. EXPERIMENTAL The polarization curves of anodic dissolution of Mo, U3Si, and UMo were taken with a PI-50 potentiostat interfaced with a PR-8 programmer. Measurements were performed in a temperature-controlled diaphragmless cell with a working volume of 10 ml, operating in the three-electrode mode. The working electrode was a polished plate from Mo, U3Si, or UMo. The auxiliary electrode was a platinum foil of the same surface area. The working electrode potential was monitored against saturated hydrogen silver chloride electrode connected to the system under investigation through a salt bridge with a Luggin capillary. As dissolution potential we took the potential at which the anodic current started to sharply increase.

The potential sweeping rate was 8 mV s!1 in all the experiments. Before each experiment, the solution was flushed with dry Ar for 10 min; during the time of recording the polarization curves, the Ar stream was passed over the solution surface. The electrolyte solutions were prepared as for electrochemical syntheses [2, 3, 7, 8]. The conditions of the electrochemical syntheses and the cell are described in [2, 3]. The solvents (acetonitrile, ethanol), supporting electrolytes (tetraethylammonium bromide, tetraethylammonium chloride), and b-diketones [acetylacetone (2,4-pentanedione, HAA); dipivaloylmethane (2,2,6,6-tetramethyl-3,5-heptanedione, HDPM); pivaloyltrifluoroacetone (1,1,1-trifluoro5,5-dimethyl-2,4-hexanedione, HPTA)] were additionally purified as described in [7, 8]. Benzoylacetone (1-phenyl-1,3-butanedione, HBA) of analytically pure grade was recrystallized from anhydrous ethanol. Lithium chloride (ultrapure grade) was used without additional purification. Working solutions of the electrolytes were prepared in accordance with [2, 3, 7, 8]. The uranium compounds were isolated and analyzed as described in [2, 3]. In some cases, the compounds isolated were additionally studied by IR spectroscopy. The amount of the electricity passed through the cell was determined with a copper coulometer [9]. The ionizing radiation of solutions and substances was detected with a UIM 2-2 radiometer. RESULTS AND DISCUSSION The polarization curves of anodic dissolution of Mo, U3Si, and UMo in dry acetonitrile containing 0.1 M (C2H5)4NCl and in anhydrous and 96% ethanol containing 0.1 M LiCl are shown in the figure. It is seen that, in going to positive potentials, the current sharply increases in the range from 0 to +0.4 V. The

1066-3622/05/4701-0054 + 2005 Pleiades Publishing, Inc.

ANODIC DISSOLUTION OF U3Si AND UMo IN NONAQUEOUS MEDIA

potential of anodic oxidation of U3Si is independent of the solvent, and only in 96% ethanol there is certain scatter of the oxidation potential (within 45 mV, see table), accompanying an increase in the current. With UMo in ethanol, two inflections are observed: at 31848 and +370 mV (see table), which suggests a two-step mechanism of the process. With UMo in acetonitrile, the first inflection in the negative range of potentials disappears, and the potential of the onset of the alloy dissolution decreases by 230 3245 mV. With pure Mo, the dissolution potential is higher by 640 mV (see table and figure), and in ethanol it is lower by 40 320 mV. In 96% ethanol, this difference is smaller. The voltammetric data do not allow any conclusions about selective dissolution of U at anodic polarization of its alloys but suggest that the anodic dissolution occurs after the potentials from 0 to +0.4 V, indicated in the table, are reached and that the process is single-step. Preparative electrolysis using a UMo anode in alcoholic and acetonitrile solutions containing acetylacetone yields blue solutions. The qualitative reactions for Mo with disodium hydrogen phosphate and ammonium thiocyanate in the presence of SnCl2 [10] are positive for both the solution and isolated product. Uranium is also present in the solution and isolated product (detected by ionizing radiation). The IR spectra of the isolated products exhibit strong absorption at 1600 31500 cm!1 assignable to a quasiaromatic chelate ring and suggesting formation of U and Mo acetylacetonates. Electrolysis with a U3Si anode is accompanied by formation of a gray powder. The dissolution of U is registered radiometrically. To find whether silicon also passes to the solution upon anodic dissolution of U3Si alloy, we used a qualitative reaction with ammonium molybdate and benzidine [11]. A 100 3200-mg portion of the substance was decomposed by refluxing in 40% aqueous KOH for 1 h. The reaction for Si was performed after cooling and filtration. To avoid leaching of Si from the glassware, all manipulations were performed in specially fabricated copper beakers coated with Ni from the inside. The qualitative reaction shows that virtually the whole amount of Si is present in the powder falling down from the anode. No Si was detected in the solution and in the isolated electrolysis products. To avoid contamination of the solution with the Si powder, the anode in the subsequent experiments was placed in a protective bag made from a thin cotton fabric (cambric or moleskin). RADIOCHEMISTRY

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Current!voltage dependences for anodic dissolution of (1) UMo and (2) U3Si in ethanol and of (3) UMo, (4) U3Si, and (5) Mo in acetonitrile.

The preparative electrolysis with a U3Si anode in ethanol containing 0.1 M LiCl (total solution volume 25335 ml) and 6 mmol of acetylacetone for 12 h at room temperature in an inert atmosphere yields [U(AA)4], according to elemental analysis. The current density decreases in the process from 8 to 6 mA cm!2. The amount of the electricity passed was 1617 C. Uranium(IV) acetylacetonate precipitates in the course of the electrolysis as gray needle-like crystals. The completeness of product washing to remove lithium ions was checked by qualitative reactions with potassium hexacyanoferrate and 8-quinolinol [11]. The current efficiency of the anode dissolution (weight loss), as calculated for U(IV), was about 100%, and the yield of [U(AA)4] as a function of current, 60%. The IR spectrum of the product coincided with that of an authentic sample [2]. The preparative electrolysis of an ethanol solution containing benzoylacetone for 5.5 h at room temperature in an inert atmosphere results in formation of a red-brown powder in the cell. The current density was varied from 8 to 2 mA cm!2. The amount of the electricity passed was 412 C. The results of elemental analysis of the product were consistent with the composition [U(BA)4]. The IR spectrum of the product Oxidation potenitals of uranium alloys and molybdenum metal

ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ Alloy, ³ E, mV, ³ E, mV, ³E, mV, abs. metal ³ abs. ethanol ³ 96% ethanol ³ acetonitrile ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄ U3Si ³ +280 ³ +235 to +280 ³ +290 ³ ³ ³ UMo ³ 31848 ³ 31808 ³ 3 ³ +370 ³ +385 ³ +140 ³ ³³ ³³ Mo ³ +330 +364 +780 ÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄ

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coincided with that of an authentic sample [2]. The yields of U(IV) and isolated chelate as functions of currenrt were ~100 and 74%, respectively. These examples show that the electrolysis in an inert atmosphere with a U3Si anode in the presence of b-diketones yields U(IV) chelates, which agrees with data of [133] on anodic dissolution of U. In oxidative atmosphere, the formation of the uranyl chelates should be expected. A solution containing 12.5 mmol of acetylacetone and 2.2 mmol of LiCl in 25 ml of anhydrous ethanol was subjected to electrolysis for 4 h in a stream of dry ait. Orange needle-like crystals were isolated; the product was identified by elemental analysis and IR spectroscopy as (acetylacetone)bis(acetylacetonato)dioxouranium(VI). The electrolysis of ethanol solutions of dipivaloylmethane and pivaloyltrifluoroacetone in air under the similar conditions yields, according to elemental analysis and IR spectroscopy, [UO2(DPM)2] and [UO2(PTA)2]. The products were purified by vacuum sublimation. The yields of the chelates as functions of current were 74 and 69%, respectively. The current efficiency of the U(IV) formation was ~100% in all the three experiments on electrolysis in an oxidative atmosphere, which indicates that the electrochemical oxidation of U to the tetravalent state is independent of the atmosphere of the process and, in the presence of oxygen, is followed by electroless oxidation to the hexavalent state. Our results do not contradict the results of [133] as far as the composition of the uranium compounds depending on the electrolysis conditions is concerned. At the same time, Mutassa et al. [1] suggest an electrolysis mechanism based on the assumption that uranium is electrochemically oxidized to the bivalent state with the formation of intermediate compounds of the type UL2, which are subsequently oxidized with the ligand to form UL4. The suggested scheme is, on the whole, consistent with the widespread concept that anodic oxidation of a metal is a result of consecutiveparallel elementary events [12]. Within the framework of this concept, simultaneous removal of two electrons is more probable than that of three or, the more so, four electrons. In our case, with the U3Si alloy as anode, the current efficiency of uranium dissolution assuming formation of U2+ (in accordance with [1]) should be 200%. In some cases, the current efficiencies exceeding 100% are, indeed, observed in anodic dissolution of f elements (lanthanides) in nonaqueous media, e.g., 1713275% for La, 1673250% for Sm, and

220% for Gd [13315]. The current efficiencies exceeding 100% reveal a contribution of the chemical dissolution of the anode material, occurring concurrently with the electrolysis [12]. At the same time, no such effects were found in anodic dissolution of Dy [16]: The current efficiencies were close to 100%. The scatter of the current efficiencies exceeding 100% is due to the reactivity of the reaction medium, which is confirmed by the fact that the current efficiency depends on the acidity of the b-diketone [13]. Therefore, the fact that the current efficiency of the metal dissolution is reproducibly close to 100% indicates that the electrochemical process is not noticeably accompanied by concurrent chemical processes. This viewpoint is also indirectly supported by the fairly high stability of the U3Si alloy to chemical agents. Furthermore, the voltammetric data (see figure) show that the anodic dissolution of U3Si is a single-step process. Thus, anodic dissolution of U3Si in ethanol and acetonitrile in an inert medium can be described by the system of the following equations: anode: U3Si 3 12e! = 3U4+ + Si2; cathode: 2HL + 2e! = L! + H28; solution: U4+ + 4L! = UL4.

In an oxidative atmosphere, this scheme is supplemented by the subsequent chemical oxidation of uranium and partial cathodic reduction of oxygen: U4+ + 2L! + O2 + nHL + 2e! = UO2L2 . nHL,

where n = 0.531; HL is the ligand. The reaction in the oxidative atmosphere (air) is hindered, which is confirmed by low current efficiencies of formation of uranyl compounds: 5359% depending on the ligand. When the electrolysis is performed in the presence of hydrogen peroxide, these parameters become considerably higher [2, 3]. In anodic dissolution of the UMo alloy, both metals are oxidized, and a mixture of the corresponding chelates is formed. REFERENCES 1. Mutassa, L., Kumar, N., and Tuck, D.L., Inorg. Chim. Acta, 1985, vol. 109, no. 1, pp. 19321. 2. Kostyuk, N.N., Kolevich, T.A., Shirokii, V.L., and Umreiko, D.S., Koord. Khim., 1989, vol. 15, no. 12, pp. 1704 31707. RADIOCHEMISTRY

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ANODIC DISSOLUTION OF U3Si AND UMo IN NONAQUEOUS MEDIA 3. Kostyuk, N.N., Dik, T.A., Klavsut’, G.N., and Umreiko, D.S., Zh. Neorg. Khim., 1989, vol. 34, no. 11, pp. 2810 32814. 4. Tomilov, A.P., Chernykh, I.N., and Kargin, Yu.M., Elektrokhimiya elementoorganicheskikh soedinenii. Elementy I, II, III grupp periodicheskoi sistemy i perekhodnye metally (Electrochemistry of Organometallic Compounds. Elements of Groups I, II, and III of the Periodic Table and Transition Metals), Moscow: Nauka, 1985, pp. 2373252. 5. Suglobov, D.N., Sidorenko, G.V., and Legin, E.K., Letuchie organicheskie i kompleksnye soedineniya f-elementov (Volatile Organic and Coordination Compounds of f Elements), Moscow: Energoatomizdat, 1987. 6. Kostyuk, N.N. and Dik, T.A., Izbrannye trudy Belorusskogo gosudarstvennogo universiteta (Selected Works of the Belarussian State Univ.), vol. 5: Khimiya (Chemistry), Sviridov, V.V., Ed., Minsk: Bel. Gos. Univ., 2001, pp. 2833298. 7. Gordon, A.J. and Ford, R.A., The Chemist’s Companion. A Handbook of Practical Data, Techniques, and References, New York: Wiley, 1972. 8. Reichardt, Ch., Solvents and Solvent Effects in Organic Chemistry, Weinheim: VCH, 1988. 9. Praktikum po fizicheskoi khimii (Practical Course of Physical Chemistry), Budanov, V.V. and Vorob’-

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ev, N.K., Eds., Moscow: Khimiya, 1986, 5th ed. 10. Alekseev, V.N., Kurs kachestvennogo khimicheskogo polumikroanaliza (Course of Qualitative Chemical Semimicroanalysis), Moscow: Khimiya, 1973, pp. 476 3 477. 11. Murashova, V.I., Tananaeva, A.N., and Khovyakova, R.F., Kachestvennyi khimicheskii drobnyi analiz (Qualitative Chemical Fractional Analysis), Moscow: Khimiya, 1976, p. 273. 12. Kiss, L., Kinetics of Electrochemical Metal Dissolution, New York: Elsevier, 1987. 13. Kostyuk, N.N., Dik, T.A., Tereshko, N.V., and Trebnikov, A.G., Elektrokhimiya, 2003, vol. 39, no. 11, pp. 1376 31379. 14. Kostyuk, N.N., Dik, T.A., Trebnikov, A.G., and Shirokii, V.L., Elektrokhimiya, 2003, vol. 39, no. 11, pp. 137131375. 15. Kostyuk, N.N., Dik, T.A., Trebnikov, A.G., and Shirokii, V.L., in Sbornik dokladov III nauchno-tekhnicheskoi konferentsii [Resursosberegayushchie i ekologicheski chistye tekhnologii] (Coll. of Papers of III Scientific and Technical Conf. [Resource-Saving and Environmentally Clean Technologies]), Grodno, 1999, part II (VI), pp. 1433146. 16. Kostyuk, N.N., Shirokii, V.L., Vinokurov, I.I., and Maier, N.A., Zh. Obshch. Khim., 1994, vol. 64, no. 9, pp. 143231434.