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Electrochemical behavior of tetrachloro and tetrabromouranyl complexes in room temperature ionic liquids IPNO Participation: C. Cannes, C. Le Naour, M-O. Sornein, M. Mendes Collaboration : DRECAM (CEA Saclay), LISE (Paris VI, CNRS) Les liquides ioniques sont des solvants pouvant intervenir dans des étapes de séparation lors du retraitement des déchets nucléaires. Notre objectif est d’acquérir des données fondamentales sur les actinides dans ces milieux. Le comportement électrochimique de complexes tétrahalogénés d’uranyle a été étudié par voltammétrie et électrolyses quantitatives. Deux mécanismes de réduction sont proposés: le 1er mécanisme, noté DISP, comprend un transfert monoélectronique suivi d’une réaction de dismutation conduisant à la régénération du complexe uranyle(VI) initial. Le 2e mécanisme, noté ECE, correspond à un transfert monoélectronique suivi d’une dissociation partielle conduisant à la formation d’un complexe d’uranyle(V), lui-même réductible aux potentiels imposés à l’électrode selon un deuxième transfert électronique. Dans les deux cas, la réduction conduit à la formation d’espèces insolubles d’U(IV) (oxyde ou hydroxyde) et à la production d’ions halogénure. Ionic liquids (Ils) are media entirely composed of ions with a melting point bellow 100°C. They are characterized by a thermal, chemical and electrochemical stability. They posses a low vapor pressure, they are non-flammable and not toxic. Their physicochemical properties can be also «finetuned» by a judicious choice of the anion-cation combination. They are currently under study to replace organic solvents, especially the volatile organic compounds, in many fields: synthesis, catalysis, polymerization and various electrochemical devices. They are also considered in the nuclear industry as recent works have shown their good stability under , and irradiation. ILs could then be involved in the separation of actinides and fission products either by liquid/liquid extraction or electrodeposition processes. Moreover, ILs are solvents different from molecular solvents and high temperature molten salts. New complexes or unusual oxidation states could then be stabilized. To use ILs in chemical processes, it seems necessary to accumulate data on the solvation, complexation reaction and redox properties of metallic species in these media. In this context, we have focused our work on the structural and electrochemical behavior of uranium complexes in two ILs: the 1-butyl-3-methylimidazolium and the methyl-tributylammonium, both of them being associated to the bis(trifluoromethylsulfonyl)imide ([BuMeIm][Tf 2 N] and [MeBu 3 N][Tf 2 N]). Here are presented the redox properties of [UO 2 Cl 4 ] 2- in [BuMeIm][Tf 2 N]. The influence of the uranyl ligand and the IL cation are also examined via the study of [UO 2 Br 4 ] 2- and the IL [MeBu 3 N][Tf 2 N]. Electrochemical behavior of [UO 2 Cl 4 ] 2- in [BuMeIm][Tf 2 N] The cyclic voltammogram of [U VI O 2 Cl 4 ] 2– presents a cathodic peak at -1.4 V and two anodic peaks around -0.4 V and 0.2 V (fig. 1, blue curve). If the swithching cathodic potential is -2.5 V, the intensity of the second anodic peak increases and its shape corresponds to an anodic redissolution one (red curve). Then the reduction of the [UO 2 Cl 4 ] 2- seems irreversible and would lead to a deposit. Fig. 1: Voltammograms of 0.01 M [UO 2 Cl 4 ] 2– in [BuMeIm][Tf 2 N] at 25°C at a glassy carbon eletrode at 0.1 V.s -1 . When Cl - are added in the solution, the intensity of the anodic peak around -0.4 V (a 2 ) decreases while a new anodic one (a 1 ) appears around -1.3 V (fig. 2). Table 1 reports the variation of the ratio of this new anodic peak intensity (I pa1 ) over the cathodic peak intensity (I pc1 ): I pa1 /I pc1 increases with the concentration of Cl – and with the potential scan rate. These results indicate that the redox process is characterized by an electron transfer followed by a chemical reaction. The kinetic of this reaction would be function of the Cl - concentration: the highest the concentration, the slowest the reaction. This reaction would then lead to Cl - production. Fig. 2 put also in evidence a decrease of I pc1 when the Cl - concentration increases. However, if the potential scan rate is increased, that is when the chemical reaction is more and more kinetically limited, I pc1 is almost constant as a function of the Cl - amount. Two mechanisms could explain the results: either an EC (DISP) mechanism, the chemical reaction being a disproportionation reaction, or an ECE mechanism, that is two electron transfer steps separated by a chemical reaction. Thus, the 129
global electron number of the redox process is increased because of either the regeneration of the initial uranyl species (DISP) or the reduction of a second species formed by the chemical reaction (ECE). Fig. 2: cyclic voltammograms of 0.01 M [UO 2 Cl 4 ] 2- with x eq. Cl - in [BuMeIm][Tf 2 N] at 25°C at a GC electrode at 25 mV.s -1 . Effect of the IL cation: redox behavior of tetrahalouranyl complex in [MeBu 3 N][Tf 2 N] Fig. 3 presents the voltammograms of the tetrahalo uranyl complexes [UO 2 X 4 ] 2- with 10 eq. of halide ions in both ILs. In [MeBu 3 N][Tf 2 N], the reduction peaks of [UO 2 Cl 4 ] 2– and [UO 2 Br 4 ] 2– are observed respectively at -1.74 V and -1.56 V. In both ILs, the reduction potential of the bromo complex is higher than for the chloro analog. This indicates that [UO 2 Br 4 ] 2– is less stable than [UO 2 Cl 4 ] 2– . We can also observe for each complex that the reduction potential is more than 300 mV less negative in [MeBu 3 N][Tf 2 N] than in [BuMeIm][Tf 2 N]. This can be account to a difference of solvation as we have already demonstrated that [BuMeIm] + interacts more strongly than [MeBu 3 N] + by H-bonding. We have also shown that the intermediate species [U V O 2 X 4 ] 3– is less stable in [MeBu 3 N][Tf 2 N] than in [BuMeIm][Tf 2 N]. 0 eq Cl - 5 eq Cl - 10 eq Cl - 0.025 V.s -1 - 0.21 0.52 0.05 V.s -1 - 0.32 0.63 0.1 V.s -1 0.14 0.37 0.71 0.2 V.s -1 0.20 0.42 0.74 0.5 V.s -1 0.27 0.51 0.76 1 V.s -1 0.33 0.54 0.82 2 V.s -1 0.35 0.62 0.84 Table 1: I pa1 /I pc1 measured for [UO 2 Cl 4 ] 2- (0.01M) with 0, 5 and 10 eq of Cl – in [BuMeIm][Tf 2 N] as a function of the potential scan rate at 25°C. With a large excess of Cl - , the chemical reaction is no more observed by voltammetry. Our measurements have shown that in these conditions, [U VI O 2 Cl 4 ] 2- is reduced via a quasireversible monoelectronic transfer to form [U V O 2 Cl 4 ] 3- . Effect of the uranyl ligand: electrochemical behavior of [UO 2 Br 4 ] 2– in [BuMeIm][Tf 2 N] If the switching cathodic potential is -2.7 V, the cyclic voltammogram presents a single reduction peak at -1.24 V and two oxidation peaks at -0.3 and 0.3 V, this latter corresponding to a redissolution peak. Then, the reduction of the tetrabromouranyl complex leads also to the formation of a deposit at the electrode surface. In the potential region [-1.6 V ; -0.1 V], when Br - are added to the solution, a new anodic peak appears around -1.18 V. When the Br - concentration increases, the intensity of this new peak increases and the intensity of the anodic peaks at higher potentials decreases. Moreover, at a done Br – concentration, the ratio I pa (-1.18 V)/I pc (-1.24 V) increases with the potential scan rate. Nevertheless, for the same halide amount and the same potential scan rate, this ratio is lower in the case of the bromo complex than for its chloro analog. This suggests that the intermediate species [U V O 2 Br 4 ] 3- is less stable than [U V O 2 Cl 4 ] 3- . Fig. 3: Voltammograms of 0.01 M [UO 2 Cl 4 ] 2– with 10 eq of Cl - (red curves) and 0.01 M [UO 2 Br 4 ] 2– with 10 eq of Br - in [BuMeIm][Tf 2 N] at 25°C (solid curves) and in [MeBu 3 N][Tf 2 N] at 60°C (dashed curves) at a GC electrode (0.07 cm 2 ) at 0.1 V.s -1 . Controlled potential electrolysis Quantitative electrolysis of the tetrahalouranyl complexes in both ionic liquids give a deposit at a platinum cathode. XRD analysis suggests that this solid phase is composed of amorphous oxide or hydroxide uranium(IV). Moreover, the voltammetric analysis of the solution at the end of the electrolysis indicates that 4 equivalents of halide ions are produced per eq. of reduced uranyl complex. Suggested mechanism The above results allowed us to propose the two following mechanisms for the reduction of the tetrahalouranyl complexes in ILs. DISP mechanism: [U VI O 2 X 4 ] 2- + e - [U V O 2 X 4 ] 3- 2 [U V O 2 X 4 ] 3- [U IV ] + [U VI O 2 X 4 ] 2- + 4 X - ECE mechanism: [U VI O 2 X 4 ] 2- + e - [U V O 2 X 4 ] 3- [U V O 2 X 4 ] 3- [U V O 2 X (4-x) ] x-3 + x X - [U V O 2 X (4-x) ] x-3 + e - [U IV ] + (4-x) X - In both mechanisms, [U IV ] corresponds to oxide or hydroxide species. 130
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EXOTIC NUCLEI STRUCTURE AND REACTIO
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in the spectra by analyzing their t
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were taken from ref. [5]. Concernin
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keV -ray with the particles and the
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the 605.9 keV line of the 74 Zn 2+
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Figure 2 a n d analysis. Figure 2 s
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ing particles was thus obtained by
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First p-p p collisions in the ALICE
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Quarkonia physics with ALICE IPNO P
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NA50 updated puzzle IPNO Participat
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Status of the HADES experiment (I):
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Status of the HADES experiments (II
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Status of the HADES experiment (III
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Hadron physics in pbar-p p annihila
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Hadron Electrodynamics IPNO Partici
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G0 experiment at Jefferson Lab. IPN
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GEp-III and GEp-2 at Jefferson Lab
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Exotic low mass narrow baryons from
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The PVA4 parity violation experimen
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Etude des Distributions de Partons
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Latest Results from GRAAL IPNO Part
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Persistence of the Polarization in
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The northern site of the Pierre Aug
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The Pierre Auger southern Observato
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In figure 4 we plot the differentia
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THEORETICAL PHYSICS Research in the
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transfer reaction, the 34 Si(d, 3 H
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Collective excitations with SKYRME
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Pairing and nuclear incompressibili
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Pairing properties in nuclei and in
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Evaporation-residue residue cross s
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Extending the VMI model: normal and
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Alpha particle condensation in nucl
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Collective Modes in Ultracold Trapp
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Page 105 and 106: ENERGY AND ENVIRONMENT The increasi
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