Plutonium(IV) oxalate

Last updated
Plutonium(IV) oxalate
Pu(C2O4)2 6H2O layer.png
Names
IUPAC name
Plutonium(IV) oxalate
Identifiers
3D model (JSmol)
  • InChI=1S/2C2H2O4.Pu/c2*3-1(4)2(5)6;/h2*(H,3,4)(H,5,6);/q;;+4/p-4
    Key: HRBJILZCKYHUJF-UHFFFAOYSA-J
  • C(=O)(C(=O)[O-])[O-].C(=O)(C(=O)[O-])[O-].[Pu+4]
Properties
Pu(C2O4)2
Molar mass 580 g·mol−1
Related compounds
Other cations
Thorium(IV) oxalate
Uranium(IV) oxalate
Neptunium(IV) oxalate
Related plutonium oxalates
 Plutonium(III) oxalate
Plutonyl oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Plutonium(IV) oxalate is a compound consisting of plutonium and oxalate with the formula Pu(C2O4)2. It is produced by addition of oxalic acid to plutonium solution, and is widely used in the synthesis of plutonium(IV) oxide for nuclear reprocessing, plutonium recovery from radioactive waste, or lab use. It is also a starting material for the production of other plutonium compounds, such as plutonium(III) fluoride, plutonium(IV) fluoride, or plutonium(III) chloride.

Contents

Synthesis

Plutonium(IV) oxalate is prepared by the combination of oxalic acid solution and plutonium(IV)-nitric acid solution, after which it precipitates as the hexahydrate, Pu(C2O4)2·6H2O. Hydrogen peroxide can added to prevent reduction to plutonium(III): [1] [2] [3]

Pu(NO3)4 + H2C2O4 + 6 H2O → Pu(C2O4)2·6H2O ↓ + 4 HNO3

There are three ways that these solutions can be combined: direct strike (adding oxalic acid to nitric acid solution), reverse strike (adding nitric acid solution to oxalic acid), and continuous (adding nitric acid solution and oxalic acid simultaneously). [4]

Properties

Plutonium(IV) oxalate forms several hydrates, notably the hexahydrate, Pu(C2O4)2·6H2O, which has been variously described as tan [5] : 837  or yellow-green. [6] Other hydrates are known, such as the monohydrate (Pu(C2O4)2·H2O) and dihydrate (Pu(C2O4)2·2H2O), and the compound can also be found without water as Pu(C2O4)2. [5] : 1174 

Decomposition

The thermal decomposition of plutonium(IV) oxalate hexahydrate starts by it losing water. It goes through the dihydrate, Pu(C2O4)2·2H2O, and the monohydrate, Pu(C2O4)2·H2O, as intermediates before arriving at anhydrous plutonium(IV) oxalate: [7] [8] [9]

Pu(C2O4)2·6H2O → Pu(C2O4)2·2H2O + 4 H2O (between 60–100 °C)
Pu(C2O4)2·2H2O → Pu(C2O4)2·H2O + H2O (between 100–120 °C)
Pu(C2O4)2·H2O → Pu(C2O4)2 + H2O (between 120–180 °C)

The anhydrous form loses carbon dioxide to produce anhydrous plutonium(III) oxalate, Pu2(C2O4)3. Upon heating, it loses carbon monoxide, going through several carbonate oxalate phases with compositions Pu2(C2O4)2CO3 and Pu2C2O4(CO3)2, before arriving at plutonium(IV) oxycarbonate PuOCO3. PuOCO3 finally releases carbon dioxide to form plutonium dioxide: [7] [8] [9]

2 Pu(C2O4)2 → Pu2(C2O4)3 + 2 CO2 → Pu2(C2O4)2CO3 + 2 CO2 + CO (between 180–210 °C)
Pu2(C2O4)2CO3 → Pu2C2O4(CO3)2 + CO (between 210–235 °C)
Pu2C2O4(CO3)2 → 2 PuOCO3 + 2 CO (between 235–260 °C)
PuOCO3 → PuO2 + CO2 (between 260–400 °C)
Diagram of the reactions which occur in the thermal decomposition of plutonium(III) oxalate and plutonium(IV) oxalate Decomposition of plutonium oxalates.svg
Diagram of the reactions which occur in the thermal decomposition of plutonium(III) oxalate and plutonium(IV) oxalate

Plutonium(IV) oxalate also slowly degrades under ambient conditions. The end product is proposed to be either a colloidal PuO2 polymer or PuOCO3. [4]

Structure

Two structures are predicted for anhydrous Pu(C2O4)2. In the first one, each plutonium atom is at the center of a cube formed by eight oxygen atoms coming from four oxalate groups (giving it a coordination geometry of cubic). The plutonium atoms are bonded with both oxygen atoms coming from each oxalate. In the second one, each plutonium atom is at the center of a distorted square antiprism formed by eight oxygen atoms coming from four oxalate groups (giving it a coordination geometry of square antiprismatic). Unlike in the first one, only one oxygen from each oxalate group bonds with the plutonium center, as the other oxygen atom is too far away to form a bond. In both structures, each oxalate group bridges between two plutonium atoms, and both structures consist of two-dimensional plutonium-oxalate layers, though layer-layer interactions are stronger in the second one than in the first one. The second structure has been calculated to be more stable. [7] [10]

The structure of plutonium(IV) hexahydrate (Pu(C2O4)2·6H2O) consists of alternating layers of Pu(C2O4)2(H2O)2 and interstitial water molecules (four waters per formula unit). Within the Pu(C2O4)2(H2O)2 layers, each plutonium atom is coordinated to ten oxygen atoms, eight from four oxalate groups and two from two water molecules. Three-quarters of the oxalate groups lie perpendicular to the layers, while one quarter of them lie parallel to the plane, providing enough space to fit the two water molecules. [11]

Pu coordination in Pu(C2O4)2 6H2O.png
Coordination of plutonium in plutonium(IV) oxalate hexahydrate. Blue is plutonium, grey is carbon, red is oxygen in oxalate, and sea green is oxygen in water. Hydrogen atoms are omitted.
Pu(C2O4)2 6H2O layer.png
Single layer of plutonium(IV) oxalate hexahydrate, showing all possible positions for oxalate groups and water molecules. Blue is plutonium, grey is carbon, and red is oxygen. Hydrogen atoms are omitted.

Uses

Plutonium(IV) oxalate is widely used to produce plutonium(IV) oxide (PuO2) via thermal decomposition for applications such as nuclear reprocessing, recovery of plutonium from waste and residues, laboratory use, or plutonium(III) chloride production. The related compound plutonium(III) oxalate can also be used to produce PuO2. [8] [5] : 1031–1032, 1093  After it is synthesized (see synthesis details above), it is first slowly heated up to 700 °C, and then heated afterwards at 1000 °C to remove any leftover carbon. Plutonium(IV) oxide produced by this method appears as a yellow-buff bulky powder. [5] : 1031–1032  In addition, it can also be converted to plutonium(IV) oxide by hydrothermal methods. This has been proposed as an alternative to thermal decomposition. [12] Because it can be synthesized from nitrate solution and then be converted to the oxide, it can be used in the conversion of plutonium(IV) nitrate to PuO2 as an intermediate. [3]

It can also be used for the production of plutonium fluorides. Upon reaction with hydrogen fluoride, it either produces plutonium(III) fluoride (PuF3) or plutonium(IV) fluoride (PuF4). When reducing agents such as hydrogen gas are present, PuF3 is formed: [5] : 1077–1078 

2 Pu(C2O4)2 + 6 HF → 2 PuF3 + 3 CO + 5 CO2 + 3 H2O (at 600 °C)

References

  1. ↑ https://www.osti.gov/servlets/purl/4634035
  2. ↑ https://www.osti.gov/servlets/purl/7227473
  3. 1 2 Greintz, R.M.; Neal, D.H. (1978). "Plutonium(IV) oxalate precipitation and calcination process for plutonium nitrate to oxide conversion". doi:10.2172/709947. OSTI   709947.
  4. 1 2 Corbey, Jordan F.; Sweet, Lucas E.; Sinkov, Sergey I.; Reilly, Dallas D.; Parker, Cyrena M.; Lonergan, Jason M.; Johnson, Timothy J. (2021). "Quantitative Microstructural Characterization of Plutonium Oxalate Auto-Degradation and Evidence for PuO2 Nanocrystal Formation". European Journal of Inorganic Chemistry (32): 3277–3291. Bibcode:2021EJIC.2021.3277C. doi:10.1002/ejic.202100511. OSTI   1808901.
  5. 1 2 3 4 5 Clark, David L.; Hecker, Siegfried S.; Jarvinen, Gordon D.; Neu, Mary P. (2011). "Plutonium". The Chemistry of the Actinide and Transactinide Elements (PDF). doi:10.1007/978-94-007-0211-0_7. ISBN   978-94-007-0211-0.
  6. ↑ Kirk-Othmer Encyclopedia of Chemical Technology. Vol. 19. p. 203.
  7. 1 2 3 South, Christopher J.; Roy, Lindsay E. (2021). "Insights into the thermal decomposition of plutonium(IV) oxalate – a DFT study of the intermediate structures". Journal of Nuclear Materials. 549 152864. Bibcode:2021JNuM..54952864S. doi:10.1016/j.jnucmat.2021.152864. OSTI   1805215.
  8. 1 2 3 Orr, R.M.; Sims, H.E.; Taylor, R.J. (2015). "A review of plutonium oxalate decomposition reactions and effects of decomposition temperature on the surface area of the plutonium dioxide product" . Journal of Nuclear Materials. 465: 756–773. Bibcode:2015JNuM..465..756O. doi:10.1016/j.jnucmat.2015.06.058.
  9. 1 2 Christian, Jonathan H.; Foley, Bryan J.; Ciprian, Elodia; Dick, Don D.; Said, Meena; Darvin, Jason; Hixon, Amy E.; Villa-Aleman, Eliel (2022). "Raman and infrared spectra of plutonium (IV) oxalate and its thermal degradation products". Journal of Nuclear Materials. 562 153574. Bibcode:2022JNuM..56253574C. doi:10.1016/j.jnucmat.2022.153574. OSTI   1844185.
  10. ↑ Isbill, Sara B.; Ciprian, Elodia; Christian, Jonathan H.; Hixon, Amy; Foley, Bryan J.; Villa-Aleman, Eliel; Miskowiec, Andrew J. (2023). "Computational insights into the lattice dynamics of Pu(IV) oxalates". Journal of Nuclear Materials. 573 154106. Bibcode:2023JNuM..57354106I. doi:10.1016/j.jnucmat.2022.154106. OSTI   1899009.
  11. ↑ Sockwell, A. Kirstin; Sweet, Teagan F. M.; Barth, Brodie; Isbill, Sara B.; Diblasi, Nicole A.; Szymanowski, Jennifer E. S.; Sigmon, Ginger E.; Oliver, Allen G.; Miskowiec, Andrew J.; Burns, Peter C.; Hixon, Amy E. (2023). "Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries*". Chemistry – A European Journal. 29 (47) e202301164. Bibcode:2023ChEuJ..29E1164S. doi:10.1002/chem.202301164. OSTI   1983863. PMID   37227412.
  12. ↑ Baumann, Viktoria; Popa, Karin; Walter, Olaf; Rivenet, Murielle; Senentz, Gérald; Morel, Bertrand; Konings, Rudy J.M. (2023). "Synthesis of Nanocrystalline PuO2 by Hydrothermal and Thermal Decomposition of Pu(IV) Oxalate: A Comparative Study". Nanomaterials. 13 (2): 340. doi: 10.3390/nano13020340 . PMC   9865700 . PMID   36678093.
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