Trimolecular Reactions of Uranium Hexafluoride with Water

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• 作者：Maria C. Lind ; Stephen L. Garrison ; James M. Becnel
• 刊名：Journal of Physical Chemistry A
• 出版年：2010
• 出版时间：April 8, 2010
• 年：2010
• 卷：114
• 期：13
• 页码：4641-4646
• 全文大小：217K
• 年卷期：v.114,no.13(April 8, 2010)
• ISSN：1520-5215

文摘

The hydrolysis reaction of uranium hexafluoride (UF₆) is a key step in the synthesis of uranium dioxide (UO₂) powder for nuclear fuels. Mechanisms for the hydrolysis reactions are studied here with density functional theory and the Stuttgart small-core scalar relativistic pseudopotential and associated basis set for uranium. The reaction of a single UF₆ molecule with a water molecule in the gas phase has been previously predicted to proceed over a relatively sizable barrier of 78.2 kJ·mol⁻¹, indicating this reaction is only feasible at elevated temperatures. Given the observed formation of a second morphology for the UO₂ product coupled with the observations of rapid, spontaneous hydrolysis at ambient conditions, an alternate reaction pathway must exist. In the present work, two trimolecular hydrolysis mechanisms are studied with density functional theory: (1) the reaction between two UF₆ molecules and one water molecule, and (2) the reaction of two water molecules with a single UF₆ molecule. The predicted reaction of two UF₆ molecules with one water molecule displays an interesting “fluorine-shuttle” mechanism, a significant energy barrier of 69.0 kJ·mol⁻¹ to the formation of UF₅OH, and an enthalpy of reaction (ΔH₂₉₈) of +17.9 kJ·mol⁻¹. The reaction of a single UF₆ molecule with two water molecules displays a “proton-shuttle” mechanism, and is more favorable, having a slightly lower computed energy barrier of 58.9 kJ·mol⁻¹ and an exothermic enthalpy of reaction (ΔH₂₉₈) of −13.9 kJ·mol⁻¹. The exothermic nature of the overall UF₆ + 2H₂O trimolecular reaction and the lowering of the barrier height with respect to the bimolecular reaction are encouraging.