Theoretical Mechanism Study of UF₆ Hydrolysis in the Gas Phase

Abstract

The mechanism of the gas-phase reaction UF 6 + H 2O --> UOF 4 + 2HF is explored using relativistic density functional theory calculations. Initially, H 2O coordinates with UF 6 to form a 1:1 complex UF 6.H 2O. Over an activation energy barrier of about 19 kcal/mol, H 2O transfers a H atom to a nearby ligand F, resulting in UF 5OH + HF. The eliminated HF or another H 2O molecule may form a hydrogen bond with UF 5OH. Starting from UF 5OH, the second HF elimination results in UOF 4. If UF 5OH is in the isolated form, UF 5OH --> UOF 4 + HF takes place over a barrier of 24 kcal/mol. If UF 5OH is hydrogen-bonded with H 2O or HF, the conversion barrier is less than 10 kcal/mol. Once formed, the unstable UOF 4 tends to associate with additional ligands and hydrogen-bonding donors. The calculated binding energies indicate the significance of such interactions, which may have profound impact on further HF eliminating reactions. The IR spectra features can be used to indicate the formation and interaction type of the intermediates and products.

Kinetic investigation of the hydrolysis of uranium hexafluoride gas

“…Previous efforts to characterize this reaction have focused on spectroscopic measurements [5][6][7] and computational studies. [8][9][10][11][12][13] The experimental studies have focused on the spectroscopic detection of uranium species but lacked the temporal resolution to observe the reaction or identify any transient species. Unfortunately, there are major discrepancies between the experimentally observed reaction and computationally predicted mechanisms.…”

“…Unfortunately, there are major discrepancies between the experimentally observed reaction and computationally predicted mechanisms. For example, the computational studies suggest large thermodynamic energy barriers that would likely prevent the reaction of UF 6 and water vapor, [9][10][11][12][13] yet the hydrolysis reaction is known to occur quickly and completely under ambient conditions. [5][6][7] Additionally, one of the long-suspected intermediates of this reaction, UOF 4 , has not been successfully identied under extreme conditions designed to optimize its production such as reacting water with a 5000-fold excess of UF 6 .…”

Formation of Aerosol Nanoparticles by Gas-Phase Hydrolysis Reaction of Uranium Hexafluoride

“…Recently, Hu et al (2008Hu et al ( , 2009Hu et al ( , 2014 used the relativistic density function theory to computationally show that intermediate species containing a U-O-U bond were found because of several probable reactions involving UF5OH and UF6 or UF5OH and UF5OH under the condition of enriched UF6 and small amounts of H2O (i.e., small ω condition) where ω is the molar ratio of H2O to UF6. In large ω conditions, the coordination energy between UF6 and (H2O)n in UF6-nH2O (e.g., where n = 1-3) increases as water changes from single molecules to the associated dimer and trimer.…”

“…So far, these molecular dynamics modeling works (Hu et al 2008(Hu et al , 2009(Hu et al , 2014 suggested only the pathway of the UF6 hydrolysis in the gas phase, but none did for the condensed phase in which the formation of UO2F2 aerosol particles exist. The aerosol formation process and kinetics remain unknown.…”

Morphologic and chemical characterization of products from hydrolysis of UF6

“…These studies suggested a complex reaction pathway from UF 6 to UOF 4 through a UF 5 OH intermediate, and reported binding energies of formation and vibrational frequencies of molecular fragments [7][8][9]. A subsequent study using DFT with a small-core, scalar relativistic pseudopotential proposed that trimolecular mechanisms are likely involved in formation of the UF 5 OH intermediate [10].…”