Online monitoring the hydrolysis of uranium hexafluoride for intermediates by cryogenic layering and FTIR

Uranium hexafluoride (UF₆) is a commonly utilized material feedstock in uranium enrichment processes due to its high vapor pressure and ease of sublimation. When exposed to air, UF₆ undergoes spontaneous hydrolysis to form uranyl fluoride (UO₂F₂) particulates which are utilized for the detection of undeclared nuclear activities by nuclear safeguards organizations. The kinetics of the hydrolysis reaction and how they relate to particle morphology of the product are still debated in the literature. Here, we report the direct, in situ observation of UF₆ reaction intermediates by cooling the reaction to cryogenic temperatures to significantly reduce the rate of hydrolysis. The reaction is then observable by Fourier transform infrared (FTIR) spectroscopy. The conversion of UF₆ to UOF₄ is observed as well as several other bands associated with possible long lived intermediate complexes. Chemometrics are used to further elucidate the reaction pathway from UF₆ to UO₂F₂.

1 Introduction

In the 1980s, the Oak Ridge Gaseous Diffusion Plant conducted several characterization experiments on the spontaneous reaction of uranium hexafluoride (UF₆) hydrolysis in a controlled environment (Bostick et al., 1983; Pickrell, 1984; Bostick et al., 1985; Pickrell, 1985). These studies show a correlation between physical features observed in the aerosol, the relative humidity, and the temperature of the UF₆ at the time of release. This phenomenon, observed over 40 years ago, has yet to be fully understood and plays a key role in nuclear forensics and safeguards. There is consensus across the scientific literature that the simplified reaction:

${\text{UF}}_6\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\to {\text{UO}}_2{\mathrm{F}}_2\left(\mathrm{s}\right)+4\text{HF}\left(\mathrm{g}\right)$

can be used to describe the hydrolysis of UF₆ with products of hydrofluoric acid and uranyl fluoride (UO₂F₂). Armstrong, Bostick, and Fletcher isolated the product of hydrolysis, UO₂F₂, and assigned the observed features in the IR spectrum (Armstrong et al., 1991). Sherrow and Hunt published a seminal spectroscopic study where they observed the formation of UOF₄ by co-condensing water and UF₆ in an argon matrix then initiating the hydrolysis reaction by photolysis (Sherrow and Hunt, 1992). Over the next several decades, multiple computational studies were published suggesting a UF₅OH intermediate is more favorable than UOF₄ (Garrison and Becnel, 2008; Hu et al., 2008; 2009; Lind et al., 2010; Hu et al., 2014; Lutz et al., 2022). Experimentalists have continued to attempt to elucidate the kinetics and identify intermediate compounds formed during the hydrolysis reaction, but the mechanism behind UF₆ hydrolysis remains heavily debated in the literature (Paine et al., 1975; Klimov et al., 1992; Kips et al., 2007; Wagner et al., 2015; Richards et al., 2020; Pastoor et al., 2021). To date, there has been no experimental evidence for the formation of UF₅OH and no in situ observations of any intermediate formation during the UF₆ hydrolysis reaction.

Here, we demonstrate the ability to monitor the generation of reaction intermediates by cryogenic layering. We explore the hydrolysis of UF₆ through temperature dependent infrared spectroscopy based on methods recently developed to better understand the hydrolysis of molybdenum hexafluoride (MoF₆) (McNamara et al., 2023). By layering the reactant at cryogenic temperatures, the hydrolysis reaction is effectively halted until the sample layers are warmed sufficiently to overcome the activation barrier of each step in the hydrolysis reaction. Continuous monitoring of the sample layers by FTIR allows for the direct, in situ, observation of intermediates forming and being consumed as the reaction progresses. The data presented here is the initial findings from applying this technique to UF₆.

2 Methods

The experimental design has been previously reported for studies of MoF₆ hydrolysis (McNamara et al., 2023). In brief, a Jasco FT-IR 6300 (Jasco Inc, Easton, MD, USA) was employed for the acquisition of infrared spectra. To control the initiation of the hydrolytic reaction, the UF₆ and water vapor were cryogenically condensed under vacuum by means of a helium cryostat (Janis Research, VPF Series). The cryogenic layers were held under a vacuum of 100 mTorr (13 Pa) during the duration of the entire experiment. The cell was constructed with 2-inch diameter uncoated zinc selenide windows (Edmund Optics, ZC-W-50-2) with a thin polished polycrystalline diamond window for cryogenic layering (Ap-plied Diamond, Inc., 18 mm diameter, 150 microns thin). The cell was brought under vacuum (Edwards Oil-Free Scroll Pump) to evacuate air and water vapor prior to and while achieving cryogenic conditions. A scan was taken periodically with a 2 cm⁻¹ resolution, 8 averages, and a 500 to 5,000 cm⁻¹ spectral range while the cell warmed to room temperature.

3 Results

The final product of the reaction aligns well with previously reported IR spectra of uranyl fluorides. A strong absorption band at 940 cm⁻¹ is observed which is attributed to U-O stretching of UO₂F₂. Two distinct bands around 1,700 cm⁻¹ and a broad absorption band from 2,500–3,500 cm⁻¹ are observed, which are attributed to UO₂F₂^…xH₂O^…yHF complexes. The resulting product is spectroscopically identical to the product observed by Armstrong et al., suggesting we have formed the same final product despite the reaction being performed in the condensed phase.

From time zero, it takes roughly 10 min for the layered reactants to warm enough to initiate the reaction. No temperature data could be recorded for these experiments, but temperatures have been estimated based on temperature data from previous experiments: time zero ∼ 10 K, after 30 min. ∼ 180 K, after 60 min. ∼ 240 K, after 72 min. ∼ 250 K. Based on the 100 mTorr pressure of the chamber and the phase diagrams of pure water and UF₆, it seems likely the reaction progressed primarily in the solid state with excess H₂O and UF₆ sublimating away near the 60 min. (240 K) point. Figure 1 summarizes the observed spectroscopic signals during the reaction in the 700–1,000 cm⁻¹ region. Initially, the intensity of the bands associated with UF₆ from 590–630 cm⁻¹ and the ice libration band at 850 cm⁻¹ are observed to be decreasing while a much narrower peak at 850 cm⁻¹ begins to grow. The growth of the feature at 850 cm⁻¹ is quickly followed by the formation of a band at 930 cm⁻¹. Next, three smaller bands at 715, 780, and 815 cm⁻¹ appear while the bands at 850 cm⁻¹ and 930 cm⁻¹ blue shift. Finally, all bands in this region disappear except for the strong band at 940 cm⁻¹ and small band at 865 cm⁻¹ which are associated with the product.

FIGURE 1

FIGURE 1. FTIR spectra from 700–1,000 cm⁻¹ over 4 different time intervals: (A) 0–30 min (B) 31–40 min (C) 41–60 min (D) 61–72 min. Spectra in dark blue are the earliest while dark red represents the last spectra in that time interval.

The region between 1,000–4,000 cm⁻¹ also shows significant changes during the reaction, Figure 2. As the reaction proceeds a broad continuum of absorption bands from 1,000–3,500 cm⁻¹ is observed. This continuum is indicative of an aqueous proton network and depletes as the final product is formed. Before the reaction is initiated, the strong absorption due to OH stretching of amorphous solid water is observed at 3,250 cm⁻¹. As the reaction proceeds five distinct bands are observed to shift, increase in intensity, and decrease in intensity over the duration of the experiment: 1,200 cm⁻¹, 1,500 cm⁻¹, 1700 cm⁻¹, 2,200 cm⁻¹, and 2,650 cm⁻¹. The bands at 1,200 cm⁻¹, 1,700 cm⁻¹, and 2,650 cm⁻¹, have been observed in cryogenic mixtures of HF with water and their intensities are reported as directly proportional to HF concentration (Ayotte et al., 2005; Ayotte et al., 2008; Iftimie et al., 2008). By the end of the experiment, the only bands that remain are associated with the UO₂F₂ product.

FIGURE 2

FIGURE 2. FTIR spectra from 1,000–4,000 cm⁻¹ over 4 different time intervals: (A) 0–30 min (B) 31–40 min (C) 41–60 min (D) 61–72 min. Spectra in dark blue are the earliest while dark red represents the last spectra in that time interval.

4 Discussion

Figure 3 shows the spectral range between 500 and 700 cm⁻¹, which has been characterized here as the reactant region in which the UF₆ antisymmetric stretching mode is centered at 624 cm⁻¹; however, a more dominant feature is observed at 590 cm⁻¹ during the initial cryogenic conditions. Modes in this region have previously been attributed to free UF₅ and also to UF₆ complexes with water (Sherrow and Hunt, 1992). Neither of these assignments seems likely here as the feature does not resemble previously reported spectra of UF₅ and is observed even before the addition of water (Kim et al., 2008; Krohn et al., 2008; Paine et al., 2008). Sherrow and Hunt also observed the formation of a broad absorption band centered at 550 cm⁻¹ when co-depositing UF₆ and water, which they attributed to the initial formation of UF₆^…xH₂O complexes and is not observed here (Sherrow and Hunt, 1992). We are attributing the feature at 590 cm⁻¹, and the smaller feature at 610 cm⁻¹, to cryogenic UF₆ as we observed a similar red shifted band in cryogenic layers of MoF₆ (McNamara et al., 2023). As the layers warm and the reaction proceeds, these two features give way to the expected UF₆ absorption profile.

FIGURE 3

FIGURE 3. FTIR spectra from 500–700 cm⁻¹ over 4 different time intervals: (A) 0–30 min (B) 31–40 min (C) 41–60 min (D) 61–72 min. Spectra in dark blue are the earliest while dark red represents the last spectra in that time interval.

The spectral region between 700 and 1,000 cm⁻¹ is comprised of transient intermediate species and product (UO₂F₂). The spectral region between 1,000 and 3,800 cm⁻¹ holds a rich dynamic signature of proton transfer processes that have been attributed to the fluoride-hydronium ion pair. The intermediate relationship of the negative fluoride ion and the positive hydronium ion has been described by Giguère and Turrell (1976) and Giguere and Turrell (1980). The canonical OH stretching region expresses a red-shifting progression to a continuous absorption from 3,800 cm⁻¹ to 2000 cm⁻¹. This is a well-characterized signature of proton transfer but is more obvious in the dissolution of hydrofluoric acid (Cabarcos et al., 1999; Ayotte et al., 2000; Ayotte et al., 2005; Ayotte et al., 2008; Iftimie et al., 2008).

Figure 4 shows peak fits for the spectral features observed between 700 and 1,000 cm⁻¹ where m) is the initial spectrum of the reactants and a) is the spectrum of the final product. The features observed for the product display two major components centered at 955 cm⁻¹ and 933 cm⁻¹ (labelled as a₁ and a₂) and a minor feature at 857 cm⁻¹ (a₃). The minor feature at 857 cm⁻¹ overlaps spectrally with the intermediate features in this region but can be observed in the final product. While the origin of the features can be attributed to the presence of UO₂F₂ with the crystalline environment of residual water and hydrofluoric acid, the temporal onset to these features may provide a mechanistic understanding of to the a₁, a₂, and a₃ origins. Three features are observed most clearly at the end of hydrolysis; however, a₁ and a₃ have significant changes in intensity across the temperature dependent spectra. These features are likely the result of multiple uranyl ions being present in the aqueous environment. UO²⁺, (UO₂)₂OH₂²⁺, and (UO₂)₃OH₅⁺ have previously been assigned to features of aqueous uranyl compounds in this region (Quilès and Burneau, 1998).

FIGURE 4

FIGURE 4. Temperature dependent infrared spectra of the uranium hexafluoride hydrolysis and the formation of uranyl fluoride in the spectral range of 700–1,000 cm⁻¹. m) is the initial spectrum of the reactants and a) is the final product. a₁ = 955 cm⁻¹, a₂ = 933 cm⁻¹, and a₃ = 857 cm⁻¹ represent bands associated with the final product. b₁ = 865 cm⁻¹, b₂ = 780 cm⁻¹, b₃ = 715 cm⁻¹, and c₁ = 815 cm⁻¹ represent bands associated with intermediate complexes.

The spectral features of UOF₄, assigned at 868 cm⁻¹ and 857 cm⁻¹ by Sherrow and Hunt, are observed as intermediate complexes (Sherrow and Hunt, 1992). The feature labeled b₁ in Figure 3C is centered at 865 cm⁻¹ and expressed a strong spectral signature throughout hydrolysis. The feature is first observed immediately upon hydration of UF₆ but redshifts ∼10 cm^-1 and becomes more dominant in the infrared spectrum as water reacts and is later evaporated from the matrix. The two bands around 860 cm⁻¹ were previously observed and attributed to the axial and equatorial isomers of the UOF₄. Interestingly, the doublet was previously observed at a 50/50 ratio while we are observing a clear shift from one isomer to another. This shift between the isomers is attributed to the kinetically more favorable formation of one isomer followed by the conversion to the more thermodynamically favorable isomer. Since the experiment takes place in the condensed phase, intermediate and product compounds are likely complexed with HF and water. The frequencies of these compounds will be affected by both the temperature of the film and the relative abundance of HF and water in the observed aqueous proton network. The resolution of the current experiment does not allow the different possible complexes to be easily distinguished from one another.

During the intermediate reaction stage, a band at 815 cm⁻¹ is observed to grow and then deplete while two other bands at 715 cm⁻¹ and 780 cm⁻¹ grow. These two new bands, the UOF₄ stretching mode at 865 cm⁻¹, and the remaining UF₆ then all deplete simultaneously as the final product continues to form. The new band observed at 815 cm⁻¹ seems to be correlated with the conversion between the two UOF₄ isomers as it is depleted by the time the 865 cm⁻¹ component of the UOF₄ band has reached its maximum intensity. The bands at 715 cm⁻¹ and 780 cm⁻¹ do not appear to depend on the formation of any other new bands in the spectrum but do deplete alongside the remaining UF₆ and UOF₄ bands. These two bands may be attributed to a previously unobserved intermediate complex. The region from 700 to 800 cm⁻¹ is indicative of U-O-U bridging compounds. This region lines up well with several computationally predicted intermediates with U-O-U bonds (Hu et al., 2008; 2009; Lind et al., 2010; Lutz et al., 2022); however, these bridged compounds are typically predicted to have formed through UF₅OH intermediates which there is not strong evidence of in this experiment. Other possibilities are U₂O₃F₆ and U₃O₅F₈, which have been shown to form as intermediates with sub-stochiometric mixtures of water and UF₆, though their IR spectra are not well documented (Otey and LeDoux, 1967). PCA was performed on the data set and the results support the previous discussion, see Supplementary Material for more detail.

The observations from this experiment support previous claims that UF₆ hydrolysis is composed of two half reactions with a UOF₄ intermediate:

${\text{UF}}_6\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\to {\text{UOF}}_4\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+2\text{HF}\left(\mathrm{s}\right)$

${\text{UOF}}_4\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+2\text{HF}\left(\mathrm{s}\right)\to {\text{UO}}_2{\mathrm{F}}_2\left(\mathrm{s}\right)+4\text{HF}\left(\mathrm{g}\right)$

In excess water, the final product is a hydrate of the form: UO₂F₂^…xH₂O. The formation of the three previously unobserved bands at 715 cm⁻¹, 780 cm⁻¹, and 815 cm⁻¹ suggest the formation of bridged O-U-O intermediates which can directly convert to UO₂F₂. These preliminary results show promise for this technique to further elucidate the chemistry governing UF₆ hydrolysis by varying reactant isotopologues and employing other spectroscopic techniques.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

LM: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Visualization, Writing–original draft, Writing–review and editing. JK: Data curation, Formal Analysis, Writing–original draft, Writing–review and editing. AW: Conceptualization, Investigation, Methodology, Writing–review and editing. EV-A: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing–review and editing. KS: Funding acquisition, Resources, Supervision, Writing–review and editing.

Funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. Financial support for this work was provided by the National Nuclear Security Administration under Defense Nuclear Nonproliferation R&D.

Acknowledgments

This work was produced by Battelle Savannah River Alliance, LLC under Contract No. 89303321CEM000080 with the US Department of Energy. Publisher acknowledges the US Government license to provide public access under the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). Assistance from Jason Richards at ORNL for UF₆ expertise and samples is greatly appreciated.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fnuen.2023.1300544/full#supplementary-material

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