Journal of Molecular Structure 1062 (2014) 29–34

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Journal of Molecular Structure

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Effect of H2O on the hydrolysis of UF6 in the gas phase

0022-2860/$ - see front matter � 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2014.01.015

⇑ Corresponding author. Tel.: +86 1062765560.E-mail address: swhu@pku.edu.cn (S.-W. Hu).

Shao-Wen Hu ⇑, Hao Lin, Xiang-Yun Wang, Tai-Wei ChuBeijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science, College of Chemistry and MolecularEngineering, Peking University, Beijing 100871, China

h i g h l i g h t s

� UF6 hydrolyzes in excess of H2O.� Dimer or trimer of H2O coordinates with UF6 to form UF6�2H2O or UF6�3H2O.� H2O molecules catalyze the reactions and stabilize the transition states.

a r t i c l e i n f o

Article history:Received 1 November 2013Received in revised form 8 January 2014Accepted 9 January 2014Available online 18 January 2014

Keywords:Uranium hexafluorideHydrolysisRelativistic density functional theorycalculationWater catalysis mechanism

a b s t r a c t

In our previous works (Hu et al., 2008, 2009), we found the theoretical evidence that indicates UF5OH isan intermediate produced in the first step of UF6 hydrolysis, UF6 + H2O. When UF6/H2O ratio is high,UF5OH may react with UF6 or another UF5OH, forming species containing UAOAU bond. In this work,we considered another situation – the hydrolysis of UF6 in excess of H2O, and explored the probableinitial reactions of UF6 + 2H2O and UF6 + 3H2O systems using the same relativistic density functionaltheory calculations. Water molecules may form dimer or trimer and then coordinate with UF6, formingcomplexes UF6�2H2O or UF6�3H2O, which are more stable than UF6�H2O. Compared to the conversionUF6�H2O ? UF5 OH�HF, the energy barriers of reactions UF6�2H2O ? UF5OH�H2O�HF and UF6�3H2O ?UF5OH�2H2O�HF are 4 and 6 kcal/mol lower respectively. The additional H2O molecules catalyze thereactions by bridging the hydrogen which transfers from ligand H2O to F and stabilizes the transitionstates. In addition, as water content increases, the reaction step UF6 + 3H2O becomes exothermic andthe products-HF, UF5 OH and two H2O molecules-tend to bond tightly into a stable complex.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Uranium hexafluoride (UF6) is a key compound in the nuclearfuel industry. Because of its low boiling point, UF6 is used to enrichfissionable isotope through gas phase diffusion [1,2]. The reactionof enriched UF6 with water is a controlled step to produce purenuclear fuel, uranium dioxide [3,4]. The initial UF6/H2O ratio wasfound affecting on the composition, the size and morphology ofthe final product [5–8]. The depleted UF6 should be stored andtransported for reprocessing. In all the cases, the potential releaseof UF6 and its hydrolysis with atmosphere moisture can causeenvironment hazards [9–15]. Besides the species containing ura-nium, hydrogen fluoride (HF), as a very erosive byproduct of UF6

hydrolysis, should be collected and recycled properly [16].Despite the importance of the title reaction, the studies of

kinetic or mechanism are rare and highly dependent on the specificexperimental conditions. Kessie studied the hydrolysis of UF6 andPuF6 in packed bed reactor, and found that the reaction rate

increased with an increases of the partial pressure of either wateror UF6. Reactions in the gas phase were detected at moist partialpressures greater than 1.0 mmHg [17]. Klimov et al. examinedthe reaction under different pressures in excess of H2O. Assuminga two-step process monitored by measuring the spectra of HF pro-duced, they determined the rate constant of the first step, which ismuch slower than the second one [18].

UF6 þH2O! UOF4 þ 2HF ð1Þ

UOF4 þH2O! UO2F2 þ 2HF ð2Þ

Sherrow et al. studied the probable intermediates of the reac-tion using infrared spectra measurements in three conditions. Theyfound that when UF6 and H2O co-deposited in excess of argon at12 K, a 1:1 complex UF6�H2O formed and UOF4 could be producedby photolysis. When UF6 and H2O co-deposited without argon,UF6�nH2O (n P 1) complexes may form and UOF4 could be producedby annealing at 242 K. While in a gas cell at low pressure and ambi-ent temperature, they did not detect any signal of gaseous uraniumoxyfluorides, only an unknown pale yellow solid product wasfound on the nickel surface of the cell [19].

30 S.-W. Hu et al. / Journal of Molecular Structure 1062 (2014) 29–34

To the best of our knowledge, several computational worksfocused on relevant reactants or probable products of UF6 hydrolysis.Privalov et al. calculated the enthalpy changes of the hydrolysis ofUO3, UO2F2 and UF6 [20]. Kovacs et al. studied the structure andbonding of the probable intermediate UOF4 and other halogen sub-stituents [21]. Shamov et al. investigated various known uraniumoxofluorides and evaluated the effect of theoretical methods onthe geometry and bonding energy of these species [22]. In a recentwork, Garrison et al. found a transition state of the first rate-limit-ing step of UF6 hydrolysis [23,24].

Previously, we investigated the reaction mechanism usingrelativistic density functional theory calculations in a series ofworks. In the first work [25], we focused on the initial stage ofthe reaction, assuming that it starts from collision between twomolecules, UF6 and H2O. The results showed that the reaction (1)is composed of two steps.

UF6 þH2O! UF5OHþHF

UF5OH! UOF4 � HF

Both the steps should overcome a substantial energy barrierand are endothermic, although the second step can be catalyzedby H2O or HF. UOF4 can hardly form as an isolated molecule. Inthe second work [26], we found that the probable reactions ofUF5OH/UF6 and UF5OH/UF5OH systems can lead to the formationof several species containing a UAOAU bond. This is a simulationof reactions between concentrated UF6 with small amounts ofH2O. As an extension of our previous works, we started to studythe behavior of UF6 in excess of H2O. In this work, we focused onhow water affected the initial step. The reactions of UF6 with twoand three H2O molecules were considered.

2. Methods and calculations

The method choice was based on former theoretical works [22].More details of the calculations have been described in our previ-ous work [25]. Briefly, the geometry structures of all stationarypoints were fully optimized using the density functional withgeneral gradient approximation (GGA) methods incorporated withall-electron TZ2P basis set. The relativistic effect was evaluatedusing scalar zero order regular approximation (ZORA). Frequency

Table 1Relative energies and dipole moments of the species.

Species Syma E_GGAb EZ

UF6�H2OUF6�H2O (1) C1(0) 0.00 0.TS1 C1(1) 17.64 �UF5OH�HF (5) Cs(0) 8.55 �

UF6�2H2OUF6�2H2O (2) C1(0) 0.00 0.TS2 C1(1) 17.00 �TS3 C1(1) 13.08 �UF5OH�H2O�HF (6) C1(0) 5.71 �UF5OH�H2O�HF (7) C1(0) 11.34 �

UF6�3H2OUF6�3H2O (3) C1(0) 0.00 0.UF6�3H2O (4) C1(0) �1.46 0.TS4 C1(1) 11.97 �TS5 C1(1) 14.72 �UF5OH�2H2O�HF (8) C1(0) �3.98 �UF5O�H3O�H2O�HF (9) C1(0) �1.19 �

e Dipole moments are in Debye, the hybrid PBE0 calculation.a Point group of the species, number of imaginary frequency is in parentheses.b Relative energy (kcal/mol) of the GGA calculation.c Relative ZPE (kcal/mol) of the GGA calculation.d Relative energy (kcal/mol) of the hybrid PBE0 calculation.

calculations using the same GGA method were performed tocharacterize their nature as well as to provide zero point vibrationenergy (ZPE). Intrinsic reaction coordinate (IRC) calculations wereperformed for each transition state to ensure the correct connec-tions between relevant reactants and products. Single pointcalculation was evaluated by using each GGA optimized structureand hybrid density functional PBE0 incorporated with all-electronTZ2P basis set and scalar relativistic ZORA. The final reportedenergies were at the hybrid PBE0 level with GGA calculated ZPEcorrections. The relative calculated energies using the twomethods were listed comparatively in Table 1.

The interaction energy of molecular fragments in complexeswas calculated and corrected by basis set superposition error(BSSE) and ZPE. This was referred to as binding energy (Ebi, Table 2).For all the energy items, the results calculated with hybrid densityfunctional PBE0 were used except for ZPE, for which the GGAcalculated results were used.

ADF2007 program package [27] was employed. The accuracycriterion of 6.5 was used for all the numerical integration, whichis a rough indication of the number of significant digits.

3. Results and discussion

This section consists of three parts. In part I, we discussed theformation probability of complexes UF6�nH2O. In part II, we dis-cussed the conversion mechanism of UF6�nH2O to UF5OH�mH2O�HFand HF elimination. In part III, total reaction pathways wereoverviewed.

Each minimum structure of the complexes was named accord-ing to its structure and numbered as it appears in the discussion.The symbol ‘‘TS’’ plus a number was used to name each transitionstate.

3.1. Part I: Formation of UF6�nH2O (n = 1–3) complexes (Fig. 1)

We have found that UF6 forms a weak 1:1 complex with H2O,UF6�H2O (1), which is the precursor of further reactions betweenthe two molecules [25]. If more H2O molecules are close to UF6, itis possible that UF6�nH2O (n > 1) forms. For UF6 associating withtwo H2O molecules, two initial structures were considered. First,starting from 1, another H2O was positioned as an additional

c E_PBE0d E_PBE0 + EZ Dipolee

00 0.00 0.00 3.3422.04 20.63 18.59 2.7981.40 8.62 7.21 2.840

00 0.00 0.00 4.4022.84 20.15 17.30 5.1921.92 16.24 14.32 5.8401.76 5.47 3.71 5.0591.14 11.56 10.42 4.896

00 0.00 0.00 5.40632 �1.85 �1.53 3.8572.25 14.31 12.06 5.6872.96 17.98 15.02 6.2391.16 �3.61 �4.77 3.2940.76 �2.51 �3.27 6.532

Table 2Binding energies of molecular fragments in complexes.

Associations DEa DEb BSSEb DEza Ebi

c

UF6 + H2O?UF6�H2O (1) �1.67 �2.65 �0.63 1.83 �0.19UF6 + (H2O)2?UF6�2H2O (2) �6.04 �6.93 �0.92 2.11 �3.89UF6 + (H2O)3?UF6�3H2O (3) �10.92 �11.09 �1.19 2.65 �7.26UF5OH + HF?UF5OH�HF (5) �3.37 �3.53 �0.53 1.88 �1.12UF5OH�H2O + HF?UF5OH�H2O�HF (6) �4.56 �4.89 �0.54 1.16 �2.82UF5OH + H2O�HF?UF5OH�H2O�HF (7) �0.52 �0.60 �0.95 1.84 2.19UF5OH�H2O + H2O�HF?UF5OH�2H2O�HF (8) �13.37 �12.54 �0.99 1.36 �10.30UF5OH�2H2O + HF?UF5O�H3O�H2O�HF (9) �6.89 �8.30 �0.36 2.79 �5.15

a Energies (kcal/mol) of the GGA calculations.b Energies (kcal/mol) of the hybrid PBE0calculations.c Binding energies of hybrid PBE0 calculation. See Section 2 for definition.

Fig. 1. Structures of UF6�nH2O and the composing molecules UF6 and (H2O)n

(n = 1–3); bond lengths are in Å; angles are in degrees; symmetry of the structuresare indicated in parentheses except for C1.

S.-W. Hu et al. / Journal of Molecular Structure 1062 (2014) 29–34 31

ligand to uranium. During geometry optimization, however, noclosely associated final structures were obtained, indicating thatonly one H2O molecule can be accepted by UF6 as an additionalligand. Second, two H2O molecules can associate into a dimer,(H2O)2, which then coordinates with uranium of UF6, forming acomplex UF6�2H2O (2). The second H2O acts both as a hydrogenbonding donor and an acceptor, connecting the ligand H2O andF. As a result, the coordination becomes stronger from 1 to 2. Thiscan be seen from the shorter UAO bond length and the largerbinding energy of 2. This effect is enhanced further as the thirdH2O is added to form complex UF6�3H2O (3). Alternatively, whenthree H2O molecules associate into a chain structure and form aring with UF6 as shown in complex UF6�3H2O (4), the energy willbe slightly lower. The UAO bond in 4, however, is slightly longerthan that in 3, implying that the coordination of H2O with U isweaker and the stable effect may come from the stronger hydro-gen bonding.

Complex 1 is assumed to form through collision of moleculesUF6 and H2O. For complexes 2, 3 and 4, the formation processes,involving more molecules, may have several ways. The first wayis that three or four molecules meet together and associate into acomplex. The possibility of such processes is quite low in the gasphase. The second way is that complex 1 forms firstly, and it thenassociates with additional H2O molecules one after another to form2, 3 or 4. This may happen as the molecules co-deposit in some so-lid surface. The third way is that water dimer or trimer formsfirstly, and it then coordinates with UF6 to form 2 and 3. Thismay happen because H2O is abundant in the form of steam. Watertrimer has three conformers [28]. The one shown in Fig. 1 is not themost stable but it can act as a ligand. Complex 4 may not formthrough water trimer and UF6 since water trimer cannot existstably in a chain structure. When calculating binding energy, weassumed the complexes form through the third way, because thecoordination energies between UF6 and water fragments are ourmajor concern. It can be seen that for 1, 2 and 3, the binding ener-gies between UF6 and H2O, (H2O)2, (H2O)3 are �0.19, �3.89, and�7.26 kcal/mol respectively, indicating the significant effect ofmultiple H2O molecules on the coordination.

3.2. Part II: Conversion of UF6�nH2O (n = 1–3) to UF5OH�mH2O�HF(m = 0–2) (Figs. 2 and 3)

Our previous work [25] showed that the reaction of UF6 andH2O started from complex 1, and a hydrogen atom transferred

from H2O to a fluorine ligand of UF6. Relative to 1, the energyof the transition state TS1 (Fig. 2) is 18.59 kcal/mol. In complexUF5OH�HF (5) (Fig. 3), UF5OH and HF are weakly bonded andreadily dissociate overcoming a modest binding energy. Whenstarting from the UF6�2H2O complex 2, two probable convertingchannels exist. One of them is through transition state TS2, oneH2O molecule transfers an H atom to F, and the other H2O stabi-lizes the system by joining a hydrogen bonded ring structure.Relative to 2, the energy barrier is 17.30 kcal/mol. Due to thehydrogen bonding, the binding energy of HF and UF 5OH�H2 Oin the product UF5OH�H2O�HF (6) is slightly larger than that ofHF and UF 5OH in 5. The other channel is through transition stateTS3 where H does not transfer directly from the ligand H2O to F,but is bridged by another H2O. Involving the H2O bridge, a six-member ring structure forms in TS3, and it causes less strain thanthe four-member ring in TS1 and TS2. As a result, the energy bar-rier substantially lowers. Relative to 2, the energy of TS3 is14.32 kcal/ mol. The product is UF5OH�H2O�HF (7) which can beseen as a weakly bonded complex of UF5OH and H2O�HF. Thebinding energy of the two molecular fragments in 7 is negligibleand even negative after ZPE and BSSE corrections. Starting fromthe water trimer coordinated with complex 3, the H transfer pro-cess becomes much easier. In transition state TS4, besides theligand H2O, the other two H 2O molecules play roles to stabilize

1.590 1.029

2.0082.027

2.071 2.039

2.286

0.97

7

85.5

UF5OH H2O.

1.119

1.3341.002

1.9582.028

1.759

0.988

2.114 2.047

87.3

1.592

UF5OH 2H2O.

0.978

2.023

88.1

2.095

1.818

2.0202.028

0.94

6

5

0.9491.559

1.0341.783

0.974

2.044 2.024

2.0302.055

2.560

85.9

6

1.84

9

0.989 1.4801.001

2.758

2.139

66.5

2.034

2.0282.027

7

2.0152 .02 9

2.047

2.043

1.844

1.00

5

1.3881.095

1.611

1.461

1.010

0.99

1

96.3

8

1.9 622.02 4

2.0532.159

1.005

1.599

1.6740.972

1.571

87.3

78.7

1.802

1.377

1.101

0.99

7

1.036

9

H2O HF (Cs)

1.668 0.959

0.97

2HF (C )

0.934

2.0282.053

2.034

2.029

0.976

UF5OH (Cs)

.

8

Fig. 3. Structures of the products, UF5OH�mH2O�HF(m = 0–2), produced in the initialreactions between UF6 and nH2O (n = 1–3); bond lengths are in Å; angles are indegrees.

1.102

2.478

2.293

1.257

2.030 2.018

2.029

55.5

TS1

1.133

2.466

1.22

0

1.636

2.027

2.03356.1

2.260

73.7

2.065

1.012

2.232

0.976

TS2

1.50

0

2.061

66.5

2.040

1.160

1.046

2.3941.206

2.248

2.037

TS3

2.092

1.2191.657

2.364

2.039

2.041

68.7

1.41

0

0.97

9

1.982

1.081

2.193

1.153

1.013

TS4

2.089

2.183

1.216

2.468

2.03 7

1.842

56.6

1.466

1.66

8

0.982

1.139

2.030

1.0541.007

TS5

Fig. 2. Structures of transition states involved in the initial reactions between UF6and nH2O (n = 1–3); bond lengths are in Å; angles are in degrees.

32 S.-W. Hu et al. / Journal of Molecular Structure 1062 (2014) 29–34

the system as a bridge of H transfer and as a ring memberrespectively. As a result, the energy of TS4 is only 12.06 kcal/ molrelative to 3. Because there seems no space to add more H2Omolecules to the ring structure, this is perhaps the reaction channelwith the lowest activation energy among the probable initial stepsof UF6 hydrolysis. Furthermore, the product, UF5OH�2H2O�HF (8), ismore stable than the reactant 3 due to the tight association of theleaving HF with both H2O molecules and the OH group of UF5OH.The dissociation of HF from 8 is clearly not easy. It causes lessstructural change that 8 dissociate into UF5OH�H2O and H2O�HF.Still, however, the dissociation needs to overcome a bindingenergy as large as 10 kcal/mol. Starting from UF6�3H2O (4), Htransfers in a similar way as in the second channel, and the onlydifference is that the hydrogen bonding involves two H2Omolecules in TS5. As a result, the energy of TS5 is 16.55 kcal/ molrelative to 4, only slightly lower than the barrier of the secondchannel. The product, UF5O�H3O�H2O�HF (9), however, is morestable than the reactant 4. Due to the hydrogen bonding effect,structure of 9 is quite different from the usual UF5OH�2H2O�HF.In 9, OAH bond of OH group lengthens significantly and thecomplex seems to consist of UF5O, H3O, H2O and HF. The dipolemoments of TS5 and 9 are especially large, indicating some ionic

nature of the species. The binding energy of HF with the restfragments in 9 is �5.15 kcal/ mol.

TS1(18.39)

UF6 (0.00)

3 (-7.26)

TS2(13.41)

5 (7.02)

1 (-0.19)

2 (-3.89)

TS3(10.43)

UF6 H2O

7 (6.52)

9 (-10.53)

4 (-8.79)

UF6 2H2O

UF6 3H2O

(8.14)

6 (-0.18)

(2.64)UF5OH H2O+HF

UF5OH+H2O HF (4.33)TS4(4.80)

8 (-12.03)

TS5(7.76)

UF5OH H2O HF

UF5O H3O H2O HF

UF5OH 2H2O HF

UF5OH HF

UF5OH+HF

(-1.73)

(-5.38)UF5OH 2H2O+HF

UF5OH H2O+H2O HF

,

,, , ,

,,

,

, ,

,

,

,

, ,

,

Fig. 4. The initial reactions of UF6 + H2O, UF6 + 2H2O, and UF6 + 3H2O; the relative energies (in parentheses) calculated at the PBE0 level with the GGA ZPE corrections are inkcal/mol.

S.-W. Hu et al. / Journal of Molecular Structure 1062 (2014) 29–34 33

3.3. Part III. Overview of the reaction channels

The initial steps of the reactions UF6+nH2O (n = 1–3) are drawnschematically in Fig. 4. From the figure, we can see several featuresof UF6 hydrolysis at this stage.

The coordination energy between UF6 and (H2O) n in UF6�nH2O(n = 1–3) increases as water changes from separated single mole-cules to associated dimer and trimer.

Conversion of UF6�nH2O to UF5OH�mH2O�HF requires H transferfrom H2O to F ligand. The H transfer bridged by H2O (TS3 and TS4)can significantly lower the energy barrier. Less effectively, H2O canalso stabilize the transition states through hydrogen bonding.

For reactions between UF6 and 2H2O or (H2O)2, the products areless stable than the corresponding reactants. The eliminated HFcan dissociate from the major part UF5OH with modest energy.Especially for product 7, dissociation into UF5OH and HF�H2O evenstabilizes the system.

For reactions between UF6 and 3H2O or (H2O)3, the products aremore stable than the corresponding reactants. The eliminated HF istightly hydrogen bonded to the major part UF5OH. The dissociationis quite energy demanding.

The most probable reaction channel is as follows.

3H2O! ðH2OÞ3

UF6 þ ðH2OÞ3 ! UF6 � 3H2Oð3Þ DE ¼ �7:26kcal=mol

UF6 � 3H2Oð3Þ ! UF5OH � 2H2O � HFð8Þ Ea ¼ 12:06

DE ¼ �4:77 kcal=mol

The reactions are exothermic overcoming low energy barrier,but it may not be easy to remove the byproduct HF.

UF5OH � 2H2O � HFð8Þ ! UF5OH � H2OþH2O �HF

DE ¼ 10:30 kcal=mol

The easiest pathway to removing the HF is as follows.

2H2O! ðH2OÞ2

UF6 þ ðH2OÞ2 ! UF6 � 2H2Oð2Þ DE ¼ �3:89kcal=mol

UF6 � 2H2Oð2Þ ! UF5OH �H2O � HFð7Þ Ea ¼ 14:32DE ¼ 10:42kcal=mol

UF5OH � H2O � HFð7Þ ! UF5OHþH2O � HF DE ¼ �2:19 kcal=mol

4. Conclusions

The effect of H2O/UF6 ratio in the initial step of UF6 hydrolysiswas summarized as follows. First, one H2O molecule coordinateswith uranium as a ligand of UF6. Additional one or two H2O mole-cules can enhance the coordination through hydrogen bondingwith ligand H2O and F. Second, the additional H2O moleculescatalyze UF6 hydrolysis by bridging the hydrogen which transfersfrom ligand H2O to F and stabilizes the transition state throughhydrogen bonding. Third, the additional H2O molecules connectthe products of the hydrolysis, UF5OH and HF, through hydrogenbonding. The eliminated HF becomes easier or more difficult to re-move depending on the water content and bonding types of theproducts. When the H2O/UF6 ratio is more than three, the first stepof hydrolysis becomes exothermic, but the HF tends to associatewith UF5OH and H2O more tightly. These results can roughly inter-pret the experiment [19]. When UF6 and H2O co-deposited inargon, argon molecules might separate water molecules. Only 1:1UF6�H2O complex formed and the reaction started from photolysis.When UF6 and H2O co-deposited without argon, there were morechances that UF6�nH2O (n > 1) formed and thermo reaction couldstart due to water catalysis. In the pure gas phase, reactions mightproceed in an uncontrolled way towards formation of solid prod-ucts, so it was hard to detect the signal of probable intermediates.This also implies that the further reactions may be fast stepsinvolving catalysts.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.molstruc.2014.01.015.

34 S.-W. Hu et al. / Journal of Molecular Structure 1062 (2014) 29–34

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