Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX-XXX
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Reaction of CO2 with UO3 Nanoclusters Luis A. Flores, Julia G. Murphy, William B. Copeland, and David A. Dixon* Department of Chemistry, The University of Alabama, Shelby Hall, Tuscaloosa, Alabama 35487-0336, United States S Supporting Information *
ABSTRACT: Adsorption of CO2 to uranium oxide, (UO3)n, clusters was modeled using density functional theory (DFT) and coupled cluster theory (CCSD(T)). Geometries and reaction energies were predicted for carbonate formation (chemisorption) and Lewis acid−base addition of CO2 (physisorption) to these (UO3)n clusters. Chemisorption of multiple CO2 moieties was also modeled for dimer and trimer clusters. Physisorption and chemisorption were both predicted to be thermodynamically allowed for (UO3)n clusters, with chemisorption being more thermodynamically favorable than physisorption. The most energetically favored (UO3)3(CO2)m clusters contain tridentate carbonates, which is consistent with solid-state and solution structures for uranyl carbonates. The calculations show that CO2 exposure is likely to convert (UO3)n to uranyl carbonates.
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(DFT) level, finding that the presence of short, uranyl-like bonds in the structure affects the properties of the UO3 as the polymorphs containing the uranyl bonds are more thermodynamically stable.7 Cluster models have been particularly helpful with the study of the properties of these uranyl salts using high levels of theory.11,12 We have previously reported a DFT study of uranyl carbonates, nitrates, and acetates; these computational results showed that the geometries calculated for uranyl carbonates at the DFT level are in good agreement with experiment.13 We report the results of a computational study of the formation of uranyl carbonates from the reaction of (UO3)n clusters with carbon dioxide to better understand how carbonation might occur. Calculations at the DFT and CCSD(T) levels are employed for this study, building on work in our group on the addition of H2O and alcohols14−17 to transition metal oxide clusters and the addition of CO2 and other acid gases with metal−organic frameworks (MOFs).18
INTRODUCTION Uranium oxides are important in nuclear fuels as well as in dealing with environmental issues from past nuclear weapon production practices. Uranium oxide is commonly mined as UO3 before being processed to UO2, which is used as a fuel in nuclear fuel cycles. UO2 is exposed to oxidizing conditions at various steps in nuclear fuel cycles, leading to the formation of further uranium oxides.1 In practice, UO2 from spent fuel is not completely oxidized to UO3 and is instead oxidized to U3O8. Exposure of UO2 to O2 has been shown to oxidize the surface to form UO3 or UO3 hydrates in humid conditions. UO3 may be converted into uranyl carbonate when exposed to CO2 under aqueous conditions.2 The uranyl dication has been the subject of many studies due to its stability. Carbonates and other naturally occurring anions may form complexes with actinide cations, increasing their solubility in water. This can allow actinides from nuclear waste repositories, or sites with nuclear material, to migrate through natural water sources in the event of release of material. The uranyl carbonate structure is found in several naturally occurring uranyl carbonates. Rutherfordine is the most typical uranyl carbonate3 (UO2CO3) with an orthorhombic structure with linear uranyl carbonate chain species. Liebigite 4 (Ca2[UO2(CO3)3]·10−11H2O), and andersonite5 (Na2Ca[UO 2 (CO 3 ) 3 ]·6H 2 O) contain tricarbonate uranyl UO2(CO3)34− species. A crystal structure containing a larger trimeric (UO2)3(CO3)66− anionic cluster has been reported by Zwick et al. 6 They also detected UO 2 (CO 3 ) 3 4− and (UO2)3(CO3)64− anionic species in aqueous solutions using several spectroscopic techniques, including NMR, Raman, and EXAFS. Several computational studies have been conducted on uranium oxides and uranyl salts.7−10 Brincat et al. studied several bulk UO3 polymorphs at the density functional theory © XXXX American Chemical Society
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COMPUTATIONAL METHODS Geometries for all clusters were optimized at the DFT level using the B3LYP functional.19,20 with the aug-cc-pVDZ basis set for C and O21 and the Stuttgart small core RECP and associated Stuttgart orbital basis sets for U.22 Vibrational frequencies were calculated to ensure that the structures were energy minima. The DFT calculations were done with the Gaussian 09 program system.23 Additional single-point energy calculations were performed at the CCSD(T) level24−27 using the B3LYP optimized geometry. CCSD(T) calculations were performed using the Received: September 12, 2017 Revised: October 12, 2017 Published: October 13, 2017 A
DOI: 10.1021/acs.jpca.7b09107 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A aug-cc-pVnZ basis set for C and O21 and the cc-pVnZ-PP basis set for U (n = D, T, Q).28 (Basis sets are abbreviated as aN for the combination aug-cc-pVNZ (N = D, T, Q for C and O and cc-pVNZ-PP for U). Dimer and trimer reaction energies were calculated up to the CCSD(T)/aT and CCSD(T)/aD levels, respectively. CCSD(T) enthalpies and free energies were obtained by using vibrations calculated at the B3LYP/aug-ccpVDZ/Stuttgart(U) level with electronic energies calculated at the CCSD(T) level. As a check on the use of the B3LYP/augcc-pVDZ/Stuttgart(U) quantities for the zero-point energy enthalpic thermal corrections and the entropy contributions to the free energies, we also calculated these quantities at the B3LYP/aug-cc-pVnZ/cc-pVTZ-PP(U) level for the monomer reactions. The results (Supporting Information) show that the differences in the contributions are very small and below any errors in the electronic energies or in the use of the harmonic approximation; therefore, the composite method that was used is appropriate. Reaction energies with the UO3 monomer were extrapolated to the complete basis set limit using a mixed Gaussian/exponential formula.29 The CCSD(T) calculations were performed with MOLPRO 2012.1 package.30,31 Ligand binding energies (LBEs) were calculated for the following reaction (UO3)n + mCO2 → (UO3)n (CO2 )m
1). The OUO bond angle in the 1-P1 cluster shows little change in the core UO3 structure. Chemisorption of CO2
Figure 1. UO3CO2 monomer clusters: physisorbed UO3CO2 and (1P1) chemisorbed UO3CO2 (1-C1).
converts the equatorial U−O bond into a C−O bidentate carbonate bond, leading to a C2v cluster that can be described as a bidentate carbonate CO32− bound to a uranyl dication UO22+ (1-C1). The OUO bond angle in 1-C1 increased by almost 4°, and the UO bond decreased by 0.03 Å (Table 2). The carbonate thus has less back-donation than does the equatorial O2−, on the basis of the geometry changes. A natural population analysis (NPA) based on the natural bond orbitals (NBOs)33,34 using NBO635,36 was performed for the lowest-energy structures to provide additional insights. The populations are given in the Supporting Information. The UO22+ has a charge of 0.67 e for UO3, and that for UO2CO3 is 0.98 e, showing less charge transfer from the CO32− than from the O2−, consistent with the geometry changes. The LBEs for the monomer are given in Table 3. Physisorption is an exoergic process, and the free energy shows that CO2 binds to the UO3. As expected, the B3LYP LBE for this Lewis acid−base interaction is too small and CO2 would not be predicted to bind at 298 K at the DFT level with this functional, although CCSD(T)/CBS shows that this binding will occur. Chemisorption forms the more energetically favorable cluster 1-C1, with ΔH298 K 2−3 kcal/mol more exothermic than the 1-P1 cluster at all levels. This preference for forming uranyl carbonate is consistent with delocalizing the two negative charges on the ligand over more centers. Additional CO2 molecules were not added to the monomer carbonate given that carbonation of a uranyl oxygen is unlikely to occur. Binding energies calculated at the DFT and CCSD(T) levels, up to the CBS limit (Table 3), are in reasonable agreement in terms of the absolute values but do lead to different qualitative conclusions for the physisorption process. The LBEs at the DFT/B3LYP level are less negative by 3−4 kcal/mol as compared to the CCSD(T)/CBS for 1-P1 and 1-C1. Dimer. Physisorption of CO2 to (UO3)2 produces a single cluster, 2-P1, where one of the CO2 oxygen atoms binds to a U center and orients its C center toward the closest bridging O atom (Figure 2). The reaction of CO2 with the (UO3)2 dimer produces three chemisorbed clusters. The 2-C1a geometry is generated when the CO2 molecule binds to one of the bridging O atoms in the dimer cluster, forming a bridging bidentate carbonate where one of the carbonate O atoms bridges the two U centers and another carbonate O binds to one of the U centers. In 2-C1a, the UO bond contracts by 0.01 Å and a OUO bond angle increases to 5° (Table 2), the largest OUO bond angle increase for the (UO3)2CO2 clusters. The 2C1b cluster forms when the CO2 forms a carbonate with a bridging dimer O atom but breaks one of the M−O bridge bonds, forming a bidentate carbonate that binds both of the metal bond O atoms to a single U center. The 2-C1c cluster is generated by a CO2 reacting with one of the terminal O atoms in the (UO3)2 cluster, forming a terminal bidenate carbonate bound to a single U center. The other terminal O changes its
(1)
Clusters will be named using the abbreviation n-xm, where n is the size of the oxide cluster (n = 1, 2, 3), m is the amount of CO2 bound in the system (m = 1, 2, 3), and x is the type of binding occurring (C = chemisorption, P = physisorption).
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RESULTS AND DISCUSSION Before we discuss the addition of CO2 to the UO3 monomer, dimer, and trimer, we examine the clustering energies for formation of the dimer and trimer from the monomer. There is some dependence on the basis set, as noted in Table 1, but the Table 1. Reaction Energies (ΔH298 K in kcal/mol) to Form (UO3)2 and (UO3)3 at the CCSD(T) Level reaction
aD
aT
aQ
CBS
NCE(aT)a
2UO3 → (UO3)2 3UO3 → (UO3)3 UO3 + (UO3)2 → (UO3)3
−87.2 −156.4 −69.2
−83.7 −152.1 −68.4
−82.7
−82.2
41.8 50.7
a
NCE = E(UO3) − E((UO3)n)/n = normalized clustering energy.
values for the dimerization energy at the aug-cc-pVTZ-PP level are in reasonable agreement with the CBS limit value. The dimerization reaction is more exothermic than that for the addition of a monomer to the dimer to form the trimer. The heat of formation of solid γ-UO3 is −292.5 ± 0.3 kcal/mol, and the gas-phase value is −190.0 ± 2.4 kcal/mol, giving a cohesive energy of 102.5 kcal/mol.32 Clearly the normalized clustering energy (NCE) of 50.7 kcal/mol for the trimer is still far from the cohesive energy, as would be expected for such a small system. The dimer and trimer values for ΔHf(g,298 K) can be derived from the reaction energies and the heat of formation of the monomer as −462.2 kcal/mol for the dimer and −722.1 kcal/mol for the trimer. Monomer. The UO3 monomer has a C2v structure with two axial O atoms with a ∠OUO of 159°; the third O occupies an equatorial site. Physisorption binds one of the CO2 O atoms to the U center of the UO3, forming the Cs 1-P1 cluster (Figure B
DOI: 10.1021/acs.jpca.7b09107 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Table 2. UO Bond Lengths and OUO Bond Angles and for (UO3)n and (UO3)n(CO2)m (n = 1, 2, 3; m = 1, 2, 3) Clusters from the B3LYP Calculationsa U-1 ∠OUO (deg)
cluster UO3 1-P1 1-C1 (UO3)2 2-P1 2-C1a 2-C1b 2-C1c 2-C2a 2-C2b 2-C2c 2-C2d (UO3)3 3-P1 3-C1a 3-C1b 3-C1c 3-C2a 3-C2b 3-C3a 3-C3b 3-C3c a
159.1 159.5 162.8 161.7 163.0 167.0 164.3 100.8 168.0 167.8 167.5 167.3 163.7 165.0 166.8 165.9 101.5 170.9 169.2 171.8 172.9 170.3
×2
×2 ×2 ×2 ×2 ×2 ×3 ×2
×2 ×2
U-2
U-3
∠OUO (deg)
UO (Å) 1.81 1.81 1.78 1.79 ×2 1.79 1.78 1.78 ×2 1.78; 2.18 1.77 ×2 1.77 ×2 1.77 ×2 1.77 ×2 1.78 ×3 1.79 1.78 ×2 1.78 1.79; 2.19 1.76 1.77 1.76 ×2 1.76 1.77 ×2
∠OUO (deg)
UO (Å)
161.6 162.9
1.79 1.78
165.3
1.77; 1.78
163.6 165.7 165.7 166.0 167.4 ×2 168.0
1.79 1.75 1.78 1.78; 1.79 1.77 ×2 1.77
169.9 ×2 168.5
1.77 ×2 1.77
UO (Å)
163.4
1.79
165.4 165.9
1.77; 1.78 1.77; 1.78
166.6
1.77; 1.78
Bond angles and bond lengths that are shared by multiple uranyls are identified by ×Z, where Z identifies the number of identical uranyls (Z = 2, 3).
Table 3. Calculated Binding Energies (ΔH298 K) and Gibbs Free Energies of Binding (ΔG298 K) at 298 K for the Addition of CO2 to UO3 at the CCSD(T) Level (kcal/mol) 1-P1
1-C1
energy
B3LYP
basis set
aD
aD
aT
CCSD(T) aQ
CBS
B3LYP aD
aD
aT
CCSD(T) aQ
CBS
ΔH298 K ΔG298 K
−7.5 0.0
−12.7 −5.2
−11.7 −4.2
−11.1 −3.6
−10.8 −3.3
−9.2 2.4
−15.0 −3.4
−14.4 −2.8
−13.7 −2.1
−13.3 −1.7
charge of −1.05 e. For 2-C2a, the UO22+ has a charge of 0.98 e, again showing less charge transfer from the carbonates. The LBEs for the dimers (Table 4) show that physisorption is an exoergic process with an energy comparable to the physisorption LBE for the monomer. Chemisorption forming the 2-C1a cluster is more energetically favorable than physisorption by only 1 kcal/mol. The remaining chemisorbed clusters are not predicted to form. Although the 2-C1b cluster has a weakly exothermic ΔH298 K, the change in free energy of that reaction shows that this reaction will not occur. A second CO2 can react with these monocarbonate clusters, yielding four chemisorbed structures (Figure 2). The 2-C2a cluster forms when CO2 reacts with the bridging secondary O atom on the 2-C1a cluster, forming a bidentate carbonate where only one carbonate O atom is bound to each U center. This allows the first bridging carbonate to adopt a more relaxed geometry with the remaining free carbonate O atom binding to the closest U center and the OUO bond increasing to 168.0°. The remaining (UO3)2(CO2)2 clusters experience smaller changes in the OUO bond angle. The 2-C2b cluster is also a product of a reaction between the 2-C1a cluster and CO2. In the 2-C2b cluster, the second carbonate has one carbonate O bridging the two U centers and a second carbonate O atom being bound to one of the U centers. The 2-C2c and 2-C2d clusters are products of a reaction between CO2 and the 2-C1b
Figure 2. (UO3)2CO2 dimer clusters: chemisorbed (UO3)2CO2 (2C1a, 2-C1b, and 2-C1c) and physisorbed (UO3)2CO2 (2-P1). (UO3)2(CO2)2 dimer clusters: chemisorbed (UO3)2(CO2)2 clusters (2-C2a, 2-C2b, 2-C2c, and 2-C2d).
position with a large increase in the UO bond distance to 2.18 Å, and the OUO bond angle changes to 100.8°. Thus, one of the uranyl dication moieties is no longer present. In (UO3)2, the charge on the UO22+ is 0.81 e, with each bridging O having −0.80 e. For 2-C1a, each UO22+ has a charge of 0.90 e, with the bridging O atom charge essentially unchanged with a charge of −0.79 e and the CO32− having a C
DOI: 10.1021/acs.jpca.7b09107 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Table 4. Calculated Binding Energies (ΔH298 K) and Gibbs Free Energies of Binding (ΔG298 K) at 298 K for the Addition of CO2 to (UO3)2 at the CCSD(T)/aug-cc-pVTZ Level (kcal/mol) CCSD(T)/aT
CCSD(T)/aD
B3LYP/aD
product
reactant
ΔH298 K
ΔG298 K
ΔH298 K
ΔG298 K
ΔH298 K
ΔG298 K
2-P1 2-C1a 2-C1b 2-C1c 2-C2a 2-C2b 2-C2c 2-C2d
(UO3)2 (UO3)2 (UO3)2 (UO3)2 2-C1a 2-C1a 2-C1b 2-C1b
−11.0 −11.6 −3.2 5.5 −16.2 −11.6 3.1 3.0
−3.7 −1.4 5.4 15.1 −4.3 −0.6 13.3 12.4
−12.2 −11.7 −1.7 4.8 −16.4 −12.1 3.7 4.7
−4.8 −1.5 6.9 14.4 −4.6 −1.1 14.0 14.1
−5.8 −5.0 −0.9 7.4 −8.7 −4.0 5.5 6.3
1.5 5.2 7.7 17.1 3.2 7.0 15.8 15.7
cluster. Both clusters form a second bridging bidentate carbonate that binds one carbonate O to each U center. Products from reacting CO2 with the 2-C1a are both likely to occur as both have exoergic ΔG298 K values (Table 4). The 2C2a cluster is the lowest-energy cluster for this reaction. The LBE for binding a second CO2 is larger than that for binding the first CO2 to the dimer. The 2-C2b cluster is 5 kcal/mol higher in energy than 2-C2a. The ΔG298 K values for the 2-C2c and 2-C2d clusters are endoergic, showing that these clusters will not form. Binding energies calculated at the CCSD(T) level for the 2P1 and 2-C1a are 5−6 kcal/mol more negative than the energies calculated with DFT/B3LYP. The CCSD(T) energies for 2-C2a and 2-C2b are 8 kcal/mol more negative than the corresponding DFT energies. This is consistent with the issues found using the B3LYP functional for the Lewis acid/base addition of water and alcohols to the group 6 trimers (MO3)3 for M = Mo and W.14,15 DFT binding energies for the remaining dimer clusters are within 2−3 kcal/mol of CCSD(T) energies. The CCSD(T) calculations with the aD basis set are usually within 2 kcal/mol of the results with the aT basis set. For most cases, the aD basis set provides reasonable results at the CCSD(T) level except when the energy differences are small, as found for the energy difference between 2-P1 and 2C1a where the order between physisorption and chemisorption changes with the larger basis set. Trimer. The reaction of CO2 with the (UO3)3 trimer cluster results in three chemisorbed clusters and one physisorbed cluster (Figure 3). The 3-P1 cluster physisorbs CO2 to one of the U centers and orients the C center toward one of the bridging (UO3)3 oxgyen atoms without forming a carbonate. The 3-C1a cluster chemisorbs CO2 to one of the trimer bridging O atoms, forming a tridentate carbonate that bridges two of the U centers. The 3-C1b cluster chemisorbs CO2 to one of the (UO3)3 bridging O atoms, forming a bidentate bridging carbonate that forms one U−O bond with two of the U centers. The 3-C1c chemisorbs the CO2 to a terminal O of the (UO3)3 cluster, forming a terminal bidentate carbonate and breaking one of the “uranyls” in the (UO3)3 cluster. The 3-C1a and 3-P1 clusters are both predicted to experience an increase in OUO bond angle from 163.7° in (UO3)3 to 165° for the CO2 bound uranyl in 3-P1, 166.8° for the carbonate bound uranyl, and 165.7° for the remaining uranyl of 3-C1a. In (UO3)3, the charge on the UO22+ is 0.87 e, with each bridging O having ca. −0.87 e. For 3-C1a, each UO22+ has a charge of 0.90 e, with the bridging O atom charge essentially unchanged and the CO32− having a charge of −1.06 e. For 3C2a, the UO22+ has a charge of 0.96 e, with the negative charge
Figure 3. (UO3)3(CO2) trimer clusters: physisorbed (UO3)3(CO2) clusters (3-P1), chemisorbed (UO3)3(CO2)2 clusters (3-C1a, 3-C1b, and 3-C1c). (UO 3 ) 3 (CO 2 ) 2 trimer clusters: chemisorbed (UO3)3(CO2)2 clusters (3-C2a and 3-C2b). (UO3)3(CO2)3 trimer clusters: chemisorbed (UO3)3(CO2)2 clusters (3-C3a, 3-C3b, and 3C3c).
on the bridging O reduced to −0.82 e and charges on the carbonates of −1.03 e. For 3-C3a, the UO22+ has a more positive charge of 1.01 e, which is balanced by the carbonates. Thus, in all stable chemisorbed clusters, the bridging (or equatorial for the monomer) O2− transfers more electron density to the UO22+ moieties than does the CO32−. Physisorption of CO2 to (UO3)3 is likely to occur because the ΔH298 K for the 3-P1 cluster is exoergic (ΔG298 K) at −4.6 kcal/mol (Table 5). Again, this energy is comparable to that for physisorption to the monomer and dimer even though the equatorial environments at the UO22+ are different in all three structures. Chemisorption produces the most energetically preferred cluster, 3-C1a, which is 19 kcal/mol more stable than the 3-P1 cluster, with a ΔH298 K of −31.3 kcal/mol. Both remaining chemisorbed clusters, 3-C1b and 3-C1c, are predicted not to form as their ΔG298 K values are both endoergic. Two clusters were optimized for the chemisorption of two CO2 moieties, with both being possible products of CO2 chemisorption to the 3-C1a cluster. Both clusters bind CO2 to one of the remaining bridging (UO3)3 O atoms. The 3-C2a cluster forms a second tridentate carbonate bridging two of the U centers. The 3-C2b cluster forms a bidentate carbonate that forms only two carbonate O−U bonds. Structure 3-C2a experiences a greater increase in the OUO bond angle than 3D
DOI: 10.1021/acs.jpca.7b09107 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
the adsorption of two CO2 moieties to the (UO3)2 dimer is predicted to first form 2-C1a upon chemisorption of the first CO2. Chemisorption of the second CO2 is predicted to form 2C2a. Each chemisorbed cluster becomes more stable as more carbonate−U bonds are formed. The bridging bidentate carbonate in 2-C1a forms three O−U bonds. Reducing the amount of U−O carbonate bonds to two, as found for the 2C1b cluster, raises the cluster’s energy by 8 kcal/mol. Carbonation of one of the terminal O atoms of (UO3)2 is not favored because this causes the cluster to break one of the uranyl UO bonds in the parent cluster. Binding all three carbonate O atoms is even more favorable as tridentate carbonates are more stable than either form of the bidentate carbonate. The tridentate carbonate is the only major structural difference between the 2-C2a cluster and the 2-C2c and 2-C2d clusters, yet the 2-C2a cluster is 23 kcal/mol more stable than either of the latter two clusters. The lowest-energy reaction pathway for the (UO3)3 trimer begins with the formation of 3-C1a, followed by formation of 3-C2a and then formation of 3-C3a. Each step of the lowestenergy reaction pathway forms a tridentate carbonate (Figure 4). The final product, 3-C3a, is structurally similar to the crystal structures of uranyl carbonates.3,6 Reaction energies for the monomer and dimer are comparable, but the binding energies for the trimer clusters are 15−20 kcal/mol more negative. This is because all three carbonate oxygens can participate in binding to the uranyls in the trimer clusters. Uranyl Vibrational Frequencies. The uranyl stretching modes are given in Table 6. Physisorption of CO2 to the dimer and trimer clusters causes little if any change in the uranyl vibrational frequencies, consistent with a Lewis acid/base interaction of the CO2 with the U. This interaction does not lead to any real electron density donation to the U; therefore, the uranyl stretches do not exhibit much change. Chemisorption causes the uranyl stretching modes to shift to a higher wavenumber, with the clusters with endothermic binding energies having the greatest shift in these vibrational frequencies. Chemisorption causes the uranyls to become more cationic as the carbonate donates less electron density to the uranyl than does the O2− ligand, as noted above.
Table 5. Calculated Binding Energies (ΔH298 K) and Gibbs Free Energies of Binding (ΔG298 K) at 298 K for the Addition of CO2 to (UO3)3 at the CCSD(T)/aug-cc-pVDZ Level (kcal/mol) CCSD(T)/aD
B3LYP/aD
product
reactant
ΔH298 K
ΔG298 K
ΔH298 K
ΔG298 K
3-P1 3-C1a 3-C1b 3-C1c 3-C2a 3-C2b 3-C3a 3-C3b 3-C3c
(UO3)2 (UO3)2 (UO3)2 (UO3)2 3-C1a 3-C1a 3-C2a 3-C2a 3-C2b
−12.0 −31.3 3.9 5.6 −30.8 4.4 −34.2 3.4 6.9
−4.6 −19.2 12.6 15.8 −18.9 13.5 −21.7 13.0 16.7
−5.7 −19.7 5.1 8.2 −19.8 3.9 −21.6 3.6 8.1
1.8 −7.6 13.8 18.3 −7.9 13.1 −9.0 13.3 17.9
C1a to 170.9° for the uranyl bound to two carbonates and 167.4° for the remaining uranyls. Formation of a second tridentate carbonate is also energetically allowed as 3-C2a binds a CO2 by −30.8 kcal/mol (ΔH298 K) and has ΔG298 K = −18.9 kcal/mol. Formation of a bidentate carbonate is predicted not to occur as the 3-C2b cluster has an endoergic ΔG298 K of 13.5 kcal/mol. Chemisorption of a third CO2 to the (UO3)3 cluster forms three (UO3)3(CO2)2 clusters. Two of these clusters (3-C3a, 3C3b) are products of CO2 chemisorption to the bridging O of the 3-C2a cluster. Structure 3-C3a forms a third tridentate carbonate that bridges two U centers. Structure 3-C3a experiences an increase in bond angle with respect to 3-C2a to 171.8° for all uranyls. The 3-C3b cluster forms a bidentate carbonate where one carbonate O atom is bound to two of the U centers. The 3-C3c cluster is a product of chemisorption to the bridging O atom of the 3-C2b cluster, forming a second bidentate carbonate that bridges two U centers. The 3-C3a cluster is the most energetically favorable cluster and is likely to form because it has a very exoergic ΔG298 K of −21.7 kcal/mol. The 3-C3b and 3-C3c clusters are predicted not to form as both have endoergic ΔG298 K values. Trimer clusters with exothermic binding energies have DFT binding energies that are 6−12 kcal/mol less negative than their CCSD(T) binding energies. The remaining clusters have DFT binding energies that are within 3 kcal/mol of the CCSD(T) energies. This is similar to what we found for the monomer and dimers, except that this effect is larger for the trimers. Reaction Pathway. Formation of the 1-C1 cluster is the lowest-energy pathway for adsorption of CO2 to the UO3 monomer (Figure 4). The lowest-energy reaction pathway for
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CONCLUSIONS DFT and CCSD(T) were used to model the physisorption and chemisorption of CO2 to (UO3)n (n = 1, 2, 3) clusters. Physisorption and chemisorption, in most cases, of CO2 to the (UO3)n clusters result in an increase in the OUO bond angles of the uranyl moieties. This effect increases as more CO2 moieties are added to the cluster. The OUO uranyl bond angle also increases with increasing stability of the cluster. The 3-C3b cluster has a larger OUO bond angle for one of its uranyls, but 3-C3a increases the bond angles of all three of its uranyls to a greater degree than 3-C3b. The calculations show that CO2 exposure is likely to convert (UO3)n to uranyl carbonates. Physisorption of CO2 is predicted to occur for all (UO3)n clusters as the ΔG298 K values are all exoergic. Chemisorption of a single CO2 is predicted to occur for all (UO3)n clusters and to be more energetically preferred than physisorption as the UO3CO2 monomer cluster, as well as the dimer and trimer clusters, with the tridentate carbonates or bridging bidentate carbonates with three O−U bonds, have exoergic ΔG298 K values. The addition of a second and third CO2 moiety is exoergic when forming tridentate carbonates or bridging carbonates with three O−U bonds. Formation of a terminal
Figure 4. Lowest-energy reaction pathway for the adsorption of CO2 moieties to UO3, two CO2 moieties (UO3)2, and three CO2 moieties to (UO3)3. Reaction energies in the figure are (ΔH298 K/ΔG298 K). E
DOI: 10.1021/acs.jpca.7b09107 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 6. Unscaled Uranyl Stretching Frequencies, ν (cm−1), with IR Intensities (km/mol) from (UO3)n(CO2)m Clusters Calculated at the B3LYP/aug-cc-pVDZ Level asymmetric stretch ν
IR
ν
IR
UO22+ UO3 1-P1 1-C1 (UO3)2
1128.0 884.7 876.2 958.3 927.3 911.5 926.6 909.6 961.8 943.7 950.9 938.9 954.2 981.2 969.5 984.9 973.3 972.3 966.1 972.1 966.2 940.0 923.0 923.0 938.0 921.8 920.0 962.7 952.0 941.3 955.8 947.0 936.2 957.4 941.2 913.0 988.0 966.3 956.7 978.9 965.4 953.0 997.4 987.1 986.5 994.3 980.7 978.6 986.1 978.5 973.1
172 482 476 395 789 0 743 30 624 85 738 0 147 596 65 645 0 677 0 660 25 1083 0 0 1055 9 0 974 0 38 861 58 111 330 140 244 538 420 0 616 256 108 902 1 0 664 252 8 763 0 158
1029.0 773.5 763.8 889.7 869.0 861.0 866.5 858.1 884.8 862.0 876.0 871.0 884.3 895.1 885.7 893.7 883.3 886.1 879.1 886.6 879.4 882.3 882.3 873.2 882.0 879.2 870.5 893.2 886.9 885.5 892.2 878.7 872.9 901.5 889.0 885.6 905.4 895.3 890.7 896.7 892.5 879.3 911.2 906.5 906.0 909.2 896.5 890.8 897.5 893.1 884.4
0 220 215 52 0 32 1 34 30 296 55 118 80 17 176 0 296 0 274 0 286 0 0 0 3 1 1 3 28 4 17 24 138 108 100 62 14 0 38 3 40 150 0 32 31 19 2 171 41 52 171
2-C1a 2-C1b 2-C1c 2-C2a 2-C2b 2-C2c 2-C2d (UO3)3
3-P1
3-C1a
3-C1b
3-C1c
3-C2a
3-C2b
3-C3a
3-C3b
3-C3c
■
symmetric stretch
compound
2-P1
CO2. The types of tridentate carbonates predicted to from upon addition of CO2 to UO3 clusters resemble those found in uranyl carbonate crystal structures.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b09107. Complete refs 6, 23, and 31, total energies, monomer benchmark thermal corrections, and Cartesian coordinates for all clusters (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
David A. Dixon: 0000-0002-9492-0056 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported as part of UNCAGE-ME, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012577. D.A.D. also thanks the Robert Ramsay Chair Endowment, University of Alabama, for support. We thank Dr. Monica Vasiliu and Dr. Virgil Jackson for assistance with the calculations.
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REFERENCES
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