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Cluster-Models for Uranyl(VI) Adsorption on R-Alumina Vassiliki-Alexandra Glezakou*,† and Wibe A. deJong*,‡ †
Chemical Physics and Analysis, Fundamental and Computational Sciences Directorate, ‡W. R. Wiley, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ABSTRACT: Aqueous complexation, adsorption, and redox chemistry of actinide species at mineral surfaces have a significant impact on their transport and reactive behavior in chemically and physically heterogeneous environments. The adsorption configurations and energies of microsolvated uranyl dication species, UO2(H2O)n2þ, were determined on fully hydroxylated and proton-deficient R-alumina(0001)-like finite cluster models. The significant size of the models provides faithful representations of features that have emerged from periodic calculations, but most importantly, they afford us a systematic study of the adsorption mechanism, the effect of secondary solvation shells and an explicit treatment of the total charge. Based on this cluster representation, the energetics computed from the difference between the optimized structures and the appropriate reference states point to a preference for an inner-sphere type complex.
1. INTRODUCTION Understanding the chemical behavior and properties of actinides interacting with mineral surfaces and water is of paramount importance for the effective mediation of radioactive waste.1,2 Their complex transport and reactivity in such heterogeneous environments is driven by processes at the molecular level such as speciation, solvation, and their oxidation and reduction chemistry. Better understanding of such phenomena at the molecular level will enable us to make long-term predictions of the fate of radionuclides in the environment. Actinide chemistry is intimately connected to the oxidation state of the ion, and across a wide range of the chemical environments, the uranyl dication, UO22þ, dominates uranium chemistry in solution and on surfaces. The identity, structure, and dynamics of the hydration shells of UO22þ have been investigated in a number of experimental studies such as X-ray,3-5 IR,6 and NMR7 spectroscopies, and by theoretical ab initio molecular dynamics studies.8 Similarly, actinide adsorption on mineral surfaces has also been the subject of numerous experimental studies, in particular, by X-ray absorption fine-structure (EXAFS) studies that provide local structural information.9-12 There are two principal mechanisms for metal complex adsorption on surfaces: inner-sphere, when there is direct interaction of the metal centers with surface atoms, and outer-sphere, when the complexes bind through their ligands.13 In either, the complexes may form single (monodentate) or multiple bonds (polydentate) to the surface. X-ray adsorption experiments of Pb(II) and Co(II) on single-crystal R-alumina suggest an outer-sphere adsorption mechanism on the (0001) surface and inner-sphere on (1102) surfaces. In addition, Co(II) adsorption is rather sensitive to surface defects and local morphology.14,15 XAFS studies of U(VI) sorption on silica, γ-alumina, and montorilonite suggest adsorption via an inner-sphere mechanism.16 Although these studies are indispensable, r 2011 American Chemical Society
the signal is often averaged and they do not give much detail on the relative orientation, type of bonding, that is, mono- versus polydentate, or change of the oxidation state and coordination as a function of the substrate, surface orientation, and the environment. In this respect, theoretical studies are unique in elucidating the atomistic and mechanistic details of these processes, which are still lacking. Alumina (aluminum oxide, Al2O3) is a very important material with many industrial applications as catalyst or catalytic support.17 R-Alumina(0001) is of particular importance because of its thermodynamic stability and common occurrence in soil phases such as goethite, gibbsite, ferrihydrite, and so on. A growing number of theoretical studies18-28 have made progress in this domain, but relatively few studies address the modeling of actinide chemisorption and reactivity.29-34 The majority of the theoretical studies involve density functional theory (DFT) with periodic boundary conditions (PBC). Given the complexity of the systems, timeintensive calculations with extended periodic cells are often required. Hydroxylated R-Al2O3(0001) has been well characterized by experimental studies such as those by Ahn and Rabalais35 or in the more recent study by Eng et al.36 In the latter study, it was found that a fully hydrated surface is an intermediate between R-Al2O3 and γ-Al(OH)3 and is essentially oxygen terminated, in contrast to the clean surface, which is Al-terminated. Its reactivity is greatly influenced by the degree of hydroxylation. Water reactivity on clean R-alumina has also been studied with periodic DFT calculations,37,38 cluster,37-39 or embedded-cluster models.40,41 The emerging picture from these studies is a mixture of physisorbed water molecules, chemisorbed H and OH species and a complex, dynamic network of inter-planar hydrogen bonding interactions. Received: September 27, 2010 Published: January 26, 2011 1257
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The Journal of Physical Chemistry A Small cluster models comprised of 3-4 stoichiometric Al2O3 units did not give particularly good agreement with periodic calculations, while embedded-type models were significantly better. In one of the first detailed periodic DFT studies on R-alumina(0001), Moskaleva et al.32 evaluated the adsorption energies of both inner- and outer-sphere uranyl-water complexes and found that a defect-free hydroxylated surface favors the outersphere complex with five equatorial ligands, two of which are assumed to have lost two of their hydrogens to surrounding water, becoming a (surface)-U(VI)O2(OH)2(H2O)3 species. These periodic boundary super cell calculations are indeed indispensable as they provide a more realistic representation of these systems, but they suffer from the inherent inability of PBC models to handle an overall charged super cell. Generally, this type of problem is circumvented by the judicious choice of model or charge compensation by extra ions to result in an overall zero cell charge. However, the comparison between the two types of complexes in this last study, inner- versus outer-sphere, is not straightforward nor is it executed on equal footing, in the sense that the preference for the outer-complex is based on the comparison between two rather different systems, the three-coordinate UO2(H2O)32þ on a proton-deficient hydroxylated surface (with nominal charge of -2), with the five-coordinate UO2(H2O)3(OH)2 on a neutral, fully hydroxylated surface. In the current study, we present our results for adsorption of uranyl species on the commonly encountered R-alumina(0001), based on finite-size models. The substantial, albeit finite, size of our models affords us a realistic representation of these types of systems and explicit treatment of the overall charge. In this way, we are able to investigate local geometry, coordination, and oxidation state of solvated uranyl dication, as a function of cluster size, degree of hydration, defect presence, and type of terminating layer in a systematic way, enhancing the existing literature on the subject by determining some new sorbed complexes that have not been reported before. Based on several theoretical studies,42,43 solvation energetics,27,28 and other experimental evidence,3,42,44-46 the first solvation shell of uranyl dication contains, on average, five ligands in equatorial positions. In our calculations we will assume this to be the case and we will compare other coordination configurations to that: more specifically, when considering three-coordinate species, we include the remaining two waters in the second solvation shell. When we consider the neutral, five-coordinate species UO2(OH)2(H2O)3, we also compare with the UO2(OH)2(H2O) 3 2H2O complex, where two of the waters are included in the second solvation shell. In all calculations, the upper layer of atoms or hydroxy groups was allowed to relax together with the sorbent molecules. This paper is organized as follows: in section 2, we briefly discuss the computational details. In section 3 we present results evaluating the size of the surface cluster models (section 3.1), the gas-phase geometries of free uranyl and the three- and five-coordinate species of the type U(VI)O2(OH)m(H2O)nq, where m = 0 or 2, n = 3, 4, and 5, and q = þ2 or 0 (section 3.2), sorption complexes on a fully hydroxylized surface (section 3.3), and finally, sorption complexes on hydrolyzed, protondeficient surface clusters to assess the role of defects (section 3.4). In all cases, surface defects and total charge can be explicitly accounted for and can be directly correlated to the local geometry, coordination number, and oxidation state. Finally, conclusions are presented in conjunction with important findings of the literature.
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2. COMPUTATIONAL DETAILS Our models consist of finite, O-terminated stoichiometric clusters of the type (Al2O3)n, where n = 8, 11, 14, and 18 carved out of R-alumina(0001) surface, with adsorbed UO22þ and a varying number of water molecules to evaluate the influence of degree of hydration on the adsorption energies and adsorption sites. We also used a 17-unit Al-terminated cluster for H2O adsorption as a benchmark calculation for direct comparison with the literature.38,47 A model of the R-Al2O3(0001) surface was built using the cell parameters determined by a detailed study of Hinnemann and Carter.48 The optimized hexagonal unit cell parameters from this study correspond to a = 4.809 Å and c = 13.115 Å and an extended slab model was built using these parameters. In all cases, the clusters are stoichiometric and neutral, that is, they consist of an integer number of Al2O3 units. For the fully hydroxylated clusters, a layer of protons was added to all surface oxygens and the appropriate number of oxygen atoms was left as an extra bottom layer to maintain the overall neutrality of the cluster. Proton-deficient clusters were produced by removal of two protons from the cluster surface resulting in a total charge of -2. The surface areas of the 14-unit O-terminated cluster and of the fully hydroxylated 18-unit cluster are approximately 11 11 and 14 14 Å2 and seem to be adequate to study the adsorption of even the fully hydrated uranyl dication. The larger, 18-unit hydroxylated clusters consisted of ∼140 atoms, including the adsorbed complexes, of similar surface and thickness of slabs used in periodic boundary calculations in the literature. Geometry optimizations were carried out with density functional theory using the B3LYP hybrid functional, implemented in the NWChem suite of codes43 with the upper atomic layer and adsorbing molecules being fully relaxed, while the remaining of atoms remained frozen. All optimizations were done considering the singlet spin state of the overall system. In the case of the hydroxylated clusters, all surface hydroxy groups were allowed to relax. For U, we used the Stuttgart 1997 small core relativistic potential for the representation of the 60 inner electrons, while the valence electrons were described in terms of Gaussian functions, by the companion basis set. For O and Al, we also used the Stuttgart effective core potentials and companion basis sets, keeping 2 and 10 electrons, respectively, in the core, while the valence electrons were represented by (4, 5) contracted to (2, 3) Gaussian basis functions. For H, we used Dunning’s cc-pVDZ basis.49 The binding energies of the complexes are calculated as energy differences BE ¼ E½S-UO2 ðOHÞm ðH2 OÞn q E½Sq1 - E½UO2 ðOHÞm ðH2 OÞn q2 where q = q1 þ q2 and E[S-UO2(OH)m(H2O)nq], E[Sq1] and E[UO2(OH)m(H2O)nq2] the energies of the adsorbed uranyl species on the surface cluster, the isolated surface cluster and the gas-phase energy of the corresponding uranyl complex configuration with the appropriate total charges, respectively. In the case where H-transfer occurred during the optimization from the complex to the surface, the energy difference was calculated with respect to the corresponding surface cluster.
3. RESULTS AND DISCUSSION Free uranyl dication UO22þ has not been observed; however, theoretical predictions by Pyykko et al. suggest that the U-Oax distance of the free, linear species would be in the range 1.66-1.70 Å, 1258
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The Journal of Physical Chemistry A
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Table 1. Structural Parameters of Small Clusters UO2(OH)m(H2O)nqa molecule
R(U-Oax)
R(U-Oeq)
R(U-OH)
