Anticorrelated Contributions to Pre-Edge Features of Aluminate Near

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Spectroscopy and Photochemistry; General Theory

Anticorrelated Contributions to Pre-Edge Features of Aluminate NearEdge X-ray Absorption Spectroscopy in Concentrated Electrolytes Andrew Wildman, Ernesto Martinez-Baez, John L. Fulton, Gregory K. Schenter, Carolyn I. Pearce, Aurora Evelyn Clark, and Xiaosong Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00642 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry Letters

Anticorrelated Contributions to Pre-Edge Features of Aluminate Near-Edge X-Ray Absorption Spectroscopy in Concentrated Electrolytes Andrew Wildman,† Ernesto Martinez-Baez,‡ John Fulton,¶ Gregory Schenter,¶ Carolyn Pearce,¶ Aurora Clark,∗,‡ and Xiaosong Li∗,† †Department of Chemistry, University of Washington, Seattle, WA, 98195 ‡Department of Chemistry, Washington State University, Pullman, WA, 99164 ¶Pacific Northwest National Laboratory, Richland, WA, 99354 E-mail: [email protected]; [email protected]

Abstract

spectroscopies. Given the nature of the transitions involved, this observation may be extended to other systems where ion-ion interactions dominate, however a complete understanding of the contributing transitions is necessary for accurate analysis of XANES pre-edge features in concentrated electrolytes.

Ion pairing within complex solutions and electrolytes is a difficult phenomenon to measure and investigate, yet it has significant impact upon macroscopic processes, such as crystal formation. Traditional methods of detecting and characterizing ion pairing are sensitive to contact ion pairs, may require minimum concentrations that limit applicability, and can have difficulty in characterizing solutions with many components. Because of its element specificity and sensitivity to local environment, X-ray absorption near edge structure (XANES) is a promising tool for investigating ion pairing in complex solutions. In concentrated sodium aluminate solutions, a shift in the pre-edge shoulder correlated to sodium concentration is observed, and the physical origins of that shift are investigated using energy specific time-dependent density functional theory of sub-ensmbles obtained from ab-initio molecular dynamics. Two transitions are found to contribute to the pre-edge feature, yet they are anticorrelated with respect to the sodium...aluminate distance. Unexpectedly, this causes Al XANES to be an effective probe for longer-range ion interactions than the traditional counterparts of NMR or vibrational

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The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Highly concentrated aqueous electrolytes represent an increasingly important suite of industrially relevant solutions that exhibit unique speciation, hierarchical structure, and dynamics. At the molecular scale, water-water hydrogen bonding is disrupted due to ion solvation, and new (often stable) molecular species that include contact- and solvent-separated ion pairs emerge. 1,2 At the macroscale water behavior also begins to deviate from the bulk. 3 Understanding ion-ion interactions and ion pairing is essential to explaining the mechansisms of chemical transformations within industrially relevant concentrated electrolyte solutions. A relevant example derives from the underlying chemistry of the Bayer process, where a central step in aluminum production depends upon the transformation of the predominant tetrahedral aluminate (Al(OH)− 4 ) in concentrated NaOH to an octahedral crystalline material, like gibbsite (Al(OH)3 ). 1 4 During the process of Al(OH)3 crystal formation, stable cation and Al(OH)− 4 ion pairs have been reported. 1,5 These ion pairs have been shown to mediate the mechanism of crystal growth, 6 and the cations stabilize the initial microstructures well enough to be incorporated into the final crystal lattice, 7 suggesting that ion pairing plays an essential role in the mechanism of solid formation. Ion pairs can also act to stabilize high concentrations of aluminum in solution, with the balance between dissolution and precipitation depending upon the relative concentrations of cationic and anionic species in the system. 8 Investigating ion pairing within any electrolyte is difficult. Traditional methods include NMR and Raman spectroscopy, which rely upon peak shifts in the spectra that result from the ion-ion interactions. However, sensitivity is typically limited to identification of the presence of contact ion pairs (ion pairs that have no solvent molecules between them), which are only one of the ensemble of ion-ion interactions that may be present in solution, Solvent shared and solvent separated ion pairs - containing one or two layers of solvent between ions, respectively - can exist in solution as well, influencing solution properties and processes. 2 As a promising alternative, X-ray absorption

near edge structure (XANES) spectroscopy is an element specific technique that is highly sensitive to local environment, making it a good candidate for probing both the different forms of ion pairing and the subtle differences in solution structure arising from different cations. In this paper, XANES is explored as a tool for characterizing the predominant type of ion pairing present in highly concentrated sodium aluminate solutions. First, the experimental spectra of sodium aluminate solutions are investigated. Solutions containing 0.5 M aluminum in a 2.7 M hydroxide solution containing either 2.7 M sodium or 8.5 M sodium (augmented with sodium nitrate) were prepared. Solutions were measured at the PHOENIX I beamline at the Swiss Light Source (SLS), at the Paul Scherrer Institute, 9 and the trends observed were reproduced in measurements at BL6.3.1.2 at the Advanced Light Source (ALS), at Lawrence Berkeley National Laboratory. Representative Aluminum K-edge X-ray absorption spectra are shown in Figure 1 with the region of interest highlighted in the inset. A KTiOPO4 (011) double-crystal monochromator provided an energy resolution of 0.6 eV and, in addition, the corehole lifetime broadening for aluminum is expected to be about 0.42 eV to give a resolution on the order of 1.0 eV for comparison with calculated spectra. The spectra exhibit a subtle and reproducible red shift of the preedge shoulder with increasing sodium concentration, suggesting that XANES is sensitive to changing ion-ion interactions. The correlation of the change in the aluminum pre-edge shoulder with changes in its extended environment is consistent with observations very recently made in the literature. 10,11 Complementary ab-initio molecular dynamics simulations, with ensemble sampling, were used to create a set of configurations to study the role of ion-pairing upon the XANES pre-edge features. In order to simulate different types of ion pairing, three ensembles of structures were generated based upon the distribution of distances observed in the equilibrated system. A sodium aluminate ion pair was placed in a periodic box of 90 water molecules. An unconstrained system was equilibrated, in addition to con-


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density functional theory (LR-TDDFT). Core excitation spectroscopies have been previously calculated in the TDDFT framework by two methods: restricted energy window TDDFT, 17–19 which approximates the exciations by only considering contributions from occupied orbitals below an energy threshold, and the method used in this paper, energy specific TDDFT (ES-TDDFT). 20 ES-TDDFT has been shown to give excitation energies in exact agreement with the full TDDFT framework 21 and produces XANES spectra in good agreement with experiment. 22 Each cluster used the BHandH functional, 23 previously shown to reproduce XANES spectra with low error. 21 A split basis set was used: 6-31G 24 on all oxygen and hydrogen atoms and Def2TZVP 25 on aluminum atoms and sodium atoms when present. ES-TDDFT calculations were performed in the G16.A03 version of the Gaussian software package. 26 To present the spectra, the excitation energies were broadened with a lorentzian distribution and averaged across the ensemble. Each lorentzian has a full width half max of 0.4 eV, derived from the core-hole lifetime of aluminum. A larger broadening parameter, 1.0 eV, must be used to reproduce the experimental spectra, due to the previously mentioned resolution restrictions of the detector. The calculated spectra were shifted up 23 eV in energy to account for self-interaction energy errors. 19,27,28 The experimental and calculated spectra cannot be directly compared, since the experimental spectra take into account the ensemble of all ion-ion interactions present in solution, and the relative abundance of each is unknown. In addition, the broadening parameter for the calculation must be comparable with the energy resolution of the experimental measurement. However, increased ion pairing is expected in the more concentrated solution, which is red shifted from the lower concentrated solution. A red shift is then expected in the calculated spectra for those ensembles with the sodium closer to the aluminate. The calculated pre-edge region of the aluminum K-edge is shown with a broadening parameter of 0.4 eV corresponding to the corehole

1.0 0.8 0.6

Intensity (Arb. Units)

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0.4 0.2 0.0

a 1564.2

1564.6

1565.0

2.0 1.5 1.0 0.5 0.0 1562

b 1564

1566

1568

Energy (eV) Figure 1. Aluminum K-edge spectra for solutions containing 0.5 M aluminum and either 2.7 M sodium (blue) or 8.5 M sodium (green). The bottom (b) shows the whole spectrum, and the top (a) is the pre-edge shoulder, delimited by the ellipse on the bottom.

strained simulations that restricted the sodiumaluminum distance to 2.5 ˚ A, 4.0 ˚ A, and 6.0 ˚ A. For reference, an ensemble without sodium was also calculated. To generate the structures, ab-initio molecular dynamics (AIMD) simulations were then carried out in the canonical (NVT) ensemble using the Quickstep module of the CP2K software and periodic boundry conditions. 12,13 The temperature was targeted at 300 K, using the canonical sampling through velocity rescaling thermostat 14 and a time constant of 20 fs. The system was treated at the DFT/revPBE 15 level of theory. Dispersion contributions were calculated with D3-DFT. 16 The AIMD simulations were allowed to equilibrate prior to structural sampling. Aluminum K-Edge spectra were then generated for a subset of 100 structures from each ensemble, using linear response time-dependent


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approximately the same energy acress ensembles, but different intensities. Interestingly, preedge feature intensity does not vary monotonically with distance; the most intense feature occurs with the 2.5 ˚ A distance, decreases at ˚ 4.0 A, then increases again at 6.0 ˚ A. Furthermore, the ensemble without sodium is lower in intensity than any of the structures with sodium. The decrease for the structures without sodium is expected, since those structures have fewer electrons, which leads to a smaller band contribution to the peak height. On the other hand, the non-monotonic relationship between the sodium-aluminum distance and the strength of the transition was not expected. Assuming that the transition can be characterized as a transition from an aluminum 1s orbital to an orbital localized around the sodium, the intensity of the feature should decrease with increasing distance because the overlap between the two orbitals decreases, decreasing the transition dipole moment. To discover the origin of this complex relationship, the pre-edge feature was separated into its contributing transitions. The pre-edge feature is dominated by the first two transitions in the aluminum K-edge energy regime. For each transition, the validity of the assumption about the structure of starting and ending state is tested, and the spectra of each excitation is calculated. The starting and ending states for a transition can be visualized through natural transition orbitals (NTOs), which provide a method of transforming the transition density into an “electron” and a “hole” orbital that dominate the transition. 29 For both transitions in all ensembles, the hole NTO is an aluminum 1s orbital, fitting with the prior assumption. However, the electron NTOs deviate from the assumption above. The orbitals are primarily centered around the sodium, but their spatial extent varies significantly with aluminumsodium distance, as can be seen in Figure 3. Furthermore, while the orbitals in the sodiumfree cluster have primarily S-type character, only the orbitals for the first transition have Stype character for the sodium containing clusters. The orbitals for the second transition in

7 6 5

No 6.0 4.0 2.5

Na A A A

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2 1 0 1563

a 1564

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8 7 6 5 4 3

b

2 1564.0 1564.2 1564.4 1564.6 1564.8 1565.0

Energy (eV) Figure 2. Pre-edge region of calculated aluminum Kedge spectra for an aluminate ion with a sodium ion at various distances from the aluminum. The top spectrum (a) was created with only the core hole lifetime broadening of 0.4 eV, whereas the bottom spectrum (b) was created with the experimental broadening of 1 eV, and the axes have been scaled to match Fig. 1a.

lifetime broadening for aluminum in Figure 2a, and with 1.0 eV broadening to compare with the energy resolution of the experimental spectrum (Figure 1) in Figure 2b. In these calculations, only the electric dipole contributions to oscillator strength are considered. Higher order contributions, such as electric quadrupole, were calculated to be two orders of magnitude smaller than the electric dipole contributions, so we expect no significant change from including these higher order terms. A preedge feature is observed in all ensembles, with


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Figure 3. Representative natural transition orbitals (NTOs) for the first and second transitions in the aluminum K-edge region for aluminate with a sodium ion at various distances away from the aluminum.

all the sodium containing clusters have a P-type character, with the nodal plane centered on the

sodium. When the sodium is close to the aluminum,


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the first two transitions have mixed contributions from both the sodium and aluminum. The S-type transition dominates the pre-edge feature because the overlap between the starting and ending orbitals is much greater than the overlap for the P-type transition. As the sodium moves away from the aluminum, the degree of mixing between the sodium and aluminum decreases, which causes the S-type transition to localize primarily on the sodium and the P-type transition to localize primarily on the aluminum.

No 6.0 4.0 2.5

2.5 2.0

creases as the sodium moves away from the aluminum, which leads to a decreasing intensity, as seen in Figure 4a. The assumptions stated above hold for the first transition, and the results are consistent with what is expected. In contrast, because of the increased localization of the transition density on the aluminum with increasing distance, the second transition actually increases in intensity with increasing distance, as observed in Figure 4b. In the limit that the sodium is at medium distance from the aluminum (6 ˚ A), the S-type transition localized around the sodium ought not be observed, since the transition dipole moment is so small. Aditionaly, the P-type transition cannot mix with the sodium, so it is entirely localized around the aluminum, and recovers an S-type character which is consistent with the first excitation in the sodium-free cluster. The anticorrelated behavior of these two transitions causes the overall pre-edge feature to be sensitive to ion-ion interactions that span a contact ion pair to a medium-range solvent solvent separated pair, though there is a a minimum intensity at the intermediary distance of 4 ˚ Adue to the cross-over regarding which of the two transitions is dominant (Figure 2). These simulations present an interesting and intriguing quandry for the general study of ion-pairing processes using XANES - specifically, that multiple contributions can lead to observed pre-edge features, and that these contributions can have different behavior as a function of distance between a set of ions. Further, they lead to an unexpected ability of the Al XANES to identify longerrange ion-ion interactions than would be identified by NMR or vibrational spectroscopies. Indeed, this feature is incredibly useful as when combined with NMR or IR/Raman, a more holistic characterization of ion-ion interactions across length scales may be obtained. In summary, an increase in the aluminum preedge shoulder is shown experimentally to be correlated with sodium concentration. Energyspecific TDDFT of sub-ensembles of solution configurations from AIMD reveal that the preedge shoulder is dominated by two transitions, and the strength of these transitions are functions of aluminate-sodium distance up to 6 ˚ A.

Na A A A

1.5


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1.0 0.5

a

0.0 2.5 2.0 1.5 1.0 0.5 0.0 1563

b 1564

1565

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Energy (eV) Figure 4. Peaks created by the first (a) and second (b) excitations in the aluminum K-edge region for aluminate with a sodium ion at various distances away from the aluminum.

The overlap for the S-type transition de-


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References

Importantly, the two transitions are anticorrelated with respect to distance - meaning that one transition is primarily sensitive to contact ion pairs, while the other transition is sensitive to ion-ion distances of around 6 ˚ A. In combination, the two contributing transitions indicate that XANES is sensitive to medium range ion-ion interactions, unlike other spectroscopic signatures from NMR or IR/Raman. The sensitivity of XANES to medium range ion-ion interactions is encouraging, yet the complex relationship between the ion pair distance and the shoulder height demonstrates that a theoretical understanding of the features present is incredibly important for accurate interpretations of the XANES spectrum.

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Acknowledgement This work was supported by IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). J.L.F. was supported by U.S. Department of Energys (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. The Al XANES measurements were performed at the PHOENIX beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland and at the Advanced Light Source (ALS), supported by the Office of Science BES of the DOE under Contract No. DE-AC02-05CH11231. Kevin Rosso, Jinghua Guo and Per-Anders Glans-Suzuki are gratefully acknowledged for their assistance in collecting solution Al Kedge XANES data at ALS. The development of energy-specific TDDFT method was supported by the National Science Foundation (CHE1565520 to X.L). Computations were facilitated through the use of advanced computational, storage, and networking infrastructure provided by the Hyak supercomputer system at the University of Washington, funded by the Student Technology Fee and the National Science Foundation MRI-1624430).EMB would like to thank Chris Mundy and Marcel Baer for guidance setting up the CP2K ensemble calculations for this system.

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Anticorrelated Contributions to Pre-Edge Features of Aluminate Near

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