James R. Rustad

Interaction of water with the (1×1) and (2×1) surfaces of α-Fe2O3(012)

Abstract The interaction of water with the (1×1) and (2×1) surfaces of α-Fe2O3(012) was examined with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS), low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) in the temperature range between 100 and 950xa0K. The (1×1) surface is fully oxidized and has a bulk-like concentration and structure of cation and anion sites. After vacuum annealing at 950xa0K a (2×1) pattern is observed in LEED. Although the structure of the (2×1) surface is not fully understood, it possesses a greater surface concentration of cation sites than the (1×1) surface, some of which are probably reduced. H2O adsorbs dissociatively on both surfaces as evidenced by HREELS losses at 3625 and 960xa0cm−1 due to the stretching and bending modes of terminal hydroxyl groups. These losses shift as expected for D2O. Bridging hydroxyls are also formed by proton transfer to bridging oxygen anion sites from dissociating water molecules, but have a poorly resolved O–H stretch at 3400xa0cm−1 suggesting they are involved in hydrogen-bonding interactions. Further evidence for water dissociation on both surfaces comes from isotopic scrambling of oxygen between these hydroxyls and the 18 O -enriched surfaces. Although both surfaces dissociate water, the structural differences between the (1×1) and (2×1) surfaces result in different ratios of molecular-to-dissociative water. Terminal hydroxyls occupy roughly 6×1014 sitesxa0cm−2 on the (1×1) surface, but only about 4.5×1014 sitesxa0cm−2 on the (2×1) surface. The balance of available cation sites bind molecular water that evolves in TPD below 300xa0K from either surface. This molecular water is readily detected in both HREELS and SSIMS. The surface structure also influences the hydroxyl recombinative desorption kinetics. Terminal and bridging hydroxyls recombine to liberate water in TPD at 350xa0K from the (1×1) surface and at 405xa0K from the (2×1) surface. The recombinative desorption state of water at 350xa0K from the (1×1) surface exhibits first-order desorption kinetics with an activation energy of about 120xa0kJxa0mol−1 and a pre-exponential of 1×1017xa0s−1. The first-order behavior and the high pre-exponential suggests that recombinative desorption from the (1×1) surface involves pairing of bridging and terminal hydroxyl groups. In contrast, recombinative desorption from the (2×1) surface is pseudo-zeroth order in appearance suggesting that hydroxyls are bound in one-dimensional arrays with desorption occurring preferentially at the ends of each array.

Molecular modeling of the surface charging of hematite: II. Optimal proton distribution and simulation of surface charge versus pH relationships

A parameterized classical potential model for the interaction of water and hydroxide with iron oxide was used to calculate the optimal proton arrangement and proton binding energies on the (012) surface of hematite. Energy minimization calculations with the parameterized potential model indicate that approximately 75% of adsorbed water molecules are dissociated on this surface, in agreement with recent TPD and HREELS measurements. Surface protonation/deprotonation energies were calculated from the predicted optimal arrangement of protons on the neutral (012) surface. A supercell geometry with translational symmetry in two dimensions and finite in the third dimension (2-D PBC) was assumed. The calculated surface protonation energies were then used to model the experimentally observed surface-charging curve of hematite in aqueous solution. Excellent agreement was found between the calculated and measured surface charge for ionic strengths ranging from 0.001 to 0.1 M. Our calculations favor the value of 8.5 for the pH of zero charge of hematite over the more recent result of 6.7.

Interaction potential of Al3+ in water from first principles calculations

We present a parametrization of the interaction potential for Al3+ in water from first principles calculations. We have performed a critical study of the Al3+–water interaction using sequences of correlation consistent basis sets that approach the complete basis set limit and include core-valence correlation effects. We suggest as minimum theoretical requirements treatment of the electron correlation at the MP2 level of theory using a triple zeta quality basis set that accounts for the effect of core-valence correlation. The latter amounts for an increase of ∼5 kcal/mol (3%) to the stabilization energy, a shortening of 0.015 A in the Al–O distance, and an increase of 22u2009cm−1 in the harmonic frequency of the Al–O vibration. This is the first time that core-valence effects were investigated for this system. The stabilization energy of the Al3+(H2O) cluster is 201 kcal/mol and the corresponding Al–O bond length is 1.719 A at the MP2 level of theory with the cc-pwCVQZ basis set. This minimum is metastable wit...

Molecular simulation of the magnetite-water interface

This paper reports molecular dynamics simulations of the magnetite (001)-water interface, both in pure water and in the presence of a 2.3 molal solution of NaClO4. The simulations are carried out using a potential model designed to allow the protonation states of the surface functional groups to evolve dynamically through the molecular dynamics trajectory. The primary structural quantities investigated are the populations of the surface functional groups, the distribution of electrolyte in the solution, and the surface hydrogen bonding relationships. The surface protonation states are dominated by extensive hydrolysis of interfacial water molecules, giving rise to a dipolar surface dominated by FeOH2+-OH2-OH− arrangements. Triply coordinated, more deeply buried, surface sites are inert, probably due to the relative lack of solvent in their vicinity. The electrolyte distribution is oscillatory, arranging preferentially in layers defined by the solvating water molecules. The presence of electrolyte has a negligible effect on the protonation states of the surface functional groups. Steady-state behavior is obtained for the protonation states of the surface functional groups and hydrogen-bonding network. Although the overall structure of the electrolyte distribution is fairly well established, the electrolyte distribution has not fully equilibrated, as evidenced by the asymmetry in the distribution from the top to the bottom of the slab.

Ab initio quantum mechanical modeling of infrared vibrational frequencies of the OH group in dioctahedral phyllosilicates. Part II: Main physical factors governing the OH vibrations

Abstract The physical factors responsible for the variability observed in OH infrared (IR) fundamentals in dioctahedral phyllosilicates, due to octahedral substitution of Al3+ by Mg2+, Fe2+, and Fe3+, are discussed here. The data analyzed consist of experimental frequencies as well as frequencies modeled using Density Functional Theory (DFT) calculations. The charge of the octahedral cations surrounding the OH is one of the main factors affecting both the OH stretch and the in-plane bend; cationic electronegativity and ionic radius play important roles in the stretch and bend modes, respectively. The mass of the octahedral cations does not affect the OH fundamental vibrations. The nature of the octahedral cations alone can explain most of the variability observed in the OH in-plane bend, making this fundamental vibration the most suitable for assessing octahedral composition. Discrepancies between modeled and experimental OH stretch frequencies indicate the existence of other factors governing this fundamental vibration. Further DFT calculations indicate that apical O atoms of the tetrahedral sheet with unsatisfied charges due to octahedral and/or tetrahedral substitutions can explain these discrepancies. The modeling results are utilized to predict the frequency of the OH stretch and in-plane-bend combination band that occurs near 4545 cm-1 (2.2 μm) in phyllosilicates. This band can be observed in imaging spectrometer data, allowing for the detection and analysis of phyllosilicates and other minerals in large natural systems. The modeling results confirm that the variability observed in the combination band of dioctahedral phyllosilicates reflects octahedral and, to a certain degree, tetrahedral composition, but not interlayer composition.

Molecular modeling of the surface charging of hematite: I. The calculation of proton affinities and acidities on a surface

Calculation of the energy of a charged defect on a surface in supercell geometry is discussed. An important example of such a calculation is evaluation of surface proton affinities and acidities, as adding or removing a proton creates a charged unit cell. Systems with periodic boundary conditions in three spatial directions and a vacuum gap between slabs are demonstrated to be inadequate for unit cells having non-zero ionic charge and uniform neutralizing background. In such a system the calculated energy diverges linearly with the thickness of the vacuum gap. A system periodic in two directions and finite in the direction perpendicular to the surface (2-D PBC) with the neutralizing background distributed as the surface charge density is free from this problem. Furthermore, the correction for the interaction of the charged defect with its own translational images is needed to speed up the convergence to the infinite dilution limit. The expression for the asymptotic correction for the energy of interaction of a charged defect with its translational images in 2-D PBC geometry has been developed in this study. The asymptotic correction is evaluated as the interaction energy of a 2-D translationally periodic array of point charges located above and below the plate of non-uniform dielectric. This is a generalization of the method of M. Leslie and M.J. Gillan [J. Phys. C, 18 (1985) 973] for the calculation of the energy of a charged defect in bulk crystals. The usefulness of this correction was demonstrated on two test cases involving the calculation of proton affinity and acidity at the (012) surface of hematite. The proposed method is likely to be important in ab initio calculations of the energy effect of the surface protonation reactions, where computational limitations dictate a small size for the unit cell.

A molecular dynamics investigation of surface reconstruction on magnetite (001)

Abstract Molecular dynamics calculations using analytical potential functions with polarizable oxygen ions have been used to identify a novel mode of reconstruction on the half-occupied tetrahedral layer termination of the magnetite (Fe 3 O 4 ) (001) surface. In the proposed reconstruction, the twofold coordinated iron ion in the top monolayer rotates downward to occupy a vacant half-octahedral site in the plane of the second-layer iron ions. At the same time, half of the tetrahedral iron ions in the third iron layer are pushed upward to occupy an adjacent octahedral vacancy at the level of the second-layer iron ions. The other half of the third-layer iron ions remain roughly in their original positions. The proposed reconstruction is consistent with recent low-energy electron diffraction and X-ray photoelectron spectroscopy results. It also provides a compelling interpretation for the arrangement of atoms suggested by high-resolution scanning-tunneling microscopy studies.

Intrinsic acidity of aluminum, chromium (III) and iron (III) μ3-hydroxo functional groups from ab initio electronic structure calculations

Density functional calculations are performed on M{sub 3}(OH){sub 7}(H{sub 2}O){sub 6}{sup 2+} and M{sub 3}O(OH){sub 6}(H{sub 2}O){sub 6}{sup +} clusters for M {double_bond} Al, Cr(III), and Fe(III), allowing determination of the relative acidities of the {mu}{sub 3}-hydroxo and aquo functional groups. Contrary to previous predictions and rationalizations, {double_bond}Fe{sub 3}OH and {double_bond}Al{sub 3}OH groups have nearly the same intrinsic acidity, while {double_bond}Cr{sub 3}OH groups are significantly more acidic. The gas-phase acidity of the Fe{sub 3}OH site is in good agreement with the value predicted by the molecular mechanics model previously used to estimate the relative acidities of surface sites on iron oxides. Acidities of aquo functional groups were also computed for Al and Cr. The {double_bond}AlOH{sub 2} site is more acidic than the {double_bond}Al{sub 3}OH site, whereas the {double_bond}Cr{sub 3}OH site is more acidic than the {double_bond}CrOH{sub 2} site. These findings predict that the surface charging behavior of chromium oxides/oxyhydroxides should be distinguishable from their Fe, Al counterparts. The calculations also provide insight into why the lepidocrocite/boehmite polymorph is not observed for CrOOH.

Ab initio quantum mechanical modeling of infrared vibrational frequencies of the OH group in dioctahedral phyllosilicates. Part I: Methods, results and comparison to experimental data

Abstract The infrared (IR) spectra of small clusters of atoms ([MM(OH)2] and [MM(OH)2 (H2O)6], where M, M = Al3+, Mg2+, Fe2+, Fe3+) mimicking the environment of the OH group in dioctahedral phyllosilicates have been modeled using ab initio quantum mechanical calculations. These modeling results are relevant to establishing the connections between IR spectra of phyllosilicates and their composition, and to investigate the utility of quantum mechanical models for calculating IR frequencies of minerals. This study focused on the OH stretch and in-plane bend fundamentals, because they give rise to a combination band near 4545 cm-1 (2.2 μm) that can be observed in imaging spectrometer or hyperspectral remote sensing data. A comparison among results obtained using both ab initio [Hartree-Fock (HF) and Density Functional Theory (DFT)], and semi-empirical [PM3(tm)] methods, showed that the DFT model approaches IR frequency experimental values most closely. IR spectra of phyllosilicates were modeled using the DFT method. The modeled frequencies were scaled using a mode-dependent linear transformation, and experimental frequencies were reproduced satisfactorily. The modeling results show that most of the variability observed in the OH in-plane bend fundamental of dioctahedral phyllosilicates can be explained by the effects of neighboring octahedral cations alone. Discrepancies between modeling and experimental results in the case of the OH stretch point to the existence of factors other than the nature of the neighboring octahedral cations, such as tetrahedral substitution, affecting this fundamental mode

Ab initio investigation of the structures of NaOH hydrates and their Na+ and OH− coordination polyhedra

Abstract Plane-wave pseudopotential density functional methods using the Perdew-Burke-Ernzerhof exchange- correlation functional were used to investigate theoretically the structures of five NaOH hydrate phases through optimization of lattice parameters and atomic coordinates. Although all the calculations were carried out with P1 symmetry, we find in four of the five cases that the experimentally determined space group is maintained to high accuracy. Particular focus is placed on the coordination environments of Na+ and OH-. The Na-O distances are, in general, overestimated; however, the sodium ion coordination polyhedra are well reproduced by the theoretical calculations, including the fivefold coordinated sodium atom in the α-NaOH·4H2O structure. The theoretical calculations correctly predict that α-NaOH·4H2O is lower in energy than the metastable β-NaOH·4H2O phase; thus, the a phase is stable even in the absence of proton disorder. The octahedral coordination environment around OH- is calculated accurately, including the distances of the weak OH--OH2 hydrogen bonds in which the hydroxide ion acts as the proton donor. This work provides further evidence of the reliability of the Perdew-Burke-Ernzerhof exchange-correlation functional in hydrogen bonded systems, providing a direct, unambiguous test of the elusive hydroxide-water interaction.