J. Phys. Chem. 1996, 100, 16989-16995
16989
Structure and Dynamics of the Water/MgO Interface Maureen I. McCarthy,* Gregory K. Schenter, Carol A. Scamehorn, and John B. Nicholas EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, MS K1-96, POB 999, Richland, Washington 99352 ReceiVed: May 13, 1996; In Final Form: July 18, 1996X
A pairwise additive potential energy expression for the water/MgO interaction was obtained by fitting the parameters to ab initio electronic structure energy data, computed using correlation-corrected periodic HartreeFock (PHF) theory, at selected adsorbate/surface geometries. This potential energy expression was used in molecular dynamics and Monte Carlo simulations to elucidate the water/MgO interaction. Energy minimization reveals a nearly planar adsorbate/surface equilibrium geometry (-15° from the surface plane with the hydrogens pointing toward the surface oxygens) for an isolated water on perfect MgO (001), with a binding energy of 17.5 kcal/mol; subsequent PHF calculations on this system confirmed that this is a potential minimum. Rate constants for desorption (kdsorb), intersite hopping (khop), intrasite rotation (krot), and intrasite flipping (kflip) were estimated for an isolated water on the surface using simple transition state theory. The computed rates (at T ) 300 K) are kdsorb ) 1.1 × 105 s-1, khop ) 3.7 × 1010 s-1, krot ) 5.7 × 1011 s-1, and kflip ) 4.6 × 1011 s-1. The motion of a single water on the surface is described by an effective diffusion constant (Deff ) 8.0 × 10-6 cm2/s), computed from the surface rate constants combined with Monte Carlo simulations. The structure of the liquid water/MgO interface was determined from simulations with 64 and 128 water molecules on the surface. Simulations (at T ) 300 K) of the two-dimensional water overlayers reveal a densely packed first layer, Z(Ow-surf) ) 2-3 Å, with one water per surface magnesium, with a nearly equal distribution of water molecules aligned -17° and +30° with respect to the surface plane. A more diffuse second layer exists, Z(Ow-surf) ) 4-5.5 Å, with a much broader distribution of water angular orientations with respect to the surface plane. The region Z(Ow-surf) > 6 Å resembles bulk water, with the density profile approaching a constant as a function of distance above the surface and a uniform distribution in water/surface angular orientations. At the water/vacuum interface (top of the multilayer) the waters assume a “planar orientation” (0° with respect to the surface plane). During the timescale of these simulations very little interlayer exchange of water molecules occurs between the first monolayer (n ) 1) and the additional overlayers (n g 2). In contrast, the water molecules in the multilayers (n g 2) display motion similar to bulk liquid water at this temperature.
I. Introduction Understanding the chemistry of water-oxide interfaces is crucial for modeling a variety of industrial and environmental processes. Many oxide surfaces function either as catalysts or as supports for heterogeneous metal catalysts.1,2 Depending on the system, reactions with water may impede or enhance the catalytic properties of these materials. In addition, watermineral oxide interface chemistry is critical in determining the hydrodynamic properties of the Earth’s subsurface.3-6 Magnesium oxide (MgO) is a known catalyst and a fundamental component of many minerals found in the subsurface. It is commonly used as a model system for understanding interfacial processes on oxide materials. Several experimental2,7-21 and theoretical22-30 studies have investigated the properties of the water/MgO interface. The diversity in the empirical data probing the nature of this interface indicates that MgOslike many metal oxidessis a “structure sensitive” material whose properties are very dependent on the experimental approach and sample preparation procedures used. Processes such as molecular adsorption and chemidissociation at the water/MgO interface are very sensitive to the local atomic structure of the material. Water chemidissociation has been shown to be particularly sensitive to the presence of lowcoordination defect sites, i.e. step edges and corners. Most experimental methods probe numerous surface sites simultaX
Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01373-1 CCC: $12.00
neously; hence the observed empirical data represent configuration-averaged quantities. Earlier theoretical studies investigated the structure and energetics of water on MgO (001) at several well-defined surface sitessflat surface sites, step edges, and cornerssusing ab initio periodic Hartree-Fock (PHF) theory.23,29,30 Those studies revealed that at low coverages water is molecularly physisorbed but not dissociated on perfect MgO (001). The presence of low-coordinated defect sitessstep edges and corners, 4-foldand 3-fold-coordinated, respectivelysallowed the chemidissociation channel to become energetically accessible. At corner sites, which occur in low concentrations on most surfaces, surface hydroxylation (resulting from chemidissociation of water) is the energetic minimum configuration. In contrast, at the more prevalent edge sites the molecular absorption and chemidissociated states are nearly isoenergetic. For many oxides, including MgO, the similarity of the hydroxylation and adsorption energetics makes it very difficult to deduce the interfacial structure using experimental methods that only probe adsorbate/surface binding energies. This study uses the data from our earlier work to examine the dynamics of water on a perfect MgO (001) surface. The object of this work is to model processes that can be observed on “nearly flat” MgO (001), e.g. on surfaces containing large step plateaus and low defect concentrations. The minimum energy adsorbate/surface geometry was determined and the rates for surface hopping, desorption, and intrasite rotation and © 1996 American Chemical Society
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flipping were computed from transition state theory (TST) coupled to Monte Carlo simulations for an isolated water molecule on the surface. The parameters of the potential energy expression used in the dynamics simulations were fit to the original ab initio data. A pairwise additive form of the potential was chosen, so it could readily be incorporated into classical simulations of the water/MgO interface dynamics. In addition to the isolated water/surface dynamics, the structure and dynamic properties of the liquid water interface were simulated. II. Method A. Potential Energy Surface. The structure and energetics of the water/MgO interface were previously investigated23,29,30 using periodic-Hartree-Fock (PHF) theory31 and correlationcorrected PHF theory,32-34 as implemented in the program CRYSTAL92.35 In our quantum mechanical calculations the MgO (001) surface was modeled as a 2-D slab, periodic in x and y.23,29,30 A three-layer slab (along the z-axis) was used to represent the (001) surface because it is the minimum thickness slab in which the charge density of the middle layer resembles that found in the bulk material. A mixed split valence set (861G on magnesium and 8-51G on oxygen) was used for MgO,36 and the standard Pople 6-31G* set37 was used for water. We computed binding energies for 10 geometries of water over the MgO (001) surface, computed at 0.25 monolayer surface coverage.23,29,30 These calculations indicated that water is weakly bound (physisorbed) to MgO (001) and the interaction energy is very dependent on the nature of the surface site. For the 10 adsorbate/surface geometries probed, the lowest energy was found when the water dipole is parallel to the surface plane and the water oxygen is over a surface magnesium.23 The binding energies of these configurations ranged from 4.1 to 9.7 kcal/mol at the PHF level and 6.3 to 17.0 kcal/mol when correlation corrections were included. (n. b. Differences between the reported binding energies and the well-depths shown in Figure 2 are due to intermolecular interactions between waters at the fixed coverage of 0.25 monolayer coverage.) The correlation-corrected energies are based on an a posteriori correction to the PHF energies, estimated from the ColleSalvetti correlation functional of the charge density.34 A quantitative assessment of the accuracy of the correlationcorrected PHF approach is difficult due to the disparity in empirical data for these types of systems. Experimental measurements of adsorbate/surface binding energies are difficult and usually indirect. However, empirical estimates of the water/ MgO (001) binding energy are in the range 14-17 kcal/mol,2,21 consistent with the correlation-corrected values. The present work uses the correlation-corrected PHF data to fit the force-field parameters described by the potential energy expression:
V)∑ i 5 Å for the 64-water simulation (Figure 8c-d) and Z > 13 Å for the 128-water run (Figure 9g-h)sthe distribution is peaked near cos(θ) ) 0 (planar molecules) with very little intensity at cos(θ) ) (1. This tendency of the water dipoles to align parallel to the surface plane has also been observed in simulations of the vacuum/water interface.48 The density profiles and angular distributions shown in Figures 7-9 indicate that the influence of the surface on the structure of the water multilayers diminishes with distance. The first two monolayers of water on this surface form the “interface”, and subsequent increases in surface coverage produce a “liquid-like” overlayer. At lower temperatures these overlayers are expected to be more “ice-like”, either amorphous or crystalline. Because the forces at the vacuum/water interface will influence the angular distributions of the outermost layer, greater than two monolayers of coverage are required to produce bulk water-like properties in the overlayers. The near zero density regime around Z ) 3 Å indicates that there is little interchange of water molecules between the first and second layers of the interface. Strong water-surface interactions (compared to water-water binding) reduce the tendency for the waters to interchange or mix between the first monolayer and the additional layers. In contrast, the interlayer mixing in the liquid overlayers (n g 2) is more characteristic of liquid water.
Structure and Dynamics of the Water/MgO Interface IV. Conclusions This study used ab initio quantum mechanical data to fit a pairwise additive potential energy expression for the interaction of water with MgO (001). This potential was used in classical mechanics simulations to compute the structural and dynamical properties of an isolated water and water multilayers on defect-free MgO (001). At equilibrium, an isolated water is aligned nearly parallel to the surface plane with the hydrogens pointing toward the surface oxygens. The binding energy in the minimum energy configuration is 17.5 kcal/mol. The predicted geometry and binding energy are consistent with recent experiments. The eigenfrequencies at the stationary points on the potential energy surface were computed and used in simple transition state theory to obtain the rate constants for intersite hopping, intrasite rotation and flipping, and desorption. Intersite hopping is the ratelimiting step for diffusion on this surface, and the computed effective diffusion constant is Deff ) 8.0 × 10-6 cm2/s. The desorption rate is around 105 times slower than the diffusion rate, indicating that adsorbed water molecules are very mobile on MgO (001). Empirical measurements of this observable will be effected by the presence of surface defects and collective effects due to other coadsorbed water molecules. Simulations of water multilayers on MgO (001) reveal a tightly bound monolayer at the interface with approximately one water per surface magnesium. In this first monolayer (Z ) 2-3 Å) the waters assume a staggered configuration with nearly equal probability of the hydrogens pointing toward and away from the surface. A less dense second layer exists between 4 and 6 Å, with a much broader angular distribution. Above the second layer the water molecules become more “liquid-like” with near constant density (as a function of distance above the surface plane) and approach an isotropic angular distribution. An “interface” region can be defined that includes the first two monolayers, beyond which the surface has little influence on the structure of the water overlayers. Acknowledgment. Pacific Northwest National Laboratory is a multiprogram laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC06-76RLO 1830. The authors were supported by the Divisions of Chemical Sciences and Geosciences of the Office of Basic Energy Sciences, U.S. Department of Energy, and (CAS) by the Northwestern College and University Association for Science (Washington State University) under Grant No. DEFG06-89ER-75522 with the Department of Energy. We would like to thank the Scientific Computing Staff, Office of Energy Research, U.S. Department of Energy, for a grant of computing time at the National Energy Research Supercomputing Center. We gratefully acknowledge helpful discussions and consultations with Prof. D. Wayne Goodman (Texas A&M University) and Drs. Stephen Joyce and Bruce Kay of PNNL. References and Notes (1) Gates, B. C. Catalytic Chemistry; John Wiley and Sons: New York, 1992). (2) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, England, 1994. (3) Hochella, M. F.; White, A. F. Mineral-Water Interface Geochemistry; Mineralogical Society of America: Washington, DC, 1990. (4) Sposito, G. The Surface Chemistry of Soils; Oxford University Press: Oxford, 1984. (5) Greenland, D. J.; Hayes, M. H. B. The Chemistry of Soil Constituents; John Wiley and Sons: New York, 1978. (6) Stumm, W. Chemistry of the Solid-Water Interface: Processes at the Mineral-Water and Particle-Water Interface in Natural Systems; John Wiley and Sons: New York, 1992.
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