Adsorption of Methane on the (100) Surface of MgO: Insight into Surface-Adsorbate and Adsorbate-Adsorbate Interactions f
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0928-GG13-02
Adsorption of Methane on the (100) Surface of MgO: Insight into Surface-Adsorbate and Adsorbate-Adsorbate Interactions from First-Principles Calculations Michael L Drummond1, Bobby G Sumpter1, William A Shelton, Jr. 1, and John Z Larese2 1 Computer Science and Mathematics Division, Oak Ridge National Laboratory, Bethel Valley Road, P.O. Box 2008, Bldg. 6012, Oak Ridge, TN, 37831-6367 2 Department of Chemistry, University of Tennessee, Knoxville, 319 Buehler Hall, 1420 Circle Dr., Knoxville, TN, 37996-1600
ABSTRACT First principles calculations using density functional theory (DFT) are reported for two layers of methane adsorbed on the (100) surface of MgO. The lowest energy structure has a first layer with C2v methane adsorbed above magnesium, hydrogens pointed toward neighboring oxygen atoms, and a rotation of 90° in between each neighboring methane. The second methane layer has a similar structure, except the hydrogens are directed toward nearest neighbor magnesiums. It is found that the structure of the first layer has a large effect on the relative energies of proposed bilayer structures, as does the calculated separation between the two layers. Competing roles of surface-adsorbate and adsorbate-adsorbate interactions are also discussed. INTRODUCTION The behavior at the interface of an extended solid surface and an adsorbed gaseous phase is an area of intense research, owing to its relevance in diverse phenomena such as heterogeneous catalysis, corrosion, electronics, and adhesion. The resultant system depends primarily on the relative strengths of surface-adsorbate and adsorbate-adsorbate interactions, and a full description of the interface therefore requires a proper understanding of each. To this end, our research has involved thorough investigations of the adsorption of CH4 onto the (100) face of MgO, using both first-principles density functional theory (DFT) [1] and a variety of experimental techniques, principally inelastic neutron scattering (INS) [2-3]. This solid/gas combination serves as a prototypical surface, as MgO(100) has a simple (both conceptually and computationally) square lattice, and as methane is the simplest hydrocarbon – a class of chemicals whose adsorption properties are obviously of great interest. Furthermore, the closeness in the lattice parameters of both solids (4.18 Å for methane [4] and 4.21 Å for MgO [5]) suggests the possibility of commensurate interactions. We have previously described, using DFT [1], the structure of a monolayer of CH4 on the MgO(100) surface. In the present study, we extend this DFT-based description to a bilayer of CH4. After describing the computational methods used, salient details of the monolayer structure are briefly recapitulated. The structural and energetic details of the bilayer will then be given, followed by a discussion of the relative strengths of surface-adsorbate and adsorbate-adsorbate interactions, as well as the effect of adding more layers on these interactions. Finally, conclusions and areas of ongoing and future researc
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