PHYSICAL ADSORPTION OF GASES ON IONIC-CRYSTALS - ADSORPTION POTENTIAL OF CO2 ON (100) NACL

An empirical adsorption potential VA of CO2 on the (100) surface of NaCl is reported expressing the solid molecule potential as the sum of the dispersion, repulsion and Coulomb interaction for all atom pairs and of the induction interaction. VA(ϑ,φ, x, y, z) was calculated for 20 orientations (ϑ,φ) of the molecule translating above the surface unit cell. Perspective three-dimensional plots of the adsorption potential, around the important sites with Gzv and Civ symmetry, respectively, and for the migration of the molecule parallel to the surface are displayed. Among the numerous potential minima distributed in a narrow energy range, the lowest one is found to possess Czv site symmetry, CO2 being parallel to the surface above the cell center. The corresponding adsorption potential energy reproduces the isosteric heat of adsorption at zero coverage (- 6.07 kcal/mol), which had been obtained by Hayakawa from measured adsorption isotherms. From the calculated frequencies of all the external vibrational normal modes and the frequency shifts of the internal modes the contributions of the zero point energy to the energy of adsorption were determined for the sites c2v and With increasing distance from the surface the electrostatic potential falls off exponentially. The van der Waals potential on the other hand decays more slowly and becomes of greater importance than the electrostatic at long distances, an exchange of their usual roles. In surface migration the molecule favors a translation-rotation along the line above the positive ions, whereby the site symmetry changes from C2v to dv. This preliminary conclusion inferred from the potential surface is found to be in agreement with the result of a normal coordinate analysis. The dispersion and repulsion potential coefficients for carbon and oxygen atoms in carbon dioxide and the ions in the (100) face of all NaCl-type alkali halide crystals are calculated. Whereas the dispersion coefficients, obtained in the Kirkwood-Müller-approximation, depend only on the properties of the atoms and ions and can be applied for any face, the repulsion coefficients were merely calculated for the (100) face of the crystals by minimization of the interaction energies at distances given by the van der Waals and ionic radii and a small parameter. The potential provides the basis for the determination of the equilibrium configurations, adsorption energies, molecular migrations on the surface, vibrational shifts and splittings, thermodynamic quantities of the above class of systems and for the recognition of relationships between them. © 1978, Walter de Gruyter. All rights reserved.