A quantum chemical cluster study of hydrated halide adsorption on the cathodic Al(111) surface

Ab-initio cluster calculations are used to simulate water, fluorine and iod ine adsorption on a negatively charged Al(111) surface. In contrast to our earlier work using neutral Al clusters, we determine the water to be only w eakly adsorbed above the negatively charged Al clusters, with the water H a toms being closest to the metal surface. A II-bond network is readily forme d when more than one water molecule is adsorbed on the Al cluster surface. Analogous to the recent in-situ surface X-ray scattering experiments on Ag( lll) surfaces, we find the separation between the water and the cathodic su rface to be approximately 1.5 times greater than that found previously for the neutral Al(111) surface. In addition, there is a strong repulsion preve nting the water molecules from being closer than 3.0 Angstrom to the negati vely charged surface. For the halides, in line with gas-phase adsorption ex periments and other calculations, we find that fluorine is much more strong ly bound to the Al clusters than iodine, with the Al(111) atop site being t he most favored surface site for both halides. By performing calculations o n Al clusters with a halide ion and one or more water molecules coadsorbed, we are able to develop an explanation as to why solvated iodine is more re adily able to specifically adsorb on a cathodic surface than fluorine. The larger atomic size of iodine enables it to adsorb on the cathodic Al(111) s urface at a higher Vertical height than fluorine. Water molecules can then bond to iodine without being drawn into the region of repulsive interaction from the negatively charged surface. Thus we find the adsorption energy fo r I-.(H2O)(3) adsorbed on A(19)(-) to be very similar to the I- adsorption energy, suggesting that iodine can be specifically adsorbed on the cathodic Al(111) surface without destabilizing any coadsorbed water molecules, wher eas any water molecules hydrogen-bonding to fluorine are pulled towards the Al(111) surface and destabilized when the fluorine atom is also bonded to a surface Al atom. As a consequence, even though we find for the clusters w ith F-.(H2O)(3) adsorbed on Al-19(-) that the Al-F bond is still formed, th e calculations show the Al-F bond to be now severely strained. (C) 1999 Pub lished by Elsevier Science B.V. All rights reserved.