Adsorption of phosphonic and ethylphosphonic acid on aluminum oxide surfaces

Phosphonic acid and ethylphosphonic acid chemisorbed on aluminum oxide surfaces have been investigated using a density-functional based tight-binding method. We have in particular focused on hydroxylated surface models based on corundum α-Al2O3 (0 0 0 1), bayerite β-Al(OH)3 (0 0 1) and boehmite γ-AlOOH (0 1 0). On these we have studied monodentate, bidentate and tridentate adsorption of the acids on all possible adsorption sites on the surfaces considering different surface coverages. By comparing the energies of the adsorption complexes we determined the favored adsorption sites for each coordination mode and surface structure. We found that the preference of an adsorption site is strongly influenced by its geometry and the regioselectivity increases when going from mono- to tridentate adsorption complexes. Moreover, the ethyl chain has no influence on the selection of the preferred adsorption sites.