Reaction mechanism in fluoride adsorption to aluminum oxide

Question

Despite a fair amount of research (excluding non-open access journals, to which I have not got access), I cannot seem to find an explanation of the process that takes place when $\ce{F^-}$ adsorbs to activated alumina ($\ce{Al_2O_3}$).

I do know that the process involves both chemisorption (monolayer formation) and physisorption, but since the latter is explained solely by van der Waals forces, I am primarily looking for an explanation of the former.

EDIT: Does adsorption take place only at the aluminium ions? If not, does that mean that all aluminium compounds with oxidation state +3 will be equally good adsorbents?

EDIT: I have found an article (https://link.springer.com/article/10.1023/A:1012929900113) which proposes a mechanism that involves aluminium-fluoride complex formation. If this is the case - why is this even classed as an adsorption reaction at all, and not just a normal chemical reaction? And what role do the $\ce{OH^-}$ ions play? Do they keep the aluminum in the ionic lattice of the alumina?

Answer 1

Besides the presence of Al and O atoms on an Aluminum Oxide surface, there are also H atoms, which are bounded to the solid by OH bonds, - O --- H+ . Direct evidence for the presence of H atoms, and OH bonds, come from XPS or FTIR (grazing incidence) data. Indirect evidence comes from several sources, but especially from the reactivity of the surface to some compounds, like Silanes, where Si --- O --- R groups react readily with OH. Based on what is said above, I´d say the best explanation for the F– anion interaction with the Aluminum Oxide surface is a chemisorption process, through an indirect process, bonding of the F– anion to the H+ of the surface, forming an -O --- H+ --- F– , anion (F–) – to – dipole (+H --- O– ) complex structure. A real solid – state Hydrogen bonding.