Consider an initially non-hydrated gamma-alumina surface. I pass water over the surface hoping it to dissociatively adsorb on the surface giving me hydroxyl radicals adsorbed on alumina surface. What happens to the proton thus formed? How does the chemistry actually look like when this process occurs on the gamma-alumina surface? How does co-ordination number of the aluminium atoms in the lattice play a role in the process? Most studies I cam across start with something like "Imagine an alumina surface with hydroxyl ions present on the surface". I wish to know where and how do these hydroxyl ions come from.
Thanks in advance!
First of all, how do you come to the assumption that there would be hydroxyl radicals? The expectation someone should have is that O2- gets protonated forming two OH- (so both H+ are already used, plus you should refer to this as "hydroxide", not "hydroxyl" ion), no radical intermediate. The preferred coordination number of Al3+ is always 4 with oxygen, the only questionable part is with how many Al3+ have to share an oxygen atom. Concerning the source of hydroxyl groups at the initial state: unless you cleave your Al2O3 under UHV conditions, every surface atom will have "seen" more than enough water molecules from the ambient humidity, so if they want protons, they get them. You can estimate that by primitive kinetic gas theory (see https://en.wikipedia.org/wiki/Kinetic_theory_of_gases#Collisions_with_container).
Aluminium oxides are amphoteric in nature and therefore, possess a surface charge that varies with the pH, known as pH-dependent surface charge. The surface charge on the mineral surface in an aqueous medium arises due to the following five different mechanisms:
(i) The surface charge is created due to the protonation and dissociation (proton release ) of the surface functional group (reactions 1 and 2).
(ii) By the sorption or binding of ions, colloidal particles may acquire a charge. This process involves the attachment of hydrophobic species or surfactant ions on the surface by van der Waals interaction and H-bonding.
(iii) By ion replacement, the particle obtains a net permanent charge. For instance, a negatively charged framework is established when Si (IV) is replaced by Al(III) in SiO2 having a tetrahedral crystal structure:
[SiO2 + Al(III) → Si(IV) + [AlO2-]
(iv) Crystal defects in a mineral such as a defect in Al crystal can give rise to surface charge.
(v) Unsatisfied bonds at the variable charge surface by means of ionization create a surface charge exposing oxide and hydroxide ions.
Al-oxide surfaces in water are covered with hydroxyl or aqua groups (ionizable functional groups). The hydrated surface then protonates (H+ attached) giving the surface a net positive charge at low pH or deprotonates where protons are lost from the surface to produce a net negative charge at higher pH.
This can be represented by the following reactions
In acidic pH, ≡AlOH + H+ ↔ ≡AlOH2+ (1)
In basic pH, ≡AlOH ↔ ≡AlO- + H+ (2)
Upon protonation or deprotonation, the mineral surface thus possesses the electric charge in aqueous solutions. At an intermediate pH (known as the point of zero charge-pHPZC), the concentration of positive and negative surface sites is equal giving a zero surface charge (at pHPZC):
At pHPZC, [≡AlOH2+] = [≡AlO-] (3)
Below the pHPZC, the mineral is positively charged which enhances anion sorption whereas, above the pHPZC, it is negatively charged, enhancing cation sorption.
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