Mobile v. Adsorbed Hydroxyl Radical

[citw]

In the linked article,

http://pubs.acs.org/doi/abs/10.1021/jp036735i

the authors describe the generation of "free" or "mobile" OH radicals on surface fluorinated TiO₂,

$$\text{Ti}-\text{F}+\text{H}_2\text{O}~(\text{or }\text{OH}^-)+h_\text{vb}^+\longrightarrow \text{T}-\text{F}+\text{OH}^{\cdot}_\text{free}+\text{H}^+$$

and adsorbed OH radicals on naked TiO₂,

$$\text{Ti}-\text{OH}+h_\text{vb}^+\longrightarrow\text{Ti}-\text{OH}^{\cdot+}$$

The authors also go on to say that surface fluorination inhibits hole transfer paths because "... adsorption or complexation is prohibited on F-TiO₂." If this is true, how does the first equation work? In other words, if OH^- or H₂O isn't adsorbed on F-TiO₂, how is the OH radical formed? Doesn't the hole transfer have to occur on the surface? How can the hole oxidize OH^- to the OH radical if it's never adsorbed?

 

[jbsteele]

The authors explain that the OH radicals are formed through a two-step reaction. In the first step, H2O molecules (and/or OH-) become adsorbed on the surface of the F-TiO2, where they are oxidized by hole transfer. Then, in the second step, the oxidized species can desorb from the surface as the free OH radical. Thus, while adsorption is prohibited on F-TiO2, the reaction still proceeds due to the intermediate adsorption and oxidation of H2O (or OH-).