Alcohol Dissociation at Oxygen Vacancies

Like water, the interaction of 02 with Ti02 also has implications for photocatalysis. As with water again, reactions of 02 with Ti02 are also important because 02 will form part of the environment in many Ti02 applications. 

It has been known for some time that the spectroscopic signature of Ob-vacs can be healed by exposure to 02 [42-46], In addition, Epling etal. [47] show that temperature-programmed desorption (TPD) spectra of water and ammonia are perturbed when the surface is predosed with 02. This implies that oxygen is left on the surface in some form when Ob-vacs are healed by 02, As such, Epling et al. proposed that one Ob-vac is healed per 02 molecule with the other O atom being adsorbed at a Ti5c site (Oad), a dissociation mechanism supported by theoretical calculations [48, 49]. 

This suggests that 02 dissociates in the Ob-vacs, one O atom filling the vacancy and the other forming Oad on a nearby Ti5c site. Wendt et al. [18] performed similar experiments at low temperature, showing that 02 dissociates in Ob-vacs at least down to 120 K. 

Detailed analysis of the Oad positions following dissociation at room temperature shows that most Oad are found one lattice constant (in the [0 0 1] direction) away from the Ob-vac that is filled, the rest being immediately adjacent and two lattice constants away [50]. As there is little thermal diffusion of Oad on Ti02(l 1 0), the separation of Oad from the closest positions to the reacting Ob-vacs was attributed to the energy released during the exothermic dissociation of 02 (calculated at 3.5 eV [18]) in a similar way to that observed, for example, for Cl2 dissociation on Ti02(l 1 0) [51], 

The reactivity of Ti02 with alcohols is important in a number of technological applications. For example, alcohols can be used as energy carriers in renewable sources and they are also employed as model pollutants so that environmental cleaning strategies can be tested. 

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