The reduction and re-oxidation of sub-monolayer coverages of TiO2 deposited onto MCM-48 were investigated. The deposited TiO2 was characterized by Raman and UV-visible spectroscopy. Raman spectra showed that Ti atoms were bonded to the silica support by Ti-O-Si bonds and that crystalline TiO2 was not formed. The results of the Raman and UV-visible spectroscopy suggested that the dispersed TiO2 was present as two-dimensional oligomeric structures. Reduction in H2 at 923K produces Ti3+ cations observable by EPR (g = 1.932), suggesting the formation of oxygen vacancies. The fraction of Ti that could be reduced increased with TiO2 surface concentration. This observation was attributed to the ease with which O atoms could be removed from the TiO2 overlayer as the size of the titania patches increased. The amount of oxygen removed during reduction was quantified by pulsed re-oxidation. It was observed that the temperature required for complete re-oxidation decreased with increasing surface coverage of the silica support by TiO2. This trend was explained with a proposed model of the re-oxidation process, in which the rate limiting step was the migration of peroxide species through or between the deposited TiO2 patches. A linear correlation was established between the intensity of the EPR signal for Ti3+ and the amount of oxygen removed from TiO2/SiO2. This relationship was then used to determine the oxygen vacancy concentration present on the surface of TiO2/SiO2 after temperature-programmed oxidation of methanol.

A Study of Oxygen Vacancy Formation and Annihilation in Submonolayer Coverages of TiO2 Dispersed on MCM-48. Strunk, J., Vining, W.C., Bell, A.T.: Journal of Physical Chemistry C, 2010, 114[40], 16937-45