A study of oxygen vacancy formation and annihilation in submonolayer coverages of TiO₂ dispersed on MCM-48

The reduction and reoxidation of submonolayer coverages of TiO₂ deposited onto MCM-48 were investigated. The deposited TiO₂ was characterized by Raman and UV-visible spectroscopy. Raman spectra show that Ti atoms are bonded to the silica support by Ti-O-Si bonds and that crystalline TiO₂ is not formed. The results of the Raman and UV-visible spectroscopy suggest that the dispersed TiO₂ is present as two-dimensional oligomeric structures. Reduction in H₂ at 923 K 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 is attributed to the ease with which O atoms can be removed from the TiO ₂ overlayer as the size of the titania patches increases. The amount of oxygen removed during reduction was quantified by pulsed reoxidation. It was observed that the temperature required for complete reoxidation decreased with increasing surface coverage of the silica support by TiO₂. This trend is explained with a proposed model of the reoxidation process, in which the rate limiting step is 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 TiO ₂/SiO₂ after temperature-programmed oxidation of methanol.