A Study of Oxygen Vacancy Formation and Annihilation in Submonolayer Coverages of TiO₂ Dispersed on MCM-48

Abstract

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 TiO2 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 TiO2/SiO2 after temperature-programmed oxidation of methanol.

Features of Surface Structures of Alumina and Titanium Dioxide Nanoparticles Produced Using Different Synthesis Methods

“…For sample (9) (Figure 7), two absorption bands can be clearly identified with peaks at 30,000 and 35,500 cm −1 . According to [17,18], these bands can be attributed to oxygen defects (O 2 − ), which may appear in the TiO 2 structure, depending on the method of sample synthesis. The analysis of the intensity of the absorption bands at 30,000, 35,500, and 12,700 cm −1 in the UV-Vis-DRS of all the samples showed that as the number of Ti 3+ cations with coordination numbers less than 6 increased, the number of oxygen defects (O 2 − ) in the titanium dioxide structure decreased.…”

“…It is well known that TiO 2 is an indirect-gap semiconductor [18]. It is characterized by a fundamental absorption edge (or band gap, ).…”

High dielectric constant nickel-doped titanium oxide films prepared by liquid-phase deposition

“…In this study, the crystallization and electrical characteristics of Ni-doped LPD-TiO 2 films grown on p-type (100) silicon substrate were investigated. [17]. Moreover, LPD-TiO 2 has more concentration oxygen vacancy than that by other deposition methods (sol-gel, magnetron sputtering and metal organic chemical vapor deposition) due to high concentration of water molecules examined by FTIR in LPD-TiO 2 film [9].…”

Layered sphere-shaped TiO 2 capped with gold nanoparticles on structural defects and their catalysis of formaldehyde oxidation

“…Crystal steps and edges are important defect features in TiO 2 surfaces (Derdour and Skliar, 2012) and are involved in several processes, including nucleation of Au particles and adsorption of molecules. The relatively low density of point defects, such as surface oxygen vacancies on TiO 2 , which play an important role in catalytic oxidation (Strunk et al, 2010;Namai and Matsuoka, 2005), is also related to these perimeter sites (Fujitani and Nakamura, 2011). Both theoretical calculations and experimental results concerning the effects of oxygen vacancies in TiO 2 on catalytic reactions have been widely investigated.…”