Formation of Schottky Defects at the Surface of MgO, TiO2, and SnO2: A Comparative Density Functional Theoretical Study

Formation of O adatom pairs and charge transfer upon O(2) dissociation on reduced TiO(2)(110)

Scanning tunneling microscopy and density functional theory have been used to investigate the details of O(2) dissociation leading to the formation of oxygen adatom (O(a)) pairs at terminal Ti sites. An intermediate, metastable O(a)-O(a) configuration with two nearest-neighbor O atoms is observed after O(2) dissociation at 300 K. The nearest-neighbor O(a) pairs are destabilized by Coulomb repulsion of charged O(a)'s and separate further along the Ti row into energetically more favorable second-nearest neighbor configuration. The potential energy profile calculated for O(2) dissociation on Ti rows and following O(a)'s separation strongly supports the experimental observations. Furthermore, our results suggest that the itinerant electrons associated with the O vacancies (V(O)) are being utilized in the O(2) dissociation process at the Ti row. Experimentally this is supported by the observation that not all V(O)'s can be healed by O(2) exposure at 300 K, as some V(O)'s becoming less reactive due to supplying certain charge to O(a)'s. Further, theoretical results show that at least two oxygen vacancies per O(2) molecule are required in order for the O(2) dissociation at the Ti row to become viable.

Effect of NOM on arsenic adsorption by TiO(2) in simulated As(III)-contaminated raw waters

The effect of natural organic matter (NOM) on arsenic adsorption by a commercial available TiO(2) (Degussa P25) in various simulated As(III)-contaminated raw waters was examined. Five types of NOM that represent different environmental origins were tested. Batch adsorption experiments were conducted under anaerobic conditions and in the absence of light. Either with or without the presence of NOM, the arsenic adsorption reached steady-state within 1h. The presence of 8 mg/L NOM as C in the simulated raw water, however, significantly reduced the amount of arsenic adsorbed at the steady-state. Without NOM, the arsenic adsorption increased with increasing solution pH within the pH range of 4.0-9.4. With four of the NOMs tested, the arsenic adsorption firstly increased with increasing pH and then decreased after the adsorption reached the maximum at pH 7.4-8.7. An appreciable amount of arsenate (As(V)) was detected in the filtrate after the TiO(2) adsorption in the simulated raw waters that contained NOM. The absolute amount of As(V) in the filtrate after TiO(2) adsorption was pH dependent: more As(V) was presented at pH>7 than that at pH<7. The arsenic adsorption in the simulated raw waters with and without NOM were modelled by both Langmuir and Frendlich adsorption equations, with Frendlich adsorption equation giving a better fit for the water without NOM and Langmuir adsorption equation giving a better fit for the waters with NOM. The modelling implies that NOM can occupy some available binding sites for arsenic adsorption on TiO(2) surface. This study suggests that in an As(III)-contaminated raw water, NOM can hinder the uptake of arsenic by TiO(2), but can facilitate the As(III) oxidation to As(V) at TiO(2) surface under alkaline conditions and in the absence of O(2) and light. TiO(2) thus can be used in situ to convert As(III) to the less toxic As(V) in NOM-rich groundwaters.