Nano-TiO₂‑Catalyzed Dehydrochlorination of 1,1,2,2-Tetrachloroethane: Roles of Crystalline Phase and Exposed Facets

Nanoscale titanium dioxide (<i>n</i>TiO₂) is one of the most widely used metal oxide nanomaterials. Once released into the environment, <i>n</i>TiO₂ may catalyze abiotic transformation of contaminants and consequently affect their fate and effects. Here, we show that the overall catalytic efficiency of <i>n</i>TiO₂ for the dehydrochlorination reaction of 1,1,2,2-tetrachloroethane, a commonly used solvent, depends on the crystalline phase and exposed facets of <i>n</i>TiO₂, which significantly affect the adsorption capacity and surface catalytic activity of <i>n</i>TiO₂. Specifically, under all three pH conditions tested (7.0, 7.5 and 8.0), the overall catalytic efficiency of eight <i>n</i>TiO₂ materials (as indicated by the surface-area-normalized reaction kinetic constants) followed the order of rutile > anatase > TiO₂(B). For anatase and TiO₂(B) materials, the overall catalytic efficiency increased with the increasing percentage of exposed {001} and {010} facets, respectively. Crystalline phase and exposed facets significantly affected adsorption affinities of <i>n</i>TiO₂, likely by modulating surface hydrophobicity of <i>n</i>TiO₂. Crystalline phase and exposed facets also determined the activity of surface catalytic sites on <i>n</i>TiO₂ by dictating the concentration and strength of surface unsaturated Ti atoms, as the deprotonated hydroxyl groups chemisorbed to these reactive Ti atoms served as bases to catalyze the base-promoted reaction