Substoichiometric titanium oxide Ti2O3 exhibits greater efficiency in enhancing hydrolysis of 1,1,2,2-tetrachloroethane than TiO2 nanomaterials

Abstract

Oxygen-deficient substoichiometric titanium oxides, or “titanium suboxides,” are produced incidentally from coal combustion and are environmentally abundant. Additionally, titanium suboxide nanomaterials are promising new materials with likely future environmental release. How these materials may affect contaminant fate differently than stoichiometric TiO2 (nano)materials is largely unknown. Here, we show that Ti2O3 (selected as a model titanium suboxide) exhibits significantly greater efficiency in enhancing the hydrolysis of 1,1,2,2-tetrachloroethane (TeCA), a common groundwater contaminant, than the stoichiometric anatase and rutile TiO2. At environmentally relevant pH (6.5–7.5), the surface area-normalized pseudo-first-order hydrolysis rate constant in the presence of Ti2O3 is approximately an order of magnitude higher than those associated with TiO2. The superior catalytic efficiency of Ti2O3 can be attributed to both its higher surface hydrophobicity, which renders higher adsorption affinity for TeCA, and its higher concentration of Lewis acid sites (mainly the Ti3+ and the five-coordinated Ti4+). Particularly, the deprotonated hydroxyl groups attached to Ti3+ (a weaker Lewis acid than Ti4+) exhibit higher basicity and thus, are more effective in catalyzing the base-promoted hydrolysis reaction. The findings call for further understanding of the environmental implications of titanium suboxide (nano)materials, which may not be readily predictable based on the knowledge acquired for TiO2.