Photoinduced Production of Chlorine Molecules from Titanium Dioxide Surfaces Containing Chloride

Titanium dioxide (TiO₂) is extensively used with the process of urbanization and potentially inflfluences atmospheric chemistry, which is yet unclear. In this work, we demonstrated strong production of Cl₂ from illuminated KCl-coated TiO₂ membranes and suggest an important daytime source of chlorine radicals. We found that water and oxygen were required for the reactions to proceed, and Cl₂ production increased linearly with the amount of coated KCl, humidity of the carrier gas, and light intensity. These results suggested that water promotes the reactivity of coated KCl via interaction with the crystal lattice to release free chloride ions (Cl⁻). The free Cl⁻ transfer charges to O₂ via photoactivated TiO₂ to form Cl₂ and probably the O₂⁻ radical. In addition to Cl₂, ClO and HOCl were also observed via the complex reactions between Cl/Cl₂ and HOx. An intensive campaign was conducted in Shanghai, during which evident daytime peaks of Cl₂ were observed. Estimated Cl₂ production from TiO₂ photocatalysis can be up to 0.2 ppb/h when the TiO₂-containing surface reaches 20% of the urban surface, and this is highly correlated to the observed Cl₂. Our results suggest a non-negligible role of TiO₂ in atmospheric photochemistry via altering the radical budget.