Generation of Gaseous ClO₂ from Thin Films of Solid NaClO₂ by Sequential Exposure to Ultraviolet Light and Moisture

We report that thin films of solid sodium chlorite (NaClO₂) can be photochemically activated by irradiation with ultraviolet (UV) light to generate gaseous chlorine dioxide (ClO₂) upon subsequent exposure to moisture. The limiting role of water in the reaction is evidenced by an increase in yield of ClO₂ with relative humidity of the gas stream passed over the UV-activated salt. The UV-activated state of the NaClO₂ was found to possess a half-life of 48 h, revealing the presence of long-lived UV activated species that subsequently react with water to produce gaseous ClO₂. The yield of ClO₂ was determined to be proportional to the surface area of NaClO₂ particles projected to the incident illumination, consistent with activation of a ∼10 nm-thick layer of NaClO₂ at the surface of the micrometer-sized salt crystals (for an activation wavelength of 254 nm). We also found that the quantity of ClO₂ released can be tuned ∼10-fold by varying wavelength of UV irradiation and relative humidity of the gas stream passed over the UV-activated NaClO₂. The UV-activated species were not detectable by electron paramagnetic resonance spectroscopy, indicating that the activated intermediate is not an excited triplet state of ClO₂^-. Additionally, neither X-ray photoelectron spectroscopy, nor Raman spectroscopy, nor attenuated total reflection infrared spectroscopy revealed the identity of the activated intermediate species. The ability to preactivate solid phase chlorite salt for subsequent generation of ClO₂ upon exposure to moisture suggests the basis of new materials and methods that permit triggered release of ClO₂ in contexts that use its disinfectant properties.