TiO2 photocatalyst for indoor air remediation: Influence of crystallinity, crystal phase, and UV radiation intensity on trichloroethylene degradation

Nanocrystalline TiO2 photocatalysts prepared through a simple sol–gel process followed by calcination at 100–800 °C were evaluated for the decomposition of trichloroethylene (TCE) in simulated indoor air. Catalysts made of pure anatase, anatase/rutile mixture and pure rutile with crystal sizes of 5.8–75.8 nm and surface areas of 4.3–498 m2/g were immobilized as thick films on borosilicate glass and installed in a gas-phase, flat-plate photocatalytic reactor. Pure anatase TiO2 treated at 300–600 °C exhibited higher activity than commercial TiO2 (P-25) under UV-A radiation with intensity of 5.1, 14.0 and 20.8 W/m2. Conversely, pure rutile and mixed phase TiO2 materials showed significantly lower activity. The photocatalytic activity of fully grown anatase TiO2 with crystal size of 27 nm was up to three-fold higher than that of P-25 despite exhibiting lower surface area. The increase in the crystallinity of anatase TiO2 upon calcination overcame the depreciation in its structural properties (decrease in surface area and porosity) for the decomposition of TCE. The dependence of the TCE oxidation rate on the intensity of the incident radiation in the range 5.2–20.8 W/m2 was found to be first-order for the samples containing a rutile phase (including P-25) and 0.75-order for the sample containing pure anatase suggesting that “absorbed” photons are more effectively used when a rutile phase is present. However, less number of incident photons are absorbed by samples with pure rutile compared to samples with pure anatase. The crystallographic properties of TiO2 over its structural properties were found to be essential factors which determined the higher photocatalytic activity of TiO2 anatase for the photocatalytic decomposition of TCE. Rate equations for the photocatalytic oxidation of TCE over TiO2 films derived from an elementary reaction mechanism and with explicit effect of the incident photon flux were correlated with the catalyst properties.