Solar photocatalytic degradation of gaseous formaldehyde by sol-gel TiO₂ thin film for enhancement of indoor air quality

Solar photocatalytic degradation of formaldehyde in the gaseous phase has been investigated. The tested photoreactor is made of a borosilicate glass tube with the inner surface coated with a sol-gel TiO₂ thin film. In a pseudo-first-order Langmuir-Hinshelwood (L-H) model, the maximum reaction rate constant obtained is 0.148 min^-1 under an exposure to sunlight with solar UVA irradiance of 1.56 mW/cm². The solar photolysis effect is found to be negligible. It is also found that the sol-gel TiO₂ thin film has a lower apparent photonic efficiency of solar photocatalysis than a Degussa P25 TiO₂ coating. However, for the photonic efficiency taking into account the absorbed and scattered photons only and, in other words, excluding the transmitted photons, the thin film has a higher value. Based on a total of 28 measured data, an empirical-correlation equation has been developed to express the reactant residue with respect to the solar UVA irradiance and exposure time. A reasonable agreement between the correlation and experimental data is obtained. The findings of this investigation can be applied to design optimization of a honeycomb photoreactor made up of TiO₂-coated glass tubes or polygonal cells. © 2004 Elsevier Ltd. All rights reserved.