A kinetic model for photocatalytic degradation of organic contaminants in a thin-film TiO2 catalyst

The thin-film technique is becoming a standard for the preparation of TiO2-based photocatalysts for organic degradation. The catalyst alleviates the drawback of poor settleability associated with the powder TiO2 traditionally used. In addition, the thin-film catalyst can be connected to an external power source to reduce the recombination of UV-activated electrons and holes, thereby increasing the quantum efficiency. The immobilization of TiO2 on a solid carrier as a thin-film catalyst introduces several mechanisms not normally found in conventional TiO2 slurry process. These mechanisms have been identified to include at least liquid–film transfer, adsorption, diffusion and photocatalytic reaction in a thin-film. A mathematical model was developed to incorporate these mechanisms for the photodegradation of organic molecules in a batch reactor. The model was verified using the data of 4-chlorophenol degradation obtained from the literature. The thin-film photocatalytic model was then used to investigate the effect of catalyst properties on the photodegradation of organics. The properties investigated included adsorption capacity, diffusion in the catalyst, UV attenuation and film thickness. The results of model simulation show that the effects of catalyst properties on the degradation of organics are highly nonlinear. There is an optimal film thickness that yields a maximum rate of photodegradation. The model not only provides insights into the effect of these underlying mechanisms but also can be used as a tool to assist the design of a thin-film photocatalyst.