Photocatalytic oxidation of gas phase volatile organic compounds using undoped and metal ions doped titanium dioxide thin films

Abstract

Heterogeneous photocatalytic oxidation allows the oxidation of airborne volatile organic compounds (VOCs) into carbon dioxide and water in the presence of a semiconductor catalyst and UV light source. Titanium dioxide (TiO2), due to its chemical stability, non-toxicity and low cost represents one of the most efficient photocatalyst. However, only the ultraviolet fraction of the solar irradiation is active in the photoexcitation processes using pure TiO2 and although, TiO2 can treat a wide range of VOCs, the effectiveness of the process for pollution abatement is still low. A more effective and efficient catalyst therefore must be formulated. Doping TiO2 with metal ions was considered with the aim of improving TiO2 photocatalytic properties. In this study transparent TiO2 thin films were prepared using the sol-gel and dip-coating method. Various ratios of doped TiO2 thin films were also prepared using Cr3+, Fe3+, Cu2+, Ni2+, Co2+, Zn2+, Mn2+ and Ag+ ions. The photoactivity of the catalyst was evaluated by the photodegradation of VOCs, conducted in a home built glass reactor. Photocatalytic oxidation of benzene using various ratios of metaldoped TiO2 showed an optimum dopant to metal ion ratio that indicates the dependency of TiO2 photocatalytic reactivity on dopant concentration. Photocatalytic oxidation of benzene, toluene, m-xylene, acetone and tetrachloroethylene was conducted using the catalyst with the optimum ratio. It was observed that the photoactivity of doped TiO2 substantially depends on the type and concentration of dopant and VOCs. Dopants affect the photoreactivity of TiO2 by acting either as electron/hole trap or electron-hole recombination center. High photocatalytic degradation of all the VOCs was observed with pure TiO2. However adding Fe3+ and Ag+ into TiO2 increased the photodegradation of benzene and acetone while Zn2+ in toluene and m-xylene degradation. Dopants such as Cr3+, Co2+, Cu2+, Ni2+ and Mn2+ decreased the photoreactivity of TiO2 in the photodegradation of all VOCs under studied. Dopants have no effect in TeCE degradation. However, TeCE shows highest degradation compared to the non-chlorinated hydrocarbon. This observation was attributed to the participation of chlorine radical, which induced a chain reaction mechanism. Preliminary studies on the mineralization of VOCs showed that CO2 was formed during the photooxidation process using pure or doped photocatalyst. The formation of new compounds, however, was not detected. The experimental ratio of VOCs degraded against the amount of CO2 formed showed that the photodegradation of VOCs is a partial oxidation process. Structural and optical properties of the thin films were characterized using XRD, SEM/EDAX, XPS, UV-Vis and PL spectroscopy. All thin films showed primarily the anatase phase. However, the presence of rutile in doped TiO2 contributed to the reduced photoreactivity of TiO2. Surface species such as Ti4+, surface hydroxyl and physically adsorbed water increased the photoreactivity of TiO2 but Ti3+ reduced its activity. Dopants such as Zn2+, Mn2+ and Ag+ shift the absorption edge of TiO2 into the visible region indicating the possibility of photocatalytic process using visible light