TiO2 photocatalyst for removal of volatile organic compounds in gas phase – A review

Heterogeneous photocatalytic oxidation process (PCO) is a promising technology for removing indoor volatile organic compounds (VOCs) contaminants. Titanium dioxide (TiO2) has been regarded as the most suitable photocatalyst for its cost effectiveness, high stability and great capability to degrade various VOCs. However, no TiO2-based photocatalysts completely satisfy all practical requirements given photoexcited charge carriers’ short lifetime and a wide band gap requiring ultraviolet (UV) radiation. Strategies for improving TiO2 photocatalyst activities by doping with different metal and/or non-metal ions and by coupling with other semiconductors have been examined and reported. These techniques can improve PCO performance through the following mechanisms: i) by introducing an electron capturing level in the band gap that would generate some defects in the TiO2 lattice and help capture charge carriers; ii) by slowing down the charge carrier recombination rate and increasing VOCs degradation. This paper reports the outcomes of a comprehensive literature review of TiO2 modification techniques that include approaches for overcoming the inherent TiO2 limitations and improving the photocatalytic degradation of VOCs. Accordingly, it focuses on the recent development of modified-TiO2 used for degrading gas phase pollutants in ambient conditions. Modification techniques, such as metal and non-metal doping, co-doping, and the heterojunction of TiO2 with other semiconductors, are reviewed. A brief introduction on the basics of photocatalysis and the effects of controlling parameters is presented, followed by a discussion about TiO2 photocatalyst modification for gas phase applications. The reported experimental results obtained with PCO for eliminating VOCs are also compiled and evaluated.