Titanium Dioxide for Visible-Light Photocatalytic Purification of Gaseous Indoor Air Pollutants

摘要

Indoor air pollution from toxic gaseous chemicals poses adverse effects on human health, comfort and productivity. This is a serious concern nowadays since people spend more time (over 80%) indoors. In addition, recent studies on indoor air quality show that certain gaseous contaminants have greater concentration levels indoor than in the outdoor environment. Strategies to address this problem include increased ventilation and air cleaning. The former involves the transfer of pollutants from indoor to outdoor while conventional air cleaning methods utilize air filtration and adsorption. Meanwhile, heterogeneous photocatalysis, an advanced oxidation process (AOP), can completely degrade gaseous organics to carbon dioxide and water. In the past two decades, photocatalysis has emerged as a promising technology for the purification of indoor air and exhibited greater effectiveness over conventional air cleaning technologies. A catalyst that has taken center stage in the development of this technology is titanium dioxide (TiO2), also known as titania, mainly due to these advantages: (1) TiO2 is relatively inexpensive, non-toxic and chemically stable, (2) it promotes ambient temperature oxidation of the major classes of indoor air pollutants, (3) the complete degradation of a broad class range of pollutants can be achieved under certain operating conditions, and (4) chemical additives are not required for the process to proceed. The photogenerated holes of TiO2 are highly oxidizing while its photogenerated electrons are reducing enough to produce superoxide from dioxygen. However, TiO2 has a low solar photoconversion efficiency because of the large band gap of 3.2 and 3.0 eV for the anatase and rutile phases, respectively. Thus, TiO2 can only be excited by irradiation using ultraviolet (UV) light, which covers only ~5% of the light spectrum. The development of photocatalysts exhibiting high reactivity under visible light (λ> 400 nm) is of great interest because it would allow the main part of the light spectrum to be utilized and photocatalytic activity would be observed even under poor illumination of interior lighting. This requires engineering the band-gap of TiO2 to less than 3.2 eV. Studies on TiO2 modification have been carried out to increase its photocatalytic activity under visible light illumination through doping metals and non-metals onto TiO2.