Degradation of Rhodamine B Using Cr-doped TiO₂ under Visible Light Irradiation

Dye pollutants produced from the textile industries are becoming a major source of environmental contamination. Traditional methods such as flocculation, carbon adsorption, reverse osmosis and activated sludge process have difficulties in the complete destruction of dye pollutants for the treatment of dye-containing. In the past decades, scientists and engineers are all interested in developing the semiconductor photocatalytic reactions. TiO₂ has widely been investigated for photocatalyzed degradation of organic dyes because of its strong photoactive ability, stability, non-toxic and low cost. TiO₂ photocatalyst is well known to generate various active oxygen species such as hydroxyl radicals, hydrogen peroxide, superoxide radical anions, etc. by redox reactions under UV irradiation. Anatase-type of TiO₂ has highly photocatalytic activity for decomposition of various environmental pollutants in both gas and liquid phases. However, TiO₂ can only utilize a small part (less than 5%) of the solar light energy for photocatalytic oxidation and the artificial UV light sources are unstable and expensive. In order to achieve photooxidation of pollutants with efficient utilization by titania particles under visible light irradiation, i.e., the band gaps must be smaller than 3.2 eV, the development of photocatalysts with high activity under visible light irradiation has been required. In attempt to devise the photocatalysts that can be operated at lower energies, the methods of modified TiO₂ are ion implantation using Cr or V ions, various synthesis techniques and substitutional doping of non-metals such as N. In this study, Cr-doped TiO₂ powder was prepared by sol–gel method with the aim of extending the light absorption spectrum toward the visible light region. The photocatalytic activity under visible light irradiation was evaluated using rhodamine B as a model organic compound and was carried out in a recycle fluidized bed reactor. The effects of initial acid dye concentration, particle size and loading amounts of photocatalyst were investigated.