Photocatalytic degradation of methyl red dye

Introduction Most dyes used in the pigmentation of textiles, paper, leather, ceramics, cosmetics, inks and food-processing products are derived from azo dyes, which are characterised by the presence of one or more azo groups (-N=N-) in their structure. Approximately 15% of the dyes produced worldwide are lost within waste water during synthesis and processing. This waste represents a great hazard to human and environmental health due to the toxicity of azo dyes. Although silica is essentially inert for many reactions, it shows noticeable activity towards certain catalytic and photocatalytic reactions. Pure silica was proven to promote photocatalytic reactions such as the photo-oxidation of CO 13 and the photometathesis of propene. Additionally, silica-based photocatalysts, such as silica-alumina, silica-supported zirconia, silica-supported magnesia, and silica-alumina-titania, exhibit activity under UV irradiation at room temperature. The photoactive sites were formed on the surface of silica (SiO2) prepared by a sol-gel method. 19 These active sites were revealed by vacuum (V)UV-UV, infrared (IR), electron spin resonance (ESR) and photoluminescence spectroscopy. The IR symmetric stretching vibration of the Si-O non-bridging bond appeared at 950 cm. Furthermore, the enhancement of the peak intensity at 950 cm indicated an increase in the amount of the non-bridging oxygen, i.e. a change in the structural units from SiO2 (three-dimensional network structure) to SiO4 (isolated tetrahedron). The silica nanoparticles could be photoexcited under UV light (258 nm), which corresponded to a charge transfer from the bonding orbital of Si–O to the 2p non-bonding orbital of non-bridging oxygen. There are conflicting reports in the literature describing the effects of metal ions in the presence of semiconductor photocatalysts. In some cases they may act as a rate accelerator but, in other cases, the rate was found to decrease upon the addition of such metal ions. In this study, we prepared silica nanoparticles and modified the surface by doping with metal ions or coating with metallic nanoparticles. We also studied the effect of additives on the kinetics and the mechanism of photocatalytic degradation of the MR dye.