Comparison of azo dye degradation efficiency using UV/single semiconductor and UV/coupled semiconductor systems

Area of Science:

• Environmental Chemistry
• Materials Science
• Chemical Engineering

Background:

• Azo dyes are widely used synthetic colorants posing environmental challenges.
• Photocatalytic oxidation offers a promising method for degrading persistent organic pollutants like azo dyes.
• Semiconductor photocatalysts such as TiO2, ZnO, and SnO2 are explored for their efficacy in dye degradation.

Purpose of the Study:

• To evaluate the photocatalytic degradation of Procion Red MX-5B and Amaranth azo dyes using TiO2, ZnO, and SnO2 under UV irradiation.
• To investigate the influence of catalyst type, pH, and dye structure on degradation efficiency.
• To elucidate the reaction mechanisms involved in azo dye photodegradation.

Main Methods:

• Photocatalytic oxidation experiments were conducted using UV light (365 nm) with TiO2, ZnO, and SnO2 as photocatalysts.
• UV-Vis spectroscopy was employed to determine the band gap energies of the semiconductor materials.
• Kinetic analysis using a first-order reaction model was performed to quantify degradation rates.
• Dye adsorption studies on ZnO were conducted to understand the role of dye structure.

Main Results:

• The band gap energies of TiO2, ZnO, and SnO2 were determined as 3.17, 2.92, and 4.13 eV, respectively. SnO2's band gap was insufficient for UV-initiated photocatalysis.
• The combined TiO2+SnO2 catalyst showed a higher reaction rate constant (0.31 h-1) for Procion Red MX-5B degradation than TiO2 (0.24 h-1) or SnO2 alone at pH 10.
• Degradation rates were generally higher at pH 10 compared to pH 7, indicating the significance of OH radical attack.
• Amaranth exhibited higher adsorption onto ZnO than Procion Red MX-5B due to its greater number of sulfonate groups.
• The degradation of Amaranth at pH 7 with UV/ZnO suggested initial cleavage of C-S bonds, leading to sulfate ion formation.

Conclusions:

• TiO2 and ZnO are effective semiconductor photocatalysts for azo dye degradation, with optimal performance at pH 10.
• The synergistic effect of TiO2 and SnO2 enhances degradation efficiency by utilizing SnO2 as an electron sink.
• Dye structure, specifically the presence of sulfonate groups, influences adsorption and degradation pathways, with C-S bond cleavage being a key step for Amaranth.