Photodegradation mechanism of the RB5 dye: A theoretical and spectroscopic study

Abstract

Plasmonic photocatalysis has been widely applied in the photodegradation of organic dyes, allowing the use of visible light in the process. Specifically, Ag nanoparticles associated with ZnO have been used for the photodegradation of the azo dye RB5 under visible light irradiation. Experimental Raman and UV–vis spectral profile strongly indicated changes in the structure of the molecule, but the mechanism involved was not fully elucidated. In the present study, a photodegradation mechanism of the RB5 dye assisted by hydroxyl radicals was proposed, using computational simulation based on DFT method. Initially, a theoretical-experimental study of the tautomeric azo-hydrazo balance of RB5 was carried out, indicating predominance of the hydrazo form in aqueous solution greater than 99 %. Two sites were identified as favorable for hydroxyl radical attack, involving the CN bond breaking of the hydrazo species, resulting in two proposed products. The potential energy surface (PES) for the overall degradation process presented a favorable profile based on thermodynamic and kinetic parameters, indicating a spontaneous and rapid photodegradation, with a rate limiting barrier of 5.3 kcal mol−1 and –79.6 kcal mol−1 free energy. The theoretical spectral results for the proposed reagents and products are supported by previous experimental data and adds an efficient approach to the photodegradation mechanism of RB5, which could be extended for other azo dyes.