Cálculo por DFT del potencial redox del cristal violeta para la aplicación en procesos fotocatalíticos

Abstract

In the microbiological and / or industrial laboratories, large volumes of colored liquid wastes originate. These are the products of cellular stains to classify bacteria by Gram method, paper coloring, textile dyeing, fingerprint development (in forensic medicine) among others. Thus, there is a wide range of dyes from different chemical families, such as anthraquinones, azo and, especially, triphenylmethanes, such as violet crystal (CV) (Lou & Chang, 2007). These dyes are capable of generating serious disorders in living beings. Faced with this, photocatalytic processes have demonstrated great effectiveness in the treatment of a wide variety of colored aqueous wastes (Sabnis, 2010). That is why titanium dioxide is one of the most commonly used semiconductor compounds (SC) for the oxide-reductive degradation of triphenylmethane contaminants - such as CV-. In the theory of the redox processes on which the photocatalysis is based, one of the fundamental parameters of the system is the redox potential of the pollutant, which can be theoretically calculated through the use of different computational methods. This parameter can be used to help explain the physical-chemical processes involved in the degradation of CV through the photocatalysis of TiO2. In this paper we report the calculation of the redox potential of the CV and its energy levels measured with respect to the standard reference electrode (SHE), to study the electron transfer between the CV substrate and the TiO2 catalyst. To calculate the redox potential, the CV + (cationic) molecule was reduced to CV0 (neutral) and CV- (reduced) by using the Gaussian09 programs and the GaussView05 visualizer. Subsequently, the Gibbs thermodynamic cycle was applied using the direct method (Fernández, 2012). Specifically in our case we divided the process into two parts: 1) transfer of two electrons in a single step (from CV + to CV-) and, 2) transfer of one electron per step (from CV + to CV 0 and from CV 0 to CV-) . Two theory levels were evaluated: Hartre-Fock and DFT (with B3LYP hybrid), the latter method being one of the most widely used for its accuracy and low computational cost. Both approaches were performed with the base function 6-311 + G (d). Additionally, the solvation was performed implicitly with the Solvent Density Model (SMD) and the Polarizable Conductor Continuous Model (CPCM), which are the most commonly used solvents (Arumugam & Becker, 2014; Meing, Hu, & Zhang, 2013 ); With water and acetonitrile as solvents.