Estudo da degradação de pesticidas fotocatalisada por AgNP/TiO2 e monitorada por espectroscopia SERS

Abstract

The present work aimed at the synthesis and characterization of a plasmonic catalyst based on silver nanoparticles (AgNP) and titanium dioxide (TiO2) semiconductor, and its application in photocatalytic degradation by radiation in the visible region of pesticides Tebuconazole (TEB) and 2,4-dichlorophenoxyacetic acid (2,4-D). AgNP and the hybrid catalyst were synthesized according to methodologies already reported in the literature, and different concentrations of AgNP were added to TiO2 in the manufacture of the latter. In addition to the original synthesis, six adaptations of AgNP synthesis were carried out, which were mainly characterized by UV–VIS absorption spectroscopy, where it was verified the successful reproduction of the synthesis reported in the literature, and by dynamic light scattering (DLS), from which the hydrodynamic diameters of each of the seven syntheses performed were determined. The synthesized catalysts, with different Ag:TiO2 molar ratios, were also properly characterized by Raman scattering spectroscopy, X-ray diffraction by polycrystals, DLS, isoelectric point pH (by potential-ζ measurements), and UV–VIS diffuse reflectance; from these techniques, it was verified that the characteristic properties of the semiconductor did not suffer great alterations with the modification by AgNP. Semiquantitative analysis of the catalyst with the highest level of AgNP (AT320) by X-ray fluorescence spectroscopy showed that it had about 4.8% w/w of Ag. The catalysts were initially tested in crystal violet degradation procedures, where it was observed high efficiency in analyte removal (> 90%) with only 100 minutes of visible irradiation of the AT320 catalyst. In a study of the isolated systems (AgNP/Vis and TiO2/Vis), no catalytic action was observed, which confirms the great synergy between the two materials. Unexpectedly, the AT320/Vis system was not efficient in removing the studied pesticides. It was verified that the exchange of incident radiation from visible to UV-A, as well as the adjustment of other pH values of the reaction medium, proved to be effective in the degradation of 2,4-D and TEB. The SERS spectroscopy monitoring of these reactions allowed to confirm the degradation, also observed through UV–VIS, but proved to be inconclusive with the proposal of products and reaction intermediates formed. However, some products could be inferred, such as chlorinated and non-chlorinated phenolic compounds (for both pesticides), formation of 1,2,4-triazole for degradation of TEB and glycolic and glyoxylic acids to 2,4-D.