Estudo da fotodegradação dos corantes reactive black 5, azul de metileno e cristal violeta por irradiação no visível utilizando catalisadores plasmônicos híbridos nanoestruturados

Abstract

The present doctorate work was dedicated to studying new nanostructured plasmonic photocatalysts and their application in photodegradation processes of organic dyes, using irradiation with visible light. Silver (AgNP), bimetallic (Au@Ag), and copper (CuNP) plasmonic nanoparticles were synthesized, as well as zinc oxide (ZnO). The hybrid metalsemiconductor catalysts were obtained through the association of metallic nanoparticles with ZnO (NP/ZnO) and commercial niobium pentoxide (NP/Nb2O5). The characterization of photocatalysts was performed using Raman spectroscopy, UV-VIS, polycrystalline XRD, SEM, and ξ-potential techniques. The photodegradation processes were carried out in a labmade reactor containing aqueous dye solution (1.0×10-5 mol L-1 ), which was irradiated using a white (15 W) LED lamp and∕or incandescent (100 W) as sources of irradiation in the visible. The type A Ag/ZnO photocatalyst was applied to the visible photodegradation of the dye Reactive Black 5, under the incidence of a tungsten filament incandescent lamp. The process was monitored by electronic spectroscopy in UV-VIS, resonant Raman (RR), and surface enhanced Raman spectroscopy (SERS), and it was possible to observe spectral changes related to the formation of intermediates and products. The plasmonic catalyst showed a photocatalytic performance 40% higher than the pure semiconductor oxide, promoting a 57% faster photodegradation, indicating the influence of the plasmonic metal in the better efficiency of the process. Computational chemistry, through the DFT method, was used in the study of the RB5 azo-hydrazo balance as well as in the proposal of the dye degradation assisted by hydroxyl radicals mechanism. The theoretical-experimental results indicated a predominance of the hydrazo form in solution. The proposed mechanism indicated favoring of the C-N attack, in the region of azo-hydrazo equilibrium, giving rise to a energy profile favorable both thermodynamically and kinetically. The type B Ag/ZnO catalysts with three different concentrations of AgNP were applied to the photodegradation of RB5 (anionic), Methylene Blue (MB) and Crystal Violet (CV) (both cationic) dyes under the LED lamp. The ξ-potential of the photocatalysts indicated a negative surface charge for all samples. The catalysts were inefficient in the photodegradation of the RB5 dye. For both cationic dyes, the catalyst with the lowest concentration of AgNP (B210) showed efficiency 10% (MB) and 29% (CV) higher than in the presence of pure ZnO, proving the influence of plasmonic metal in the intensification of visible photodegradation. The monitoring of the processes using UV-VIS and SERS spectra showed more significant structural changes in the processes involving the CV dye. Through pseudo-first order kinetic adjustments, it was concluded that the plasmatic photocatalysis of the CV dye was 87% faster when compared to the process in the presence of pure ZnO.