SÍNTESE E CARACTERIZAÇÃO DE NANOTUBOS DE TiO2 DECORADOS COM ESTRUTURAS METAL-ORGÂNICAS PARA FOTOELETROCATÁLISE

Abstract

The depletion of fossil fuel reserves, rapid population growth, and increasing concerns about environmental pollution have led to the search for biofuels. Biodiesel is one of the most promising, and during its production, 10% by mass of glycerol is obtained as a byproduct in the process. Due to its widespread availability and low cost, the valorization of glycerol is not only encouraged by the global consensus on carbon neutrality but also highly economically profitable. This is relevant when it comes to the selective production of products such as glyceraldehyde (GAD) and dihydroxyacetone (DHA), which are key precursors in the synthesis of cosmetics, polymers, and emulsifiers. In this sense, a promising pathway for obtaining higher-value byproducts using glycerol is using photoelectrocatalysis (PEC). Thus, the use of metal-organic frameworks (MOFs), which are a new class of materials, is noteworthy due to their physicochemical properties. IRMOF, MIL, ZIF, and UiO are the most well-known and employed MOFs in the study of gas storage and catalysis. Furthermore, Fe-based MOFs have great potential as highly effective photocatalysts for water treatment applications. In this study, the synthesis, characterization, and application of TiO2 nanotube electrodes decorated with UiO-66 and UiO-66 modified by melamine (ZrMOF-1, Zr-MOF-2, and Zr-MOF-3) in the PEC of glycerol and MIL-53(Fe) for the photoassisted electrochemical degradation of 2,4-dimethylaniline (2,4-DMA) under visible light irradiation were investigated. The TiO2 nanotubes were prepared by anodization and calcined at 450 °C for 2 h. The MOFs were prepared by solvothermal synthesis and applied onto the TiO2 nanotubes by the drop casting technique and fixed with 70 µL of Nafion®. The morphological, structural, and thermal stability properties of the synthesized materials were analyzed. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs showed the presence of cubic nanoparticles of approximately 100 nm. The use of melamine caused an increase in the surface area determined by the BET technique. In the FTIR spectra, the peak at 1089 cm-1 present in the modified MOFs can be attributed to the C=N bond of melamine, and the presence of Zr3p, O1s, and C1s in the XPS results confirmed the structure of UiO-66. The absorption band of carboxyl groups coordinated to iron centers is observed by FTIR at 1550 cm-1, while a peak at 538 cm-1 is related to the formation of metal-oxo bonds between the carboxylic group of terephthalic acid and Fe3+. The characteristic diffraction peaks of MIL-53(Fe) are found at 9.1° and 11.1°. The adsorption isotherm presents a type I curve, which informs about the microporosity of MIL-53(Fe). The byproducts of glycerol PEC were monitored by high-performance liquid chromatography (HPLC). The result of PEC applying a potential of +1.2 V at pH 3 showed a selectivity of 50% for the DHA byproduct in just 3 h of experiment for the TiO2 nanotube electrode decorated with 250 µg cm-2 of Zr-MOF-1. The photoelectrochemical activity of MIL-53(Fe) at pH 3.0 was optimized using a central composite design (CCD) experimental plan, with H2O2 concentration / mg L-1, applied electrical potential / V, and reaction time / min as variables under study. After the addition of 10 mg L-1 H2O2, a degradation of 74.4% of 2,4-DMA was achieved at 1.0 V after 110 min, surpassing the 57% achieved using an unmodified TiO2 nanotube photoanode. Further modification of MIL-53(Fe) with melamine allowed for nearly complete degradation. A plausible degradation pathway for 2,4-DMA is discussed based on NMR and HPLC-MS/MS results. This work demonstrates the promising photoelectrocatalytic activity of TiO2 photoanodes decorated with Fe-MOFs.