Full Length ArticlePhotocatalytic activity of Al2O3 improved by the addition of Ce3+/Ce4+ synthesized by the sol-gel method. Photodegradation of phenolic compounds using UV light
Graphical abstract
Introduction
Phenolic contaminants are derived from several sources as pharmaceutical, plastics, pesticides, dyes, etc. These organic wastes are of particular interest and concern for the environment for their toxicity to aquatic organisms and humans. The US Environmental Protection Agency (EPA) has listed 11 phenolic compounds classified as priority pollutants. Phenol is a highly toxic pollutant found to be mutagenic and carcinogenic; in addition, the concentration of phenolic compounds in drinking water should not exceed 0.1 μg L−1 [1], [2].
The heterogeneous photocatalysis has been proposed as an alternative to resolve the problem of contamination by organic compounds, which involves the absorption of photons by a semiconductor material that causes the electron-hole separation which migrate to the material surface and are precursors to the formation of OH. Then, these radicals react with the organic molecules to perform their degradation [3].
It has been proposed by the recent literature that Al2O3, obtained by the sol-gel synthesis, can be a novel alternative for the organic pollutants photodegradation [4], and has been proposed that hydroxyl groups on the Al2O3 surface are responsible of its photoactivity; this mechanism involves a charge transfer phenomenon on the surface when illuminated with UV light. Karunakaran and coworkers [5], [6], [7] have discussed the photodegradation of carboxylic acids and phenol with commercial Al2O3. These authors suggest that the light absorption by the carboxylic acid chemisorbed on Al2O3 transfers an electron to the defective solid matrix; this could be the initial step in the photodegradation process. Then, there will occur an electron transfer to a neighboring adsorbed oxygen molecule and the rest of the mechanism could be similar to a photocatalyst. Although, Al2O3 is considered an insulator, there are indications that it may be applied in organic contaminants photodegradation. It is interesting to follow the research on this topic since the literature is not thorough enough about it.
For this novelty of this material in organic contaminants degradation, it is proposed to add Ce3+/Ce4+ impurities to improve its photoactivity. Cerium oxide is one of the more reactive oxides of rare earth; it has been employed as a photocatalyst in water splitting reactions, wastewater treatment, and for methane decomposition [8], [9]. The CeO2 has strong absorption in the UV region of the electromagnetic spectrum [8]; it is an n-type semiconductor [10] and it has a band gap of 3.2 eV [11]. The combination of ceria with other semiconductors has been an interesting study by showing high efficiency in the photodegradation of harmful organic pollutants, by using materials such as Bi2WO6–CeO2, SrTiO3–CeO2, TiO2–CeO2, graphene/CeO2, LDHs/CeO2, BiVO4/CeOx among others [12], [13], [14].
It has been reported that the addition of 5.0 wt.% CeO2 on TiO2 there is an increase of the specific surface area of this material [15]; also, all lanthanide ions have a particular electronic structure 4fx 5dy which might provide different optical properties [16]. In particular, ceria is attractive due to its optical properties, associated to the Ce3+/Ce4+ redox couple acting as an electron trap [17], [18], and it would be able to form defects by oxygen vacancies to keep the electroneutrality in the material [16], [19], [20].
In this work, the synthesis of the materials Al2O3:Ce3+/Ce4+ was carried out by sol-gel method, wherein the amount of Ce3+ added (0.5, 1.0, 3.0, and 5.0 wt.%) was the variable in the synthesis process, in order to improve the photocatalytic activity of aluminum oxide. The synthesized materials were characterized by techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), N2 physisorption, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), Mott-Schottky curves, open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). These materials were evaluated for the photodegradation of phenol and the most active photocatalyst was selected for the photo-oxidation of phenol derivatives as 4-chlorophenol, p-cresol, and 4-nitrophenol. A UV lamp with a wavelength of 254 nm (4.4 mW cm−2) was used as photons source; the reaction process was monitored by the quantification of total organic carbon and UV–Vis spectroscopy.
Section snippets
Synthesis process
The materials Al2O3:Ce3+/Ce4+ were obtained by the sol-gel process; the amount added of Ce3+ was 0.5, 1.0, 3.0, and 5.0 wt.%. The starting precursor was aluminum-tri-sec-butoxide (Sigma Aldrich, 97%) which was hydrolyzed with distilled water at a molar ratio alkoxide/water 1:5 using 1-butanol (J.T. Baker, 99.92%) as solvent. Pure Al2O3 was prepared by using 0.2 mol of alkoxide which was dissolved in 600 mL of 1-butanol at 60 °C (Solution 1) and adjusted to pH 5.0 with nitric acid. Another solution
TGA and DSC analysis
The calcination temperature was selected based on the TGA and DSC results of the uncalcined materials, which are shown in Fig. 1 (only AC0 and AC5.0 as representative materials are shown). These results show a weight loss from room temperature up to approximately 170 °C together with a broad endothermic peak (about 75 °C). This signal is assigned to the loss of solvent and water. Additionally, two exothermic peaks (a sharp and another broad) accompanied of a weight loss are shown between 170 and
Conclusions
Phenol photodegradation was carried out by using Al2O3:Ce3+/Ce4+ materials prepared by the sol-gel method where different amounts of cerium were evaluated. These materials improved the photoactivity of bare Al2O3 by adding Ce3+/Ce4+. The material with the amount of 1.0 wt.% of cerium impurities proved to be the most efficient for this application and it was evaluated also for the degradation of other organic molecules such as 4-chlorophenol, p-cresol and 4-nitrophenol. The couple Ce3+/Ce4+ acts
Acknowledgments
The author acknowledges CONACYT – Mexico for the scholarship given during the development of this project (CVU/No. of scholar 507712/286023), the CONACyT project 154994, and the SEP proyect 103.5/15/14156.
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