TiO2-Acetylacetone as an Efficient Source of Superoxide Radicals under Reduced Power Visible Light: Photocatalytic Degradation of Chlorophenol and Tetracycline

Abstract

Visible light-sensitive TiO2-based nanomaterials are widely investigated for photocatalytic applications under high power (≥300 W) UV and visible light. The formation of charge transfer complexes (CTCs) between bidentate ligands and nanocrystalline TiO2 promotes visible light absorption and constitutes a promising alternative for environmental remediation under reduced visible light power. However, the efficiency of photodegradation, the volatilization profile of bidentates, and the role of reactive oxidizing species (ROS) are not fully understood. In this study, thermogravimetric analyses coupled with mass spectroscopy (TGA-MS) were performed on TiO2-Acetylacetone (ACAC) CTC. TiO2-ACAC CTC calcined at 300 °C (TiO2-A300) was applied for the photocatalytic degradation of chlorophenol (4-CP) and tetracycline (TC) under low power visible light (26 W). Furthermore, the ROS scavengers isopropanol and benzoquinone were added for studying the photocatalytic role of •OH and •O2− radicals. The TGA-MS showed the release of ACAC fragments, such as ethyl ions and acetone, in the range between 150 °C and 265 °C, while between 300 °C and 450 °C only CO2 and H2O were released during oxidation of ACAC. The photocatalytic abatement of tetracycline (68.6%), performed by TiO2-A300, was ~two times higher than that observed for chlorophenol (31.3%) after 6 h, indicating a distinct participation of ROS in the degradation of these pollutants. The addition of the ROS scavenger revealed •O2− radicals as primarily responsible for the high efficiency of TiO2-ACAC CTC under reduced visible light. On the other hand, the •OH radicals are not efficiently generated in the CTC. Therefore, the development of heterostructures based on TiO2-ACAC CTC can increase the generation of ROS through coupling with semiconductors capable of generating •OH under visible light.

Extrinsic Point Defects in TiO2–Acetylacetone Charge-Transfer Complex and Their Effects on Optical and Photochemical Properties

“…20,37 However, a higher percentage of ACAC was detected for CTCs calcined under reducing atmospheres (7.4% for hydrogen atmosphere and 5.5% for argon atmosphere) compared to the CTC calcined under static air (3.5%). This result is in accordance with the observation by Almeida et al, 26 who reported that the ACAC fragmentation corresponds to an oxidative process. Therefore, as observed in Figure 3, calcination under reducing atmospheres was expected to lead to the presence of a higher percentage of ACAC in the TiO 2 -ACAC CTC.…”

“…On the other hand, only one paper that reported TC photocatalytic degradation using CTCs based on TiO 2 was found. In this paper, Almeida et al reported a TC photocatalytic abatement of 68.6% using 200 mg·L –1 of TiO 2 -ACAC 300, a CTC calcined at 300 °C as a photocatalyst, under low-power visible-light excitation (26 W) during 6 h. Nonetheless, to our knowledge, the influence of oxygen vacancies on the photocatalytic performance of TiO 2 -ACAC CTCs has not yet been investigated.…”

“…Measurement of the Photocatalytic Performance for TC Photodegradation and the Efficiency of ROS Production. The photocatalytic performance of TiO 2 -ACAC CTCs was measured using the same equipment and following a similar procedure to that reported by Almeida et al 26 The measurements were carried out using 5 mg•L −1 of TC, as a testing pollutant, and 50 mg•L −1 of the photocatalysts (TiO 2 -ACAC CTCs).…”

“…The photocatalysts were activated through photons supplied by a DULUX D/E 26 W residential fluorescent lamp with visible-light emission (wavelength range from 400 to 700 nm) and an irradiance of 0.23 W•cm −2 . 26 The lamp was placed inside a transparent quartz sleeve with a cylindrical geometry (internal diameter 3.2 cm, thickness 0.15 cm, and length 11 cm).…”

Heterogeneous Photocatalysis: A Solution for a Greener Earth

“…In a contribution by Almeida et al [1], the efficiency of photodegradation, the volatilization profile of bidentates, and the role of reactive oxidizing species (ROS) were explored for nanocrystalline TiO 2 modified with bidentate ligands (acetylacetone). In this study, TiO 2 -ACAC CTC calcined at 300 • C (TiO 2 -A300) was applied for the photocatalytic degradation of chlorophenol (4-CP) and tetracycline (TC) under low power visible light (26 W).…”