Bicarbonate-enhanced generation of hydroxyl radical by visible light-induced photocatalysis of H2O2 over WO3: Alteration of electron transfer mechanism
Graphical abstract
Introduction
Semiconductor photocatalysis has been widely studied for the application in versatile areas such as energy production, chemical synthesis, and environmental cleanup [1], [2]. Among various semiconductor photocatalysts, titanium dioxide (TiO2) and zinc oxide (ZnO) have been most frequently studied for water and wastewater treatment, owing to their high photo-oxidizing power and low toxicity [3], [4]. Upon light illumination, TiO2 and ZnO in aqueous suspension can generate reactive oxygen species (ROS) that are capable of degrading refractory organic contaminants and inactivating microorganisms [5], [6]. Electron-hole pairs generated on the surface of these photocatalyst can induce different redox reactions, including the reduction of oxygen into superoxide radical anion (O2•−) and the oxidation of water into hydroxyl radical (•OH). Photo-excitation of TiO2 and ZnO requires UV light absorption due to their high band gap energies (3.0 – 3.3 eV) [7], [8]. This limits the practical application of these photocatalysts under sunlight illumination.
Tungsten oxide (WO3) has been investigated as an alternative photocatalyst applicable under visible light illumination [9], [10]. WO3 possesses less band gap energy (2.4 – 2.8 eV [11], [12]) than TiO2 and ZnO, allowing fractional utiization of visible light (ca. λ < 500 nm). In addition, WO3 exhibits a high oxidation power of the valence band (VB) (3.1 – 3.2 VNHE) and a low toxicity [12], [13]. However, the conduction band (CB) of WO3 has a reduction power (0.3 – 0.5 VNHE) that unfavors the trapping of CB electrons by oxygen and water [10], [11]. As a result, recombination of electron-hole pairs proceeds fast on the surface of illuminated WO3, subsequently generating little ROS. To enhance the charge separation on the WO3 surface, several modification methods have been proposed, including junction with other semiconductiors [14], introduction of co-catalysts [15], [16], and addtion of external electron acceptors [17]. However, hydrogen peroxide (H2O2) did not result in significant enhancement in photocatalytic activity of WO3 compared to other electron acceptors [17], possibly due to its low electron-trapping efficiency and nonradical decomposition pathways. Meanwhile, the addition of H2O2 along with Fe(III) (i.e., Fenton-like reagents) successfully improved the generation of •OH from illuminated WO3, not only by trapping CB electrons via Fe(III) reduction, but also by producing additional •OH from the Fenton reaction (i.e., the reaction of Fe(II) with H2O2) [18].
This study newly found that the addition of H2O2 in combination with bicarbonate ion (HCO3−) could greatly enhance the generation of •OH by visible light-illuminated WO3 and resultingly accelerate the degradation of organic contaminants. The enhancing effect of HCO3− is considered to be very unique in advanced oxidation processes because HCO3− generally serves as a sink for •OH that inhibits degradation of contaminants [19]. Since HCO3− is ubiquitous in natural water and wastewater, this effect is expected to prevail in field applications of WO3 photocatalysis. This study aimed to explrore the effect of simultaneous addition of H2O2 and HCO3− on the generation of •OH by WO3 photocatalysis. For this purpose, benzoic acid (BA) was selected as a probe compound, and its oxidative degradation and transformation into hydroxybenzoic acids (HBAs) were examined under different conditions. To elucidate the underlying mechanism, a series of experiments using radical scavengers and probes, and an isotope tracer were conducted. In addition, electron paramagnetic resonance (EPR) spectroscopy and various (photo-)electrochemical analyses were performed.
Section snippets
Reagents and characterization of WO3
All chemicals (including WO3 powder from Sigma-Aldrich, methanol and acetonitrile from J. T. Baker) were of reagent grade and used without further purification. Deionized (DI) water (>18.2 MΩ⋅cm) was produced with a Milli-Q Integral Water Purification System (Millipore), and used for the preparation of solutions. All stock solutions were stored at 4℃ until use. Stock solutions of NaHCO3 (0.1 M) and H2O2 (1 M) were prepared freshly prior to experiments. Pristine and treated WO3 materials were
Enhanced photocatalytic oxidation of BA and organic contaminants by hν/WO3/H2O2/HCO3−
Oxidative degradation of BA by WO3 and its different combinations with H2O2 and HCO3− (i.e., H2O2, H2O2/HCO3−, WO3, WO3/HCO3−, WO3/H2O2, WO3/H2O2/HCO3−) was examined under visible light illumination (Fig. 1a). hν/H2O2, hν/H2O2/HCO3−, hν/WO3, and hν/WO3/HCO3−did not degrade BA for the entire reaction time. hν/WO3/H2O2 caused partial degradation of BA (27% removal in 120 min). In contrast, the ternary system, hν/WO3/H2O2/HCO3−, resulted in 91% BA degradation. Without illumination (dark), the BA
Conclusions
This study newly found that photocatalytic generation of •OH by visible light-illuminated WO3 could be greatly enhanced by the presence of H2O2 and HCO3−. Compared to hν/WO3/H2O2, hν/WO3/H2O2/HCO3− resulted in greater BA degradation but less H2O2 decomposition, consequently exhibiting higher H2O2 utilization efficiency. In hν/WO3/H2O2, H2O2 was mainly decomposed via pathways through which ROS were hardly generated (likely two-electron redox reactions). In contrast, in the presence of HCO3−
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by a National Research Foundation of Korea (NRF) Grant (NRF-2021R1A4A1026364), and by Korea Environment Industry & Technology Institute (KEITI) through Prospective Green Technology Innovation Project (2020003160008).
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These authors equally contributed to this work.