Bicarbonate-enhanced generation of hydroxyl radical by visible light-induced photocatalysis of H2O2 over WO3: Alteration of electron transfer mechanism

https://doi.org/10.1016/j.cej.2021.134401Get rights and content

Highlights

  • HCO3 greatly enhances the generation of OH by photocatalysis of H2O2 over WO3.

  • /WO3/H2O2/HCO3 exhibits less consumption of H2O2 than /WO3/H2O2.

  • One-electron reduction of H2O2 into OH is dominant in /WO3/H2O2/HCO3.

  • Roles of HCO3 in electron-transfer interaction between WO3 and H2O2 are discussed.

Abstract

The generation of hydroxyl radical (OH) by visible light-illuminated tungsten oxide (WO3) was found to be significantly improved in the presence of hydrogen peroxide (H2O2) and bicarbonate ion (HCO3). A ternary system of /WO3/H2O2/HCO3 showed synergistic enhancement in oxidation of benzoic acid (BA, a OH probe compound) into hydroxybenzoic acids (HBAs), exhibiting even less consumption of H2O2 than /WO3/H2O2. Analyses of HBAs from BA oxidation (three HBA isomers and 18O-labelled HBA from H218O2) suggested that /WO3/H2O2/HCO3, contrary to /WO3/H2O2, generated OH mainly via one-electron transfer from the conduction band of WO3 to H2O2. The dominant one-electron reduction of H2O2 over HCO3-treated WO3 was further evidenced by Koutecký–Levich plots obtained with a rotating disk electrode setup. Based on different experiments using electron paramagnetic resonance spectroscopy, radical scavengers and probes, (photo-)electrochemical measurements, and density functional theory calculations, the mechanisms underlying the enhanced generation of OH by /WO3/H2O2/HCO3 were discussed.

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). /H2O2, /H2O2/HCO3, /WO3, and /WO3/HCO3did not degrade BA for the entire reaction time. /WO3/H2O2 caused partial degradation of BA (27% removal in 120 min). In contrast, the ternary system, /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 /WO3/H2O2, /WO3/H2O2/HCO3 resulted in greater BA degradation but less H2O2 decomposition, consequently exhibiting higher H2O2 utilization efficiency. In /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).

References (35)

  • J.H. Kim et al.

    Facile surfactant driven fabrication of transparent WO3 photoanodes for improved photoelectrochemical properties

    Appl. Catal. A: Gen.

    (2016)
  • F. Tian et al.

    DFT study of CO sensing mechanism on hexagonal WO3 (001) surface: The role of oxygen vacancy

    Appl. Surf. Sci.

    (2014)
  • G.S. Timmins et al.

    Trapping of free radicals with direct in vivo EPR detection: a comparison of 5,5-dimethyl-1-pyrroline-N-oxide and 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide as spin traps for HO and SO4•−

    Free Radi. Bio. Med.

    (1999)
  • Y. Gao et al.

    Accelerated photocatalytic degradation of organic pollutant over metal-organic framework MIL-53(Fe) under visible LED light mediated by persulfate

    Appl. Catal. B: Environ.

    (2017)
  • L. Wojnárovits et al.

    Rate constants of carbonate radical anion reactions with molecules of environmental interest in aqueous solution: A review

    Sci. Tot. Environ.

    (2020)
  • G. Liu et al.

    Photoassisted degradation of dye pollutants. 8. Irreversible degradation of alizarin red under visible light radiation in air-equilibrated aqueous TiO2 dispersions

    Environ. Sci. Technol.

    (1999)
  • J. Zhao et al.

    Photoassisted degradation of dye pollutants. 3. Degradation of the cationic dye rhodamine B in aqueous anionic surfactant/TiO2 dispersions under visible light irradiation: evidence for the need of substrate adsorption on TiO2 particles

    Environ. Sci. Technol.

    (1998)
  • Cited by (16)

    • Novel ultrasensitive Raman assay method based on enzyme mimetics for ultra trace of H<inf>2</inf>O<inf>2</inf>

      2023, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
    View all citing articles on Scopus
    1

    These authors equally contributed to this work.

    View full text