Constructing highly catalytic oxidation over BiOBr-based hierarchical microspheres: Importance of redox potential of doped cations

Highlights

• Modified BiOBr microspheres were facilitated and used in heterogeneous Fenton-like assisted photocatalysis process.

• Cu-BiOBr and Fe-BiOBr exhibited higher catalytic performance on organic contaminants degradation in Vis/catalyst/H₂O₂ system.

• The possible catalytic mechanisms of modified BiOBr in Vis/catalyst/H₂O₂ systems are compared.

Abstract

In this work, we prepared BiOBr-based hierarchical microspheres by a simple solvothermal method. The phase structure, morphology and optical properties of catalysts were well characterized by XRD, FESEM, FTIR, UV-DR spectra, XPS valence band and BET surface area analysis. Among the Vis/catalyst/H₂O₂ system, Cu-BiOBr is found to be the most effective for rhodamine b degradation while Fe-BiOBr exhibits the highest catalytic activity for the mineralization of 2-chlorophenol. Hydroxyl radicals generation rate and H₂O₂ decomposition rate follow: Fe-BiOBr > BiOBr > Zn-BiOBr = Ni-BiOBr = Ag-BiOBr > Cu-BiOBr, and Cu-BiOBr > Fe-BiOBr > BiOBr = Zn-BiOBr = Ni-BiOBr > Ag-BiOBr, respectively. The catalytic mechanisms under Vis/catalyst/H₂O₂ systems are proposed and compared, as following: (1) for BiOBr/Zn-BiOBr/Ni-BiOBr/Ag-BiOBr, the activation of H₂O₂ by photoelectrons to generate hydroxyl radical; (2) for Fe-BiOBr, the reaction of Fe(II) or photoelectrons with H₂O₂ to produce hydroxyl radical, and Fe(III) is reduced by photoelectrons to Fe(II); (3) for Cu-BiOBr, the activation of H₂O₂ by photoelectrons to generate hydroxyl radical that probably oxides Cu(II) to Cu(III), and the reaction of Cu(I) with H₂O₂ to generate Cu(III). The trapping experiments display that holes and hydroxyl radicals (or Cu(III)) have dominant roles.

Introduction

Nowadays, the removal of organic contaminants from industrial wastewater has attracted increasing interest. Stable organic dye and chlorophenols molecules are prominent in their contribution towards water pollution. Large amounts of wastewater containing dyes are generated from textile dyeing industries. These dyes and pigments not only give colour to water but also inhibit biological treatment processes [1], [2], [3]. Chlorophenols are widely used as raw materials or intermediates in chemical industries such as the production of pesticides, synthetic resins, and pharmaceuticals [4], [5], [6]. Due to their potential carcinogenic and mutagenic activity, they can bring a potential negative impact on the environment, particularly damaging livers’ physiological processes. Various techniques are adopted to remove these contaminants, including adsorption, coagulation, electrochemical oxidation, semiconductor photocatalysis, degradation by strong oxidizing agents etc [7], [8], [9], [10]. Among these techniques, semiconductor photocatalysis has been great attention in this area due to its attractive features such as environmentally friendly and readily available materials, ease of operation as well as efficient degradation.

Recently, BiOBr has been widely concerned because of outstanding optical and electrical properties. It has a tetragonal matlockite structure, a layer structure characterized by [Bi₂O₂] slabs interleaved by double slabs of Br atoms [11]. The band gap of BiOBr is round 2.7 eV [12], which can adsorb parts of visible light (Vis) and has high recombination chance of holes and photoelectrons. Researchers have devoted intensive efforts to find out some methods that reduce their recombination opportunity and increase the visible light adsorption range, for instance, various hierarchical nanostructures [13], halogen-mixing solid solutions [14], semiconductor coupling [15], [16] and metal doping [17], [18]. The modified BiOBr photocatalyst showed the high photocatalytic activity for methylene blue [19], [20], while lower catalytic performance for those resistant pollutants (i.e., 2-chlorophenol and bisphenol A).

Fenton-like reaction system is also proven to be an efficient system for the mineralization of organic pollutants in wastewater. Li et al. reported that FeO_x/NiO_y/SBA-15 showed the high Fenton-like catalytic activity for acid red 73, and nickel played an important role in the catalytic activity [21]. The acetaminophen was completely degraded within 24 h by Cu–Zn–Fe LDH in the Fenton-like reaction as Lu reported [22]. Park et al. found that the fully dispersed PVP-AgNPs exhibited fast degradation kinetics for EE2 in the presence of H₂O₂ [23]. This attributes that low transition metal can react with H₂O₂, producing the hydroxyl radicals. However, in the Fenton-like reaction, the reduce rate of high-valent metal to low-valent metal by H₂O₂ is extremely slow [24], [25]. Light irradiation can improve the catalytic activity of some semiconductor heterogeneous Fenton catalysts [26], [27], [28], [29]. The decomposition rate of H₂O₂ becomes one of the crucial factors on the photocatataytic degradation efficiency [30]. Thus, the development of transition metal modified BiOBr with enhanced Vis-light-driven activity is of high interest.

The specific objective of this work was to evaluate the Vis-light-catalytic activity of transition metal modified BiOBr system for rhodamine (RhB) and 2-chlorophenol (2-CP) degradation with coupling heterogeneous Fenton-like oxidation. Herein, the M (Ag/Ni/Cu/Fe/Zn) modified BiOBr catalyst was prepared by a solvothermal method and characterized by XRD, FESEM, XPS valence band, UV–vis absorption spectroscopy and N₂ adsorption-desorption. The possible mechanisms for organic contaminants degradation by metal modified BiOBr were proposed and explained by EPR technology, H₂O₂ consumption and hydroxyl radical measurement experiments. The difference of oxidation process among Vis/modified BiOBr/H₂O₂ systems was also elucidated by XPS valance band analysis for the first time.