Regular ArticleSelf-grown oxygen vacancies-rich CeO2/BiOBr Z-scheme heterojunction decorated with rGO as charge transfer channel for enhanced photocatalytic oxidation of elemental mercury
Graphical abstract
Introduction
With the continuous development of industrial technology, a large amount of fossil energy is consumed, bringing a series of energy and environmental problems. Mercury, as a typical atmospheric pollutant, seriously affects human health and the ecological environment [1], [2], [3]. Coal-fired power plants are the main source of anthropogenic mercury emissions [4], [5]. The mercury in the flue gas mainly exists in the form of gaseous elemental mercury (Hg0), oxidized mercury species (Hg2+) and particulate-bound mercury (Hgp) [3], [4], [5], [6], [7]. The gas-phase Hg0 present in coal-fired flue gas is the most difficult to control through existing power plant equipment [6], [7], [8]. Therefore, it is necessary to explore an effective scheme to remove low-concentration Hg0 pollutants in the gas phase. Photocatalysis is one of the most promising technologies to solve energy and environmental problems [9], [10]. It has attracted the attention of scholars all over the world in the field to degrade pollutants, hydrogen evolution, reduction of CO2, removal of heavy metals, etc. [11], [12], [13], [14]. Nevertheless, the capability of existing photocatalysts is far from meeting the requirements of industrial applications, which has become a bottleneck restricting the development of this conspicuous technology. For instance, wide band gap of photocatalyst limits light absorption efficiency [15]. Surface reactions usually have lower quantum yields due to the high activation energy barrier and rapid recombination of charge carriers [16]. The redox reaction of the catalyst is limited by the position of the valence band and conduction band [17]. Therefore, developing green catalysts with high quantum efficiency and redox capacity is of great meaningful.
So far, BiOX (X = Cl, Br, I) has risen in photocatalysis owing to its narrow band gap and unique layered structure, which has a layered structure of (BiO)22+ plates interlaced with double X-atom plates, interacting through non-bonding (van der Waals forces) [18]. Due to the weak force of van der Waals, the layered structure of BiOX can be peeled off along the (0 0 1) direction, which has an open type structure [19], which provides enough space for the polarization of atoms and orbits to form a built-in electric field (B-IEF) to promote the separation of charge carriers [19], [20]. Particularly, BiOBr has a suitable band gap energy (2.70 eV), which makes it have unique photocatalytic activity. But the high recombination rate of photogenerated electron and hole limits its application. To achieve a higher activity, great efforts have been devoted via surface modification, structural defects, ion doping, metal doping, interface heterojunction, etc. [21], [22], [23], [24], [25], [26], [27]. In particular, the formation of Z-scheme heterojunction via interface engineering is considered as an effective modification strategy to enhance the photocatalytic efficiency.
As a nontoxic and inexpensive rare earth material, cerium dioxide (CeO2) has attracted extensive attention in catalysis [28], [29], [30], which is considered as a potential photocatalyst with relatively narrow band gap and small size. Furthermore, CeO2 exhibits a pair of Ce4+/Ce3+ redox centers with oxygen storage capacity, which can be taken as a photocatalytic reaction centers and beneficial to the photocatalytic reaction [30], [31], [32]. Taken together, CeO2 is suitable for constructing heterojunction with BiOBr. In addition, reduced graphene oxide (rGO) with a large surface area, excellent optical properties and high conductivity has been reported as a good electronic mediator for constructing Z-scheme heterojunctions [33], [34]. For instance, Noda et al. reported that the C3N4/rGO/C-TiO2 ternary Z-scheme heterojunction with rGO as an electron mediator significantly enhanced the photocatalytic activity [34]. Thus, it is worth looking forward to the construction of CeO2/BiOBr Z-scheme heterojunction with rGO as an electron mediator to promote charge transfer and photocatalytic activity.
Currently, Surface defects are widely considered to tune the electronic structure, charge density and charge separation of the photocatalyst [35], [36], [37], [38]. Sun et al. reported that BiOIO3 with oxygen vacancies promoted the separation of charge carriers and facilitated the photocatalytic oxidation of elemental mercury [35]. Oxygen vacancy as an inherent defect of the photocatalyst is of an important influence on the charge distribution and band structure of the photocatalyst [36], [37], which could trap electrons in photocatalytic process to hinder the recombination of photogenerated electron pairs. Li et al. proved that BiOCl with oxygen vacancies can effectively capture electrons and selectively oxidize NO to nitrate [38]. Meanwhile, oxygen vacancies could facilitate the adsorption and activation of molecular oxygen and lessen charge transfer barriers owing to the formation of abundant localized electrons [39], [40], [41]. Zhu et al. studied that oxygen vacancies can capture and activate surface adsorbed oxygen, which is conducive to the catalytic oxidation of Hg0 [39]. Therefore, the introduction of abundant surface oxygen vacancies is expected to enhance charge transfer and molecular oxygen activation.
Herein, oxygen vacancies-rich CeO2/BiOBr decorated with rGO were successfully synthesized, which was used for photocatalytic removal of gaseous elemental mercury. It was found that the efficiency of photocatalytic mercury removal was significantly improved, which is 1.29 times of BiOBr and 1.91 times of CeO2. Meanwhile, the effect of actual flue gas components (SO2, NO and HCl) on the performance of photocatalytic Hg0 removal was investigated [42]. The physicochemical properties of the catalyst were evaluated via characterization method, combined with DFT calculation, the effect of oxygen vacancies on the photocatalyst was discussed, and the Z-scheme heterojunction photocatalytic mechanism was proposed. This study could provide a feasible strategy for exploring high efficiency Z-scheme heterojunction photocatalysts.
Section snippets
Experimental section
The details of Experimental section are given in the Supplementary Information. The schematic illustration of the preparation process is displayed in Scheme 1. The schematic diagram of the experimental system is shown in Fig. S1. The photocatalytic removal efficiency was calculated according to the Eq. (1) in the Supplementary Information.
Characterization of photocatalysts
The crystal structures and phase of the prepared CeO2, BiOBr, CeO2/BiOBr and CeO2/BiOBr/rGO were identified via XRD patterns. From Fig. 1a, it can be observed that the main diffraction peaks at 28.55°, 33.08°, 47.48°, and 56.34° correspond to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of CeO2 (fluorite cubic phase, JCPDS # 43-1002), respectively [43]. The main diffraction peak of BiOBr can be indexed in JCPDS # 09-0393 to show the tetragonal crystal phase [30]. The intensity of (1 1 1), (2 0 0), (2 2
Conclusions
In summary, oxygen vacancies-rich CeO2/BiOBr decorated with rGO was synthesized via solvothermal method for removal gas-phase elemental mercury. Herein, CeO2 nanoparticles can uniformly self-grow on BiOBr flower-ball decorated with rGO to form a Z-scheme heterojunction. This significantly promotes the electron-hole separation efficiency and accelerates the charge carrier transfer of the single catalyst. Meanwhile, the surface oxygen vacancies and Ce3+/Ce4+ redox centers tend to capture
CRediT authorship contribution statement
Tao Jia: Writing - original draft, Writing - review & editing, Formal analysis, Software, Data curation, Conceptualization. Jiang Wu: Writing - review & editing, Project administration. Yixuan Xiao: Software, Validation. Qizhen Liu: Resources. Qiang Wu: Supervision. Yongfeng Qi: Supervision. Xuemei Qi: Supervision.
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.
Acknowledgments
This work was partially sponsored by National Natural Science Foundation of China (52076126), Natural Science Foundation of Shanghai (18ZR1416200).
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