Oxygen vacancy mediated surface charge redistribution of Cu-substituted LaFeO3 for degradation of bisphenol A by efficient decomposition of H2O2
Graphical abstract
Introduction
Strategies for efficient and rapid degradation of organic pollutants in wastewater are of great research interest in many disciplines related to environmental science (Zhu et al., 2019; Xing et al., 2018). The vigorous development of heterogeneous Fenton-like catalysts in recent years has benefited from the advance of novel techniques to tackle many issues of traditional Fenton reactions (Zhang and Zhou, 2019; Liu et al., 2016). Therefore, heterogeneous Fenton catalysts such as iron minerals (Wang et al., 2019), transition metal oxides (Su et al., 2019) and their composites (Li et al., 2018a) were studied to replace soluble Fe2+ for the catalysis of H2O2 decomposition reaction, to expand the reaction pH (Lu et al., 2018; Xiang et al., 2019; Huang et al., 2019; Canals et al., 2013) and avoid the precipitation of Fe2+ ions (Jin et al., 2019).
Unsaturated coordination is an effective way of improving the catalytic capability of Fenton-like reactions with transition metals (Shen et al., 2016; Liu et al., 2018). However, the preparation of such substances, usually involving organic ligands with unique structures, is relatively complex (Liu et al., 2018; Ranji-Burachaloo et al., 2019; Li et al., 2016a). In contrast, catalysts with oxygen-containing compounds with oxygen defects are comparatively easier to prepare (Li et al., 2018b, 2017; An et al., 2019; Lyu et al., 2018a). Peroxide species generated on the surface of catalysts are formed by the interaction between oxygen vacancies (OVs) and H2O2 (Wang et al., 2018a). Metal-organic frameworks (Tang and Wang, 2018; Guo et al., 2019), polyoxometalates (An et al., 2019), solid solutions (Yang et al., 2018), g-C3N4 (Jiang et al., 2018) and oxyhalides (Li et al., 2017; Wang et al., 2018b) have been explored as advanced materials for generating OVs. Unlike traditional bimetallic or polymetallic catalysts, these materials can catalyze the decomposition of H2O2 through electron-rich and electron-deficient regions on the surface of materials, rather than relying solely on the valence changes of transition metals. Efficient Fenton-like catalysis can be achieved even without transition metals where defects can induce sufficient differences of charge densities distribution (Lyu et al., 2018b). Therefore, the existence of oxygen vacancy is a reliable means to realize charge redistribution (Wang et al., 2016, 2018c; Li et al., 2018c). Thus, the design of catalyst with significant difference in charge density of the surface active sites is critical in promoting the formation of reactive oxygen species (ROS) (Li et al., 2019).
Perovskite oxide (ABO3-type oxide) is a class of transition metal oxide with unique crystal structure that the atoms at position A and B can be substituted by other atoms with similar radius and charge number (Wang et al., 2018d). As a heterogeneous catalyst, its unique crystal structure provides flexibility for doping with transition metal (Chandrasekaran et al., 2017), which would form defect structures having some elements with mixed valence state, with the resulting lattice defects facilitate the generation of ROS (Wang et al., 2018d; Jin et al., 2011; Wang et al., 2018e; Zhu et al., 2018). This provides a variety of effective ways for the realization of oxygen vacancy mediated advanced oxidation processes (Sun et al., 2019). Density functional theory (DFT) calculations show that H2O2 molecules are mainly absorbed on the surface vacancy sites of BiFeO3 perovskite facets. The O − O bonds were elongated at the OV sites with strong interaction with iron atoms and thereby the O − O bond strength was significantly weakened, leading to the accelerated generation of •OH (Wei et al., 2010). With the formation of LaFeO3-H2O2 complex, the decomposition of H2O2 into O2 and ROS, as well as the cycling of Fe3+/Fe2+ are greatly accelerated (Nie et al., 2015; Jauhar et al., 2015; Pecchi et al., 2011). Charge compensation in perovskite structure was the critical factor for their superior performances (Wang et al., 2018e; Safakas et al., 2019). However, a high concentration of H2O2 is still required due to the low utilization rate, where H2O2 is more intended to decompose and produce O2 rather than ROS. As a result, most perovskite Fenton-like catalysis still needs to be carried out at weak acidic conditions (pH3-5) (Nie et al., 2015), and the elucidation of the role of oxygen defects on the electron distribution of the catalyst surface is essential for the design and synthesis of perovskite structural oxides.
Here in, we fabricated a defective Cu-substituted LaCuxFe1-xO3-δ perovskite catalyst by manipulating the copper doping ratio with a citric acid complexation method. Bisphenol A (BPA), a representative endocrine disrupting chemical pollutant (Yu et al., 2019; Zhang et al., 2019), was degraded at near neutral pH condition in the presence of H2O2 to evaluate the oxidation degradation activity of the catalyst. The degradation efficiency of the synthesized LaCuxFe1-xO3-δ was characterized and the stability under weakly acidic conditions was also evaluated compared to the undoped LaFeO3. Both chemical characterizations and DFT calculations were used to elucidate the functions of defects and electron transfer to shed light on the catalytic mechanisms.
Section snippets
Chemicals and materials
Bisphenol A (97 %), Fe(NO3)3·9H2O, Cu(NO3)2·3H2O, La(NO3)3·6H2O, NH3·H2O (30 %, w/w), citric acid, H2O2 (30 %, w/w), NaOH and H2SO4 were purchased from the Sinopharm Chemical Reagent Co., Ltd (analytical grade) and used without further purification. Deionized water was used throughout this study. The reactants pH was adjusted by aqueous solution of NaOH or H2SO4.
Preparation of the LaCuxFe1-xO3-δ perovskite
Cu-substituted LaCuxFe1-xO3-δ (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by a modified citric acid complexation method (Zhao
Characterization of LaCuxFe1-xO3-δ
Phase evolutions of the as prepared perovskite oxides were analyzed using the powder XRD technique (Fig. 1a). XRD peaks became slightly narrower and sharper with the increased calcination temperature from 400 to 800 °C. The XRD patterns of LaCu0.5Fe0.5O3-δ annealed at 700 °C and 800 °C showed a highly crystalline structure and were well correspond to the standard card of Lanthanum ferrite (ICSD:74-2203). In addition, based on the change of thermogravimetry (TG) curves of LaFeO3 and LaCu0.5Fe0.5O
Conclusion
Copper substituted LaCu0.5Fe0.5O3-δ perovskite structure was prepared with controlled OVs, which exhibited excellent catalytic activity and stability for the degradation of BPA. BPA removal of 92.1 % at near neutral pH after 120 min of reaction time was successfully achieved. The catalyst is capable of maintaining its composition and retaining high catalytic efficiency in the range of pH3-6. •OH was identified as the main ROS involved in the degradation process, which originated from the
CRediT authorship contribution statement
Keliang Pan: Methodology, Investigation, Writing - original draft. Changzhu Yang: Data curation. Jingping Hu: Software. Wenlong Yang: Formal analysis. Bingchuan Liu: Validation. Jiakuan Yang: Supervision. Sha Liang: Investigation. Keke Xiao: Data curation. Huijie Hou: Conceptualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
The authors would like to express their gratitude for the financial support for this research from the project funded by Natural Science Foundation of China (51608217, 51878309, 51508213), National Key Research and Development Program of China (2018YFC1900105), Innovative and Interdisciplinary Team at HUST (0118261077) and the Fundamental Research Fund for the Central Universities (2017KFYXJJ217). The authors would like to appreciate the Analytical and Testing Center of Huazhong University of
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