Persulfate activation by two-dimensional MoS2 confining single Fe atoms: Performance, mechanism and DFT calculations

https://doi.org/10.1016/j.jhazmat.2020.122137Get rights and content

Highlights

  • Two-dimensional MoS2 confining single Fe atoms was synthesized.

  • Sulfate radicals were generated via persulfate activation by FexMo1-xS2 catalyst.

  • Dual Fe and Mo catalytic sites and their interaction enhance catalytic performance.

  • The catalytic performance of FexMo1-xS2 was interpreted by DFT calculations.

  • Various organic pollutants can be removed in a real water environment.

Abstract

Developing efficient catalysts for persulfate (PS) activation is important for the potential application of sulfate-radical-based advanced oxidation process. Herein, we demonstrate single iron atoms confined in MoS2 nanosheets with dual catalytic sites and synergistic catalysis as highly reactive and stable catalysts for efficient catalytic oxidation of recalcitrant organic pollutants via activation of PS. The dual reaction sites and the interaction between Fe and Mo greatly enhance the catalytic performance for PS activation. The radical scavenger experiments and electron paramagnetic resonance results confirm and SO4radical dot rather than HOradical dot is responsible for aniline degradation. The high catalytic performance of Fe0.36Mo0.64S2 was interpreted by density functional theory (DFT) calculations via strong metal-support interactions and the low formal oxidation state of Fe in FexMo1-xS2. FexMo1-xS2/PS system can effectively remove various persistent organic pollutants and works well in a real water environment. Also, FexMo1-xS2 can efficiently activate peroxymonosulfate, sulfite and H2O2, suggesting its potential practical applications under various circumstances.

Introduction

Developing technologies for efficient removal of persistent organic pollutants is strongly desired during water treatment and wastewater reclamation process. Advanced oxidation processes (AOPs) are believed as one of the most promising technologies to obtain this goal. Hydroxyl radical (HOradical dot), which is highly reactive towards nearly all persistent organic pollutants, is the most used oxidants generated from ozone or H2O2 in AOPs. Using ozone as a HOradical dot source require complex and high-cost ozone generation system which hinders its large scale application (Lee et al., 2017). H2O2 generate HOradical dot via Fenton process (Fe2+ + H2O2→ Fe3+ + HOradical dot + OH), which requires acidic pH (Chakma and Moholkar, 2014). Thus, a large amount of acid and base is needed for pH adjustment. Sludge production and disposal during Fenton process is also a great environmental concern (Zeng et al., 2019).

Sulfate radical (SO4radical dot) has been recently recognized as one of the most highly reactive oxidants, as superior as hydroxyl radical (HOradical dot), for degradation of organic pollutants in AOPs. Persulfate (PS) has been widely utilized as a precursor to generate sulfate radicals. PS can be activated via various catalysts, alkaline, heat, UV irradiation and electrochemical method to generate SO4radical dot (Ahmad et al., 2013; Johnson et al., 2008; Furman et al., 2010; Kim et al., 2018; Matzek et al., 2018). In contrast to the high energy consumption and high chemical dosage for most of these activation methods, transitional metal-based catalyst has been proved to be efficient and cost-effective to activate PS in potential practical applications (Zhang et al., 2013). Transitional metal ions dissolved in homogeneous solutions are one type of efficient catalysts for PS activation (Huang and Huang, 2009). Unfortunately, the formation of metal sludge and the potential health hazards caused by free metal ions in water is of great concern. Thus, a variety of metal nanomaterials have been developed and used as heterogeneous catalysts for the activation of PS (Rong et al., 2019; Xu et al., 2019; Zhou et al., 2019; Liu et al., 2014). Metal nanomaterials are relatively stable for catalysis and can be reused after separation from the treated water. However, the low activity is always the issue compared to homogeneous metal ions (Zhang et al., 2018). The intrinsic reason is that PS activation only occurs on the surface of metal nanomaterials, and any metal atoms inaccessible by PS molecules are not involved in the catalytic process. Single-atom catalysts (SACs), with atomically distributed active sites on supports, are believed to have the advantages of both homogeneous catalysts (high reactivity) and heterogeneous catalysts (stable, easy to separate and reuse, no secondary pollution) in water treatment applications (Chen et al., 2018). Application of SACs in AOPs has been scarcely studied although it is one of the most promising strategies to maximize the efficiency of AOPs in potential practical applications (Li et al., 2018; Guo et al., 2019; Yin et al., 2019; An et al., 2018).

The surface free energy is extremely high for SACs, thus the aggregation of SACs is a big problem during their application. This problem can be solved by anchoring single metal atoms on suitable catalyst support (Zhang et al., 2018). Various supports have been developed to confine SACs, including 3D (carbon, metal oxide, MoC, metal-organic frameworks etc.) and 2D supports (graphene, g-C3N4, and MoS2) (Lin et al., 2013; Pei et al., 2015; Qiu et al., 2015; Yan et al., 2015; Jones et al., 2016; Li et al., 2016; Liu et al., 2016; Yin et al., 2016; Wang et al., 2019; Sun et al., 2019). SACs confined in 2D supports have several unique features compared to 3D supports such as more coordinatively unsaturated single atoms, expedited mass-transfer on both sides of the 2D structure, and the well-defined 2D motif allowing catalytic performances interpreted theoretically (Wang et al., 2019). Also, the interaction between SACs and supports significantly influence the activity, selectivity, and stability of the catalysts (Zhang et al., 2018). Recent studies have shown that MoS2 can act as co-catalyst in homogeneous Fenton-like reaction (Xing et al., 2018; Liu et al., 2018). Thus, we hypothesize that Fe SACs confined in 2D MoS2 support may lead to strong SACs-supports interaction and lead to high activity in AOPs.

In this work, we report the in-plane doping of single Fe atoms in 2D MoS2 with various Fe content (designated as FexMo1-xS2) and demonstrate that FexMo1-xS2 is a highly active catalyst for PS activation, thereby leading to complete mineralization of aniline, a persistent organic pollutant widely detected in surface and ground waters. The high activity of FexMo1-xS2 derives from the synergistic catalysis between the atomically distributed Fe and Mo sites, as revealed by experiments and theoretical calculations. Sulfate radicals are demonstrated to be the major reactive oxygen species responsible for the oxidative degradation of aniline. Furthermore, the FexMo1-xS2/PS system can degrade a wide range of other persistent organic pollutants and work well in a real water environment, testifying the great potential of FexMo1-xS2/PS system for practical water treatment applications.

Section snippets

Chemicals and materials

The FexMo1-xS2 nanosheets were synthesized via a biomolecule-assisted hydrothermal synthetic route (Chang and Chen, 2011; Miao et al., 2015). FeSO4·7H2O, Na2MoO4·2H2O, and L-cysteine were used as iron, molybdenum and sulfur source, respectively. A 200 mL aqueous solution consists of Na2MoO4·2H2O, FeSO4·7H2O and L-cysteine were used as a precursor for hydrothermal synthesis. The FexMo1-xS2 with different x was synthesized by varying Na2MoO4·2H2O/FeSO4·7H2O ratios of 1/1, 1/3 and 3/1 with the sum

Experimental and theoretical characterization of FexMo1-xS2

STEM images show FexMo1-xS2 has a nanosheet morphology similar to MoS2 (Fig. 1a). No nanoparticles or distinct clusters can be observed by TEM, implying impurities such as ferrous sulfide does not exist in FexMo1-xS2. EDS elemental mapping in HAADF-STEM images demonstrates the homogeneous distribution of Fe, Mo, and S in the FexMo1-xS2 nanosheets (Fig. 1b-d). The atomically dispersion of Fe atoms in MoS2 matrix was evidenced by the dispersed white dots in the MoS2 matrix observed by spherical

Conclusion

Two-dimensional MoS2 confining single Fe atoms (FexMo1-xS2) was synthesized to activate PS for aniline degradation. Aniline can be complete mineralized to CO2 and H2O as confirmed by the TOC analysis. Fe0.36Mo0.64S2 shows good reusability and stability during PS activation. FexMo1-xS2/PS system can effectively remove various persistent organic pollutants and FexMo1-xS2 show high reactivity in most AOP systems. The slightly decreased degradation efficiency of aniline in real polluted water can

CRediT authorship contribution statement

Li-Zhi Huang: Conceptualization, Methodology, Writing - original draft, Project administration, Funding acquisition. Chu Zhou: Validation, Investigation. Miaolong Shen: Validation, Investigation. Enlai Gao: Formal analysis. Chunbo Zhang: Formal analysis. Xin-Ming Hu: Writing - review & editing. Yiqun Chen: Supervision, Funding acquisition. Yingwen Xue: Supervision. Zizheng Liu: Supervision, Funding acquisition.

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 funded by the National Natural Science Foundation of China (Grant No. 51978537, 41807188, 51508423 and 51508423), the National Natural Science Foundation of China and the Russian Foundation for Basic Research (NSFC–RFBR 51811530099).

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