Heterogeneous Fe-Co dual-atom catalyst outdistances the homogeneous counterpart for peroxymonosulfate-assisted water decontamination: New surface collision oxidation path and diatomic synergy
Graphical abstract
Introduction
The ever-growing demand for clean water and safe eco-environment stimulates the upgradation of wastewater treatment technologies (Dias et al., 2023). Among them, the advanced oxidation process (AOP) is powerful in degrading refractory organic pollutants to reduce the effluent toxicity (Hodges et al., 2018). Peroxymonosulfate (PMS), with a reactive OO bond in the asymmetric structure, is widely employed to initiate AOP owing to the simplicity of its activation (Li et al., 2023). Compared with homogeneous systems utilizing heat, light or metal ions to activate PMS, the heterogeneous catalysis possesses prominent merits including neglectable energy expenditure and convenient catalyst recycling (Gujar et al., 2022). However, limited by the low site intrinsic activity and sluggish mass transfer, heterogeneous catalysts still lag far behind homogeneous ones in terms of the catalytic efficiency (Qian et al., 2021). This dims their application prospects because more catalysts are required to ensure the treatment effectiveness. Promoting the kinetic rate of heterogeneous catalysts to approach or even surpass that of homogeneous catalysts is thus of great necessity, but remains challenging due to the lack of appropriate structural design strategy.
Recently, the single-atom catalysts (SACs) featuring atomically dispersed metal sites have demonstrated excellent PMS activation efficiency because of the unsaturated metal coordination structure and maximum atom utilization (Li et al., 2018). Moreover, SACs can bridge the gap between homogeneous and heterogeneous catalysts by integrating the well-defined quasi-homogeneous sites into solid materials (Weon et al., 2020). Consequently, an ideal platform can be constructed by SACs to precisely explore the heterogeneous structure-activity relationship at the atomic level, which in turn provides valid guidance for the catalyst structure design (Guo et al., 2019). Gao et al. tuned the metal center atom (Mn, Fe, Co, Ni, Cu) of carbon-supported SACs, and established the relation between the metal kind and catalytic performance (Gao et al., 2021). The Fe-SAC was found to possess higher activity than Co-SAC, while Co2+ was better than Fe2+ in homogeneous systems for PMS-assisted pollutant degradation (Gao et al., 2021; Zhu et al., 2021). Although this work and other frontier researches proposed various regulatory strategies to enhance the intrinsic activity of SACs, the inherent defects of SACs in catalyzing complex reactions could not be overcome (An et al., 2023). Due to the structural monotony of active sites, SACs suffer from the limitation of scaling relations between the adsorption energies of similar adsorbates (i.e. the adsorptions of an adsorbate and its derived molecules on the identical site are either all strong or all weak) (Zhang et al., 2023). This makes it difficult for SACs to find a thermodynamic balance between strongly adsorbing reactants, efficiently activating intermediates and easily desorbing products (Ying et al., 2021). Therefore, in the complicated PMS-based catalytic oxidation process involving multiple intermediates (e.g. *SO4, *OH, *O) and reaction steps, energetically unfavorable step or high energy barrier may exist, which causes the hardship of SACs to break the kinetic constraint and further improve the catalytic efficiency (Gao et al., 2021; Mi et al., 2021).
A promising approach to tackle this issue is introducing another metal atom into the active center of SACs to form heteronuclear dual-atom catalysts (HDACs), which contain two different metal atoms in one site (Sun et al., 2023). When the metal spacing matches with the size of PMS molecule, the metal dimers can act as two collaborative grippers to strongly adsorb PMS, consequently generating high surface concentration of oxidants (Cheng et al., 2023; Wang et al., 2022). Meanwhile, the increased site area and asymmetric electron distribution of the paired metal center can provide a variety of options for the bonding and activation of different intermediates/products derived from PMS, which is conducive to accelerating the rate-determining step and optimizing the global energy configuration (Sun et al., 2022). Besides the construction of above HDACs to boost the intrinsic activity of catalytic sites, the selection of supports is also crucial because ideal supports can supply suitable microenvironments for the efficient operation of active centers (Zhu et al., 2020). Among various candidates, the carbon matrix derived from metal-organic frameworks (MOFs) can inherit the large surface area and abundant pores, which enable high degree of site exposure and unimpeded mass transfer (Wang et al., 2020a). Although MOFs-derived carbon-supported HDACs have recently displayed enhanced catalytic activity in the field of energy conversion, the research on their feasibility in activating PMS to fleetly decompose contaminants is still in its infancy (Fan et al., 2023; Liu et al., 2022). More importantly, whether bimetallic atoms have synergistic effect in promoting reaction kinetics and the associated catalytic mechanism remain ambiguous, which deserves further exploration.
In this work, a porous carbon matrix with Fe-Co diatomic sites was synthesized by a co-adsorption-pyrolysis strategy using low-crystallinity NH2-UIO-66 as the precursor. The as-obtained composite not only owned higher efficiency than Fe/Co SACs and its homogeneous counterpart for sulfamethoxazole (SMZ, as model pollutant) degradation via PMS activation, but also exhibited superior performance beyond reported state-of-the-art catalysts. Besides, a fluidized-bed reactor was constructed to evaluate its dynamic SMZ removal effectiveness in actual waters. Chemical quenching tests, electron paramagnetic resonance spectroscopy and molecular probe experiments were conducted to identify the predominant reactive oxygen species (ROS). By means of dissolved oxygen monitoring, electrochemical analysis and theoretical calculation, a new surface collision oxidation path for SMZ elimination was discovered. The interaction behavior between PMS and SMZ on the dual-atom site, and Fe-Co synergistic mechanism for ultrafast catalytic kinetics were thoroughly revealed. The SMZ degradation pathways and effluent biotoxicity were also investigated. Overall, this work could guide the rational design of HDACs for efficient PMS activation and advance the atom-level mechanism understanding of PMS-based AOP.
Section snippets
Material and methods
The details of chemicals and materials, material preparation and characterizations, catalytic degradation procedure, analytic methods and computational methods were presented in Texts S1-S6.
Synthesis and characterization
The HDAC with atomically dispersed Fe-Co sites on porous carbon support was synthesized by a co-adsorption-pyrolysis strategy. Firstly, zirconium chloride (ZrCl4) and 2-amino-1,4-benzenedicarboxylic acid (NH2-H2BDC) were assembled to yellowish NH2-UIO-66 via the solvothermal reaction (Fig. S1). Subsequently, Fe2+ and Co2+ were synchronously adsorbed into the MOF with -NH2 group on its ligand to yield a yellow-green composite (U-FeCo), laying the foundation for forming bimetallic sites. In the
Conclusion
This work developed a MOF-derived carbon-supported HDAC by the co-adsorption-pyrolysis method. The obtained U-FeCo-C outperformed its homogeneous counterpart in activating PMS to degrade SMZ. The catalyst-dose normalized k value reached 99.26 L min−1 g−1, exceeding reported values by 1∼2 orders of magnitude. Moreover, the FeCoN6 diatomic site possessed higher TOF (13.07 min−1) than single-atom FeN4 and CoN4 sites. The catalytic system maintained excellent efficiency in the presence of anions
CRediT authorship contribution statement
Changqing Zhu: Conceptualization, Data curation, Funding acquisition, Methodology, Formal analysis, Investigation, Software, Writing – original draft, Visualization. Fenxian Cun: Investigation, Visualization, Methodology, Resources, Validation. Zhongwei Fan: Investigation. Yu Nie: Investigation. Qing Du: Investigation. Fuqiang Liu: Funding acquisition, Project administration, Supervision, Writing – review & editing. Weiben Yang: Supervision. Aimin Li: 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 supported by the National Natural Science Foundation of China (no. 51908273, 51522805).
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