受上述回顾性分析的启发，本研究通过简便的煅烧策略，制备了含 Fe-NC 和其他铁构型（铁氧化物和金属铁）的铁基催化剂。即使在高盐度、高浓度腐植酸和宽 pH 值范围内，合成的催化剂也能在 PMS 活化难降解污染物的过程中发挥出令人赞叹的催化活性。尤其是本研究中的高浓度含盐废水刷新了目前研究的极限，表现出了优异的环境耐受性。根据淬灭实验的 EPR 分析，1O2 被确认为主要的活性物质，它直接产生于与 PMS 的末端 O 原子结合的活性 Fe-吡啶 N 分子上。此外，实验结果和密度泛函理论（DFT）计算详细揭示了 1O2 直接生成过程的有效性和合理性，1O2 是由吸附的 HSO5- 上的两个 O 原子伴随着两个 HSO4- 分子的形成而产生的。这项工作将为了解 Fe-NC/PMS 系统中 1O2 的直接生成机制提供重要的启示和指导。
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Fig. 1. XRD patterns (a) and N2 adsorption-desorption isotherm (b) of Fe-NC catalysts; TEM images (c-d) and HAADF-STEM images (e-f) of MP-3; HAADF image (g) and corresponding EDS images (h-j) of MP-3.![]()
Fig. 2. K-edge XANES spectra of MP-1, MP-3, MP-5, Fe foil, FePc (a); Fourier transforms of the k3-weighted K-edge EXAFS spectra of MP-1, MP-3, MP-5, Fe foil, FePc (b); High resolution XPS (a) Fe 2p spectra (b) N 1s spectra of MP-1, MP-3 and MP-5 catalysts; (c) Low - temperature and (d) Room - temperature 57Fe Mössbauer spectra of MP-1, MP-3 and MP-5 catalysts.![]()
Fig. 3. Degradation of BPA in different systems (a); the corresponding pseudo-first order kinetics curves (b); (Conditions: [BPA] = 100 µM, [PMS] = 1 mM; [Catalyst] = 300 mg L−1).![]()
Fig. 4. Fenton-like activity towards BPA degradation by MP-3 catalyst with different quenching agents (a); BPA degradation of MP-3/PMS/BPA with methanol as solvent (b); EPR spectra for DMPO-OH/DMPO-SO4 of different system (c); BPA degradation of MP-3/PMS/BPA with D2O as solvent (d); Quenching effect of different concentration of L-histidine (e); EPR spectra of TEMP-1O2 in MP-3/PMS and PMS alone systems (f); EPR spectra of TEMP-1O2 in the different systems with D2O and H2O as solvent (g); Decomposition rates of PMS and BPA in MP-3/PMS/BPA system (h); Decomposition rates of PMS in MP-3/PMS/BPA system and MP-3/PMS system (i). (Conditions: [BPA] = 100 µM, [PMS] = 1 mM; [Catalyst] = 300 mg L−1).![]()
Fig. 5. Electrostatic potential map of N-doped graphene (a); Relationship between different N species content and kinetic constant of reaction of as-synthesized catalysts (b); Low temperature EPR of MP-3/PMS and MP-3/H2O (c); XPS analysis of used MP-3 (d). (Conditions: [BPA] = 100 µM, [PMS] = 1 mM; [Catalyst] = 0.3 g L−1).![]()
Fig. 6. Quenching experiment in MP-3/PMS system with methanol solvent (a) Relationship between organics degradation efficiency and corresponding ionization potentials (IP) (b); The charge density difference around the Fe atom embedded on Fe-graphitic N (c), Fe-pyridinic N (d) and Fe-pyrrolic N (e). The isosurface contour is 0.03 e/Bohr3. The blue and yellow colors denote electron accumulation and depletion, respectively. The electrostatic potentials of PMS and Fe-graphitic N (f), Fe-pyridinic N (g) and Fe-pyrrolic N (h). The isosurface value is 0.15 e/Bohr3. The spin density of PMS and Fe-graphitic N (i), Fe-pyridinic N (j) and Fe-pyrrolic N (k). The iso-surface value is 0.025 e/Bohr3.
总之，通过调整 PB 和三聚氰胺的比例，制备出了铁基催化剂。得益于铁纳米团簇物种继承的导电性能和单个铁原子的高活性，MP-3 催化剂在高盐度、高浓度腐植酸和宽 pH 值范围下，仍能在活化 PMS 降解难降解污染物方面发挥出优异的催化活性，解决了传统 PMS 基 AOPs 难以解决的难题。淬火实验和 EPR 测量结果表明，非自由基 1O2 种类占主导地位，而电子穿梭和高价铁-氧物种则起辅助作用。与以往通过前驱体（-O2-、SO5--）转化间接演化 1O2 的途径不同，实验和计算结果共同证实了 FeN4 分子的重要作用。根据对不同 FeN4 构型下 1O2 的三种生成途径的比较，提出了详细的 1O2 直接生成途径，即通过吸附的 PMS（HSO5-）分子中的两个 O 原子相互作用，伴随两个 HSO4-分子生成，且能垒最低。这项研究从原子尺度上揭示了 PMS 通过 FeN4 位点辅助生成 1O2 的催化机理，为未来开发高效率、高选择性和高适应性的 Fenton 类催化剂提供了新的思路。
Liang Zhang, Yun Sun, Rile Ge, Wenhui Zhou, Zhimin Ao, Junhu Wang, Mechanical insight into direct singlet oxygen generation pathway: Pivotal role of FeN4 sites and selective organic contaminants removal, Applied Catalysis B: Environmental, 2023, https://doi.org/10.1016/j.apcatb.2023.123130
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