Research ArticleChemistry

A single iron site confined in a graphene matrix for the catalytic oxidation of benzene at room temperature

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Science Advances  04 Dec 2015:
Vol. 1, no. 11, e1500462
DOI: 10.1126/sciadv.1500462
  • Fig. 1 Structural analysis of graphene-embedded FeN4 (FeN4/GN) catalysts.

    (A to D) High-resolution transmission electron microscopy (HRTEM) images of FeN4/GN-2.7. The area with arrows and the dashed circles shows some typical single Fe atoms in the nanosheets. (E and F) Atomic models (E) and the corresponding simulated HRTEM images (F) for the structures in (D), where the FeN4/GN structures have been optimized. (G and H) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of FeN4/GN-2.7. (I) The electron energy loss spectroscopy (EELS) atomic spectra of Fe and N elements from the bright dots as shown by the red arrow in (H). The red circles show Fe and N signals, respectively. a.u., arbitrary units. (J) Low-temperature scanning tunneling microscopy (LS-STM) image of FeN4/GN-2.7, measured at a bias of 1.0 V and a current (I) of 0.3 nA (2 nm × 2 nm). (K) Simulated STM image for (J). The inserted schematic structures represent the structure of the graphene-embedded FeN4. The gray, blue, and light blue balls in (E), (J), and (K) represent C, N, and Fe atoms, respectively. (L) dI/dV spectra acquired along the white line in the inset image. U, 1.0 V; I, 0.3 nA; modulation frequency, 500 Hz; amplitude, 20 millivolts peak to peak; RC, 7 Hz.

  • Fig. 2 Chemical state and coordination information of FeN4/GN catalysts.

    (A and B) Fe K-edge x-ray absorption near-edge structure (XANES) (A) and Fourier transform (FT) extended x-ray absorption fine structure (EXAFS) (B) signals of FeN4/GN samples with various Fe content in comparison to FePc, Fe foil, and Fe2O3. (C and D) C K-edge (C) and N K-edge (D) x-ray absorption spectroscopy (XAS) spectra of FeN4/GN samples with various Fe content in comparison to that of FePc. (E) N 1s x-ray photoelectron spectroscopy (XPS) spectra of FeN4/GN samples with various Fe content in comparison to FePc. The inserted schematic structures represent the FePc molecule, where the pyrrolic N with Fe bonding is denoted as Nα and the pyridinic N with carbon bonding on the outside macrocycle is denoted as Nβ.

  • Fig. 3 The performance and reaction process of the catalytic oxidation of benzene to phenol over FeN4/GN catalysts.

    (A) The performance of the direct oxidation of benzene to phenol by FeN4/GN samples compared with GF, GN, and FePc. Reaction conditions: 50 mg of catalyst, 0.4 ml of benzene, 6 ml of H2O2 (30%), and 3 ml of CH3CN in a pressure vessel at 25°C for 24 hours. (B) The phenol yield of FeN4/GN-2.7 for the direct oxidation of benzene to phenol with different reaction times. (C and D) Fe K-edge XANES (C) and FT EXAFS (D) signals of FeN4/GN samples with H2O2 treatment in comparison to their corresponding original samples.

  • Fig. 4 Theoretical analysis of the FeN4/GN structure and the catalytic reaction process by DFT calculations.

    (A) The formation energies of FeN4/GN and Fe/GN structures. The formation energy is calculated as follows: EFe-embeddedEFe-bulkE(N)GN, where EFe-embedded and EFe-bulk are the total energies of FeN4/GN and the Fe/GN structure and an Fe atom in Fe bulk, respectively, and E(N)GN is the total energy of the optimized structure of FeN4/GN or Fe/GN with the Fe atom removed from the system. (B) Free energy diagram of the oxidation of benzene to phenol on FeN4/NG. The gray, blue, light blue, red, and white balls represent C, N, Fe, O, and H atoms, respectively. (C) Scheme for the reaction mechanism of the oxidation of benzene to phenol on FeN4/NG.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/11/e1500462/DC1

    Fig. S1. HRTEM images of FeN4/GN-2.7.

    Fig. S2. HRTEM image of FeN4/GN-2.7 with the red circles showing some typical single Fe atom positions in the graphene network.

    Fig. S3. HAADF-STEM image of FeN4/GN-2.7.

    Fig. S4. HRTEM image of FeN4/GN-2.7 with the red circles showing some Fe atoms with different defects in the surroundings.

    Fig. S5. XRD patterns of graphite, GN, FeN4/GN-1.5, FeN4/GN-2.7, FeN4/GN-4.0, and FePc.

    Fig. S6. Raman spectra of FeN4/GN samples in comparison to their parent materials FePc, GN, and graphite.

    Fig. S7. XPS spectra of FePc, FeN4/GN-4.0, FeN4/GN-2.7, and FeN4/GN-1.5.

    Fig. S8. Scheme of a proposed mechanism for synthesis of FeN4/GN via a facile ball milling method.

    Fig. S9. Low-temperature O2 TPD profiles of FeN4/GN-2.7, GN, and GF.

    Fig. S10. The recycling experiments of FeN4/GN-2.7.

    Fig. S11. Models of FeN4/GN and the FePc monomer in the DFT calculations.

    Fig. S12. Free energy profile of the benzene oxidation reaction intermediates on the iron site of the FePc monomer and FeN4/GN.

    Fig. S13. Fe K-edge XANES signal of FeN4/GN samples with H2O2 treatment in comparison to their corresponding original samples.

    Fig. S14. The Fe K-edge EXAFS analysis of FeN4/GN samples before and after H2O2 treatment.

    Fig. S15. Room-temperature 57Fe Mössbauer spectra of FeN4/GN-2.7, FeN4/GN-2.7-H2O2, and FeN4/GN-2.7-H2O2-Ben.

    Table S1. The elemental compositions of FePc, FeN4/GN-4.0, FeN4/GN-2.7, and FeN4/GN-1.5 estimated from XPS and ICP measurements.

    Table S2. Catalytic performance of different samples for the direct oxidation of benzene to phenol.

    Table S3. Catalytic performance of FeN4/GN-2.7 for the direct oxidation of benzene to phenol with different reaction times.

    Table S4. Catalytic performance of different samples for the direct oxidation of benzene to phenol at 0°C.

    Table S5. Fitting parameters for the analysis of the EXAFS spectra of FeN4/GN samples with H2O2 treatment in comparison to their corresponding original samples.

    Table S6. Fitting parameters for the 57Fe Mössbauer spectra in fig. S15.

    References (5961)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. HRTEM images of FeN4/GN-2.7.
    • Fig. S2. HRTEM image of FeN4/GN-2.7 with the red circles showing some typical single Fe atom positions in the graphene network.
    • Fig. S3. HAADF-STEM image of FeN4/GN-2.7.
    • Fig. S4. HRTEM image of FeN4/GN-2.7 with the red circles showing some Fe atoms with different defects in the surroundings.
    • Fig. S5. XRD patterns of graphite, GN, FeN4/GN-1.5, FeN4/GN-2.7, FeN4/GN-4.0, and FePc.
    • Fig. S6. Raman spectra of FeN4/GN samples in comparison to their parent materials FePc, GN, and graphite.
    • Fig. S7. XPS spectra of FePc, FeN4/GN-4.0, FeN4/GN-2.7, and FeN4/GN-1.5.
    • Fig. S8. Scheme of a proposed mechanism for synthesis of FeN4/GN via a facile ball milling method.
    • Fig. S9. Low-temperature O2 TPD profiles of FeN4/GN-2.7, GN, and GF.
    • Fig. S10. The recycling experiments of FeN4/GN-2.7.
    • Fig. S11. Models of FeN4/GN and the FePc monomer in the DFT calculations.
    • Fig. S12. Free energy profile of the benzene oxidation reaction intermediates on the iron site of the FePc monomer and FeN4/GN.
    • Fig. S13. Fe K-edge XANES signal of FeN4/GN samples with H2O2 treatment in comparison to their corresponding original samples.
    • Fig. S14. The Fe K-edge EXAFS analysis of FeN4/GN samples before and after H2O2 treatment.
    • Fig. S15. Room-temperature 57Fe Mössbauer spectra of FeN4/GN-2.7, FeN4/GN-2.7-H2O2, and FeN4/GN-2.7-H2O2-Ben.
    • Table S1. The elemental compositions of FePc, FeN4/GN-4.0, FeN4/GN-2.7, and FeN4/GN-1.5 estimated from XPS and ICP measurements.
    • Table S2. Catalytic performance of different samples for the direct oxidation of benzene to phenol.
      Table S3. Catalytic performance of FeN4/GN-2.7 for the direct oxidation of benzene to phenol with different reaction times.
    • Table S4. Catalytic performance of different samples for the direct oxidation of benzene to phenol at 0°.
    • Table S5. Fitting parameters for the analysis of the EXAFS spectra of FeN4/GN samples with H2O2 treatment in comparison to their corresponding original samples.
    • Table S6. Fitting parameters for the 57Fe Mössbauer spectra in fig. S15.
    • References (59–61)

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