4.3. R ESULTS AND D ISCUSSION
4.3.2. XAS Analysis
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five-coordination square pyramidal structure. The peaks at 7132 eV (peak C) and 7140 eV (peak D) at the edge region can inform the degree of distortion of the Fe–N4 site with the D4h symmetry.38 Fe–N/C catalysts have a larger portion of distorted Fe–N4 sites because the intensity of peak C is relatively higher than that of peak D.29,38,51 For FePc and FeTMPPCl, the intensity of peak C is lower than that of peak D, suggesting a near-planar structure for both the references. Furthermore, the CNT/PC(LT) and CNT/PC(SiO2) samples showed similar relative intensity ratios of peaks C and D to those of the references. However, after high-temperature pyrolysis, CNT/PC(HT) and CNT/PC had a reversed intensity ratio of peaks C and D, indicating that the Fe atom was off-centered with distortion.
Comparing the XANES spectra of CNT/PC(HT) and CNT/PC, the relative intensity of peak C to that of peak D is higher for CNT/PC, indicating that the degree of distortion is more pronounced for CNT/PC. This change in the distortion could be attributed to the presence or absence of the SiO2 layer, which could suppress the distortion of the Fe center atom in the CNT/PC(HT) sample.
Figure 4.2. (a) Fe K-edge XANES spectra for CNT/PC intermediates, along with Fe foil, FePc, and FeTMPPCl. (b) XANES spectra for CNT/PC(LT), CNT/PC(SiO2), CNT/PC(HT), and CNT/PC samples: 7110–7120 eV pre-edge region (up) and XANES spectra for CNT/PC(LT) and CNT/PC(SiO2) 7125–7150 eV region (down).
To track the structural changes in more detail after silica coating, pyrolysis, and silica etching, the XANES spectra of samples were closely observed in pre-edge region (7110–7120 eV) (Figure
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4.2b, upper panel). The changes in the XANES spectra revealed that the intensity of peak B decreased upon silica coating, suggesting that new atoms other than four surrounding N atoms could be axially bonded to Fe. However, after the pyrolysis, the intensity of pre-edge peak B at 7118 eV increased in CNT/PC(HT) and CNT/PC, which is similar to that of CNT/PC(LT), indicating that Fe has a four- coordination structure. In CNT/PC(HT), the pre-edge peak A at 7114 eV is also increased in intensity due to the presence of Fe clusters. Another change in their XANES spectra can be found in the region between peaks C and D (Figure 4.2b, lower panel), where CNT/PC(SiO2) showed a slightly higher intensity than CNT/PC(LT). This intensity increase could be associated with the increased Fe oxidation state in CNT/PC(SiO2) due to an increase in the d-band vacancy of the Fe center atom.60,61 The increase in oxidation state of Fe after silica coating could be observed more clearly with a higher intensity of 1st derivative peak (7123–7129 eV) in XANES spectrum of CNT/PC(SiO2) than CNT/PC(LT). These changes in the XANES spectra suggest that the silica coating could generate a new interaction with the Fe center atom, accompanied by an increase in the oxidation state. The nature of the new interaction after the silica coating will be further elucidated with Mӧssbauer spectroscopy (vide infra).
The coordination structure around the Fe center atom during the synthesis was tracked by Fourier-transformed EXAFS profile analyses (Figure 4.3). The EXAFS profile of the FePc reference showed a major peak at 1.45 Å , which corresponded to the Fe–N coordination, whereas that of Fe foil presented a peak at 2.18 Å due to Fe–Fe metallic scattering. These two peaks provided bases for identifying the presence of atomically dispersed Fe–Nx sites or Fe-based clusters (or agglomerated large particles) in the samples. The samples before high-temperature pyrolysis—CNT/PC(LT) and CNT/PC(SiO2)—only showed a peak at approximately 1.45 Å , indicating that the Fe–N scattering in FeTMPPCl was preserved after annealing at 400 °C and silica coating. A closer observation of the EXAFS profiles of CNT/PC(LT) and CNT/PC(SiO2) (Figure 4.3b, upper panel) revealed a slightly higher intensity peak at 1.45 Å for CNT/PC(SiO2), which could stem from the additional interaction around the Fe center atom. However, it is difficult to determine the type of newly bonded atoms based only on the EXAFS analysis. Pyrolysis at 800 °C resulted in the emergence of a new peak at 2.18 Å for Fe–Fe scattering along with another peak at 1.45 Å for CNT/PC(HT). Upon etching the SiO2 layer, the EXAFS profile of CNT/PC exhibited a Fe–Fe peak with a significantly decreased intensity (Figure 4.3b, lower panel).
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Figure 4.3. Fourier-transformed EXAFS spectra of the CNT/PC intermediates, along with those of Fe foil (gray), FePc (black), and FeTMPPCl (purple) references. (b) EXAFS spectra for the CNT/PC(LT)–CNT/PC(SiO2) (up) and CNT/PC(HT)–CNT/PC (down) samples.
We performed EXAFS curve fitting to quantitatively analyze coordination environment of Fe in each sample (Figure 4.4 and Table 4.1). The average Fe−N coordination number and bond length for CNT/PC(SiO2) are 5.4 and 2.030 Å , respectively, while those of CNT/PC(HT) are 3.3 and 1.936 Å , respectively. The relatively lower Fe−N coordination number for CNT/PC(HT) indicates the breakage of Fe−Si bond during the heat treatment. The Fe−N coordination number and bond length for CNT/PC catalyst after silica etching are 4.5 and 1.972 Å , suggesting that CNT/PC catalyst has Fe–N4
sites as active sites.
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Figure 4.4. EXAFS fitting curves of (a) CNT/PC(LT), (b) CNT/PC(SiO2), (c) CNT/PC(HT), and (d) CNT/PC in R space.
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Table 4.1. Structural parameters of CNT/PC(LT), CNT/PC(SiO2), CNT/PC(HT), and CNT/PC obtained from curve fitting of their Fe K-edge EXAFS spectra.
Catalyst Name
k range (Å−1)
R range
(Å ) scattering CNa R (Å )b
σ2 (×103 Å−2)c
ΔE0
(eV) R factor CNT/PC(LT) 1.8–11.0 1.0–3.0 Fe–N1 4.8
(0.6)
2.032 (0.011)
10.9 (2.4)
3.4
(0.8) 1.5%
Fe–C1 4.0 (1.2)
3.044 (0.025)
5.7 (4.8) CNT/PC(SiO2) 1.8–11.0 1.0–3.0 Fe–N1 5.4
(0.5)
2.030 (0.007)
11.4 (1.9)
2.3
(0.6) 1.2%
Fe–C1 2.9 (0.7)
3.030 (0.014)
2.3 (3.4) CNT/PC(HT) 2.1–12.5 1.0–3.2 Fe–N1 3.3
(0.5)
1.936 (0.009)
8.3 (1.6)
−2.8
(0.8) 0.2%
Fe–C1
4.1 (1.1)
2.728 (0.017)
3.4 (2.5) Fe–Fe1 2.8
(0.4)
2.543 (0.010)
8.0 (1.2)
−2.2 (1.5) CNT/PC 2.3–12.5 1.0–3.2 Fe–N1
4.5 (1.0)
1.972 (0.012)
10.0 (3.0)
0.2
(1.3) 2.1%
Fe–C1 3.8 (2.4)
2.925 (0.045)
10.8 (10.8) Fe–Fe1
−0.5 (0.3)
2.348
(0.024) 8.0d −2.2d
a Coordination number. b Interatomic distance. c Debye–Waller factor. d Denotes a fixed value. The numbers in parentheses indicate uncertainties of the fitting. The double-shell fitting gave a very small and negative coordination number for Fe–Fe (CN = −0.5 ± 0.3), which is not physically much meaningful. This result confirms the insignificant presence of Fe clusters in CNT/PC sample. S02
fixed at 0.89 as obtained by fitting the reference Fe foil.
The CNT/PC_w/o SiO2 and its intermediate sample, CNT/PC_w/o SiO2(HT), were prepared to further clarify the role of silica coating. The XANES spectra of the CNT/PC_w/o SiO2(HT) and CNT/PC_w/o SiO2 samples (Figure 4.5a) evidently showed a broad peak at 7115 eV (peak A’), which was a characteristic feature of a Fe foil and differed from the pre-edge peak A shown by CNT/PC(LT).
The presence of peak E near 7160 eV for the CNT/PC_w/o SiO2(HT) and CNT/PC_w/o SiO2 samples further attested to the formation of the metallic phase in these samples. The EXAFS data of both the samples (Figure 4.5b) exhibited two peaks for Fe–N (1.45 Å ) and Fe–Fe (2.18 Å ) scattering. Notably, the intensity of the Fe–Fe peak was much higher than that of the Fe–N peak, indicating the formation of Fe particles. Indeed, the XRD patterns of CNT/PC_w/o SiO2(HT) and CNT/PC_w/o SiO2 (Figure 4.8) confirmed the formation of crystalline Fe species after high-temperature pyrolysis. In addition, the TEM images of CNT/PC_w/o SiO2(HT) and CNT/PC_w/o SiO2 (Figure 4.9) apparently reveal Fe-based particles with diameters of 30 nm.
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Figure 4.5. (a) Fe K-edge XANES spectra and (b) Fourier-transformed EXAFS spectra for the CNT/PC_w/o SiO2 intermediates, along with those of Fe foil (gray), FePc (black), and FeTMPPCl (purple) references.