5.3. R ESULTS AND D ISCUSSION
5.3.3. Structural Difference between Fe-Phen_X Catalysts
The Fe-Phen_X mixtures was pyrolyzed at 800 °C and the silica template was etched to obtain the Fe-Phen_X catalysts. Scanning electron microscopy (SEM) images of Fe-Phen_X catalysts show that they have similar morphology regardless of the amount of precursors. Small-angle X-ray diffraction (XRD) patterns of the Fe-Phen_X catalysts exhibit diffraction peaks commensurate with hexagonal p6m symmetry resulting from the replication of mesoporous silica template (Figure 5.3a).
Wide-angle XRD patterns of the Fe-Phen_X catalysts exhibit a broad peak for amorphous carbon (20–
30°) as well as a peak corresponding to (002) plane (26°) of graphitic carbon without peaks for Fe- based crystalline phases (Figure 5.3b). During high temperature pyrolysis, amorphous carbon is formed by thermally decomposed Fe(Phen)3 complex, and graphitic carbon is formed by Fischer- Tropsch reaction between some carbon and Fe.27 As the amounts of Fe and N precursors increase, graphitic carbon peaks gradually increases, which is thought to be due to the carbon nanotubes (CNTs) observed in the SEM images. However, amorphous carbon is the major carbon species for each catalyst, and CNTs observed in SEM images are only a small fraction. In C 1s XPS spectra of Fe-Phen_X catalysts (Figure 5.4a), we also confirmed that there were no significant differences in carbon species between Fe-Phen_X catalysts.
Figure 5.3. (a) Small-angle and (b) wide-angle XRD patterns of the Fe-Phen_X catalysts.
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Figure 5.4. (a) C 1s XPS spectra and (b) O 1s XPS spectra of the Fe-Phen_X catalysts.
Table 5.1. Elemental analysis results in the Fe-Phen_X catalysts.
Catalyst Name Fe a C b H b N b O b
Fe-Phen_1.0 1.9 70.0 1.3 9.1 8.8
Fe-Phen_1.25 1.9 70.8 1.5 8.6 10.0
Fe-Phen_1.5 2.0 72.7 1.4 8.3 9.1
Fe-Phen_1.75 1.9 73.6 1.7 8.0 9.2
Fe-Phen_2.0 2.0 73.9 1.6 7.2 7.9
a values are obtained by ICP-OES.
b values are obtained by element analyzer.
The porous structures of the Fe-Phen_X catalysts and silica template were investigated by nitrogen adsorption-desorption analyses (Figure 5.5 and Table 5.2). The Fe-Phen_1.0 showed two isotherms indicating dual pore structure, and the Fe-Phen_1.25, 1.5, and 1.75 exhibited similar hysteresis loops to Fe-Phen_1.0. Correspondingly, the pore size distributions of the Fe-Phen_1.0, 1.25, 1.5, and 1.75 catalysts showed a maximum peak at 4.8 nm and another peak at 10.0 nm. On the other hand, the Fe-Phen_2.0 showed only one isotherm in the relative pressure ranges of 0.45–0.65. The pore size distribution curve of Fe-Phen_2.0 indicates that Fe-Phen_2.0 has a 4.8 nm-sized mesopore.
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The nitrogen adsorption-desorption results for the Fe-Phen_X catalysts suggest that the catalyst structure changes from a tubular structure to a rod structure as the amount of precursor increases, as shown in the schematic diagram of Figure 5.5b.
Figure 5.5. (a) N2 adsorption-desorption isotherms of the Fe-Phen_X catalysts. The isotherms of the Fe-Phen_1.25, Fe-Phen_1.5, Fe-Phen_1.75 and Fe-Phen_2.0 are offset by 500, 1000, 1500, and 2000 cm3 g−1, respectively. (b) The pore size distribution curves obtained from the adsorption branches of corresponding isotherms with the schematic diagram.
Table 5.2. Textural properties of the Fe-Phen_X catalysts.
Catalyst Name BET Surface Area (m2 g−1) a
Pore Volume (cm3 g−1) b
Pore Size (nm) c
Fe-Phen_1.0 1350 1.56 4.5, 10.0
Fe-Phen_1.25 1200 1.31 4.5, 8.0–12.5
Fe-Phen_1.5 1070 1.16 4.5, 8.0–12.5
Fe-Phen_1.75 1030 1.00 4.5, 8.0–12.5
Fe-Phen_2.0 950 0.84 4.5
a BET surface area was calculated in the relative pressure range of 0.05–0.3.
b Pore volume was calculated at the relative pressure of 0.98–0.99.
c Pore size was calculated from the adsorption branch of the corresponding isotherm using the BJH method.
According to the nitrogen adsorption-desorption analysis, the two catalysts with the greatest difference in pore structures are Fe-Phen_1.0 and Fe-Phen_2.0 (Figures 5.7a,b). The former is a tube
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structure with empty inner pores, and the latter is a rod structure filled with inner pores. The distinct structural difference between the two catalysts was also confirmed through TEM images (Figure 5.6).
Figure 5.6. TEM images of (a) Fe-Phen_1.0 and (c) Fe-Phen_2.0. High resolution TEM images of (b) Fe-Phen_1.0 and (d) Fe-Phen_2.0.
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Figure 5.7. (a) N2 adsorption-desorption isotherms of Fe-Phen_1.0 and Fe-Phen_2.0 catalysts. The isotherms of the Fe-Phen_2.0 is offset by 600 cm3 g−1. (b) The pore size distribution curves obtained from the adsorption branches of corresponding isotherms with representations for each catalyst. (c) Fe K-edge XANES spectra for Fe-Phen_1.0 and Fe-Phen_2.0 catalysts and (d) Fourier-transformed EXAFS spectra of the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts, along with those of Fe foil (black), FePc (blue), and FeTMPPCl (gray) references.
Interestingly, the differences in the catalyst structure were evident, but the content and chemical state of elements including Fe did not change significantly (Figure 5.4 and Table 5.1). In particular, there was little difference in the coordination environment around the Fe center of the two catalyst when comparing the X-ray absorption spectroscopy (XAS) analysis of the two catalysts (Figures 5.7c,d). Figure 5.7c shows the X-ray absorption near edge structure (XANES) spectra of the Fe- Phen_1.0 and Fe-Phen_2.0 catalysts, as well as references (FePc, FeTMPPCl, and Fe foil). In the pre-
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edge region (7110–7120 eV), the reference FePc evidently showed peaks at 7114 eV (peak A) and 7118 eV (peak B), whereas FeTMPPCl exhibited only peak A. This difference between the two references originates from the coordination structure. In FePc, a central Fe atom is coordinated with four N atoms in a square planar structure, whereas FeTMPPCl has an additional Cl atom that is axially coordinated to the central Fe atom to form a square pyramidal structure. Hence, the presence or absence of peak B can serve as a fingerprint to distinguish the coordination structure of the central Fe: four-coordination square planar structure or 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.24,28 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.28 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. However, the Fe-Phen_1.0 and Fe-Phen_2.0 catalysts subjected to high temperature pyrolysis showed a reversed intensity ratio of peaks C and D, indicating that the Fe atom was off-centered with distortion. The coordination structure around the Fe center atom during the synthesis was tracked by Fourier-transformed EXAFS profile analyses (Figure 5.7d). 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 bonding. 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. RDFs of the EXAFS spectra for Fe-Phen_1.0 and Fe-Phen_2.0 (Figure 5.7d) exhibited only a peak near 1.45 Å , indicating the exclusive existence of Fe−N/O bonds. Compiling the ICP and XAS results suggest that Fe-Phen_1.0 and Fe-Phen_2.0 have similar amount of the same Fe–Nx active sites. In addition, considering the nitrogen adsorption- desorption analysis, it can be intuitively seen that the degree of exposure of Fe–Nx sites in Fe- Phen_1.0, which has a higher surface area, is greater than that of Fe-Phen_2.0.