2. I MPACT OF T EXTURAL P ROPERTIES OF M ESOPOROUS P ORPHYRINIC C ARBON
2.3. R ESULTS AND D ISCUSSION
2.3.2. Physicochemical Characterizations
Figure 2.1 displays the morphologies and pore structures of the Fe-MPC catalysts. The SEM images show that the morphology of each silica templates is well preserved in the Fe-MPC catalysts (Figures 2.1a,c,e,g), suggesting that the nanocasting process occurs inside the mesopores of silica templates without the deposition of FeIIITMPPCl precursor on the exteriors of the silica particles.
Transmission electron microscopy (TEM) images of Fe-MPC(SBA-15-150) and Fe-MPC(SBA-15- 100) catalysts (Figures 2.1b,d) clearly suggested the formation of interconnected arrays of uniform, Fe- and N-containing carbogenic nanorods as well as the presence of uniform mesopores between the nanorods, similar to CMK-3.63 Compared to Fe-MPC(SBA-15-150) and Fe-MPC(SBA-15-100), Fe- MPC(SBA-15-35) exhibits less ordered nanorods arrays (Figure 2.1f). TEM observations suggest that, in Fe-MPC(SBA-15-150) and Fe-MPC(SBA-15-100), the templated nanorods form 3D interconnected networks, whereas in Fe-MPC(SBA-15-35), the individual nanorods are loosely held together by van der Waals interactions without interconnection.
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Figure 2.2. (a) Small-angle and (b) wide-angle XRD patterns of Fe-MPC catalysts.
The small-angle XRD patterns from the Fe-MPCs (Figure 2.2a) confirm these TEM observations. The patterns of Fe-MPC(SBA-15-150) and Fe-MPC(SBA-15-100) exhibit three distinct diffraction peaks for the (100), (110), and (200) planes of hexagonal p6m symmetry. In contrast, Fe- MPC(SBA-15-35) shows nearly featureless small-angle XRD pattern, indicating the absence of long- range mesostructural order. For Fe-MPC(KIT-6), the TEM image (Figure 2.1h) and small-angle XRD pattern (Figure 2.2a) both reveal the formation of a uniform mesoporous structure with cubic Ia3d symmetry. Wide-angle XRD patterns of all four Fe-MPC catalysts (Figure 2.2b) commonly show a broad peak for amorphous carbon (20–30°) as well as peaks for graphitic carbon (~26° and ~44°) without peaks for a Fe crystalline phase.
The porous structures of the Fe-MPC catalysts were investigated by N2 adsorption analyses (Figure 2.3). The N2 adsorption isotherms of Fe-MPC(SBA-15-150) exhibits two hysteresis loops in the relative pressure ranges of 0.45–0.65 and 0.65–0.90, suggesting the formation of dual mesopores.
Accordingly, the pore size distribution curve of Fe-MPC(SBA-15-150) shows a maximum at 4.8 nm as well as a long tail in the larger pore size range. Fe-MPC(SBA-15-100) and Fe-MPC(SBA-15-35) each show a single hysteresis loop at a similar relative pressure range (0.45–0.65) in their N2
adsorption isotherms. The corresponding pore size distribution curves reveal that Fe-MPC(SBA-15- 100) has mesopores with the average size of 4.8 nm paired with smaller-sized mesopores, while Fe- MPC(SBA-15-35) predominantly contains small mesopores less than 3 nm. Hence, the pore sizes of the Fe-MPC(SBA-15-X) catalysts progressively decrease in the order of Fe-MPC(SBA-15-150), Fe- MPC(SBA-15-100), and Fe-MPC(SBA-15-35). Fe-MPC(KIT-6) exhibits a single hysteresis loop and the mesopore size of 3.3 nm. The textural properties of the Fe-MPC catalysts are summarized in
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Table 2.1. The BET surface areas of the Fe-MPC catalysts are generally very high, ranging from 820 m2 g–1 to 1180 m2 g–1. The total pore volumes of the catalysts were in the range of 0.66 to 1.09 cm3 g–1. It is noted that all Fe-MPC catalysts show steep N2 uptake at low relative pressure ranges, indicating the presence of micropores, which have been suggested to host active Fe–Nx sites and can greatly affect the ORR activity.26,66
Figure 2.3. (a) N2 adsorption-desorption isotherms of Fe-MPC catalysts. The isotherms of the Fe- MPC(SBA-15-100), Fe-MPC(SBA-15-35), and Fe-MPC(KIT-6) are offset by 300, 600, and 800 cm3 g−1, respectively. (b) The pore size distribution curves obtained from the adsorption branches of corresponding isotherms.
Table 2.1. Textural properties of Fe-MPC catalysts prepared with mesoporous silica templates.
Sample BET Surface Area (m2 g−1) a
Pore Volume (cm3 g−1) b
Particle Size (nm)
Fe-MPC(SBA-15-150) 820 1.09 950
Fe-MPC(SBA-15-100) 1180 0.87 900
Fe-MPC(SBA-15-35) 1080 0.66 750
Fe-MPC(KIT-6) 1150 0.88 1000–2000
a BET surface area was calculated in the relative pressure range of 0.05–0.2.
b Pore volume was calculated at the relative pressure of 0.98–0.99.
The combination of SEM, TEM, XRD, and N2 adsorption results suggests that the three Fe- MPC(SBA-15-X) catalysts have controlled pore sizes and differing pore connectivities, but similar particle sizes; and Fe-MPC(SBA-15-100) and Fe-MPC(KIT-6) exhibit dramatically different particle sizes yet similar pore size, surface areas, and pore volumes. Therefore, the Fe-MPC catalysts could be used as model systems for investigating the effects of pore size, pore connectivity, and particle size on the catalytic activity of the materials.
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Finally, to understand the origin of the excellent ORR activities of Fe-MPC(SBA-15-X), the local structure around the Fe atoms was investigated by X-ray absorption spectroscopy (XAS). The Fe K-edge X-ray absorption near-edge structure (XANES) of the Fe-MPC(SBA-15-150) catalyst, along with those of references (Fe foil, Fe(II) phthalocyanine (FeIIPc), and FeIIITMPPCl), are shown in Figure 2.4a. FeIIPc and FeIIITMPPCl represent references for Fe–N4 moieties while Fe foil provides that for Fe clusters. The Fe-MPC(SBA-15-150) catalyst exhibits a similar XANES spectrum to that of FeIIITMPPCl with a pre-edge peak at 7114 eV (denoted as peak A); peak B at 7118 eV shown by FeIIPc is absent for Fe-MPC(SBA-15-150). These XANES features of Fe-MPC(SBA-15-150) indicates the breakage of the square planar D4h symmetry and possible formation of a square pyramidal geometry around the Fe atoms. A major difference between the XANES spectrum of Fe- MPC(SBA-15-150) and those of FeIIITMPPCl and FeIIPc is the relative intensities of the XANES signals at 7132 eV and 7140 eV (peaks C and D, respectively). While FeIIITMPPCl and FeIIPc show higher intensities for peak D than for peak C, the Fe-MPC(SBA-15-150) catalyst shows a higher peak C intensity. Mukerjee et al. suggested that a higher intensity of peak C than that of peak D could explain the translocation of the central Fe atom away from the in-plane Fe–N4 structure, which was shown to contribute to high ORR activity.69 Hence, the active sites of the Fe-MPC catalysts could mainly consist of distorted Fe–Nx sites. Figure 2.4b displays the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra of the Fe-MPC(SBA-15-150) catalyst and the references.
The Fe-MPC(SBA-15-150) catalyst exhibits major peaks at 1.4 Å and 2.4 Å , which correspond to Fe–
N coordination and Fe–(N)–C coordination, respectively. No peaks for Fe metal are found for the Fe- MPC(SBA-15-150) catalyst. The EXAFS results suggest that the Fe–Nx sites could be major active sites for Fe-MPC(SBA-15-X) catalysts, consistent with the XANES results. The abundance of Fe–Nx
sites was further identified with X-ray photoelectron spectroscopy (XPS). A deconvoluted N 1s XPS spectrum of the Fe-MPC(SBA-15-150) revealed that Fe–Nx sites were a major species (29.2%).
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Figure 2.4. (a) Fe K-edge XANES spectra. b) Fourier transform of k3-weighted Fe K-edge EXAFS spectra of Fe-MPC(SBA-15-150) catalyst, Fe foil, FeIIPc, and FeIIITMPPCl.