Physicochemical Characterizations

2.3. R ESULTS AND D ISCUSSION

2.3.2. Physicochemical Characterizations

Fe3O4 NPs of ca. 12 nm diameter were prepared following a previously reported method (Figure 2.2a,c),²⁹ and dispersed on CNT to yield a Fe3O4/CNT as a precursor (Figure 2.2b,d). The Fe3O4/CNT precursor was converted firstly by CO gas as a conversion agent, and the product was labeled Fe–

Fe3C@C/CNT_CO. XRD pattern of Fe–Fe3C@C/CNT_CO (Figure 2.3) shows that most of Fe3O4 NPs were converted into metallic Fe–Fe3C phase, but a trace amount still remained. The residual Fe3O4 NPs were eliminated by treating sequential acid leaching with 6 M HCl and 6 M HNO3 at 80 °C (Fe–

Fe3C@C/CNT_CO_A). The acid treatment could remove the Fe3O4 NPs, exclusively, because the Fe–

Fe3C particles were protected by the graphitic carbon layers from acid.¹⁹ The resulting XRD pattern (Figure 2.3) confirmed the preservation of Fe–Fe3C phase as well as the etching of Fe3O4 phase. TEM image of Fe–Fe3C@C/CNT_CO_A (Figures 2.4a) also shows that Fe–Fe3C@C sites were supported on CNT. In addition, HAADF-STEM image of Fe–Fe3C@C/CNT_CO_A in the region free of Fe–Fe3C@C particles (Figures 2.4d) exhibits no other Fe-containing species. These observations confirmed that Fe–

Fe3C@C/CNT_CO_A catalyst contained only Fe–Fe3C@C sites.

Acid Leaching

Fe–Fe₃C@C/CNT_CO_A Fe₃O₄ /CNT

Fe–Fe₃C@C/CNT_Urea

:Fe–N_xsite

Fe–Fe₃C@C/CNT_Urea_A : Fe–Fe₃C@C : Fe–Fe₃C@C : Fe–Fe₃C@C

: C–N_x : C–N_x

Figure 2.2. TEM images of (a) Fe3O4 NPs and (b) Fe3O4/CNT. Their high-resolution TEM images are shown in (c) and (d), respectively.

Figure 2.3. XRD patterns of Fe3O4/CNT, Fe–Fe3C@C/CNT_CO, Fe–Fe3C@C/CNT_CO_A, Fe–

Fe3C@C/CNT_Urea, and Fe–Fe3C@C/CNT_Urea_A.

(c)

(a) (b)

(d)

100 nm 100 nm

5 nm 5 nm

2 (degrees)

10 20 30 40 50 60 70

Intensity (a.u.)

Fe Fe3C

▼Fe₃O₄

●

▼

▼

◆

▼

▼ ▼

● ●

●

● ●

● ● ●

◆

◆

●

● ●

◆

▼

◆

●

Fe–Fe3C@C/CNT_Urea_A Fe–Fe3C@C/CNT_Urea

Fe–Fe3C@C/CNT_CO_A Fe–Fe3C@C/CNT_CO

Fe3O4/CNT

Fe JCPDS: 71-4409 Fe3C JCPDS: 34-0001 Fe3O4JCPDS: 72-8151

Figure 2.4. (a) Fe–Fe3C@C/CNT_CO_A, (b) Fe–Fe3C@C/CNT_Urea, and (c) Fe–

Fe3C@C/CNT_Urea_A. HAADF-STEM images of (d) Fe–Fe3C@C/CNT_CO_A, (e) Fe–

Fe3C@C/CNT_Urea, and (f) Fe–Fe3C@C/CNT_Urea_A. (g) Atomic resolution TEM image of Fe–

Fe3C@C/CNT_Urea. (h) EELS spectrum obtained from the black box region in panel (i).

Table 2.1. Elemental analysis results of Fe3O4/CNT and Fe–Fe3C@C/CNT catalysts.

Sample Content (wt%)

C N O H

Fe3O4/CNT 80.9 0 6.9 2.6

Fe–Fe3C@C/CNT_CO_A 91.9 0 2.2 0.2

Fe–Fe3C@C/CNT_Urea 86.0 1.4 1.6 0.1

Fe–Fe3C@C/CNT_Urea_A 88.4 1.6 5.0 0.3

In the conversion reaction with urea,³⁰ Fe3O4/CNT was mixed with urea and agar, and pyrolyzed at 900 °C under N2 flow. The resulting catalyst was named as Fe–Fe3C@C/CNT_Urea. The XRD pattern of Fe–Fe3C@C/CNT_Urea (Figure 2.3) indicates that the Fe3O4 phase was fully converted into Fe and Fe3C phases. TEM image of Fe–Fe3C@C/CNT_Urea (Figure 2.4b) also revealed that Fe–

Fe3C@C sites were generated and deposited on the CNT. In Fe–Fe3C@C/CNT_Urea, not only Fe–

Fe3C@C sites, but Fe–Nx and C–Nx sites were additionally generated as discussed above. The HAADF- STEM image of Fe–Fe3C@C/CNT_Urea (Figures 2.4e) shows numerous bright spots of sub-nanometer

Energy Loss (eV)

280 350 420 490 560 630 700 770

Intensity (a.u.)

C–K

N–K

Fe–L_2,3

(d) (e) (f)

(g) (h)

(a) (b) (c)

100 nm

5 nm

10 nm 10 nm 10 nm

100 nm 100 nm

5 nm 5 nm

2 nm

size that correspond to ultra-small Fe-based species. EELS spectrum (Figure 2.4g) taken at dotted square section of atomic-resolution TEM image (Figure 2.4h) also indicated the existence of very small Fe and N species, indicating the generation of Fe–Nx sites. Elemental analysis results identified the nitrogen species in Fe–Fe3C@C/CNT_Urea (Table 2.1), and the detailed analysis of the nitrogen species was performed by XPS (Figure 2.5). The XPS spectrum of Fe–Fe3C@C/CNT_Urea shows three types of nitrogen species with deconvoluted peaks at 398.1 eV, 398.9 eV, and 400.8 eV, ascribed to pyridinic Nβ (bond with outer carbon of Fe phthalocyanine), pyrrolic Nα (Fe–N bond), and graphitic nitrogen moieties, respectively, confirming the formation of Fe–Nx species and N-doped carbon species.³⁵ We note that the nitrogen can also exist in the carbon shells encapsulating Fe–Fe3C particles.

Figure 2.5. XPS N 1s spectra of Fe phthalocyanine, Fe–Fe3C@C/CNT_CO_A, Fe–

Fe3C@C/CNT_Urea, and Fe–Fe3C@C/CNT_Urea_A.

To identify the catalytic role of Fe–Nx sites, they were selectively removed by acid leaching from Fe–

Fe3C@C/CNT_Urea catalyst, and the product was labeled as Fe–Fe3C@C/CNT_Urea_A. This treatment can leave the C–Nx and Fe–Fe3C@C sites intact,³⁶ whereas most of Fe–Nx species, if not all, can be etched.^37,38 The TEM (Figures 2.4c) and HAADF-STEM (Figures 2.4f) images of Fe- Fe3C@C/CNT_Urea_A present that the white dots in Fe–Fe3C@C/CNT_Urea were almost eliminated after the acid treatment, while Fe–Fe3C@C species remained. The depleted Fe–Nx sites in Fe–

Fe3C@C/CNT_Urea_A could be corroborated by the N 1s XPS spectrum (Figure 2.5). The pyridinic Nβ and graphitic nitrogen peaks were preserved, whereas the pyrrolic Nα peak corresponding to Fe–Nx

sites was diminished significantly, verifying that the Fe–Nx sites in Fe–Fe3C@C/CNT_Urea were mostly removed by the acid leaching with preserving the Fe–Fe3C@C and C–Nx sites.

Binding Energy (eV)

394 396 398 400 402 404 406

Intensity (a.u.)

β α

N_β Nα

Graphitic N

Fe–Fe3C@C/CNT_Urea_A

Fe–Fe3C@C/CNT_Urea

Fe–Fe3C@C/CNT_CO_A

Fe phthalocyanine