Trapping-and-Immobilizing Strategy to Atomically Dispersed Catalysts

3.3. R ESULTS AND D ISCUSSION

3.3.1. Trapping-and-Immobilizing Strategy to Atomically Dispersed Catalysts

Figure 3.1. Trapping-and-immobilizing strategy for producing atomically dispersed precious metal catalysts and structural characterization of the catalysts. (a) Illustrations of the three types of catalysts:

Pt/CNT, Pt/CNT_IL, and Pt/CNT_IL_SiO2. (b–d) HAADF-STEM images of 3Pt/CNT (b), 3Pt/CNT_IL (c), and 3Pt/CNT_IL_SiO2 (d). (e–g) Pt L3-edge k³-weighted EXAFS spectra and their best fits for 3Pt/CNT (e), 3Pt/CNT_IL (f), and 3Pt/CNT_IL_SiO2 (g).

During the preparation of supported metal catalysts by the impregnation–activation method, metallic species can agglomerate during the impregnation step, the thermal activation step, or both. To mitigate agglomeration into NPs during each step, we designed a synthetic approach for atomically

0 1 2 3 4 5 6

FT Magnitude (a.u.)

Raw data Fit Window

Pt/CNT

Impregnation

Thermal activation

IL coating 450 ^oC

SiO2

coating

(e) (a)

Pt–Pt Pt–C

Reduced Distance (Å )

Pt/CNT_IL

Impregnation

Thermal activation

Pt/CNT_IL_SiO₂

Thermal activation

SiO2

removal : Graphitic carbon : Amorphous carbon : Heteroatoms : Pt precursor : Pt after activation

2 nm

0 1 2 3 4 5 6

FT Magnitude (a.u.)

Raw data Fit Window

(f) (c)

Pt–Pt Pt–NPt–Cl

Reduced Distance (Å )

2 nm

0 1 2 3 4 5 6

FT Magnitude (a.u.)

Raw data Fit Window

(g) (d)

Pt–NPt–Cl

Reduced Distance (Å )

2 nm

dispersed precious metal catalysts, as illustrated in Figure 3.1a. First, IL containing various heteroatoms was coated on a CNT support, which was then annealed at 450 °C. The heteroatoms, including N and O, contained in the resulting heteroatom-doped carbonaceous layer could act as favorable anchoring sites for trapping metal precursor molecules, thus inducing an isolated geometry of adsorbed precursors.

After impregnation of a metal precursor, a SiO2 protective layer was coated on the surface to

‘immobilize’ the isolated metal precursors during the thermal activation step.^58,59

Table 3.1. Fitting parameters for Pt L3-edge k³-weighted EXAFS spectra of 3Pt/CNT, 3Pt/CNT_IL, and 3Pt/CNT_IL_SiO2.

Sample Shell CN^a ΔE0

(eV)^b

R (Å)^c

σ² (10⁻³ Å²)^d

R factor (%)^e 3Pt/CNT Pt–C 1.95 ± 0.35 −2.9

± 1.9

2.06 ± 0.02 10.0^f Pt–Pt 6.38 ± 1.40 2.69 ± 0.01 11.7 ± 1.7 0.7

3Pt/CNT_IL

Pt–N 2.86 ± 0.84

−5.6

± 1.7

2.03 ± 0.02 8.5 ± 3.1

2.3

Pt–Cl 0.67 ± 0.14 2.30 ± 0.02 5.0^f

Pt–Pt 1.40 ± 0.59 2.74 ± 0.01 7.8 ± 1.6 3Pt/CNT_IL

_SiO2

Pt–N 3.03 ± 0.95 3.4

± 1.9

2.01 ± 0.02 5.8 ± 2.8 Pt–Cl 1.03 ± 0.18 2.28 ± 0.02 5.0^f 2.7

aCoordination number. ^bEnergy shift. ^cBond distance. ^dDebye–Waller factor. ^eR factor was obtained from the best fit for the respective catalyst. ^fDenotes a fixed value.

The effectiveness of our strategy was visualized using HAADF-STEM images of a series of Pt catalysts (3 wt% Pt loading, denoted as 3Pt) prepared by three different protocols (Figure 3.1b–d). When the Pt precursor (H2PtCl6‧6H2O) was impregnated onto the CNT and activated without the IL and SiO2

coating steps (3Pt/CNT), Pt NPs of ca. 1 nm were generated (Figure 3.1b). The addition of the IL coating step before the impregnation of H2PtCl6‧6H2O (3Pt/CNT_IL) resulted in the generation of atomically dispersed Pt sites as well as Pt NPs (Figure 3.1c). Interestingly, when both the IL and SiO2 coating steps were exploited (3Pt/CNT_IL_SiO2), only atomically dispersed Pt sites were generated (Figure 1d). The Pt L3-edge k³-weighted EXAFS analysis of the three catalysts (Figure 3.1e–g and Table 3.1) corroborated this trend. In the EXAFS spectra of the catalysts, 3Pt/CNT showed scattering peaks corresponding to Pt–Pt and Pt–C, indicating the formation of supported Pt NPs (Figure 3.1e), whereas 3Pt/CNT_IL_SiO2 only exhibited scattering peaks for light elements (Pt–N and Pt–Cl) that could stabilize atomically dispersed Pt (Figure 3.1g).²⁹ 3Pt/CNT_IL showed EXAFS peaks for both NPs and atomically dispersed sites (Figure 3.1f). The HAADF-STEM and EXAFS analyses clearly demonstrate the validity of our rationale for preparing atomically dispersed precious metal catalysts.

Figure 3.2. Verification of the developed synthetic strategy for catalysts with various Pt loadings. (a–

d) HAADF-STEM images of Pt/CNT catalysts with 1 wt% (a), 3 wt% (b), 5 wt% (c), and 10 wt% (d) Pt loadings. (e–h) HAADF-STEM images of Pt/CNT_IL catalysts with 1 wt% (e), 3 wt% (f), 5 wt%

(g), and 10 wt% (h) Pt loadings. (i–l) HAADF-STEM images of Pt/CNT_IL_SiO2 catalysts with 1 wt%

(i), 3 wt% (j), 5 wt% (k), and 10 wt% (l) Pt loadings.

(a) (e) (i)

(d)

(b) (f) (j)

(h)

(c) (g) (k)

(l) 1 wt%

3 wt%

5 wt%

10 wt%

5 nm

Pt/CNT Pt/CNT_IL Pt/CNT_IL_SiO₂

5 nm 5 nm

5 nm 5 nm 5 nm

Figure 3.3. Pt particle size distribution histograms of catalysts with various Pt loadings. (a–d) Size distribution histograms of Pt/CNT catalysts with 1 wt% (a), 3 wt% (b), 5 wt% (c), and 10 wt% (d) Pt loadings. (e–h) Size distribution histograms of Pt/CNT_IL catalysts with 1 wt% (e), 3 wt% (f), 5 wt%

(g), and 10 wt% (h) Pt loadings. (i–l) Size distribution histograms of Pt/CNT_IL_SiO2 catalysts with 1 wt% (i), 3 wt% (j), 5 wt% (k), and 10 wt% (l) Pt loadings. The histograms were obtained by counting at least 150 sites in the HAADF-STEM images of the catalysts. The numbers shown in the upper right corner of each histogram indicate the average size ± standard deviation of Pt.

We extended our approach to the preparation of Pt catalysts with a broad range of Pt loadings (1, 3, 5, and 10 wt%). The HAADF-STEM images and corresponding size distribution histograms of Pt/CNTs (Figure 3.2a–d and Figure 3.3a–d) clearly revealed that Pt NPs form on the bare CNT, regardless of the Pt loading. In the case of the Pt/CNT_IL catalysts, atomically dispersed Pt sites were generated at 1 wt%

loading (Figure 3.2e and Figure 3.3e). However, an increase in Pt loading resulted in a gradual increase