3.3. R ESULTS AND D ISCUSSION
3.3.2. Role of IL and SiO 2 Coatings
Table 3.2. ICP-OES results for catalysts with various Pt loadings.
Sample Loading (wt%)
1Pt/CNT 0.9
1Pt/CNT_IL 0.9
1Pt/CNT_IL_SiO2 0.9
3Pt/CNT 2.7
3Pt/CNT_IL 2.6
3Pt/CNT_IL_SiO2 2.7
5Pt/CNT 4.8
5Pt/CNT_IL 4.8
5Pt/CNT_IL_SiO2 4.7
10Pt/CNT 9.8
10Pt/CNT_IL 9.6
10Pt/CNT_IL_SiO2 9.5
suggesting the formation of amorphous carbon. Similarly, the XPS spectra of the two supports indicated that CNT mostly consisted of sp2 C=C bonds, whereas CNT_IL contained a significant amount of sp3 C–C bonds and C–O(N) bonds, along with sp2 C=C bonds, which is in accordance with HRTEM observations. The elemental analysis results for CNT and CNT_IL (Table 3.3) indicated that the heteroatom contents increased significantly in CNT_IL (13.6 wt% nitrogen and 10.8 wt% oxygen) compared with those in CNT (0 wt% nitrogen and 2.5 wt% oxygen).
Figure 3.6. Formation of amorphous carbon coating layer after IL coating on CNT. XRD patterns of CNT and CNT_IL.
Figure 3.7. XPS analysis of carbon bonding states before and after IL coating on CNT. (a,b) Carbon 1s XPS spectra of CNT (a) and CNT_IL (b).
2 (Degrees) 10 20 30 40
2 (degrees) 10 20 30 40
Intensity (a.u.)
2 (degrees)
10 20 30 40 50 60 70 80 90
Intensity (a.u.)
CNT CNT_IL
C (002)
Binding Energy (eV)
280 284 288 292 296
Intensity (a.u.)
Raw data Fitting sum sp2 C=C sp3 C-C C-O(N)
Binding Energy (eV)
280 284 288 292 296
Intensity (a.u.)
Raw data Fitting sum sp2 C=C sp3 C-C C-O(N)
(a) (b)
CNT C 1s
CNT_IL C 1s
Table 3.3.Elemental analysis results for CNT and CNT_IL.
Sample Content (wt%)
C H N O S
CNT 94.8 0.1 0 2.5 0
CNT_IL 72.6 1.6 13.6 10.8 0
Figure 3.8. XANES spectra for the Pt precursor and impregnated samples. (a–c) Pt L3-edge XANES spectra of H2PtCl6, 3Pt/CNT_Imp, and 3Pt/CNT_IL_Imp (a) and expanded spectra between 11,560 and 11,580 eV (b) and 11,572 and 11,700 eV (c).
The XAS analysis (Figures 3.8 and 3.9) provided important evidence for enhanced interactions between the Pt precursor and the support after the formation of the heteroatom-enriched carbon layer.
The Pt L3-edge X-ray absorption near edge structure (XANES) spectra of H2PtCl6‧6H2O and H2PtCl6‧6H2O-impregnated CNT (3Pt/CNT_Imp) and CNT_IL (3Pt/CNT_IL_Imp) revealed clear differences between the precursor and impregnated states in the edge and post-edge regions (Figure 3.8).
A comparison of the XANES spectra for H2PtCl6‧6H2Oand the impregnated samples (Figures 3.8a,b) indicated that the edge (peak A) was shifted to a lower energy and the white line intensity was decreased after impregnation with H2PtCl6‧6H2O. This change revealed that the Pt precursor was reduced from Pt4+ to Pt2+ accompanied by partial decomposition of the Cl ligands.62 The precursor decomposition after impregnation was also suggested by the decreased intensities of peaks C, D, and E (Figure 3.8), which represent the octahedral configuration of H2PtCl6.60 Although H2PtCl6‧6H2O was reduced similarly on both CNT and CNT_IL, a clear difference observed for peak B, which originates from the hybridization of Pt 5d and Cl 3d states.60 While a small decrease of peak B was observed for 3Pt/CNT_Imp relative to H2PtCl6‧6H2O, a marked decrease was found for 3Pt/CNT_IL_Imp (Figure
11580 11620 11660 11700 11540 11580 11620 11660
Normalized Absorption
H2PtCl6 3Pt/CNT_Imp 3Pt/CNT_IL_Imp
(a)
Pt L3
11560 11580 (b)
B
C E
D
(c)
Photon Energy (eV)
A
3.8c), indicating the additional breaking of Pt–Cl bonds in Pt/CNT_IL_Imp. The EXAFS analysis of this series of samples (Figure 3.9 and Table 3.4) indicated that the peak for Pt–N scattering was newly evolved in 3Pt/CNT_IL_Imp along with Pt–Cl scattering, whereas 3Pt/CNT_Imp had only Pt–Cl scattering. We suppose that H2PtCl6‧6H2O could be further stabilized by ligand exchange from Cl to N in CNT_IL during the impregnation process, which would provide strong adsorption sites for Pt precursors.
Figure 3.9. EXAFS spectra for the Pt precursor and impregnated samples. (a–c) Pt L3-edge k3-weighted EXAFS spectra and their best fit for H2PtCl6 (a), 3Pt/CNT_Imp (b), and 3Pt/CNT_IL_Imp (c).
Table 3.4. Fitting parameters for Pt L3-edge k3-weighted EXAFS spectra of H2PtCl6, 3Pt/CNT_Imp, and 3Pt/CNT_IL_Imp.
Sample Shell CNa ΔE0
(eV)
R (Å)
σ2 (10−3 Å2) b
R factor (%) H2PtCl6 Pt–Cl 6.05 ± 0.348 11.5
± 0.9 2.32 ± 0.00 2.3 ± 0.6 0.6 3Pt/CNT_Imp Pt–Cl 4.09 ± 0.37 −12.0
± 1.2 2.31 ± 0.01 4.0 ± 0.6 1.2 3Pt/CNT_IL
_Imp
Pt–N 1.33 ± 0.44 11.0
± 1.2
2.04 ± 0.03 1.0 f Pt–Cl 4.49 ± 0.52 2.31 ± 0.00 2.7 ± 0.7 0.7
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 strong interaction between H2PtCl6‧6H2O and 3Pt/CNT_IL_Imp could influence the activation process of impregnated Pt precursors, thus affecting the dispersion state of the final Pt species.
We analyzed the thermal activation processes of the impregnated catalysts by monitoring the consumed and desorbed gases in a quartz flow reactor coupled to a mass spectrometer while raising the temperature from RT to 500 °C under 2% H2/Ar gas flow (Figure 3.10). When activating 3Pt/CNT_Imp, a sharp peak for H2 consumption was observed near 150 °C (Figure 3.10a) accompanied by a peak for HCl generation (Figure 3.10b), implying that H2PtCl6‧6H2O decomposed into HCl. Further decomposition into Cl2 was observed near 200 °C (Figure 3.10c) without H2 consumption. These MS results suggest that the H2PtCl6‧6H2O molecules decomposed by H2 became mobile enough to agglomerate into Pt NPs via the reaction Pt–Cl + Cl–Pt → Cl2 + Pt–Pt, thus generating Cl2 gas. Contrary to the rapid activation observed on 3Pt/CNT_Imp, this process was slowed down for 3Pt/CNT_IL_Imp,
0 1 2 3 4 5 6
FT Magnitude (a.u.)
Raw data Fit Window
0 1 2 3 4 5 6
FT Magnitude (a.u.)
Raw data Fit Window
0 1 2 3 4 5 6
FT Magnitude (a.u.)
Raw data Fit Window
(c) Pt–N
Pt–Cl
(b) Pt–Cl
(a) Pt–Cl
Reduced Distance (Å ) Reduced Distance (Å ) Reduced Distance (Å )
as revealed by the broad peaks for H2 consumption (Figure 3.10a) and HCl production (Figure 3.10b).
In addition, for 3Pt/CNT_IL_Imp, Cl2 was evolved at higher temperatures and the peak area for Cl2
decreased drastically compared with that for 3Pt/CNT_Imp (Figure 3.10c). It appears that strong anchoring of H2PtCl6‧6H2O on 3Pt/CNT_IL_Imp retards its decomposition, allowing the generated atomically dispersed sites to be stably isolated while suppressing agglomeration into NPs.
Figure 3.10. MS analyses of the evolved gas products during thermal activation. (a–d) MS spectra of impregnated 3Pt/CNT, 3Pt/CNT_IL, and 3Pt/CNT_IL_SiO2 during thermal activation under 2% H2/Ar:
H2 (a), HCl (b), and Cl2 (c,d).
Next, we investigated the role of the SiO2 coating. The SiO2 coated sample was prepared by mixing the H2PtCl6‧6H2O-impregnated powders with TEOS, and the resulting sample was denoted as 3Pt/CNT_IL_Imp_SiO2. We note that the polymerization of TEOS was catalyzed by impregnated H2PtCl6‧6H2O without the use of any acid or base catalyst, inducing the formation of a SiO2 layer exclusively on the surface of Pt/CNT_IL. The HAADF-STEM images of 3Pt/CNT_IL_SiO2 before HF etching showed that the SiO2 protective layer coated the atomically dispersed Pt supported on CNT_IL (Figure 3.11a–c). The HAADF-STEM and EDS mapping images of 3Pt/CNT_IL_SiO2 also indicated the presence of Si and O, along with Pt, C, and N (Figure 3.11d–i).
Temperature (oC)
0 100 200 300 400 500
Intensity (a.u.)
Temperature (oC)
0 100 200 300 400 500
Intensity (a.u.)
3Pt/CNT_Imp 3Pt/CNT_IL_Imp 3Pt/CNT_IL_Imp_SiO2
Temperature (oC)
0 100 200 300 400 500
Intenstiy (a.u.)
Temperature (oC)
0 100 200 300 400 500
Intenstiy (a.u.)
36 (HCl)
2 (H2) 70 (Cl2)
70 (Cl2)
(a) (c)
(b) (d)
Figure 3.11. Morphological and compositional analyses of 3Pt/CNT_IL_SiO2 before SiO2 protective layer etching. (a–d) HAADF-STEM images of 3Pt/CNT_IL_SiO2 before SiO2 etching. (e–i) EDS mapping images of 3Pt/CNT_IL_SiO2 before SiO2 etching: carbon (e), nitrogen (f), oxygen (g), silicon (h), and platinum (i).
XANES and XPS analyses indicated no significant chemical interaction between Pt and the SiO2
layer (Figure 3.12). The XANES spectra were recorded after the CNT_IL support was (i) impregnated with the Pt precursor, (ii) coated with the SiO2 layer, (iii) activated with H2, and (iv) etched by HF (samples denoted as 3Pt/CNT_IL_Imp, 3Pt/CNT_IL_Imp_SiO2, 3Pt/CNT_IL_Imp_SiO2_Act, and 3Pt/CNT_IL_Imp_SiO2_Act_HF, respectively). A comparison of the Pt L3-edge XANES (Figure 3.12a) and Pt 4f XPS (Figure 3.12b) spectra of 3Pt/CNT_IL_Imp and 3Pt/CNT_IL_Imp_SiO2 shows no significant changes in the edge or peak positions, indicating that the addition of the SiO2 layer caused a minimal change to the electronic structure of Pt. Similarly, the XANES and XPS spectra of 3Pt/CNT_IL_Imp_SiO2_Act and 3Pt/CNT_IL_Imp_SiO2_Act_HF were similar with no edge or peak shifts, implying that the removal of the SiO2 layer also did not alter the electronic structure of Pt. Notable changes in the XANES and XPS spectra were only observed between the 3Pt/CNT_IL_Imp_SiO2 and 3Pt/CNT_IL_Imp_SiO2_Act samples, suggesting that the electronic structure of Pt was only affected by the activation process. We also compared the Si 2p XPS spectra of 3Pt/CNT_IL_Imp_SiO2 and silica-layer-coated CNT_IL (CNT_IL_SiO2). As shown in Figure 3.12c, the presence of Pt had no significant influence on the chemical states of Si. Thus, the XANES and XPS results indicate that there
(a) (b) (c)
(d) (e) C (f) N
O Si Pt
(g) (h) (i)
Amorphous silica layers
20 nm
50 nm 30 nm 10 nm
20 nm 20 nm
20 nm 20 nm 20 nm
was no significant chemical interaction between Pt and Si. However, the SiO2 layer could influence the thermal activation behavior of impregnated H2PtCl6‧6H2O. The MS spectrum of 3Pt/CNT_IL_Imp_SiO2 recorded under H2/Ar gas flow showed a broad peak for HCl with concomitant consumption of H2, similar to that of 3Pt/CNT_IL_Imp (Figure 3.10a,b). Interestingly, the Cl2 evolution peak of 3Pt/CNT_IL_Imp_SiO2 was shifted to higher temperatures compared with that of 3Pt/CNT_IL_Imp (Figure 3.10c,d), suggesting that the SiO2 coating could further mitigate the agglomeration of Pt species.
Figure 3.12. XANES and XPS analyses of intermediate samples to probe the chemical interaction between Pt and the SiO2 protective layer. (a) Pt L3-edge XANES and (b) Pt 4f XPS spectra of 3Pt/CNT_IL_Imp, 3Pt/CNT_IL_Imp_SiO2, 3Pt/CNT_IL_Imp_SiO2_Act, and 3Pt/CNT_IL_Imp_SiO2_Act_HF. (c) Si 2p XPS spectra of 3Pt/CNT_IL_Imp_SiO2 and CNT_IL_SiO2.