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Title Page

Abstract

Contents

List of Abbreviations 18

Chapter 1. Introduction 19

1.1. Conventional heterogeneous catalysts 19

1.2. Single-atom catalysts 20

1.3. Characteristics of CeO₂ 25

1.4. CO oxidation mechanism 31

1.5. Motivation and organization of dissertation 33

Chapter 2. Investigation of CO oxidation over Pd/CeO₂(100) single-atom catalysts 35

2.1. Introduction 35

2.2. Experimental methods 37

2.2.1. Preparation of catalysts 37

2.2.2. Characterization of the catalyst 37

2.2.3. CO oxidation reaction 37

2.2.4. DFT calculations 38

2.3. Results and discussion 39

2.4. Conclusion 50

Chapter 3. Surface density dependent catalytic activity of single Pd atoms supported on CeO₂ 51

3.1. Introduction 51

3.2. Experimental methods 53

3.2.1. Catalysts preparation 53

3.2.2. CeO₂ characterization 53

3.2.3. Pd/CeO₂ and Pd/Al₂O₃ characterization 53

3.2.4. Catalytic performance 54

3.2.5. DFT calculations 54

3.3. Results and discussion 56

3.4. Conclusion 77

Chapter 4. Catalytic behavior of Pt single atoms supported on CeO₂ 78

4.1. Introduction 78

4.2. Experimental methods 79

4.2.1. Catalysts preparation 79

4.2.2. CeO₂ characterization 79

4.2.3. Pt/CeO₂ characterization 79

4.2.4. Catalytic performance 80

4.3. Results and discussion 81

4.4. Conclusions 101

Chapter 5. Summary 102

References 104

Publications 118

List of Tables

Table 1.1. Coordination number of Ce and O of bulk and low index surfaces. 26

Table 2.1. Catalytic activity of 0.1% Pd/CeO₂ catalysts in CO oxidation 41

Table 3.1. Pd contents in prepared Pd/CeO₂ catalysts. 57

Table 3.2. Pd contents in prepared Pd/Al₂O₃ catalysts. 57

Table 3.3. Structural parameters from Pd K-edge EXAFS analysis for 0.8 and 4 Pd/CeO₂. 60

Table 3.4. Summary of H₂-TPR over Pd/CeO₂. 64

Table 3.5. Summary of H₂-TPR over fresh and spent Pd/CeO₂. 65

Table 4.1. Pt contents in prepared Pt/CeO₂ catalysts. 82

Table 4.2. Structural parameters from Pt L3-edge EXAFS analysis for 0.4, 0.8 and 4 Pt/CeO₂. 88

Table 4.3. Summary of H₂-TPR over Pt/CeO₂. 91

Table 4.4. Summary of CO-TPD over oxidized 0.4, 0.8, 1.2 and 4 Pt/CeO₂ catalysts. P1 and P2 is... 94

Table 4.5. Summary of CO-TPD over oxidized 0.1 and 0.4 Pd/CeO₂ catalysts. P1 and P2 are desorption... 95

Table 4.6. Summary of CO-TPD over 0.4 Pt/CeO₂ catalysts after oxidation and reduction with CO.... 97

Table 4.7. Summary of H₂-TPR over 0.4 Pt/CeO₂ and 0.8 Pt/CeO₂. 100

List of Figures

Figure 1.1. Geometric and electronic structures of single atom, clusters, and nanoparticles. 20

Figure 1.2. Structural features of metal catalysts based on supported nanoparticles or single atoms, or... 21

Figure 1.3. Characterization of single-atom catalyst. (a) HAADF-STEM image of Ptiso/CeO₂ and (b)...[이미지참조] 23

Figure 1.4. (a) Operando HERFD-XANES spectra of Pt-SS in the reaction mixture (1,000 ppm CO,... 24

Figure 1.5. Crystal structure of (a) ideal crystal CeO₂ and (b) CeO₂ with oxygen vacancy. 25

Figure 1.6. Top, side, and perspective view of CeO₂ (100), (110), and (111) surfaces. Gray and red... 26

Figure 1.7. TEM and HRTEM images of (a, d) CeO₂ rods enclosed by (110) and (100) crystal planes,... 28

Figure 1.8. (a) Schematic representation of the differences in the energetics for the sintering of Ag... 29

Figure 1.9. Calculated number of sites with a particular geometry (surface and perimeter or corner... 30

Figure 1.10. The LH mechanism on a metal surface. 31

Figure 1.11. The MvK mechanism on a metal oxide surface. 32

Figure 1.12. Schematic illustration for a surface of (a) conventional support and (b) morphology-... 33

Figure 2.1. (a) XRD patterns of synthesized CeO₂ (100) and CeO₂ (111). SEM and TEM images of (b,... 39

Figure 2.2. AC-HAADF-STEM images of (a) 0.1% Pd/CeO₂ (100) and (b) 0.1% Pd/CeO₂ (111). (c)... 40

Figure 2.3. Profiles of temperature-programmed CO oxidation (inset: Arrhenius plots for CO oxidation). 41

Figure 2.4. Profiles of CO-TPR. 42

Figure 2.5. in situ Raman spectra of (a) 0.1% Pd/CeO₂ (100) and (b) 0.1% Pd/CeO₂ (111). Inset:... 43

Figure 2.6. Structure of Pd/CeO₂ and the CO adsorption barriers. (a) Tasker's stacking sequence types... 45

Figure 2.7. Relative energy of Pd/CeO₂ (100) with different positions of Pd compared to (a) which is... 46

Figure 2.8. O₂ adsorption barriers and energy profiles on Pd/CeO₂. Red circle is used to emphasize... 47

Figure 2.9. Mechanism of low-temperature CO oxidation on a Pd/CeO₂ (100) surface. The initial state... 49

Figure 3.1. (a) XRD pattern of synthesized CeO₂. (b) N₂ sorption isotherms for CeO₂ (Closed circle:... 56

Figure 3.2. (a) SEM and (b) TEM images of CeO₂. 56

Figure 3.3. AC-HAADF-STEM images of (a, b) 0.4 Pd/CeO₂, (c, d) 0.8 Pd/CeO₂, (e, f) 1.2 Pd/CeO₂... 58

Figure 3.4. AC-HAADF-STEM images of (a, b) 0.068 Pd/Al₂O₃ and (c, d) 0.2 Pd/Al₂O₃. Pd atoms are... 59

Figure 3.5. EXFAS spectra obtained for (a) 0.8 Pd/CeO₂ and (b) 4 Pd/CeO₂. The circle and solid line... 60

Figure 3.6. (a) Specific activity of CO oxidation at steady state as a function of surface Pd density of... 61

Figure 3.7. (a) AC-HAADF-STEM images of spent 4 Pd/CeO₂ (CO oxidation at 75℃ for 2 h), (b)... 62

Figure 3.8. (a) H₂-TPR profiles of Pd/CeO₂ and (b) reduction temperatures as a function of surface Pd density 63

Figure 3.9. H₂-TPR profiles of fresh and spent catalysts. (a) 0.2 Pd/CeO₂, (b) 0.4 Pd/CeO₂, (c) 0.8... 65

Figure 3.10. H₂-TPR profiles of Pd/Al₂O₃ 66

Figure 3.11. in situ Raman spectra of (a, d) 0.2 Pd/CeO₂, (b, e) 0.4 Pd/CeO₂, and (c, f) 4 Pd/CeO₂. (d),... 68

Figure 3.12. In situ IR spectra of (a) 0.2, (b) 0.4 and (c) 4 Pd/CeO₂. The spectra were obtained while... 70

Figure 3.13. (a) DFT surface energy of the Pd 1 /CeO₂ (100) model, seen in the first panel of (b), as a... 71

Figure 3.14. (a) Top-down view of the square planar [PdO 4] active site on the CeO₂ (100) surface. (b)... 73

Figure 3.15. Quantities computed as a function of Pd Loading: (a) oxygen vacancy formation energies,... 75

Figure 3.16. (a) total OVFE (extensive property) as a function of the number of O atoms removed from... 76

Figure 4.1. (a) XRD pattern of synthesized CeO₂. (b) N₂ sorption isotherms for CeO₂ (Closed circle:... 81

Figure 4.2. (a) SEM and (b) TEM image of CeO₂. 81

Figure 4.3. (a) XRD patterns of CeO₂ and Pt/CeO₂ catalysts and (b) magnification from 35 to 45°.... 82

Figure 4.4. (a) Temperature-programmed CO oxidation activity of Pt/CeO₂ catalysts. (b) T10 for...[이미지참조] 84

Figure 4.5. AC-HAADF-STEM images of 0.4 Pt/CeO₂. 85

Figure 4.6. AC-HAADF-STEM images of 0.8 Pt/CeO₂. 86

Figure 4.7. (a) AC-HAADF-STEM image of 4 Pt/CeO₂, (b) magnified image and (c) EDS linescan result. 87

Figure 4.8. k 3 -weighted chi (k) and the corresponding fourier transform for (a, b) 0.4 Pt/CeO₂, (c, d)... 88

Figure 4.9. XANES spectra (Pt L3-edge) of the 0.4 Pt/CeO₂, 0.8 Pt/CeO₂ and 4 Pt/CeO₂. 89

Figure 4.10. (a) H₂-TPR profiles of CeO₂ and Pt/CeO₂ catalysts and (b) reduction temperature as a... 90

Figure 4.11. Comparison of reduction temperature and isothermal reaction temperature of Pt/CeO₂... 92

Figure 4.12. (a) CO-TPD results of Pt/CeO₂ catalysts. (b-d) TPD results of CO (m/z＝28) and CO₂... 93

Figure 4.13. (a) CO-TPD (inset: TPD results of CO (m/z＝28) and CO₂ (m/z＝44) on 0.4 Pd/CeO₂) and... 95

Figure 4.14. (a) CO-TPD and (b) temperature-programmed CO oxidation results of 0.4 Pt/CeO₂... 97

Figure 4.15. (a) in situ Raman spectra of 0.4 Pt/CeO₂ during CO-TPR (1% CO/He). (b) magnified spectra. 98

Figure 4.16. H₂-TPR profiles of 0.4 Pt/CeO₂ reduced under CO at 75 ℃ and 100 ℃ (designated as 0.4... 99

Figure 5.1. Catalytic behavior of Pd/Al₂O₃, Pd/CeO₂, and Pt/CeO₂ catalysts in CO oxidation reaction. 103

초록보기

Heterogeneous catalyst is important in the chemical industry. To design and synthesize more efficient catalysts, it is necessary to understand catalytic behavior at the atomic level. However, despite its wide applicability, understanding the catalytic behavior of heterogeneous catalysts at the atomic level is challenging owing to the inherent heterogeneity of the active sites. In contrast, single-atom catalysts contain uniform and well-defined active sites, and they can serve as model catalysts. This makes it possible to understand catalytic behavior at the atomic level. CeO₂ has been frequently used as support for single-atom catalysts because it strongly interacts with supported metal and prevents metal agglomeration. CeO₂ is also well known for its excellent redox properties and oxygen storage capacity. Its reducibility is significantly enhanced when a noble metal is supported. Due to its redox properties, CeO₂ has been an essential component of environmental catalysts to remove CO and hydrocarbons from vehicle emissions. In this study, we investigated how the redox behavior of CeO₂ is involved in catalytic behavior of single-atom catalysts. CO oxidation reaction was adopted as a model reaction because of its simplicity and practical importance.

Initially, we compared catalytic behavior of atomically dispersed Pd supported on CeO₂(111) and CeO₂(100). Analysis of in situ Raman spectroscopy and detailed DFT revealed the origin of the superior CO oxidation activity of atomically dispersed Pd on CeO₂(100), compared with CeO₂(111). We postulated that O₂ adsorbed on the Pd/CeO₂(100) surface has an important role in lowering the activation barrier of the CO oxidation reaction.

We also investigated the effect of the surface density of single Pd atoms supported on CeO₂ on catalytic behavior. The Pd/CeO₂ single-atom catalysts show increasing CO oxidation specific activity with surface Pd density, which has not been observed with atomically dispersed Pd on Al₂O₃. The unique behavior of the Pd/CeO₂ single-atom catalysts was rationalized by cumulative enhancement of the reducibility with increased surface Pd density.

In addition, we investigated the catalytic behavior of atomically dispersed Pt supported on CeO₂. Contrary to Pd/CeO₂ single-atom catalysts, the CO oxidation activity of Pt/CeO₂ single-atom catalysts was very low despite the reducibility of the catalysts increasing with surface Pt density, which originated from negligible CO interaction. The CO interaction and CO oxidation activity were significantly enhanced upon Pt cluster formation.

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