Title Page
ABSTRACT
Contents
CHAPTER I. General Introduction 26
1.1. Catalytic Activity and Selectivity of Colloidal Nanocrystals 27
1.2. Construction of Active Sites in the Nanocatalysts 28
1.2.1. Increasing the Number of Active Sites by the Formation of Nanoframe Structures 28
1.2.2. Forming Nanoscale Hetero-interfaces between Metals and Metal Compounds with Synergistic Effect in Catalytic Reactions 38
1.2.3. Broadening Horizons of Active Sites in the Multiphasic Nanocrystal via Directing Atom Migration 56
1.3. Applications of Nanocatalysts in Selective Conversion to Value-Added Products 72
1.3.1. Hydrogenation of Biomass-Derived Furfural 72
1.3.2. Electrochemical CO₂ Reduction 74
1.4. Work Described in This Thesis 81
1.5. References 82
CHAPTER II. Suppressing Over-Hydrogenation of Furfural on the Abundant Active Sites in Surface Energy-Tuned Pd₃Pb Alloy Nanosponges 99
2.1. Introduction 100
2.2. Experimental Section 102
2.2.1. Reagents 102
2.2.2. Preparation of the Pd₃Pb Multiframes 102
2.2.3. Preparation of the Carbon-Supported Catalyst 103
2.2.4. Preparation of the 24 nm Pd₃Pb Nanoparticles 104
2.2.5. Material Characterization 104
2.2.6. Catalytic Hydrogenation of Furfural 105
2.2.7. Calculation of Turnover Frequency for Each of the Catalysts 106
2.2.8. Density Functional Theory 106
2.3. Results and Discussion 109
2.3.1. Characterizations of Sponge-Like Pd₃Pb Multiframes 109
2.3.2. Formation Mechanism of the Pd₃Pb Multiframes 116
2.3.3. Catalytic Performance of Pd₃Pb MF/C Catalyst toward Furfural Hydrogenation 125
2.3.4. Origin of Enhanced Catalytic Selectivity in Pd₃Pb MF Catalyst 136
2.4. Conclusion 138
2.5. References 139
CHAPTER III. Promoting C-C Coupling on the Hetero-Interfaces between Cu and Cu₂O Derived from Surface Reconstruction of Hierarchical Cu₂O Nanocrystals during the Electrochemical CO₂ Reduction 144
3.1. Introduction 145
3.2. Experimental Section 147
3.2.1. Reagents 147
3.2.2. Microfluidic Devices 148
3.2.3. Flow Synthesis of h-Cu₂O ONSs (h-Cu₂O Catalyst) and f-Cu₂O Catalyst 148
3.2.4. Synthesis of b-Cu₂O Catalyst 149
3.2.5. Material Characterization 150
3.2.6. Computational Fluid Dynamics (CFD) Simulation of Microfluidic Device 151
3.2.7. Working Electrode Preparation 152
3.2.8. Electrochemical Measurement and CO₂ Reduction Product Analysis 153
3.3. Results and Discussion 156
3.3.1. Synthesis and Characterization of h-Cu₂O ONS 156
3.3.2. Growth Mechanism of h-Cu₂O ONS 163
3.3.3. Unprecedented Fast Reaction Kinetics and CFD Simulation 167
3.3.4. Electrocatalytic Performance of h-Cu₂O ONS for the CO₂RR 175
3.3.5. Structural Changes of h-Cu₂O ONS during the CO₂RR 181
3.4. Conclusion 191
3.5. References 192
CHAPTER IV. Conclusion 198
Vita 201
Table 2.1. Summary of Pb 4f XPS fitting parameters and relative peak areas for unetched Pd₃Pb MF and etched Pd₃Pb MF. 124
Table 2.2. Summary of Pd 3d XPS fitting parameters and relative peak areas for unetched Pd₃Pb MF and etched Pd₃Pb MF. 124
Table 2.3. Hydrogenation of furfural catalyzed by commercial Pd/C catalyst. Reaction conditions: furfural (1.2 mmol), 1.6 mol% catalyst in 5 mL i-PrOH under room temperature and... 130
Table 2.4. Hydrogenation of furfural catalyzed by Pd₃Pb MF/C catalyst. Reaction conditions: furfural (1.2 mmol), 1.1 mol% catalyst in 5 mL i-PrOH under room temperature and 1 atm H₂... 132
Table 2.5. Comparison of the catalytic performance of Pd-based catalysts for the hydrogenation of furfural. Time on steam (TOS) in min. total pressure. thermal reduction. chemical... 133
Table 2.6. Cycle test of furfural hydrogenation catalyzed by Pd₃Pb MF/C catalyst in optimized condition. Reaction conditions: furfural (1.2 mmol), 1.1 mol% catalyst in 5 mL i-PrOH under... 134
Table 3.1. Comparison of Faradaic efficiency and partial current density of C2+ product in Cu based catalyst in the flow cell system.[이미지참조] 180
Table 3.2. Comparison of electrochemical surface area through Pb UPD of each catalyst pre and post CO₂RR. 183
Figure 1.1. Simulated evolution profiles of nanocrystals by heat-up method from precursors with different dissociation energies: Precursor X has a high reactivity (top), precursor Y has an... 34
Figure 1.2. AuPd binary nanocrystal under different halide anion concentration. a,b) EDX elemental mapping analyses of Au (red) and Pd (green) contents in AuPd alloy (a) and Au@Pd... 37
Figure 1.3. (A) Schematic illustration of the synthesis of two-types of heterodimer isomers prepared by repeating the seed-mediated growth, Fe₃O₄-Pt-Ag and the Ag-Fe₃O₄-Pt, and the... 42
Figure 1.4. (A-C) Pt-Co(OH)₂/CC catalyst. (A) Schematic illustration of the synthetic process of Pt-Co(OH)₂/CC, (B) TEM and (C) HRTEM images of Pt-Co(OH)₂/CC. Reproduced with... 45
Figure 1.5. (A) Schematic illustration of the mechanism for surfactant-controlled Co deposition on TiO₂ nanorod seeds. TEM images of the case of (B) homogeneous nucleation, (C) selective... 48
Figure 1.6. Ni/Mo₂C-PC catalyst. (A) Schematic illustration of the synthesis of Ni/Mo₂C-PC from NiMoO₄ nanorods. (B) TEM image and corresponding EELS elemental mapping images... 53
Figure 1.7. Ag₂S-hollow Pt heterostructures. (A) Schematic illustration of the synthetic process of Ag₂S-hollow Pt heterostructures through phase segregation. (B) TEM images of core-shell... 55
Figure 1.8. Summary of various factors that can affect the atom energy state in the nanocrystal, which are categorically called "chemical fields." 58
Figure 1.9. Schematic lattice potential contour maps for a pristine crystal under radial, anisotropic, and asymmetric chemical fields. The fluid movement depicts the atom migration... 61
Figure 1.10. (a) Schematic illustration and corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images for the structural evolution of PtNi octahedral... 66
Figure 1.11. (a, b) TEM images and schematic illustration of Cu2-xS hexagonal nanoplates and the Au+-exchanged Cu2-xS nanoplates. Reproduced with permission. Copyright 2016, Wiley-...[이미지참조] 70
Figure 1.12. Various types of high-value products can be obtained through the conversion of furfural. Reproduced with permission from ref. 175, copyright 2019 Multidisciplinary Digital... 73
Figure 1.13. Proposed electrochemical CO₂RR mechanism for the formation of major (a) C₁ products (formate, carbon monoxide, and methane) and (b) C₂ products (ethylene and ethanol).... 76
Figure 1.14. (a) Cropped periodic table of major product classification of metal catalysts for electroreduction of CO₂ with colors and major product faradaic efficiency. Four groups are... 78
Figure 1.15. Current efficiency as a function of V vs. RHE for CO₂ reduction products on metallic Cu surfaces. From top to bottom: Major, intermediate, and minor products. Reproduced... 80
Figure 2.1. Top and side view of the adsorption configurations of furfuryl alcohol (FOL) on Pd₃Pb. 108
Figure 2.2. Morphology and structure characterization of the sponge-like Pd₃Pb multiframes (Pd₃Pb MFs). Representative a) HAADF-STEM image and structural illustration of the Pd₃Pb... 110
Figure 2.3. TEM analysis of Pd₃Pb MFs. (a) TEM image of Pd₃Pb MFs and (b) histogram for particle size distribution. The particle size has measured the diameter of nanoparticles. The mean... 111
Figure 2.4. HAADF line profile analysis of Pd₃Pb MF from Figure 2.2d. 113
Figure 2.5. EDS spectrum of the Pd₃Pb MFs. Sample for EDS measurement was prepared on Cu TEM grid. 113
Figure 2.6. N₂ isotherms of Pd₃Pb MFs and Pd₃Pb nanoparticles at 77 K. Before sorption analysis, the samples were degassed at 120 ℃ under vacuum for 5 hours. All gases used in... 114
Figure 2.7. Morphology and structure characterization of Pd₃Pb nanoparticles (Pd₃Pb NPs). (a) TEM image of Pd₃Pb NPs and (b) histogram for particle size distribution. The particle size has... 115
Figure 2.8. Temporal evolution of the sponge-like Pd₃Pb multiframes (Pd₃Pb MFs). a) Schematic illustration of the structural evolution into a Pd₃Pb MF. HAADF-STEM and... 119
Figure 2.9. Temporal TEM images of reaction intermediates of Pd₃Pb MF at (a) 2 min, (b) 4 min (inset: chemically etched by acid treatment). 120
Figure 2.10. (a) HRTEM images of reaction intermediates of Pd₃Pb MF at 2 min. (b) and (c) Corresponding FFT patterns in the marked area. 120
Figure 2.11. Summary of crystallography information and corresponding unit cell models of monoclinic Pd13Pb9 and fcc Pd₃Pb.[이미지참조] 121
Figure 2.12. TEM image of the cleaved Pd₃Pb MF intermediates. 121
Figure 2.13. (a-c) Temporal TEM images and (d-f) particle size distribution histograms for reaction intermediates of Pd₃Pb MF at (a,d) 10 min, (b,e) 30 min, and (c,f) 60 min. (g)... 122
Figure 2.14. EDS spectrum of the unetched Pd₃Pb MFs. Sample for EDS measurement was prepared on Cu TEM grid. 122
Figure 2.15. (a) Pb 4f and (b) Pd 3d XPS spectra of unetched Pd₃Pb MF and etched Pd₃Pb MF. 123
Figure 2.16. Catalytic performance of the Pd₃Pb MF/C and commercial Pd/C catalysts in furfural hydrogenation. a) Reaction pathways in the furfural hydrogenation reaction. Catalytic... 126
Figure 2.17. Reaction pathways for the hydrogenation of furfural. 127
Figure 2.18. (a) Low-magnification and (b) high-magnification TEM images of carbon-supported Pd₃Pb MF (Pd₃Pb MF/C) catalyst. 129
Figure 2.19. TEM image of aggregated Pd₃Pb MFs loaded on carbon supports after acid treatment. 129
Figure 2.20. (a) Low-magnification and (b) high-magnification TEM images of commercial Pd/C (10 wt%, Sigma-Aldrich). 130
Figure 2.21. (a) TEM image of Pd₃Pb MF/C catalyst after five cycles of furfural hydrogenation. (b) Comparison of PXRD patterns for Pd₃Pb MF/C catalyst before and after five cycles of hydrogenation. 135
Figure 2.22. The adsorption configurations of furfuryl alcohol (FOL) on a) Pd and b) Pd₃Pb. Eads is the FOL adsorption energy. Gibbs free energy diagrams of the three competitive...[이미지참조] 137
Figure 3.1. Schematic diagram and operation photo of the flow cell system. 154
Figure 3.2. Structural and composition characterizations of hierarchical Cu₂O octopods with multi-steps (h-Cu₂O ONSs). a) Schematic diagram of the synthesizing h-Cu₂O ONSs in a flow... 157
Figure 3.3. Representative FE-SEM images at different magnifications of the h-Cu₂O ONS nanocrystals. 159
Figure 3.4. Structural models and corresponding TEM images viewed from different zone axes for h-Cu₂O ONS nanocrystal. 160
Figure 3.5. TEM analysis of h-Cu₂O ONS nanocrystals. (a) TEM image of h-Cu₂O ONSs and (b) histogram for particle size distribution. The particle size is measured at the edge of... 161
Figure 3.6. (a) TEM image and (b) PXRD pattern of the Cu₂O nanocubes obtained by the conventional batch reaction. 161
Figure 3.7. Structure analysis of h-Cu₂O ONSs at different reaction times. a-d) Schematic illustration and e-h) TEM images of intermediates at different reaction times of (a,e) 2 min, (b,f)... 164
Figure 3.8. Illustration of the crystal structures of oriented to show the (a) {100}, (b) {110} and (c) {111} plane. The red spheres are oxygen atoms, and the blue spheres are copper atoms. The... 166
Figure 3.9. UV-vis absorption spectra of the h-Cu₂O ONS obtained at different aging times. The h-Cu₂O ONS absorption band maxima are located at 504, 587, 650, and 658 at aging times... 166
Figure 3.10. (a) Schematic diagram of the system and (b) TEM image of the Cu₂O nanocrystals in a flow microreactor without M2 mixing. 167
Figure 3.11. Computational fluid dynamics (CFD) simulation and analysis of mixing performance in different total flow rates. (a, b) Streamlines in the T-shaped micromixer for total... 169
Figure 3.12. Computational fluid dynamics (CFD) simulation and analysis of mixing performance in different total flow rates of 16.8 mL/min, 12.6 mL/min, 8.4 mL/min, and 4.2... 171
Figure 3.13. TEM images of Cu₂O nanocrystals obtained at different reducing agents and the total flow rates. (a) L-glucose - 2.1 mL/min, (b) L-glucose - 10.5 mL/min, (c) L-glucose - 21... 173
Figure 3.14. TEM images of Cu₂O nanocrystals obtained at different Cu precursors and the total flow rates. (a) CuBr₂ - 2.1 mL/min, (b) CuBr₂ - 10.5 mL/min, (c) CuBr₂ - 21 mL/min, (d)... 174
Figure 3.15. TEM images of (a) cube-shaped b-Cu₂O catalyst and (b) f-Cu₂O catalyst filled with all step sites. 177
Figure 3.16. Electrocatalytic performance of the b-Cu₂O, f-Cu₂O, and h-Cu₂O catalysts in CO₂RR. a) Total current densities depend on the elapsed time. b) Faradaic efficiencies of C2H6,...[이미지참조] 178
Figure 3.17. Potential-dependent (a-c) total current densities and (d-f) Faradaic efficiencies (FEs) of (a,d) b-Cu₂O, (b,e) f-Cu₂O, and (c,f) h-Cu₂O catalysts. 179
Figure 3.18. C₂H₄/CH₄ product ratio of the b-Cu₂O, f-Cu₂O, and h-Cu₂O catalysts within 1 h and after 4 h of CO₂RR. 179
Figure 3.19. Morphological change of the h-Cu₂O catalyst during the CO₂RR. a-c) TEM images and d-f) corresponding SAED patterns of h-Cu₂O catalyst (a,d) before the CO₂RR, (b,e)... 182
Figure 3.20. Comparison of Pb monolayer peak size before and after reaction of each catalyst. The peak around -0.3 V vs. Ag/AgCl indicated Pb monolayer stripping. 183
Figure 3.21. (a) HRTEM image and (b) corresponding FFT pattern of h-Cu₂O catalyst after 2 h of the CO₂RR at -1.1 V vs. RHE in CO₂-saturated 0.1 M KHCO₃. The elongation of spots in... 185
Figure 3.22. Morphological change of the b-Cu₂O catalyst and f-Cu₂O catalyst during the CO₂RR. (a,b) TEM images and (c,d) corresponding SAED patterns of b-Cu₂O catalyst (a,c)... 186
Figure 3.23. HRTEM images of a) b-Cu₂O and c) h-Cu₂O catalyst after the CO₂RR. Auger Cu LMM spectra of b) b-Cu₂O and d) h-Cu₂O catalyst before and after the CO₂RR. The dotted lines... 188
Figure 3.24. Auger Cu LMM spectra of f-Cu₂O catalyst before and after the CO₂RR. The dotted line represents the binding energy of Cu+ species....[이미지참조] 190