General Introduction
Metal-Support Interaction
- Introduction to Metal-Support Interaction
- Main Phenomena of Metal-Support Interaction
- Tuning Strategy Ⅰ: Metal Nanoparticles
- Tuning Strategy Ⅱ : Support Structures
In other words, the MSI depends on the size of the metal NPs and is maximized at the size of 1~1.5 nm. In particular, the nanostructure of titanium dioxide differs in the ratio of the exposed (101) plane to (001) plane depending on the shape.
Catalytic Properties of Ceria
- Introduction to Catalytic Properties of Ceria
- Redox Property and Oxygen Storage Capacity of Ceria
- Nanostructured Ceria
- Catalytic Applications
In many catalytic reactions, the activity of the CeO2 catalyst is closely related to the energy that forms oxygen vacancies on the ceria surface. Besides the main role of surface oxygen vacancies, some of the bulk vacancies promote the redox properties of CeO2 in the catalytic combustion of toluene.
Outline of This Dissertation
The DFT calculations confirmed the activation of the Au-CeO2 interface-mediated M-vK mechanism for CO oxidation. We confirmed the superior assisting power of the CeO2(100) surface to activate CO oxidation at the Au-CeO2 interface.
Shape Effect of Ceria in Gold-Ceria Catalyst for Carbon Monoxide Oxidation Reaction
Experimental Methods
- Preparation of CeO 2 Nanocrystals
- Preparation of Au/CeO 2 Catalysts
- Experimental Characterization
- Catalytic CO oxidation Activity of Au/CeO 2
- Density Functional Theory Calculations
A microwave-assisted reduction method was used to accelerate the homogeneous nucleation and deposition of Au NPs on CeO2.49 Typically, 40 mg of shape-controlled CeO2 powder was dissolved in 40 mL of ethylene glycol, and the mixture solution was sonicated for 1 h. for complete mixing. Prior to ICP-OES measurements, the Au concentration was calibrated with a standard Au solution (Sigma-Aldrich), and the catalyst was dissolved in aqua regia and diluted with water. The reaction was carried out at 473–513 K, and the CO conversion was determined by the converted amount of CO to CO2 per unit weight of catalyst determined by ICP-OES.
The partial pressure of CO2 was calculated as the integral of the areas indicated by the GC and taking into account the reaction temperature and the volume (1 L) inside the reactor.
Results and Discussion
- DFT-studied CO Oxidation Pathway
- Structural Analysis of Au/CeO 2 Catalysts
- Catalytic Performance of Au/CeO 2 in CO Oxidation
- Experimental TOF Maps and DFT-estimated Rate Maps
Because the Au NPs were covered with CO molecules, CO oxidation occurred on Au-CeO2. Based on the DFT-estimated energetics of CO oxidation by Au/CeO2(100) and Au/CeO2(111), we performed microkinetic modeling and presented rate maps for both catalysts as a function of p(CO) and temperature (discussed further below). Au NPs were activated on CeO2 nanocrystals, qualitatively showing the critical role of Au-CeO2.
We are confident that the interface-mediated M-vK mechanism of CO oxidation occurs at the Au-CeO2.
Conclusion
Since the Au-CO interaction is stronger than the Au-O2 interaction, the total CO oxidation rate predicted by the L-H mechanism should increase as an inverse function of p(CO) as the binding sites of O2 on Au NPs become secured in case of increases. in the p(O2)/p(CO) ratio. At higher p(CO), additional surface sites of the Au NPs can be occupied by CO molecules, and thus further O–C–O formation occurs easily. Although the CO preference of Au NPs caused CO poisoning, a positive correlation between p(CO) and the experimental TOF of CO oxidation was observed in both the Au/CeO2 cubes and the Au/CeO2 octahedrons.
This finding suggests that the oxygen for CO oxidation is provided by the Au-CeO2 interface and that the increased CO concentration at the Au-CeO2 interface increases the CO oxidation activity of Au/CeO2 catalysts.
The CO oxidation was carried out using a fixed bed reactor and the CO conversion of the Pt/CeO2. TEM images and CO oxidation results of the 5 nm Pt/CeO2 catalysts are shown in Figure 3.5. As the size of the Pt NPs increases, the catalytic activity increases, saturation occurs, and the CeO2 shape effect decreases.
The active surface site of Co was determined based on the geometric dispersion of Co3O4/CeO2.
Combinational Effect of Particle Size and Support Morphology at the Platinum-Ceria
Experimental Methods
- Synthesis of CeO 2 Nanocrystals and Pt/CeO 2 Catalysts
- Characterization Methods
- Catalytic Tests
- Density Functional Theory Calculations
The reaction was monitored during the microwave-assisted reaction at 800 W of power (2.45 GHz) for 30 s. Inductively coupled plasma-optical emission spectrometry (ICP-OES) using a 700-ES model instrument (Varian) was used to determine the Pt concentration of the catalysts. CO-temperature programmed reaction (CO-TPR) tests to verify the operation of the Mars-van Krevelen.
HAADF-STEM images, TEM images, and histograms of Pt particle size distribution (inset) of Pt/CeO2 catalysts.
Results and Discussion
- Structure Identification of Pt/CeO 2 Catalysts …
- Catalytic Performance of the Pt/CeO 2 Catalysts
- DFT Calculation for the Mechanism of CO Oxidation
- Theoretical Interpretation of Size Effects
Clearly, as the size of the Pt NPs increases, the amount of adsorbed CO increases gradually. As a result, small Pt NPs (1 nm or 2 nm) have a slightly oxidized form, which is decisive evidence for the precise control of the Pt size. As the size of Pt NPs increases, the ratio of the atom located at the interface with CeO2 decreases.
When comparing the reduction temperature of CO (CO2 formation), the CeO2 cube had a lower reduction temperature regardless of the presence of Pt or the size of the Pt NPs.
Conclusion
Our findings suggest that understanding the change of the electronic interaction at the Pt-CeO2 interface depending on the size of the Pt NPs is one of the keys to achieving maximum catalytic performance.
Electrochemical characterization of the catalysts was performed using an electrochemical analyzer (CH Instruments, CHI760E) in a three-electrode electrochemical cell with a rotating ring-disk electrode (RRDE, Pine Research Instrumentation, AFE7R9GCPT). CO oxidation was performed as a model reaction to investigate the interfacial effect on the catalytic performance of heterostructured oxide NCs. As the temperature increases in the case of Co3O4 NC, the oxidation state easily transforms to metallic Co at 673 K.
Through the MvK mechanism, a reversible change in the oxidation state of the Co3O4 NCs enables oxygen migration for enhanced CO oxidation (Figure 5.5g). NCs, the TOFs achieved for the CO oxidation reaction were higher than those of the pristine NCs due to the interfacial effect. The Co3O4 NCs fully covered by CeO2 (CoCe-6F) showed a lower TOF than that of CoCe-3F, showing that the CO oxidation activity is directly related to the amount of the exposed Co3O4-CeO2- interface.
Synergistic Effect of Ceria Morphology and Copper Doping for Water-Gas Shift Reaction
Experimental Methods
- Catalyst Preparation
- Characterization Methods
- Catalytic Activity Testing
- Computational Details
Results and Discussion
- Characterizations of Pd/CDC Catalysts
- Catalytic Activity by Morphology Engineering and Cu Doping
- Increase of DPd Depending on Morphology and Cu Doping
- Reducibility of Pd Catalysts by Morphology Change and Cu Doping
- Reinforcement of Cu Doping Effect and D Pd CeO 2 Facet
- Quantification of Morphology and Cu Doping Effects on WGSR
CO chemisorption analysis determined that Pd/CDC-C had the highest DPd among the samples tested, as a result of morphology modification and Cu doping. TOF showed increased reducibility by morphology engineering and Cu doping (Figure 4.2c), but whether TOF was increased by improved reducibility remains to be determined precisely. This comparison is further evidence that the improvement of Pd/CDC-C by morphological engineering and Cu doping gave enhanced reducibility.
They also demonstrate that VO formation is easier on Pd/CDC-C, as a result of morphological engineering and Cu doping.
Conclusion
Thus, the doping sensitivity of the rate in the cube is always >3.13 times higher than in the octahedron. Although the rate of octahedra can be improved by Cu doping even without morphological engineering, the rate can be increased more significantly by Cu doping when the morphology is changed to cube (Figure 4.6b), because the rate doping sensitivity is higher in the cube than in the octahedron (Figure 4.6a). Due to the simplicity of the rate equations above, the quantitative analysis shown here can easily be used to assess the contribution of each of the surface atom control methods (e.g., morphology engineering, metal doping) on the improvement of material properties (e.g., reducibility, metal dispersion ) ) and reactivity of catalytic materials in the form of supported metal.
XPS and ToF-SIMS with DFT calculations of ECu,seg further revealed that the two support-tuning strategies can provide activity increase greater than just the sum of two effects, because increased ease of Cu segregation in the cube enhances the effect of Cu doping; thus, simultaneous application of morphology engineering and doping was clearly verified to synergistically increase the efficiency of supported metal catalyst for reactions involving redox process.
Based on these results, the CeO2 layer appeared to prevent the reduction of Co3O4 NCs by supplying Co3O4 oxygen at the Co3O4-CeO2 interface. Oxidation state changes of Co3O4-CeO2 NCs observed by XANES and NAP-XPS analyzes in different reaction environments. The hydrogen oxidation reaction (H2 + O2 → H2O) was performed as an exothermic reaction to investigate the TOF-dependent hot electron flow of Co3O4-CeO2 NCs.
The chemical current yield shown in Figure 5.6e also shows that the highest TOF of CoCe-3F originated from the highest charge transfer at the Co3O4-CeO2 interface.
Interface Control between Spinel Oxides and Ceria to Understand the Role of Ceria during
Experimental Methods
- Synthesis of Spinel Oxide NCs and Selective Deposition of CeO 2
- Catalytic CO Oxidation
- Characterization
- In Situ XRD, XAS, and NAP-XPS
- Catalytic H 2 Oxidation on a Nanodiode
For example, when 5 mmol of oleylamine and 4.5 mmol of formic acid were used during the preparation of Co3O4 NC, CoCe-1F was formed. XAS measurements for the Co K-edge and Ce L3-edge were performed at the Pohang Accelerator Laboratory (PAL) 7D beamline. XAS data were processed using the ATHENA program of the IFEFFIT package.48 In situ XANES measurements were performed in transmittance detection mode on the 6D PAL beamline.
The binding energies of the obtained Ce 3d were corrected according to the position of the Ce4+ component (917.4 eV).
Results and Discussion
- Spinel Oxide NCs with Controlled CeO 2 Layers by Selective Deposition
- Catalytic CO Oxidation
- Changes in the Crystal Structure under Different Reaction Conditions
- Changes in the Oxidation State under Different Reaction Environments
- Hot Electron Detection at the Co 3 O 4 -CeO 2 Interface
As indicated, the TOF values of Co3O4-CeO2 NCs were significantly improved at all temperatures compared to the original Co3O4 NCs without CeO2 deposition. The TOF was significantly reduced at a CO/O2 ratio of 6.0 for all Co3O4-CeO2 NC catalysts. However, the oxidation state of cobalt in CoCe-6F is more robust than that of Co3O4 NCs and is preserved even at high temperatures.
The shift of the absorption edge (E0) from the derivative as a function of the coverage of CeO2 in NC Co3O4 is shown in Figure 5.5c.
Conclusion
연구 방향을 선택하고 완료율을 높이는 데 많은 도움이 되었습니다. 또한, 실험적, 이론적으로 제가 수행하기 어려웠던 분야의 공동연구를 도와주신 여러 교수님들께도 감사드립니다. 또한 제가 연구 외의 삶을 살아갈 수 있도록 도와준 친구들에게도 감사의 말씀을 전하고 싶습니다.
마지막으로 사랑하는 가족들에게 감사의 마음을 전하고 싶습니다.
