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Elucidation of Structure-Activity Relationship in Heterogeneous Catalysis by in situ/operando Surface Science Techniques
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| Title: | Elucidation of Structure-Activity Relationship in Heterogeneous Catalysis by in situ/operando Surface Science Techniques |
| Other Titles: | in situ/operando表面科学手法による不均一触媒構造活性相関の解明 |
| Authors: | LU, Bang Browse this author |
| Issue Date: | 25-Sep-2023 |
| Publisher: | Hokkaido University |
| Abstract: | Catalysts play a critical role in industrial chemistry since they can increase the rate of chemical reactions. Most of the industrial catalysts consist of active metal species that are highly dispersed on powdery oxide supports and the catalytic performance largely depends on the morphological and electronic states of the active metal species. Therefore, it is important to characterize them at the atomic level to elucidate the “structure-activity relationship” for the development of further active catalysts. However, since the powdery catalysts are “ill-defined” with polycrystalline and microporous structures, it is hard to obtain an accurate structure. Using single-crystal surfaces can solve this problem by providing “well-defined” surfaces and applying the ultra-high vacuum (UHV) surface science techniques, which are mainly electron probe techniques, give an atomic-level understanding of the surface processes in heterogeneous catalysis. The catalytic reaction does not occur without reactants (i.e., UHV condition). Thus, the development of in situ/operando surface science techniques, where surface characterization is carried out under working conditions, is highly demanded to unravel the surface chemical processes on the model catalysts under realistic reaction conditions.
In this work, firstly I used single-crystal surfaces to investigate the surface processes of plasma-assisted catalysis. Then I analyzed the 3-dimensional structure of Cu species on an α-Al₂O₃(0001) single crystal by polarization-dependent total reflection fluorescence (PTRF)-XAFS. Finally, I developed an in situ/operando surface science technique, which is named “operando PTRF-XAFS technique”, to obtain the structureactivity relationship in heterogeneous catalysis.
The first chapter of the dissertation is the introduction. The second chapter describes experimental methods. It mainly includes the details of the development of the operando PTRF-XAFS technique and the construction of an integrated UHV system for the preparation and multiple characterizations of model catalyst surfaces.
The third chapter describes the investigation of surface processes of plasmaassisted catalysis by surface science techniques. An XPS study on Co(0001) surface found that the adsorption of nitrogen species occurred when the surface was exposed to the electron cyclotron resonance (ECR) N₂ plasma at room temperature (RT) while no nitrogen adsorption without the plasma. They can be further hydrogenated to ammonia at RT by exposure to the H₂ plasma. The corresponding kinetics and mechanism study was
carried out on the Co(0001) surface.
In the fourth chapter, the 3D structure of Cu atoms on an α-Al₂O₃(0001) singlecrystal surface at low coverage (9.7×10¹³ /cm²) was studied in the UHV conditions using the newly developed operando PTRF-XAFS cell to test its performance on XAFS measurements. The PTRF-XANES spectra in both s-and p-polarizations showed the formation of monovalent Cu species. Furthermore, an iterative method using FEFF code to simulate PTRF-EXAFS spectra based on a real-space model structure suggested that the Cu species was atomically dispersed and stabilized by the surface oxygens of the α-Al₂O₃(0001) surface.
The fifth chapter focuses on an operando study of a Pt/α-Al₂O₃(0001) model catalyst during the CO oxidation reaction using the batch-type operando PTRF-XAFS cell. The cuboctahedral Pt clusters consisting of 147 Pt atoms were found to be the main species under the CO oxidation reaction condition at 493K. Its corresponding turnover frequency (TOF) of CO₂ formation was determined to be 0.06 s‐¹ based on the structure determined by the operando PTRF-XAFS and the simultaneous activity measurements
using the quadrupole mass spectrometry (QMS).
In the sixth chapter, I found there were two temperature regimes in the activation energies towards CO oxidation on the Pt/α-Al₂O₃(0001) by using the flow-type cell. The in situ PTRF-XAFS measurements were applied to reveal the origin of two different activation energies.
The seventh chapter is the general conclusions of the work. I have successfully developed a novel operando PTRF-XAFS technique to reveal the well-defined catalyst surface structure to unravel the surface processes. Consequently, my Ph.D. work has opened a new horizon of operando surface science in the heterogeneous catalysis field. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15620号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 朝倉 清高, 教授 佐々木 浩一, 教授 小崎 完, 教授 高草木 達 (本学大学院環境科学院) |
| Degree Affiliation: | 工学院（量子理工学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/90838 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （工学）
課程博士 (Doctorate by way of Advanced Course) > 工学院(Graduate School of Engineering)
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