利用光觸媒通過光子活化，將CO2轉變為碳氫化合物的光催化還原CO2已被認為是一種有前景的生物模擬方法，並用於減量CO2。人工光合作用不僅促進碳循環中的CO2轉變，且同時產生高價值燃料（CH4、CO和甲醇等）。為開發人造光催化劑以提高CO2轉化效率，已作出了許多努力。本研究以探討整體結晶度的表面性質對於CO2還原的影響，並全面了解表面控制活性和選擇性。本研究的第一部分已證明，以溶膠-凝膠法所製備的無定形TiO2在液相還原CO2系統中，其光催化活性高於熱還原銳鈦礦納米晶體。高密度的OH基團(12.45 /nm2) 和高表面積 (274 m2/g) 導致非晶態TiO2顆粒具有極高的反應活性。在所有系統中雖然CH4為主要產物，但仍測量了次要產物，包括CH4、CO、C2H4 和C2H6。經過10小時放射線照射後，無定形TiO2的產量達到17.88 µmol/g，分別為熱還原銳鈦礦納米晶體和市售P25的16倍和9倍。豐富的OH基團表面不僅將CO2分子轉化為碳酸氫鹽/碳酸鹽物質，以改善CO2的化學吸附 (2.94 mmol/g)，並捕獲空穴，形成Ti-O-O-Ti產物，並在CO2還原中提供質子。這樣的協同作用促進了界面電荷轉移，進而提高量子效率。齒狀碳酸鹽和 •CO2- 兩種活性物種，在羥基化表面上生成CH4和CO產物的過程中，進行CO32- Ti-OOCH2 Ti-O-CH3 CH4和 •CO2- CO22- Ti-COOH CO。高度以化學性吸附的碳酸鹽表面，以進行CO2的還原為主而不是產生H2。然而隨著煅燒溫度升高，OH基團和表面積皆顯著降低。此時，在所得的銳鈦礦晶體中產生氧空位作為替代活性位點。儘管對於吸附的CO2物種，氧空位表現出比OH基更高的轉化活性，但是活性位點的數量不足仍導致結晶粉末的效率低於無定形粉末的效率。通過將雜離子摻入TiO2晶格中引入表面缺陷，以降低電子電洞再結合機率，從而提高光催化活性。結果顯示，高溫下將磷酸鹽摻入TiO2中會導致表面缺陷的產生，如氧空位和Ti3+。溶膠-凝膠法所製備的磷酸化TiO2 (PT-700)在氣相還原方面表現出更高的光催化活性，CO的產率為30.57 µmol/g，約為市售P25的5倍。表面缺陷（Ti3+/氧空位）改善了CO2的化學吸附 (0.22 mmol/g)。此外，P5+離子和表面缺陷可協同改善界面電荷轉移。根據漫反射紅外光譜 (DRIFT) 在黑暗和光照條件下，表面缺陷可藉由形成CO過程中所產生的CO2 - 和 HCOO- 中間物種，來改善的CO2的化學性吸附。我們探討了表面缺陷和P5+ 離子對表面反應的貢獻，包括CO2 化學吸附、電荷捕獲、磷酸鹽TiO2 光催化物的電荷轉移等介面反應。從動力學的觀點來看，磷酸鹽和焦磷酸鹽物質可以穩定更多的電子並清除電洞，使CO2 有強烈及更穩定的化學性吸附。反應的中間產物是另一個控制產物選擇性和反應性的重要因素。在這項研究中，以溶膠-凝膠法製備的磷酸化TiO2 (PT-500)顯示出最高的光催化活性，CO2產率為25.11 μmol/g，在液相反應中的比商用P25高約6倍。含有高度碳酸鹽類的表面，其主要反應為CO2的還原，而非產生H2 。此外，在CO2還原反應中的選擇性，Ti3+ 和氧空缺十分重要，並導致在液相反應中有更高的CO + CH4選擇性（57.61％）。 P5+ 離子可以幫助穩定電子，表面缺陷可以捕獲電子並實現有效的電荷分離，這有助於改善液相中的光催化活性。

Photocatalytic CO2 reduction that utilizes photocatalysts to convert CO2 into hydrocarbons via photon activation has been considered as a promising bio-mimic approach for CO2 mitigation. This artificial photosynthesis can not only promote CO2 transition in the natural carbon cycle but also generate value added fuels (CH4, CO, methanol, etc.) at the same time. Numerous efforts have been made to develop artificial photocatalysts to increase the CO2 conversion efficiency. In this thesis, we studied the influence of surface properties over the bulk crystallinity on CO2 reduction and comprehensively understand the surface-controlled activity and selectivity. The first part of this study, sol-gel-derived amorphous TiO2 powders have been demonstrated to exhibit superior photocatalytic activity than their thermally-induced anatase nano-crystals for CO2 reduction in aqueous phase. The high density of OH groups (12.45 /nm2) and high surface area (274 m2/g) were responsible for the extraordinarily high reactivity of the amorphous TiO2 particles. While CH4 was identified as the major products in all the systems, minor products including CO, C2H4, and C2H6 were also measured. After 10-hr irradiation, the accumulated production yield in the amorphous TiO2-based system reached 17.88 µmol/g, which is 16 and 9 times higher than the their thermally derived anatase crystals and commercially P25 powders. The abundant surface OH groups not only converted CO2 molecule into bicarbonate/ carbonate species to improve CO2 chemisorption (2.94 mmol/g) but also trapped holes to form Ti-O-O-Ti species and provide protons for CO2 reduction. The effects synergistically promoted interfacial charge transfer, thus improving the quantum efficiencies. Bidentate carbonate and •CO2- were two active species that were able to underwent CO32- Ti-OOCH2 Ti-O-CH3 CH4 and •CO2- CO22- Ti-COOH CO sequences on the hydroxylated surface to produce CH4 and CO products, respectively. High coverage of the chemisorbed carbonate species selected CO¬2 reduction rather than H2 evolution. The OH groups and the surface area, however, dramatically decreased at the elevated calcination temperatures. In turn, oxygen vacancies were produced as the alternative active sites in the resulting anatase crystals. Although the oxygen vacancies exhibited higher transformation activity than the OH groups for the adsorbed CO2 species, insufficient amounts of the active sites still caused the efficiency of crystalline powders to be lower than that of the amorphous powder. The introduction of surface defects via the incorporation of hetero-ions into TiO2 lattice enhanced the photocatalytic activity as a results of retardation of electron-hole recombination. It was demonstrated that incorporated phosphate into TiO2 at high temperature resulted in creation of surface defects as oxygen vacancies and Ti3+. Sol-gel derived phosphated TiO2 (PT-700) have been exhibited higher photocatalytic activity for the reduction of CO2 at gaseous phase with production yield 30.57 µmol/g of CO, which is about 5 times higher than commercial P25 catalyst. Surface defects (Ti3+/oxygen vacancies) improved the CO2 chemisorption/adsorption (0.22 mmol/g). In addition, P5+ ions and surface defects synergistically improved interfacial charge transfer. According to diffuse reflectance infrared spectra (DRIFT), surface defects could improve CO2 chemisorption both in dark and under light illumination by generating CO2- and HCOO- intermediate species for CO formation. We explored the contribution of surface defects and P5+ ion to the surface reactivity, interfacial reaction including CO2 chemisorption, charge trapping, charge transfer on phosphated TiO2 photocatalyst. On kinetic point of view, phosphate and pyrophosphate species can stabilize more electron and scavenges hole, which allows CO2 to chemisorbed strongly and stabilized more CO2 chemisorption. The reaction medium is another important factor which control the product selectivity and reactivity. In this study, sol-gel derived phosphated TiO2 (PT-500) sample shows the highest photocatalytic activity with a CO yield of 25.11 μmol/g, which is ∼6 times higher than that of commercial P25 in an aqueous system. High coverage of the surface with the carbonaceous species selected CO2 reduction over H2 evolution as the dominant reaction. Moreover, Ti3+ and oxygen vacancies were essential for selection of CO2 reduction, and results in greater CO+CH4 selectivity (57.61%) in aqueous phase. P5+ ions can help in charge carrier stabilization, and surface defects can trap the charge carrier and facilities efficient charge separation, which is responsible for improved photocatalytic activity in the aqueous phase.

Chapter 1. Introduction 1
1.1. Background and Motivation 1
1.2. Research objectives 3
Chapter 2. Literature Review 5
2.1. Photocatalytic CO2 reduction 5
2.1.1. Basic CO2 reduction 5
2.1.2. Types of photocatalysts 7
2.1.3. Influences of band position on CO2 reduction 7
2.1.4. Particle size and crystallinity 9
2.1.5. Surface properties 10
2.2. Reaction medium 13
2.2.1. Aqueous phase reaction 13
2.2.2. Gaseous phase reaction 13
2.2.3. Hole scavenger 13
2.3. Reaction Mechanism 14
2.3.1. CO2 reduction to different products 14
2.3.2. Reaction pathways 15
2.4. TiO2-based photocatalysts for CO2 reduction 20
2.4.1. Metal and non-metal doping 21
2.4.2. Heterojunction construction 23
2.4.3. Surface modification with basic sites 24
2.4.4. Carbon based TiO2 modification . 25
2.4.5. Metal organic framework modification . 27
2.5. Conclusions, Challenges and Limitation for CO2 Reduction 27
Chapter 3. Experimental Section 29
3.1. Sol-derived TiO2 photocatalysts 30
3.1.1. Materials 30
3.1.2. Preparation of sol-gel derived TiO2 material 31
3.1.3. Materials characterizations (TiO2) 31 3.1.4. Apparent Quantum Efficiency (AQEs) Calculation 33
3.1.5. Turnover Number (TON) Calculation . 34
3.1.6. Quantification of H2O2 species . 34
3.1.7. Photocatalytic CO2 reduction in aqueous phase 35
3.1.8. Photocatalytic Degradation of BPA 36
3.2. Sol-gel derived Phosphated TiO2 36
3.2.1. Materials 36
3.2.2. Preparation of phosphate TiO2 photocatalysts 38
3.2.3. Characterization of Phosphated TiO2 38
3.2.4 Apparent Quantum Efficiency (AQE) 41
3.2.5 Photocatalytic CO2 reduction in gaseous phase 41
Chapter 4. Highly Active Sol-Gel-Derived Amorphous TiO2 for CO2 Reduction-Contributions of Abundant OH Groups 43
4.1 Introduction 43
4.2. Results and Discussion 44
4.2.1. Crystalline structure and Microstructure (XRD, TEM image and BET) 44
4.2.2. Optical properties and Band Gap 45
4.2.3. Surface Properties (XPS, CO2-TPD) 46
4.2.4. CO2 Photocatalytic Activities 49
4.2.5 Interfacial Charge Transfer (EPR) 53
4.2.6 Identification of Intermediates Species (DRIFT) 54
4.2.7 Role of Surface Properties in Kinetics and Mechanism . 56
4.3 Conclusion 61
Appendix A- Sol-gel derived TiO2 samples 62
Chapter 5. Photocatalytic CO2 Reduction on Phosphated TiO2 Photocatalysts in a Gas System 74
5.1 Introduction . 74
5.2. Results and Discussion 75
5.2.1. Microstructure (XRD and BET) 75
5.2.2. Optical Properties (UV-vis spectroscopy) 78
5.2.3. Surface properties (FTIR) 78
5.2.4. CO2 adsorption (CO2-TPD) 79
5.2.5 Surface Chemical Properties (XPS) 81
5.2.6. Photoluminescence (PL) Spectra 83
5.2.8. Interfacial Charge Transfer (EPR measurement) 88
5.2.9 Chemisorbed Species After CO2 Reduction 91
5.2.10. Identification of Intermediate Species (DRIFT) 95
5.3 Conclusion 98
Appendix B- Sol-gel derived phosphated TiO2 samples in gas system 99
Chapter 6. Photocatalytic CO2 Reduction using Phosphated TiO2 Catalysts in Aqueous Medium 100
6.1 Introduction . 100
6.2 Results and Discussion 101
6.2.1 Photocatalytic Activity in Aqueous Phase 101
6.2.2 Effect of Reaction Medium 103
6.2.3 Role of Surface Properties 104
6.3 Summary 104
Appendix C- Sol-gel derived phosphated TiO2 samples in Aqueous system 105
Chapter 7. Conclusion and Perspectives 106
References 108

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