臺灣博碩士論文加值系統

本文利用密度泛函理論(DFT)探討在釕-鉑(111)表面上的甲醇裂解、甲醇蒸氣重組和水氣轉移反應。由計算結果中可知甲醇最穩定的吸附位置為在釕原子上，而裂解反應經由O–H鍵裂解開始，接著連續三個C–H鍵裂解，最後形成一氧化碳是比較容易發生的路徑，其中速率決定步驟為甲醇中O–H鍵裂解，反應能障為0.73 eV，但是此能障與甲醛基(HCO)的脫氫反應能障相當地接近。此外，共吸附水的存在對於甲醇的裂解反應有明顯地影響，它降低了甲醇中O–H鍵裂解反應所需的能障，卻增加了其他中間產物中C–H鍵裂解反應所需的能障。當一氧化碳於表面生成時，可以藉水氣轉移反應將其氧化。根據我們的計算結果，在釕-鉑(111)表面上的一氧化碳可能透過表面上氧原子進行氧化(redox mechanism)，也有可能先形成羧基(COOH)為中間產物(carboxyl mechanism)，最後皆形成二氧化碳。此兩種反應路徑的速率決定步驟皆為由水在釕上生成羥基(OH)，而最可能的反應路徑為從水雙體中所產生羥基，其反應能障為0.55 eV。

First principle periodic DFT calculations have been used to study the thermodynamic and kinetic characterization of methanol decomposition, methanol steam reforming reaction (SRM), and water-gas shift (WGS) reaction on Ru-Pt(111) surface. Our calculated results indicate that the favorable adsorption site for methanol decomposition is on Ru top site. Methanol decomposition prefers to begin with O–H bond scission rather than C–H bond scission. The rate-determining step (RDS) for this reaction pathway is the cleavage of O–H bond in methanol with barrier of 0.73 eV, but it is very close to the barrier for abstraction of H from formyl (HCO), 0.70 eV. Moreover, the presence of co-adsorbed water has a significant influence on methanol decomposition, which reduces the O–H bond breaking activation barrier of methanol but enlarges the C–H bond breaking activation barrier of methoxy intermediates. The formed CO on surface can be oxidized via WGS reaction. According to our results, two mechanisms of WGS reaction on Ru-Pt(111) surface, redox and carboxyl mechanisms, can happen. However, redox mechanism is limited by the coverage of OH groups on Pt sites. The RDS for both two mechanisms of WGS reaction are formation of OH groups on Ru top site, which a reaction barrier of 0.55 eV is calculated in water dimer activation.

ABSTRACT I
ACKNOWLEDGEMENTS III
CONTENTS IV
INDOX OF FIGURE VI
INDOX OF TABLE X
CHAPTER 1. INTRODUCTION 1
1.1 Fuel Cells 1
1.1.1 Development and Applications 1
1.1.2 The Direct Methanol Fuel Cells (DMFCs) 4
1.1.3 The Hydrogen Generation via Methanol 6
1.1.4 Water-Gas Shift Reaction 8
1.1.5 Bimetallic Catalysts Applications 11
1.2 This Research 14
CHAPTER 2. METHODOLOGY 16
2.1 Theoretical Background 16
2.1.1 Quantum Chemistry 16
2.1.2 Density Functional Theory 16
2.1.3 Periodic Systems 19
2.1.4 Brillouin Zone Sampling 22
2.1.5 Plane Wave Basis Set 25
2.1.6 Pseudopotential 28
2.1.7 Ultrasoft-pseudopotential 32
2.1.8 Projected Augmented Wave (PAW) 34
2.1.8 Generalized Gradient Approximation (GGA) 36
2.1.9 Nudged Elastic Band Method (NEB) 37
2.1.10 Linear Synchronous Transit (LST) 39
2.1.11 Synchronous Transit Methods 40
2.2 Computational Details 42
2.2.1 Method 42
2.2.2 Surface Model 44
a. Bulk 44
b. Pt(111) Surface 44
c. Pt(111) Surface Alloyed Atomic Ru 46
CHAPTER 3. RESULTS AND DISCUSSION 48
3.1 Methanol Dehydrogenation on Ru-Pt(111) Surface 48
3.1.1 Adsorption Properties of Reaction Intermediates 48
a. Adsorption of Hydrogen Atom 51
b. Adsorptions of Methanol Intermediates 52
3.1.2 Reaction Mechanism of Methanol dehydrogenation 61
a. Behavior of H Atom on Ru-Pt(111) Surface 63
b. Methanol Decomposition on Pt’ top Site 64
c. Methanol Decomposition via O−H Bond Scission on Ru top Site 67
d. Methanol Decomposition via C−H Bond Scission on Ru top Site 70
e. Summary 75
3.2 Methanol Steam Reforming Reaction 78
3.2.1 Co-adsorption Properties of Reaction Intermediates 78
a. Adsorption of H2O 79
b. Co-adsorption of CH3OH and H2O 80
c. Co-adsorption of Other Intermediates and H2O/OH 83
3.2.2 Reaction Mechanisms of Methanol Steam Reforming Reaction 89
a. Methanol Steam Reforming Reaction of H2ORu + CH3OHPt’ 89
b. Methanol Steam Reforming Reaction of CH3OHRu + H2OPt’ 95
c. Summary 103
3.3 Water-Gas Shift Reaction 108
3.3.1 Adsorption Properties of Water-Gas Shift Reaction Intermediates 108
a. Adsorptions of WGS Reaction Intermediates 108
b. Adsorptions of Water Dimer 114
3.3.2 Reaction Mechanisms of Water-Gas Shift Reaction 116
a. Water Activation 117
b. Redox Mechanism 121
c. Carboxyl Mechanism 126
d. Hydrogen Recombination 132
e. Summary 133
CHAPTER 4. CONCLUSION 137
REFERENCE 141

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