以密度泛函理論研究乙醇在Ni/α-Al2O3 (0001)觸媒表面上之裂解反應

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研究生：	程雅琴 Ya-chin Cheng
論文名稱：	以密度泛函理論研究乙醇在Ni/α-Al2O3 (0001)觸媒表面上之裂解反應 Density functional theory study of ethanol decomposition reaction over Ni/α-Al2O3(0001) surface
指導教授：	江志強 Jyh-Chiang Jiang
口試委員:	王伯昌 Bo-Cheng Wang 何嘉仁 Jia-Ien Ho 黃炳照 Bing-Joe Hwang
學位類別：	碩士 Master
系所名稱：	工程學院 - 化學工程系 Department of Chemical Engineering
論文出版年：	2008
畢業學年度：	96
語文別：	英文
論文頁數：	107
中文關鍵詞：	密度泛函理論、乙醇水蒸氣重組反應、鎳、裂解反應
外文關鍵詞：	DFT, Ethanol steam reforming, Nickel, Decomposition
相關次數：	點閱：377 下載：2
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本文利用密度泛函理論(DFT)探討乙醇在純α-Al2O3(0001)與Ni/α-Al2O3 (0001)表面上的裂解反應，並將計算結果與乙醇水蒸氣重組反應的實驗現象做比較。當n個鎳原子(n=3–4)沉積在純α-Al2O3 (0001)表面上，其結構傾向形成團簇而非分散地吸附在表面上。三角形的鎳團簇吸附在2 × 2純α-Al2O3 (0001)表面上之結構被用來代表Ni/α-Al2O3 (0001)。乙醇在純α-Al2O3 (0001)與Ni/α-Al2O3 (0001)表面上所進行的裂解反應包括脫氫以及斷碳—碳鍵，其中乙醇脫水形成乙烯是在純α-Al2O3 (0001)表面上主要的反應路徑，其次則是形成穩定的CH2CHO中間產物且進而覆蓋觸媒表面上的活性點；Ni/α-Al2O3(0001)表面則明顯的抑止積碳現象發生，依據計算結果可提出一個可行的斷碳—碳鍵之反應途徑，其中CH2CO分子可藉由改變其吸附結構，增強碳與鎳團簇表面的作用力，進而減弱碳—碳鍵的鍵能，總反應的速率決定步驟是CH2CH2O(a) → CH2CHO (a) + H(a)，其反應能障為1.20 eV。最後進一步研究一氧化碳在Ni/α-Al2O3(0001)表面進行的氧化反應以及其吸附結構的電子態密度(DOS)，主要為一氧化碳分子的5σ 軌域提供電子給鎳, 而鎳反饋電子給一氧化碳分子的2π軌域。

Ethanol decomposition on clean α-Al2O3 (0001) and Ni/α-Al2O3 (0001) surface was studied using periodic DFT calculations. Our results are compared with the available experimental findings of ethanol steam reforming reaction. For n Ni atoms (n=3–4) deposition on α-Al2O3(0001) surface, the preferential structure is forming cluster rather than dispersion on surface. Triangle Ni3 cluster adsorption on 2 × 2 α-Al2O3(0001) surface is used to represent Ni/α-Al2O3(0001) surface. The considered possible pathways for ethanol decomposition on clean α-Al2O3 (0001) and Ni/α-Al2O3 (0001) surface include dehydrogenation and C－C bond cleavage. Clean α-Al2O3 (0001) surface favors the reaction of ethanol dehydration to ethylene or leading to stable intermediate (CH2CHO) which finally occupies the active site of surface. Ni/α-Al2O3 (0001) surface shows high activity to inhibit coke formation, one feasible channel leading to C－C bond breaking was proposed. The C－C bond in CH2CO intermediate can be weaken via transforming the adsorption structure to increase the coordination number of the two carbon atoms with the surface of Ni cluster. The CH2CH2O(a) → CH2CHO (a) + H(a) reaction is the rate-determining step for the overall reaction (‡E = 1.20 eV). CO oxidation on Ni/α-Al2O3 (0001) surface is also investigated and the DOS analysis for CO adsorption on Ni bridge site shows that the interaction is mainly contributed from CO(5σ)－Ni charge donation and Ni(d)－2π* backdonation.

Contents

Abstract………………………………………………………………….I
Contents………………………………….……………………...………II
List of Tables...……………………………………………………..IV
List of Figures……………………………………………………….VI
CHAPTER 1   INTRODUCTION…………………………………1
1	 Hydrogen Fuel Initiative………………………………….………………….1
2	 Hydrogen Production…………………………………………………..……. 2
3	 Ethanol Steam Reforming Reaction…………………………….....................4
4	 Experimental Analysis………………………………………………………..7
5	 Ni/α-Al2O3(0001)……………………………………………………………11
6	 This research………………………………………………………………...13
CHAPTER 2   THEORETICAL STUDY………………………….14
1  Hartree Approximation……………………………………………………...14
2  DFT Method………………………………………………………………...14
2.1  Description of Theory…………………………………………………15
2.2  Derivation and Formalism……………………………………………..17
3  Basis Set…………………………………………………………………….21
4  Bloch’s Theorem and Plane Wave Basis Set………………………………..22
4.1  Bloch’s Theorem………………………………………………………22
4.2  Plane Wave Basis Set………………………………………………….26
4.3  K-Point Sampling……………………………………………………...28
5  PAW Pseudopotential……………………………………………………….29
5.1  Pseudopotential………………………………………………………..29
5.2  Projected Augmented Wave (PAW)……………………………………34
6  GGA Approximation………………………………………………………..36
7  NEB method………………………………………………………………...37

CHAPTER 3   COMPUTATIONAL DETAILS……………………40
CHAPTER 4   RESULTS AND DISCUSSIONS…………………...42
1	 Clean α-Al2O3(0001) Surface……………………………………………… 42
2	 Ni/α-Al2O3(0001) Surface ………………………………………………….43
2.1	Ni Adsorption on α-Al2O3(0001) Surface...............................................43
2.2	Ni Cluster Adsorption on α-Al2O3(0001) Surface…………..…………46
3	 Ethanol Decomposition on α-Al2O3(0001) Surface ………………………...52
3.1	Ethanol Adsorption on α-Al2O3(0001) Surface………………………..52
3.2	Reaction Paths for Ethanol on α-Al2O3(0001) Surface………………..53
3.2.1	Ethanol Dehydrogenation Reaction………………………...…..53
3.2.2	Ethoxy Decomposition Reaction………………………………..56
3.2.3	Water Effect……………………………………………………..61
4	 Ethanol Decomposition on Ni/α-Al2O3(0001) Surface……………………...64
4.1	Ethanol Adsorption on Ni/α-Al2O3(0001) Surface…………………….64
4.2	Reaction Paths for Ethanol on Ni/α-Al2O3(0001) Surface…………….65
5	 CO Oxidation on Ni/α-Al2O3(0001) Surface……………………………...82
6	 Density of States Analysis of CO－Ni/α-Al2O3(0001) System…………...84
CHAPTER 5   CONCLUSIONS…………………………………..91
References……………………………………………………………..93

                                

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