密度泛函理論對PtxRu55-x團簇上一氧化碳氧化反應之研究

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研究生：	蔡孟哲 Meng-Che Tsai
論文名稱：	密度泛函理論對PtxRu55-x團簇上一氧化碳氧化反應之研究 DFT study of CO oxidation reaction on PtxRu55-x clusters
指導教授：	黃炳照 Bing-Joe Hwang
口試委員:	魏金明 Ching-Ming Wei 江志強 Jyh-Chiang Jiang 林智汶 Chi-Wen Lin
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：	中文
論文頁數：	94
中文關鍵詞：	密度泛函理論、奈米尺寸效應、CO氧化、鉑原子團簇、CO2生成
外文關鍵詞：	DFT, nano-sized effect, CO oxidation, Pt clusters, CO2 formation
相關次數：	點閱：324 下載：1
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本研究以密度泛函理論(DFT)探討一氧化碳(CO)與氫氧基(OH)在鉑為主之團簇表面上的共吸附氧化反應。為了充分了解奈米尺寸效應對一氧化碳氧化反應的影響，對於CO、OH基及反應中間物在團簇上的吸附性質亦做了相關的計算。本文採用兩種計算模型：分別以55顆鉑原子團簇(Pt55)及鉑金屬層狀系統(Pt(111) 3×3 slab)模擬奈米粒子與塊材性質，研究結果顯示採用的模型尺寸大小對CO吸附能及其氧化反應能障影響相當大。最近文獻亦證實鉑金屬粒子尺寸的減小使得CO氧化反應能障增加。根據CO+OH共吸附氧化機制，分成以下幾個步驟：CO與OH基共吸附在觸媒表面上最穩定的活性位置，接著CO與OH基發生反應，產生cis-COOH後轉換成trans-COOH分子組態，最後氫原子分解至表面，生成CO2。Pt55上的CO+OH共吸附氧化能障為0.92 eV，高於Pt(111)上的反應能障(0.75 eV)，表示CO與OH基在奈米粒子表面較難反應。此外，釕原子加入後，誘導兩金屬間發生電荷轉移，部份電子從釕原子傳至鉑原子，此現象使Pt-CO及Pt-OH鍵結強度減弱。以釕原子取代鉑原子團簇之核層，在相同的CO+OH共吸附氧化機制下，其反應能障為0.81 eV。

Density functional theory (PAW-GGA) calculations are employed to study CO+OH coadsorption oxidation reaction on Pt-based clusters. In order to get insight into the nano-sized effect toward CO oxidation, adsorption properties of CO, OH group and reaction intermediate have also been investigated. In this study, we adopted two model systems: 55 atoms Pt clusters (Pt55) and Pt(111) 3×3 slab as nanoparticle and bulk properties, respectively. The results show that adsorption energy and reaction barrier of CO oxidation are strongly influenced by the size of Pt nanocatalysts. Recent literature shows that the energy barrier of CO oxidation reaction increases with a decrease in Pt particle size. According to CO+OH coadsorption oxidation mechanism, it occurs through following steps: CO and OH group coadsorb at the most stable sites on the catalyst surface, and CO reacts with OH group, forming cis-COOH then transforms configuration into trans-COOH. At last the H on COOH transfers to surface, leading to CO2 formation. The reaction barrier of CO+OH on Pt55 clusters (111) surface is 0.92 eV, and it is higher than reaction barrier on Pt(111) slab (0.75 eV) which indicates that CO is not easy to react with OH group on the surface of nanoparticles. On the other hand, the presence of second metal (Ru) induces charge transfer from Ru to Pt leading to weaker bond of both CO and OH on the Pt site. By similar CO oxidation mechanism, reaction barrier of CO+OH on Pt-based cluster (shell: Pt, core: Ru) is 0.81 eV.

摘要..........................................................................I
Abstract.....................................................................II
目錄........................................................................III
第一章	緒論.................................................................1
1 燃料電池(Fuel Cell)簡介...................................................1
2 直接甲醇燃料電池(DMFC) ...................................................4
3 CO吸附的理論研究..........................................................6
3.1 CO在過渡金屬上的吸附....................................................6
3.2 CO在雙元金屬觸媒(bimetal catalyst)上的吸附.............................10
3.2.1 金屬應變效應(strain effect)對於CO吸附、氧化反應之影響................10
3.2.2 CO在Pt表面及其合金結構上吸附之理論研究...............................12
4 CO在Pt表面及其合金結構上氧化反應之理論研究...............................16
4.1 H2O對CO吸附及氧化的影響..............................................20
5 觸媒尺寸效應對於CO吸附及氧化反應之影響...................................22
6 研究動機.................................................................24
第二章 理論背景..............................................................25
1 第一原理計算(Ab initio methods)..........................................25
2 密度泛涵理論 (Density Functional Theory，DFT)............................27
2.1絕熱近似...............................................................27
2.2 Hartree近似...........................................................28
2.3 Hartree-Fork近似......................................................30
2.4 密度泛函理論..........................................................30
2.5 局部密度泛函近似法 (LDA)..............................................33
2.6 LDA與GGA (General Gradient Approximation)的差異.......................36
2.7 能帶理論計算..........................................................38
2.7.1 虛位勢法( pseudopotential method)..................................40
3 NEB法(Nudged elastic band method)........................................41
第三章 計算方法..............................................................43
第四章 結果與討論............................................................45
1 團簇能量計算(bare clusters) .............................................45
1.1 團簇(111)表面原子電荷分佈..............................................48
2 在不同團簇結構上的CO、OH基及其他反應中間物的吸附.........................50
2.1 CO吸附計算.............................................................50
2.2 OH基吸附計算...........................................................56
2.3 CO與OH基的共吸附計算...................................................61
2.4 COOH中間物吸附.........................................................66
3 CO氧化反應 (CO + OH reaction)............................................67
3.1 反應過渡態(T.S.)......................................................67
3.2 Pt(111)層狀模型及Pt55團簇模型上的CO+OH反應路徑........................68
3.2.1 Pt(111)層狀模型及Pt55團簇模型上的CO+OH反應能量分佈.................69
3.3 Pt55團簇(111)及Pt55團簇(100)上的CO+OH反應.............................72
3.4 鉑-釕核殼團簇(Ru core Pt shell, PR3)結構上的CO+OH反應.................74
第五章 結論..................................................................76
參考文獻.....................................................................78

                                

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