以理論計算探討:I.氮氧化物在鎳(111)表面的反應機構II.一氧化氮在鎳-鉑雙金屬的分解反應III.二氧化碳在碳化鎢與碳化鎢-鈷合金表面反應探討

Theoretical Calculation to study the Following Reactions:I.The Interaction of NOx on Ni(111) surface.II.The NO Dissociation on Ni-Pt bimetallic SurfacesIII.The CO2 Reaction on WC and WC-Co alloy Surfaces

吳亘曜(shiuan-yau Wu)

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/博士(2012年)

摘要

摘 要
第一部分 :氮氧化物(NOx)在Ni(111)表面的反應機構之探討

利用空間週期性來探討不同的氮氧化物(包含NO、NO2和N2O)在Ni(111)表面的反應機構，進一步討論到不同的覆蓋率下可能的變化。其中，在覆蓋率小的情況下，吸附的分子無論是NO、NO2和N2O都會完全分解成吸附態的N和O原子，而克服了2.34 eV的活化能之後，表面的N原子會再結合成N2分子從表面脫附。但是當覆蓋率不斷的提升之後，還沒有完全分解的NO和表面的N 原子會進行再結合，在高覆蓋率的情況下，N2O可能會進行脫附或者進一步斷N-O鍵形成N2分子。而在高覆蓋率的情況下會有N2O的副產物也可以從實驗的觀察得到證實。

第二部分: 一氧化氮(NO)在鎳-鉑雙金屬表面分解反應的探討

利用空間週期性來探討一氧化氮在Ni-Pt雙金屬表面的吸附與分解反應。其中，我們利用到的Ni-Pt雙金屬表面有: xNi@Pt(111), NixPt4-x(111), 和(4–x)Pt@Ni(111) ( x = 0~4)。 在所有的雙金屬表面當中，NO傾向被吸附在表面上有較多Ni原子的位置，而吸附能會隨著表面上Ni原子的數量增加而上升。另外，在我們所探討的所有雙金屬組成當中，當出現了表層的組成相同而內層不同的情況下，依不同的內層，NO分子吸附能的順序依次為xNi@Pt(111) > NixPt4-x(111) > (4 – x) Pt@Ni(111)表面，而NO斷鍵所需的活化能則剛好相反，換言之，在我們所有的表面當中，吸附能越大，斷NO鍵所需要的能障就越小。另外，我們也利用了局部電子態密度的分析來探討不同內層組成所造成雙金屬效益的原因。

第三部分:二氧化碳在碳化鎢WC(0001)和碳化鎢-鈷合金WC-Co表面反應探討

利用空間週期性探討二氧化碳在碳化鎢(0001)和碳化鎢-鈷合金表面的吸附。並進一步探討在不同鎢鈷比例的情況下，二氧化碳分解與氫化的趨勢。其中，碳化鎢(0001)表面有明顯的局域化現象，而當表面的組成結構改變，伴隨鈷原子的比例增加，會改變表面的局域化情形，進一步影響到吸附與反應的趨勢。而當鈷的覆蓋率為0.25ML的情況下，二氧化碳在WC-Co(0.25ML)有最佳的吸附能，而當鈷的覆蓋率增加到0.50ML，二氧化碳的吸附能雖然略減，但在該表面有最小的分解活化能。而氫化反應的活化能則是隨著表面鈷原子的比例增加而遞減，顯示鈷原子對氫化反應的幫助。而在這個部分，我們利用了電子局域化函數分析來探討表面局域化情況對二氧化碳催化反應的影響。

關鍵字

理論計算 ； 催化 ； 氮氧化物 ； 二氧化碳 ； 表面

並列摘要

Abstract

1st part: The interaction of NOx on Ni(111) surface investigated with quantum-chemical calculations

We applied periodic density-functional theory to investigate the interaction of NOx on Ni(111) surface for small and large coverages. For a small coverage, adsorbed species such as NO, N2O and NO2 tend to dissociate to form atomic N and atomic O on the surface, but a large barrier, 2.34 eV, hinders the recombination of adsorbed N to form N¬2. At a large coverage, the recombination of N and NO to form N2O is favorable; this species might either desorb or break the N-O bond to form N2. Our calculated results agree satisfactorily with experimental observations. The formation of N2 via paths that vary with coverage is analyzed and discussed.

2nd part: The NO reaction on Ni-Pt bimetallic surfaces investigated with theoretical calculations

We applied periodic density-functional theory to investigate the adsorption and dissociation of NO on bimetallic surfaces, including the xNi@Pt(111), NixPt4-x(111), and (4–x)Pt@Ni(111) surfaces ( x = 0~4). For all bimetallic surfaces, NO is preferentially adsorbed on Ni-rich sites, and the adsorption energies increase with the increasing number of top-layer Ni atoms on the surface. When the top-layer compositions are equal (but with varied composition of inner layers), the adsorption energy of NO on these surfaces decreases in the order xNi@Pt(111) > NixPt4-x(111) > (4 – x) Pt@Ni(111), whereas the NO dissociation barriers increase in the opposite order; a larger adsorption energy of NO leads to a smaller NO dissociation barrier. We employed the local density of states to study the inner-layer effect of the various surfaces, and found that the inner-layer Pt atoms of the 4Ni@Pt(111) surface caused the greatest up-shift of the d-band center (of top-layer Ni atoms) toward the Fermi energy.

3rd part: The CO2 reaction on WC(0001) and WC-Co alloy surfaces investigated with theoretical calculations

We applied periodic density-functional theory to investigate the adsorption of CO2 on WC(0001) and various WC-Co alloy surfaces, and discussed the reaction trend of CO2 dissociation or hydrogenation on these surfaces. We employed the electron localization function, ELF to study the electron localization or delocalization effect of the various Co-ratio WC-Co alloy surfaces, and found that the partial-delocalization surfaces (WC-Co(0.25ML) surface) exhibit largest adsorption energy to CO2 molecule (–1.61 eV) in all of our calculated surfaces. Incidentally, when we increased the Co-ratio to form WC-2Co(0.50ML) surface, the activation energy of CO2 dissociation (CO2→CO+O) was reduced to 0.57 eV; it also decrease the CO2 hydrogenation, CO2+H → HCOO (formate), due to the cause of electron delocalization on the increased Co-ratio WC-Co alloy surfaces.

並列關鍵字

theoretical calculation ； catalysis ； NOx ； CO2 ； surface

參考文獻

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