二氧化碳氫化反應、Fischer-Tropsch 合成反應在M(111) (M = Fe、W) 表面之理論計算研究

Abstract

摘 要 第一部分 : 二氧化碳在Fe(111) 及W(111) 表面氫化反應探討 我們使用密度泛函理論來研究二氧化碳在Fe(111)及W(111) 表面的氫化反應，包括: 產物的結構、吸附能並計算氫化反應之位能圖。計算結果顯示，在Fe(111) 及W(111) 表面，二氧化碳的氫化反應有很相似的路徑，但在能量上卻有很大的不同。在Fe(111)表面，氫化產生Formate (HCOO) 和Carboxyl (COOH) 的反應能障分別為0.72 和1.75 eV，在W(111)表面則為0.79 和 2.91 eV。兩者都傾向生成Formate (HCOO-vertical) 產物。而Formate後續的氫化反應，結果顯示在Fe 及W表面都不傾向產生甲酸，而若要產生甲醇，則速率決定步驟在兩表面皆為最後一個氫化步驟，CH3O + H → CH3OH，反應能障分別為1.71 eV(Fe) 和2.18 eV(W)。為了瞭解吸附物和表面的交互作用，我們也提出電子結構等相關資訊輔以討論。 第二部分 : Fischer-Tropsch合成反應在Fe(111) 及W(111) 表面可能的反應機構探討 我們使用密度泛函理論來研究Fischer-Tropsch合成反應在Fe(111)及W(111) 表面可能的反應機制，其中包括CO的活化反應、CHx (x = 0~3) 的氫化反應及C-C單體結合反應。結果顯示不論在Fe(111)或W(111)表面，CO並不會直接斷C-O鍵，反而傾向先和表面的H原子進行氫化反應，生成中間物CHO，接著才解離C-O鍵。 CHx (x = 0~3)的選擇性，在Fe(111) 及W(111)表面，皆以CH佔大多數。最後，在Fe(111)表面，C-C 單體結合反應，傾向以CH + CH的方式進行，反應能障為0.54 eV。而在W(111)表面則可能以CO + CH 或CH + CH方式進行，能障分別為0.26 eV 和0.35 eV。為了瞭解吸附物和表面的交互作用，我們也提出電子結構等相關資訊輔以討論。Abstract 1st part: Density-Functional Calculations on the Hydrogenation of Carbon Dioxide on Fe(111) and W(111) Surfaces With quantum-chemical calculations we investigated the hydrogenation of a CO2 molecule on Fe(111) and W(111) surfaces using the density functional theory (DFT) with the projector-augmented wave (PAW) approach in periodic boundary condition. The structures and geometric parameters of the hydrogenation products, and the potential-energy surfaces (PESs), were calculated. It was shown that the similar reaction paths for the hydrogenation of CO2 on Fe(111) and W(111) surfaces were found but with disparate reaction energy barriers. The barriers from M-CO2 (M = Fe, W) plus H atom to form formate (HCOO) and carboxyl (COOH) on a Fe(111) surface are 0.72 and 1.75 eV, respectively, but 0.79 and 2.91 eV, respectively, on a W(111) surface. The most probable path for the hydrogenation of a CO2 molecule on either the Fe(111) or the W(111) surface is the formation of a formate-vertical structure. To understand the interaction between adsorbates and surfaces, we calculated the Bader charges and analyzed the local densities of states (DOS). 2nd part: Density Functional Calculations to Study the Mechanism of Fischer-Tropch Reaction on Fe(111) and W(111) Surfaces Density-functional theory calculation is employed to study the Fischer-Tropsch synthesis on Fe(111) and W(111) surfaces, which contains the mechanism investigations of CO activation, hydrogenation of CHx (x = 0~3) species, and C-C coupling processes. It shows that CO prefers to form the HCO intermediate before C-O bond scission rather than direct dissociation of its C-O bond on both Fe(111) and W(111) surfaces. In addition, the CH will be the most abundant adsorbing species on these two surfaces, which would induce the coupling reaction of CH + CH to be the most probable processes on the Fe(111) surface with a calculated barrier of 0.54 eV; while it might induce two favorable coupling reactions: CO + CH and CH + CH on the W(111) surface with the calculated reaction barriers of 0.26 and 0.35 eV, respectively. To understand the interaction between adsorbates and surfaces, we calculated the Bader charges and analyzed the density of states.