利用密度泛函理論與動態蒙特卡羅法研究Fischer-Tropsch合成反應

Abstract

本篇論文是探討Fischer-Tropsch合成反應(以下簡稱F-T合成反應)路徑機構，利用密度泛函理論與動態蒙特卡羅模擬實驗上以Ru(0001)和Co(0001)表面作為催化劑的化學機制探討。我們先以密度泛函理論計算所有事件中間產物，從CO和H2反應形成C1(甲烷)和C2(乙烯、乙烷)等產物的吸附最佳位置，以及利用微動彈性帶方法找出各反應事件的活化能。所得吸附能和活化能讓我們適用於動態蒙特卡羅模擬實驗，並且可以揭示反應機制變化以及反應產物的選擇率。由動態蒙特卡羅模擬找出Ru和Co催化反應機構的差異，發現Ru(0001)與Co(0001)表面進行F-T反應在起始活化CO反應皆選擇氫化產生CHO路徑，在Co(0001)表面上我們可以看到CHO中間產物可以進一步氫化產生CH2O或CH3O，或是直接型CO鍵斷裂行成CHx產物群聚於表面，所得CHx具有氫化生成CH4的更好機會。而Ru表面趨向CHO的斷氧反應產生CH中間產物聚集於表面利於碳長鏈加成為C2產物，與Co表面最大的不同便在於此，我們也看到在同溫度的情況下，產率方面以Ru表面較好。因此我們探討了400~520K的溫度對兩表面的產率變化，發現隨著溫度上升，長碳鏈的選擇率也上升，以及針對於Co表面的改良，讓表面能脫離卡在CHO與CH2O的加脫氫反應的產率干擾現象，也得到了更高的長碳鏈產率，從動態蒙特卡羅模擬的結果，我們合理推測：在CHO中間產物的加氫生成CH¬2O以及斷氧產生CH的路徑與F-T反應長碳鏈產物的選擇率關係密切，因此只要在這個步驟上能有效的改良便能使F-T反應的轉換效率提升。

This thesis is focused on mechanistic study of Fischer-Tropsch(F-T) synthesis reaction on Ru(0001) and Co(0001) surfaces by density functional theory(DFT) and kinetic Monte Carlo (kMC) method. DFT was employed to optimize the local minima and transition states for a series of elementary steps in the formation of C1 (CH4) and C2 (C2H4 and C2H6) products from reactants of CO and H2. The resulted energetics, including adsorption energy and activation barriers, were applied for kMC simulation to reveal the mechanism and product selectivities. The computations found that the first step in F-T is CO hydrogenation in the formation of CHO on both Ru(0001) and Co(0001) surfaces. CHO could further hydrogenated to CH2O or CH3O before the C-O bond breaking forming CHx on Co(0001) surface. The resulted CHx has better chance for hydrogenation to CH4. On the other hand, CHO immediately breaks its C-O bond forming CH on Ru(0001) surface. The intermediates favors C-C coupling reaction and forming C2 products. Thus, hydrogenation or C-O bond dissociation of CHO are the key step to control C1 and C2 product selectivity. In the temperature effect study, both surfaces have more C2 products at higher temperature.