理論計算探討乙醇在2Ru/ZrO2(111)表面之脫氫反應

Abstract

本論文分為兩大主題： 第一部分：乙醇在2Ru/ZrO2(111)表面之脫氫反應 我們使用週期性的密度泛函理論來研究乙醇在2Ru/ZrO2(111)表面催化下之脫氫反應，我們計算出來乙醇有最大吸附能的結構是以乙醇的O原子接在表面的Ru原子上，而這個結構接續的反應會經由O-Ru路徑，即斷鍵的順序是：O-H鍵→βC-H鍵→C-O鍵而最後得到乙烯吸附在表面上；另外一個有第二大吸附能的結構是以乙醇的αC原子吸附在表面的Ru原子上，這個結構接續的反應會經由αC-Ru路徑，即斷鍵的順序是：αC-H鍵→O-H鍵→(βC-H鍵) →C-C鍵而最後得到氫氣。最後，我們也計算了吸附在表面上的H原子結合成氫氣的反應位能面，其所計算出來的能障大約是20-30 kcal/mol。這個結果象徵著使用參雜Ru的ZrO2表面可能是個頗為有效的催化劑來催化乙醇的脫氫反應。

第二部分：在ZrO2表面參雜Ru與否對催化乙醇脫氫反應的影響 我們使用週期性的密度泛函理論來研究乙醇在ZrO2(111)表面以及2Ru/ZrO2(111)表面催化下之脫氫反應的差別，發現在ZrO2(111)表面脫氫反應所需克服的活化能比在2Ru/ZrO2(111)表面還要高，特別是斷βC-H鍵的過程，其活化能的差距為36.05 kcal/mol，這導因於斷βC-H鍵產生的吸附物非常的不穩定。試著了解造成這個現象的原因，我們做了態密度以及變形能的分析，而分析的結果發現這導因於兩個因素：(1) 乙醇的O、C原子與2Ru/ZrO2表面的Ru原子的作用力強過與ZrO2表面的Zr原子的作用力；(2) 乙醇在ZrO2(111)表面催化下斷βC-H鍵所得到的吸附結構，其表面的變形能比起在2Ru/ZrO2(111)表面催化下的情形大很多(30.41 kcal/mol)。There are two major themes in this thesis: Part 1: Dehydrogenation of Ethanol on a 2Ru/ZrO2 (111) Surface We applied periodic density-functional theory (DFT) to investigate the dehydrogenation of ethanol on a 2Ru/ZrO2 (111) surface. A structure with ethanol adsorbed with its O atom attached to a Ru atom is calculated to exhibit the largest energy of adsorption; it reacts via an O-Ru path: the sequence of bond scission is O-H  βC –H  C-O that eventually forms ethene and coke. Another structure adsorbed via the α-C atom onto Ru that exhibits the second largest adsorption energy dissociates via a αC-Ru path. The sequence of bond scission is αC-H  O-H  αC-H  (βC-H) C-C, and eventually forms H2. Possible potential energy surfaces to form H2 from a combination of adsorbed H atoms were calculated at the final stage, subject to a barrier about 20~30 kcal/mol. These results indicate that a Ru-doped ZrO2 surface might be a fairly effective catalyst to dehydrogenate ethanol.

Part 2: The Comparison of Catalytic Effect of Ethanol Dehydrogenation on ZrO2 (111) Surface with or without Doped Ru We applied periodic density-functional theory (DFT) to investigate the difference between dehydrogenation of ethanol on 2Ru/ZrO2 (111) surface and ZrO2 (111) surfaces. We found that the barriers of dehydrogenation that must overcome were higher on ZrO2(111) surface as compared to the 2Ru/ZrO2 (111) counterparts. The difference of barrier is 36.05 kcal/mol on the process of βC-H bond scission, due to that the adsorbed species of βC-H bond scission being unstable and causing different catalytic effect between these two surfaces. To understand the reason, we performed the calculation of density of states and the deformation energy. The outcomes show that it results form two factors: (1) the interaction between the O, C atoms of ethanol with the Ru atom of 2Ru/ZrO2 (111) is stronger than with Zr atom on its ZrO2 (111) counterparts, and (2) the deformation energy comes from the adsorbed structure of βC-H bond scission on ZrO2 (111) surface is much larger than on its 2Ru/ZrO2 (111) counterparts by about 30.41 kcal/mol.