理論計算探討在 2Ru/γ-Al2O3(110) 表面之乙醇脫氫及水氣轉移反應機構

Theoretical Studies of the Dehydrogenation of Ethanol and Water-Gas Shift Reaction Mechanisms on a 2Ru/γ-Al2O3(110) Surface

廖正豪(Cheng-Hao Liao)

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/碩士(2009年)

摘要

本篇論文我們利用週期性密度泛函數理論(DFT)的計算方法，探討在2Ru/γ-Al2O3(110)表面上對於乙醇脫氫以及水氣轉移(WGS)的反應機構。我們計算出乙醇最穩定的吸附結構是乙醇以氧端吸附於表面的Al原子上，βC端靠近表面的Ru原子，我們將此位向的乙醇脫氫路徑稱為βC path。此路徑的斷鍵順序為βC-H鍵 → C-O鍵，而其活化能為：0.109 → 1.159 eV，最後形成CH2CH2(a) + OH(a) + H(a)在表面上。第二穩定的乙醇吸附位向是以氧端吸附於表面的鋁原子上，αC端靠近表面的Ru原子，此脫氫路徑稱為αC path，此路徑最主要的斷鍵順序為αC-H鍵 → O-H鍵 →αC-H鍵 → C-C鍵 → βC-H鍵，而其活化能為：0.234 → 0.992 → 0.349 → 0.899 → 0.223 eV，最後產生CH2 (a) + CO(a) + H(a)在表面上。結果顯示理論計算與實驗上相符合。
水氣轉移反應的機制主要分為兩種:(1) carboxyl mechanism; (2) redox mechanism。在進行水氣轉移反應前我們先計算出一個一氧化碳與一個水分子在表面吸附能最佳的位置。水氣轉移反應第一步的水分子解離後不管是經由carboxyl mechanism或是redox mechanism反應都會遇到2 eV以上的能障導致反應無法繼續。於是採用將三個水分子同時吸附在表面上第一層的三個鋁原子上與一氧化碳進行水氣轉移反應。
計算三個水分子的系統後我們發現在2Ru/γ-Al2O3(110)表面上的水氣轉移反應較傾向經由redox mechanism路徑。此路徑會先進行
OH(a) → H(a) + O(a)步驟，活化能大小為1.219 eV；接下來會經由
CO(a) + O(a) → CO2(a) 產生二氧化碳，其活化能為1.497 eV。而carboxyl mechanism路徑的活化能比redox mechanism高，且中間產物也較不穩定。

關鍵字

表面 ； 催化 ； 乙醇 ； 脫氫 ； 理論計算 ； 水氣轉移反應

並列摘要

We applied periodic density-functional theory (DFT) to investigate the mechanism of ethanol dehydrogenation and water gas shift (WGS) reaction on a 2Ru/Al2O3 (110) surface. A structure with ethanol adsorbed with its O atom attached to the Al atom and βC terminal near by the Ru atom is calculated to exhibit the most stable adsorbed structure, which we name the βC path. The sequence of bond scission is βC–H → C-O that eventually forms CH2CH2(a) + OH(a) + H(a) on the surface, with respect to the barriers：0.109 → 1.159 eV. Another structure adsorbed via the O atom attached to the Al atom and αC terminal near by the Ru atom that exhibits the second stable adsorbed structure, which we name the αC-Ru path. The sequence of bond scission is αC-H → O-H → αC-H → C-C → βC-H, and eventually forms CH2(a) + CO(a) + 4H(a) on the surface, with respect to the barriers： 0.234 → 0.992 → 0.349 → 0.899 → 0.223 eV. These calculated results indicate that the DFT calculation corresponds with the experiment.
There are two mainly mechanisms in water-gas-shift reaction:(1) carboxyl mechanism; (2) redox mechanism. Before water gas shift reaction we calculate the site with greatest adsorption energy of one CO and H2O molecule on the surface. After the H2O molecule dissociate into H(a) + OH(a), the reaction is blocked by a huge barrier over 2 eV to proceed the WGS reaction of next step. Then we put three H2O molecules on the Al atoms of the first layer on the surface, trying to discuss the water-gas-shift reaction.
Based on DFT calculations, it is found that the water gas shift reaction prefers the redox mechanism on a 2Ru/γ-Al2O3(110) surface. It starts with the path: OH(a) → H(a) + O(a), which has a 1.219 eV barrier. And then form carbon dioxide: CO(a) + O(a) → CO2(a) with a barrier of 1.497 eV. The barriers of carboxyl mechanism are higher than that of the redox mechanism, and its local minimum is also less stable.

並列關鍵字

surface ； catalyst ； ethanol ； dehydrogenation ； Theoretical studies ； water-gas shift reaction

參考文獻

(2) Holladay, J. D.; Wang, Y.; Jones, E. Chem. Rev. 2004, 104, 4767.

(3) Logan, B. E. Environ. Sci. Technol. A-Pages. 2004, 38, 160A.

(6) Garcia, E.; Laborde, M. Int J. Hydrogen Energy 1991, 16, 307.

(7) Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. Int J. Hydrogen Energy 1996, 21, 13.

(8) Fishtik, I.; Alexander, A.; Datta, R.; Geana, D. Int J. Hydrogen Energy 2000, 25, 31.