理論計算探討Rh/CeO2(111)表面上

Computational Studies of Reaction Mechanisms of Dehydrogenation of Ethanol on the Rh/CeO2(111) Surface

劉士弘(Shih-Hung Liu)

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/碩士(2006年)

摘要

隨著化石燃料的日漸枯竭，人們正在找尋可以替代石油的能源來源。而氫氣(H2)是目前較佳的燃料之一，將氫氣使用於內燃機引擎中，其燃燒產生的物質對環境是沒有污染性的。此外，氫氣也可以應用於H2/O2燃料電池的高效率發電方面，且具有相當的實用價值。乙醇可以當做是氫氣主要的來源之一，乙醇在合適的氧化物表面上時，加以合適的溫度，可以有絕佳的催化效果，而產生高效率的脫氫反應。在本文中，我們利用週期性的電子密度泛函理論的計算方法，探討乙醇在Rh/CeO2(111)表面上之可能的分解反應機構。結果發現，乙醇若以該分子的氧端吸附在Rh/CeO2(111)表面的Ce原子上，比起其他表面原子而言（例如Rh與O原子），將有著較高的吸附能。是故乙醇首先將會以該氧端吸附在Rh/CeO2(111)表面的Ce上，而形成CH3CH2O(H)–Ce(a)，再藉由接續的脫氫反應（斷去O–H以及H2C–H），計算所得能障分別為12.00以及28.57 kcal/mol，之後形成一個穩定六員環的中間物Rh–CH2CH2–Ce(a) (oxametallacycle)。此外，分別計算所脫附的氫原子在表面的不同原子上(Ce, Rh和O)，發現氫原子吸附在氧上有著最高的吸附能(Eads = 101.59 kcal/mol)。再者，該中間物會先在α-碳上接連斷去兩個C–H鍵而形成吸附中間物
Rh–CH2CO–Ce(a)，計算所得能障分別為34.26和40.84 kcal/mol。最
後Rh–CH2CO–Ce(a)將藉由斷去C–C鍵結(TSC-C ; Ea = 49.54 kcal/mol)而形成Rh–CH2(a) + 4H(a) + CO(g)的產物，最後這些產物在高溫下再產生脫附反應，而形成CH4(g) + H2(g) + CO(g)。
另一方面，我們在有水分子存在時之乙醇的脫氫反應能障相較於無水分子存在時，討探是否有降低的趨勢，結果發現有水分子的幫助，脫氫反應能障大致上有降低的趨勢。

關鍵字

乙醇 ； 氫氣 ； CeO2

並列摘要

Hydrogen is considered a desirable fuel for several reasons, among which hydrogen is the least polluting fuel that one can use in an internal–combustion engine, and it can serve in a highly efficient hydrogen/oxygen fuel cell to produce electricity. As a result, we applied periodic density–functional theory (DFT) to investigate the dehydrogenation of ethanol on a Rh/CeO2(111) surface. Ethanol is calculated to have the greatest energy of adsorption when the oxygen atom of the molecule is adsorbed onto a Ce atom in the surface, relative to other surface atoms (Rh or O). Before forming a six-membered ring of an oxametallacyclic compound (Rh–CH2CH2O–Ce(a)), two hydrogen atoms from ethanol are first eliminated; the barriers for dissociation of the O-H and the β-carbon (CH2–H) hydrogens are calculated to be 12.00 and 28.57 kcal/mol, respectively. The dehydrogenated H atom has the greatest adsorption energy (Eads = 101.59 kcal/mol) when it is adsorbed onto an oxygen atom of the surface. The dehydrogenation continues with loss of two hydrogens from the α-carbon, forming an intermediate species Rh–CH2CO–Ce(a), for which the successive barriers are 34.26 and 40.84 kcal/mol. Scission of the C–C bond occurs at this stage with a dissociation barrier Ea = 49.54 kcal/mol, to form Rh–CH2(a) + 4H(a) + CO(g). At high temperatures, these adsorbates desorb to yield the final products CH4(g), H2(g) and CO(g).
The reaction C2H5OH + 3H2O → 2CO2 + 6H2 on the Rh/CeO2(111) surface with the aid of one water molecule have been examined. From our calculated results, if the ethanol molecule is catalyzed by one water molecule, the energy barrier of such processes could be diminished around 20 kcal/mol. It is plausible to suggest that the existence of a large water-catalytic effect for the dehydrogenation processes on the Rh/CeO2(111) surface could be possible.

並列關鍵字

ethanol ； hydrogen ； CeO2

參考文獻

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