乙醇在Rh/Ce0.5Zr0.5O2(111)氧化金屬表面脫氫的可能反應機構

Computational Studies of Reaction Mechanisms of Dehydrogenation of Ethanol on the Rh/Ce0.5Zr0.5O2 (111) Surface

鍾國偉(Guo-Wei Zhong)

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

中文摘要
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)是目前較佳的的料燃之一，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/Ce0.5Zr0.5O2(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)表極面的Ce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）
，之後形成一個穩定環狀的中間物Rh–CH2CH2–Ce(a) (oxametallacycle)。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)。最後Rh–CH2CO–Ce(a)將藉由斷去C–C鍵結而形成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)。

關鍵字

乙醇脫氫 ； 化學含氧量

並列摘要

Abstract
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/Ce0.5Zr0.5O2(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 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 15.2 and 26.9 kcal/mol, respectively. The dehydrogenation continues with loss of two hydrogens from the α-carbon, forming an intermediate species Rh–CH2CO–Ce(a), . Scission of the C–C bond occurs at this stage 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).

參考文獻

(2) Taylor, K. C. Catal. Rev. Sci. Eng. 1995, 35, 457.

(3) Basic Research Needs for Vehicles of the Future; Eisenberger, P. M., Ed; Princeton Materials Institute: Princeton, NJ, 1995.

(4) Kaspar, J. ; Fornasiero, P.; Graziani, M. Catalysis Today .1999, 50, 285

(13) Handbook of Chemistry and Physics, 76th ed., CRC Press,

Baton Rouge, FL, 1995.