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

Abstract

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)。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).