理論計算探討乙醇於Rh/γ-Al2O3(110)表面上之吸附結構與脫氫的反應機構

Abstract

利用periodic DFT的計算方法， 我們探討乙醇於Rh/γ-Al2O3(110)表面之吸附結構與分解路徑。相較於乙醇吸附於Rh上的結果，乙醇吸附於表面的鋁離子上具有較好的吸附能，然而此酸性性質的γ-Al2O3載體會幫助乙醇脫水形成乙烯，因此我們考慮了其他的吸附結構來探討乙醇的分解路徑。由乙醇以α碳端吸附於Rh原子上的結構作為起始架構，相繼的反應路徑為脫附兩個α氫原子;能障分別為2.59與7.35 kcal/mol。下一步驟的脫氫反應是脫附O-H之氫形成中間產物CH3C(a)O + 3H(a)，所需克服的能障為10.13 kcal/mol，由此中間產物斷去C-C鍵，所需克服的能障為12.06 kcal/mol，形成C(a)H3 + C(a)O + 3H(a)等中間產物吸附於表面，於此所計算的乙醇分解路徑，在實驗上有類似的觀察結果。We applied periodic density-functional theory (DFT) to investigate the adsorption configurations and decomposition paths of ethanol on a Rh/γ-Al2O3(110) surface. Adsorption of ethanol onto a Al atom in the surface performs a larger adsorption energy than it adsorbed onto a Rh atom. But it is well known that the acidic support, γ-Al2O3 promotes dehydration of ethanol to ethylene. Therefore, we consider another ethanol adsorption configuration for the calculation of ethanol dehydrogenation pathways. In this configuration, ethanol adsorbed onto a Rh via α-cabon, two α-hydrogen atoms from ethanol are first eliminated; the barriers for abstraction of this two α-hydrogen atoms are calculate to be 2.59 and 7.35 kcal/mol, respectively. The dehydrogenation continues with the loss of one hydrogen from the O-H, forming an intermediate species CH3C(a)O + 3H(a), for which the successive barrier is 10.13 kcal/mol. Cleavage of the C-C bond occurs at this stage with a dissociation barrier Ea = 12.06 kcal/mol, to form C(a)H3 + C(a)O + 3H(a). The aforementioned reaction mechanism was also proposed and discussed by experiment.