tert-Butylacetylacetate分子在Si(100)表面與乙醇在Pd(111)表面解離吸附反應之理論研究

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Title:	tert-Butylacetylacetate分子在Si(100)表面與乙醇在Pd(111)表面解離吸附反應之理論研究
Other Titles:	Theoretical studies of desorption of tert-Butylacetylacetate on Si(100) surface and ethanol on Pd(111) surface
Authors:	李泳霆;Lee, Yung-ting
Contributors:	淡江大學化學學系碩士班林志興;Lin, Jyh-shing
Keywords:	密度泛函理論;贋位勢;部份結構限制法;態密度;Density Functional Theory;Pseudopotential;partial structural constraint path minimization method;density of state
Date:	2007
Issue Date:	2010-01-11 02:48:56 (UTC+8)
Abstract:	我們利用密度泛函理論搭配ultrasoft pseudopotential、平面波基底函數以及supercell模型計算模擬tert-Butylacetylacetate分子在Si(100)表面的解離吸附反應與乙醇單分子在Pd(111)表面第一步驟O-H鍵斷裂的解離反應，並藉由部份結構限制法建立反應過渡態的活化能和結構資訊。tBAA在Si(100)表面的解離吸附計算結果說明，tBAA分子以ester模式吸附在Si(100)表面，以較低的活化能障反應生成中間物isobutane。所以我們歸納tBAA的解離吸附反應主要在ester官能基上的O-tertButyl鍵透過表面催化促使鍵弱化所致。我們也將tert-Butyl取代為methyl進行相同的解離吸附反應，探討不同碳取代所造成的影響。接續第一步反應所產生的中間物isobutane利用修飾過的部份結構限制法計算β-hydrogen elimination反應脫附形成isobutene需較高的活化能，並與實驗在100~400℃左右所觀測結果相近。透過過渡態結構參數的比較、DOS與電荷密度的分析，C=C雙鍵的形成是為反應活化能障降低的關鍵因素。至於單一乙醇分子吸附在Pd(111)表面透過O-H鍵的解離吸附產生氫和Ethoxide第一步反應，其計算所得活化能較實驗所測量的結果要高。另外，我們將乙醇分子吸附在Pd(111)表面各位置的結果發現，乙醇透過氧原子的孤對電子與表面形成鍵結將較為穩定，但是O-H鍵長並無明顯的改變，這顯示出吸附在表面的乙醇發生O-H鍵的解離反應需要較高的能量。 Density Functional Theory with ultrasoft pseudopotential, plane wave basis sets and supercell models were used to simulate the dissociative adsorption of tert-Butylacetylacetate and ethanol onto Si(100) surface and Pd(111) surface, respectively. Activation energies and relative structural variations were calculated through partial structural constraint path minimization method. Computational result shows that the dissociative absorption of ester model of tert-Butylacetylacetate onto Si(100) has quite low activation energy to produce the isobutane. This low energy is attributed to the weakening of O-tertButyl bond through the catalytical effect of the Si(100) surface. Furthermore, it is found that the much higher activation energy is needed for β-hydrogen elimination of adsorbed isobutane leading to the isobutene. Finally, our calculated partial density of state in connection with the structural variation showed that the formation of C=C double bond within the isobutane is the main point to reduce activation energy. Regarding to the dissociation of absorbed ethanol on Pd(111) surface, the lone pairs of oxygen within the ethanol will initially bond to the Pd atom on the Pd(111) surface. However, the O-H bond length is not significantly changed at this initial stage. Then the breaking of O-H bond within the adsorbed ethanol will occur leading to the adsorbed H and ethoxide on Pd(111) surface. Finally, this process of dissociation of adsorbed ethanol on Pd(111) surface is energetically favourable.
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