利用第一原理探討水煤氣在過渡金屬上的轉移反應趨勢

Trends of Water Gas Shift Reaction on Transition Metal Surfaces from First Principles calculation

黃世昌(Huang Shih-Chang)

指導教授 ： 王禎翰

國立臺灣師範大學/理學院/化學系/碩士(2010年)

摘要

利用密度泛函理論(DFT)可有系統的檢視最密堆積的過渡金屬Co, Ni Cu (第三週期) Rh, Pd, Ag (第四週期) 和 Ir, Pt, Au (第五週期)上水煤氣轉移反應(water gas shift，簡稱WGS)機構，計算出的能量態顯示WGS的活性主要受到兩種步驟類型的影響：O-H鍵的斷裂和C-O鍵的生成。本次研究中在過渡金屬上的活化能障和反應熱有很好的線性關係，從能量上的觀察O-H鍵斷裂和C-O鍵形成在週期表上的趨勢是互為相反的，以週期表趨勢來看第9 族< 10族 < 11族 ； 第3週期 < 第4週期 < 第5週期，換句話說，越往右下的金屬表面如Cu、Ag、Pt、Au，擁有相對較低的C-O鍵結活化能障和反應熱，表示對於WGS反應有較好的活性。此外，能量基礎上的趨勢也利用吸附能、density of state (DOS) 和 charge density來檢視，結果顯示左上的過渡金屬有較高的能量和較狹窄的d軌域的分佈，藉以造成強吸附能來穩定解離物，O-H解離反應有較低的活化能障和反應熱。此能量趨勢上的預測對於其他催化反應工作，如：乙醇催化裂解和CO氧化，可有效的設計預期的反應。

關鍵字

水煤氣轉移 ； 過渡金屬 ； 催化劑

並列摘要

The mechanism of water gas shift reaction (WGSR) on the close-packed transition metal surfaces of Co, Ni Cu (from the 3d row) Rh, Pd, Ag (from the 4d row) and Ir, Pt, Au (from the 5d row) has been systematically examined by periodic density functional theory (DFT) calculations. The computed potential energy surface (PES) shows that the activity of WGSR is influenced by two kinds of elementary steps: O-H bond dissociation and C-O bond formation. Activation barriers (Ea) and reaction energies (H) on a series of metal surfaces show good BEP relationship; the energetic trends in periodic table are opposite in these two kinds of steps. In O-H bond dissociation steps, trends of Ea and H are groups 9 < 10 < 11 and 3d < 4d < 5d. On the other hand, the lower-right metal surfaces in the Periodic Table, Cu(111), Ag(111), Pt(111) and Au(111), have relatively lower Ea and H in C-O bond formation steps, which is responsible for their highly WGSR activity of metal/oxide catalysts. In addition, the fundamental of energetic trends has been examined from the analyses of adsorption energy, density of state (DOS) and charge density. The result shows that the surfaces of upper-left metals in the Periodic Table with higher energy and narrower delocalization of their d orbitals yield a stronger adsorption energy that will stabilize dissociating fragments and lower Ea and H in O-H bond dissociation steps. The prediction of energetic trends in the present work is also appropriate for other catalytic reactions, such as ethanol decomposition and CO oxidation, and can help us scientifically design a better catalyst for the desired reaction.

並列關鍵字

water gas shift ； transition metal ； catalyst

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