本篇論文我們利用週期性電子密度泛函數理論(periodic density–functional theory)的計算方法，探討在4Rh/CeO2(111) 表面上對於水氣轉移反應(water-gas-shift reaction): CO + H2O CO2 + H2 的催化效用和可能的反應機構，以及貴重金屬(如: Pt，Pd，Rh)添加於CeO2(111)表面時，可能造成的電荷轉移現象和電荷分布情形。首先，我們探討四個Rh原子於表面最可能的吸附結構。經由計算結果發現，四個Rh金屬於CeO2(111) 表面的吸附能大小順序為4Rh/CeO2(Rh-cluster) > 4 Rh/CeO2(Rh-Od) > 4Rh/CeO2(Rh-Ou) > 4Rh/CeO2(Rh-Ce)，因此我們選用最穩定的表面4Rh/CeO2(4Rh-cluster) 做為催化表面。CO分子對於此表面的吸附能在Rha(OC-Rha(a)) 位置時最高(Eads = 67.44 kcal/mol)，吸附方式為C端吸附於表面上。而H2O分子在類似的位置(H2O-Rha(a))也有最高的吸附能(Eads = 24.2 kcal/mol)。由文獻中實驗結果，當CO與H2O分子吸附於表面上後，我們可將表面催化水氣轉移反應的機制主要分為兩種:(1) carboxyl mechanism; (2) redox mechanism。在理論計算中，我們利用下列路徑來模擬此兩種反應機構﹕

SCHEME 1: The Postulated Two Different Pathways of Carboxyl and Redox Mechanisms

計算結果我們發現，反應經由carboxyl mechanism路徑的速率決定步驟為COOH分子脫H，活化能大小為25.87 kcal/mol，而經由redox mechanism路徑的速率決定步驟為OH脫H步驟，活化能大小為32.60 kcal/mol。所以，反應會較傾向經由carboxyl mechanism路徑。另外，我們也探討了OH基對於COOH和另一OH分子脫H反應的幫助效應(water assisted effect)，結果發現OH基協助OH脫H可降低活化能6.94 kcal/mol，而幫助COOH脫H反應則是不需要能障。因此，完整的水氣轉移反應在4Rh/CeO2(111) (4Rh-cluster)的反應路徑已經浮現 : 首先，CO和H2O分子吸附於表面，接著H2O(a)越過能障大小為17.43 kcal/mol產生 OH(a) + H(a)，而下一步經由TS2: OH(a)和CO(a)結合產生COOH(a) ( 活化能4.28 kcal/mol)，此COOH(a)分子在有OH(a)的存在下能夠進一步分解產生CO2(a)，整個反應所需越過最高的活化能大小則為17.43 kcal/mol。兩個反應路徑皆生成H和CO2吸附於表面，在高溫下，吸附物可以脫附產生H2(g)和CO2(g)。在關於貴重金屬添加於CeO2(111)表面時，金屬電荷分布於表面的計算探討中我們發現，雖然金屬主要與CeO2表層的O原子接觸，但其電子會經由穿隧效應移轉至第二層的Ce離子，造成Ce四價離子的部分還原。因此，添加貴重金屬於表面將有助於提升CeO2的儲氧能力(Oxygen Storage Capacity, OSC)，以及幫助CO的氧化。

Abstract
We applied periodic density–functional theory (DFT) to investigate the water–gas shift reaction, CO + H2O à CO2 + H2, on a 4Rh/CeO2(111) surface. Our calculated results indicate that, in decreasing order, the adsorption energies are 4Rh/CeO2(Rh–cluster) > 4Rh/CeO2(Rh–Od) > 4Rh/CeO2(Rh–Ou) > 4Rh/CeO2(Rh–Ce). A molecule of CO is calculated to have the greatest adsorption energy when its C–terminus toward the 4Rh/CeO2(Rh-cluster) surface connects to the Rha atom: OC–Rha(a), Eads = 67.44 kcal/mol. A similar adsorption conformation of greatest adsorption energy was found for the H2O molecule: H2O–Rha(a), 24.2 kcal/mol. A favorable reaction path, through carboxyl intermediate, producing H2(g) is identified; the calculated maximum barrier that must be overcome in this reaction, 25.87 kcal/mol, involves the detachment of an H atom to release CO2(g) from an adsorbed intermediate HOOC–Rha(a). We also investigated water assisted effect on OH-Rha(a) and COOH-Rha(a) dehydrogenation reaction. We find that this effect can decreased 6.94 kcal/mol on OH-Rha(a) dissociated reaction, but empty the reaction barrier on COOH-Rha(a) dissociation. At high temperature, these adsorbates desorb to yield the ultimate products CO2(g) and H2(g). Further, we
calculated the charge distribution when precious metals load on CeO2(111) surface. Our calculation results show Ce ion will partially reducted by the loaded precious metals and may further enhance oxygen storage capacity on CeO2.

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