異相催化劑反應之理論計算研究： 乙醇重組反應與Fischer-Tropsch合成反應

Abstract

乙醇蒸氣重組被視為一種產氫的重要反應。在本篇論文第三章中，我們利用理論計算探討貨幣金屬(Cu、Ag、Au)在進行乙醇蒸氣重組反應中的強氧化性與高乙醛選擇率的反應路徑研究。從計算的能量發現，貨幣金屬傾向選擇活化能障較低且放熱較多的部分氧化路徑。利用電子結構分析，發現d軌域被填滿的貨幣金屬能有效率地將電子傳遞至O和OH，使O和OH的p軌域提升，降低氧化反應步驟的能障。在本篇論文第四章中，我們探討Rh金屬的蒸氣重組反應機構以及氧在反應中所扮演的角色。從反應機構的計算發現，CH3CHO*和CH2CH2O*為關鍵的中間產物，中間物反應生成CO(g)、 CO2(g)的氧化步驟能障高，被視為速度決定步驟。氧扮演旁觀者所造成的adsorbate effect，可以降低速度決定步驟的活化能。在動力學上的分析也獲得與實驗上一致的結果。因此，我們合理推測：Rh-based催化劑的可藉由合適的添加物，提高載氧能力，作為乙醇重組反應催化劑時可獲得更好的效能。 本篇論文的第五章是探討Fischer-Tropsch合成反應(以下簡稱F-T合成反應)，在Ru(0001)和Co(0001)表面，計算CO的活化反應、CHx(x=1~3)的氫化反應、C-C單體結合反應及C-H鍵結/解離終止反應等機構的探討，找出Ru和Co催化反應機構的差異，並添加1A金屬Na於Co表面，探討Na對吸附物的影響，進一步提出Co催化劑的改良辦法。從計算結果顯示，不論在Ru(0001)還是Co(0001)表面，CO 並不會氫化生成COH，反而傾向直接解離或是生成中間物CHxO再解離C-O鍵。CHx (x = 0~3)的選擇性，在Ru(0001)以CH 佔大多數，在Co(0001)表面則是CH和CH3。在Ru(0001)表面C-C 單體結合反應，傾向以CH2 + CH2 的方式進行。而在Co(0001)表面則可能以CH2 + CH2、CH + CH或CH+CHO的方式進行。從終止反應的探討發現，不論是CHx還是C2Hy的氫化終止反應，Co表面皆為動力學與熱力學上穩定的。最後，添加Na金屬於Co可以使含氧化物的吸附能提升，穩定CO、HCO、HCHO等吸附物，降低C-O解離的能障。另外，添加Na並不會增強CHx(x=1-3)的吸附，可以保留Co表面上C-C鍵結速率較快的優勢。綜合上述結果，合理推測：添加對氧吸附能力大於碳的金屬，或是將Co金屬吸附於擁有氧空缺的氧化物支撐物上，能有效提升Co催化劑對高碳數產物的選擇率及減少含氧化物的產生。Steam reforming of ethanol is considered as an important reaction for producing hydrogen. In Chapter 3, we computational examined the high oxidative capability and acetaldehyde selectivity for coinage metals in the reforming reaction. The energetic results show that coinage metals have much lower activation energies and higher exothermicities for the oxidative dehydrogenation steps. In the electronic structure analysis, coinage metals with saturated d bands can efficiently donate electrons to O* and OH* and better promote their p bands to higher energy levels. The negatively charged O* and OH* with high-lying p bands are responsible for lowering the energies in oxidative steps. In Chapter 4, we investigated the catalytic mechanism and oxygen effect in the oxidative steam reforming (OSR) of ethanol on Rh(111). The mechanistic results show that both surface acetaldehyde (CH3CHO*) and oxametallacycle (CH2CH2O*) species are key intermediates. The formation of major products CO(g) and CO2(g) through high-barrier oxidation steps are considered the rate-determining steps in the overall catalytic process. Surface oxygen (O*) species can lower the barriers for the rate-limiting oxidation steps. The kinetic calculations also well-predict the experimental observations. These results concluded that the catalytic performance of Rh-based catalysts can be improved by using promoters with better oxidative capability. In Chapter 5, we examined Fischer-Tropsch synthesis (F-T synthesis) reaction mechanism on Ru(0001) and Co(0001) surfaces, which contains investigations of CO activation, hydrogenation of CHx (x = 0~3) species, C-C coupling processes, and termination reaction and found the difference of the mechanism. We also discussed the effect of promoter, Na, and found its effect in the mechanism. According to our calculations, CO prefers to direct dissociate or form the HCO intermediate before C-O bond scission rather than forming COH on both Ru(0001) and Co(0001) surfaces. In addition, the CH will be the most abundant adsorbing species on both two surfaces and CH3 is also trapped on Co(0001) surface. The favorable coupling reactions on both surfaces are CH2 + CH2. CH + CH and CH+CHO are also possible coupling reaction on Co(0001) surface.The hydrogenation reaction (termination reaction) of CHx and C2Hy are kinetically and thermodynamically favored on the Co(0001) surface. Finally, Na enhances the adsorption of O-containing species, such as CO, HCO and HCHO, and decreasing the barrier for C-O bond cleavage. On the other hand, Na has no effect on CHx, keeping the advantage of the faster C-C coupling reaction rate on Co(0001). In summary, adding metal which attracts O better than C to Co or covering Co on oxide supports may improve the selectivity of long-chain hydrocarbon to heavier molecular weight and decrease the production of oxygenate.