利用反射式紅外光譜吸收儀及熱脫附質譜術來研究甲醇於白金奈米粒子的的分解反應之催化模型系統;Methanol Decomposition on Pt Nanoparticles supported by Al2O3/NiAl(100):A combined IRAS and TPD study

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题名:	利用反射式紅外光譜吸收儀及熱脫附質譜術來研究甲醇於白金奈米粒子的的分解反應之催化模型系統;Methanol Decomposition on Pt Nanoparticles supported by Al2O3/NiAl(100):A combined IRAS and TPD study
作者:	趙振盛;Chao,Chen-sheng
贡献者:	物理研究所
关键词:	甲醇;奈米粒子;白金;模型系統;熱脫附;紅外光;催化;毒化;catalyst;TPD;IRAS;poison;model system;methanol;nanoparticle;cluster;platinum;Pt
日期:	2012-07-25
上传时间:	2012-09-11 18:41:03 (UTC+8)
出版者:	國立中央大學
摘要:	我們利用熱脫附質譜技術及反射式紅外線光譜吸收儀來研究甲醇於白金奈米粒子上的分解反應之觸媒模型。白金以蒸鍍的方式，在θ相的氧化鋁成長成白金奈米粒子。在此模型系統上有兩種甲醇的反應途徑被發現:(1)甲醇(CH3OH)的去氫化產生一氧化碳(CO)和氫氣(H2)；(2)甲醇(CH3OH)內的碳氧鍵斷裂而產生甲烷(CH4)。在去氫化這條反應途徑上，低配位數白金可在較低溫的情況下，使甲醇分解成一氧化碳，在Pt(100)奈米粒子上發生於150 K，而在Pt(111)奈米粒子上則發生於200 K。由實驗結果顯示位於平坦及低配位處上的白金，皆可對甲醇進行催化反應。甲醇在白金奈米粒子上分解成一氧化碳的效能約為一般白金單晶面上的2～6倍。除此之外，我們發現由甲醇分解而來的氫原子具有降低一氧化碳裂解反應發生的機率。在另一反應路徑-碳氧鍵斷裂，甲醇首先分解成甲基(CH3)吸附於表面。加溫到300 K以後，甲基只會與氫原子合併，形成甲烷脫附；而不會進一步分解成其它的碳氫化合物。我們發現此分解途徑可能與奈米粒子的晶格常數有關，當晶格常數隨著奈米粒子增大而減少時，此反應途徑的發生機率會隨之降低。在一氧化碳毒化的白金實驗上，這兩種反應途徑仍持續發生，由此顯示除了吸附在單顆白金原子上的甲醇可反應外，兩顆或三顆白金原子的中間位置亦可對甲醇進行反應。我們亦對甲醇於氧化白金(Pt2+)的催化反應進行研究。將300 K時成長的白金奈米粒子加溫至650 K使之氧化，並造成底層氧化鋁往上包覆住白金奈米粒子，僅有30 %的白金原子（低配位數的白金）裸露於表面上。在氧化白金上，依然可觀察到甲醇的兩種反應路徑。相較於純白金上的反應，甲醇在氧化白金上的去氫化的反應行為雖然相似，但產能僅剩一半左右；然而對於氫氧鍵斷裂的反應途徑，卻沒有太大的影響。此外，一氧化碳在氧化白金上容易裂解，發生溫度約為200 K；而一般在低配位的純白金上，此反應溫度約為500 K。Methanol decomposition on Pt nanoparticles supported by Al2O3/NiAl(100) as a model system is studied by IRAS and TPD. The Pt nanoparticles are grown from vapor deposition. Two channels of methanol decomposition are revealed: dehydrogenation and C-O bond scission. The adsorbed methanol are dehydrogenated to CO first at low-coordinated Pt sites, at 150 K on Pt(100) clusters and 200 K on Pt(111) clusters, whereas both terrace and low-coordinated Pt sites are reactive toward the dehydrogenation, despite of the cluster size. The produced CO per surface Pt on the clusters are 2 - 6 times more than those on the single-crystal counterparts. Additionally, the co-adsorbed atomic hydrogen from dehydrogenated methanol prevent CO from dissociating further to elemental carbon. In the alternative reaction channel, the C-O bond break to form intermediate CH3; the CH3 combine with the atomic hydrogen above 300 K to form methane, rather than dehydrogenating to other hydrocarbons. The C-O bond scission channel exhibits evident dependence on the lattice constant of the clusters: the reaction probability declines when the lattice constant decreases with the cluster size or coverage. The CO blocking experiments suggest that not only the atop sites but also the bridge and/or hollow sites are reactive toward the two reaction channels. The two reaction channels are also observed for methanol adsorbed on oxidized Pt (Pt2+) nanoclusters. The oxidized Pt clusters grown at 300 K and annealed to 650 K are partially encapsulated by the alumina, and only 30 % bare Pt remain (low-coordinated Pt). The dehydrogenation to CO starts at 150 K, resembling that on the pristine Pt clusters, but the produced CO per surface Pt on the oxidized clusters are only 50 % of those on the pristine Pt clusters. In contrast, the oxidation of Pt has little effect on the C-O bond scission: the produced methane per surface Pt are similar on both oxidized and pristine clusters. Moreover, CO dissociation is enhanced on the oxidized low-coordinated Pt, which occurs above 200 K, significantly lower than that on the pristine low-coordinated Pt (500 K).
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