甲醇在金與白金形成的合金奈米粒子上之分解反應：紅外光吸收能譜和熱脫附質譜

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姓名	李昱達(Yu-da Li) 查詢紙本館藏	畢業系所	物理學系
論文名稱	甲醇在金與白金形成的合金奈米粒子上之分解反應：紅外光吸收能譜和熱脫附質譜 (Methanol decomposition on Au-Pt bimetallic nanoclusters supported by Al2O3/NiAl(100): A combined IRAS and TPD study)
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摘要(中)	我們利用反射式紅外吸收光譜及熱脫附質譜術研究金與白金形成的合金奈米粒子之表面結構。我們以蒸鍍的方式，讓金與白金原子在θ相的氧化鋁薄膜上成長，形成金與白金的合金奈米粒子。不論成長的先後順序，表面皆形成金與白金混合的狀態。當金鍍在白金奈米粒子上時，金原子優先佔據白金粒子的邊緣處，而後鍍的金原子會聚集在已存在的金原子周圍，所以奈米粒子表面依然有裸露的白金原子；當白金鍍在金奈米粒子時，白金原子會佔據所有的位置並且輕易地將金奈米粒子包覆住，但是表面會殘留少數孤立的金原子。在合金上的金觀察到的一氧化碳紅外光吸收增強的現象，使我們推斷表面上的金不僅被白金圍繞而且處於低配位數的狀態。除表面形貌外，我們也研究甲醇在金與白金形成的合金奈米粒子上的分解反應之觸媒模型。在合金模型上，甲醇分解途徑分為兩種：(1)甲醇的去氫化反應，產生一氧化碳和氫氣；(2)甲醇內的碳氧鍵斷裂，產生甲烷。去氫化後的產物量(一氧化碳、氫氣)和表面白金原子數成正比；對於碳氧鍵斷鍵的機率而言，表面白金數目的影響較小。與純白金奈米粒子相比，甲醇在合金奈米粒子上分解後的產率較低。在一氧化碳毒化的實驗上，兩種分解反應依然持續進行，但反應發生的機率下降；而毒化的合金奈米粒子上的反應機率和毒化後的純白金奈米粒子相同，因此形成合金無法有效改善一氧化碳毒化的問題。
摘要(英)	We investigate the surface structures of Au-Pt bimetallic clusters by probe molecule CO and IRAS and TPD. Au and Pt deposited sequentially from vapor onto thin-film Al2O3/NiAl(100) at 300 K form preferentially Au-Pt intermixed bimetallic nanoclusters, despite the order of the metal deposition. For the deposition of Pt followed by Au, the deposited Au atoms prefer to decorate the edges of Pt clusters (low coordination sites) and aggregates with the following deposited Au; the 2 ML Au deposited cover only a small portion of the Pt cluster surface. For the reverse order of deposition, the deposited Pt atoms have no preferential sites and cover the Au cluster easily, but there are still few isolated Au atoms on the surface. The enhancement of infrared absorption for CO on Au sites of Au-Pt bimetallic cluster was observed. The effect indicates that the cluster surface consists of both Au and Pt atoms and the surface Au is not only coordinated to Pt but is also preferentially low coordinated. Methanol decomposition on Au-Pt bimetallic nanoclusters supported by Al2O3/NiAl(100) is studied by IRAS and TPD. Two decomposition channels are observed for methanol adsorbed on Au-Pt bimetallic nanoclusters: dehydrogenation to CO and C-O bond scission. The produced CO and D2 from methanol dehydrogenation increased in proportion to the Pt sites on the surface of Au-Pt bimetallic clusters, but the varied Pt sites of the surface change little probability of C-O bond scission. Comparing the desorption of molecularly adsorbed CO to CO from methanol decomposition on Au-Pt bimetallic clusters, we found that the produced CO amounts to about 70 - 80 % of CO saturating the clusters, smaller than that (above 90 %) on pure Pt clusters. It indicates that the productivity, number of CO produced per surface Pt sites, is smaller than that on pure Pt clusters. The pre-adsorbed CO can not prohibit entirely the dehydrogenation and the C-O bond scission of methanol, but decrease the probability of these reactions. The unblocked sites, such as bridge and hollow sites, are still reactive. The effect of alloying Pt with Au to reduce the effect CO poison is insignificant; the probability of methanol decomposition on CO-poisoned Au-Pt bimetallic clusters is similar to that on CO-poisoned pure Pt clusters.
關鍵字(中)	★ 甲醇分解 ★ 金與白金合金奈米粒子 ★ 熱脫附質譜技術 ★ 傅立葉轉換紅外光譜儀	關鍵字(英)	★ Methanol decomposition ★ IRAS ★ TPD ★ Au-Pt bimetallic nanoclusters
論文目次	Chapter 1 Introduction 1 Chapter 2 Literature Survey 5 2.1 The structure of Au-Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100) 5 2.1.1 Alloying of Au with Pt in nanoclusters supported on thin film of Al2O3/NiAl(100) at 300 K 5 2.1.2 Temperature-dependent structuring of Au-Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100) 12 2.2 CO-induced changes in the surface composition of bimetallic clusters: Pt-Au on TiO2(100) 18 2.2.1 The formation of bimetallic Pt-Au cluster 18 2.2.2 The surface composition of the Pt-Au bimetallic cluster by LEIS 19 2.2.3 The desorption of CO 21 2.2.4 Density functional theory calculation 23 2.3 Intensity enhancement of IR absorption 26 2.4 Decomposition of methanol on pure Au and Pt nanoparticles supported on a thin film of Al2O3/NiAl(100) 31 2.4.1 Decomposition of methanol on Au nanoparticles 31 2.5 Reference: 44 Chapter 3 Experimental Methods and Apparatus 46 3.1 Experimental methods 46 3.1.1 Cleaning NiAl(100) 46 3.1.2 θ-Al2O3 ultrathin film growth 47 3.1.3 Vapor deposition of Au and Pt 48 3.1.4 Methanol adsorption and reaction 48 3.2 Temperature programmed desorption (TPD) 50 3.3 Infrared reflection absorption spectroscopy 55 3.3.1 Principle of IRAS 55 3.3.2 Fourier transform interferometers 60 3.4 LEED and AES 63 3.4.1 Low energy electron diffraction (LEED) 63 3.4.2 Auger electron spectroscopy (AES) 63 3.5 Reference: 65 Chapter 4 Result and discussion 66 4.1 The surface structure of Au-Pt bimetallic clusters/Al2O3/NiAl(100) 66 4.1.1 Au-Pt clusters formed by Au deposited on Pt clusters on Al2O3/NiAl(100) 66 4.1.2 Au-Pt clusters formed by Pt deposited on Au clusters Al2O3/NiAl(100) 74 4.2 The decomposition of methanol on Au-Pt/Al2O3/NiAl(100) 81 4.2.1 The thermal desorption spectra of methanol-d4 on Au-Pt bimetallic clusters on Al2O3/NiAl(100) 81 4.2.2 The infrared absorption spectra of methanol on Au-Pt bimetallic clusters on Al2O3/NiAl(100) 91 4.2.3 The reaction of methanol on CO-preadsorbed Au-Pt bimetallic clusters 97 4.3 Reference 102 Chapter 5 Conclusion 104
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指導教授	羅夣凡(Meng-fan Luo)	審核日期	2013-5-13
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