Methanol Decomposition on Rh Nanoclusters supported by Al2O3/NiAl(100)：A combined IRAS, TPD and PES study

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/64448

Title:	Methanol Decomposition on Rh Nanoclusters supported by Al2O3/NiAl(100)：A combined IRAS, TPD and PES study
Authors:	洪碇傑;Hung,Ting-Chieh
Contributors:	物理學系
Keywords:	甲醇;銠;觸媒模型;反應;Methanol;Rh;UHV;IR;PES;TPD
Date:	2014-06-30
Issue Date:	2014-08-11 18:23:28 (UTC+8)
Publisher:	國立中央大學
Abstract:	我們以反射吸收式傅立葉轉換紅外線光譜儀(FT-RAIRS)、熱脫附質譜術(TPD)與以同步輻射作為光源的光電子能譜(PES)研究一氧化碳(CO)與甲醇(CH3OH)在銠(Rh)奈米粒子上的吸附、脫附與分解反應之觸媒模型。銠以蒸鍍的方式，在θ相氧化鋁成長為奈米粒子。從IR吸收光譜上以CO作為探測物的結果發現CO傾向於吸附在Rh奈米粒子的on-top site上，沒有其他像是bridge或是hollow的吸附site被探測到。由CO的TPD脫附譜線則顯示CO的脫附有兩個峰值：一個位於430 K（在Rh單晶上也有出現），一個則位於360 K（我們認為是來自於較小的Rh奈米粒子上脫附的CO）。由PES光譜則指出CO於Rh奈米粒子上分解成為原子碳(C)的比率約介於21到55%（與Rh奈米粒子的覆蓋率及大小有關）。在Rh奈米粒子在加熱至700 K之後，IR吸收光譜上顯示其吸收譜線並未改變，但在TPD譜線上則顯示低溫脫附的CO比率提升，我們認為這是因為Rh奈米粒子的尺寸所造成的。甲醇吸附於Rh奈米粒子上將會進行脫氫反應，而脫氫反應開始於200 K之前且部分CO將會分解成為原子碳。甲醇分解產生的CO會在300 K開始脫附，而來自於CD3OD的D2則會於200 K開始脫附，我們沒有發現任何的中間產物。而在PES成果中顯示有43.1 ± 2.0 %的甲醇會分解成原子碳，由此結果顯示甲醇分解成CO的機率約為86.9 ± 4.5 %。而將樣品加熱至700 K後並不會造成Rh奈米粒子的氧化。在TPD的脫附譜線中顯示，由甲醇分解產生的CO與直接吸附的分子CO的脫附量的比率較未經過加熱處理的樣品有所提升。1 ML的Rh其比率由47.2 ± 1 % 提升至54.4 ± 10 %，而4 ML的Rh其比率由39.9 ± 10 % 提升至77.9 ± 10 %。而甲醇分解反應的CO產率也有所提升，在4 ML的Rh上甲醇分解反應由原本的64.9 %提升至83.9 %，與我們的CO比率的成果一致。 ;Methanol decomposition on Rh nanoclusters supported by Al2O3/NiAl(100) as a model system is studied by IRAS, TPD and PES. The study contained two parts: surface structures of Rh clusters probe with CO and methanol decomposition on Rh nanoclusters. The Rh nanoclusters are grown from vapor deposition. The IRAS spectra with CO as a probe show that the CO adsorbed on on-top site of Rh nanoclusters; no other site such as bridge or hollow site have been detected. The CO TPD spectra show that CO desorbed with two distinct peaks, one at 430 K, which is observed for CO on Rh single crystal results and the other at about 360 K, which is observed for CO on small Rh clusters. The PES spectra show that the CO dissociation rate ranges between 21 and 55 %, depending on the coverage and hence the size of the clusters. For the clusters annealed to 700 K, the IRAS spectra show the same CO absorption band but the CO TPD spectra show the proportion of low temperature desorption increases. We argue that the desorption temperature depends on the cluster size. Adsorbed methanol was dehydrogenated on the nanoclusters. The dehydrogenation to CO began below 200 K, and some of the CO formed from the dehydrogenated methanol dissociated into atomic carbon. Our PES results confirm that about 21 - 55 % of molecularly adsorbed CO dissociates into atomic carbon, depending on the size of the clusters. The produced CO desorbed above 300 K and D2 from methanol-d4 above 200 K; no intermediate species were detected in the dehydrogenation process. The PES results show the ratio of methanol dissociating into atomic C is about 43.1 ± 2.0 %. This result suggests that the probability of methanol decomposed into CO is about 86.9 ± 4.5 %. Annealing the sample to 700 K does not result in oxidation of the Rh clusters. The ratio of CO TPD desorption intensities from dehydrogenated methanol and molecularly adsorbed CO on the annealed Rh clusters is greater than that on the pristine Rh clusters. For 1 ML Rh, the ratio increases from 47.2 ± 1 % to 54.4 ± 10 %, and for 4 ML Rh, the ratio increases from 39.9 ± 10 % to 77.9 ± 10 %. The fraction of monolayer methanol undergoing dehydrogenation on the annealed Rh clusters is also greater than that on the pristine Rh clusters. On the annealed 4 ML Rh, the fraction of methanol undergoing dehydrogenation increased to 83.9 % (from 64.9 %), consistent with the result of CO ratio.
Appears in Collections:	[Graduate Institute of Physics] Electronic Thesis & Dissertation

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