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Preparation of Copper Based Catalyst Supported on Carbon Nanotubes for Steam Reforming of Methanol
|關鍵字:||carbon nano-tubes;奈米碳管奈;nano-catalyst;Ni-Cu alloys;chemical reduction method;methanol steam reforming;米觸媒Cu-Ni合金;化學還原法;蒸汽重組反應||出版社:||化學工程學系所||引用:||. S. Iijima, "Helical microtubules of graphitic carbon", Nature 354 (1991) 56-58 . S. Iijima, T. Ichihashi, "Single-shell carbon nanotubes of 1-nm diameter", Nature 363 (1993) 603-605 . D. S. Bethune, C. H. Klang, M. S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, "Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls", Nature 363 (1993) 605-607 . M. S. Dresselhaus, G. Dresselhaus, R. Saito, "Physics of carbon nanotubes", Carbon 33 (1995) 883-891 . R. S. Ruoff, D. C. Lorents, "Mechanical and thermal properties of carbon nanotubes", Carbon 33 (1995) 925-930 . V. N. Popov, "Carbon nanotubes: properties and application", Materials Science and Engineering: R: Reports 43 (2004) 61-102 . C. H. Li, K. F. Yao, J. 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由TEM分析結果，Cu、Ni與Cu-Ni合金觸媒之金屬顆粒大小約為10-20nm，且金屬顆粒分散性良好。將此觸媒應用在甲醇蒸汽重組反應，金屬含量為23~30wt% Cu80ZnO20/CNTs之觸媒具有高活性，其反應性隨著溫度的增加可得氫氣產率隨之增加。使用23 wt% Cu80ZnO20/CNTs觸媒在溫度為280℃時氫產率可達到83%，而在溫度360℃以上，氫產率則可達到100%，以水與甲醇之莫耳比為1.5為最佳，20 wt% 之Ni20-Cu80/CNTs觸媒在溫度360℃以上，氫氣產率亦可幾乎達到100%。以奈米碳管為載體之觸媒(Ni20-Cu80/CNTs)活性明顯高於以活性碳為載體之觸媒(Ni20-Cu80/C)，且亦高於非合金狀態觸媒(Ni20/Cu80/CNTs)。另外，Cu/ZnO/CNTs觸媒雖然表現出高活性，但是觸媒穩定性較低，當Cu觸媒加入Ni使之形成合金狀態，且加入ZnO為促進劑並擔持在奈米碳管後，所形成之(Cu4-Ni1)80/ZnO20/CNTs觸媒則具有很高之反應活性與穩定度，在360℃之甲醇轉化率亦可以達到100 %。其中Ni金屬會影響氫氣吸附與抑制Cu顆粒燒結，而加入ZnO則可以分散並提高觸媒表面積。以40-60 nm管徑之CNTs當作載體，提供了較良好的環境使金屬顆粒使顆粒分散，而增加觸媒觸媒活隨著管徑的增加而提升。在甲醇裂解反應中，Ni的含量與CNTs管徑都會影響觸媒活性與CO的選擇率，反應溫度在200-400℃間隨著Ni含量增加其甲醇轉化率隨之降低，但是CO選擇率隨之增加。
In this dissertation, the purpose of this study is to prepare copper based catalyst supported on carbon nanotubes (CNTs) and to apply it on steam reforming of methanol. Mathanol, compared to natural gas or other hydrocarbons, is a more efficient energy source to produce hydrogen in fuel cell reformer, and steam reforming of methanol reaction is a method to produce hydrogen frequently. Besides, carbon nanotubes can be used as supports of catalysts because of their specific properties, such as high ratio of length to diameter, appropriate pore size distribution, etc. The preparation of catalysts with carbon nanotubes as supports is investigated for production of hydrogen in this study.
Cu, Cu-Ni alloys and Ni catalysts with ZnO as a promoter supported on CNTs are prepared by chemical reduction and wet impregnation methods. The effects of different preparing conditions, including acid-treatment of CNTs, additions of ethanol, type of dispersants and supports of catalyst, metal content, weight ratio of alloys and ratio of metal/ZnO on catalytic activity are explored. Operating parameters in steam reforming of methanol are included reaction temperature, weight hourly space velocity, molar ratio of H2O/CH3OH, metal content, weight ratio of alloys, ratio of metal/ZnO, and stability of catalysts. Before the chemical reduction step, the CNTs should be pre-treated by mixed acids (nitric acid/sulfuric acid= 3/1 v/v) to remove impurities, and to create defects on its surface to form functional groups. The thermal property of CNTs was then enhanced. The hydrophilicity of CNTs was improved by adding a suitable amount of ethanol to make the metal precursors contacting with the surface of CNTs more easily. The addition of dispersant can make metal particles to diperse well on surface of CNTs and increase surface area of metal particles. The Cu-Ni alloys were anchored on the surface of CNTs by co-reduction of Ni- and Cu-precursors under the use of tetra-n-methylammonium hydroxide to reduce the aggregation of Cu-Ni particles. Ni-Cu catalyst supported on activated carbon (Ni-Cu/C) was prepared as well, and bimetal Ni and Cu supported on CNTs (Ni/Cu/CNTs) was attained by successive reduction of first Cu- and then Ni- precursors in this study. The catalysts were characterized by transmission electron microscope (TEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive spectrometer (EDS), Brunauer-Emmett-Teller method (BET), field emission scanning electron microscope (FESEM) and thermogravimetric analysis (TGA).
From TEM analysis a good dispersion of Cu, Ni and Cu-Ni alloys particles with about 10-20 nm on the surface of CNTs was observed. The catalysts with 23 to 30 wt% Cu80ZnO20/CNTs showed high activities, increased with an increase of temperature to produce hydrogen. By using 23 wt% Cu80ZnO20/CNTs as the catalyst, the hydrogen yield was obtained up to 83% at 280oC and nearly 100% at temperatures greater than 360oC with 1.5 of molar ratio of water to methanol. The hydrogen yield in steam reforming of methanol was near 100% at 360oC. Ni affected the chemisorption of H2 catalysts and restrained the sintering of Cu particles. Moreover, ZnO dispersed Cu80-Ni20 particles well and increased surface area of catalysts. CNTs with 40-60 nm of tube diameters provided better environment to disperse metals and to enhance activity of catalysts. Both of the Ni content of catalysts and tube diameters of CNTs affected activities and CO selectivity in decomposition of methanol. The increasing Ni content of catalysts made lower CH3OH conversion and higher CO selectivity at 200-400 oC.
In this study, we successfully prepared Cu, Ni, Cu-Ni alloys and ZnO supported on CNTs with chemical reduction and impregnation method, and applied in steam reforming of methanol efficiently.
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