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|標題:||Applications in Hydrogenation/Dehydrogenation over Nano-intermetallic Mg-Ni Alloys: PEG Amination and CNT Growth
|關鍵字:||奈米介金屬鎂-鎳合金;nano-intermetallic Mg-Ni alloy;醇還原法;胺化反應;奈米碳管成長反應;奈米碳管純化;polyol process;amination;carbon nanotube growth;carbon nanotube purification||出版社:||化學工程學系||摘要:||
奈米介金屬鎂-鎳合金的粒徑大小主要受保護劑含量及鎂鎳前驅物莫耳比（Mg/Ni）的影響。不同條件下所製備之鎂-鎳合金具有高比表面積，約為113 ~ 315 m2/g，其成分組成主要為Mg2Ni、Ni、Mg及MgO等四種。其在合金中的含量多寡取決於製備時鎂鎳前驅物莫耳比，鎳前驅物莫耳比越高會使單獨相的鎳容易被還原出來並造成鎳結晶度的增加，使鎂-鎳合金的整體粒徑因而增大。
奈米介金屬鎂-鎳合金於加氫/脫氫反應催化效能的研究，主要以進行聚環氧乙烯二元醇（PEG）之胺化反應及奈米碳管的成長反應為主。PEG胺化反應的催化研究結果顯示，以1 g的nano-Ni及30% Ni/Al2O3進行胺化反應時，其總胺產率分別為24.7%及40.0%；而以1 g不同條件下所製備的奈米介金屬鎂-鎳合金為觸媒時，其總胺產率較高，可達到38.8% ~ 58.4%。當提高鎂鎳前驅物中鎳的莫耳比為2時，其總胺產率更達到一最大值58.4%，推測催化活性的增加是因為有較多單獨相的鎳形成所致。所合成之奈米介金屬鎂-鎳合金中所存在的Mg2Ni合金及MgO成份可以防止鎳金屬的聚集，也使其具有較高的胺化反應催化活性。根據上述的結果發現，觸媒的粒徑大小及Mg2Ni、單獨相的鎳及鎂等活性相的存在決定了胺化反應的催化活性。
對以奈米介金屬鎂-鎳合金為催化劑成長奈米碳管的反應而言，根據XRD、TGA及甲烷轉化率和碳產率的計算結果顯示，成長奈米碳管的最適化條件為：取50 mg的奈米鎂-鎳合金為催化劑，於650℃下通入流量為100 ~ 120 sccm的純甲烷，進行熱裂解30分鐘以生成奈米碳管。經由拉曼光譜分析顯示，所製備之奈米碳管皆屬多壁奈米碳管，而碳產率及初產物純度分別高達900%及91%，且由TGA分析結果得知，不定形碳含量極少。將反應時間拉長為一小時，其奈米碳管初產物的碳產率及純度分別從甲烷裂解過程中無添加氫氣的232%及67%增加至添加氫時的1200%及92%。研究結果顯示，在長時間的反應下，氫氣的添加不僅可以避免催化劑的失活還可以提升奈米碳管的石墨化程度。純化研究結果顯示，將奈米碳管初產物以空氣氧化及鹽酸處理後，其純度幾乎達到100%，但若僅以空氣氧化或鹽酸處理則純化後碳管純度較低。在一系列的純化步驟中，奈米碳管的純化完全與否取決於空氣氧化步驟，因為空氣氧化可以使石墨層進行修飾，並使鹽酸容易進入石墨層中將殘存的催化劑溶出。
The intermetallic alloys have been applied as a catalyst in hydrogenation/ dehydrogenation reactions to produce hydrocarbons or organics; intermetallic alloys such as lanthanum-nickel or magnesium-nickel have been used as hydrogen storage materials or electrodes due to their capability of hydrogen adsorption. The intermetallic alloys were traditionally produced by mechanical alloying (also named high-energy ball milling) which consumes much energy and results in higher cost. In this study, a chemical reduction method such as a polyol process has been used to prepare the nano-intermetallic Mg-Ni alloys. There are two advantages for this method: (1) a higher quantity of sample can be prepared, (2) the preparation conditions such as reduction temperature, reaction time, protect reagent (PVP) concentration can be adjusted to control the particle size and to produce the nano-single metal or nano-intermetallic alloy.
The particle size and composition of the synthetic Mg-Ni alloy was affected by the amount of the protect agent and the Mg/Ni molar ratio in the corresponding metal salts. All synthetic Mg-Ni alloys possess as higher surface area about 113 ~ 315 m2/g and consists with four different components: Mg2Ni, Ni, Mg and MgO. The relative concentration of Mg2Ni, Ni, Mg and MgO are affected by the Mg/Ni molar ratio. Higher molar ratio of Ni makes the single phase of Ni easy to reduce and results in the increase of Ni crystalline degree and the particle size of the synthetic nano-Mg-Ni alloys.
The catalytic performance of hydrogenation/dehydrogenation over nano- Mg-Ni alloys was studied by the PEG amination and CNT growth reaction. As for the catalytic studies of PEG amination show that the total amine yield over 1 gram of nano-nickel and 30% Ni/Al2O3 is 24.7% and 40.0%, respectively, however, the total amine yield reaches higher between 38.9% and 58.4% over 1 gram of nano-MgNi alloys with various Mg/Ni ratios. The amination activity over nano-MgNi alloys is higher than those of the other two types of catalysts. The amine yield reaches to the highest value of 58.4% when the molar ratio of Ni has been increased to Mg : Ni = 1 : 2; the enhancement of the activity of PEG amination is due to the formation of higher concentration single phase of Ni. In addition, the presence of Mg2Ni alloy and MgO can prevent the aggregation of Ni and results in a higher amination activity. According to the previous results, the particle size of catalyst and the active phases of catalyst, such as Mg2Ni alloy or single phase of Ni and Mg, control the amination activity.
As for the CNT growth over nano-Mg-Ni alloys, the results of XRD, TGA and the calculations of methane conversion and carbon yield indicate that the optimum reaction condition is using about 50 mg of nano-Mg-Ni alloy as the catalyst to perform pyrolysis of the pure CH4 gas to form CNTs with the flow rate about 100 ~ 120 sccm at 650℃ for 30 min. Raman results also indicate that the CNTs are formed as the multi-walled structure. To use nano-Mg-Ni alloys as the catalyst to growth CNT for 30 min, the carbon yield and purity can reach to about 1200% and 91%, respectively, and almost no amorphous carbon in the synthetic CNTs by TGA analysis. If the growing time has been extended to 1 hour, the carbon yield and purity of raw CNTs increase respectively from 232% and 67% (without H2 addition) to 1200% and 92% (with H2 addition) during pyrolysis of CH4. It appears that the addition of hydrogen not only prevents deactivation of catalysts but also enhances the graphitization degree of CNT. After the purified procedures with both air oxidation at 550℃ and HCl treatment, the purity of CNT reaches to ~100%. However, the purity of CNT is lower with using only air oxidation or HCl treatment Thus, the air oxidation step is crucial for the completed purification. The oxidation with flowing of air at high temperature can reorganize the graphite interlayers and makes the HCl easily to penetrate into the graphite interlayers to dissolve the residual catalyst.
From the previous catalytic results, it indicates that the nano-Mg-Ni alloys possess higher activity in hydrogenation/dehydrogenation catalysis such as PEG amination and CNT growth. The nano-Mg-Ni alloys can be a potential catalyst for a hydrogenation/dehydrogenation processes due to its specific properties and structure.
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