- 學位論文
高溫處理和化學改質之多壁奈米碳管對有機蒸氣吸附效能研究
Study of the absorption of organic vapor on multi-walled carbon nanotubes with heat treatment or chemical modification
摘要
奈米科技的新材料－奈米碳管具有奈米尺度的中空孔隙、大的比表面積和許多優異的特性，被視為是一種極具潛力的儲氣材料。若大量氫氣吸附在碳管管壁，則可成為燃料電池的儲氫材料。若將奈米碳管有效地用以吸附氣體或有機蒸氣，將對能源儲存、氣體感測和污染防治有相當地貢獻。本研究的目的將用改質技術提高奈米碳管吸附有機蒸氣的吸附行為和效能。 本研究選用商用的多壁奈米碳管以高溫處理和化學改質進行改質，並可藉由許多表面的檢測技術得知奈米碳管之表面物化特性。隨後，以苯蒸氣當作吸附質進行奈米碳管對有機蒸氣的吸附實驗，進而探討有機蒸氣的濃度對奈米碳管的吸附效益，由此吸附實驗的結果利用過去研究學者提出的經驗模式進行模擬，並建立適合的經驗模式來說明奈米碳管之動態吸附曲線。 由研究結果發現：由FESEM影像得知樣本為細長且彎曲的表面結構，進一步從TEM影像能觀察到奈米碳管為中空管結構，部分改質後的樣本因受到破壞而出現長度變短。所有奈米碳管樣本的晶面間距約為3.4 Å，該空間可視為平行板所構成的狹縫形微孔；此間距會因改質過程而出現些微改變。奈米碳管樣本表面、末端開口或是缺陷上的含氧官能基對樣本吸附苯蒸氣之效能可能會造成相當的影響。由拉曼光譜得知改質可能破壞奈米碳管石墨化程度。 氮氣等溫吸附/脫附曲線得知奈米碳管為多孔性的吸附劑，其吸附的位置可能為類似平板狀構造或裂縫狀孔隙；經高溫處理和部分經化學改質的樣本，其比表面積和總孔體積都有增加的趨勢；化學改質的樣本改善了微孔之特性，微孔逐漸擴大，且出現新微孔。 原奈米碳管或是各種改質條件的樣本在16400 ppm苯蒸氣的吸附環境中，以高溫處理或是混酸溶液進行改質的奈米碳管擁有較佳的吸附效能。在不同濃度的吸附環境中亦以高溫處理的樣本（CBT-CO2）呈現較理想的吸附結果。由本研究的吸附結果發現：此商用多壁奈米碳管經高溫處理或是混酸溶液進行改質將能有效地改善奈米碳管吸附苯蒸氣的吸附量。本研究成功地以經驗模式描述奈米碳管吸附苯蒸氣（16400 ppm, 25 ℃）的動態吸附曲線。由資料統計分析的相關性分析和因子分析討論奈米碳管的表面物化特性、苯蒸氣（16400 ppm, 25 ℃）吸附量和模擬係數間的相關性。
並列摘要
Carbon nanotubes, one of new materials in nanotechnology, with many excellent properties such as high porosity and specific surface area, have been regarded as a promising adsorbent for hydrogen storage. Once the adsorptivity of carbon nanotubes for gases and vapors has been proven, their applications on energy storage, gas sensing and pollution control would be prosperous. The objectives of this research were to understand the influence of heat treatment or chemical modification on material properties of carbon nanotubes and further to evaluate their adsorption capacity for benzene. One commercial multi-walled carbon nanotubes (MWNTs) was selected and several heat treatments or chemical modifications were applied. The properties of as-received and modified MWNTs were analyzed using several surface techniques. Next, the adsorption capacities of benzene of 16400 ppm at 25 ℃ on carbon nanotubes were determined and the adsorption curves with elapsed time were collected. Moreover, the adsorptions of benzene at several concentrations on selected MWNTs were measured. An empirical model has been attempted to be proposed to describe the adsorption curves. Finally, correlation analysis and factor analysis were used to investigate the relationships among the properties, adsorption capacities and model parameters. The results show that the MWNTs had curved and tangled morphology by FESEM images, and their tubular structure has been verified from TEM images where some nanotubes have been cut after treatment. The interlayer spacing of all samples was about 3.4 Å, and the distance might be changed after treatment. Some surface functional groups have been found to be occurred at the defects on the surface or at the entrance of the open-ended tube, which might influence the absorption. The Raman spectrum revealed that modification would decrease the degree of graphitic crystallinity. The N2 adsorption-desorption isotherms of MWNTs indicated that they were porous materials, and their pore structure was plate-like or slit-shaped holes. After heat-treated and chemically modified, the increase in the specific surface area and total pore volume were observed. Especially, the chemical modification has significantly improved the porosity; besides generation of new micropores, the mesopore developed from micropores were happened. The adsorption capacities of benzene (at 16400 ppm, 25 ℃) on MWNTs have been improved after heat treatment or chemical modification with mixed acidic solutions, especially for the samples treated with CO2. It should be noted that the samples treated with HNO3 had the largest surface area but an inferior adsorption capacity. In addition, an empirical model has been successfully proposed to describe the adsorption curves. The results from statistical analyses have suggested the critical properties affecting the adsorption capacity of benzene and the model parameters. To sum up, the results from this study have recommended that MWNTs treated should be a promising adsorbent.