多壁奈米碳管的氧化和活化及其對SF6吸附效能評估

Oxidation and Activation of Multi-walled Carbon Nanotubes and Their Adsorption Capacities for SF6

吳柏昀(Po-Yun Wu)

指導教授 ： 江右君

元智大學/工程學院/機械工程學系/碩士(2007年)

摘要

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。六氟化硫(SF6)為產業界常用之氣體，用途廣泛，同時亦為溫室氣體。未適當加以控制，可能加速全球暖化。因此，本研究的目的乃利用氧化或活化技術對多壁奈米碳管的特性進行改質，並加以評估奈米碳管吸附SF6的吸附行為和效能。
本研究利用CVD法合成多壁奈米碳管，並配合商用多壁奈米碳管，經過氧化或活化程序後作為吸附劑，SF6作為吸附質，利用批次式測定法進行吸附實驗。主要探討在不同濃度與溫度下奈米碳管對SF6的吸附行為，並利用Freundlich等溫吸附模式進行曲線擬合，最後根據Clausius-Clapeyron equation求得等量吸附熱，進而瞭解其吸附機制。
研究結果發現，多壁奈米碳管樣本外形細長且彎曲，結構為中空管狀型態。部分改質後的樣本因受到破壞而出現長度變短、末端開口的情形。原多壁奈米碳管樣本包含Fe觸媒成分，經由純化或氧化處理程序後似乎沒有明顯的改善，但是表面卻增加了O元素成分。反觀活化後的樣本，發現Fe觸媒能有效的被移除且K元素明顯的增加在碳管表面。多壁奈米碳管經過H2SO4/H2O2氧化程序後，表面出現了Carbonyl groups (C=O)官能基，但是原有的??-??* transitions官能基卻消失了。然而，活化後之樣本不但可以保留??-??* transitions官能基存在，還增添了Carboxyl groups (C-OOH)官能基。氮氣等溫吸附/脫附曲線得知奈米碳管可視為多孔性的吸附劑，其吸附的位置可能為類似平板狀構造或裂縫狀孔隙。由吸附實驗可瞭解，吸附行為及其機制是很複雜的。實驗結果顯示，活化後的奈米碳管有利於吸附SF6。此外，低溫較有利於吸附。同時也發現，Freundlich等溫吸附模式較適於描述多壁奈米碳管吸附SF6實驗結果。等量吸附熱的分析中觀察出，本研究所用之多壁奈米碳管的等量吸附熱皆隨吸附量的增加而降低，且之間存在著線性關係。

關鍵字

多壁奈米碳管；氧化；活化；特性分析；等量吸附熱；SF6

並列摘要

Abstract
The new nanomaterials - carbon nanotubes (CNTs) with pore structure in nano scales, great specific surface area, and many excellent characteristics have been considered as an alternative adsorbent for gas adsorption. It should give a great contribution to energy storage, gas sensor and pollution control if CNTs can be used to adsorb gases or organic vapors effectively. Sulfur hexafluoride (SF6) is widely used in several industry, meanwhile it is also a kind of greenhouse gases. If SF6 emission is out of control, it will accelerate the global warming. Therefore, the objective of this research is to modify the properties of multi-walled carbon nanotubes (MWNTs) by oxidation or activation techniques, and further to evaluate the SF6 adsorption on MWNTs.
The chemical vapor deposition method was used to synthesize MWNTs, and one of commercial product was also used for comparison. After oxidation or activation process, the modified MWNTs were characterized and served as adsorbents to adsorb SF6 gas with batch experiments under desired environments. It was focused on the effects of inlet concentration and temperature. Finally, the Clausius-Clapeyron equation was utilized to determine the isosteric heat of adsorption.
Results show that the MWNT samples are long and hollow, and some modified samples suffer from partial damages such as shortened and open-ended. The as-prepared MWNTs contain significantly Fe catalysts. Purification and oxidation processes seem not to remove the catalyst effectively but can increase O content. On the other hand, activation can eliminate Fe content and introduce K element. After H2SO4/H2O2 oxidation, the carbonyl functional groups on the surface of MWNTs are present but the ??-??* transitions are disappeared. However, activation can generate more carboxyl groups and retain the ??-??* transitions. The N2 adsorption/desorption isotherms at 77 K show that MWNTs are porous, which probably are composed of slit-shaped pores. The adsorption data indicate that the adsorption behavior and mechanism of SF6 on MWNTs are complicated. The activated samples possess the highest adsorption capacities of SF6. Moreover, low temperature also gives an advantage of adsorption. Freundlich isotherm is suitable for the curve fitting of SF6 adsorption on MWNTs. It can be seen from the data that the isosteric heat of adsorption decreases with increasing the amount adsorbed, and a linear relationship can be obtained.

並列關鍵字

Multi-walled carbon nanotubes ； Oxidation ； Activation ； Characterization ； Isosteric heat of adsorption ； Sulfur hexafluoride.

參考文獻

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