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A Study on the Adsorption Properties of Benzene Vapor onto Multi-walled Carbon Nanotubes
Lee, Meng Shan
|關鍵字:||Multi-walled carbon nanotubes;多壁奈米碳管;adsorption;benzene;adsorption model;吸附;苯;吸附模式||出版社:||環境工程學系所||摘要:||
經過氫氧化鈉活化預處理與高溫熱處理後之多壁奈米碳管，因去除金屬觸媒及非奈米碳管之雜質碳等，使得多壁奈米碳管之物化特性穩定並且吸附苯蒸氣之效能亦被提昇。於奈米碳管表面特性與吸附效能之研究中，顯示微孔表面積與微孔孔洞體積與吸附效能呈現正相關，進一步證實苯分子吸附反應之主要位置。於苯蒸氣之進流濃度為50至800ppmv範圍中，環境溫度及濕度分別為25℃及0%時，發現其等溫吸附曲線呈現第一類型(type I)吸附曲線，由此可說明奈米碳管吸附苯蒸氣之行為為單層吸附且奈米碳管為微孔型吸附劑。同時，使用BET、Langmuir及Freundlich等等溫吸附模式模擬之結果，發現BET吸附模式較適用於描述多壁奈米碳管吸附苯蒸氣之行為，而經純化改質後之多壁奈米碳管其最大吸附容量為65.24 mg/g，大於未經改質之多壁奈米碳管(59.50mg/g)。在環境因子對吸附行為之影響研究中，苯蒸氣於多壁奈米碳管之吸附量隨著溫度之增加而呈現線性遞減，然而，當氣流中之相對濕度增加時，與吸附量呈現多項式關係遞減之現象。苯蒸氣於奈米碳管之等容量吸附熱皆為負值且大約為苯蒸發熱之1至4倍，其結果顯示奈米碳管吸附苯蒸氣之反應為物理性放熱反應。
Vapor adsorption is an important process for environmental protections. The unique microporous structure of carbon nanotubes (CNTs) is speculated to have remarkable adsorption properties. Therefore, commercially available multi- walled carbon nanotubes (MWCNTs, external diameter <10 nm) were employed as adsorbents for benzene vapor adsorption from air stream in an inlet concentration range of 50-800 ppmv, and the environmental conditions were controlled at 25℃ and 0% relative humidity. This study is part of an attempt to understand vapor adsorption by systematically studying the influence of adsorbent, adsorbate, temperature, and relative humidity on the adsorption equilibrium.
A purification process, activating (concentrated sodium hydroxide) and then heating treatments, was used to improve the properties of MWCNTs such as purity, structure, and nature of the surface. These improvements made MWCNTs become more stable and suitable for adsorption of benzene vapor. The isotherm on both raw and purified MWCNTs were generalized as typical type I adsorption isotherm, which describes monolayer adsorption mechanisms exhibited by microporous adsorbents. The adsorption equilibrium data were found to be best correlated with the BET isotherm model. The maximum adsorption amount of benzene adsorbed at equilibrium calculated by BET isotherm were found to be 65.24 mg/g for purified MWCNTs, and it was greater than that of raw MWCNTs(59.50 mg/g). In the temperature range of 5 to 45 oC, the adsorption capacity linearly decreased with a rise in temperature. Negative values of isosteric heats of adsorption for both MWCNTs were presented as 1-4 times to the heats of vaporization, which is typical exothermic physical adsorption of organic vapors on porous carbons. Meanwhile, a polynomial decrease in the adsorption capacity was observed between 0 to 91% relative humidity.
A numerical model using basic dynamic reaction theory for simulating the kinetics of adsorption in column operations was also conducted. Results showed that the model could well predict the adsorption isotherms and removal performances as well under various conditions.
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