奈米碳管合成技術及其污染防治應用

Synthesis and Environmental Application of Carbon Nanotube

李鑫濃(Shin-Nung Li)

指導教授 ： 林錕松

元智大學/工程學院/化學工程與材料科學學系/碩士(2004年)

摘要

摘 要
由於奈米碳管(CNTs)之特殊結構，使其具備特異的物理化學性質，尤其在吸附特性應用於污染防治方面，如水中重金屬污染物去除，深具研究及實用的潛力。因此，本研究之主要目的為探討最佳多壁CNTs(MWCNTs)合成方法、結構特性鑑定及其在廢水重金屬鉛污染物之吸附去除。實驗是以溶劑熱法配合使用鉀及鈉金屬為還原劑來合成MWCNTs，經場發掃描式電子顯微鏡(FE-SEM)及穿透式電子顯微鏡(TEM)測試結果為以鉀為還原劑所合成之K-MWCNTs管徑約為30~100 nm，管壁為多層石墨化結構，較佳合成條件為350℃反應12 hr；以鈉為還原劑所合成之Na-MWCNTs管徑約為20~60 nm，管壁為非晶質碳(amorphous carbon)所構成，較佳合成條件為250℃反應20 hr，並可得到碳雜質較少且純度較高之MWCNTs。並以X射線能量散佈分析儀(EDS)、X光粉末繞射儀(XRPD)、熱重量分析儀(TGA)、拉曼光譜(Raman spectroscopy)、延伸細微結構X光吸收光譜(EXAFS)及X光吸收邊緣結構光譜(XANES)來驗證溶劑熱法合成MWCNTs之反應機構及分析其碳管結構之結晶性。拉曼光譜分析結果顯示K-MWCNTs之G-band強度大於Na-MWCNTs，而D-band的強度大小則相反，顯示K-MWCNTs石墨結晶性較強。TGA分析結果顯示K-MWCNTs之氧化分解溫度為220~400℃，Na-MWCNTs之氧化分解溫度為220~370℃。XRPD分析結果顯示以溶劑熱法合成之MWCNTs結構結晶性與電弧放電法(arc discharge)或噴霧熱解法(spray pyrolysis)法相異。
為了探討MWCNTs經改質處理後對水中鉛離子吸附效果之差異，選擇arc discharge(AD)或spray pyrolysis(SP)合成之MWCNTs及K-MWCNTs或Na-MWCNTs進行比較，並對以上不同製程方法之MWCNTs進行HNO3、KMnO4、Na2S及3:1 concentrated H2SO4/HNO3等改質處理，進一步處理50 mg/L的Pb2+溶液以探討以上處理對MWCNTs於Pb2+吸附能力之提升效果。並以原子吸收光譜儀(AAS)及紅外線光譜(FTIR)檢測MWCNTs對Pb2+吸附之效果及改質處理後碳管表面結構之變化。由實驗結果得知經處理後AD-MWCNTs吸附Pb2+效果依序為3:1 concentrated H2SO4/HNO3 >KMnO4 >HNO3 >Na2S；SP-MWCNTs、K-MWCNTs及Na-MWCNTs吸附Pb2+提昇效果依序皆為KMnO4 >HNO3 >Na2S；而經Na2S及HNO3處理後對Pb2+的吸附效果依序為Na-MWCNTs > K-MWCNTs > SP-MWCNTs > AD-MWCNTs，經KMnO4處理後對Pb2+的吸附效果依序為Na-MWCNTs >K-MWCNTs >AD-MWCNTs > SP-MWCNTs。FTIR分析結果顯示，K-MWCNTs及Na-MWCNTs經HNO3處理後，碳管表面缺陷結構產生-COOH官能基，而K-MWCNTs的表面缺陷結構亦產生-NO官能基。由XANES及EXAFS光譜分析結果顯示MWCNTs合成反應中之Co/Ni前趨物為零價金屬，鈷(Co-Co)鍵長為2.49 Å ± 0.05，配位數為2.92 ± 0.05，鎳(Ni-Ni)鍵長為2.48 ± 0.05 Å，配位數為5.97 ± 0.05，顯示MWCNTs可能以奈米Co/Ni金屬為前導物，包覆上頂式地向前線性成長，形狀可能為彎曲、單開或雙開方式存在。

關鍵字

奈米碳管 ； 溶劑熱法 ； 鉛離子 ； 吸附 ； 表面改質 ； 拉曼光譜 ； 延伸細微結構X光吸收光譜 ； X光吸收邊緣結構光譜

並列摘要

ABSTRACT
Carbon nanotubes (CNTs) have attracted increasing attention because of their unique structural, mechanical, and electronic properties. Surface chemistry modifications are also useful and critical to manipulate the adsorptive properties of CNTs and develop their potential of environmental application. Therefore, the main objectives of this study were to investigate the optimal synthesis methods or characteristics identification of multiwall CNTs (MWCNTs) and the environmental application of lead ions adsorptive removal by CNTs. Experimentally, the MWCNTs were produced from the catalytic-assembly benzene-thermal routes to MWCNTs by using reduction of hexachlorobenzene by metallic K or Na in the presence of Co/Ni catalyst precursors at moderate temperatures of 503-623 K. The MWCNTs of well-graphited walls were obtained with reductive K metals of catalytic hexachlorobenzene-thermal routes at 623 K for 12 hrs. Similarly, the amorphous MWCNTs with fewer impurities were also formed from the reductive Na metals of hexachlorobenzene-thermal catalytic pathways at lower temperature of 523 K for 20 hrs. The diameters of K-MWCNTs and Na-MWCNTs ranged of 30-100 and 20-60 nm, respectively by using field-emission scanning microscopy (FE-SEM) and transmission electron microscopy (TEM) microphotos. In addition, the reaction mechanisms, fine structures, surface chemical modification or crystalline properties of MWCNTs were further identified by using energy dispersive spectrometer (EDS), X-ray powder diffractometer (XRPD), thermal gravimetric analyzer (TGA), X-ray absorption near edge structural (XANES) or extended X-ray absorption fine structural (EXAFS) spectroscopy. From Raman spectra of MWCNTs, the G-band peak of K-MWCNTs was more intensive than the one of Na-MWCNTs but for the D-band peak of MWCNTs oppositely. The decomposition temperatures of K- and Na-MWCNTs were 493-673 K and 493-643 K, respectively measured by TGA analyses. They both indicated that the K-MWCNTs had stronger well-graphited structures than Na-MWCNTs. Moreover, the optimal crystalline or surface modification properties of MWCNTs via solvothermal routes compared with arc discharge (AD) or spray pyrolysis (SP) method were observed by XRPD patterns of MWCNTs.
In order to more thoroughly examine the adsorptive efficiencies of 50-100 mg/L Pb2+ for different kinds of AD-MWCNTs, SP-MWCNTs, K-MWCNTs or Na-MWCNTs were used for comparison. The chemical modifications of MWCNTs surfaces for adsorption enhancement included HNO3, KMnO4, Na2S or 3:1 concentrated H2SO4/HNO3 processes. Moreover, the fine structures and the nature of the active species involved in Pb2+ adsorption, the analyses of atomic adsorption spectroscopy (AAS) and Fourier transform infrared spectroscopy (FTIR) were performed. Experimentally, the Pb2+ adsorptive efficiency of AD-MWCNTs was 3:1 concentrated H2SO4/HNO3 > KMnO4 > HNO3 > Na2S modification processes in series. Similarly, the Pb2+ adsorptive efficiencies of SP-MWCNTs, K-MWCNTs, and Na-MWCNTs were KMnO4 > HNO3 > Na2S modification processes in series. Moreover, according to the chemical modification of HNO3 and Na2S, the Pb2+ adsorptive efficiencies were Na-MWCNTs > K-MWCNTs > SP-MWCNTs > AD-MWCNTs and that of KMnO4 was Na-MWCNTs > K-MWCNTs > AD-MWCNTs > SP-MWCNTs, respectively. From FTIR spectra, the K- and Na-MWCNTs possessed COOH functional groups on the surface of defect structures during chemical oxidation of HNO3 and the —NO functional groups were observed on the K-MWCNTs surface of defect structures. By using XANES spectra, the Co/Ni catalyst precursors of the MWCNTs syntheses were all zero-valent Co or Ni species. The EXAFS data revealed that the nanophase Co(0) (or Ni(0)) particles had a central Co (or Ni) atom of a coordination number of 2.92 ± 0.05 (or 5.96 ± 0.05) primarily Co-Co with a bond distance of 2.49 ± 0.05 Å (or 2.48 ± 0.05 Å). However, these results might offer a further explanation of how the yield and structure of MWCNTs formed by catalytic-assembly hexachlorobenzene-solvothermal route depend on the species of the Co/Ni metal catalysts. They also might indicate that some MWCNTs having bending structure, and at the one- or two-open end of the tubes, the catalytic Co/Ni metal particles exist clearly, which might suggest that the Co/Ni metal particles are responsible for the nucleation and growth of the MWCNTs.

並列關鍵字

Carbon nanotube ； Solvothermal method ； Lead ion ； Adsorption ； Surface modification ； Raman spectroscopy ； EXAFS ； XANES

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