甲烷催化裂解法多壁碳纳米管的除杂提纯

Impurity removal of multi-walled carbon nanotubes from catalytic pyrolysis of methane

针对甲烷催化裂解法自制多壁碳纳米管(MWCNTs)中的催化剂残留(灰分)和碳杂质,研究常温HCl或NaOH溶液溶灰与高温空气氧化或H_(2)甲烷化碳杂质相结合的除杂效果。表征提纯前后MWCNTs样品的空气氧化热失重曲线,结合GB/T 36065-2018中观点和处理方法,对特征组分和关键温度进行量化,并在扫描电子显微镜下观察样品形貌变化。结果表明,常温HCl溶液(36.0%~38.0%)可溶解去除大部分Fe[O]/γ-Al_(2)O_(3)灰分,而常温NaOH溶液(2 mol·L^(-1))对较为活泼的还原态Fe/Al灰分有较好的去除效果;500℃和530℃空气氧化均无法选择性去除碳杂质,且会同步消耗较多MWCNTs,而660℃和730℃H2甲烷化能去除部分碳杂质,并还原氧化态灰分;730℃H_(2)搭配常温NaOH溶液处理可获得纯度较佳的MWCNTs产品;部分MWCNTs与杂质紧密结合/接触,除杂的同时其结构也会被破坏。常温液相除灰和高温气相除碳杂质的工艺和设备存在较大差异,无法连续化操作,且非目标杂质的存在往往会影响目标杂质的去除效果,因此有必要探索MWCNTs的“制备-提纯”一体化方法。

With respect to catalyst residues(ash)and carbon impurities in multi-walled carbon nanotubes(MWCNTs)prepared by catalytic pyrolysis of methane,impurity removal efficiency of the combination of dissolving ash in HCl or NaOH solution at room temperature with oxidation in air or carbon impurities methanation in H_(2) at high temperature was investigated.The air oxidation thermogravimetric(TG)curves of MWCNTs samples before and after purification were characterized,and characteristic components and key temperatures were quantitatively determined referring to the viewpoints and treatment methods in GB/T 36065-2018,and morphology of the samples was characterized using SEM.The results showed that HCl solution at room temperature(wHCl=36.0%~38.0%)dissolved and removed most of Fe[O]/γ-Al_(2)O_(3) ash,while NaOH solution(cNaOH=2 mol·L^(-1))at room temperature exhibited higher removal efficiency for more active Fe/Al ash in reduced state.Air oxidation at 500℃and 530℃can not selectively remove carbon impurities while more MWCNTs are consumed simultaneously.H_(2) methanation at 660℃and 730℃can remove some carbon impurities and reduce ash in oxidation state.MWCNTs with higher purity can be obtained by treatment with H_(2) at 730℃combined with NaOH solution at room temperature.Some MWCNTs are closely bound or in contact with impurities,leading to structures collapse while removing impurities.Since there is great difference in the process and equipment between liquid phase ash removal at room temperature and gas phase carbon impurities removal at high temperature,the two processes can not be operated continuously.Existence of non target impurities often affects the removal efficiency for target impurities.Therefore,it is necessary to seek for an integration of preparation and purification of MWCNTs.