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纖維素在炭化和活化過程中的結構變化

Structural evolution of cellulose during carbonization and activation

  • 摘要: 以纖維素為原料,通過在氮氣氛下炭化和水蒸氣活化得到纖維素基炭。采用熱分析、傅里葉紅外光譜、X射線衍射及低溫N2吸附測試手段研究了纖維素的炭化和活化過程以及過程中炭微晶結構和比表面積的變化。纖維素分子結構中的C-OH、C-O-C、C-H等基團在280~380℃之間大量分解,380℃后少量裂解產生的小分子碎片或基團持續分解,同時碳元素發生結構重排,形成石墨微晶。炭化溫度是影響纖維素基活性炭微晶結構及孔結構的關鍵因素,隨炭化溫度的升高,石墨微晶尺寸變大,孔結構得到發育,但活性炭的比表面積則呈先增加后下降趨勢,當炭化溫度為600℃時所得活性炭比表面積最大;炭化時間對炭微晶結構及比表面積的影響不顯著;隨著活化時間的延長,先是炭結構中的非微晶碳被氧化,比表面積及總孔容積變大,然后微晶碳被氧化,微晶結構被破壞,炭中部分微孔變成中孔或大孔,導致比表面積及總孔容積變小,當微晶間的非微晶碳被充分氧化而又不破壞原微晶結構時得到的炭孔隙最豐富。

     

    Abstract: Carbon samples were prepared from cellulose by carbonization under the nitrogen atmosphere and water steam activa-tion. Their structure and specific surface area during carbonation and activation processes were studied by thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, and nitrogen adsorption at low temperature. The results show that groups in the cellulose molecular structure like C-OH, C-O-C and C-H are mostly pyrolyzed completely between 280-380℃. A few fragments or surface groups produced during pyrolysis decompose continuously above 380℃. Meanwhile, carbon atoms rearrange within the solid sample and form graphite crystallites. Carbonization temperature exerts a crucial influence on the microcrystalline carbon structure and pore structure. With the rise of carbonation temperature, the size of graphite crystallites increases and the pore structure develops, but the specific surface area of the carbon prepared first increases and then decreases, reaching maximum at 600℃. Carbonization time has less significant influence on the structures. With increasing activation time, non-crystalline carbon is oxidized, the specific surface area and total pore volume of the carbon sample increase simultaneously. However, a longer activation time causes that the original crystalline carbon structure is destroyed, the specific surface area and total pore volume of the carbon sample decrease. The porosity is mostly abundant when non-crystalline carbon is fully oxidized and the original crystalline carbon structure is not destroyed.

     

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