玉米穗軸以KOH化學活化法製備高表面積活性碳及其應用

Abstract

本文以KOH活化法和KOH伴隨CO2複合活化法兩種方法，製備高表面積之玉米穗軸活性碳。物理特性是以BET表面積儀和SEM量測活性碳之比表面積、孔隙容積、孔徑分佈和外觀之變化。觀察活性碳之物理與化學性質變化，發現以KOH活化法KOH/char值在0.5至2範圍，反應是以物理性熱裂解與KOH蝕刻作用並存；KOH/char值在3至6範圍，僅存在化學性KOH蝕刻作用，當KOH/char為6之BET比表面積可達2595m2/g。KOH伴隨CO2複合活化法在KOH/char為1和4，探討活性碳之性質與CO2氣化作用時間之關係，瞭解在低KOH/char值時CO2氣化作用對比表面積和中孔容積比皆有顯著的增加，當KOH/char為4和CO2氣化30分鐘獲得最高表面積2844m2/g。利用元素分析和程溫脫附量測活性碳之化學組成與表面官能基變化，佐證上述反應機構之存在。以循環伏安法量測活性碳電極，比電容與KOH用量之關係呈現 高原型態，其中以KOH/char值為3活性碳之比電容 127 F/g (0.5M H2SO4) 最高，另由表面電阻之變化，瞭解KOH活化法發展高表面積活性碳時，導致碳導電性不連續，影響在電化學之應用。活性碳在溶液中等溫平衡吸附染料和酚類，以Langmuir和Freundlich兩種等溫式解析，計算吸附質在活性碳表面之覆蓋率和單位表面積之貢獻度；以吸附動力學模式探討孔隙質量傳遞，利用t50方法推算顆粒之有效擴散係數，瞭解本文活性碳之吸附性能，並藉此說明，在各種新穎科技用途之潛力。

Two methods, KOH activation and a combination KOH plus CO2 activation, were employed to prepare of corncob derived activated carbon with high surface area in this study. Physical properties of activated carbon including specific surface area, pore volume, pore size distribution, and morphologies were evaluated with the BET surface area analysis and SEM observation. Through the investigations on the variations of physical and chemical properties of activated carbon, as the KOH/char ratios from 0.5 to 2, both physical pyrolysis and KOH etching coexisted during activation; as the KOH/char ratios from 3 to 6, only chemical KOH etching existed. At the KOH/char ratio equal to 6, BET surface area reached 2595 m2/g. The relationships between activated carbon characteristics and CO2 gasification time were investigated at the KOH/char ratios of 1 and 4 for studying the effect of the combined activation process. At the low KOH/char ratio, long CO2 gasification time significantly increased the specific surface area and mesopores volume ratio. At KOH/char equal to 4 and CO2 gasification time equal to 30 minutes, the highest surface area of 2844 m2/g was obtained. Elemental analysis and temperature programmed desorption were employed to measure the variations of chemical properties and functional groups of activated carbon. The results supported the above proposals. Cyclic voltammetry was employed to measure the carbon electrode. Capacitive-like curves with a maximum specific capacitance of 127 F/g (0.5 M H2SO4) was obtained at KOH/char ratio equal to 3. Furthermore, from the variation of surface resistivity, KOH activation for preparing the high surface area activated carbon caused discontinuities of electrical conductivity in carbon, affecting the applicability in electrochemical field. Both Langmuir and Freundlich equations were employed to analyze the adsorption isotherms of dyes and chlorinated phenols in solutions on activated carbons. Adsorbate coverage on activated carbon surface and unit area contribution was calculated. Adsorption kinetics was employed to investigate mass transfer within pores. Method of t50 was employed to calculate the effective diffusion coefficients of particles to understand the adsorption performance of the activated carbons studied in this work, and explained the application potential in various fields of new technology.