| Being expected in various applications such as catalysts or catalyst supports, selective adsorption agents, and energy storage devices such as electric double layer capacitor, ultrafine activated carbon fiber (UFACF) has been recognized as one of the promising materials based on its micro- and nano-structure and special properties. As a microporous carbonaceous material, it is developed from organic precursors by either physical activation or chemical activation. To obtain Polyacrylonitrile (PAN) based ultrafine activated carbon fibers (PAN based UFACFs), electrospun technique is used to produce PAN ultrafine fiber web. Then, the web was stabilized, carbonized and activated. In the present work it was concentrated on the influence of the conditions of pre-oxidation stabilization and chemical activation in the adsorption performance of the fibers.The initial step of the preparation of PAN based UFACFs is electrospinning. The morphology of the ultrafine fibers was analyzed by SEM, and the effect of the parameter, such as the concentration of Polyacrylonitrile /Dimethylfomamide (PAN/DMF) solution, spinning voltage and needle-collector distance, on fiber diameter was discussed. The results show that all the parameters affect fiber diameter and distribution. The fiber webs achieve the desired morphology when the solution concentration was 12%, voltage 25 kV, and the distance 180 mm.Based on the above result, the electrospun PAN ultrafine fiber was stabilized and chemically activated. Through the change of pre-oxidation temperature, heating rate and time, the effects of pre-oxidation conditions on the structure of pre-oxidized fibers and the adsorption performance of the UFACFs have been studied. The results indicate that with the improvement of temperature, heating rate and time, more linear PAN molecules are converted to thermoduric ladder-shaped structure, which hinders the formation of surface functional groups and micropore structure. Therefore, the moderate pre-oxidation condition is necessary to prepare UFACF with better adsorption capacity. In the present work it is demonstrated that the optimal oxidation conditions are the temperature increased from room temperature to 230℃with a heating rate of 0.75℃/min, held then at the peak temperature for 30 min. The electrospun PAN pre-oxidized fiber webs were soaked in KOH, ZnCl2, H3PO4, KH2PO4 solution with the same impregnation ration for 12 h. The adsorption properties of the UFACFs prepared under the same activation conditions were characterized by nitrogen adsorption isotherm. The results show that the UFACFs activated by KOH have the most developed capillary structure and the Brunauer/Emmett/Teller (BET) surface area reaches 2256.59 m2/g. The samples activated by KH2PO4and ZnCl2have abundant micropore structure. However, KH2PO4 produced residual substance left on the fibers'surface during the activation reaction. Under the given conditions, H3PO4 obtained the least desired catalysis action during the activation, so that the fibers develop less pore structure. Therefore, KOH is the most appropriate chemical activation agent.Finally, the relationships between the activation conditions and the samples' adsorption properties are discussed. Through Taguchi experimental design method, the experimental results demonstrate the influence of different factors on the yield, the BET surface area and the adsorption of methylthionine chloride, and the degrees of the activation factors affecting them: impregnation ration> activation temperature> activation time. Considering cost and the adsorption properties of UFACFs, the optimal activation conditions are that impregnation ration is 5, activation temperature is 800℃, and the activation time is 30 min. |
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