Abstract
The CO2 adsorption selectivity of plain activated carbon nanofibers (ANF) is generally low. For enhancement, nitrogen functionalities favorable for CO2 adsorption are usually tethered to the ANF. In the current study, we adopted chemical impregnation using 0.5 wt% tetraethylenepentamine (TEPA) solution as an impregnant. To enhance the impregnation of TEPA further, preliminary oxidation of the nanofibers with 70% HNO3 was conducted. The effects of HNO3 and TEPA treatments on the modified ANFs were investigated for physical (using N2 monosorb, thermogravimetric analyzer, scanning electron microscopy) and chemical (X-ray photoelectron spectrometer) changes. From the results, we found that although TEPA impregnation reduced the specific surface area and pore volume of the ANFs (from 673.7 and 15.61 to 278.8 m2/g and 0.284 cm3/g, respectively), whereas the HNO3 pre-oxidation increased the number of carboxylic groups on the ANF. Upon TEPA loading, pyridinic nitrogen was tethered and further enhanced by pre-oxidation. The surface treatment cumulatively increased the amine content from 5.81% to 13.31%. Consequently, the final adsorption capacity for low (0.3%) and pure CO2 levels were enhanced from 0.20 and 1.89 to 0.33 and 2.96 mmol/g, respectively. Hence, the two-step pre-oxidation and TEPA treatments were efficient for improved CO2 affinity.
Highlights
Besides the highest contributor to anthropogenic global warming, CO2 could be harmful at relatively low concentrations, especially in confined indoor spaces, depending on whether they are, stationary or mobile environments [1, 2]
We observed that the SBET and Vtotal of activated carbon nanofibers (ANF) were enhanced by either activation time or the quantity of the activation reagent
We inferred that ANFs prepared by physical activation are more suitable for surface modification by SBET (m2/g)
Summary
Besides the highest contributor to anthropogenic global warming, CO2 could be harmful at relatively low concentrations, especially in confined indoor spaces, depending on whether they are, stationary (such as offices, homes, and subway stations) or mobile environments (such as cars, airplane, and submarines) [1, 2]. The results showed that if TEPA was used as a chemical impregnant for a solid support, it could improve CO2 adsorption of the support. TEPA is a linear molecule bearing five amine groups per molecule: two primary amines (RNH2) and three secondary amine (R2NH) groups. To achieve optimum wet impregnation, adequate knowledge of certain properties of the support such as the specific surface area, pore size, pore structure, and surface pH is essential. A porous support with a relatively high specific surface area and large pore volume has a good potential for organic amine immobilization [11]. All the ANFs were impregnated with TEPA solution. To improve the TEPA loading, some ANFs were pre-oxidized with nitric acid solution prior to impregnation
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