# Carbon Dioxide Adsorption on Carbon Nanofibers with Different Porous Structures
## 3 Results and Discussion
### CO\({}_{2}\) Adsorption Breakthrough Curves  
Figure 9 shows the adsorption breakthrough curves of CO\({}_{2}\) at different temperatures on the samples. The CO\({}_{2}\) concentration was 15%. The [PERSON] and [PERSON] equation (Equation (2)) was adopted for breakthrough data fitting, which could predict the whole breakthrough curve. The fitting results are listed in Table 4 and also plotted in Figure 9. The values of R-squared suggest that the [PERSON] and Nelson equation fitted the data well in the whole range. The adsorption breakthrough wave was not sharp for PANf (at 25 \({}^{\circ}\)C) and PANh15 (at 25 and 40 \({}^{\circ}\)C), which could account for the larger pore diffusion resistance [50] due to smaller nanofiber sizes or a large micropore volume. This feature was in relation to the values of the rate coefficient, k'. The rate coefficients in this work did not exhibit large variations. Moreover, the highest rate coefficient took place on the curves at 40 \({}^{\circ}\)C except  
Figure 8: The isosteric heat of adsorption of CO\({}_{2}\) on the samples.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**adsorbent** & **Precursor** & **SSA (m\({}^{2}\)/g)** & **Conc. of CO\({}_{2}\)** & **Temp. (\({}^{\circ}\)C)** & **CO\({}_{2}\) Uptake** & **Reference** \\ \hline Hollow ACNFs \({}^{\alpha}\) & PAN & 884 & 1 atm & 25 & 3.16 & This study \\  & & & 0.15 atm & 25 & 0.94 & This study \\ ACNFs & PAN & 872 & 1 atm & 25 & 3.47 & [44] \\  & & & 0.15 atm & 1.01 & [44] \\ CNFs \({}^{\beta}\) & PAN & 306 & 1 atm & 25 & 2.52 & [44] \\  & & & 0.15 atm & 1.11 & [44] \\  & & & 1 bar & 2.25 & 2.25 & [45] \\  & PAN & 486 & 0.15 bar & 25 & 1.09 & [45] \\ CNFs & PAN & 12 & 1 bar & 25 & 0.55 & [46] \\  & PAN, polyvinylidene & 925 & 1 bar & 25 & 2.21 & [46] \\  & fluoride & & & & & \\  & PAN & 412 & 1 bar & 25 & 0.92 & [47] \\  & PAN & 547 & 1 bar & 25 & 1.44 & [47] \\  & Melamine & & & & & \\  & PAN & 966 & 1 bar & 25 & 2.9 & [48] \\  & & & 1 atm & 25 & 3.17 & [49] \\  & PAN & 897 & 0.15 atm & 1.00 & [49] \\  & ACFs \({}^{\gamma}\) & PAN & 966 & 1 atm & 25 & 2.33 & [41] \\ \hline \hline \end{tabular}  
* \({}^{\alpha}\) ACNFs: activated carbon nanofibers; \({}^{\beta}\) CNFs: carbon nanofibers; \({}^{\gamma}\) ACFs: activated carbon fibers.  
\end{table}
Table 3: Comparisons of CO\({}_{2}\) uptakes on the adsorbents in this study with various support materials using electrospinningning in the literature.  
for PANh15, which implies that the pore diffusion resistance was weak at 40 \({}^{\circ}\)C. In order to determine the regeneration of these samples, PANh15 was selected as an example to carry out the adsorption-desorption runs 10 times. Successive cyclic adsorption breakthrough curves and the variation of time at C/C0 = 0.1 and 0.5 are shown in Figure 10. The data demonstrates that after 10 adsorption-desorption cyclic tests, this adsorbent maintained its CO2 adsorption capacities without appreciable degradation.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Temp. (\({}^{\circ}\)C)** & **\(\tau\) (min)** & **K\({}^{\prime}\) (I/min)** & **R\({}^{2}\)** \\ \hline \multirow{3}{*}{PANf} & 25 & 16.40 & 0.553 & 0.99009 \\  & 40 & 14.07 & 0.904 & 0.99453 \\  & 55 & 10.52 & 0.655 & 0.99294 \\ \hline \multirow{3}{*}{PANh15} & 25 & 13.01 & 0.547 & 0.99352 \\  & 40 & 11.04 & 0.515 & 0.99093 \\  & 55 & 8.87 & 0.779 & 0.99147 \\ \hline \multirow{3}{*}{PANh12} & 25 & 13.17 & 0.895 & 0.99363 \\  & 40 & 10.54 & 0.918 & 0.99206 \\  & 55 & 8.14 & 0.827 & 0.98918 \\ \hline \multirow{3}{*}{PANp} & 25 & 19.65 & 0.977 & 0.99107 \\  & 40 & 17.34 & 1.057 & 0.99462 \\ \cline{1-1}  & 55 & 14.80 & 0.980 & 0.98692 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fitted parameters of the Yoon and Nelson equation for the CO2 adsorption breakthrough data on the samples at different temperatures.  
Figure 9: Adsorption breakthrough curves of CO2 at different temperatures on the samples: (**a**) PANf; (**b**) PANh15; (**c**) PANh12; (**d**) PANp. The lines are the fitted results using the Yoon and Nelson equation.