# Carbon Spheres as CO\({}_{2}\) Sorbents
## 3 Results and Discussion
### Adsorption Studies  
According to the low-temperature nitrogen adsorption-desorption studies, for samples modified with oxalate only, the increase of carbonization temperature resulted in a higher volume of adsorbed nitrogen. The opposite effect was observed for samples modified with EDA only. The addition of both modificators gave a similar result to the use of oxalate only. Some examples of low-temperature nitrogen adsorption isotherms are shown in Figure 7.  
The isotherms are of type Ia [35], characteristic for microporous materials, however a slight increase of adsorbed nitrogen volume at the highest P/P\({}_{0}\) can be attributed to the presence of macropores (type II). Spheres, modified with EDA only, adsorbed the lowest nitrogen volume. Modification with potassium oxalate resulted in higher nitrogen adsorption, slightly increasing with dopant concentration. The highest amount of nitrogen was adsorbed in the sample modified with both potassium oxalate and EDA. Physico-chemical properties of the samples were measured, and the results are shown in Table 4. In almost all cases, except samples modified with EDA only, an increase in carbonization temperature resulted in an increase of the samples' density, specific surface area, total pore volume, and CO \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} **Start of Mass Loss** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \
\begin{tabular}{c} **Max of Mass Loss** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} &
\begin{tabular}{c} **End of Mass Loss** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & **Resolute (\%)** \\ \hline RF 700 & 364 & 600 & 712 & 7.03 \\ RF 7/1 600 & 181 & 420 & 547 & 16.71 \\ RF 7/1 700 & 181 & 420 & 547 & 20 \\ RF 7/1 800 & 181 & 432 & 573 & 14.53 \\ RF + EDA 700 & 364 & 589 & 728 & 8.6 \\ RF 7/1 + EDA 700 & 149 & 391 & 519 & 21 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of the thermogravimetric studies. Figure 7: Low-temperature nitrogen adsorption isotherms of carbon spheres modified with potassium oxalate and/or with EDA. adsorption. An unusual increase in density, simultaneously with an increase in surface area and porosity can be explained by the decomposition of modificators and removal of gaseous decomposition products. The same phenomenon was reported for activated carbon produced from palm shell and modified with potassium carbonate [36] or phosphoric acid [37]. An extremely high increase in specific surface area and CO\({}_{2}\) adsorption was observed for the samples modified with both potassium oxalate and EDA. In contrast, samples without potassium oxalate carbonized at higher temperatures did not exhibit larger surface area, and a lower amount of CO\({}_{2}\) was adsorbed (because of a lower microporosity). However, a higher concentration of the activator did not improve the specific surface area. Due to the higher saturation of the mixture, bigger spheres were formed, but oxalate moieties were not well dispersed within the volume of the sample (as shown before in SEM images). For adsorption of carbon dioxide, the presence of the micropores below 1 nm is considered to be most important, and the pore size distribution in this area was calculated from CO\({}_{2}\) adsorption at 0 \({}^{\circ}\)C and is shown in Figure 8.