## 3 Results and Discussion  
### Sample Characterisation  
Table 1 presents the results obtained from adsorption isotherms of nitrogen to 77 K. Surface area was calculated using the M.B.E.T. model; Figure 1 shows the optimization of the fit of the adsorption data to the BET model; here, it is possible to see that the optimal range is between 3.08 \(\times\) 10\({}^{-3}\) and 4.8 \(\times\) 10\({}^{-2}\) of P/P\({}^{0}\); this range is different to the typical range between 0.05 and 0.35 used for calculate the BET area, and the adjustment permit to obtain positive values for the C parameter related to adsorption energy [12]. The micropore volume was obtained with (DR) and (DA) models [13]; the mesopore volume was obtained by applying the BJH model; the pore radius and the total volume of pore V\({}_{\rm T}\) were calculated by applying Density Functional Theory (DFT). Table 1 shows that the activated carbons presented pore radius lower than 0.20 nm, which indicates that these materials are microporous according to the classification of IUPAC [14]. It is also noted that the different impregnations modified the surface area of the original activated carbon G. The most significant changes were seen when the activated carbon was impregnated with nitric acid (sample GN) and when it was carbonised at 1073 K (sample GC\({}_{1073}\)). The treatment with nitric acid at 291 K generated a decrease in the surface area of around 54%; different studies show that the impregnation with this agent produces oxidation on the surface of the activated carbon accompanied by  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{ \begin{tabular}{c} **SBET** \\ **(m\({}^{2}\)-g\({}^{-1}\))** \\ \end{tabular}  } & \multirow{2}{*}{**C**} & \multicolumn{2}{c}{**DR method**} & \multicolumn{2}{c}{**DA-Method**} & \multicolumn{2}{c}{**BJH**} & \multicolumn{2}{c}{**QSDFT [1]**} \\ \cline{3-10}  & & & **Vmicro** & **Eo** & **Best E** & **Vmicro** & **Vmacro** & **Pree Radius** & **VTr** \\  & & & **(m\({}^{2}\)g\({}^{-1}\))** & **k\({}_{\text{J}}\)-mol\({}^{-1}\)** & **j\({}_{\text{J}}\)-mol\({}^{-1}\)** & **j**-mol\({}^{-1}\)** & **j**-mol\({}^{-1}\)** & **(cm\({}^{2}\)g\({}^{-1}\))** & **(Å)** & **(cm\({}^{2}\)g\({}^{-1}\))** \\ \hline G & 864 & 1729 & 0.34 & 25.8 & 8.3 & 2.1 & 0.33 & 0.02 & 6.0 & 0.35 \\ GN & 469 & 1149 & 0.18 & 22.2 & 7.4 & 2.1 & 0.18 & 0.03 & 10.1 & 0.21 \\ GNA & 786 & 1519 & 0.30 & 26.8 & 8.6 & 2.1 & 0.30 & 0.03 & 7.6 & 0.33 \\ GP & 863 & 1287 & 0.33 & 24.1 & 7.7 & 2.2 & 0.33 & 0.04 & 8.0 & 0.37 \\ GC\({}_{1073}\) & 1113 & 1073 & 0.43 & 24.6 & 7.7 & 2.1 & 0.42 & 0.07 & 6.8 & 0.49 \\ GC\({}_{1173}\) & 813 & 1130 & 0.32 & 25.0 & 8.0 & 2.2 & 0.30 & 0.03 & 8.0 & 0.34 \\ GC\({}_{1273}\)a decrease of surface area and microporosity, and an increase of oxygen containing functional groups, particularly carboxylic groups [15]. When the pre-oxidised activated carbon GN is modified by reflux with ammonium hydroxide solution (sample GNA), it is observed that the area and the microporosity increase again, which suggests that there is an accumulation of nitric acid trapped in the micropores in the GN sample, which might mean that during the nitrogen adsorption, this cannot access all surfaces and would interfere with nitrogen molecules' homogeneous coverage, and consequently with area and micropore volume calculations. When acid groups are modified through the formation of amides, amines, imides, and pyrrol and quaternary nitrogen groups [16; 17], by reflux with ammonia solution (sample GNA), this favours the adsorption of nitrogen more evenly over the surface. Regarding activated carbon heat treatment to 1073 K, the surface area increases to 28%; this indicates that the carbonisation process at this temperature allows access to new microporosity. This can be seen in Table 1 by the values obtained for this parameter.