*3.1. Characterization*

The results of the proximate analysis are compiled in Table 1. It can be seen that Stipa Tenacissima leaves (STLs) contain 62.81% volatile matter, 24.50% fixed carbon, and 1.19% ash. This composition follows the general trend of a typical biomass composition [34,52–54]. The high volatile matter and low ash content of biomass resources make them good starting materials for preparing activated carbons [55].


**Table 1.** Proximate analysis of Stipa tenacissima leaves (STLs).

Table 2 summarizes the elemental composition of the precursor and activated carbons prepared from STLs at different activation temperatures and impregnation ratios. The elemental composition, H/C, and O/C atomic ratios results indicate remarkable chemical changes in the surface after the activation process, while no sulfur (S) traces were detected for all the samples.

**Table 2.** Elemental analysis of the precursor and activated carbons produced at different activation temperatures and impregnation ratios (wt.%).


(\*): by difference.

The results demonstrated that carbon is the major constituent of the obtained ACs confirming the carbonaceous nature of the materials [55]. An increase in the carbon content from 47.74 wt.% for raw STLs to more than 70 wt.% could be observed in all the activated carbons with increasing activation temperature. As for the impregnation ratio of 1, the carbon content in activated carbons increased from 76.46 to 91.04 wt.% with increasing temperatures from 400 to 600 ◦C, which could be attributed to the increasing release of volatile matter. On another hand, hydrogen and oxygen content highly decreased, respectively, from 2.43 to 1.57 wt.%, and 18.56 to 6.99 wt.%, mainly as a result of the cleavage and breakage of bonds within the ACs structure that occurs during the activation process [56]. Moreover, the progressive decrease in the H/C and O/C atomic fractions (see Figure 1) observed for the obtained activated carbons with the different activation conditions, is indicative of the carbonization and activation processes. The results demonstrated that carbon is the major constituent of the obtained ACs confirming the carbonaceous nature of the materials [55]. An increase in the carbon content from 47.74 wt.% for raw STLs to more than 70 wt.% could be observed in all the activated carbons with increasing activation temperature. As for the impregnation ratio of 1, the carbon content in activated carbons increased from 76.46 to 91.04 wt.% with increasing temperatures from 400 to 600 °C, which could be attributed to the increasing release of volatile matter. On another hand, hydrogen and oxygen content highly decreased, respectively, from 2.43 to 1.57 wt.%, and 18.56 to 6.99 wt.%, mainly as a result of the cleavage and breakage of bonds within the ACs structure that occurs during the activation process [56]. Moreover, the progressive decrease in the H/C and O/C atomic fractions (see Figure 1) observed for the obtained activated carbons with the different activation conditions, is indicative of the carbonization and activation processes.

R1-500 0.50 82.60 1.90 15.00 18.16 2.30 R1-600 0.40 91.00 1.60 7.00 7.69 1.76 R2-400 2.30 76.20 2.00 19.50 25.59 2.62 R2-450 0.40 78.40 1.70 19.50 24.87 2.17 R2-500 0.40 79.10 1.40 19.10 24.15 1.77 R2-600 0.30 81.60 1.40 16.70 20.47 1.72 R3-400 1.50 70.30 1.80 26.40 37.55 2.56 R3-450 0.20 72.00 1.50 26.30 36.53 2.08 R3-500 0.20 75.20 1.30 23.30 30.98 1.73 R3-600 0.20 78.30 1.30 20.20 25.80 1.66

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(\*): by difference.

**Figure 1.** Effect of impregnation ratio and activation temperature on H/C and O/C fractions (H3PO<sup>4</sup> concentration: wt. 85%; flow N2 100 mL/min; activation duration: 1 h). **Figure 1.** Effect of impregnation ratio and activation temperature on H/C and O/C fractions (H3PO<sup>4</sup> concentration: wt. 85%; flow N<sup>2</sup> 100 mL/min; activation duration: 1 h).

Indeed, during the activation process, polymeric structures decompose and liberate most of the non-carbon elements, mainly hydrogen, oxygen, and nitrogen in the form of liquid and gases, leaving behind a rigid carbon material with a short-range order [57,58]. Indeed, during the activation process, polymeric structures decompose and liberate most of the non-carbon elements, mainly hydrogen, oxygen, and nitrogen in the form of liquid and gases, leaving behind a rigid carbon material with a short-range order [57,58].

Furthermore, as the impregnation ratio increases, carbon, and hydrogen contents decay, whereas the oxygen content increases from 15.53 wt.% for R1-500 to 34.80 wt.% for R3-500. This increase can be due to the progressive incorporation of phosphorus species with increasing the impregnation ratio. Furthermore, as the impregnation ratio increases, carbon, and hydrogen contents decay, whereas the oxygen content increases from 15.53 wt.% for R1-500 to 34.80 wt.% for R3-500. This increase can be due to the progressive incorporation of phosphorus species with increasing the impregnation ratio.

Figure 2a–c, respectively, show the N<sup>2</sup> adsorption–desorption isotherms at −196 ◦C of the prepared activated carbons from STLs with different impregnation ratios and at different activation temperatures. Figure 2a revealed that the isotherms of samples prepared with an impregnation ratio of (1:1) at different activation temperatures are of type I (b) based on IUPAC classification [59], showing a significant increase in the adsorption at low P/P<sup>0</sup> values, with barely defined knee, and long plateau which extends to P/P<sup>0</sup> ≈ 1.0. This is indicative of the presence of large micropores and mesopores. In addition, an absence of hysteresis suggests that the obtained activated carbons contained mostly micropores with only a small contribution of mesopores.

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cropores with only a small contribution of mesopores.

Figure 2a,b, and c, respectively, show the N2 adsorption–desorption isotherms at −196 °C of the prepared activated carbons from STLs with different impregnation ratios and at different activation temperatures. Figure 2a revealed that the isotherms of samples prepared with an impregnation ratio of (1:1) at different activation temperatures are of type I (b) based on IUPAC classification [59], showing a significant increase in the adsorption at low P/P0 values, with barely defined knee, and long plateau which extends to P/P<sup>0</sup> ≈ 1.0. This is indicative of the presence of large micropores and mesopores. In addition, an absence of hysteresis suggests that the obtained activated carbons contained mostly mi-

**Figure 2.** Adsorption–desorption isotherms of N<sup>2</sup> at −196 ◦C (**a**–**c**); and micropore size distribution (**d**–**f**) of activated carbons from STLs at different impregnation ratios and activation temperatures.

Figure 2b represents the adsorption isotherms of samples prepared with an impregnation ratio of (2:1) at different activation temperatures. The activated carbon obtained at 400 ◦C provides isotherm type I(b) which is typical of microporous materials where micropore filling may take place by primary filling at very low relative pressure. The activated carbons obtained at higher temperatures, exhibit a combination of type I and type IV(a) isotherms [59]. This indicates the presence of micro and mesoporosity leading to a gradual increase in adsorption after the initial filling of the micropores. The isotherms exhibit type H4 hysteresis, typical for slit-shaped pores.

For the impregnation ratio of (3:1) (Figure 2c) the activated carbons prepared at 400 ◦C exhibit type I (b) and the activated carbons prepared at 450, 500, and 600 ◦C a combination of type I and type IV (a) isotherms [59], with the presence of a hysteresis loop type H4. A small hysteresis in the shape was observed in the R3-400 and R3-450 samples. It means that the mesopores are developed during activation with an increasing of impregnation ratio to 3. Additionally, a larger hysteresis loop was observed for R3-500 which suggests a higher contribution of mesopores in their porosity.

The effect of activation temperature and impregnation ratio on the BET surface area, total pores volume, micropores, mesopores volume, and average pore diameter are given in Table 3. The optimum result in terms of surface area (1503 m2/g) was obtained for an impregnation ratio of 2 as can be clearly seen in Table 4. The development of porosity goes through a maximum with the activation temperature, which is typically observed in phosphoric acid activation and is in agreement with our previous results [48]. It is known that phosphoric acid treatment accelerates structural alteration at low temperatures [60]. In fact, it has been reported [61] that at temperatures above 500 ◦C, the carbon structure shrinks, and the surface area decreases.


**Table 3.** Textural properties of the obtained activated carbons produced at different activation conditions.

SBET: BET specific surface area; VTotal: total pore volume; Vµp: micropore volume; Dp: average pore diameter.

Girgis et al. have explained that the acid introduced into the material plays a double role [62]: (i) it produces hydrolysis of the lignocellulosic material with subsequent partial extraction of some components, thus weakening the particle which swells, and (ii) the acid occupies a volume which inhibits the contraction of the particle during the heat treatment, thus leaving a porosity when it is extracted by washing after carbonization [62].

Additionally, Jagtoyen et al. have reported that the phosphoric acid combines with organic species forming phosphate and polyphosphate bridges that connect biopolymer fragments and partially hindering the contraction in materials when the temperature increases [63]. Above 450 ◦C, these bridges become thermally unstable, and their loss produces a contraction in the material, which will result in a decrease in porosity.

From this point of view, keeping the activation temperature at around 500 ◦C leads to better development of the adsorbent porosity. Several investigators have established that in the case of phosphoric acid activation of lignocellulosic material, temperatures neighboring 500 ◦C were suitable to obtain optimal properties of the activated carbons.

Impregnation ratio has been identified as one of the most important factors in the chemical activation process. With the increase in ratio from 1 to 3, the surface area and, mainly, total pore volumes also increased. The growth in porosity was attributed to the release of tars from the cross-linked framework generated by the treatment with phosphoric acid [64,65]. In fact, porosity is generated with phosphoric acid remaining in the internal structure of the biopolymer material in the form of phosphate and polyphosphate

compounds. As the amount of H3PO<sup>4</sup> used increases, the volume filled by it and various polyphosphates will increase, resulting in larger pore volume and pore size [57].

Figure 2d–f display the micropore size distribution of the different activated carbon obtained from the N<sup>2</sup> adsorption at −196 ◦C. As clearly observed the activated carbon with an impregnation ratio of 1 contain micropores in the range of 12–14 nm (Figure 2d). The increase in the impregnation ratios (Figure 2e,f) results in the appearance of a multimodal pore size distribution.

On the other hand, the experimental data in Table 3 also shows an increase in the percentage of mesoporosity with an increasing impregnation ratio, showing that the development of porosity is also accompanied by a widening of the porosity as the amount of H3PO<sup>4</sup> is increased. These results support those extracted from the above-discussed pore size distribution. When the temperature exceeds 500 ◦C for the samples prepared with the impregnation ratio of (1:1) and 450 ◦C for the samples prepared with the impregnation ratio of (2:1) and (3:1), this trend is reversed. This change may be attributed to the increased merging and collapse of micropores which contributes to the reduction of surface area. J. Donald et al. have reported that the phosphate ester cross-links reach their limit of thermal stability at temperatures around 450–500 ◦C [66]. At higher temperatures, the breakdown of these cross-links would cause contraction and consequent reduction in porosity development.
