3.6. Pore Structure Analyses
Figure 7 shows the relationship between the incremental pore areas and pore widths of RFG
n and RFG
n-AC samples. It is worth noting that these results are in view point of the low-to-high scale of pore sizes for all samples. A summary of the pore structure characteristics of RFG
n and RFG
n-AC samples is given in
Table 5.
It can be observed from
Figure 7a that each of the samples RFG3 through RFG7 exhibit two main peaks, whereas the samples RFG1, RFG2, RFG8 and RFG9 are relatively nonporous. RFG3 and RFG4 exhibit two key peaks at 1.3 and 1.5 nm. The corresponding incremental pore areas of these two peaks were, respectively, 27.01 and 12.70 m
2/g (for RFG3) and 44.42 and 8.77 m
2/g (for RFG4). RG5, RFG6 and RFG7 expose two key peaks at 1.4 and 1.7 nm. The corresponding incremental pore areas of RG5, RFG6 and RFG7 at 1.4 nm are 18.16, 17.86 and 21.40 m
2/g, respectively; whereas, those at 1.7 nm are 3.77, 4.66 and 5.13 m
2/g, respectively. The samples of RFG1, RFG2, RFG8 and RFG9 exhibited nil incremental pore areas of less than 1 m
2/g. Overall, RFG4 denotes the highest incremental pore area; whereas RFG1, RFG2, RFG8 and RFG9 exhibited the lowest incremental pore areas < 1 m
2/g, which can be considered relatively nonporous. Moreover, the highest incremental pore area for each RFG
n is localized in the microporous scale.
It can be noticed from
Figure 7b that RFG3-AC shows four key peaks at 0.8, 0.9, 1.1 and 1.3 nm with the corresponding incremental pore areas of 225.09, 39.61, 56.47 and 22.55 m
2/g, respectively. RFG9-AC exhibits two key peaks at 0.8 and 1.0 nm with the corresponding incremental pore areas of 167.25 and 139.16 m
2/g, respectively. RFG4-AC, RFG5-AC, RFG7-AC and RFG8-AC illustrate two key peaks at 0.7 and 1.1 nm. The corresponding incremental pore areas of RFG4-AC, RFG5-AC, RFG7-AC and RFG8-AC at 0.7 nm are 114.31, 20.0, 17.46 and 99.66 m
2/g, respectively. Furthermore, the corresponding incremental pore areas of RFG4-AC, RFG5-AC, RFG7-AC and RFG8-AC at 1.1 nm are 93.73, 114, 156.05 and 109.65 m
2/g, respectively. RFG2-AC displays two key peaks at 0.8 and 1.1 nm with the corresponding incremental pore areas of 13.02 and 76.20 m
2/g, respectively. RFG6-AC exposes three key peaks at 0.8, 1.0 and 1.2 nm with the corresponding incremental pore areas of 42.59, 106.68 and 72.56 m
2/g, respectively. RFG1-AC shows two key peaks at 1.1 and 1.2 nm with the corresponding incremental pore areas of 98.41 and 67.47 m
2/g, respectively.
A summary of various pore structure characteristics of RFG
n and RFG
n-AC samples is presented in
Table 5. The average pore sizes of RFGn samples range from 3.5 nm (for RFG3 and RFG4) to 62.9 nm (for RFG8). Other related details of average pore sizes are registered in
Table 5. The percentage of micropores ranges from 0.0 to 1.2%, which corresponds to the samples RFG2 and RFG1, respectively. The mesopores of RFG
n samples range from 9.0 to 48.5% for the samples of RFG2 and RFG1, respectively. Accordingly, the percentage of macropores in RFG samples ranges from 50.3 to 91.0% for RFG1 and RFG2, respectively. Overall, the order of mesopore percentage was RFG2 < RFG9 < RFG4 < RFG 5 < RFG6 < RFG7 < RFG8 < RFG3 < RFG1; whereas, the order of macropore percentage was RFG1 < RFG3 < RFG8 < RFG5 < RFG6 < RFG7 < RFG4 < RFG9 < RFG2. The average pore widths of RFG-AC samples range from 0.9 nm (for RFG8-AC and RFG9-AC) to 2.0 nm (for RFG7-AC). Furthermore, the percentage of micropores of RFG
n-AC samples ranges from 0.3% (for RFG1-AC, RFG2-AC, RFG3-AC, RFG5-AC and RFG8-AC) to 0.6% (for RFG4-AC and RFG9-AC). The percentage of mesopores of RFG
n-AC ranges from 11.2% (for RFG1-AC, RFG2-AC, RFG3-AC, RFG5-AC and RFG6-AC) to 19.0% (for RFG9-AC), and the percentage of macropores ranges from 80.4% (for RFG9-AC) to 88.5% (for RFG1-AC, RFG2-AC, RFG3-AC and RFG5-AC). The percentages of micropores, mesopores and macropores follow the orders of RFG1-AC = RFG2-AC = RFG3-AC = RFG5-AC = RFG8-AC < RFG6-AC = RFG7-AC < RFG4-AC = RFG9-AC, RFG1-AC = RFG2-AC = RFG3-AC = RFG5-AC = RFG6-AC < RFG8-AC < RFG7-AC < RFG4-AC < RFG9-AC and RFG9-AC < RFG4-AC < RFG7-AC < RFG8-AC < RFG6-AC = RFG1-AC = RFG2-AC = RFG3-AC = RFG5-AC, respectively.
Figure 8 displays the relationship between the cumulative pore area and pore width of RFG
n and RFG
n-AC samples. The maximum cumulative surface areas of RFG
n and RFG
n-AC samples are also listed in
Table 5. It is observed that the cumulative pore area of RFG1 is 0.01 m
2/g, which denotes the lowest cumulative pore area; that of RFG4 is 176.15 m
2/g, which denotes the highest cumulative pore area. Further, the sequence order of cumulative surface areas of RFG samples are RFG4 > RFG6 > RFG3 > RFG7 > RFG5 > RFG8 > RFG9 > RFG2 > RFG1. The increment trend between the cumulative pore area and pore width is noticeable in the cases of RFG3, RFG4, RFG5, RFG6, RFG7 and RFG8 whereas a weak trend of the cumulative pore areas is found in the cases of RFG1, RFG2, RFG8 and RFG9. The cumulative pore areas increase by the augmenting of pore width of RFG3, RFG4 and RFG5 up to 12.7 nm then level off at the corresponding cumulative areas of 145.40, 176.15 and 67.48 m
2/g, respectively. The cumulative surface area of RFG6 increases up to a pore width of 25.3 nm, then levels off with the corresponding cumulative pore area of 147.66 m
2/g. Moreover, the cumulative surface area of RFG7 increases up to the pore width of 34.3 nm, then levels off at the corresponding cumulative pore area of 118.26 m
2/g. More details of surface area of these RFG samples are recorded in
Table 5.
It can be noticed from
Figure 8b that cumulative pore areas of RFG
n-AC samples range from 160.93 to 511.46 m
2/g for RFG2-AC and RFG3-AC respectively. The sequence of cumulative areas’ order is RFG3-AC > RFG6-AC > RFG4-AC > RFG9-AC > RFG7-AC > RFG5-AC > RFG8-AC > RFG1-AC > RFG2-AC. Overall, the cumulative pore area of RFG
n-AC samples increase by augmenting the pore width. The cumulative pore area increments up to 8.6 nm then levels off for the samples of RFG3-AC and RFG6-AC at 511.46 and 370.81 m
2/g, respectively. The cumulative pore areas of RFG1-AC, RFG2-AC, RFG4-AC, RFG5-AC, RFG7-AC, RFG8-AC and RFG9-AC increase up to 1.2 nm then level off at the corresponding cumulative pore areas of 165.92, 160.93, 368.89, 333.31, 356.26, 308.49 and 368.51 m
2/g, respectively. Other related details are recorded in
Table 5.
Figure 9 displays the incremental pore volume distributions (as a function of pore widths) of RFG
n and RFG
n-AC samples. RFG
n samples contain RFG1, RFG2, RFG3, RFG4, RFG5, RFG6, RFG7, RFG8 and RFG9. Furthermore, RFGn-AC samples comprise RFG1-AC, RFG2-AC, RFG3-AC, RFG4-AC, RFG5-AC, RFG6-AC, RFG7-AC, RFG8-AC and RFG9-AC-AC. The discussion of results right here begins from the low scale of pore width then goes up to the high scale in gradient, regardless of sample number either in RFG
n or RFG
n-AC samples. It can be observed from
Figure 9a that both of RFG3 and RFG4 exhibit three key peaks at 1.3, 6.8 and 8.6 nm. The corresponding incremental pore volumes of RFG3 are 0.016, 0.013 and 0.014 cm
3/g, respectively; those of RFG4 are 0.028, 0.015 and 0.015 cm
3/g, respectively. RFG7 displays five key peaks at 1.4, 27.3, 34.3, 54.4 and 117.2 nm with the corresponding incremental pore volumes of 0.014, 0.024, 0.025, 0.010 and 0.003 cm
3/g, respectively. RFG6 exposes two key peaks at 1.4 and 17.2 nm with the corresponding incremental pore volumes of 0.012 and 0.031 cm
3/g, respectively. RFG5 shows two key peaks at 1.4 and 6.8 nm with the corresponding incremental pore volumes of 0.012 and 0.008 cm
3/g, respectively. On the other hand, the trends in RFG1, RFG2, RFG8 and RFG9 are weak and can be ignored; this agrees with the results induced from the incremental surface area distributions (
Figure 7a). The differences in incremental surface areas or incremental pore volumes of various RFG
n samples are due to the differences in the molar ratios of resorcinol-to-formaldehyde utilized in the preparation process as outlined in
Table 1.
In the course of the results in
Figure 9b, it can be concluded that RFG3-AC exposes four key peaks at 0.8, 1.1, 6.3 and 8.6 nm with the corresponding incremental pore volumes of 0.103, 0.031, 0.011 and 0.010 cm
3/g, respectively. RFG9-AC displays two key peaks at 0.8 and 1.0 nm with the corresponding incremental pore volumes of 0.067 and 0.071 cm
3/g, respectively. RFG4-AC, RFG5-AC and RFG8-AC exhibit two key peaks at 0.7 and 1.1 nm. The corresponding incremental pore volumes of RFG4-AC are 0.041 and 0.005 cm
3/g; those of RFG5-AC are 0.008 and 0.006 cm
3/g; and those of RFG8-AC are 0.037 and 0.058 cm
3/g, respectively. RFG1-AC exposes only one key peak at 1.1 nm with an incremental pore volume of 0.054 cm
3/g. RFG6-AC shows three key peaks at 0.8, 1.1 and 14.4 nm with the corresponding incremental pore volumes of 0.017, 0.055 and 0.012 cm
3/g, respectively. RFG7-AC refers to three key peaks at 0.7, 1.1 and 14.8 nm with corresponding incremental pore volumes of 0.006, 0.082 and 0.009 cm
3/g, respectively. Moreover, RFG2-AC displays two key peaks at 0.8 and 1.1 nm with the corresponding incremental pore volumes of 0.004 and 0.042 cm
3/g, respectively. These differences in incremental pore volumes are due to the differences in the molar ratios of resorcinol-to-formaldehyde through the initial preparation process.
Figure 10 shows the relationship between the cumulative pore volume and pore width of RFG
n and RFG
n-AC samples. The maximum cumulative pore volumes of RFG
n and RFG
n-AC samples are also listed in
Table 5. Overall, it can be noticed from
Figure 10a that by augmenting the pore width, the cumulative pore volume augments. The lowest cumulative pore volume (of 0.00038 cm
3/g) is for RFG1, and the highest cumulative pore volume (of 0.546 cm
3/g) is for RFG6. The sequential order of RFG
n samples are RFG6 > RFG7 > RFG4 > RFG3 > RFG5 > RFG8 > RFG2 > RFG9 > RFG1. The trend of cumulative versus pore widths of RFG6, RFG7, RFG4, RFG3, RFG5 and RFG8 ratchet up to 54.4, 108.6, 13.7, 12.7, 10.1 and 108.6 nm, respectively; and the total pore volumes of these samples are 0.546, 0.522, 0.292, 0.253, 0.127 and 0.028 cm
3/g, respectively. The cumulative pore volumes of RFG1, RFG2 and RFG9 are practically insignificant. Furthermore, the results in
Figure 10b expose that by augmenting pore widths, the cumulative pore volumes of RFG
n-AC samples augment. The sequential order of these RFG
n-AC samples are RFG3-AC > RFG7-AC > RFG6-AC > RFG4-AC > RFG5-AC > RFG9-AC> RFG8-AC > RFG1-AC > RFG2-AC. Further, RFG2-AC and RFG3-AC denote the lowest and the highest cumulative pore volumes among samples, which are equal to 0.086 and 0.366 cm
3/g, respectively. The cumulative pore volumes of RFG3-AC, RFG6-AC RFG7-AC, RFG4-AC, RFG5-AC, RFG9-AC-AC, RFG8-AC, RFG1-AC and RFG2-AC augment with augmenting pore widths up to 17.2, 40.0, 73.7, 13.7, 10.1, 0.5, 0.5, 0.5 and 0.5 nm, respectively, then level off with the corresponding total pore volumes of 0.366, 0.342, 0.348, 0.220, 0.215, 0.162, 0.150, 0.095 and 0.086 cm
3/g, respectively, as registered in
Table 5.
3.8. Thermal Stability
Figure 13 a,b exposes the TGA thermograms of RFG
n and RFG
n-AC samples, respectively; from room temperature up to 800 °C. The corresponding decomposition (%weight loss) values of RFG
n and RFG
n-AC samples at different temperatures are also listed in
Table 6. Overall, it is observed that the thermal stability of RFG
n-AC samples is higher than that of RFG
n samples at the same temperature. It can be noticed from
Table 6 that the lowest and the highest thermal decomposition percentages of RFG
n samples at 150 °C are 0 and 4.94% for RFG3 and RFG5, respectively. The lowest and the highest weight losses among RFG
n samples at 200 °C are 1.26 and 6.72% for RFG6 and RFG5, respectively. The lowest and the highest weight losses among RFG
n samples at 300 °C are 3.69 and 11.59% for RFG5 and RFG1, respectively. The lowest and the highest weight losses among RFG
n samples at 400 °C are 10.63 and 22.63% for RFG4 and RFG9, respectively. The lowest and the highest weight losses among RFG
n samples at 500 °C are 19.34 and 36.15% for the RFG3 and RFG5, respectively. The lowest and the highest weight losses among RFG
n samples at 600 °C are 31.83 and 55.22% for RFG3 and RFG5, respectively. The lowest and the highest weight losses among RFG
n samples at 700 °C are 45.09 and 71.26% for RFG3 and RFG5, respectively; at 800 °C, they are 55.16 and 80.25% for RFG3 and RFG5, respectively. Overall, RFG3 and RFG4 exhibit the best thermal stabilities over almost the entire range of temperatures (as indicated in
Table 6 by mostly green color-coded cells at different temperatures), whereas RFG5 exhibits the poorest overall thermal stability (as indicated in
Table 6 by mostly red color-coded cells at different temperatures).
Moreover, it is observed from the data listed in
Table 6 that the lowest and the highest weight losses of RFG
n-AC samples at 150 °C are 0 and 4.46% for RFG6-AC and RFG9-AC, respectively. The lowest and the highest weight losses at 200 °C are 0 and 3.29% for RFG6-AC and RFG2-AC, respectively. The lowest and the highest weight losses at 300 °C are 0 and 5.00% for RFG6-AC and RFG2-AC, respectively. The lowest and the highest weight losses at 400 °C are 0.62 and 6.79% for RFG6-AC and RFG2-AC, respectively. The lowest and the highest weight losses at 500 °C are 1.95 and 8.46% for RFG6-AC and RFG2-AC, respectively. The lowest and the highest weight losses at 600 °C are 5.76 and 11.77% for RFG4-AC and RFG2-AC, respectively. The lowest and the highest weight losses at 700 °C are 11.52 and 22.21% for RFG4-AC and RFG6-AC, respectively. Furthermore, the lowest and the highest weight losses at 800 °C are 19.28 and 36.41% for RFG4-AC and RFG6-AC, respectively. Overall, the thermal stability of RFG4-AC exhibits the widest range of thermal stability (as indicated in
Table 6 by mostly green color-coded cells at various temperatures), especially at (and beyond) ~600 °C. On the other hand, RFG6-AC exhibits the best thermal stability at temperatures below ~600 °C, but its stability degrades considerably starting from 700 °C. Indeed, the poorest thermal stability at high temperatures (of ~700 °C and beyond) was for RFG6-AC. In addition, the poorest thermal stability at intermediate temperatures (from ~200 to ~600 °C) was for RFG2-AC, whereas the poorest thermal stability at low temperatures (up to ~150 °C) was for RFG9-AC.