## 4 Results and Discussion
The ratio of XRD peaks (\(I_{26.1^{\circ}}/I_{21.4^{\circ}}\)) increases with the increasing MWCNT content; thereby increasing the relative crystallinity and reducing the relative amorpticity into the matrix of the product. Figure 9c exhibits the impact of sample composition on FWHM of XRD peaks at 20 of 42\({}^{\circ}\) and 54\({}^{\circ}\), which evidently show that no significant influence on the intensity of these peaks is noticed when adding AC-RFA to MWCNTs, whereas the pure AC-RFA (i.e., the sample of S0) has no such peaks. Figure 8: XRD patterns of different MWCNT/AC-RFA compositions; namely samples of S100, S60, S50, S40 and S0. The values on pattern peaks indicate to their corresponding intensities. Figure 10 shows the isotherms of CO\({}_{2}\) and CH\({}_{4}\) gases adsorption/desorption isotherms at 40 \({}^{\circ}\)C versus pressure onto hybrid MWCNT/AC-RFA adsorbents with various compositions, namely S100, S60, S50, S40, and S0. It is observed that the adsorption capacities of CO\({}_{2}\) and CH\({}_{4}\) increase by increasing the ratio of AC-RFA into the hybrid MWCNT/AC-RFA adsorbent, which can be attributed to the increased surface area (see Table 1), and enhance the interactions between the adsorbing surface and gas molecules. The sequential order of hybrid MWCNT/AC-RFA samples towards either CO\({}_{2}\) or CH\({}_{4}\) is as S0 \(>\) S40 \(>\) S50 \(>\) S60 \(>\) S100. Furthermore, the adsorption affinity of all samples towards CO\({}_{2}\) is higher than that towards CH\({}_{4}\). All isotherms described in Figure 10 are classified as Type-I according to IUPAC [46]. However, the sample S100 exposes an almost linear adsorption isotherm for both CH\({}_{4}\) and CO\({}_{2}\). Figure 9: Impact of hybrid MWCNT/AC-RFA sample composition on the XRD (**a**) intensities of the peaks at 20 of 26.1\({}^{\circ}\) and 21.4\({}^{\circ}\) (**b**) intensity ratio (I26.1\({}^{\circ}\)/I21.4\({}^{\circ}\)) and (**c**) intensities at 20 of 42\({}^{\circ}\) and 54\({}^{\circ}\). From Raman results, the trend of uniformity deduced from \(I_{D}/I_{G}\) ratio leads to affecting the adsorption capacity; which is also supported by pore and area structure data listed in Table 1. Overall, the higher ratio of \(I_{D}/I_{G}\) indicates more defects/disorders into the matrix is, which results in higher adsorption capacities. On other words, the by increasing the amount of AC-RFA into the adsorbent matrix, the adsorption capacity of adsorbent increases. Further, the XRD profiles indicate that as more RFA-AC is added to MWCNTs, a higher adsorption capacity is attained due to the increasing amorphicity of the structure. It is important to mention that the crystallinity of MWCNTs is higher than that of RFA-AC. The elemental analysis, either from EDX or XPS, indicates that as the oxygen percentage is higher, the adsorption capacity increases as shown in Table S1 (Supplementary Materials). In addition to the former results based on Raman and XRD patterns discussed previously, it could be said that by increasing the AC-RFA, the increasing non-uniformity of the adsorbent corresponds to higher adsorption capacities towards gas molecules. Therefore, the adsorption capacity of adsorbents is enhanced by increasing the non-uniformity order into the structure of sample utilized, which also corresponds to a higher surface area and pore volumes as presented in Table 1. Therefore, structural characteristics such crystallinity, composition, and orders/disorders have a significant impact on the adsorption behavior of CH\({}_{4}\) and CO\({}_{2}\) gases. Figure 10: Adsorption/desorption isotherms of (**a**) CO\({}_{2}\) and (**b**) CH\({}_{4}\) on different hybrid MWCNT/AC-RFA sample compositions; namely, S100, S60, S50, S40 and S0 at 40 \({}^{\circ}\)C. Solid and empty symbols indicate, correspondingly, the experimental adsorption and desorption data.