## 3 Results and Discussion
### Catalytic Activity
However, the decrease in Al content of the support leads to a markedly enhanced catalytic activity of the delaminated ITQ-2,  
Figure 5: XRD patterns of the 5 wt.% Ni catalysts supported on ITQ-2 and ZSM-5 with different Si/Al ratios after the reaction test. Figure 6: High-resolution transmission electron microscopy (HR-TEM) images and Ni\({}^{0}\) particle size distribution of the 5 wt.% Ni catalyst supported on ITQ-2 and ZSM-5 with different Si/Al ratios. Images obtained after the methanation reaction. which is probably related to the small NiO crystallite size found for this sample by XRD. The catalyst supported on pure silica ITQ-2 was the one which allows obtaining a higher CO\({}_{2}\) conversion and CH\({}_{4}\) selectivity. This leads to a distinctly greater reaction rate for the sample 5 Ni/ITQ-2(\(\infty\)) with values at least 150% higher than those of the rest of the samples of this study (Figure 4b inset). The similar performance of the materials with higher Al content (lower Si/Al ratio) might be explained for the similar crystallite size of NiO in these two samples (10.6 and 12.0 nm for the ITQ-2(15) and ZSM-5(15) supported catalysts, respectively). In addition, the Si/Al ratio of the ITQ-2(15) was found to be lower than the expected value, which is probably detrimental for its activity. These results suggest that the Al content of the zeolite has a more relevant effect than the zeolite type in the properties of the final catalyst. Si/Al ratio is reported to have influence in Brensted acidity and, thus, in hydrophobicity for a given zeolite type [38; 39; 40; 41]. In this way, more hydrophobic materials would remove water from active sites more efficiently, favoring the development of the reaction [24; 25; 42]. The Si/Al ratio might also affect the dispersion and stability of Ni nanoparticles. [PERSON] et al. [44] claimed that for Ni supported on Beta zeolite, a high Si/Al ratio favored both an initial dispersion of Ni and the redispersion with subsequent thermal treatments. The results obtained in our study show that for both zeolite types (ITQ-2 and ZSM-5), the higher the Si/Al ratio, the better catalytic activity, which is in accordance to the reported results for Ni catalysts supported on zeolite Y [25] and Beta [24]. Among the catalysts considered, the catalyst supported on the pure silica ITQ-2 allow to obtain the highest yield to CH\({}_{4}\). In Table 3, its activity is compared with the values reported for other similar catalysts. First, it is important to mention that a delaminated zeolite is considered as support for CO\({}_{2}\) methanation for the first time in this reaction. This is noteworthy as, according to previous studies, zeolite type has an influence on the final activity of the material [15; 20; 24] and delaminated zeolites present unique and particular features worth studying for this reaction. Intrinsic properties of each zeolite type, such as its pores system, pore sizes or hydrophobicity, are reported to affect Ni dispersion and, thus, catalytic activity. In particular, ITQ-2 presents a large external surface area, which allows obtaining a very small NiO crystallite size in comparison with other studies. The decrease in crystallite sizes of Ni species has been achieved in previous studies with the use of support with high surface area, such as MSN [17], different precursor [20] or by addition of promoters [15; 21; 22]. For example, a decrease for Ni\({}^{0}\) from 20.1 to 8.7 nm was achieved by the addition of La\({}_{2}\)O\({}_{3}\)[15] and from 19.4 to 2.4 nm with Ce incorporation [22]. This decrease in Ni\({}^{0}\) crystallite size resulted in an improved catalytic activity, evidencing that Ni particle size is one of the key factors for the successful development of methanation reaction. The comparison of the results of catalytic activity of different studies is not straightforward as the experimental conditions differ. GHSV, temperature and Ni content strongly affect the catalytic activity obtained.