# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 2 Recent Advances on Catalysts for Syngas Production
### Transition Metals-Based Catalysts
#### 2.2.1 Nickel Catalysts
However, this increase was not followed by a higher H\({}_{2}\) production, and the overall best H\({}_{2}\)/CO ratio (0.90) was achieved at CH\({}_{4}\):CO\({}_{2}\) = 1:1. This observation points to the favoring of parallel reactions on higher CH\({}_{4}\):CO\({}_{2}\) ratios. Ceria (CeO\({}_{2}\)) promotion was also found favorable for H\({}_{2}\) production in a recent study by [PERSON] et al. [11]. The authors evaluated the effects of La\({}_{2}\)O\({}_{3}\) and CeO\({}_{2}\) promotions on 10 wt% Ni/Al\({}_{2}\)O\({}_{3}\) catalysts synthesized by the sol-gel method. The authors reported that La-promotion may present a dual role on DRM catalysts by increasing their resistance to coke as well as decreasing the acidic character of the support. However, CeO\({}_{2}\) promotion was more advantageous when compared to La\({}_{2}\)O\({}_{3}\) promotion when H\({}_{2}\)/CO was considered. CeO\({}_{2}\) promotion led to a syngas with a H\({}_{2}\)/CO ratio of 0.95, while a H\({}_{2}\)/CO ratio of 0.89 was obtained when La\({}_{2}\)O\({}_{3}\) was used as a promoter in the same test conditions. The authors highlighted that the presence of the CeO\({}_{2}\) improved the active phase dispersion and reducibility, which led to higher H\({}_{2}\)/CO on the produced syngas. [PERSON] et al. [68] evaluated the impact of a direct synthesis method via Ni-phylosilicates for the production of Ni-SiO\({}_{2}\) catalysts in comparison to the incipient wetness impregnation method on the catalytic activity and stability of Ni-based catalysts on DRM. The authors reported that the Ni-SiO\({}_{2}\) catalysts synthesized via Ni-phylosilicates presented a higher metal dispersion even for high Ni loadings (15 wt% Ni), which led to a reduced carbon formation on short reaction times. However, for longer reaction times (TOS > 180 min), significant carbon formation was observed in the temperature range of 600-900 \({}^{\circ}\)C. The results indicate that for T < 700 \({}^{\circ}\)C, the Boudouard reaction was favored, leading to a high carbon deposition. For T = 800-900 \({}^{\circ}\)C, the high H\({}_{2}\) yields (above 90%) were followed by H\({}_{2}\)/CO ratio > 1.0 and high carbon formation, indicating the occurrence of CH\({}_{4}\) cracking and CO disproportionation reactions. [PERSON] et al. [69] used KIT-6 mesoporous silica as support for Ni catalysts synthesis via wet impregnation and reported a high surface area of S\({}_{\rm BET}\) = 422 m\({}^{2}\) g\({}^{-1}\) for the 15 wt% Ni/KIT-6 catalyst. The catalytic tests at T = 800 \({}^{\circ}\)C showed CH\({}_{4}\) and CO\({}_{2}\) conversions of 88% and 90%, respectively, and the production of syngas with a H\({}_{2}\)/CO ratio of 0.92. Moreover, the authors evaluated that, although cerium promotion (ceria/KIT-6 ratio = 3/2) on the 15 wt% Ni/KIT-6 showed a slight increase on CH\({}_{4}\) and CO\({}_{2}\) conversion, the H\({}_{2}\)/CO ratio at higher temperatures (>700 \({}^{\circ}\)C) was not altered. Furthermore, [PERSON] et al. [70] have recently reported the synthesis of Ni-based catalysts over nano-sized mesoporous silica (MSN) as an alternative support. The MSN was prepared by alkaline leaching of silica-rich pumice rock. Different Ni loadings varying between 5 and 20 wt% were added to the MSN support by wet impregnation and tested in DRM at 550-750 \({}^{\circ}\)C. This study showed that increasing the amount of Ni on the catalysts did not improve CH\({}_{4}\) conversion due to the formation of Ni particles clusters with non-reactive aspects for hydrogen production. On the other hand, low Ni loading (5-10 wt%) led to high metal dispersion and strong metal-support interaction, favoring H\({}_{2}\) yields close to 70%. One-pot microwave-assisted combustion synthesis was reported to be a fast and cost-effective method for producing high H\({}_{2}\)-yielding 10 wt% Ni/Al\({}_{2}\)O\({}_{3}\) catalysts by [PERSON] et al. [71].