# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 2 Recent Advances on Catalysts for Syngas Production
### Transition Metals-Based Catalysts
#### 2.2.4 Bi- and Trimetallic Catalysts  
Bi- and trimetallic catalysts have been widely explored for DRM (Table 1). Studies have shown that the presence of more than one metal on the catalyst may result in a synergistic effect, with all metals acting as active phases or, by a promotion effect, when the active metal has its intrinsic activity enhanced by the presence of a second/third doping metal. Such synergistic effects could be explained by the electronic modifications on the catalyst surface as a result of the presence of a second/third metal, which could translate as increasing reactants chemisorption, for example [88]. Those effects can range from the stabilization of metal particles to improvements in metal-support interactions, metal dispersion and reducibility [89; 90]. Increased metal dispersion and strong metal-support interactions, for example, have already been linked to performance improvements on DRM, such as higher H\({}_{2}\) yields and suppressing of coke formation [91]. Furthermore, recent DFT modeling and calculations have considered that the appropriate fine-tuning active phase composition can lead to the suppressing of DRM side reactions, such as RWGS, which would contribute to higher H\({}_{2}\)/CO ratio on the final syngas [92]. [PERSON] et al. [93] evaluated the effect of calcination temperature and reduction protocols on the DRM activity of Ni-Co bimetallic catalysts supported on Al\({}_{2}\)O\({}_{3}\)-ZrO mixed oxide. The catalysts were prepared by co-precipitation and subjected to different in situ and ex situ calcination and reduction protocols before being tested on DRM reaction at 500-800 \({}^{\circ}\)C. The authors demonstrated that the catalysts calcined at 800 \({}^{\circ}\)C and reduced in situ prior to catalytic evaluation at 700 \({}^{\circ}\)C yielded the best performance, with CH\({}_{4}\) and CO\({}_{2}\) conversions of 67% and 76%, respectively, and a H\({}_{2}\)/CO ratio of 0.91. The authors reported that the calcination at a high temperature triggered sintering and particle growth to a degree that increased both catalytic activity and stability. Bimetallic Ni-Co catalysts were also investigated by [PERSON] et al. [60]. The catalysts were prepared by co-impregnation using CeO\({}_{2}\)-ZnAl\({}_{2}\)O\({}_{4}\) mixed oxide as support. In this study, the authors reported that the presence of cobalt increased the catalyst's surface area when compared to Ni monometallic catalysts from 39.87 m\({}^{2}\) g\({}^{-1}\) to 46.27 m\({}^{2}\) g\({}^{-1}\). The number of basic sites also increased from 180.3 umol g\({}^{-1}\) to 190.1 umol g\({}^{-1}\) when Co was added as a promoter. The metal-support interaction was also favored and prevented the sintering of the active phase. Finally, Ni-Co/CeO\({}_{2}\)-ZnAl\({}_{2}\)O\({}_{4}\) catalyst yielded a CH\({}_{4}\) and CO\({}_{2}\) conversion of 76% and 88% at T = 700 \({}^{\circ}\)C, respectively, with a high H\({}_{2}\)/CO ratio of 0.99. [PERSON] et al. [94] investigated different Co/Ni ratios on the preparation of CeO\({}_{2}\)-supported Co-Ni catalysts for DRM application. The authors prepared a series of bimetallic catalysts by co-impregnation with Co/Ni ratios of 0.3-1.0 with a fixed 10 wt% Ni content and evaluated the catalytic activity for DRM between 600 and 850 \({}^{\circ}\)C. Reducibility studies showed that at the ratio of 0.8 Co/Ni, the Co addition improved catalyst reduction capacity, increased the metal-support interaction, when compared to Co- and Ni-based monometallic catalysts, and changed the configuration of bulk oxygen from CeO\({}_{2}\) support. In addition, the 0.8 Co-Ni/CeO\({}_{2}\) catalyst presented the best catalytic performance at 850 \({}^{\circ}\)C, reaching CH\({}_{4}\) and CO\({}_{2}\) conversions of 99% and 100%, respectively, and a H\({}_{2}\)/CO ratio of 0.98. Trimetallic NiFeCu alloys with a Ni/Fe = 3.0 and different amounts of Cu were prepared by co-precipitation and investigated as a catalyst for hydrogen production via DRM in a recent work by [PERSON] et al. [95].