# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 3 Alternative Processes for Increased H\({}_{2}\)/CO Ratio
### Methane Tri-Reforming (TRM)  
The methane tri-reforming (TRM) process refers to a combination of the SRM, DRM and POM processes, in which methane is reacted with a mixture of steam, CO\({}_{2}\) and oxygen to produce syngas [143]. According to its thermodynamic aspects (Figure 6), TRM is favored at higher temperatures (T \(\geq\) 700 \({}^{\circ}\)C) due to the already discussed endothermic aspects of the individual reforming reactions (SRM and DRM) and the stability of the reactants. Moreover, lower pressures (1 bar) also favor the TRM equilibrium, although typical reforming reactors usually operate at higher pressures (3-30 bar) to decrease reactor size and meet high-pressure requirements for syngas downstream processes [115; 144]. Under stoichiometric conditions (2 mol of CH\({}_{4}\), 1 mol of H\({}_{2}\)O, 1 mol of CO\({}_{2}\), 0.1 mol of O\({}_{2}\)), CH\({}_{4}\) is almost completely converted (>98%) above 850 \({}^{\circ}\)C, oxygen is completely consumed via POM and high syngas selectivity of 1.65 is reached above 800 \({}^{\circ}\)C [115]. However, the main advantage of the TRM process is the adjustable H\({}_{2}\)/CO ratio of the produced syngas according to the feed composition, which makes TRM a versatile CH\({}_{4}\) reforming process that can be controlled for various downstream applications, such as methanol or Fischer-Tropsch synthesis [145]. As a combination of different catalytic reforming processes, TRM also profits from the low cost and known catalytic activity of Ni-based catalysts for reaction with C-H bonds. [PERSON] et al. [146] evaluated the performance of 5.75 wt% Ni/Al\({}_{2}\)O\({}_{3}\) catalyst for TRM under 1 bar and feed composition CH\({}_{4}\)CO\({}_{2}\):H\({}_{2}\)O:O\({}_{2}\) = 1.0:0.49:0.3.04. The authors reported that the highest hydrogen yield of 37% was achieved at temperatures higher than 850 \({}^{\circ}\)C. The competing RWGS reaction was predominant for T \(<\) 850 \({}^{\circ}\)C and decreased the H\({}_{2}\) yield to 4.4%. [PERSON] and [PERSON] [147] prepared core-shell-type 11 wt% Ni/SiO\({}_{2}\) catalysts for TRM and reported high hydrogen production and low coke deposition for T = 750 \({}^{\circ}\)C. At this temperature, SRM and DRM reactions were favored, which increased H\({}_{2}\) production, while for T \(\geq\) 800 \({}^{\circ}\)C, RWGS took place and decreased the overall H\({}_{2}\)/CO ratio to 1.5-1.7. In this study, the authors reached a H\({}_{2}\)/CO ratio of 2.6 with 73% and 56% of CH\({}_{4}\) and CO\({}_{2}\) conversion, respectively, and low carbon formation (5.0 mg g\({}^{-1}\) catalyst) for a feed composition of CH\({}_{4}\):CO\({}_{2}\):H\({}_{2}\)O:O\({}_{2}\):He = 1.0:0.5:0.5:0.1:0.4. [PERSON] et al. [148] investigated hydrogen production by TRM over 1.76 wt% Ni/CeO\({}_{2}\) catalyst at T = 800 \({}^{\circ}\)C and described the impact of La-doping and different feed compositions on the final H\({}_{2}\)/CO ratio. The authors reported that La modifications on the catalysts were able to increase the presence of oxygen vacancies on the ceria support and enhanced Ni dispersion, which account for high CH\({}_{4}\) (96%) and CO\({}_{2}\) (86%) conversions. However, the La-doping did not show any significant impact on hydrogen production. The modifications on feed composition, on the other hand, were able to modulate the final H\({}_{2}\)/CO ratio of the  
Figure 6: Thermodynamic equilibrium plot for TRM as a function of temperature (T = 100–1000 \({}^{\circ}\)C) at 1 bar with CH\({}_{4}\):CO\({}_{2}\):H\({}_{2}\)O:O\({}_{2}\) = 2:1:1.0.1. (**A**) Equilibrium amounts for all species; (**B**) CH\({}_{4}\) and CO\({}_{2}\) conversions, H\({}_{2}\)/CO ratio profile. Reprinted with permission from [115]. Copyright 2018 Elsevier. syngas. The authors demonstrated that the H\({}_{2}\)/CO ratio could drop from 2.8 to 1.3 when the inlet molar ratio of CO\({}_{2}\)/H\({}_{2}\)O varied from 0.0 (in the absence of CO\({}_{2}\)) to 2.6. According to [PERSON] et al.