# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 3 Alternative Processes for Increased H\({}_{2}\)/CO Ratio
### Partial Oxidation of Methane (POM)  
The catalytic partial oxidation of methane (POM) has attracted attention for being a more energy-efficient process when compared to commercial SRM. The fast kinetics of POM lead to a good response time and system compactness, while its exothermic character is less energy demanding and avoids the requirements of high temperature steam [138]. In the partial oxidation process, CH4 (or other hydrocarbons) is reacted with stoichiometric amounts of pure oxygen and yields a H2/CO ratio of 2.0 for the produced syngas (Equation (10)), which is suitable for Fischer-Tropsch synthesis [139]. However, controlling the  
Figure 4: Thermodynamic equilibrium plot for POM as a function of temperature (T = 200–1000 \({}^{\circ}\)C) at 1–10 bar with CH4 CO2:H2O = 3:1:2. (**a**) CH4 and CO2 conversion levels; (**b**) H2O conversion and H2/CO ratio profile. Reprinted with permission from [135]. Copyright 2020 Elsevier. reaction selectivity to avoid total combustion can be challenging [140]. Figure 5 presents the thermodynamic equilibrium for POM as a function of temperature for different operating pressures for a stoichiometric feed (CH4:O2 = 2.0, Equation (10)). [PERSON] et al. [141] highlight that, at higher pressures, higher temperatures are required for obtaining a high selectivity towards H2 and CO (Figure 5b,c). In addition, the authors also comment that high H2 selectivity (>80%) can be obtained for temperatures as low as 550 \({}^{\circ}\)C if CH4:O2 ranges between 2.0 and 5.0. Classical noble- and transition-metal-based catalysts are the most reported for POM, and carbon deposition is also a major trigger for catalyst deactivation. Moreover, the temperature gradients resulting from the exothermic POM reaction increase the probability of catalyst deactivation due to particle sintering, which leads to more challenges on catalyst development [139]. \[\text{CH}_{4}+\frac{1}{2}\text{O}_{2}\to 2\text{H}_{2}+\text{CO}\qquad \text{$\Delta$H${}_{298K}$}=-36\text{ kJ.mol}^{-1} \tag{10}\]  
[PERSON] et al. [139] evaluated the performance of different Ni-based catalysts for hydrogen production via POM. Catalysts containing 5 wt% of Ni doped on Al2O3, CeO2, La2O3, MgO and ZrO2 supports were tested for POM activity under stoichiometric feed conditions at T = 750 \({}^{\circ}\)C and atmospheric pressure. Among the prepared catalysts, 5 wt% Ni/MgO and 5 wt% Ni/La2O3 presented low activity and rapid deactivation, due to the formation of non-catalytic phases between the metal and the oxide support. The aluminan-supported 5 wt% Ni/Al2O3 revealed to be the most efficient and most stable catalytic system, with >80% CH4 conversion and H2 yields throughout the catalytic tests. In addition, the authors showed that Rh-promotion over the Ni/Al2O3 catalyst was responsible for a slight increase in CH4 conversion and H2 yield up to 90%. Finally, the H2/CO ratio for the obtained syngas was stable around 2.5 for the Ni/Al2O3 catalyst with or without Rh-promotion. [PERSON] et al. [142] also investigated the hydrogen production via POM over Ni-based catalysts and reported the use of mesoporous oxides La2O3, Yb2O3, ZrO2 and CeO2 for the modification of SiO2 support. The catalysts were prepared with a 10 wt% Ni loading and tested for their POM activity under stoichiometric feed conditions, atmospheric pressure and T = 800 \({}^{\circ}\)C. The authors demonstrated that the modifications with mesoporous solid oxides were able to increase Ni dispersion on the support, which increases catalytic activity and CH4 conversion > 75% (reaching 90% for 10 wt% Ni/ZrO2-SiO2). Moreover, the authors reported that hydrogen production via POM can profit from oxygen mobility of oxide supports such as ZrO2, which promotes the combustion-reforming mechanism. In this mechanism, CH4 and O2 first react to form CO2 and H2O, followed by a combined steam and dry reforming promoted by the combustion products and the excess CH4 [139]. In this case, H2 selectivity increased almost 20% and the final H2/CO ratio of produced syngas stabilized at 3.0.