# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 3 Alternative Processes for Increased H\({}_{2}\)/CO Ratio
### Steam Reforming of Methane (SRM)  
Steam reforming of methane (SRM) is the most mature and currently applied technology for hydrogen production. This highly endothermic process (Equation (1)) is operated at high temperature (700-1000 \({}^{\circ}\)C) at around 3-40 bar to produce a hydrogen-rich syngas with H\({}_{2}\)/CO = 3.0 [113]. Figure 2 presents the thermodynamic equilibrium of the SRM process when operated at two different H\({}_{2}\)O/CH\({}_{4}\) ratios. When an equimolar H\({}_{2}\)O/CH\({}_{4}\) is considered (Figure 2A), H\({}_{2}\) is produced above 180 \({}^{\circ}\)C, and this production increases with the temperature increase. Solid carbon is usually obtained in the range between 450 and 800 \({}^{\circ}\)C, which can compromise the operation and lead to catalyst deactivation. For H\({}_{2}\)O/CH\({}_{4}\) = 3.0 (Figure 2B), the carbon deposits are potentially eliminated and CH\({}_{4}\) can be completely consumed above 700 \({}^{\circ}\)C, which justifies the usual operation with excess steam. For hydrogen production purposes, the SRM is usually combined with water-gas shift reaction (WGS; Equation (7)) operated simultaneously to reduce the CO content and boost the hydrogen concentration [114]. Alumina-supported Ni-based catalysts are also commonly used in SRM. For this reaction, similar problems with coke deposition and catalyst sintering are also observed, with the aggravating factor that the carbon deposits can not only reduce the catalytic performance but contribute to an increased pressure drop on the reforming reactors. The operation of SRM under excess steam conditions (H\({}_{2}\)O:CH\({}_{4}\) = 3-5) has been known to help in controlling the carbon deposition over the catalysts [116]. \[\text{CO}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+\text{H}_{2}\quad \Delta\text{H}_{298}=-41\text{ kJ}.\text{mol}^{-1} \tag{7}\]  
Figure 2: Thermodynamic equilibrium plot for SRM as a function of temperature (T = 100–1000 \({}^{\circ}\)C) at 1 bar with (**A**) H\({}_{2}\)O/CH\({}_{4}\) = 1.0 and (**B**) H\({}_{2}\)O/CH\({}_{4}\) = 3.0. Reprinted with permission from [115]. Copyright 2018 Elsevier. [PERSON] et al. [117] evaluated the performance of Ni-based catalysts supported over calcium aluminate modified SiC for SRM. In this study, the authors aimed to stabilize the H\({}_{2}\) production via SRM by taking advantage of the Lewis basicity of calcium aluminate support to avoid coke deposition. Results revealed that the structured catalysts produced delivered a CH\({}_{4}\) conversion above 80% during 20 h long tests, with H\({}_{2}\) yields above 95% and negligible carbon deposition (T = 850 \({}^{\circ}\)C, H\({}_{2}\)O:CH\({}_{4}\) = 3). Controlling the coke deposition as a means of enhancing hydrogen production was also the goal of [PERSON] et al. [118] when investigating eggshell-type Ni/MgAl\({}_{2}\)O\({}_{4}\) pellets as a catalyst for SRM. The authors showed that homotype catalyst pellets were more prone to carbon deposition and particle agglomeration due to low metal dispersion. On the other hand, superior metal distribution on eggshell-type catalysts led to lower carbon accumulation (2.5% and 39.5%, for eggshell- and homotype catalyst pellets, respectively) and more stable SRM operation. In this case, CH\({}_{4}\) conversion remained around 50% for TOS = 6 h with high H\({}_{2}\) selectivity (>85%) and overall H\({}_{2}\)/CO = 0.80-0.85. Membrane-type reactors and catalysts have recently been investigated for SRM applications towards hydrogen production. In membrane-based configurations, the produced hydrogen can be removed from the reformed gas while the reaction is still taking place. This sort of purification process can produce a high purity H\({}_{2}\) stream without the requirements of a second downstream stage and alter the reaction equilibrium to promote higher CH\({}_{4}\) conversions. [PERSON] et al.