# Prospects and Technical Challenges in Hydrogen Production through Dry Reforming of Methane
## 3 Alternative Processes for Increased H\({}_{2}\)/CO Ratio
### Chemical Looping Dry Reforming of Methane (CL-DRM)  
Chemical looping (CL) is a relatively recent process that has been investigated mainly for CO\({}_{2}\) capture and utilization (CCU) [121; 122]. In the CL process, a reaction is \"broken down\" to a series of multiple sub-reactions that take place separately. In this process, materials with a high oxygen storage capacity, here called oxygen carriers (OCs), are used as reaction intermediates that can be recycled along the process and regenerated by oxygen and energy transfer [123; 124]. Several CH\({}_{4}\) processing strategies such as partial oxidation and dry reforming can benefit from the stepwise CL strategy. For DRM, chemical looping dry reforming of methane (CL-DRM; Figure 3) consists of two steps (Equations (8) and (9)). First, CH\({}_{4}\) is reacted with an OC and is oxidized to produce a hydrogen-rich syngas mixture (H\({}_{2}\)/CO = 2.0). This step is usually called the \"reduction step\" since the OCs are reduced in the process of syngas production. Second, carbon dioxide is used as an oxidant to recycle the OCs in the usually called \"oxidant step\", producing CO in the process, which can be recovered separately or mixed with the syngas from the first step [125]. \[\text{CH}_{4}+\text{OC}_{\text{oxidized}}\to 2\text{H}_{2}+\text{CO}+\text{OC}_{ \text{reduced}} \tag{8}\]  
\[\text{CO}_{2}+\text{OC}_{\text{reduced}}\to\text{CO}+\text{OC}_{\text{ oxidized}} \tag{9}\]The CL-DRM operation offers numerous advantages when compared to the classical co-feed strategy. Overall, the global process corresponds to a classically operated DRM reaction and still presents an endothermic character. However, the equilibrium of the reaction is no longer dependent solely on the thermodynamics and the reactivity of the reactants and the catalysts, but rather on the redox properties of the OCs, which are the major focus of research and optimization [126]. [PERSON] and [PERSON] [126] evaluated the thermodynamic equilibrium of CL-DRM as a function of the OC:CH\({}_{4}\) ratio, using Fe\({}_{2}\)O\({}_{3}\) as OC. For OC:CH\({}_{4}<1.0\), POM is the main observed reaction (Equation (10)), yielding H\({}_{2}\) and CO as the main products (H\({}_{2}\)/CO \(\approx\) 2.0). For OC:CH\({}_{4}>1.0\), H\({}_{2}\) and CO production decrease as temperature increases, and CH\({}_{4}\) combustion is favored (yielding CO\({}_{2}\) and H\({}_{2}\)O) as the OC:CH\({}_{4}\) also increases, reaching total methane combustion for OC:CH\({}_{4}>12.0\). Overall, the authors evaluate that an optimal OC:CH\({}_{4}=1.0\) could be used to obtain syngas with H\({}_{2}\)/CO = 1-2.5 according to the CH\({}_{4}\)/CO\({}_{2}\) feed ratio. Furthermore, two problems observed on DRM can be avoided by the stepwise operation of CL-DRM. The accumulation of carbon on the catalysts (which is now an OC) due to CH\({}_{4}\) dissociation on the reduction step is overcome by the re-oxidation cycle. In this scenario, the deposited coke is converted to CO during the oxidation step. Moreover, since the two reactants, CH\({}_{4}\) and CO\({}_{2}\), are not in contact with CL-DRM operation, the produced H\({}_{2}\) on the reduction step is never in contact with the CO\({}_{2}\) injected on the oxidation step. Hence, the occurrence of RWGS reaction (Equation (6)), which decreases the H\({}_{2}\) selectivity, is also avoided [127; 128]. [PERSON] et al. [129] investigated the performance of a series of CeO\({}_{2}\)-based oxygen carriers with Ni, Co and Fe for CL-DRM application. The authors evaluated the influence of preparation method and metal active phase on the catalytic activity of the obtained powders. The catalytic activity of all samples was tested over 12 cycles of CL and presented a relatively stable conversion of both CH\({}_{4}\) and CO\({}_{2}\) over the reduction/oxidation cycles.