# 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)
Ni/CeO\({}_{2}\) presented the best catalytic performance (T = 800 \({}^{\circ}\)C, X\({}_{\rm CH_{4}}\) = 90%, X\({}_{\rm CO_{2}}\) = 85%) and maintained a H\({}_{2}\)/CO ratio slightly above 2.0, which is the ratio expected for CL-DRM (Equations (8) and (9)). Despite a lower reactant conversion observed for CeNi\({}_{0.3}\)O\({}_{y}\) (T = 800 \({}^{\circ}\)C, X\({}_{\rm CH_{4}}\)= 62%, X\({}_{\rm CO_{2}}\) = 62%), the catalysts prepared by  
Figure 3: Schematic representation of classic co-fed DRM operation (**A**) and stepwise chemical looping DRM (**B**). co-precipitation presented no coke formation, a steady H\({}_{2}\)/CO = 2.0 over the 12 CL cycles and no particle sintering, which was related to a better dispersion of the Ni particles on the CeO\({}_{2}\) structure. The influence of the active metal (Ni, Co and Fe) was investigated over wet impregnation-prepared catalysts. The CL-DRM experiments showed that Co/CeO\({}_{2}\) (T = 800 \({}^{\circ}\)C, \(\chi_{\text{CH}_{4}}\)= 80%, \(\chi_{\text{CO}_{2}}\)= 85%) catalysts presented a slightly lower CH\({}_{4}\) conversion when compared to Ni/CeO\({}_{2}\), while maintaining a similar H\({}_{2}\)/CO between 2.0 and 2.1. Moreover, similar results were obtained at long-term stability tests that comprised 60 CL cycles, with stable H\({}_{2}\)/CO ratio between 2.0 and 2.1. As for the Fe/CeO\({}_{2}\) catalyst, very low CH\({}_{4}\) conversion of less than 5% and rapid complete deactivation (after 6 CL cycles) were obtained. These results were related to the formation of poorly active CeFeO\({}_{3}\) mixed oxide. Interestingly, the analysis of spent catalysts showed that Ni and Co were not able to follow the oxidation/reduction cycles, i.e., once they were reduced on CL-DRM conditions, the metallic species were not oxidized again, which highlighted the importance of the oxygen storage capacity of CeO\({}_{2}\) on the operation of CL-DRM. [PERSON] et al. [130] described an efficient CL-DRM process using Ni-Fe-Al mixed oxides as oxygen carriers. In this study, the group showed the superior activity of the trimetallic mixed oxide over binary metallic mixtures, which were submitted to significant deactivation over 12 CL cycles, either by activity loss towards CH\({}_{4}\) decomposition or inefficient coke removal on the oxidation step. Overall, Ni-Fe-Al mixed oxides were able to maintain the catalysts' activity over 20 CL cycles, with no signs of deactivation, to avoid particle agglomeration and activity suppression and to deliver a stable H\({}_{2}\)/CO of 2.1. Interestingly, the authors also demonstrated the potential of Ni-Fe-Al mixed oxide for classical co-fed DRM. In this case, results showed that a H\({}_{2}\)/CO ratio slightly below 1.0 was achieved for CH\({}_{4}\):CO\({}_{2}\) = 1:1 feed at T = 650 \({}^{\circ}\)C, with over 80% conversion of both reactants. [PERSON] et al. [131] evaluated the perovskite-type La\({}_{0.9}\)Sr\({}_{0.1}\)FeO\({}_{3}\) supported on yttria-stabilized-zirconia (YSZ) for DRM and CL-DRM and reported a high H\({}_{2}\) production (H\({}_{2}\)/CO = 6.5) during the reforming step for the looping operation. The authors evaluated the DRM process at 850 \({}^{\circ}\)C for 24 h and obtained an H\({}_{2}\)/CO between 0.95 and 0.98 during the tests. For the CL operation, 20 min looping cycles were performed up to 7 h. The authors reported that for the reforming step (CH\({}_{4}\) injection), a predominant CH\({}_{4}\) cracking (Equation (3)) reaction took place and yielded high H\({}_{2}\)/CO ratios (up to 6.5) with high coke formation. On the other hand, coke was successfully removed during the oxidation step (CO\({}_{2}\) injection) and the catalysts did not present significant deactivation during CL operation.