Mechanisms Ni-based dual functional materials for ICCU-RWGS

There is extensive research on carbon capture using adsorption. The reaction mechanism for CO₂ capture could be complicated when catalytic sites (e.g. Ni) are involved. Furthermore, the utilisation of captured CO₂ is different from conventional gas-gas phase reactions for CO₂ conversion. In ICCU, gas-solid reactions are unknown. Should CO₂ be released from the sorbent first, then proceed to CO₂ reduction, or could the reduction agent (e.g. H₂) react directly with the solid phase CO₂ adsorbent?

The following sentences are copied from this reference (https://doi.org/10.1016/j.ccst.2022.100052) regarding the reaction mechanism of Ni-based DFMs for ICCU-RWGS.

“Since CH₄, CO, and CO₂ coexisted in the methanation off-gas, the CO₂ conversion was proposed to be a two-step process, namely, the release of CO₂ from CaCO₃ and the reaction with H₂ to produce CH₄ and CO (Ma et al., 2021). By comparing the CaO, 1wt%Ni/CaO, and 10wt%Ni/CaO in TPSR tests, it was found that CO was generated on carbonated CaO even though no Ni was added. With increasing the Ni from 1 to 10 wt%, the CH₄ yield significantly decreased while CO production was comparable. It was thus inferred that the methanation reaction happened on the Ni site, while the RWGS mainly happened on CaO with the generation of CO (Ma et al., 2021). The generation of CO rather than CH₄ was also reported by Sun et al. when using Ce-CaO (without addition of Ni) in the carbonation-methanation cycle (Sun, H. et al., 2022; Sun, S. et al., 2021a). On Ni/CeO₂-CaO surface, it was found formate, calcite carbonate, bicarbonate species, and CO species presented after CO₂ capture with the in-situ DRIFTS study. After introducing H₂, the intensity of formate and calcite carbonate remained unchanged, while that of CO decreased. This indicates that the CO is possibly the main intermediate being converted to CH₄ (Sun, H. et al., 2022), as illustrated in Fig. 12. By contrast, the surface formate and carbonate species intensity decreased upon H₂ injection with the generation of gas-phase CH₄ on Ni-MgO-Al₂O₃ (Zhou et al., 2020). The authors found that CO is less likely to be the intermediate for methanation as no signal of CO was detected during the FTIR test, indicating that methanation should proceed with direct hydrogenation of formate. Additionally, the carbonate is more likely to bond with metal oxides, it was assumed that the hydrogenation was accompanied by the migration of CO₂ to the metallic Ni surface (Zhou et al., 2020), namely the spillover of CO₂ as reported by Farrauto et al. (Arellano-Treviño et al., 2019b).”

“Fig. 12. Reaction mechanism on (a) CeO₂-CaO (b) Ni/CeO₂-CaO for CO₂ capture and methanation (Sun, H. et al., 2022).”