Intermediates and pathways of ICCU-RWGS

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

“For the production of CO (RWGS), the carbonyl and formate species were detected on CaNi_0.1Ce_0.033 with in-situ DRIFTS tests, indicating both the redox process and formate mechanism. Without Ce, CO₂ would spill over to Ni active sites with the formation of CO and NiO (NiO was then reduced by H₂); in the presence of Ce, CO₂ interacted with oxygen vacancy in CeO₂ and was released as CO, and then H₂ consumed the lattice oxygen from ceria to restore the oxygen vacancy (Sun et al., 2019).

Furthermore, the modification with promoters may alter the reaction pathways. For example, On Ni/ZrO₂, formate species was observed as the main intermediate for CH₄ production as suggested by the space and time-resolved operando DRIFTS investigation. After K modification, the formyl species cooperated with bidentate carbonates were found to be the active phase for methanation. While for La modification, the reaction pathway remained unchanged, as shown in Fig. 13 (Hu and Urakawa, 2018). On Ni-“Na₂O”/Al₂O₃ surface, CO₂ was adsorbed as bicarbonate and bidentate carbonates regardless of the oxidizing atmosphere. After loading with 1% Pt, CO-Pt species provided additional CO₂ sorption site during carbonation. During hydrogenation, CO₂-related species (bicarbonate, bidentate carbonates, and CO-Pt) spill over to the Ni site for methanation since Pt was inactive for the methanation reaction. While for Ru modification, CO₂ was proposed to spill over to both Ni and Ru sites for methanation via formate intermediate, thus Ru demonstrated a greater promotional effect (Proaño et al., 2020).

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“Fig. 13. Space- and time-resolved operando DRIFT spectra of ZrO₂, Ni/ZrO₂, Ni-K/ZrO₂, Ni-La/ZrO₂ under 350 °C for ICCC-Met (Hu and Urakawa, 2018).”