Evidence of bifunctional redox-mechanisms during cyclic CO₂ Storage–Regeneration technology over unsupported Ni-based catalysts

CO2 capture and catalytic conversion technologies are attracting interest for cyclic high-temperature processes aimed at carbon utilization and H2-rich syngas production. The CO2 Storage–Regeneration (CO2-SR) process follows a cyclic operation conceptually analogous to NOx Storage–Reduction (NSR) systems. Nanostructured unsupported Ni-based bimetallic catalysts promoted with Ba or K were investigated to evaluate how the alkaline promoter modifies oxygen mobility, vacancy generation and cyclic redox stability during CO2-SR operation.

NiBa and NiK catalysts (Ni:Me = 10:1; Me = Ba or K) were prepared by ultrasound-assisted coprecipitation and characterized by XRD, XPS, Raman, CO2-TPD and H2-TPR, with special attention to in situ FTIR and cyclic transient oxidation–regeneration experiments.

The combined FTIR and transient results revealed continuous reduction–reoxidation cycles. The proposed reaction pathway involves CH4/H2 activation on metallic Ni during regeneration, generating oxygen vacancies and reduced interfacial sites, followed by CO2 adsorption, carbonate formation and replenishment of these sites through reversible oxygen-transfer mechanisms during the storage step. The results suggest a bifunctional redox mechanism in which metallic Ni sites promote H2/CH4 activation, whereas Ba- or K-modified oxide domains participate in CO2 storage and oxygen exchange. In contrast to conventional DRM, CO2-SR involves sequential Storage–Regeneration steps governed by cooperative metallic/redox-active domains.

Although the apparent amount of stored CO2 was comparable for both catalysts, NiBa exhibited higher mobility of labile interfacial oxygen species and enhanced regeneration of surface/interfacial oxygen vacancies, leading to improved cyclic redox stability. FTIR carbonate speciation and transient CO/H2 evolution suggest that Ni–BaO domains behave as reversible dynamic redox-active interfacial centers, promoting oxygen exchange, carbon removal, and enhanced H₂ formation. In contrast, Ni–K2O domains exhibit more limited reversibility and higher transient CO formation, negatively affecting CO2-SR performance. These findings provide mechanistic insights for CO2-SR scale-up, currently at early laboratory development stage (TRL 2–3).