Reverse water-gas shift reaction using renewable H₂ is a promising route for CO₂ upgrade, however, it is restricted by the equilibrium. The chemical looping reverse water-gas shift reaction using oxygen carriers (i.e. Fe) has proven a more effective CO₂ utilization process to produce CO. However, CO₂ with high purity is needed to obtain concentrated CO. Herein, we propose a calcium chemical dual looping using one-pot sol-gel synthesized Ca-Fe dual functional materials (DFMs). The CO₂ in the exhaust gas (~10% CO₂) can be captured and transformed into carbonates and then in-situ converted into CO through continuous chemical looping in H₂ atmosphere. This process avoids CO₂ enrichment, storage and transportation and simultaneously realizes efficient CO₂ conversion. The Ca-Fe DFMs possessed significantly improved catalytic efficiency (enhanced real-time CO generation rate) compared to CaO. Furthermore, introducing a small amount of Ni could stabilize the crystallite size of Fe by forming an Ni-Fe alloy. It is found that Ni₁Fe₉-CaO could optimally achieve 11.3 mmol g_DFM^-1 CO yield, 82.5% CO₂ conversion and 99.9% CO selectivity at 650 °C. Notably, Ni₁Fe₉-CaO displayed high CO₂ conversion (> 80%) and CO selectivity (> 99.9%) during the cycle tests and possessed enhanced stability in relation to CO yield after 10 cycles (20.9% and 35.5% decrease for Ni₁Fe₉-CaO and CaO, respectively). Herein, Ca₂Fe₂O₅ plays two roles: acting as an oxygen carrier for in-situ chemical looping to produce CO and a thermally stable physical barrier to retard the sintering of CaO. It is noted that Fe-related species could be reduced into the metallic state at the end of hydrogenation, resulting in CO formation in the following CO₂ capture process. Introducing steam or air purge before carbonation can significantly inhibit CO formation at the carbonation stage without affecting the catalytic performance at the hydrogenation stage.
 

chemical looping, calcium looping, dual functional materials