Ni promoted Fe-CaO dual functional materials for calcium chemical dual looping

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. 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.