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

Abstract

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 gDFM−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.