Heterojunction-redox catalysts of FexCoyMg10CaO for high-temperature CO2 capture and in situ conversion in the context of green manufacturing

Abstract

The integration of carbon capture and CO₂ utilization could be a promising solution to the crisis of global warming. By integrating calcium-looping (CaL) and the reverse-water-gas-shift (RWGS) reaction, a high-temperature CO₂ capture and in situ conversion technology is successfully realized in one fixed-bed column at the same operating temperature of 650 °C. Inspired by the heterojunction photocatalytic mechanism, the heterojunction-redox catalysis strategy is proposed for the first time by doping the bimetallic Fe³⁺/Fe²⁺ and Co³⁺/Co²⁺ redox couples into a hierarchical porous CaO/MgO composite. The presence of different valence states of doped Fe and Co oxides not only provides extra oxygen vacancies to facilitate CO₂ adsorption, and hence adsorption enhanced conversion (AEC), but also significantly lowers the electric potential difference of Fe³⁺/Fe²⁺ through the newly formed Fermi level in Fe₅Co₅Mg₁₀CaO, which makes electron spillover easier to improve the catalytic activity in the RWGS reaction for CO₂ conversion. More importantly, with the high-temperature refractory MgO and the highly disperse Fe and Co oxides in Fe₅Co₅Mg₁₀CaO, the problem of CaO sintering is successfully solved. An excellent and stable high-temperature CO₂ capture capacity of 9.0–9.2 mmol g⁻¹, an in situ CO₂ conversion effeciency near 90% and a CO selectivity close to 100% are achieved in the integrated CaL/RWGS process. In addition, experimental and simulation scale-up studies further demonstrate its pratical scalability. Economic evaluation reveals that the integrated CaL/RWGS technology is much more cost-effective than the individual CaL and RWGS processes. Therefore, the heterojunction-redox strategy provides a unique way to design bifunctional adsorbent/catalyst materials. The integrated CaL/RWGS process could be a promising technology for CO₂ capture and utilization.