Efficient Fe₃O₄/Fe Redox Cycle with Biomass Waste Glycerol for Net Carbon Benefit CO₂ Reduction

CO₂ utilization is a critical aspect of achieving a sustainable carbon cycle, particularly in the context of global efforts to achieve carbon neutrality. Drawing inspiration from geological chemistry, Fe-based hydrothermal CO₂ reduction into valuable chemicals has emerged as a promising CO₂ utilization strategy. However, the lack of a sustainable and direct Fe regeneration approach presents a notable challenge to the widespread adoption of this strategy. Herein, we propose a method for the direct reduction of Fe₃O₄ to Fe using biodiesel-waste glycerol. This method yields a remarkable 97.9 wt % of reduced Fe, which exhibits a high activity for CO₂ (HCO₃ ^-) reduction to formic acid, maintaining a level of ~90 %. Our investigation reveals that the Fe₃O₄ reduction involves a direct hydrogen transfer from hydroxyl groups to lattice O atoms on the surface of Fe₃O₄, forming reductive H species. The presence of a polyhydroxy structure in glycerol facilitates the stabilization of surface H species, thereby enhancing the reduction efficiency process. Based on this mechanism, we explore the potential of using various polyols derived from woody biomass, which exhibit similar capabilities for the reduction of Fe₃O₄ as glycerol. These findings establish an efficient and sustainable Fe₃O₄/Fe redox cycle, which integrates waste biomass into circular carbon economy solutions and contributes to the overall net carbon benefit of CO₂ utilization.