Synthesis, characterization, and catalytic reduction CO₂ properties of spinel Nano-Mn_XFe_3-XO₄: Effect of X on Mn³⁺/Mn²⁺ cation occupancies and CO₂ reduction mechanism

Low-temperature catalytic reduction of CO2 to carbon (C) using spinel Nano-MnXFe3-XO4 has shown significant promise for carbon emission reduction. Previous investigations had primarily concentrated on X <= 1.0, and it was unknown the proper Mn and Fe ratios for the emerging approach of synthesizing manganese ferrites for CO2 reduction utilizing non-standard sources rather than pure substances. In this study, we designed and synthesized a series of spinel Nano-MnXFe3-XO4 (0.25 <= X <= 2.00, incrementing by 0.25) products using MnO2 and Fe2O3. The crystal structure, cation occupancy, mechanism of oxygen deficiency formation under H-2 reduction, and CO2 catalytic reduction properties of Nano-MnXFe3-XO4 were characterized. The results revealed that the optimal X-value range for CO2 catalytic reduction was 1.00-1.75. Among these values, CO2 could be completely reduced to amorphous and graphitic C, and the amount of CO2 reduction improved with increasing X values. At X = 1.75, CO2 reduction reached a peak of 5.95 mmol/g, which was better than the maximum value of 4.06 mmol/g at X = 0.78 as reported previously. The chemical valence transformation of Mn3+/Mn2+ (B-site) in the Nano-MnXFe3-XO4 lattice has been shown to be responsible for this. At X < 1.00, incomplete reduction of CO2 leading to CO byproduct generation occurred due to ionic interactions converting to Fe3+/Fe2+ (B-site); at X > 1.75, CO2 reduction significantly decreased because of space group conversion (Fd-3 m -> I41/amd) of Nano-MnXFe3-XO4. This work establishes an acceptable and feasible Mn/Fe ratio interval for synthesizing Nano-manganese ferrite from non-standard materials for efficient CO2 reduction.