Synthesis, characterization, and catalytic reduction CO2 properties of spinel Nano-Mn Fe3-O4: Effect of X on Mn3+/Mn2+ cation occupancies and CO2 reduction mechanism
Jia Wang, Zijian Su, Yuanbo Zhang, Tao Jiang
Abstract
Low-temperature catalytic reduction of CO 2 to carbon (C) using spinel Nano-Mn X Fe 3- X O 4 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 CO 2 reduction utilizing non-standard sources rather than pure substances. In this study, we designed and synthesized a series of spinel Nano-Mn X Fe 3- X O 4 (0.25 ≤ X ≤ 2.00, incrementing by 0.25) products using MnO 2 and Fe 2 O 3 . The crystal structure, cation occupancy, mechanism of oxygen deficiency formation under H 2 reduction, and CO 2 catalytic reduction properties of Nano-Mn X Fe 3- X O 4 were characterized. The results revealed that the optimal X -value range for CO 2 catalytic reduction was 1.00–1.75. Among these values, CO 2 could be completely reduced to amorphous and graphitic C, and the amount of CO 2 reduction improved with increasing X values. At X = 1.75, CO 2 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 Mn 3+ /Mn 2+ (B-site) in the Nano-Mn X Fe 3- X O 4 lattice has been shown to be responsible for this. At X < 1.00, incomplete reduction of CO 2 leading to CO byproduct generation occurred due to ionic interactions converting to Fe 3+ /Fe 2+ (B-site); at X > 1.75, CO 2 reduction significantly decreased because of space group conversion (Fd-3 m → I41/amd) of Nano-Mn X Fe 3- X O 4 . This work establishes an acceptable and feasible Mn/Fe ratio interval for synthesizing Nano-manganese ferrite from non-standard materials for efficient CO 2 reduction.