General Flame Approach to Chainlike MFe₂O₄ Spinel (M = Cu, Ni, Co, Zn) Nanoaggregates for Reduction of Nitroaromatic Compounds

Unique chainlike MFe₂O₄ (M = Cu, Ni, Co, and Zn) nanoaggregates (NAs) have been prepared by facile flame spray pyrolysis of nitrates/ethanol precursor. Synthesized MFe₂O₄ are composed of primary nanocrystallites mainly ranging from 8 to 20 nm and show uniform aggregations with size of 200-400 nm. As heterogeneous catalysts, it is clearly noted that CuFe₂O₄ NAs have the highest catalytic activity for the reduction of nitroaromatic compounds compared to NiFe₂O₄, CoFe₂O₄, and ZnFe₂O₄. The highest rate constant of 36.17 min^-1·g^-1 is achieved at a low catalyst usage (10 μL, 0.2 mg·mL^-1). The remarkably enhanced catalytic performance of CuFe₂O₄ NAs is mainly attributed to the promoted electron transfer on Cu²⁺ active sites, which is facilitated by unique spinel structures and chainlike aggregate morphology. It is demonstrated that flame spray pyrolysis technique is an effective route to produce binary or complex oxides for potential industrial use.