Nanocrystalline High-Entropy (Co,Mn,Ni,Cr,Fe)3O4 Spinels with Varying Fe Content for Environmental and Energy Catalysis.

In this work, nanocrystalline high-entropy spinels with the nominal composition (Co,Mn,Ni,Cr,Fe)3O4 and different Fe contents were synthesized by a facile solution combustion method and evaluated as catalysts for the complete oxidation of light hydrocarbons and methanol decomposition. Structural characterization by XRD, SEM, TEM, XPS, Mössbauer spectroscopy, and N2 physisorption confirmed the formation of single-phase cubic spinels and mesoporous morphology. Increasing the Fe content resulted in changes in surface elemental distribution without altering the oxidation states of the cations. Among the investigated catalysts, HES-Fe 1:1 exhibited the highest specific surface area, favorable surface enrichment in Co and Mn, and the best catalytic performance for the complete oxidation of light hydrocarbons. In methanol decomposition, HES-Fe 1:1 showed the highest methanol conversion at lower temperatures, whereas HES-Fe 1:2 exhibited the highest CO selectivity, making it the most efficient catalyst for syngas production. The results establish a relationship between Fe content, surface composition, redox properties, and catalytic performance, revealing high-entropy spinels as promising catalysts for environmental catalysis and syngas production.