Topochemical reduction of FeCo-oxide to FeCo-alloy nanosystems into a SiO2 matrix

This study focuses on the synthesis of metallic magnetic nanosystems embedded in mesoporous silica (SiO2), and the impact of matrix porosity, controlled by temperature treatment, on the efficiency of H2 reduction process. The reduction of FeCo oxides to the corresponding alloy nanosystems was first optimized, identifying FeCo with 50 at% Fe as the optimal composition due to its high saturation magnetization (∼242 A m2 kg−1) and oxidation onset temperature (∼440 °C). Then, the FeCo-oxide nanocomposites were synthesized into SiO2via sol-gel self-combustion under thermal treatments, to properly tune the surface area of the silica matrix. By controlling the annealing temperature, the specific surface area (SA) of the matrix decreases from ∼512(1) m2 g−1 to ∼345(1) m2 g−1 when annealed to 900 °C in air. Following topochemical reduction in H2, the structural properties of the obtained FeCo-SiO2 nanocomposites have been analyzed using X-ray powder diffraction and magnetic properties were evaluated to establish a correlation between matrix SA and reduction capability. The decrease of SA leads to incomplete reduction at higher temperatures, with the formation of FeYOX/CoXOY intermediates. This work underscores the critical role of matrix porosity in achieving a delicate balance to ensure both the efficient conversion of nanostructured oxide to their metallic state and the preservation of their magnetic and structural integrity.