Advances in understanding Direct Hydrogen Reduction for the Preparation of Metallic Nanoparticles

Abstract

Due to their unique properties, nanomaterials find diverse applications across various fields, including catalysis, electronics, and energy storage. In this work, the hydrogen reduction-assisted ultrasonic spray pyrolysis method was used to synthesize simple alloy (Fe-Si and Fe-Pt) and complex alloy (AgCoCuFeNi) nanoparticles. The first part of the study focused on understanding the hydrogen reduction behavior of iron trioxide (Fe2O3) in the absence and presence of additives (SiO2 or Pt). In the absence of additives, the oxygen content of iron oxide particles decreased with increasing temperature from 700 to 950 °C but significantly increased with the doping of 10 ml (40 wt.%) of SiO2 at both reaction temperatures. The inhibitory effect of Si on the hydrogen reduction of Fe2O3 was more pronounced at 950 °C than at 700 °C. In contrast, the doping of only 5 ml (15 wt.%) of Pt significantly decreased the oxygen concentration in the synthesized particles by catalyzing the reduction reaction of Fe2O3 at 700 °C. The metallic iron (Fe) product, obtained in the undoped Fe2O3 run at only 950 °C, was also formed at 700 °C in the Pt-doped Fe2O3 run. The second part of the study consisted of investigating the influence of processing parameters (reaction temperature, precursor solution concentration, and residence time) on the microstructure, composition, and crystallinity of AgCoCuFeNi high entropy alloy nanoparticles using scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The syntheses performed at 600, 700, 800, and 900 °C using a 0.05 M precursor mixture of metal salts resulted in smaller and smoother spherical particles with equiatomic elemental composition as temperature increased to 900 °C. With 0.25, 0.1, and 0.05 M precursor solutions at 900 °C, narrower size distribution and uniform AgCoCuFeNi nanoparticles were produced by reducing the solution concentration to 0.05 M. Equiatomic elemental composition was only obtained at 0.25 and 0.05 M concentrations. Increasing residence time from 5.3 to 23.8 s resulted in unclear particle microstructure. X-ray diffraction revealed that various crystal phase structures were obtained in the synthesized AgCoCuFeNi particles.