Production of rhenium nanoparticles by low-temperature aqueous-phase reduction of NH4ReO4 using NaBH4

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The synthesis of refractory rhenium (Re) powders is significant for the production of rhenic products. In this work, the ultra-fine Re nanoparticles were synthesized via a combination of aqueous-phase reduction and rapid hydrogen reduction using NH4ReO4 as Re source. The theoretical reduction route was determined by investigating the Eh-pH diagram of the Re-H2O system and conducting thermodynamic calculations. The optimal results were achieved by adjusting the experimental parameters like acid type, initial acidity, molar ratio of BH4−: ReO4−, feed rate of reductant, reaction temperature (T), and reaction time (t). The amorphous Re powders with minor oxide inclusions can be prepared under the optimized conditions of [HCl] = 1.35 mol L−1, BH4−: ReO4− molar ratio = 5.32, feed rate = 15 mL min−1, reaction temperature = 30 °C, and reaction time = 30 min. The maximum reduction efficiency reaches 99.5%. After annealing in hydrogen for 30 min, the crystalline Re nanoparticles with low oxygen content can be obtained. This work demonstrates the feasibility and efficacy of rapidly synthesizing ultra-fine Re nanoparticles in a solution-based approach.