Electrochemical Preparation of Fine Powders of Nickel-Boron Alloys in Molten Chlorides for Magnetic Hydrogenation Catalysts

Abstract

Fine powders of magnetic nickel-boron alloys were prepared for the first time by direct electrochemical reduction of the 2Ni3(BO3)2/B2O3, Ni3(BO3)2/NiO and Ni3(BO3)2/3NiO composite precursors in molten CaCl2-NaCl at 700°C. Precursors with different nickel to boron atomic ratios (RNi/B = 1, 2 or 3) were synthesized by the sol-gel process followed by thermal annealing at 750°C. The reduction pathway, phase transformation and morphology of the samples electrolyzed at different voltages and times were analyzed by XRD, SEM, and EDX. When the applied voltage reached to 3.0 V, pure Ni3B can be obtained from the oxide precursor with RNi/B = 3. However, the oxide precursors with RNi/B = 1 and 2 both resulted in mainly Ni2B. It was found that all the precursors were firstly reduced to Ni metal and Ca3(BO3)2, and then to Ni3B, and finally to Ni2B for RNi/B = 1 and 2. SEM observation showed that the prepared crystalline Ni3B and Ni2B powders consisted of uniform nodular particles with sizes of 200∼500 nm. These Ni-B alloys exhibited high catalytic activity to the hydrogenation reaction between p-nitrophenol and NaBH4. Their good ferromagnetism allows them to be easily recovered and reused.