Borohydride reduction of cobalt ions in water. Chemistry leading to nanoscale metal, boride, or borate particles

Abstract

The reduction of Co2+(aq) by NaBH4 has been studied with mechanistic and stoichiometric factors as the focus. The primary product of this reduction in aqueous solution is nanoscale C02B particles, which are formed possibly through the intermediacy of [(H20)5Co0H(BH2)HOCo(H2O)6]3+, and this complex ion is reduced by electrons provided by 3 more mol of NaBH4. The ratio of BH4*/Co2+ required is 2, and the overall balanced equation is 2CoC12 + 4NaBH4 + 9H20 -Co2B + 4NaCl + 12.5H2 + 3B(OH)3 However, a side reaction where NaBH4 reacts directly with H20 becomes important if sufficient time is available before Co24 addition or if C0/C02B particles are allowed to catalyze it. In this way NaB02 can be formed that can react with Co2+ to give Co(B02)2 as a product, which precipitates. A secondary product is metallic Co particles, which are formed by heat treatment of Co2B/Co(B02)2 mixtures, or by sacrificial oxidation of C02B. A tertiary product is C03B, and this is formed by heat treatment of a mixture of C02B and Co. These studies show that the primary product C02B is best prepared by rapid mixing of Co2+(aq) with solid NaBH4, and the C02B precipitate must be handled under anaerobic conditions. However, by adjusting conditions of reaction time, mixing procedure, and concentrations, a mixture of C02B and Co-(B02)2 can be produced, which upon heat treatment yields metallic Co nanoscale particles plus boron oxides. Magnetic and physical properties of the fresh Co2B are as follows: surface area = 15.9 m* 12

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