Cobalt precipitation by reduction with sodium borohydride

The reduction of cobalt(II) with borohydride is very complicated, as evidenced by the fact that various authors have obtained different reaction stoichiometries and have proposed a number of mechanisms. To clarify the cobalt reduction process, the reaction stoichiometry and reduction efficiency were studied using a controlled rate of addition of sodium borohydride in the temperature range 5–35°C and at pH values from 2 to 7.8. The efficiency of cobalt reduction increased with increasing concentration of NaOH in the reducing solution, the best reduction efficiency without the precipitation of cobalt hydroxide being 1 mole of sodium borohydride to reduce 1 mole of cobalt(II). The reduction efficiency increased with increasing pH, from nil at pH 2 to 96% at pH 6, and decreased with increasing temperature. X-ray diffraction patterns and TEM patterns of the recovered precipitates showed them to be amorphous. After a 2 h heat treatment at 500°C, the X-ray diffraction pattern of the precipitate showed well defined peaks due to Co2B, with the main peak attributable to cobalt. The single crystal TEM pattern obtained was consistent with that of Co2B. The particle size was about 20–100 nm. The atom ratio of Co to B increased with increasing temperature. Zinc ions have a dramatic inhibitory effect on cobalt reduction. Several tens of micromoles per litre of zinc ions completely inhibit cobalt reduction with borohydride. The main cause of inhibition is that zinc ions compete with those of cobalt for borohydride ions and zinc borohydride forms and hydrolyzes rapidly.