Langmuir 1993,9, 162-169
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Borohydride Reduction of Cobalt Ions in Water. Chemistry Leading to Nanoscale Metal, Boride, or Borate Particles George N. Glavee, Kenneth J. Klabunde,’ Christopher M. Sorensen, and George C. Hadjipanayis Departments of Chemistry and Physics, Kansas State University, Manhattan, Kansas 66606, and Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716 Received June 15,1992. I n Final Form: September 28,1992 The reduction of Co2+(aq)by NaB& has been studied with mechanistic and stoichiometricfactors as the focus. The primary product of this reduction in aqueous solution is nanoecale Co2B particles, which and this complex are formed possibly through the intermediacyof [(H~0)~CoOH(BH2)HOCo(H~O)~13+, ion is reduced by electronsprovided by 3 more mol of NaBH4. The ratio of BH4-/Co2+ required is 2, and the overall balanced equation is
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2CoC1, + 4NaBH, + 9H2O Co2B+ 4NaC1+ 12.5H2+ 3B(OH), However, a side reaction where NaJ3H4 reacts directly with H2O becomes important if sufficient time is available before CoZ+addition or if Co/CozB particles are allowed to catalyze it. In this way NaBOz can be formed that can react with Co2+to give Co(B02)2as a product, which precipitates. A secondaryproduct is metallic Co particles, which are formed by heat treatment of C02B/Co(B02)2mixtures, or by sacrificial oxidation of Goa. A tertiary product is Co$, and this is formed by heat treatment of a mixture of CozB and Co. These studies show that the primary product C e B is best prepared by rapid mixing of CoTaq) with solid NaBH,, and the Co2B precipitate must be handled under anaerobic conditions. However, by adjusting conditions of reaction time, mixing procedure, and concentrations,a mixture of Co2B and Co(BO& can be produced, which upon heat treatment yields metallic c o nanoscale particles plus boron oxides. Magnetic and physical properties of the fresh C02B are as follows: surface area = 16.9 m2/g, crystallite size =
