Amorphous ferromagnetic cobalt-boron composition reduced by sodium borohydride: Phase transformation at heat-treatment and its influence on the catalytic properties

Abstract

The cobalt-boron composition by reduction of cobalt chloride in an aqueous solution of sodium borohydride is an amorphous ferromagnetic black powder consisting of spherical particles covered with a shell. Transformations of cobalt-boron composition upon its heat treatment from 20 to 1000°С in an inert atmosphere have been studied by thermal analysis, magnetic susceptibility and HR TEM. On the basis of the obtained results the heating process was divided into five temperature intervals characterized by considerable changes in the structure of cobalt-boron composition. The end products of chemical and structural transformations are cobalt metal and cobalt borates – Co2B2O5, Co3BO5, Co3(BO3)2. In our study it was shown for the first time that the metallic phase with a face-centered cubic lattice forms from the amorphous core while the cobalt borates form as a result of transformation of an oxygen-containing shell under an inert atmosphere. The study of the effect of physicochemical properties of the calcined cobalt-boron composition on the rate of NaBH4 hydrolysis has shown that it decreases with decline specific surface area of the sample and its phase composition determines the rate of formation of the catalytic active component in the reaction medium.