Cobalt-boron catalyst for NaBH4 hydrolysis: The state of the active component forming from cobalt chloride in a reaction medium

Abstract

The structure of a cobalt-boron catalyst obtained by reduction of cobalt chloride in an aqueous solution of sodium borohydride at ambient temperature has been investigated by TEM HR, XPS, EXAFS and X-ray diffraction method. It was found that nanoparticles of the forming catalyst consisted of a core and a shell. The oxygen-containing compounds (cobalt hydroxide and cobalt borates) mainly reside in the shell. In spite of the amorphous state of the catalyst the EXAFS and X-ray diffraction analysis indicated the presence of ferromagnetic clusters with the shortest CoCo distance of about 2.5Å and coordination number of 2.7 which are regularly distributed within the core with average separations of 5.8Å. The presence in the catalyst of the CoB bonds, a short CoCo distance and the absence of long CoCo distances (4.3 and 4.8Å) as well as the presence of boron bound to hydrogen allows suggestion to be made that stabilization of the cobalt tetramers occurs due to the presence of a boron-hydrogen structure formed by BH4− ions upon the reduction of cobalt chloride. To substantiate the above model of the active phase of the cobalt-boron catalyst the DFT method with a PBE functional in the plane-wave basis has been employed which showed that the ferromagnetic tetramers may exist within a boron-hydrogen matrix isolated from the surrounding medium by an oxygen-containing shell.