Cobalt nanoparticles prepared in faujasite zeolites by borohydride reduction

Metallic cobalt nanoparticles have been prepared in the faujasite zeolite by the reduction of the Co2+-exchanged zeolite with sodium borohydride aqueous solutions. The influences of the temperature used for treating the Co2+-faujasite zeolite before reduction and the concentration of NaBH4 solution on the degree of reduction and the size of cobalt particles have been investigated. Although the treatment of the Co2+-exchanged faujasite zeolite at a higher temperature before reduction leads to a lower degree of reduction, an appropriate treatment temperature is needed for obtaining cobalt nanoparticles with sizes less than 5 nm. Cobalt particles with a maximum size distribution at 1-2 nm, which are probably located inside the supercages of faujasite zeolite, can be obtained by using a higher concentration of NaBH4 aqueous solution (10 M), while lower NaBH4 concentrations lead to the formation of larger cobalt particles (>10 nm) mainly located outside the cages of the zeolite. The smaller cobalt nanoparticles located inside the supercages of faujasite zeolite exhibit higher CO conversions in Fischer-Tropsch synthesis than the larger cobalt particles outside the supercages or the cobalt on the surface of non-porous silica. Unique product distributions are observed in the catalysis over the smaller cobalt nanoparticles in the supercages of faujasite zeolite.