Mechanism for enhancing dispersion of Co _3O _4 nanoparticles in Co/SiO _2 Fischer-Tropsch synthesis catalyst by adding glycol to impregnating solution: A quick-XAFS study

摘要

In situ Co K-edge quick-EXAFS (QEXAFS) coupled with temperature-programmed oxidation as well as ex situ XAFS was applied to investigating the mechanism for enhancing the dispersion of Co _3O _4 nanoparticles in a calcined Co/SiO _2 Fischer-Tropsch synthesis catalyst prepared by adding triethylene glycol (TEG) to a Co(NO _3) _2.6H _2O impregnating solution. Ex situ Co K-edge XAFS indicated that, regardless of whether the catalysts were prepared with or without using TEG, the hexaaqua Co (II) complex was formed in impregnated samples which then underwent the dehydration process to some extent during the subsequent drying step at 393 K. In situ QEXAFS and ex situ EXAFS results also indicated that small oxide clusters were formed in the TEG-modified catalyst calcined at 400-470 K which interacted with polymer species derived from TEG. Since the Fischer-Tropsch synthesis activity of the TEG-modified catalyst increased with an increase in the calcination temperature in a similar temperature range [Koizumi et al. (2011), Appl. Catal. A, 395, 138-145], it was suggested that such an interaction enables the clusters to be distributed over the support surface uniformly, resulting in enhancing their dispersion. After combustion of polymer species, Co _3O _4-like species were formed, and agglomeration of the Co _3O _4-like species at high calcination temperatures was suppressed by the addition of TEG to the impregnating solution. It was speculated that the addition of TEG induced the formation of some surface silicate which worked as an anchoring site for Co _3O _4 and Co ~0 nanoparticles during calcination and H2 reduction, respectively.