Mechanism for enhancing dispersion of Co3O4nanoparticles in Co/SiO2Fischer–Tropsch synthesis catalyst by adding glycol to impregnating solution: a quick-XAFS study

Abstract

In situCoK-edge quick-EXAFS (QEXAFS) coupled with temperature-programmed oxidation as well asex situXAFS was applied to investigating the mechanism for enhancing the dispersion of Co3O4nanoparticles in a calcined Co/SiO2Fischer–Tropsch synthesis catalyst prepared by adding triethylene glycol (TEG) to a Co(NO3)2.6H2O impregnating solution.Ex situCoK-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 situQEXAFS andex situEXAFS 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 [Koizumiet 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, Co3O4-like species were formed, and agglomeration of the Co3O4-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 Co3O4and Co0nanoparticles during calcination and H2reduction, respectively.