Fischer–Tropsch synthesis: Temperature programmed EXAFS/XANES investigation of the influence of support type, cobalt loading, and noble metal promoter addition to the reduction behavior of cobalt oxide particles

TPR-XANES/EXAFS carried out using a novel multi-sample holder provided key information for verifying the nature of the chemical transformations occurring during cobalt Fischer–Tropsch synthesis catalyst activation in hydrogen. In the past, assumptions had to be made regarding the nature of the cobalt species present along the trajectory of a standard TPR experiment. The new technique directly provided insight into (a) the nature of the reduction process of cobalt oxide species and (b) the resulting cobalt crystallite size, as a function of the strength of the catalyst support interaction with the cobalt oxide species. A two-step reduction process involving Co3O4 to CoO and CoO to Co0 transformations over standard calcined catalysts was observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO2) and strong interactions (e.g., Co/Al2O3) with the support. Noble metal promoter (e.g., Pt) addition strongly improved the reducibility of cobalt oxide species, most likely via a H2 dissociation and spillover mechanism. Increasing cobalt loading, on the other hand, led to a measurable, but lesser, improvement on reducibility, due to the larger resulting particle size that resulted in less surface contact with the support. Higher reduction temperatures were needed to effectively reduce cobalt oxide particles deposited on strongly interacting surfaces in comparison with unsupported Co3O4 or only weakly interacting supported cobalt catalyst. Nevertheless, despite lower extents of reduction, the smaller resulting Co particles on the more strongly interacting catalysts generally led to higher Co0 active site densities. The addition of the noble metal promoter to strongly interacting supported catalyst significantly decreased the temperature required to reduce the cobalt oxides to Co0 particles; this allows one to take advantage of the higher Co0 surface areas arising from the combination of a smaller average Co0 particle size and a higher extent of reduction.