ABSTRACT
Current understanding of Co-catalyzed Fischer-Tropsch synthesis (FTS) is that cobalt clusters should be formulated in a manner such that particles are not so large as to diminish surface Co site densities (i.e., due to excessive cobalt being locked within the bulk of the metal particle), but not too small to either hinder the reduction of cobalt oxides to the active metallic state (due to either strong interfacial interactions with the support or cobalt-support compound formation) or result in instability in the presence of water (diameter <2-4 nm suggested to oxidize Co under FTS conditions). In this contribution, a chemical vapor deposition (CVD) method was utilized to deposit nanosized Co oxide crystallites. Silica displayed significant interactions with a fraction of the Co oxide prepared through CVD, while SiC interacted strongly with the entire amount of Co oxide. This was remarkable, as SiO2 and SiC have generally been considered to be weakly interacting supports to Co oxide crystallites prepared using standard impregnation methods. CO conversion was higher for aqueous-impregnated catalysts, consistent with a higher Co site density. Despite the fact that Co clusters for CVD catalysts were much smaller, a significant fraction remained unreduced following the standard activation in hydrogen. Consistent with our earlier continuous stirred-tank reactor tests with Pt-Co/carbon and Pt-Co/TiO2 catalysts, selectivities to oxygenates were higher for both CVD catalysts relative to the ones prepared using aqueous impregnation, and the effect was more pronounced for the Pt-Co/SiC catalyst. Results are explained on the basis of Co electronic structure, crystallite size, morphology, and degree of interaction with the support. These aspects were measured through temperature-programmed reduction, synchrotronbased methods (EXAFS/XANES), and scanning transmission electron microscopy, among other techniques.
