There are several deactivation pathways over supported Co catalysts that cannot be reversed by simply reducing the catalyst.  These mechanisms include formation of refractory non-reducible mixed oxides and agglomeration of cobalt crystallites.   In this presentation, we focus primarily on the problem of cobalt crystallite growth and provide a geometrically based phase diagram that should help guide preparation of catalysts that will have a smaller amount of Co agglomeration.  Our experimental data indicates that coalescence is the predominant growth mechanism and that nano-scale homogeneity of cobalt crystallites on the support dramatically influences the degree of coalescence.  The geometric model described here attempts to quantitatively relate the physical properties of the support [surface area, pore size and pore volume] with the size and distribution of Co crystallites on both fresh and deactivated catalysts.  As Co crystallites grow, the distance between them increases until they reach a critical state of separation where the growth slows down dramatically.  Once we understand the parameters describing that state, we can attempt to create a similar Co-Co separation in fresh catalysts by adjusting support surface area, pore size, and pore volume, as well as cobalt loading and dispersion.  We assume that if we prepare a fresh catalyst with Co-Co spacing similar to those in pseudo-equilibrated catalysts, then we can remove the driving force for agglomeration.  We model the pseudo-equilibrated state from a geometric perspective using the experimentally determined pseudo-equilibrated particle distribution from bubble column pilot plant runs that have lasted for periods of 6 months or longer.  One of the key remaining research challenges involves learning how to reproducibly and homogeneously place the Co particles uniformly on supports. We show two approaches.  The first involves use of organic chelating agents to affect the size and nanoscale uniformity.  The second involves changing the decomposition kinetics and environment during cobalt nitrate decomposition.

Improved Fischer-Tropsch catalysts guided by learnings from deactivation studies

2019 Fall National Meeting

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