Abstract
This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the nanoscale to other cobalt particles on the surface. Recent efforts have shown that when promoters are used to facilitate the reduction of small crystallites with the aim of increasing surface Co0 site densities (e.g., in research catalysts), ultra-small crystallites (e.g., <2–4.4 nm) formed are more susceptible to oxidation at high conversion relative to larger ones. The choice of promoter is important, as certain metals (e.g., Au) that promote cobalt oxide reduction can separate from cobalt during oxidation-reduction (regeneration) cycles. Finally, some elements have been identified to promote reduction but either poison the surface of Co0 (e.g., Cu), or produce excessive light gas selectivity (e.g., Cu and Pd, or Au at high loading). Computational studies indicate that certain promoters may inhibit polymeric C formation by hindering C-C coupling.
Highlights
Fischer-Tropsch synthesis (FTS) making use of cobalt catalysts is the core of the gas-to-liquids (GTL) process [1,2]
H2O co-feeding can lead to improvements in activity and selectivity for certain cobalt catalysts [48], when a Pt promoter was utilized to facilitate the reduction of Co oxides in a 15%Co/Al2O3 catalyst, at 28 vol.% added H2O the catalyst underwent significant cobalt aluminate formation, as demonstrated in Figure 10 and [49,50] along with catastrophic deactivation (75% drop in XCO)
There are a number of stability issues that must be considered when selecting metal reduction promoters for use in Fischer-Tropsch synthesis catalysts
Summary
Fischer-Tropsch synthesis (FTS) making use of cobalt catalysts is the core of the gas-to-liquids (GTL) process [1,2]. This is one benefit of cobalt catalysts relative to iron catalysts for GTL, as the former typically possess low intrinsic WGS activity. Appropriate for obtaining active small crystallites of 6–15 nm, a sizeable fraction (typically in the range of 15–70%) of the cobalt remains in the oxide form, mainly as CoO. The fraction of unreduced cobalt is larger for supports such as alumina which interact strongly with cobalt oxide and for low cobalt loadings, e.g., less than 10–15% on such supports. Since higher extents of reduction (80–90%) of cobalt are highly desirable, i.e., correlated with higher activity on a per g catalyst basis, as well as improved C5+ selectivity, there is considerable interest and widespread application of noble metal promoters, which facilitate the reduction of cobalt oxides and increase the surface density of cobalt active sites. The main point of the article is that each promoter has its own advantages and set of issues that must be addressed and, in some cases, still defined
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