Cobalt Nanocrystals As Model Catalysts for the Fischer-Tropsch Synthesis

Cobalt nanocrystals as model catalysts for the Fischer-Tropsch synthesis

Colloidal synthesis of metal nanocrystals (NC) potentially offers high control over their properties (size, shape or composition) and is therefore a promising approach to prepare well-defined model catalysts. For example, the controlled synthesis of cobalt (oxide) NC might enable new research linking the catalyst structure to its performance in the Fischer-Tropsch (FT) synthesis. However, the deposition of Co-NC onto supports and thereby their utilization as catalysts is still in its infancy. Here, we present the result of two studies to advance the application of Co-NC in supported FT catalysts.

We started by investigating an alternative to high-temperature treatments often applied to remove ligands that might otherwise block active sites and lower catalytic performance. Although such high-temperature treatments are effective, the harsh conditions can compromise the well-defined Co-NC.

Therefore, we synthesized Co-NC using a hot injection method and oxidized the NC at room temperature prior to their attachment to carbon nanotubes (CNT) as support. During this low-temperature oxidation, the as-synthesized ε-cobalt NC were oxidized to spherical polycrystalline CoO-NC, which decreased the magnetic interparticle interactions and facilitated their uniform distribution over the support (Figure 1A,B). On the other hand, the as-synthesized, non-oxidized ε-cobalt NC were more cubical and formed chains of particles, indicating magnetic interactions between the NC (Figure 1C). Consequently, direct attachment of Co-NC resulted in severe clustering of the NC (Figure 1D). Part of the CoO-NC/CNT sample was subsequently oxidized at 250 °C to evaluate the effect of a high-temperature oxidative treatment. Monocrystalline Co₃O₄-NC were formed during this treatment, which were partially embedded into the CNT support, and its ligands had been removed. In situ reduction also effectively removed the ligands from CoO-NC/CNT (TGA, data not shown), yielding two highly active catalysts with comparable performance to that of catalysts prepared by conventional synthesis techniques, such as impregnation of a precursor salt. However, more extensive particle growth was observed for the high-temperature treated sample, showing the adverse effects of severe oxidation. Therefore, low-temperature oxidation was found to be the preferred pre-treatment¹.

Figure 1 (cryo-)TEM results of the Co- and CoO-NC before and after attachment to the CNT support. Low-temperature oxidized CoO-NC (A) before and (B) after attachment to CNT. As-synthesized, non-oxidized Co-NC (C) before attachment to CNT, as imaged with cryo-TEM and (D) after attachment to CNT.

Having established a promising activation procedure, we focused on preparing model Co/TiO₂ and Co/SiO₂ catalysts using 3-12 nm CoO-NC. In particular on TiO₂, Co particle sizes below 10 nm are hard to obtain using conventional synthesis techniques, so colloidal techniques could offer a clear advantage in this case.

Co-NC were synthesized using a similar hot-injection method as before and their size was regulated between 3-12 nm by adjusting the temperature at which the precursor was injected. After low-temperature oxidation, uniform CoO-NC distributions were obtained for all NC sizes on TiO₂ as well as on SiO₂. The FT activity of the TiO₂-supported Co-NC of 6 and 12 nm was similar to that of Co/TiO₂ prepared by impregnation (turnover frequency ~0.07 s^-1), showing that relevant catalysts had been obtained. However, 3 nm Co-NC on TiO₂ were less active than anticipated. After FT, TiO₂-supported Co-NC of all sizes and 3 nm Co-NC on SiO₂ had grown to ~13 nm, while 6 and 9 nm Co-NC on SiO₂ had remained stable (Figure 2). Furthermore, during reduction up to 60 % of Co(-ions) were re-dispersed over TiO₂ against 15 % on SiO₂. The high precision in initial particle size enabled us to investigate the very different growth of Co-NC in Co/TiO₂ and Co/SiO₂ catalysts, showing that both Co-NC size and interaction with the support have a major influence on the Co-NC stability.

Figure 2 Average cobalt particle sizes of the catalysts in the pristine, reduced and spent state. Surface-volume-weighted mean sizes of the Co-NC supported on (A) TiO₂ and on (B) SiO₂ from TEM analysis. The bars give the standard deviation of the particle size distribution. The reduction was performed at 350 °C (TiO₂) or 500 °C (SiO₂) for 8 h with 1 °C·min^-1 in 25 vol.% H₂ in He. FT was performed at 220 °C, 20 bar, 2 H₂/CO (V/V) for >100 h on stream.

Overall, the presented approach resulted in NC-based FT catalysts with high control over the Co-NC size on carbon and oxidic supports. We demonstrated their potential by investigating Co-NC growth and anticipate that these model catalysts will facilitate structure-performance studies.

1.           T.W. van Deelen, H. Su, N.A.J.M. Sommerdijk and K.P. de Jong, Chem. Commun. 54 (2018) 2530–2533.