The size-dependent site composition of FCC cobalt nanocrystals

• A methodology to build and optimise nanocrystals of various sizes is presented.
• Five unique surface sites were defined, identifying and counted for FCC cobalt.
• The FCC cobalt site composition shows a size-dependence up to 8 nm.
• This effect is related to the size dependence of the Fischer–Tropsch reaction rate.

Cobalt-based Fischer–Tropsch (FT) catalysts typically contain reactive cobalt metal nanoparticles on support materials. Such metal nanoparticles are expected to have properties that are strongly size-dependent. Noting the size-dependence of the Co FT rate for particle diameters below 10 nm, we aimed to see whether the FCC Co nanoparticle site composition exhibits a corresponding size-dependence. This aim was realised by: building and optimising realistic and representative close-packed Co nanoparticle models at various sizes; defining, identifying and counting the exposed surface sites; generalising these findings based on the small subset of simulated nanocrystal models. Using this approach, the site composition of five defined sites with increasing size up to 8 nm was obtained. The three defined defect/step sites increased in concentration with increasing size, with the B5-A site and the B6 site reaching a stable value at about 4 nm, while the B5-B site still increased somewhat even at 8 nm. This confirms the size-dependent change in the surface site composition for FCC Co nanocrystals. The shape and site composition ranges were also compared to previously calculated Wulff shapes and showed good agreement. Implications of the size-dependent FCC Co site composition on the FT catalysis process are also discussed.

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