Prussian Blue Iron–Cobalt Mesocrystals as a Template for the Growth of Fe/Co Carbide (Cementite) and Fe/Co Nanocrystals

Controlling the composition, size, and morphology of transition-metal carbides and metals is important for product selectivity in industrial catalytic processes, such as in the Fischer–Tropsch synthesis (FTS). The formation of iron–cobalt carbide nanocatalysts can enhance selectivity in the FTS process if the iron to cobalt ratios are controlled. Unfortunately, this is difficult to achieve in nanocrystals due to ion migration, differences in formation rates, and the requirement of perfect critical nuclei to form prior to the growth of the nanocrystal in colloidal synthesis. In this manuscript, a mixed metal iron–cobalt Prussian blue analogue (PBA) mesocrystal is shown to act as a template for isolating pure phase iron–cobalt carbide and iron–cobalt alloy nanocrystals. The formation of the individual nanocrystals from the heterometallic mesocrystal occurs through sequential decomposition and recrystallization events. The steps in the interconversion are observed by in situ and ex situ analytical techniques allowing a mechanism for carbide and metal formation to be proposed. The synthetic route is scalable and likely to be extendable to a wider range of bimetallic materials using the diverse range of Prussian blue analogues (PBAs) reported in the literature.