In‐situ and cryogenic electron microscopic study of genesis and dynamics of cobalt nanoparticle formation

Abstract

Cobalt nanoparticles have a high potential as catalysts for the Fischer‐Tropsch synthesis, i.e. to convert hydrogen and carbon monoxide, which can be derived from a variety of renewable feedstocks, into industrially useful hydrocarbons. The cobalt nanoparticles are commonly created via a deposition precipitation (DP) process In the DP process, catalyst supports such as silica and titania are suspended in solutions of cobalt precursor salts, which upon increasing/decreasing the pH precipitated a cobalt intermediates e.g. cobalt hydroxides and cobalt carbonates on the liquid‐solid interface. At present how and where the precursor salts in solution are nucleated and how they crystallize as cobalt intermediates is still unknown. In this contribution the morphological and structural development of the cobalt intermediates with and without support materials during the DP process has been investigated using advanced cryo‐EM and in situ liquid cell TEM. First, cryo‐TEM was employed to study Co nanoparticle formation during an decreasing pH induced by out‐gassing ammonia from the synthesis solution. At the early stage of the nucleation and growth, particles with a diameter of 1 to 2 nm were found. It is difficult to image these 1~ 2 nm particles through a 100 to 200 nm thick vitrified ice layer due to significant scattering of electrons in the embedding medium. Hence, we vitrified our sample on graphene oxide (GOx) supports which cover a normal TEM holy grid (figure 1a). Because of the very low background and high hydrophilicity of GOx, we generate ultra‐thin aqueous layers for high contrast cryo‐TEM imaging (figure 1b). In addition, the presence of GOx makes it possible to focus more accurately to acquire high contrast and resolution cryo images at only a few hundred nanometers of defocus. Second, liquid cell TEM was employed to study in‐situ particle formation by exposing a solution of Co 2+ ions to the vapor ‐NH 3 and CO 2. Here Co(NO 3 ) 2 solution is flown through the liquid cell to fill the system, after which wet N 2 is then flown in to remove the Co(NO 3 ) 2 solution in the tube (fig 2a, 2b). Because the chips are cleaned by O 2 plasma before mounting, the surface of two chips is highly hydrophilic. So that liquid between the two chips is not removed by the wet N 2 . This way a thin liquid layer containing Co 2+ ions is generated between two chips (fig 2b). Subsequently, a syringe containing (NH 4 ) 2 CO 3 powder is connected to the other port and NH 3 and CO 2 vapor is released from the decomposition of solid (NH 4 ) 2 CO 3 (figure 2c). In this way, we could generate a thin liquid layer with a thickness of 250 ~ 600 nm in the center in a repeatable manner (figure 3), making the edge of viewing area to be good position for (S)TEM imaging of particle nucleation and growth.