Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by in Situ Environmental TEM

Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope (ETEM) equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under reaction conditions. Analysis revealed quantitative correlation of the cobalt valence states to the particles' nanoporous structures. The in situ experiments were performed on nanoporous cobalt particles coated with silica while a 15 mTorr hydrogen environment was maintained at various temperatures (300-600\degreeC). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. In situ mapping of the valence state and the corresponding nanoporous structures allows quantitatively analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, i.e., for Fischer-Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms.Comment: ACS Nano, accepte

Morphology Changes of Co Catalyst Nanoparticles at the Onset of Fischer-Tropsch Synthesis

Cobalt nanoparticles play an important role as catalysts for the Fischer–Tropsch synthesis, which is an attractive route for production of synthetic fuels. It is of particular interest to understand the varying conversion rate during the first hours after introducing synthesis gas (H2 and CO) to the system. To this end, several in situ characterization studies have previously been done on both idealized model systems and commercially relevant catalyst nanoparticles, using bulk techniques, such as X-ray powder diffraction and X-ray absorption spectroscopy. Since catalysis takes place at the surface of the cobalt particles, it is important to develop methods to gain surface-specific structural information under realistic processing conditions. We addressed this challenge using small-angle X-ray scattering (SAXS), a technique exploiting the penetrating nature of X-rays to provide information about particle morphology during in situ experiments. Simultaneous wide-angle X-ray scattering was used for monitoring the reduction from oxide to catalytically active metal cobalt, and anomalous SAXS was used for distinguishing the cobalt particles from the other phases present. After introducing the synthesis gas, we found that the slope of the scattered intensity in the Porod region increased significantly, while the scattering invariant remained essentially constant, indicating a change in the shape or surface structure of the particles. Shape- and surface change models are discussed in light of the experimental results, leading to an improved understanding of catalytic nanoparticles