Insight by In Situ Gas Electron Microscopy on the Thermal Behaviour and Surface Reactivity of Cobalt Nanoparticles

Abstract : Here the thermal behaviour and the surface reactivity of cobalt catalysts supported by alumina‐silica and promoted by platinum were investigated by in situ transmission electron microscopy (in situ TEM) in a syngas environment. At the standard operating temperature of 220 °C, atomic diffusion and sintering processes onto the support are quite limited on the time scale of the TEM experiment. At temperatures between 350 and 450 °C, particles encapsulation occurred due to a higher CO dissociation and conversion into graphitic layers. Beyond 500 °C, carbon nanotubes (CNTs) growth is activated and the particles undergo (i) morphological changes through continuous elongation and contraction; and (ii) microstructural changes with the appearance of cobalt carbide. By comparing reactions under pure CO and a mixture CO−H2, it was shown that the addition of dihydrogen to CO increased the rate of CNTs growth and modified the structure of the nanotubes. These results clearly demonstrate the strong ability of the in situ TEM to provide the main lines of the reactivity synopsis of nanocatalysts at a nanometric scale under temperature and reactive gas.