Formation of Hollow Spheres upon Oxidation of Supported Cobalt Nanoparticles

Abstract

Oxidation of cobalt nanoparticles supported on montmorillonite was studied using transmission electron microscopy and temperature programmed reduction of the oxide particles. It has been shown that hollow shells of cobalt oxide were formed in this process, consisting of the mixture of CoO and Co 3 O 4 oxides. A linear relationship between the thickness of shells and the initial particles size was established which suggests that both diffusion of cobalt ions through the oxide layer and diffusion of the oxygen into cobalt occur, but particles sintering is negligible compared with the reverse diffusion of cobalt. The lower critical diameter of cobalt particles was estimated, above which formation of hollow shells occurred. The hollow shells formation is suggested to be the reason for cobalt redispersion upon oxidation-reduction cycling of supported heterogeneous catalysts.

Cobalt Fischer–Tropsch Catalyst Regeneration: The Crucial Role of the Kirkendall Effect for Cobalt Redispersion

“…The electron diffraction image (Fig. 2e) shows diffuse rings instead of more discreet spots, indicating that the oxide shell is highly polycrystalline, in line with the findings of other authors [16,17]. Figure 2 summarizes the results obtained after reduction of the hollow oxide shells.…”

“…4 Bright field TEM images of a a-alumina-supported cobalt model catalyst, showing the formation of hollow oxide spheres in the reoxidized stage and higher metallic cobalt dispersion in the re-reduced state first reduction step is 30 nm, while the average particle size in the reoxidized sample is 9 nm. This smaller XRD particle size in the reoxidized state can be rationalized by special morphology in the reoxidized state: the cobalt oxide is present in the form of hollow polycrystalline particles, and XRD only detects the average size of the individual crystallites that make up the hollow oxide shell [15,16]. We did not find indications for aluminate formation in either of the alumina model catalysts.…”

“…The formation of hollow oxide particles upon oxidation has been reported previously, on several metals including Co, Ni, Fe and Cu [15,16,[18][19][20], and the mechanism responsible for this is the Kirkendall effect, where diffusivity differences of the reactants in solid state reactions results in void formation [21][22][23]. In the case of oxidation of metallic nanoparticles the outward diffusion of the metal (cations) through the growing oxide layer is much faster than the inward diffusion of oxygen.…”

Morphology influence on nanoscale magnetism of Co nanoparticles: Experimental and theoretical aspects of exchange bias

“…Indeed, it has been previously stated that for the iron and cobalt particles some critical size exists above which full reaction with air is not possible at relatively low temperatures, and the oxidation of large enough nanoparticles results in yolk/shell nanostructures. 19,24,25 The 18-nm yolk/shell nanoparticles were obtained following a synthetic route identical to that used for the preparation of 13-nm core/shell nanoparticles, but maintaining the reaction solution open to the atmosphere (sample 6). To promote the complete particle oxidation, pyridine-N-oxide was introduced in the heated solution containing the nanocrystals.…”

Synthesis of Co3O4 nanotubes and their catalytic applications in CO oxidation

“…Calcination of CoC 2 O 4 ·2H 2 O nanorods produced Co 3 O 4 nanotubes with an outer diameter of about 200 nm [17]. It was proposed that the hollow nanostructure was created inside the solid nanoparticles because of the difference in the diffusion rates of the metal and oxygen ions [18][19][20][21][22]. However, little is known on the oxidation process of one-dimensional Co nanostructures to Co 3 O 4 nanotubes.…”