Stability and reactivity of anisotropic cobalt nanostructures under inert and reactive environments investigated by in‐situ
TEM
Authors: Dembélé K. , Moldovan S. , Harmel J. , Soulantica K. , Serp P. , Chaudret B. , Gay A‐S , Maury S. , Berliet A. , Ersen O. 
Published: December 2016

The use of Environmental TEM (ETEM) for investigating the materials evolution in terms of morphological, microstructural and chemical characteristics is of absolute need in catalysis. Owing to the high pressures and temperatures reached within the sealed environmental cells (E‐cell), they are suitable to mimic reaction conditions similar to the ones encountered in practice. In addition, the set‐up of a mass spectrometer at the cell exit would allow for evaluation of the reaction products and therefore the development of the “operando” methodology. This is crucial for understanding the relationship between the catalysts characteristics and their properties during its activation/operation and for accessing the mechanisms involved in its deactivation process.

The present work reports on the thermal stability and the reduction/oxidation behaviour of nanostructured metallic cobalt‐based structures with “urchin‐like” morphology synthesized by reduction of a cobalt complex in the presence of ligands.
1
These Co structures with high metallic surface area are foreseen as active phase for the Fischer‐Tropsch synthesis, in which syngas, a mixture of CO and H
2
, is converted into hydrocarbons and water.
2
The key‐question of catalysts stability under reaction is addressed in this study by using the ETEM approach. The Co structures are submitted to different atmospheres and temperatures in an effort to gather a complete knowledge of the system stability under reaction. To this end, the system is exposed to thermal constraints under vacuum/inert atmosphere, pure hydrogen and oxygen followed by hydrogen atmosphere.

Under vacuum (Figure 1) and argon, the cobalt from the “urchin” branches migrates towards the center with increasing temperature in a direction dictated by the shape of the radial needle‐like features. The Co migration is accompanied by the dissolution and subsequent rejection of the carbon atoms from the ligands on the metal surface, mechanism similar to the growth of carbon nanotubes (CNTs), but this time from an organic C‐rich precursor. Moreover, in the high temperature range, i.e. 900°C, the ligands already converted in carbon become graphitic leading to the formation of tubular structures with graphitic walls radially disposed around the Co center. This richness of this finding relies not only on the thermal stability of Co‐based urchin‐like structures, but opens a new perspective for the synthesis of graphitic structures with well‐defined tubular shapes.

Under pure hydrogen flow, the diffusion of metal atoms occurs up to 400°C as the ligands are converted into methane in this temperature range, reaction catalyzed by the Co. The “urchin”‐like microstructure collapse during the gradual temperature increment from 280°C up to 400°C (Figure 2). This morphological instability can be seen as first evidence of such nanostructures inefficiency in reactions developed at more than 350°C.

In the presence of oxygen, the ligands decompose into CO and CO
2
and the metal oxidizes according to the Kirkendall mechanism.
3
As effect, part of the Co migrates from the branches centers to the rims, recombines with the oxygen and generates tubular structures connected to the Co‐rich core and arranged in the initial configuration. The question arising is whether this structure stays stable under reactive conditions. In this sense, the oxidized system is submitted to an H
2
flow (figure 3c‐d) and progressive temperature increment. At 200°C, the Co oxide from the outer surface of “spines” reduces and migrates towards the tubes centers, leading after few hours to the formation of a metallic Co nanowire encapsulated surrounded by voids but encapsulated in a shell constituted in both Co and carbon. Obviously the mechanisms of Co oxide reduction into metallic Co upon H
2
treatment are different from the initial system to the one submitted to prior oxidation, but the latter solution is suitable to conserve the morphology of such complex system up to 700°C. From a fundamental perspective, this investigation identifies the impact of specifically reactive environments on the thermally activated diffusion and stability of nanosystems with complex geometries. From a more general perspective, this investigation explores the potential of using the E‐cell under a TEM to address complex dynamic thematic such as the mechanisms of metals reduction and oxidation under well‐defined/controlled conditions.