283722 Role of Dissolved Carbon On the Nickel-Catalyzed Growth of Carbon Nanotubes
In catalysis, the support is known to act on heat and mass transfer, but also on the active phase through strong metal-support interactions (the SMSI effect). In the case of black carbon, the structural order (graphitic character) adds one level of complexity but also one degree of freedom for logical catalyst design. It is of crucial importance to study how the graphitic character influences the anchoring of the nanoparticles and eventually modifies the catalytic activity of the metal.
In the present work, we studied how the interaction between Ni nanoparticles and carbon supports with different graphitic characters influences the Ni-catalyzed growth of carbon nanotubes (CNTs). By combining in situtechniques (XPS and XRD) with aberration-corrected HRTEM and theoretical calculations, we showed that the carbon support plays a dramatic role on the structure of the CNTs grown by catalytic chemical vapor deposition (CCVD) of ethylene. We observed that during the activation of the catalyst under hydrogen and further ramping to reaction conditions, carbon atoms from defective supports diffuse inside the Ni nanoparticles to form a non-stoichiometric metastable carbide. The insertion of carbon sub-surface generated strain and caused the reconstruction of the Ni nanoparticles. The particles became strongly faceted, which led to the growth of highly disordered carbon nanofibers during the CCVD step. In contrast, highly ordered multiwalled carbon nanotubes were grown under the same reaction conditions on Ni particles of same size and shape but supported on graphitic carbon.
Our results are relevant for the synthesis of carbon nanotubes and of hierarchical carbon-carbon composites, but we anticipate that they can also be extended to the growth of graphene. This offers also new possibilities to control the catalytic activity of metal particles and might open new routes for the rational design of heterogeneous catalysts.
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