Metastable morphological states of catalytic nanoparticles

TL;DR

This paper reveals that catalytic nanoparticles exhibit metastable states during nanostructure growth, affecting their behavior and final products, with implications for optimizing catalytic processes.

Contribution

It introduces the concept of metastable states in catalytic nanoparticles and explains their role in nanostructure growth and catalyst lifetime.

Findings

Nanoparticles elongate due to metal-carbon interactions during growth.

Nested tube formation traps particles, influencing growth cycles.

Tapering affects final structures and catalyst longevity.

Abstract

During the catalytic synthesis of graphene, nanotubes, fibers, and other nanostructures, many intriguing phenomena occur, such as phase separation, precipitation, and analogs of capillary action. We demonstrate that catalytic nanoparticles display metastable states that influence growth, reminiscent of some protein ensembles in vivo. As a carbon nanostructure grows, the nanoparticle elongates due to an energetically favorable metal-carbon interaction that overrides the surface energy increase of the metal. The formation of subsequent nested tubes, however, drives up the particle's free energy, but the particle remains trapped until an accessible free energy surface allows it to exit the tube. During this time, the nanoparticle continues to catalyze tube growth internally within the nested structure. This nonequilibrium thermodynamic cycle of elongation and retraction is heavily influenced by tapering of the structure, which, ultimately, determines the final product and catalyst lifetime. Our results provide a unifying framework to interpret similar phenomena for other catalytic reactions, such as during CO oxidation, and suggest routes to the practical optimization of such processes.