Carbon-Involved Near-Surface Evolution of Cobalt Nanocatalysts:An in Situ Study

When carbon-containing species are involved in reactions catalyzed by transition metals at high temperature,the diffusion of carbon on or in catalysts dramatically influences the catalytic performance.Acquiring information on the carbon-diffusioninvolved evolution of catalysts at the atomic level is crucial for understanding the reaction mechanism yet also challenging.For the chemical vapor deposition process of single-walled carbon nanotubes(SWCNTs),we recorded in situ the catalyst state(solid and molten)composition as well as near-surface structural and chemical evolution at the cobalt catalyst-tube interface with carbon permeation using aberration-corrected environmental transmission electron microscopy and synchrotron X-ray absorption spectroscopy.The nucleation of SWCNTs was linked with an alternating dissolving and precipitating cycle of carbon in catalysts close to the nucleation site.Understanding the dynamics of carbon atoms in catalysts brings deeper insight into the growth mechanism of SWCNTs and facilitates inferring mechanisms of other reactions.The methodologies developed here will find broad applications in studying catalytic and other processes.