Dynamic Molten Cu–In Droplets Catalyze Selective Synthesis of Multi-Walled Carbon Nanotubes During Methane Pyrolysis

Abstract

The generation of multi-walled carbon nanotubes (MWCNTs) typically utilizes solid catalyst nanoparticles. These particles often exhibit a liquid-like nature during synthesis and remain encapsulated inside the final MWCNTs. Molten Cu–In has been recently reported to produce high amounts of MWCNT in bubble column reactors during methane pyrolysis for clean H2 generation. In the present work, nanodroplets are isolated and studied on supports. The droplets are observed to deform into a head and tail geometry and generate bamboo-like MWCNTs. The compositions of 50–70 at.% Cu repeatedly generate dense bundles of MWCNTs, while higher or lower compositions yield little or no MWCNTs. The lower surface tension of the alloy at these compositions reduces the thermodynamic driving force for coalescence, stabilizing small droplets at high temperature. Graphitic structure with some defects is confirmed by transmission electron microscopy and Raman spectroscopy, showing 3.35 +/−0.08 Å interlayer spacing and an ID/IG ratio of 0.87 +/−0.13, respectively. Droplets distributed between 10 nm and 1 micron generate MWCNTs 10–400 nm in diameter, suggesting droplets above 400 nm do not generate MWCNTs at any catalyst composition. Bundles of MWCNT exceeding hundreds of microns are observed in reaction times over 1 h.