2025 AIChE Annual Meeting

(198e) Understanding Carbon Nanotube Growth and Soot Formation with Transient Kinetics

Authors

Jack Peden, Cambridge University
Adam Boies, Stanford University
Christian Reece, Harvard University
The conversion of hydrocarbons into carbon nanotubes (CNTs) is a promising method towards net-negative CO2 emissions by generating CO2-free H2 while simultaneously replacing existing CO2-intensive materials (e.g., steel). The floating catalyst CVD (FC-CVD) approach applied to combined methane pyrolysis and CNT formation is a promising method that is already operated at scale. In this method, hydrocarbon and catalyst precursors are fed into a tubular reactor at >1000 °C. During this process, most of the hydrocarbon species decompose to form soot rather than the desired CNT fibers.

The complexities of CNT formation, coupled with the extreme environment inside the reactor, make achieving fundamental understanding of the FC-CVD process a challenge. To rationalize the underlying surface chemistry, we have developed novel instrumentation to explore the kinetics of hydrocarbon precursor decomposition and CNT formation over Fe nanoparticles and CNTs using a custom-built Temporal Analysis of Products (TAP) reactor that can be heated to over 1000 °C. The TAP reactor utilizes very small and well-defined gas pulses that allow for unambiguous kinetic measurements that can resolve which species are involved in both soot formation and CNT growth.

Herein, we measure the gas decomposition kinetics over CNTs with and without Fe nanoparticles attached and Fe nanoparticles alone. In these studies, we investigate the role of known present hydrocarbon species (e.g., CH4, C2H4, C2H2), both individually and combinatorially, in CNT growth and soot formation. Importantly, through these measurements, we are able to decouple the role of gas-surface interactions from gas-gas interactions and transport effects. We report the decomposition temperatures and activation energies for these gases and observe a surface-mediated synergistic effect in their decomposition. This work contributes to fundamental understanding of CNT formation, which is directly relevant to necessary CNT synthesis optimization such that CNTs can advance as industrially competitive materials.