Identifying active surfaces for C2 decomposition relevant to the FC-CVD CNT synthesis process

Abstract

Selecting for high quality carbon nanotube (CNT) growth during the floating catalyst vapour deposition (FC-CVD) synthesis process remains a challenge due to the coupling of physical and chemical phenomena inside the reactor. While there is evidence implicating C2 species as the precursors to CNT formation in FC-CVD reactors, the heterogeneous chemistry remains poorly resolved both as it pertains to CNT growth as well as potential parasitic reactions. This work utilises a specialised Temporal Analysis of Products (TAP) reactor to isolate the kinetics for heterogeneous C2 decomposition over previously grown multi-walled CNT samples, single-walled CNT samples, supported Fe nanoparticles, and graphite. Our results indicate that the catalysed decomposition of acetylene primarily occurs over the Fe catalyst seed, whereas the catalysed decomposition of ethylene primarily occurs over the CNTs. Further, we find that as the crystallinity of a CNT sample decreases, a decreased barrier to ethylene decomposition is observed. Interestingly, both pristine and defective graphite did not catalyse ethylene decomposition, indicating that curvature or morphology may play a role. Under all conditions tested, the decomposition of C2 species lowered the CNT sample crystallinity, as measured by Raman spectroscopy. Thus, we conclude that the secondary decomposition of ethylene over defective carbon and the secondary decomposition of acetylene over Fe catalyst seeds are both drivers of decreased crystallinity of CNT samples. We also emphasise the importance of including surface catalysed decomposition alongside pyrolysis in FC-CVD reactor models, as our results demonstrate that surface processes are fundamentally distinct from previously reported gas-phase dominated pyrolysis behaviour. More broadly, this work establishes TAP as a methodology for isolating heterogeneous gas-surface chemistry from coupled physical phenomena in complex reaction systems.