(30f) Feedback Control System of Iron Catalyst in Reactors Using Deep-Injection Floating Catalyst Chemical Vapor Deposition (DI-FCCVD)

The current most promising process for scale-up of high-quality few-walled carbon nanotubes (FWCNTs) synthesis is the deep-injection floating catalyst chemical vapor deposition (DI-FCCVD) method. In our DI-FCCVD design, carbon feedstocks methane, hydrogen, and a catalyst carrying inert gas, are injected directly into the hot zone of a vertically oriented reactor. Vapor phase ferrocene and thiophene are used as catalyst precursors which react in situ to nucleate an aerosol of catalytic iron nanoparticles which serve as active sites for FWCNT growth.

Control over the catalyst particles and their formation is essential to the performance of the process, however the delivery of the catalyst precursors is poorly characterized and controlled in laboratory settings. Experimental catalyst delivery rates are often approximated using ideal bubbler assumptions of a perfectly saturated vapor, which may not be correct. To decrease the experimental variability, we have designed a feedback control system to provide an accurate and consistent ferrocene vapor delivery during synthesis using real-time acoustic measurement of ferrocene concentration. Using the process data provided by this control system we refined the design of our ferrocene sublimation system to counter sources of process variability that arise due to the characteristics of the powdered ferrocene used as a catalyst source. These sources of variability include sintering, the formation of channels, and powder entrainment in the gas flow. This design minimizes these factors while maximizing the contact of the carrier gas with the ferrocene, which remaining simple to build.

Implementing both the control system and the improved sublimator into our FCCVD reactor system has decreased run-to-run variability and allowed to begin quantitively studying how the catalyst is utilized in the system. By closing the catalyst mass balance by comparing the inlet iron flow rate to the iron recovered in the product we are able to characterize the effect of process changes on the loss of catalyst material and the efficiency of its utilization.

In this context, we alter the S/Fe ratio, injection depth, and velocities inside the injection tube to study their impacts on catalyst utilization and efficiency percentages. such percentages relationship with the methane conversion and production rate will provide information for more complex analysis of CNT formation conditions.

The improved process control provided by this system combined with the results of the quantitative catalyst studies has allowed us to exceptional CNT synthesis results. We can repeatably synthesize long (>10 µm), highly crystalline (Raman G/D > 50) FWCNTs with small diameters (1.42±0.06 nm), at high rates (~1 g/hr), with minimal residual catalysts (< 5 wt%) and non-CNT carbon content (>90% CNTs), while also achieving extremely high single pass methane conversion (>25%).