(220e) Catalyst Aerosol Dynamics in the Floating Catalyst Chemical Vapor Deposition Synthesis of High-Quality Carbon Nanotubes

Engineering materials based on carbon nanotubes (CNTs) have been demonstrated for applications ranging from strong, lightweight structural composites to cables for power and data transmission. Such applications could one day supplant steel, aluminum, and copper in many scenarios. Replacing metals with CNTs could provide a route to reducing the green-house gas emissions and other environmental impacts associated with metal extraction and refining. The success of these applications depends on the supply of long, highly crystalline, CNTs at prices low enough to enable competition with legacy materials. Currently, this is the limiting factor for many of these applications, as suitable CNTs cost more than $2000/kg.

Floating catalyst chemical vapor deposition (FCCVD) is one of the most promising methods for the scalable, continuous synthesis of high-quality CNTs for macroscale applications. FCCVD utilizes an aerosol of catalytic metal particles as active sites for CNT growth. This aerosol may either be preformed from the vaporization and condensation of a metallic source or formed in situ by the decomposition of an organometallic precursor species. Despite the critical importance of the catalyst aerosol to the FCCVD process, this key element remains poorly characterized and poorly understood.

The lack of understanding of this key process has led to an overly simplistic view of the aerosol dynamics within an FCCVD reactor. Critical aspects of the aerosol behavior, such as the potential for the evaporation of the catalyst, have been neglected until very recently. This simplified view has complicated process scale up efforts through traditional geometric similarity methods, and led to an apparent tradeoff between process yield and product quality, contributing to the high prices of high quality CNTs.

The study of the generation and population dynamics of aerosols like the FCCVD catalyst is a well-developed field; however, much of this knowledge has not been applied to the analysis and design of FCCVD processes. In the limited instances where aerosol science was successfully leveraged studies have typically been specific to particular reactors, utilizing computational fluid dynamics (CFD) to understand specific rector configurations and process recipes. The results of these studies were generally not generalizable to the FCCVD process as a whole, limiting their impact and the adoption by the broader community.

In this work we apply proven aerosol modeling techniques to develop a simplified model that captures the key aerosol dynamics present in real reactor. We reduce the reactor to a one-dimensional, plug flow reactor (1D-PFR) and consider the formation of particles by nucleation, the change in their size and concentration by coagulation, condensation, and evaporation, and the loss of catalyst material to the reactor walls. Despite the degree of simplification, the 1D-PFR model agrees well with much more complex, CFD based simulations from the literature while remaining computationally inexpensive and easy to interpret.

We use this model to interrogate the effect of common experimental parameters, such as the reaction temperature, residence time, and injection condition. We consider both the injection of preformed catalyst particles as well as the in situ formation of particles from the decomposition of a precursor species.

The results of these studies reveal that the early phases of particle formation or particle injection are critical to determining the overall dynamics of the process. The simulations show that, in general, in situ particle generation will enable higher particle concentrations due to the diminished role of particle-particle coagulation. Balancing the effects of reaction temperature and residence time is critical for maximizing the particle number concentration when forming the particles in situ. These results agree well with most of the available experimental data and provide more rigorous, aerosol-based explanations for some observations in the FCCVD literature.