Carbon nanotubes (CNTs) are nanometer-scale man-made carbon structures that can possess tensile strengths up to 50 times that of steel, can have one of the lowest resistivity values of any non-metal, and are exceptionally inert. Despite these astounding properties, CNTs have yet to be incorporated into applications for broader societal use. This stems from three core issues that plague industrial scale CNT production- low carbon yields, metal impurities, and the loss of active catalyst among the nanotubes. These factors hinder not only CNT production, but also hydrogen production processes such as catalytic methane decomposition (CMD) in which CNTs can be co-produced. As an alternative to current industrial methods of hydrogen production, particularly steam methane reforming (SMR), CMD products include CNTs rather than carbon dioxide, facilitating a potentially more sustainable future if these issues can be overcome.
My graduate research with Dr. Steven Crossley has aimed to resolve these challenges through catalyst design and kinetic studies to improve industrial feasibility for both CNT synthesis and CMD for hydrogen production. Two distinct catalysts were designed to address the previously stated challenges. The first utilizes exfoliated vermiculite, a naturally occurring lamellar clay, as a catalyst support to facilitate growth of high-yield oriented CNT arrays from ethylene. Kinetic understanding of CNT synthesis on this vermiculite support led to dramatic improvements in carbon yield, ultimately achieving 25 grams of carbon per gram of catalyst in 30 minutes of synthesis. Thus, by achieving high yields, the relative concentration of catalyst impurities was reduced to levels that are comparable to commercially sold acid-purified CNTs. Kinetics indicated the vermiculite catalyst facilitated comparable numbers of total catalyst turnovers before complete deactivation, regardless of reaction conditions. However, the vermiculite-based catalyst struggled to address the issue of catalyst reuse.
The second designed catalyst, comprised of Ni, Mo, and MgO, was intently designed to induce a base-growth mechanism for CNT synthesis with methane. This factor is key in catalyst reuse, as ongoing studies in catalyst-CNT separation could ultimately facilitate continuous production without metal impurities or catalyst loss. The base-growth mechanism results from complex catalyst evolution under reaction conditions in which molybdenum in the catalyst forms a molybdenum carbide domain that exsolves nickel nanoparticles to the carbide surface. These nanoparticles are anchored in place due to exceptionally strong interactions with the molybdenum carbide support. Various characterization techniques- XRD, XPS, TEM, Raman- have facilitated our understanding of the phases present during this catalyst evolution. The carbon yields achieved on Ni-Mo/MgO catalyst approach 15 grams of carbon per gram of catalyst after three hours on stream.
Kinetics indicate the catalyst evolution of Ni-Mo/MgO is particularly sensitive to reaction conditions, necessitating creative methods of evaluation in which the catalyst is stabilized under identical conditions before perturbating the system at steady state. This allows for measurements of reaction orders and activation energies independent of differences in Ni exsolution, which have led to our conclusion that breaking the first C-H bond in methane is rate determining step of this reaction. Further, we have evaluated methods of extending the catalyst lifetime through the introduction of co-feeds. Both water and hydrogen were found to increase the rate of methane conversion over Ni-Mo/MgO, with the produced hydrogen possessing auto-catalytic behavior and positive reaction orders. Further information regarding these co-feeds will be presented.