Carbon nanofiber growth from methane decomposition over Ni-based catalysts: a balancing act

Hydrogen plays an important role in the energy transition. However, it is important to produce this hydrogen in a green way. Unfortunately, the current production routes involve the emission of CO2. The catalytic decomposition of methane is a good alternative: during this reaction methane is split into hydrogen gas and solid carbon. The solid carbon can have different forms like, e.g. graphene, carbon black or carbon nanofibers. Carbon nanofibers (CNFs) are special because of their interesting properties (lightweight materials, good conductivity and very strong). The reaction takes place in three stages: 1) decomposition of methane, 2) diffusion of carbon atoms through the catalyst particle 3) the formation of carbon layers. The reaction is sensitive to changes and moves quickly out of balance. As a result, the catalyst might deactivate quickly. In this research, we systematically studied several factors that influence the reaction. We used Ni-based catalysts, as these are the most active for this reaction. To circumvent rapid deactivation, bimetallic NiCu catalysts were used. A graphitic support material was used to restrict support effects. We have used a thermogravimetric analyzer as reactor, operated at differential conversion. The TGA allows us to follow the carbon growth in-situ. In our studies, we were mainly interested in the initial growth rate, the final carbon yield and the catalytic lifetime. In addition, in-situ gas cell transmission electron microscopy was used, which allowed us to directly visualize CNF growth from ensembles of individual metal catalyst nanoparticles. Increasing the temperature resulted in a faster reaction, but also in a shorter catalyst lifetime. Additionally, different fiber structures formed at different temperatures. Next to an optimal temperature, we observed an optimal catalyst particle size and composition. Normally, highly active catalysts are preferred in catalysis. However, in this case, highly active catalysts also deactivated quickly, limiting the final yield, hence slow and steady wins the race.