(521by) Nonthermal Plasma Activation of Ethane/Nitrogen and Observation of Self-Ordered Nitrogen-Containing Carbon Structures at Sub-Ambient Temperatures

Nonthermal plasma activation of shale gas components (e.g., methane, ethane, and nitrogen) is a promising yet largely unexplored solution to combat the misuse of excess shale gas from remote well sites. In this process, high energy electrons from renewable resources inelastically collide with light alkanes, resulting in electronic excitation, vibrational excitation, and ultimately C-C or C-H bond dissociation to form a rich mixture of radical species, hydrogen, olefins, and larger hydrocarbons. This product stream can then be combined with a catalyst in a one- or two-stage process to improve overall activity and selectivity to the desired products. However, a significant barrier with this technology is the uncontrolled coke deposition by the plasma, which rapidly deactivates both plasma-phase and catalyst activity. Thus, an understanding of both the composition and spatial distribution of the carbon deposits is required to develop strategies to mitigate unwanted carbon formation. In this body of work, we first seek to understand the chemical and physical processes related to carbon formation by characterizing the structure and composition of the solid products from ethane plasma in the first stage of a two-stage plasma-catalytic system. Using a water-cooled dielectric barrier discharge (DBD) reactor with visual access to the plasma region, microscale carbon structures with macroscopic order were observed for the first time growing perpendicular to the inner electrode along the reactor length. The spacing between microstructures can be controlled by varying applied voltage or feed composition, and comparison with known literature on microdischarge pattern formation in DBDs led to the conclusion that microstructure growth is linked to microdischarge spacing in a pseudo-1D Type-A discharge. Upon addition of N₂ to the ethane feed, elemental analysis combined with IR and Raman spectroscopy confirm that nitrogen incorporation into both sp² and sp³ bonding phases can be controlled by varying N₂ content in the feed. Lastly, modification of the reactor design led to greater control in the amount of carbon formed. These advances demonstrate the potential of plasma activation for the decarbonization of shale-gas upgrading and provide a platform for continued research into the emerging field of nonoxidative hydrocarbon plasma-catalysis.