A Theoretical Study on the Mechanism of Small Carbon Clusters Growth in Low-Temperature Plasma

Understanding the interaction between small alkane radical ions and methane could lead to more efficient ways of hydrogen production, which is an essential component in the field of green energy. It can contribute to the development of new plasma processing methods for natural gas utilization and conversion into other useful products. In this study, using first-principles calculations we analyzed interactions between small alkane radical ions and methane molecules in low-temperature plasma resulting in formation of carbon clusters and hydrogen molecules. We found that anion-methane interaction initiates after C–H bond rupture in CH 4 with the lowest activation barrier observed for negative ions C 3 H 7 − and C 2 H 5 − undergoing a hydrogen transfer reaction. Positive alkane ion radicals C 3 H 7 + and C 2 H 5 + demonstrated a different initial step in the clustering process where CH 3 and H transfer reactions occur simultaneously. The activation barrier for the reaction between positive ethyl ion C 2 H 5 + and methane is ~ 0.4 eV in accordance with experimental studies. Our calculations showed that both negative and positive ethyl ions readily react with methane, forming hydrogen molecules and propyl ion radicals C 3 H 7 −/ C 3 H 7 + . However, the continued growth of these radicals encounters increasing activation barriers, suggesting a slowdown in the carbon ion clustering rate and hydrogen production. Estimated rate constants of the considered ion-neutral reactions are in reasonable agreement with experimental values for a wide range of temperatures. These findings are crucial for understanding of carbon nanoparticle generation and hydrogen production using plasma catalysis.