Interaction of atomic hydrogen with carbon grains

Laboratory studies on the interaction of atomic hydrogen with aliphatic and aromatic hydrogenated carbon grains are discussed. When exposed to atomic hydrogen, both types of hydrogenated carbon grains act as catalysts for molecular hydrogen formation. In the first case, an exchange reaction with hydrogen chemisorbed in aliphatic carbon sites is the formation route to H2 formation. For aromatic carbon grains, the formation of molecular hydrogen takes place through a two-step reaction sequence: 1) super hydrogenation of the aromatic carbon islands of grains 2) exchange reactions on these islands. This mechanism represents a good approximation of molecular hydrogen formation on large neutral PAHs.