Published online by Cambridge University Press: 30 March 2011
Laboratory studies on the interaction of atomic hydrogen with aliphatic and aromatichydrogenated carbon grains are discussed. When exposed to atomic hydrogen, both types ofhydrogenated carbon grains act as catalysts for molecular hydrogen formation. In the firstcase, an exchange reaction with hydrogen chemisorbed in aliphatic carbon sites is theformation route to H2 formation. For aromatic carbon grains, the formation ofmolecular hydrogen takes place through a two-step reaction sequence: 1) superhydrogenation of the aromatic carbon islands of grains 2) exchange reactions on theseislands. This mechanism represents a good approximation of molecular hydrogen formation onlarge neutral PAHs.