Polycyclic aromatic hydrocarbons--catalysts for molecular hydrogen formation.

Polycyclic aromatic hydrocarbons (PAHs) have been shown to catalyse molecular hydrogen formation. The process occurs via atomic hydrogen addition reactions leading to the formation of super-hydrogenated PAH species, followed by molecular hydrogen forming abstraction reactions. Here, we combine quadrupole mass spectrometry data with kinetic simulations to follow the addition of deuterium atoms to the PAH molecule coronene. When exposed to sufficiently large D atom fluences, coronene is observed to be driven towards the completely deuterated state (C24D36) with the mass distribution peaking at 358 amu, just below the peak mass of 360 amu. Kinetic models reproduce the experimental observations for an abstraction cross-section of sigma(abs) = 0.01 angstroms2 per excess H/D atom, and addition cross-sections in the range of sigma(add) = 0.55-2.0 angstroms2 for all degrees of hydrogenation. These findings indicate that the cross-section for addition does not scale with the number of sites available for addition on the molecule, but rather has a fairly constant value over a large interval of super-hydrogenation levels.