The possible role of neutral PAHs as catalysts for H2 formation in the
interstellar medium is investigated by a combined experimental and density function theory
study of the superhydrogenation of coronene (C24H12). The
calculations suggest efficient hydrogenation of both edge and centre sites, along with
competing abstraction reactions to form H2 in a series of catalytic cycles.
Scanning tunneling microscopy and thermal desorption measurements have been used to
provide direct evidence of the formation of superhydrogenated coronene as a result of
exposure to D atoms. Lower limit estimates for the cross-sections of
1.8 × 10-17, 5.5 × 10-18 and 1.1 × 10-18 cm2
for the formation of singly, doubly and triply hydrogenated coronene are derived. The
results suggest that superhydrogenated PAHs may play an important role in H2
formation in the ISM.