Graphene and graphite, low-temperature catalysts producing weakly-excited hydrogen molecules

A model for the low-temperature catalysis of hydrogen molecules on graphene and graphite, relevant for interstellar chemistry, is proposed: hydrogen atoms are either chemisorbed at the edges, or physisorbed on graphene and transported to a chemisorbed state at the edges. A second atom can then produce a molecule via a hot atom or an Eley-Rideal process. Since much of the energy is needed to desorb the molecule from the tightly-bound chemisorbed state, the desorbing molecules have only low internal excitation energy, in agreement with astronomical observations.