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Hydrogen interaction with polycyclic aromatic hydrocarbons – from interstellar catalysis to hydrogen storage

Final Report Summary - HPAH (Hydrogen interaction with polycyclic aromatic hydrocarbons – from interstellar catalysis to hydrogen storage)

The HPAH project.
Polycyclic aromatic hydrocarbons (PAHs) could play an important role in astrochemistry as catalysts for formation of molecular hydrogen, the most abundant molecule in the universe and the key species for formation of more complex chemical compounds under interstellar conditions. The HPAH project has demonstrated and investigated the catalytic activity of PAHs in molecular hydrogen formation under interstellar conditions, and has furthermore, in the process developed a method to engineer a band gap opening in graphene by hydrogen functionalization.

Highly superhydrogenated PAH species detected.
In the HPAH project state-of-the-art surface science techniques are combined with theoretical calculations to achieve atomic level understanding of the interaction between atomic hydrogen and PAH molecules. Thermal desorption spectroscopy allows us to monitor how H atoms are added to PAHs molecules forming superhydrogenated PAH species. Our experiments show that long time exposure of the PAH molecule coronene to a beam of atomic hydrogen results in surprisingly high levels of hydrogenation approaching the theoretical upper limit of one extra H atom pr. carbon atom. The formation of PAHs with such high degrees of superhydrogenation have important implications for interstellar molecular hydrogen formation, since calculations show that efficient formation of H2 should proceed by a combination of superhydrogenation and abstraction reactions involving neutral PAHs and their superhydrogenated forms. The measurements reported here provide the first direct observation of superhydrogenation of neutral PAHs by H atom addition [Thrower et al. Astrophysical Journal 2012].

Evidence of molecular hydrogen formation on PAHs.
By using the deuterium isotope of hydrogen it is possible to monitor the exchange between the initial H atoms on the PAH molecule and incoming D atoms from the atom beam. Our measurements show evidence of such exchange reactions. These reactions are expected to proceed via a three step process: 1. an incoming D reacts with a C atom in a C-H configuration on the edge of the PAH molecule forming a C-HD group, 2. a second incoming D atom reacts with the H atom of the C-DH group and forms deuterated molecular hydrogen (HD). 2. A new incoming D atom reacts with the PAH molecule and thereby replaces the H atom which was removed in step 2. Hence, the observation of H-D exchange reactions provides the first indirect evidence of catalytic activity of PAH molecules in molecular hydrogen formation [Thrower et al. Astrophysical Journal 2012, Menella et al. Astrophysical Journal Letters 2012].

Identification of H addition sites by submolecular imaging.
Scanning Tunneling Microscopy experiments provide images of superhydrogenated species with submolecular resolution enabling us to assign specific hydrogen reaction sites on the PAH molecule. The data show that H atoms react on several different sites on the PAH molecule. These data allow atomic level determination of H-PAH reactions.

Technological spin-off: Hydrogen induced band gap opening in graphene.
Very large PAHs containing up to hundreds of carbon atoms are expected to be present and stable in the interstellar medium. Such large PAHs are very difficult to handle experimentally. Therefore we have used graphene as a model system for studying H interaction with very large polycyclic aromatic systems. These investigations revealed that the substrate supporting graphene greatly influences the hydrogenation process. On graphene on Ir(111) the graphene-substrate interaction lead to the formation of nano-scale periodic hydrogen adsorbate structures. Periodic patterning has been predicted theoretically to induce a band gap opening in graphene. The ability to engineer a band gap in graphene is a holy grail in graphene research, since it would enable the use of graphene as a replacement material for silicon in future electronic devices. Ultraviolet photoemission spectroscopy measurements demonstrates that a bandgap of at least 450 meV is opened in graphene by the hydrogen nano-patterns. Hence, these investigations have led to the only proven method so far for opening up a band gap in large area single layer graphene which is sufficiently large for real applications [Balog et al., Nature Materials 9, 315 (2010)].