Hydrogenated polycyclic aromatic hydrocarbons (Hn-PAHs) as catalysts for hydrogenation reactions in the interstellar medium

RICARDO M. FERULLO; CAROLINA ZUBIETA; PATRICIA G. BELELLI

Montevideo

Congreso; 42nd International Congress of Theoretical Chemists of Latin Expression; 2016

It was recognized long ago that the abundance of molecules in the cold interstellar medium (ISM) cannot be explained by only considering the reactions at gas phase. Instead, it is accepted that dust grains can act as catalysts for their formation. The ISM is known to contain a variety of dust grains, such as silicates and carbon-rich particles. It was suggested that part of the carbonaceous material is formed by polycyclic aromatic hydrocarbons (PAHs). For this reason, PAH molecules are widely used as model catalysts to study the formation of molecules in the ISM. In particular, the catalytic formation of H2, the most abundant molecule in the universe, has been widely studied. Observations have suggested that high rates of H2 formation in photon-dominated regions are related to high abundances of PAHs. This has led to the suggestion that PAHs might be involved in H2 formation through a catalytic process. A variety of routes for the generation of molecular hydrogen on PAHs has been proposed. A possible channel is a two-step mechanism in which, (1) an extra H atom is added to an edge C atom of a PAH, and (2) this extra H atom is abstracted by a second H atom coming from gas phase following the Eley-Rideal (ER) mechanism. Thus, the existence of ?hydrogenated PAHs? (Hn-PAH), i.e., PAH molecules with excess H atoms, is an essential prerequisite to the occurrence of this mechanism. It was indeed suggested that Hn-PAHs are present in the ISM [1]. Since hydrogen is the most abundant atom in the universe, hydrogenation is expected to be the most common chemical reaction in the ISM. In this work we are interested in studying the formation of H2, H2O and HCO from hydrogenation of H, OH and CO, respectively. Anthracene was used as a model of a PAH. We have considered the two extreme cases of hydrogenation of a PAH molecule, i.e., with only one excess H atom, and with the maximum amount of H atoms that anthracene can accept (n=14). Using the density functional theory (DFT) approximation, we have investigated the capacity of Hn-PAH to catalize simple hydrogenation reactions by acting as a source of atomic hydrogen. For instance, the water molecule is proposed to be formed following the reaction: Hn-PAH + OH -> Hn-1-PAH + H2O. Our calculations show that whereas H2 is formed by overcoming small activation barriers (of approximately 0.02 and 0.10 eV on 1H-anthracene and 14H-anthracene, respectively), H2O is produced in a barrierless fashion with both Hn-PAH molecules. The production of HCO resulted in an unfavourable process. This efficient mechanism could therefore be a possible route for water formation in the cold interstellar medium.