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

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

Montevideo

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

Abstract: It wasrecognized long ago that the abundance of molecules in the cold interstellarmedium (ISM) cannot be explained by only considering the reactions at gasphase. Instead, it is accepted that dust grains can act as catalysts for theirformation. The ISM is known to contain a variety of dust grains, such assilicates and carbon-rich particles. It was suggested that part of thecarbonaceous material are formed by polycyclic aromatic hydrocarbons (PAHs). For this reason, PAH molecules are widely used asmodel catalysts to study the formation of molecules in the ISM. In particular,the catalytic formation of H2, the most abundant molecule of theuniverse, has been widely studied. Observations have suggested that high abundances ofPAHs in photon-dominated regions are related to high rates of H2formation. This has led to thesuggestion that PAHs might be involved in H2 formation through acatalytic process. A variety of routes for the generation of molecular hydrogenon PAHs has been proposed. A possible channel is a two-steps mechanism inwhich, (1) an extra H atom is added to an edge C atom of a PAH, and (2) thisextra H atom is abstracted by a second H atom coming from gas phase followingthe 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 theoccurrence of this mechanism. It was indeed suggested that Hn-PAHsare present in the ISM [1]. Since hydrogen is the most abundant atom in theuniverse, hydrogenation is expected to be the most common chemical reaction inthe ISM. In this work we are interested in studying the formation of H2,H2O and HCO from hydrogenation of H, OH and CO, respectively. Anthracenewas used as a model of a PAH. We have considered the two extreme cases ofhydrogenation of a PAH molecule, i.e., with only one excess H atom, and withthe 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 tocatalize simple hydrogenation reactions by acting as a source of atomichydrogen. For instance, the water molecule is proposed to be formedfollowing the reaction: Hn-PAH + OH à Hn-1-PAH + H2O.Our calculations show that meanwhile H2 is formed by overcomingsmall activation barriers (of approximately 0.02 and 0.10 eV on 1H-anthraceneand 14H-anthracene, respectively), H2O is produced in a barrierlessfashion with both Hn-PAH molecules. The production of HCO resultedto be an unfavoured process. This efficient mechanism could thereby be apossible route for water formation in the cold interstellar medium