Catalysis of addition reactions by a negatively charged silica surface site on a dust grain

Two common astrochemical surface reactions, hydrogenation and oxidation of unsaturated compounds, increase the molecular complexity in the interstellar medium. The catalytic effect of a negatively charged silica surface site on addition reactions to adsorbed unsaturated organic molecules is investigated computationally. Adsorbates bind relatively strongly to the silanolate group (SiO-) with a pronounced weakening of their multiple bonds, which results in a significant reduction in the activation barrier of the H atom addition to COads, H2C = O-ads, H2C = CH2,ads and HC CHads with respect to the corresponding gas phase barriers. More strikingly, the sizable gas phase barrier (21 kJ/mol) for the O-3 + CO reaction is almost entirely removed. This strong catalytic effect allows for effective CO2 formation in dark molecular clouds where other routes are inefficient. Furthermore, negatively charged siliceous dust particles could also play an important role in the hydrogenation of CO and hydrocarbons in the interstellar medium.