The significant role of water in reactions occurring on the surface of interstellar ice grains: Hydrogenation of pure ketene H <subscript>2</subscript> CCO ice versus hydrogenation of mixed H <subscript>2</subscript> CCO/H <subscript>2</subscript> O ice at 10 K.

Water ice plays an important role in reactions taking place on the surface of interstellar ice grains, ranging from catalytic effects that reduce reaction barrier heights to effects that stabilize the reaction products and intermediates formed, or that favor one reaction pathway over another, passing through water-involvement in the reaction to produce more complex molecules that cannot be formed without water or water-derived fragments H, O and OH. In this context, we have combined experimental and theoretical studies to investigate ketene (CH 2 CO) + H solid-state reaction at 10 K in the presence and absence of water molecules under interstellar conditions, through H-bombardment of CH 2 CO and CH 2 CO/H 2 O ices. We show in the present study that with or without water, the ketene molecule reacts with H atoms to form four reaction products, namely CO, H 2 CO, CH 4 and CH 3 CHO. Based on the amounts of CH 2 CO consumed during the hydrogenation processes, the CH 2 CO + 2H reaction appears to be more efficient in the presence of water. This underlines the catalytic role of water ice in reactions occurring on the surface of interstellar ice grains. However, if we refer to the yields of reaction products formed during the hydrogenation of CH 2 CO and CH 2 CO/H 2 O ices, we find that water molecules favor the reaction pathway to form CH 3 CHO and deactivate that leading to CH 4 and H 2 CO. These experimental results are in good agreements with the theoretical predictions that highlight the catalytic effect of H 2 O on the CH 2 CO + H reaction, whose potential energy barrier drops from 4.6 kcal mol -1 (without water) to 3.8 and 3.6 kcal mol -1 with one and two water molecules respectively.