The significant role of water in reactions occurring on the surface of interstellar ice grains: Hydrogenation of pure ketene H2CCO ice versus hydrogenation of mixed H2CCO/H2O ice at 10 K

Abstract

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 (CH₂CO) + H solid-state reaction at 10 K in the presence and absence of water molecules under interstellar conditions, through H-bombardment of CH₂CO and CH₂CO/H₂O 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, H₂CO, CH₄ and CH₃CHO. Based on the amounts of CH₂CO consumed during the hydrogenation processes, the CH₂CO + 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 CH₂CO and CH₂CO/H₂O ices, we find that water molecules favor the reaction pathway to form CH₃CHO and deactivate that leading to CH₄ and H₂CO. These experimental results are in good agreements with the theoretical predictions that highlight the catalytic effect of H₂O on the CH₂CO + 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.