Adsorption and Subsequent Reaction of a Water Molecule on Silicate and Silica Cluster Anions

Abstract

We present reactions of size-selected free silicate, Mg_<i>l</i>SiO_<i>m</i>^–, and silica, Si_<i>n</i>O_<i>m</i>^–, cluster anions with a H₂O molecule focusing on H₂O adsorption. It was found that H₂O adsorption to Mg_<i>l</i>SiO_<i>m</i>^– with <i>l</i> = 2 and 3 (<i>m</i> = 4–6) is always followed by molecular oxygen release, whereas reactivity of the clusters with <i>l</i> = 1 (<i>m</i> = 3–5) was found to be much lower. On the contrary, in the reaction of Si_<i>n</i>O_<i>m</i>^– (<i>n</i> = 3–8, 2<i>n</i> – 1 ≤ <i>m</i> ≤ 2<i>n</i> + 2), a H₂O adduct is observed as a major reaction product. Larger and oxygen-rich clusters tend to exhibit higher reactivity; the rate constants of the adsorption reaction are 2 orders of magnitude larger than those of CO adsorption previously reported. DFT calculations revealed that H₂O is dissociatively adsorbed on Si_<i>n</i>O_<i>m</i>^– to form two SiO₃(OH) tetrahedra. The site selectivity of H₂O adsorption is governed by the location of the singly occupied molecular orbital (SOMO) on Si_<i>n</i>O_<i>m</i>^–. The present findings give molecular-level insights into H₂O adsorption on silica and silicate species in the interstellar environment