How hydroxylation affects hydrogen adsorption and formation on nanosilicates

Abstract

Silicate dust constitutes one of the primary solid components of the Universe and is thought to be an essential enabler for complex chemistry in a number of astronomical environments. Hydroxylated silicate nanoclusters (MgO)_x(SiO₂)_y(H₂O)_z, where strongly absorbed water molecules are dissociated on the silicate surface, are likely to be persistent in diffuse clouds. Such precursor species are thus also primary candidates as seeds for the formation and growth of icy dust grains in dense molecular clouds. Using density functional calculations we investigate the reactivity of hydroxylated pyroxene nanoclusters (Mg₄Si₄O₁₂)(H₂O)_N (N = 1−4) towards hydrogen physisorption, chemisorption and H₂ formation. Our results show that increased hydroxylation leads to a significant reduction in the energy range for the physisorption and chemisorption of single H atoms, when compared to bare silicate grains and bare bulk silicate surfaces. Subsequent chemisorption of a second H atom is, however, little affected by hydroxylation. The H₂ reaction barrier for the recombination of two chemisorbed H atoms tends to follow a linear correlation with respect to the 2H_chem binding energy, suggestive of a general Brønsted–Evans–Polanyi relation for H₂ formation on silicate grains, independent of dust grain size, composition and degree of hydroxylation.