Competing mechanisms of catalytic H2 formation and dissociation on ultrasmall silicate nanocluster dust grains

Silicate dust grains are thought to be essential in catalysing the formation of H-2. Ultrasmall silicates (diameter < 1.5 nm) are fundamental intermediates in silicate dust formation in stellar outflows, and are ubiquitous in the interstellar medium. To investigate the catalytic formation and dissociation of H-2 on such nanosilicates, we have performed ab initio quantum chemical calculations of hydrogen interacting with a stable 21 atom nanosilicate cluster having the stoichiometry of forsterite, (MgO)(6)(SiO2)(3). Due to its small size and high percentage of surface atoms, our particle inherently does not exhibit the bulk forsterite crystal structure and possesses a range of chemisorption and physisorption sites, presumably similar to those that larger amorphous silicates would offer. We find a number of exothermic H-2 formation routes and pathways for H-2 catalytic dissociation on the nanosilicate. In particular, we discover some H-2 formation routes that are energetically more favourable than that reported for the forsterite (010) surface. Further, we find a linear correlation between the dissociative chemisorption of two H atoms and the dissociation transition state, suggestive of a general Br circle divide nsted-Evans-Polanyi relation for H-2 dissociation on bare silicates independent of dust grain size and/or crystallinity.