H Atom Adsorption on a Silicate Surface: The (010) Surface of Forsterite

We present a first-principles computational study of the interaction of an H atom with the (010) surface of forsterite (Mg2SiO4). Periodic DFT-GGA calculations (PBE) are carried out using the SIESTA code with core pseudopotentials and TZP localized basis sets. Potential energy curves are determined for the approach of the H atom toward different sites of the surface: atop, near, or in between the O, Mg, and Si atoms. An outer adsorption well is found for all investigated sites; it is deepest (162 meV) at a so-called displaced Mg-O bridge position. The binding at this well is of the weak chernisorption/strong physisorption type. An inner stronger chemisoiption well (670 meV deep) exists exclusively near an O site but not strictly atop. Depending on the path, we find activation barriers (25-170 meV high) against chemisorption, the lowest of these occurs for the top O site. General trends of the computed interaction energies qualitatively agree with the QM/MM results of Goumans et al. [Mon. Not. R. Astron. Soc. 2009, 393, 1403], but adsorption binding energies and barrier heights differ significantly.