Atomic-Scale Mechanism of H2 Formation on Hydrated Olivine Surfaces

Abiotic hydrogen gas (H2) produced by the serpentinization of ultramafic rocks has far-reaching implications, from geological H2 production to the origin of life. In spite of this importance, the atomic-scale mechanism of H2 formation remains unclear. Here, we investigate H2 generation pathways on the hydrated olivine (010) surface using density functional theory (DFT+U) calculations with van der Waals corrections. Both the M1 and M2 surface terminations─formed by cleavage through the M1 or M2 metal ion sites, respectively─were examined, each containing isolated and paired Fe(II) ions. We find that isolated surface Fe(II) resists oxidation: while Fe(II) can transiently donate an electron to adsorbed H2O and induce reductive dissociation, the resulting charge-neutral H• is unstable and loses the electron upon hopping to a neighboring O site. The energy required for simultaneous formation of two H• radicals is also prohibitively high (∼3.5 eV), rendering H2 generation via the combination of two H• radicals unlikely. In contrast, paired Fe(II) at M1–M1 (adjacent ions on the M1 surface) and M1–M2 (cross-layer pairs) are prone to oxidation, forming a [Hδ+–Hδ–] reaction intermediate (a proton-hydride pair) that facilitates H2 generation. The rate-limiting step of the overall process is the reductive dissociation of H2O on surface Fe(II), with an activation energy of ∼1.9 eV. These findings indicate that H2 can be generated directly on olivine surfaces without invoking secondary minerals such as magnetite. The site-specific oxidation of Fe(II) pairs explains why a large proportion of iron ions entering secondary minerals remain ferrous. Overall, H2 production in serpentinizing systems may hinge on the spatial proximity of Fe(II) ions.