Adsorption, diffusion, and dissociation of H ₂S on Fe(100) from first principles

We employ spin-polarized periodic density functional theory (DFT) to characterize H ₂.S and HS adsorption, diffusion, and dissociation on the Fe(100) surface. We investigate the site preference of H ₂S, HS, and S on Fe(100). H ₂S is predicted to weakly adsorb on hollow, bridge, and on-top sites of Fe(100), with the bridge site preferred. The diffusion barrier from the bridge site to the next most stable on-top site is predicted to be small (∼0.15 eV). In contrast to H ₂S, HS is predicted to be strongly chemisorbed on Fe(100), with the S atom in the hollow site and the HS bond oriented perpendicular to the surface, due to charge transfer from the surface to S p-orbitals. Isolated S atoms also are predicted to bind strongly to the hollow sites of Fe(100), with the bridge site found to be a transition state for S hopping between neighboring hollow sites. The minimum energy paths for H ₂S and HS dehydrogenation involve rotating an H atom toward a nearby surface Fe atom, with the S-H bonds breaking on the top of only one Fe atom. The barriers to break the first and second S-H bonds in H ₂S are low and about the same, ∼0.25 eV, suggesting deposition of S on Fe(100) via H ₂S is kinetically and thermodynamically facile, consistent with experiments.