Molecular and dissociative adsorption of water and hydrogen sulfide at perfect and defective Cu(110) surfaces

Abstract

We performed a density functional theory (DFT) investigation of the molecular and dissociative adsorption of H₂O and H₂S at perfect and defective Cu(110) surfaces described using supercells with c(6 × 6) periodicity. The defective surface consists of a terrace surrounded by pits. We found considerable differences in adsorption modes and energies for H₂O and H₂S. At the defective Cu(110) surface, monomers of H₂O and H₂S preferentially adsorb at the terrace site and molecular adsorption of H₂O is significantly more favorable than that of H₂S. For dissociative adsorption however, the sulfur species are considerably more stable than the oxygen species. For monolayer (ML) coverages, there are small differences in the molecular adsorption energies for H₂O and H₂S. However, for the formation of 1 ML of HO and 1 ML of HS from 1 ML of H₂O and 1 ML of H₂S, respectively, with the release of H₂(g), the differences are very large. The formation of 1 ML HO at the perfect Cu(110) surface is endoergic, while at the defective Cu(110) surface it is exoergic by −0.6 eV. For high coverages, H₂S forms stacked half-monolayers that interact with each other via a complex hydrogen bond network with a strength per H₂S molecule of −0.140 eV per H₂S and −0.120 eV per H₂S for H₂S located in the underlayer and overlayer, respectively. The large distances between hydrogen bonded H₂S molecules explain the preference for the formation of the two stacked half-monolayers of H₂S instead of a single monolayer as it happens with H₂O. Additionally, the formation of 1 ML of HS does not occur because of the spontaneous splitting of some H–S bonds resulting in surface bound HS and S and H₂S molecules. Extensive surface reconstruction and relaxation accompanies adsorption of the sulfur adsorbates. Such reconstructions with outwards pull of Cu atoms can be at the origin of the weak adhesion of sulfide films that explains the release of CuS particles from copper sulfide films at copper surfaces. Overall, the surface defects here investigated induce non-linear effects in the molecular and dissociative adsorption energies of different O and S adsorbates.