SO2 and its fragments on a Cu(1 1 0) surface

First principles simulations based on density functional theory were used to investigate the bonding and chemistry of SO2 and its fragment on a Cu(1 1 0) surface. Our results, which agree with recent experimental studies, suggest that sulphur dioxide occupies a fourfold hollow site and coordinates as a bidentate or tridentate specie. The stability of the different bonding modes increases according to the η2-O,O ≃ η3-S,O,O > η2-S,O > η1-S sequence. The bonding mechanism of SO2 on copper involves a Cu(3d, 4s) → SO2(LUMO) electron transfer that leads to a weakening and elongation of the S–O bonds. The η2-S,O conformation that exhibits one of the largest charge transfers, and the weakest S–O bonds, is an ideal precursor for the dissociation of the molecule. Of the SO2 fragments, we have studied SO, O and S. The SO molecule has a high interaction energy and the best binding sites have a η2-S,O conformation. For the atomic oxygen adsorption, at a coverage of θ = 0.25, the most stable adsorption site is the hollow one. The p(2 × 1)-O adsorption site was found to be the most stable one and the oxygen has tendency to form O–Cu chains along the [0 0 1] direction. At high coverage, the atomic interaction energy is reduced, which is in agreement with the simultaneous increase of the work function. For the sulphur the c(2 × 2) structure is the most stable. After identifying the binding locations, we further studied the mobility and dissociation of SO2 on the surface. The results obtained shows that the sulphur dioxide decomposition is a multistep process, in which the rate limiting step is the decomposition of SO2 in to O and SO. The calculated migration and dissociation energy barriers for an SO2 adsorbate on the surface are, in the worst cases, 1.16 eV and 1.18 eV respectively. Concerning the sulphur monoide produced, it immediately decomposes further, due to the large exothermicity and the very small dissociation barrier (0.06 eV) of this reaction. This generates a p(2 × 2)S and c(2 × 2)O covered surface, in agreement with the STM observation that SO dissociation proceeds rapidly and this species has not been experimentally observed on the surface.