Origin of chemoselective behavior of S-covered Cu(1Â 1Â 1) towards catalytic hydrogenation of unsaturated aldehydes

The molecular structure of acrolein and crotonaldehyde adsorbed on clean and S-covered Cu(1 1 1) surfaces has been theoretically investigated in order to explain the mechanism of selective activation of the carbonyl group induced by the presence of small amounts of S on the metal surface. While on clean Cu(1 1 1) there is an equilibrium between a physisorbed and chemisorbed species, it is found that addition of small amounts of S enhances and modifies the metal-molecule interaction, in agreement with experimental evidence. On the S-covered Cu(1 1 1) the aldehyde is bonded to the surface through the O of the carbonyl group which is slightly stretched whereas the CC bond remains undistorted and far away from the surface. This geometrical conformation induced by the presence of coadsorbed S activates the CO group and renders the CC bond inaccessible to hydrogen adatoms. We argue that this is the origin of the chemoselectivity enhancement towards the crotyl alcohol experimentally observed.