Density functional studies on the adsorption and decomposition of SO2 on Cu(100)

Density functional calculations in combination with cluster and slab models (periodic super-cell approach) were used to investigate the bonding and ch emistry SO2 on Cu(100). For small coverages of SO2, the stability of the di fferent bonding modes increases according to the eta (1)-S < eta (2)-S,O < eta (2)-O,O < eta (3)-S,O,O sequence. Large coverages make impossible eta ( 3)-S,O,O bonding, and under such conditions the most stable conformations i nvolve eta (2)-O,O or eta (2)-S,O bonding. These adsorption geometries can be expected when SO2 is coadsorbed with substantial amounts of O. The bondi ng mechanism of SO2 on copper involves a Cu(3d,4s)--> SO2(LUMO) electron tr ansfer that leads to a weakening and elongation of the S-O bonds. The eta ( 3)-S,O,O conformations exhibit the biggest adsorption energies, the largest charge transfers, and the weakest S-O bonds. These conformations are ideal precursors for the dissociation of the SO2 molecule. From a thermochemical viewpoint, it is much easier to generate SO3(3SO(2)--> 2SO(3)+S,DeltaE=sim ilar to +1 kcal/mol) than to form SO(SO2--> SO+O,DeltaE=similar to +20 kcal /mol) as an intermediate during the decomposition of sulfur dioxide on Cu(1 00). SO and SO3 behave as net electron acceptors when bonded to copper, wit h the electron density on their S atoms increasing in the following order: SO3< SO2< SO <S. At small coverages, SO3 prefers an adsorption geometry in which its C-3v axis is perpendicular to the surface and the molecule is bon ded to copper through the oxygen atoms (eta (3)-O,O,O bonding). In a crowde d surface, the SO3 can be forced into a eta (2)-S,O bonding conformation to minimize lateral adsorbate <----> adsorbate repulsions. The multidentate n ature of SO2 and SO3 opens the possibility for a complex DeSO(x) chemistry on metal surfaces. (C) 2001 American Institute of Physics.