Abstract
High-resolution synchrotron-based photoemission and X-ray photoelectron spectroscopy (XPS) have been used to study the interaction of SO 2 with the Mo(110), O/Mo(110), MoO 2/Mo(110) and Cs/Mo(110) surfaces. Ab initio self-consistent-field (SCF) calculations have been applied to provide information on the bonding of SO 2 to Mo(110) and MoO 2. At 300 K, sulfur dioxide completely decomposes on the Mo(110) surface (SO 2→S a+2O a), while molecular species (SO 2 and SO 4) are also observed at 100 K. A SO 2 multilayer, grown at large SO 2 exposures at 100 K, completely disappears from the surface upon annealing to 160 K, leaving SO 4, SO 2 and atomic S. The molecular species SO 4 and SO 2 decompose entirely upon further annealing to 250 K. Several coordination modes for adsorbed SO 2 (η 1-S a-top, η 1-S bridge, η 2-O,O bridge, η 2-S,O bridge) were examined using ab initio SCF calculations and a Mo 15 cluster model. It was found that adsorption geometries in which the molecule is di-coordinated via O,O or S,O are the most probable precursors for dissociation on Mo(110). From an electronic viewpoint, molybdenum is very well suited to adsorb and dissociate SO 2, making this metal more reactive than any late transition metal. On a Mo 7O 14/PC 4NC 8 oxide cluster (PC and NC denote positive and negative point charges, +4e and −2e), a η 2-O,O bridge conformation is the most stable, but the adsorption energies and perturbations in the geometry of SO 2 are much smaller than on the metal cluster (Mo 15). Experimental results confirm the theoretical expectations, and the Mo and O sites of the MoO 2/Mo(110) system are not reactive toward SO 2. Oxygen precovered surfaces do not facilitate the generation of SO X groups and accommodate only a small amount of atomic sulfur upon dosing of SO 2 at 300 K. Addition of cesium to the Mo(110) surface substantially enhances its reactivity towards SO 2. SO 2 readily reacts with the Cs/Mo(110) surfaces at 300 K, and SO 4 and SO 3 species are produced. The thermal stability of the SO 4 species on the Cs/Mo(110) system is much higher than that on the clean Mo(110) surface.
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