Reaction of SO₂ with Cesium and Cesium-Promoted ZnO and MoO₂

The chemistry of SO 2 on pure metallic Cs and Cs-promoted polycrystalline surfaces of ZnO and MoO 2 has been studied using high-resolution synchrotron-based photoemission. Metallic cesium reacts vigorously with SO 2 at temperatures between 100 and 300 K. At 100 K, the amount of SO 2 that fully dissociates (SO 2,gas → S a + 2O a ) on the alkali metal is relatively small, and SO x ( x = 2−4) species predominate on the surface. SO 3 and SO 4 may be formed as a result of the reaction of SO 2 with atomic oxygen (SO 2 + n O a → SO 2+ n,a where n = 1−2), or by disproportionation of chemisorbed SO 2 . At 300 K the stability of the CsSO x species is lower than at 100 K, and the formation of CsO y and CsS y becomes important. In the Cs/ZnO and Cs/MoO 2 systems, the alkali ↔ oxide interactions are very strong. Despite this, the supported Cs atoms retain a large chemical affinity for SO 2 and are able to enhance the rate of adsorption of the molecule on the oxide surfaces, even at low coverages (0.1−0.2 ML) of cesium when the alkali atoms are in a highly ionic state (Cs δ+ ). The larger the coverage of Cs on ZnO and MoO 2, the faster the rate of adsorption of SO 2, and the larger the amount of S and SO x ( x = 2−4) species present on the surface. The adsorption of SO 2 on ZnO and Cs/ZnO surfaces was examined using ab initio self-consistent-field (SCF) calculations and cluster models. The Cs adatoms provide occupied states that are very efficient for bonding interactions with the LUMO of SO 2 . Cs atoms supported on Zn-terminated terraces of ZnO respond much more strongly to the presence of SO 2 than Cs atoms supported on O-terminated terraces. The bonding interactions between the ZnO(0001)−Zn face and SO 2 are weak, and promotion with Cs adatoms considerably improves the energetics for SO 2 adsorption.