Chemistry of thiophene on ZnO, S/ZnO, and Cs/ZnO surfaces: Effects of cesium on desulfurization processes

The adsorption of thiophene (C4H4S) On clean ZnO and oxide surfaces precovered with S and Ca has been studied using synchrotron-based high-resolution photoemission and ab initio self-consistent-field (SCF) calculations. On polycrystalline ZnO, C4H4S is weakly chemisorbed, and most desorbs at temperatures below 250 K. A very small fraction of the adsorbed C4H4S (similar to 0.02 monolayer) that interacts with O-unsaturated Zn sites decomposes on the oxide surface. S adatoms weaken the bonding interactions of thiophene on ZnO, whereas Cs adatoms enhance the adsorption energy of the molecule by at least 5-10 kcal/mol and facilitate the cleavage of C-S bonds. Pure metallic Cs reacts vigorously with C4H4S decomposing the molecule at: very low temperatures (100-200 K). The: Cs atoms supported on ZnO are in an ionic state (Csdelta+), but they retain a large chemical affinity for thiophene. Small amounts of Cs (similar to 0.2 monolayer) are enough to activate the oxide surface. Results of ab initio SCF calculations indicate that the bonding interactions of thiophene with the (0001)-Zn and (000 (1) over bar)-O faces of ZnO are weak, and promotion with Cs adatoms considerably improves the energetics for C4H4S adsorption and C-S bond breaking. The Cs adatoms provide occupied states that are very efficient for bonding interactions with the frontier orbitals of C4H4S and other S-containing molecules. This should lead to an improvement in the performance of ZnO as a sorbent in desulfurization processes.