Chemistry of Thiophene on ZnO, S/ZnO, and Cs/ZnO Surfaces: Effects of Cesium on Desulfurization Processes

Abstract

The adsorption of thiophene (C4H4S) on clean ZnO and oxide surfaces precovered with S and Cs 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 (∼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 (Csδ+), but they retain a large chemical affinity for thiophene. Small amounts of Cs (∼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 (0001̄)-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.