The interaction of H2S and S2 with Cs and surfaces: photoemission and molecular-orbital studies

the surface chemistry of H2S and S2 on metallic Cs and Cs ZnO surfaces has been investigated using high-resolution synchrotron based photoemission and ab initio self-consistent-field calculations. Metallic Cs is very reactive toward H2S and S2 at temperatures between 100 and 300 K. Pure cesium decomposes H2S to form Cs2S compounds. After dosing S2 to Cs, one obtains Cs2S and Cs2S2m (m ≥1 ) compounds. The formation of cesium sulfides induces an increase in the intensity of the Cs 3d levels and large negative shifts (0.8–1.3 eV) in their peak positions. Cesium atoms supported on ZnO are in an ionic state (Csδ+), but they are still able to interact with H2S and S2 more strongly than Zn and O sites of the oxide support. A correlation is found between the electron density on the Cs adatoms and their reactivity: Cs atoms supported on Zn sites of the oxide bond S-containing species (H2S, HS, S2, S) are stronger than Cs atoms supported on O sites. H2S dissociates into HS and atomic S upon adsorption on Cs ZnO surfaces at 300 K. The HS species decompose at temperatures below 450 K leaving S atoms that are bonded to Cs and Zn. The adsorption of S2 on Cs ZnO surfaces at 300 K leads to the formation of Cs2S and Cs2S2m (m ≥ 1) compounds. Cs↔S interactions increase the thermal stability of cesium on the ZnO surface. The poisoning of Cs/Cu/ZnO catalysts is discussed in light of these results and those previously reported for the S2/Cu/ZnO system.