FT-IR study of surface properties of unsupported MoS2

Surface properties of unsupported MoS2 have been studied by means of IR spe ctroscopy of a wide range of adsorbed molecules. After contact with air, su rface of MoS2 is partially oxidised and contains adsorbed water, Mo=O group s, sulphate and thiosulphate-like anions. These species are removable by tr eatment in vacuum or in H2S-H-2 mixture. On MoS2, only molecular adsorption was established for SO2, CO2, COS and CH3SH showing weak basicity of surfa ce sulphur ions. Unsupported MoS2 presents also very weak acidic properties , as revealed by pyridine, 2,6-dimethylpyridine and ethylene adsorption. Ho wever, ammonia could be protonated or coordinately bound to MoS2 surface. C O and NO interact with the coordinately unsaturated Mo sites of the sulphid e, providing evidence for an important back donation effect. Comparison wit h surface properties of supported MoS2 points to the lower acidic and highe r metallic character of Mo sites of the unsupported sulphide. (C) 2001 Else vier Science B.V. All rights reserved.