Chemical and electronic properties of silver atoms supported on sulfur and molybdenum sulfide surfaces

The chemical and electronic properties of a series of Ag/SmaltMo(110) and Ag/MoSX systems have been investigated using X-ray photoemission, thermal desorption mass spectroscopy, hydrogen (H-2, D-2, or D) chemisorption and molecular orbital calculations. At 100 K, sulfur multilayers supported on Mo(110) react with silver to form sulfide compounds. Upon annealing to high temperature, the silver sulfides promote the sulfidation of the Mo support leading to the formation MoSX. Silver atoms deposited on molybdenum sulfide surfaces remain in a metallic state at temperatures below 300 K. The results of INDO/S and ab initio self-consistent-field calculations indicate that the Ag-MoS2 bond is best described as covalent with a small degree of ionic character. On MoS2 surfaces, Ag is a poor electron donor compared with Co and Ni. At temperatures above 400 K 4g diffuses into the bulk of molybdenum sulfide. forming AgMoSX compounds. These bimetallic sulfides decompose at high temperatures (>800 K) with Ag desorbing and MoSX remaining solid. The AgSY/MoSX and ASMoS(X) systems were unreactive towards molecular hydrogen under ultrahigh vacuum conditions. However. gas-phase atomic hydrogen reacted with the surfaces to form gaseous hydrogen sulfide and led to sorption of hydrogen by the AgSY/MoSX and AgMoSX systems. Compared with other similar systems (MoSX, NiSY/MoSX, CoSY/MoSX, ZnSY/MoSX), the AgSY/MoSX systems show the lowest rate of hydrogenation of Mo-bonded S atoms. The Ag adatoms are very efficient for blocking D<->S interactions. (C) 1998 Elsevier Science B.V.