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) chemis orption and molecular orbital calculations. At 100 K, sulfur multilaye rs supported on Mo(110) react with silver to form sulfide compounds. U pon annealing to high temperature, the silver sulfides promote the sul fidation of the Mo support leading to the formation MoSX. Silver atoms deposited on molybdenum sulfide surfaces remain in a metallic state a t temperatures below 300 K. The results of INDO/S and ab initio self-c onsistent-field calculations indicate that the Ag-MoS2 bond is best de scribed as covalent with a small degree of ionic character. On MoS2 su rfaces, Ag is a poor electron donor compared with Co and Ni. At temper atures above 400 K 4g diffuses into the bulk of molybdenum sulfide. fo rming AgMoSX compounds. These bimetallic sulfides decompose at high te mperatures (>800 K) with Ag desorbing and MoSX remaining solid. The Ag SY/MoSX and ASMoS(X) systems were unreactive towards molecular hydroge n under ultrahigh vacuum conditions. However. gas-phase atomic hydroge n reacted with the surfaces to form gaseous hydrogen sulfide and led t o sorption of hydrogen by the AgSY/MoSX and AgMoSX systems. Compared w ith 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 interact ions. (C) 1998 Elsevier Science B.V.