H-ATOM INTERACTION AND ELECTRONIC PROPERTIES OF TM/S, TM/MOS[X] (TM = AG, FE, ZN, CO AND NI) ON MO(110) AND O, RUO[X] ON RU(001)

Abstract

The chemical and electronic properties of a series of TM/Smultilayer/Mo( 110) (TM = Ag, Fe, Zn, Co, and Ni), and TM/MoSX systems (TM = Ag, Zn, Co and Ni) have been investigated using XPS, X-AES, TDS, hydrogen (H2, D2, or D) chemisorption and molecular orbital calculations. Sulfur films on a well-defined metal surface have been prepared by means of a home-made sulfur doser under UHV conditions, and our UPS, TDS and IRAS results showed that such films consisted of Sn species, S2, S4 and S8, The successful preparation of sulfur film under UHV conditions will provide scientists with a good approach lo study sulfur in materials and catalysts. Sulfur films prepared under UHV conditions are very reactive towards adsorbed metal atoms. The deposition of metals TM (TM = Ag, Fe, Zn, Co, and Ni) onto sulfur films at the substrate temperatures of 200 - 300 K led to formation of metal sulfide (TMSX) films. Our studies showed that the presence or an admetal on sulfur films would promote Mo ? S interaction with the result or sulfidation of the substrate Mo(110) forming MoSX films. The admetals promoted the sulfidation of molybdenum by facilitating the migration or S from the surface into the Mo lattice. or by increasing the reactivity or Mo towards S through metal?metal interaction. The promotion effects, based on a per admetal atom, increased in the following sequence: Ag ? Fe< Zn <Co< Ni. This trend was explained in terms or the relative stability of the sulfides and the adsorption energies or the admetals. The preparation of MoSX films on a well-defined Mo surface under UHV conditions makes it easy to study the interaction or MoSX with metals and hydrogen, so as to understand the role MoS2 played in HDS catalysts. Instead of using MoS2(0001) crystal surfaces, we have grown MoSX films on Mo(110) under UHV conditions. Of all the metals we studied, zinc was the best to be employed to prepare pure MoSX films because Zn can totally desorb from Mo(110) before MoSX films decomposed. After deposition Ag, Zn, and Co onto MoSX films at < 300 K, we found all these metals remained in metallic states and they were not able to remove sulfur from MoSX, forming metal sulfides. Annealing the TM/MoSX/Mo(110) systems (TM= Ag, Zn and Co) resulted in the formation of metal clusters of 3D islands and penetration of the admetals into the bulk of MoSX. However, the negative shifts in the binding energies of S 2p and Mo 3d core levels of molybdenum sulfide were observed only in Ag/MoSX/Mo( 110) systems. Our INDO/S and ab initio self-consistent-field (SCF) calculations showed that these negative binding energy shifts could be attributed to transfer of an electron from silver towards molybdenum sulfide forming AgMoSX compound. Although sulfur films and MoSX films were inert towards molecular hydrogen, they showed high reactivity towards atomic hydrogen. Exposure of sulfur and MoSX films to atomic hydrogen led to reduction in the amount or sulfur with the formation of hydrogen sulfides, which evolved into the gas phase as soon as they formed on the surface at the investigated temperatures. Therefore, the dissociation of molecular hydrogen was proposed to be the rate limiting step in the hydrogenation of sulfur and MoSX films. On the other hand, the rate of reaction 2Dgas + Ssolid ? D2Sgas increased in the following order: chemisorbed sulfur < MoSX films < sulfur films. On MoSX films the reaction is 3-4 times slower than on sulfur multi layers, and at least 6 times faster than on Mo( 110) precovered by chemisorbed sulfur. There was a good correlation between the rate of formation of D2S gas and the stability of the S-S or S-Mo bonds in a surface, which agreed well with the stability consequence of S-S and S-Mo bonds. The other pathway for adsorbed hydrogen on the sample was to desorb in the form of molecular hydrogen at between 400 and 500 K. Influence of admetal atoms and metal sulfides on the hydrogenation of MoSX films were also studied. The initial rates for the reaction 2Dgas + Ssolid ? D2Sgas varied with the systems. Silver and silver sulfides overlayers on or in MoSX films induced the largest drop in the rate of hydrogenation. whereas cobalt and cobalt sulfides overlayers led to a minor decrease in the rate of reduction of MoSX films by atomic hydrogen. Although the real reasons were not known, it was clear that Ag and AgSY showed the strongest inhibitory effect in the hydrogenation of MoSX films, which was proposed to associate with the poor electron donor in the MoSX interaction. In comparison, the interaction or molecular and atomic hydrogen with chemisorbed oxygen (O0.5/Ru(001)) and RuOX films (?O = 14 ML) on Ru(001) were studied by AES, TDS and LEED at surface temperatures between 120 and 500 K. At low temperatures (< 300 K). no significant reaction was observed when molecular hydrogen was dosed to the surfaces. Even at high temperatures ( 500 K ). a long induction period was experienced before the hydrogen oxidation reaction occurs. However, atomic hydrogen could remove the chemisorbed oxygen on Ru(001) and oxygen in the RuOX films even at 120 K. Water formation was observed and its desorption was the rate limiting step at surface temperatures < 200 K. while the formation of water was the rate limiting step at high temperatures. On O/Ru(001) the initial removal rate of oxygen was six times faster at 310 K than at 120 K. Atomic hydrogen was proved to be more efficient to remove oxygen from RuOX films than from O/Ru(001) surfaces. Compared with that of Sulfur and MoSX films, at the same surface temperature, the removal rate for sulfur was slower than that for oxygen, which agreed well with the stability consequence of O?Ru and S?Mo.