REACTION OF S-2 WITH X MO(110) (X=FE, PT OR AL) SURFACES - ADMETAL-PROMOTED SULFIDATION OF MO AND THE BEHAVIOR OF HYDRODESULFURIZATION CATALYSTS/

Exposure of a clean Mo(110) surface to S-2 at room temperature or at 7 00 K results in a chemisorbed layer of sulfur only, without any eviden ce for the formation of molybdenum sulfides. The effects of Fe, Pt, an d Al on the reactivity of Mo(110) toward sulfur have been studied usin g thermal desorption spectroscopy (TDS), X-ray photoelectron spectrosc opy (XPS) and X-ray excited Auger electron spectroscopy (XAES). It is found that at room temperature, the reactivity of S, with multilayers of the admetal follows the order Fe > Al >> Pt. While most of the Fe b ecomes sulfidized, only a few layers of the Al become sulfidized befor e passivation, and the least reactive Pt surface is covered by a chemi sorbed layer of sulfur. Upon exposure of the Fe/Mo(110) (theta(Fe) <2 ML) surfaces to S-2 at elevated temperatures (700 K) or annealing of F eSy/Mo(110) surfaces (700-1000 K), the formation of MoSx compounds is observed, with FeSy remaining on the surface. High-temperature S-2 exp osure of the Pt/Mol(110) system leads to the formation of a small amou nt of MoS, only in the case where theta(Pt)<1, with only a chemisorbed layer of sulfur observed on top the Pt surface in all other cases. Th ere is no evidence for PtSx formation. High-temperature S-2 exposure o f Al/Mo(110) or annealing of the AlSx/Al/Mo(110) system does not lead to formation of molybdenum sulfides. The thin layer AlSx film remains stable to further S, exposure. The effects of Fe, Pt, and Al on the ra te of sulfidation of Mo are rationalized by examining the relative sta bilities of the metal sulfides, the cohesive energies of the metals in volved and electronic interactions in bimetallic bonding. The trends i n our results are compared with trends in the activity of Mo-based bim etallic catalysts in hydrodesulfurization processes.