Synchrotron-based high-resolution photoemission has been used to study the
interaction of thiophene with pure and Ni promoted MoSx films. The films ex
hibit Mo 3d and valence spectra that are very similar to those of MoS2. On
the MoSx systems, the chemistry of thiophene closely resembles that seen on
MoS2(0002). The molecules are weakly chemisorbed, and most of them desorb
at temperatures around 200 K. A small fraction of the adsorbed thiophene is
bonded to Mo sites that have S vacancies and desorbs between 250 and 300 K
. The addition of Ni enhances the chemical activity of MoSx. On the NiMoSx
systems, the adsorption energy of thiophene is at least 5-10 kcal mol(-1) l
arger than on pure MoSx. No dissociation of the molecules is observed on th
e NiMoSx surfaces. The Ni<->S interactions reduce the reactivity of nickel,
and the presence of this metal alone is not enough to promote or facilitat
e the cleavage of C-S bonds in hydrodesulfurization (HDS) reactions. Hydrog
en seems to play an important role in this aspect. The behavior of NiMoSx c
atalysts in HDS processes is discussed in the light of these results. (C) 1
999 Elsevier Science B.V. All rights reserved.