Pj. Mangnus et al., TEMPERATURE-PROGRAMMED REDUCTION AND HDS ACTIVITY OF SULFIDED TRANSITION-METAL CATALYSTS - FORMATION OF NONSTOICHIOMETRIC SULFUR, Journal of catalysis, 151(1), 1995, pp. 178-191
Sulfided alumina-supported transition metal catalysts have been studie
d using temperature-programmed reduction (TPR). The H-2 and H2S concen
trations were determined simultaneously and the major processes occurr
ing were discriminated, namely reduction of oxidic and sulfidic specie
s and desorption of H2S. Because of the high maximum reduction tempera
ture and the high hydrogen partial pressure applied, a complete descri
ption of all species was acquired, except for the Cr and Mn catalysts
which could not be reduced below 1270 K. After sulfiding at 673 K, TPR
of alumina-supported transition metals showed that several types of s
ulfur were present, viz. (i) sulfur species with a stoichiometry in ag
reement with bulk thermodynamics, indicated as stoichiometric sulfur,
(ii) a nonstoichiometric sulfur species (S-x) which was present in exc
ess of the S/Me ratio based on bulk thermodynamics (not identical to e
lemental sulfur since it is present after purging at 673 K), (iii) S-H
groups, and (iv) adsorbed H2S. The mechanism of S-x formation is disc
ussed. The amounts of nonstoichiometric sulfur mainly depended on the
type of transition metal and the pretreatment procedure. The influence
of the reductibility of the stoichiometric and nonstoichiometric sulf
ur species on the hydrodesulfurisation activity is discussed. The resu
lts presented indicate that chemisorption measurements may depend on t
he pretreatment of sulfided catalysts. (C) 1995 Academic Press, Inc.