Lp. Zhang et al., NIMOS GAMMA-AL2O3 CATALYSTS - THE NATURE AND THE AGING BEHAVIOR OF ACTIVE-SITES IN HDN REACTIONS/, Journal of catalysis (Print), 178(2), 1998, pp. 457-465
The work presented in this paper is a continuation of our earlier work
aimed at acquiring a fundamental understanding of sulfided Ni-Mo/gamm
a-Al2O3 catalysts in HDN catalysis, A series of temperature-programmed
desorption and reduction experiments were performed over in-situ sulf
ided bare gamma-alumina support, alumina-supported monometallic Ni, Mo
, and bimetallic NiMo catalysts. The TPD and TPR results were linked t
o the active site assignment of Ni- and Mo-associated centers in HDN r
eactions. Postreaction XPS and SEM characterizations were performed fo
r catalysts used in HDN for up to 1000 h. One of the major changes dur
ing the aging of the Ni-Mo/gamma-Al2O3 catalysts was the loss of some
active sites associated with the Ni promoters, The decreased Ni conten
t over the catalyst surface was consistent with the observed decrease
in the hydrogenation function of the catalyst. Two possible mechanisms
are suggested for the loss of Ni active sites, One is coking that spr
eads from the alumina support to the nickel sites. Another is segregat
ion of some Ni atoms from the Ni-Mo-S structure due to loss of structu
ral sulfur, (C) 1998 Academic Press.