NIMOS GAMMA-AL2O3 CATALYSTS - THE NATURE AND THE AGING BEHAVIOR OF ACTIVE-SITES IN HDN REACTIONS/

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.