Calcined and sulfided Ni-Mo catalysts supported on ultrastable Y zeolite (USY), NaY zeolite, mordenite, and ZSM-5 were studied by high-resolution electron transmission microscopy (HRTEM) with selected-area electron diffraction (SAED) and energy-dispersive spectroscopy (EDS). Ni and Mo oxide aggregates were rarely observed in the USY-supported Ni-Mo catalyst, indicating that most of Ni and Mo may be incorporated into USY, e.g,, supercavities and possibly sodalite cages. However, there were a large number of(alpha-NiMoO4 aggregates of different particle sizes in NaY-, mordenite-, and ZSM-5-supported catalysts, and the mordenite-supported catalyst also contained MoO3 crystals. The alpha-NiMoO4 may be attached to the surface of substrates as individual particles or needle aggregates, it may be disseminated into mordenite particles, or it may even form an isolated sphere tin Ni-Mo/NaY) or a needle (in Ni-Mo/ZSM-5) aggregates. Thus, most of Ni and Mo in NaY-, mordenite-, and ZSM-5-supported catalysts preferentially formed aggregates, although some Mo may have been incorporated into NaY and ZSM-5. After sulfidation, small MoS2 aggregates containing some Ni were rarely but occasionally found on the surface of USY zeolite; however, there were a large number of such MoS2 aggregates on the surface of NaY. Separate and intergrown MoS2 and Ni sulfides aggregates were observed on the surface of mordenite and ZSM-5, The Ni sulfide might be identified by SAED and high-magnification images as troilite-like NiS and/or Ni9S8, but definitely not as NiS with millerite structure or Ni3S2 These results indicated that sulfidation does not extensively affect Ni and Mo that have been incorporated into the supercavity and/or sodalite cage of USY in calcined Ni-Mo/USY. After sulfidation, Ni-Mo sulfide species may have formed in the supercavity and possibly in the sodalite cage of the USY zeolite. However, in NaY-, mordenite-, and ZSM-5-supported Ni-Mo catalysts, Ni and Mo may have predominantly formed Ni-Mo sulfides or even separate Mo and Ni sulfide phases on the surface of the substrates. Thus, the USY-supported Ni-Mo catalyst had a much higher hydrodesulfurization (HDS) activity than the other zeolite-supported catalysts, which is attributed to both the highly dispersed Ni and/or Mo sulfide species in supercavities and possibly in sodalite cages of a USY zeolite, and the presence of Bronsted acidity in a USY zeolite we have demonstrated previously (Li, Dien, Nishijima, A., and Morris, D. E., J. Catal. 182, 339 (1999)). (C) 2000 Academic Press.