ON THE STRUCTURAL DIFFERENCES BETWEEN ALUMINA-SUPPORTED COMOS TYPE-I AND ALUMINA-SUPPORTED, SILICA-SUPPORTED, AND CARBON-SUPPORTED COMOS TYPE-II PHASES STUDIED BY XAFS, MES, AND XPS

In this study the local structure of the Mo and Co promoter atoms in C oMoS type I and II on alumina, and CoMoS type II on various supports i s compared using in situ XAFS spectroscopy as the main technique with XPS and MES in a supporting role. In the CoMoS phase the Co atoms are positioned at the edges of the MoS2 particles, in the same plane as th e Mo atoms. The CoMoS phase in all catalysts shows a well defined Co-M o and a Mo-Co coordination. On alumina the CoMoS type II phase is pres ent as a multilayer structure, whereas type I exists as a single-slab, i.e., monolayer, structure. The silica-supported CoMoS type II closel y resembles its alumina-supported counterpart. However, the Mo-S coord ination number, the structural ordering and degree of stacking of the carbon-supported type II CoMoS phase is much more similar to the type I CoMoS phase supported on alumina. This contradicts the common opinio n that the CoMoS type II phase is fully sulfided and not chemically bo nded to the support. Two different types of Co-sites can be distinguis hed in the suite of catalysts studied in this paper. A Co-site with an approximately fivefold Co-S coordination and possibly a single Co-Mo coordination is predominant in the least active alumina-supported CoMo S type I and CoMoS type II (with Co/Mo = 0.39 at/at) samples. The othe r Co-site has a sixfold Co-S coordination with possibly a twofold Co-M o coordination and has the highest activity for HDS. The latter site c onstitutes a large part of the Co-sites in silica-supported CoMoS phas e II and alumina-supported CoMoS type II (with Co/Mo = 0.32) and is ex clusively present in the carbon-supported CoMoS phase II. (C) 1994 Aca demic Press. Inc.