On the Structural Differences between Alumina-Supported CoMoS Type I and Alumina-, Silica-, and Carbon-Supported CoMoS Type II Phases Studied by XAFS, MES and XPS.

Abstract

In this study the local structure of the Mo and Co promoter atoms in CoMoS type I and II on alumina, and CoMoS type II on various supports is 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 the Mo atoms. The CoMoS phase in all catalysts shows a well defined Co-Mo and a Mo-Co coordination. On alumina the CoMoS type II phase is present as a multilayer structure, whereas type I exists as a single-slab, i.e., monolayer, structure. The silica-supported CoMoS type II closely resembles its alumina-supported counterpart. However, the Mo-S coordination 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 opinion that the CoMoS type II phase is fully sulfided and not chemically bonded to the support. Two different types of Co-sites can be distinguished 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 CoMoS type I and CoMoS type II (with Co/Mo = 0.39 at/at) samples. The other Co-site has a sixfold Co-S coordination with possibly a twofold Co-Mo coordination and has the highest activity for HDS. The latter site constitutes a large part of the Co-sites in silica-supported CoMoS phase II and alumina-supported CoMoS type II (with Co/Mo = 0.32) and is exclusively present in the carbon-supported CoMoS phase II.