Catalysis Letters 64 (2000) 95–99 95 Edge termination of MoS 2 and CoMoS catalyst particles Line S. Byskov a , Jens K. Nørskov a , Bjerne S. Clausen b and Henrik Topsøe b a Center for Atomic-scale Materials Physics, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark b Haldor Topsøe Research Laboratories, Nymøllevej 55, DK-2800 Lyngby, Denmark Received 28 June 1999; accepted 13 September 1999 The edge termination of MoS 2 and CoMoS catalyst particles is studied by density functional calculations. We show that for structures without vacancies Mo-terminated edges have the lowest edge energies. Creation of vacancies, which are believed to be active sites in these catalyst systems, leads to structures having mainly S-terminated edges. Thus, the results indicate that the shape of the MoS 2 and CoMoS structures may be a dynamical function of the reaction conditions. Independent of the type of edge termination, the results indicate that triangular-shaped nanocrystals may be expected in order to maximize the exposure of the favored edges. This is in contrast to the commonly assumed hexagonal morphology, but triangular-shaped MoS 2 structures have in fact recently been observed in STM studies of model systems [1]. Keywords: hydrotreating, edge termination, MoS 2 , CoMoS, density functional calculations, triangular structure 1. Introduction The commonly used industrial hydrotreating catalysts for sulfur removal from oil fractions are based on Mo struc- tures with Co or Ni added as a promoter [2]. The catalyst consists of small particles of MoS 2 , where Mo atoms are sandwiched between two hexagonal close-packed S layers. Two-dimensional structures consisting of single S–Mo–S layers are observed in Al-supported catalysts after typical sulfiding procedures [2–5]. The size of these crystallites is typically 1–3 nm [2,6,7]. The reactivity of the catalyst has been found to be related to the edges rather than the basal plane of MoS 2 [2,8–10]. The promoter atoms occupy sites at the edges of the so-called CoMoS structures which leads to an increased reactivity compared to the pure Mo edge sites [11]. Coordinatively unsaturated sites (CUS) along the edges of the catalyst particles are believed to provide the active sites, where molecules can adsorb and undergo further reactions. Based on effective medium theory stud- ies [12] and more recently density functional theory (DFT) calculations [13], it has been suggested that one role of the Co and Ni promoters is to facilitate making coordinatively unsaturated metal sites by reducing the sulfur binding en- ergy at the edge. If the edge sulfur atoms are bound weaker, the equilibrium concentration under a certain H 2 /H 2 S ratio in the gas phase will be smaller and the number of active sites correspondingly larger. With the exception of recent scanning tunneling mi- croscopy (STM) studies of MoS 2 nanostructures [1], the large experimental and theoretical efforts have not es- tablished much insight into the morphology of alumina- supported MoS 2 structures [2]. Many shapes have been considered [2,14] but most models have assumed hexagonal geometry [15]. The MoS 2 can essentially have two differ- ent edge terminations, the Mo(10 ¯ 10) and S( ¯ 1010) edges. It is expected that their equilibrium shape will be determined by the relative edge energies, which may furthermore de- pend on the edge concentration of S adatoms, and hence on temperature and gas-phase composition and on the presence of Co or Ni edge atoms. In the present letter, we apply DFT calculations to a simple model system in order to shed further light on the shape of small unpromoted and Co-promoted MoS 2 parti- cles with and without S vacancies at the edges. We show that for our model system there is a strong tendency to form Mo-terminated (10 ¯ 10) edges for both the pure MoS 2 and the CoMoS systems when no vacancies are present, but in the presence of vacancies this tendency is reversed, so that predominantly S-terminated ( ¯ 1010) edges are exposed. This means that for the Co-promoted system, where vacancies are more easily formed than on non-promoted edges [13], the S-terminated edges may become the most stable. 2. Calculational details The present study is based on a set of calculations of the energetics of MoS 2 -based structures using density func- tional theory (DFT) [16,17]. The calculations are parame- ter free and exchange–correlation effects are treated non- locally by use of the generalized gradient approximation (GGA) [18]. The wave functions are expanded in plane waves [19] with a cutoff energy of 25 Ry, and the ionic cores are treated with ultra-soft pseudo-potentials (US- PP) [20]. The calculations are performed self-consistently with 3 k-points in the irreducible Brillouin zone (IBZ). Peri- odic boundary conditions are applied and the structures are described as slabs. For each structure studied, the atomic positions are relaxed in order to obtain the minimum energy configuration.  J.C. Baltzer AG, Science Publishers