Journal of Catalysis 236 (2005) 139–152 www.elsevier.com/locate/jcat Characterization of alumina-, silica-, and titania-supported cobalt Fischer–Tropsch catalysts Sølvi Storsæter a , Bård Tøtdal b , John C. Walmsley c , Bjørn Steinar Tanem c , Anders Holmen a,∗ a Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway b Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway c SINTEF Materials and Chemistry, Synthesis and Properties, N-7491 Trondheim, Norway Received 27 July 2005; revised 12 September 2005; accepted 16 September 2005 Available online 20 October 2005 Abstract Co and CoRe supported on γ -Al 2 O 3 , SiO 2 , and TiO 2 were prepared by incipient wetness impregnation. The influence of the different supports on the shape, appearance, and size of cobalt particles, as well as on reducibility, was studied by different techniques, including X-ray diffraction, scanning transmission electron microscopy, H 2 chemisorption, N 2 adsorption measurements, temperature-programmed reduction, and O 2 titra- tion. Co was found to exist as Co 3 O 4 on the catalysts in their calcined state. Co 3 O 4 particle size, Co 0 particle size, and reducibility increased with increasing average pore diameter of the support. On both γ -alumina and silica, Co 3 O 4 appeared in clusters, with larger clusters for the silica support, which has a larger average pore diameter. However, on TiO 2 , Co 3 O 4 existed as single crystals, as it also did on α-alumina with large average pore diameter. Thus the size of Co 3 O 4 agglomerates probably increases with increasing pore size up to a certain pore size, beyond which no agglomeration will occur. Rhenium was evenly distributed over the substrate surface, but higher concentrations of rhenium were found at the cobalt-containing positions. Activity and selectivity data obtained previously are discussed in terms of cluster size, particle size, and mass transport limitations.  2005 Elsevier Inc. All rights reserved. Keywords: Fischer–Tropsch synthesis; Cobalt; Rhenium; Alumina; Silica; Titania; TEM; Pore diameter; XRD; TPR; H 2 -chemisorption; Pulse oxidation; Selectivity 1. Introduction Supported cobalt has been widely studied as a Fischer– Tropsch (FT) catalyst for the conversion of synthesis gas de- rived from natural gas, due to its high activity, high selectivity to long-chain paraffins, and low water–gas shift activity. The support seems to be an important factor in the properties of the catalysts. Cobalt in its reduced state, not cobalt precursors, is the active component in the CO/H 2 reaction. Two major factors determine catalyst activity (g prod /(g cat h)), namely the degree of reduction of the metal precursor and the shape and size of the metal particles formed, which control the number of active sites available (dispersion). If a change in dispersion does not lead to * Corresponding author. Fax: +47 73595047. E-mail address: holmen@chemeng.ntnu.no (A. Holmen). a change in turnover frequency, then, by definition, the reaction is structure-insensitive. The type and structure of the support influence the disper- sion, particle size and reducibility, and thereby the activity for Co-supported catalysts [1–19].C 5+ selectivity is also affected by the type of support and the promoter [1,10,12,16–18,20,21]. The acidity of alumina supports [22,23] and modification of silica supports by the addition of zirconia [24,25] are other fac- tors that affect FT activity through changes in reducibility and dispersion. However, zirconia increases the FT activity for an alumina-supported Co catalyst with no corresponding increase in reducibility or dispersion [26,27]. Furthermore, in most cases different promoters also affect dispersion and reducibility [2,7, 28–39]. Besides the support and promoter, several other prepa- ration variables, including cobalt precursor and solvent [6,40– 46], cobalt loading [3,4,7,44,47–49], preparation method [3,42, 50,51], and pretreatments (e.g., the conditions during drying 0021-9517/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2005.09.021