Characterization of alumina-, silica-, and titania-supported cobalt Fischer-Tropsch catalysts

Co and CoRe supported on gamma-Al2O3, SiO2, and TiO2 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 titration. Co was found to exist as Co3O4 on the catalysts in their calcined state. Co3O4 particle size, Coo particle size, and reducibility increased with increasing average pore diameter of the support. On both gamma-alumina and silica, Co3O4 appeared in clusters, with larger clusters for the silica support, which has a larger average pore diameter. However, on TiO2, Co3O4 existed as single crystals, as it also did on alpha-alumina with large average pore diameter. Thus the size of Co3O4 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. (c) 2005 Elsevier Inc. All rights reserved.