Influence of the Characteristics of γ-Aluminas on the Dispersion and the Reducibility of Supported Cobalt Catalysts

Abstract

Four commercial aluminas and different cobalt loadings have been selected to better understand the interactions between Co oxides and metallic cobalt with alumina surfaces. The solids have been characterized, after drying and calcination procedures or after reduction under H 2 , using thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The information has been complemented by specific surface area (SSA), porosimetry, and magnetic measurements. Both the effect of the cobalt content and the texture of supports have been examined. It appears that the cobalt reducibility is influenced by the cobalt loading and the alumina porosity mainly because of the heterogeneity in the Co distribution. Two cobalt reduction steps were identified. The first one which transforms Co 3 O 4 into CoO appears to be easy from 500 K whereas the second one which produces metallic Co occurs in a wide range of temperatures (623-873 K) and is highly dependent on the support characteristics. The use of high specific surface area supports leads to catalysts with a good dispersion when their macroporosity (pore diameter φ > 0.05 µm) is large. However, in all the cases, the grain edges of the powdered sample or the periphery of pellets are richer in cobalt than the bulk, and the cobalt is less dispersed. The reduction of cobalt oxide localized at the periphery is facilitated by its poor dispersion. On the other hand, the well-dispersed cobalt species in the core of the pellets are more strongly interacting with the support and are not so easily reduced.

N2O Abatement Over γ-Al2O3 Supported Catalysts: Effect of Reducing Agent and Active Phase Nature

“…The poor catalytic activity of Ni-Al 2 O 3 , as it was also observed here, was attributed to the formation of low surface area Nickel-aluminate structures (see Table 1) due to metal support interactions, phases which have been reported to exhibit very low activity towards N 2 O decomposition [16].…”

Influence of Cobalt Precursor on Efficient Production of Commercial Fuels over FTS Co/SiC Catalyst

“…Moreover, TPR profile of CoA/SiC was shifted to higher reduction temperatures (420˝C and 720˝C), confirming a lesser degree of reduction due to a stronger interaction between Co species and β-SiC support. This fact would corroborate that catalysts with larger particles are more easily reduced than those with smaller ones, which lead to a higher concentration of hardly reducible cobalt species (silicate-type) [48][49][50]. TPR profiles associated to CoCl/SiC (Figure 6d) or CoCit/SiC (Figure 6e) resulted to be more complex, showing different Co-species formation upon preparation.…”

Promotion of Cobalt Fischer-Tropsch Catalysts with Noble Metals: a Review

“…Several previous reports [31][32][33][34][35] suggest that reduction of Co 3 O 4 to metallic cobalt may proceed via intermediate formation of CoO: Figure 2 shows TPR profiles of monometallic and Ptpromoted cobalt alumina supported catalysts [17]. Promoting with Pt results in a low-temperature shift of the TPR peaks which were schematically attributed to both reduction of Co 3 O 4 to CoO at 500-650 K and to reduction of CoO to metallic cobalt at 650-973 K. Thus, it has been shown that most of noble metals (Pt, Ru, Pd, Ir) enhance the two cobalt oxide reduction steps.…”

“…This suggests that the cobalt metal particles are smaller in CoPt/Al 2 O 3 than in the monometallic catalysts. It is now admitted [32,34,35] that smaller cobalt oxide particles are more difficult to reduce than larger ones. This seems to be a reason why smaller cobalt oxide particles usually remain in the oxidized state after the pretreatment of monometallic cobalt alumina supported catalysts with hydrogen.…”