E. Bergene

Study of the deactivation mechanism of Al2O3-supported cobalt Fischer-Tropsch catalysts

The influence of water on alumina-supported cobalt catalysts has been studied. The deactivation of supported Co catalysts was studied in a fixed-bed reactor using synthesis gas feeds containing different concentrations of water vapour. Supporting model studies were carried out using H2O/H2 feeds in conjunction with XPS and gravimetry. Rapid deactivation occurs on Re-promoted CO/Al2O3 catalysts when H2/CO/H2O feeds are used, whereas unpromoted CO/Al2O3 shows more stable activity. The results from the gravimetric studies suggest that only a small fraction of the bulk cobalt metal initially present reoxidizes to cobalt oxide during reaction. However, the XPS results indicate significant reoxidation of surface cobalt atoms or highly dispersed cobalt phases, which is likely to be the cause of the observed deactivation. Rhenium is shown to have a marked effect on the extent of reoxidation of alumina-supported cobalt catalysts.

STM studies of clean, CO- and O2-exposed Pt(100)-hex-R0.7°

The structure of the Pt(100)-hex-R0.7° reconstructed phase has been studied by scanning tunneling microscopy (STM). The hexagonal first layer superstructure of this surface, written in matrix notation as N1−15, N = 12−14, was imaged with atomic resolution. The fivefold periodicity of this reconstruction was confirmed. The coexistence of differently oriented hexagonal domains on a single terrace was observed. Steps along the longer corrugation length of the surface reconstruction were found, on the average, to follow the [N 1] direction of the hexagonal Pt overlayer rather than any low-index direction of the square substrate. The transition from the N1−15 reconstruction to the Pt(100)−(1 × 1) surface structure due to exposure to CO and O2 was found to be initiated by heterogeneous nucleation, the nucleation centers being step edges and structural irregularities disrupting the hexagonal structure along a direction close to the hexagonal [1 5] direction. The growth of the (1 × 1) phase was highly anisotropic, i.e. the growth being much faster along the [011] direction (the substrate direction closer to the [N 1] direction of the hexagonal layer) than along the [011] direction. A higher density of atoms in the hexagonal than in the (1 × 1) surface layer causes islands, a single Pt(100) lattice spacing high, to form during the structural transformation. These islands were found to be a mixture of isotropic islands with diameter 10–15 A and islands highly elongated along the faster growth direction of the (1 × 1) phase.

Fischer-tropsch synthesis on monolithic catalysts of different materials

Monolithic structures made of cordierite, γ-Al 2 O 3 and steel have been prepared as catalysts and tested for Fischer-Tropsch activity. The monoliths made of cordierite and steel were washcoated with a 20 wt.% Co-1 wt.% Re/γ-Al 2 O 3 Fischer-Tropsch catalyst whereas the γ-Al 2 O 3 monoliths were made by direct impregnation with an aqueous solution of the Co and Re salts resulting in a loading of 12 wt.% Co and 0.5 wt.% Re. The activity and selectivity of the different monoliths were compared with the corresponding powder catalysts. Higher washcoat loadings resulted in decreased C 5+ selectivity and olefin/paraffin ratios due to increased transport limitations. The impregnated γ-Al 2 O 3 monoliths also showed similar C 5+ selectivities as powder catalysts of small particle size (38-53 μm). Lower activities were observed with the steel monoliths probably due to experimental problems.

Fischer-Tropsch synthesis on cobalt catalysts supported on different aluminas

Alumina with different surface area has been prepared by heat treatment of γ-alumina. Improved C5+ selectivity was obtained by using Co or CoRe supported on low surface area alumina (LSA, 15 m2/g). LSA alumina resulted in the lowest Co dispersion and in reduced activity for secondary hydrogenation of propene.

Fischer-Tropsch synthesis using monolithic catalysts

Abstract Cordierite monoliths have been washcoated with a 20 wt. % − 1 wt. % Re/γ-alumina Fischer-Tropsch catalyst and tested for Fischer-Tropsch activity. The activity and selectivity were comparable to the corresponding powder catalyst at low washcoat loadings. Higher washcoat loadings resulted in decreased C5+ selectivity and olefin/paraffin ratios due to increased transport limitations. Impregnated alumina monoliths with a Co loading of 12 wt. % (and 0.5 wt. % Re) also showed similar C5+ selectivity as the small-particle powder catalyst. A large-particle powder catalyst (425-850 lIm) showed very low C5+. selectivities due to severe intraparticle transport limitations.