J. van de Loosdrecht

Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

The oxidation of supported cobalt based slurry bed Fischer–Tropsch catalysts by means of water was studied. Water is one of the Fischer–Tropsch reaction products and can probably cause oxidation and deactivation of a reduced cobalt catalyst. Model experiments using Mossbauer emission spectroscopy and thermogravimetry as well as realistic Fischer–Tropsch synthesis runs were performed. It was demonstrated that Mossbauer emission spectroscopy can successfully be applied to the investigation of high cobalt loading Fischer–Tropsch catalysts. Strong indications were found that oxidation of reduced cobalt catalysts occurs under realistic Fischer–Tropsch conditions. Mossbauer emission spectroscopy and thermogravimetry results showed that the oxidation depends on the PH2/PH2O ratio, and that oxidation proceeds to less than complete extents under certain conditions. The formation of both reducible and less reducible cobalt oxide species was observed, and the relative ratio between these species depends on the severity of the oxidation conditions.

Preparation and properties of supported cobalt catalysts for Fischer-Tropsch synthesis

Abstract Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.

Studying Fischer–Tropsch catalysts using transmission electron microscopy and model systems of nanoparticles on planar supports

Nanoparticle model systems on planar supports form a versatile platform for studying morphological and compositional changes of catalysts due to exposure to realistic reaction conditions. We review examples from our work on iron and cobalt catalysts, which can undergo significant rearrangement in the reactive environment of the Fischer–Tropsch synthesis. The use of specially designed, silicon based supports with thin film SiO2 enables the application of transmission electron microscopy, which has furnished important insight into e.g. the mechanisms of catalyst regeneration.

The application of Mössbauer emission spectroscopy to industrial cobalt based Fischer–Tropsch catalysts

The application of Mossbauer emission spectroscopy to study cobalt based Fischer–Tropsch catalysts for the gas-to-liquids process was investigated. It was shown that Mossbauer emission spectroscopy could be used to study the oxidation of cobalt as a deactivation mechanism of high loading cobalt based Fischer–Tropsch catalysts. Oxidation was observed under conditions that are in contradiction with the bulk cobalt phase thermodynamics. This can be explained by oxidation of small cobalt crystallites or by surface oxidation. The formation of re-reducible Co3+ species was observed as well as the formation of irreducible Co3+ and Co2+ species that interact strongly with the alumina support. The formation of the different cobalt species depends on the oxidation conditions. Iron was used as a probe nuclide to investigate the cobalt catalyst preparation procedure. A high-pressure Mossbauer emission spectroscopy cell was designed and constructed, which creates the opportunity to study cobalt based Fischer–Tropsch catalysts under realistic synthesis conditions.

Stabilization of iron by manganese promoters in uniform bimetallic FeMn Fischer–Tropsch model catalysts prepared from colloidal nanoparticles

Abstract A systematic study was carried out to investigate the response of monodisperse supported Fe and FeMn nanoparticles to treatments in O2, H2 and H2/CO at temperatures between 270 and 400°C. Uniform size (7–14 nm), Fe and mixed FeMn nanoparticles were synthesised by applying thermal decomposition of Fe- and Mn-oleate complexes in a high boiling point solvent. By combining X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analysis, the phase composition and morphology of the model catalysts were studied. Energy-dispersive X-ray analysis shows that the catalyst particles have the expected composition of Fe and Mn. Well-defined crystallite phases [maghemite (γ-Fe2O3) and mixed FeMn-spinel] were observed after calcination at 350°C in Ar/O2 using XPS analysis. Upon subsequent treatments in H2 and H2/CO the crystal phases changed from maghemite (γ-Fe2O3) to metallic Fe, Fe carbide and graphitic C. Using Mn as a promoter influences the nanoparticle size achieved during the fabrication of Fe nanoparticles and improves their stability against morphological change and agglomeration during reduction and Fischer–Tropsch synthesis conditions.