P.J. van Berge

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.

Cobalt as an alternative Fischer-Tropsch catalyst to iron for the production of middle distillates

Publisher Summary Cobalt based catalysts may be considered as an alternative to iron based Fischer–Tropsch catalysts for the production of middle distillates utilizing a syngas with a H2/CO ratio of 2. This chapter presents the Fischer–Tropsch synthesis in a continuously stirred tank reactor (CSTR) with a reactor volume of 670ml. The pre-reduced catalyst was suspended in a 300ml molten Arge wax. The syngas flows were controlled by Brooks mass flow meters along with the ampoule-sampling-technique developed by Schulz. An investigation into the intrinsic kinetics of the iron catalyst revealed that the cobalt catalyst did not show any significant water-gas-shift activity and no water inhibition of the Fisher–Tropsch reaction rate. The strong advantage of the cobalt option is to be found in the application where high per pass syngas conversion levels are desired. If a comparable net conversion is to be achieved with iron, it will require gas recycle to knock out water, implying added compression costs.

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.