Niobia-supported Cobalt Catalysts for Fischer-Tropsch Synthesis

Abstract

In this thesis niobia has been shown to be an attractive support for application in Fischer-Tropsch catalysis at industrially relevant conditions without apparent deactivation up to at least 200 hours of operation. This proves that the level of potentially poisoning contaminants is sufficiently low and niobia is stable up to 260 °C and CO conversion levels higher than 80 % with concomitant high steam pressures. Niobia-supported cobalt catalysts were found to exhibit high selectivities towards liquid hydrocarbons at competitive cobalt-weight based activities. Due to the low porosity of crystalline niobia, only low cobalt loadings can be applied and consequently low catalyst-weight based activities could be obtained. However, using the reactor temperature or Pt-promotion, methods were found to compensate for the low cobalt loading and to obtain competing selectivities and catalyst-weight based activities while using a low amount of cobalt. Transition metal oxides are typical promoters added to cobalt Fischer Tropsch catalysts to enhance the selectivity towards heavy hydrocarbons. In this thesis niobia was found to be a very efficient selectivity promoter particularly at 1 bar. At 20 bar, the influence of support pore size appeared to be important. This fundamentally different role of promoters at 1 and 20 bar is very relevant to understand the necessity and role of transition metal oxide promoters in cobalt-based Fischer Tropsch catalysts but also calls for characterization methods at conditions as close as possible to industrially applied conditions, for example SSITKA at elevated pressures. Indications were found that the role of platinum in cobalt-based Fischer Tropsch synthesis was not limited to facilitation of cobalt oxide reduction but also played a role during Fischer Tropsch synthesis in PtCo/Nb2O5 catalysts. Due to the similar support nature, this observation is very relevant to Co/TiO2, a catalyst relevant for industrial Fischer Tropsch synthesis. Noble metal promoters are rarely applied in Co/TiO2 since facile and complete reduction is typical without noble metals present. Following the observations presented in this thesis, the influence of noble metal promotion on the performance of Co/TiO2 catalysts was investigated and the activity was found to be enhanced up to a factor of four upon noble metal promotion.