Catalysis Today, Vol.342, 79-87, 2020
On the passivation of platinum promoted cobalt-based Fischer-Tropsch catalyst
Passivation of reduced cobalt-based catalysts is required prior to air exposure due to the exothermicity of the Co metal oxidation, which may lead to a significant increase of the temperature of the catalyst resulting in its degradation and in a potential fire hazard. This work shows the results of the passivation process carried out on a Pt-promoted Co-based catalyst supported on stabilized alumina at different space velocities in the range of 5-50 Ncc/min/g(cat) and constant O-2 concentration of 1 vol.%. Increasing the O-2 flow fed to the reactor, the specific amount of O-2 consumed on the catalyst slightly decreases. Around 30% of Co metal particles are oxidized to CoO species in all the passivation treatments. These species are found to be much more reducible than the Co oxides species present on the calcined catalyst. Indeed, all the passivated catalysts are completely depassivated at a temperature which is significantly lower (300 degrees C) than that needed to fully reduce the calcined catalyst (400 degrees C). The temperature of the catalyst almost linearly increases with the increase of the O-2 feed flow. Indeed, an abrupt increase of the catalyst temperature is observed for high O-2 flows, resulting in a decrease of the Co degrees dispersion (i.e. increase of the average Co degrees crystallites size) after depassivation. This result is explained with the onset of sintering phenomena of the CoO species formed during passivation. The passivation treatment is found to be unsuitable for long-term catalyst protection, since a deep re-oxidation of the passivated catalyst is observed after two months of air exposure. The effectiveness of the catalyst passivation is eventually validated by running FT reactivity tests at industrially relevant process conditions. The activity of the calcined catalyst reduced in-situ at 400 degrees C is compared to that of the same catalyst reduced ex-situ at the same temperature, passivated at low O-2 flow and depassivated in-situ at 300 degrees C. Interestingly, similar stability and reactivity, expressed both in terms of activity and selectivity, are obtained.