Water-Induced Formation of Cobalt-Support Compounds under Simulated High Conversion Fischer–Tropsch Environment

Herein we present a comparative study on the water-induced formation of metal–support compounds from metallic cobalt in a simulated high conversion Fischer–Tropsch environment. Literature on the deactivation of supported cobalt catalysts via oxidation to cobalt(II) oxide or cobalt-support compounds is contradictory due to a lack of use in suitable model catalysts and insufficient direct characterization of the metallic cobalt phase under reaction conditions. The particular carrier materials stabilize the active cobalt nanoparticles, but also dictate the likelihood of the formation of nonactive cobalt-support compounds. In this study, well-defined cobalt nanoparticles of 5 nm were deposited on alumina, silica, and three titania carriers. The stability of the reduced nanoparticles against water-rich H2 atmospheres during exposure to simulated high Fischer–Tropsch conversion levels was monitored in an in situ magnetometer. Co/SiO2 was shown to be the most stable model catalyst, while various Co/TiO2 model systems readily formed large amounts of cobalt-support compounds at low ratios of the Fischer–Tropsch product H2O to reactant H2 or even during the preceding reduction of the oxidic precursor. Co/Al2O3 displayed a surprisingly high stability at industrially relevant conditions, in contradiction to thermodynamic predictions. However, cobalt aluminate forms at increased concentrations of water. The existence of hard-to-reduce metal–support compounds in the spent catalysts was confirmed and characterized by means of X-ray absorption near edge structure spectroscopy and high-resolution scanning transmission electron microscopy of the exposed and passivated model catalysts.