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

Abstract

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.

Preparation of isolated Co3O4and fcc-Co crystallites in the nanometre range employing exfoliated graphite as novel support material

“…Co 3 O 4 nanoparticles were synthesised via a surfactant-free, nonaqueous heat treatment of dissolved cobalt acetate in benzyl alcohol in the presence of ammonium hydroxide. 9,14,22,40,41 Cobalt(II) acetate tetrahydrate was dissolved in benzyl alcohol under magnetic stirring at 500 rpm in a round bottom ask (Table 1). Aer 2 h of stirring, a 25 wt% aqueous ammonium hydroxide solution was added dropwise to the pink to purple solution forming a brown emulsion.…”

“…[5][6][7][8][9] These supports typically anchor the nanoparticles via metal-support interactions sometimes even forming mixed phases. [10][11][12][13][14] On the one hand, such an interaction is desired as it increases the stability of the nanoparticles under reaction conditions, e.g. against sintering, on the other hand it is typically associated with a loss in active phase.…”

Employing Exfoliated Graphite as Novel Support Material for Heterogeneous Model Catalysts

“…Co3O4 nanoparticles were synthesised via a surfactant-free, non-aqueous heat treatment of dissolved cobalt acetate in benzyl alcohol in the presence of ammonium hydroxide. 9,14,22,40,41 Cobalt(II) acetate tetrahydrate was dissolved in benzyl alcohol under magnetic stirring at 500 rpm in a round bottom flask (Table 1). After 2 h of stirring, a 25 wt.% aqueous ammonium hydroxide solution was added dropwise to the pink to purple solution forming a brown emulsion.…”

“…[5][6][7][8][9] These supports typically anchor the nanoparticles via metalsupport interactions sometimes even forming mixed phases. [10][11][12][13][14] On the one hand, such an interaction is desired as it increases the stability of the nanoparticles under reaction conditions, e.g. against sintering, on the other hand it is typically associated with a loss in active phase.…”

Direct observation of the evolving metal–support interaction of individual cobalt nanoparticles at the titania and silica interface

“…[23][24][25][26] Wolf et al suggested that water plays an important role as far as inducing cobalt spreading on oxide supports (AlO x , SiO x and TiO x ) as well as the formation of non-reducible metal-support compounds during reduction and FTS. 27,28 In many cases encapsulation, spreading or the formation of non-reducible metal-support compounds is inimical to catalyst performance as they lead to a reduction in the number of surface active sites by either decreasing the dispersion or else the extent to which they are reduced; this would likely be detrimental to activity if Co 0 is the active phase as is generally thought. In comparison, as far as selectivity is concerned, it has been shown that the effect these changes have are minor and seemingly limited to shiing the hydrocarbon product distribution towards light hydrocarbons.…”