Microstructure of Supported Cobalt Fischer-Tropsch Catalysts

Abstract

Résumé -Microstructure de catalyseurs Fischer-Tropsch à base de cobalt supporté -La structure de particules métalliques de cobalt supporté sur silice ou sur alumine, et en présence ou non d'un promoteur (Ru), a été étudiée in situ par diffraction des rayons X conventionnelle et anomale. Par comparaison avec des diagrammes simulés, il est montré que cette structure est caractérisée par la présence de défauts d'empilement dont la densité est directement reliée à la température de réduction du cobalt. Des traitements de carburation/décarburation sous CO et H 2 nous ont permis de modifier de manière contrôlée la microstructure du cobalt métallique pour un même support tout en limitant la variation de la taille des particules. Quel que soit le support, et en présence ou non d'un promoteur, les basses températures de réduction favorisent la formation de la forme hexagonale compacte du cobalt, alors qu'une réduction à haute température favorisera la forme cubique faces centrées. Dans les conditions de la synthèse Fischer-Tropsch, les catalyseurs contenant du cobalt à forte densité d'empilements de type hexagonal compact montrent une activité en conversion du CO nettement supérieure à celle des catalyseurs majoritairement cubiques. Abstract -Microstructure of Supported Cobalt Fischer-Tropsch Catalysts -The structures of metallic cobalt particles supported on silica (with and without Ru promoter) and on alumina have been investigated in-situ by conventional and anomalous X-ray diffraction.Comparison with simulated XRD patterns shows the structure to consist of a defect phase characterised by the presence of stacking faults, the density of which is directly related to the temperature at which the cobalt oxide is reduced.

In-Situ Reduction of Promoted Cobalt Oxide Supported on Alumina by Environmental Transmission Electron Microscopy

“…Particles smaller than 20 nm were fcc, those larger than 40 nm were hcp, while the particles with the size about 30 nm were a mixture of both fcc and hcp. An increased activity for CO conversion in FT conditions for hcp as opposed to fcc cobalt supported on c-Al 2 O 3 has been reported [11]. Bulavchenko et al [12] reported a high concentration of stacking faults, according to analysis of x-ray diffraction peaks.…”

“…The corresponding volume decrease on going from Co 3 O 4 to CoO is 12% and from Co 3 O 4 to hexagonal or cubic cobalt is 49%. At temperatures above 450°C, bulk metallic cobalt has the fcc structure, while at lower temperatures the hcp cobalt structure is stable [10,11]. Supported cobalt nanoparticles tend to form with both fcc and hcp cobalt structures at low temperatures, even below the bulk transition temperature [11,12].…”

“…At temperatures above 450°C, bulk metallic cobalt has the fcc structure, while at lower temperatures the hcp cobalt structure is stable [10,11]. Supported cobalt nanoparticles tend to form with both fcc and hcp cobalt structures at low temperatures, even below the bulk transition temperature [11,12]. Lattice structures and major lattice spacings for the known cobalt phases and the alumina support are given in Table 1.…”

Effect of CO Concentration on the α-Value of Plasma-Synthesized Co/C Catalyst in Fischer-Tropsch Synthesis

“…In addition, XRD analysis revealed that the FCC crystal structure of the Co catalyst was the predominant and possibly the active phase in the FTS reaction [7], although some scholars have provided evidence for potentially greater intrinsic activity displayed by the HCP phase of the Co catalyst [44]. The catalytic activity for CO conversion was shown to greatly increase for catalysts with a majority of HCP stacking compared to those containing mostly the FCC phase particles [45]. In our plasma-synthesized Co/C sample, the existing HCP phase in the initial Co metal was completely transformed into the FCC phase and the catalyst was still very active.…”

Mars-van Krevelen-like Mechanism of CO Hydrogenation on an Iron Carbide Surface

“…Optimum metal-carbon bond strength is a key parameter in the selectivity of the CO hydrogenation. Weak and strong bonding of carbon atoms leads to mostly methane formation, while intermediate bonding leads to longer hydrocarbons [10][11][12]. In case of nickel, FTS selectivity toward C 2?…”