Promoter segregation in Pt and Ru promoted cobalt model catalysts during oxidation–reduction treatments

A flat model approach was used to study segregation phenomena in Pt and Ru-promoted cobalt catalysts prepared by co-impregnation. In the calcined state the cobalt (Co3O4) and promoter phases are well mixed and in the oxidic form (PtO2 and RuOx). Upon reduction the promoter alloys with the metallic cobalt and it is evenly distributed over the cobalt particles. During reoxidation of the metallic system Pt and Ru segregate: Kirkendall voids of Co3O4 are formed, and the promoter is left inside the hollow oxide shell. Reduction of the reoxidized state results in re-alloying of the promoter with the cobalt phase, but the promoter concentration in the surface region of the sample is lower than after reduction of the calcined catalyst. This is explained by incomplete remixing, a result of the segregation in the re-oxidized state. The results suggest that break-up of the Pt-containing hollow oxide shells leads to uneven distribution of the promoter over the cobalt phase, forming both particles with high promoter concentration and promoter-free particles. Our approach provides a simple method to generate encapsulated noble metal particles, which are potentially interesting as sinter-resistant catalysts

Promoter segregation in Pt and Ru promoted cobalt model catalysts during oxidation–reduction treatments

A flat model approach was used to study segregation phenomena in Pt and Ru-promoted cobalt catalysts prepared by co-impregnation. In the calcined state the cobalt (Co3O4) and promoter phases are well mixed and in the oxidic form (PtO2 and RuOx). Upon reduction the promoter alloys with the metallic cobalt and it is evenly distributed over the cobalt particles. During reoxidation of the metallic system Pt and Ru segregate: Kirkendall voids of Co3O4 are formed, and the promoter is left inside the hollow oxide shell. Reduction of the reoxidized state results in re-alloying of the promoter with the cobalt phase, but the promoter concentration in the surface region of the sample is lower than after reduction of the calcined catalyst. This is explained by incomplete remixing, a result of the segregation in the re-oxidized state. The results suggest that break-up of the Pt-containing hollow oxide shells leads to uneven distribution of the promoter over the cobalt phase, forming both particles with high promoter concentration and promoter-free particles. Our approach provides a simple method to generate encapsulated noble metal particles, which are potentially interesting as sinter-resistant catalysts

Ostwald ripening on a planar Co/SiO2 catalyst exposed to model Fischer-Tropsch synthesis conditions

Catalyst deactivation is an important topic for industrial catalyst development. Sintering of small cobalt crystallites is one of the deactivation mechanisms of cobalt-based Fischer–Tropsch synthesis (FTS) catalysts. This study investigates the mechanism of cobalt sintering at low-conversion FTS conditions. A Co/SiO2/Si(1 0 0) model catalyst is exposed to 20 bar dry synthesis gas (H2/CO: 2/1) at 230 °C for 10 h. Cobalt nanoparticles were characterized before and after treatment using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images of identical locations on the model catalyst showed a loss of some small crystallites and decrease in size of some crystallites. Sintering is dominated by an Ostwald ripening mechanism using our model catalyst under the present conditions. Complementary XPS measurements confirm the loss of Co dispersion. Therefore, the loss of small Co nanoparticles causes a rapid loss of metal surface area when exposed to model FTS condition

Fundamental issues on practical Fischer–Tropsch catalysts : how surface science can help

The present article highlights the contribution of surface science and molecular modeling to the understanding of Fischer–Tropsch catalysis, in particular related to carbon-induced Co Fischer–Tropsch catalyst deactivation. The role of atomic and graphitic carbon in surface restructuring is discussed. Both forms of surface carbon stabilize surface roughness, while molecular CO promotes mobility of Co surface atoms. In a proposed chain growth mechanism on Co(0 0 0 1) chain elongation proceeds via alkylidyne + CH. The resulting acetylenic species is hydrogenated to alkylidyne, the route to further growth. (Cyclo-)polymerization of acetylenic species produces (aromatic) forms of polymeric surface carbon, a slow side reaction

The formation and influence of carbon on cobalt-based Fischer-Tropsch synthesis catalysts : an integrated review

Cobalt-based Fischer-Tropsch synthesis (FTS) catalysts are the systems of choice for use in gas-to-liquid (GTL) processes. As with most catalysts, cobalt systems gradually lose their activity with increasing time on stream. There are various mechanisms that have been proposed for the deactivation of cobalt-based catalysts during realistic FTS conditions. These include poisoning, sintering, oxidation, metal support compound formation, restructuring of the active phase, and carbon deposition. Most of the recent research activities on cobalt catalyst deactivation during the FTS have focused on loss of catalyst activity due to oxidation of the metal and support compound formation. Relatively few recent studies have been conducted on the topic of carbon deposition on cobalt-based FTS catalysts. The purpose of this review is to integrate the existing open and patent literature with some of our own work on the topic of carbon deposition to provide a clearer understanding on the role of carbon as a deactivation mechanism

XANES study of the susceptibility of nano-sized cobalt crystallites to oxidation during realistic Fischer-Tropsch synthesis

The oxidation of cobalt during Fischer–Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study, wax coated samples of a Co/Pt/Al2O3 catalyst were taken from a 100-barrel/day slurry bubble column reactor operated at commercially relevant FTS conditions, i.e. 230 °C, 20 bar, (H2 + CO) conversion between 50 and 70%, feed gas composition of ca. 50 vol.% H2 and 25 vol.% CO, PH2O/PH2=1–1.5, PH2O=4–6 bar and quantitatively characterised by X-ray absorption near edge spectroscopy (XANES). The cobalt catalyst samples, carefully removed from the reactor during the course of Fischer–Tropsch synthesis, were protected from air by the FTS wax. It is clear from the XANES measurements that during realistic FTS conditions cobalt crystallites of 6 nm supported on alumina were stable against oxidation to CoO/CoAl2O4 and a gradual reduction of residual cobalt oxide (i.e. following activation in pure hydrogen) was observed. This result is in line with recent thermodynamic analysis of the oxidation and re-reduction of nano-sized cobalt crystallites in water/hydrogen mixtures

XANES study of the susceptibility of nano-sized cobalt crystallites to oxidation during realistic Fischer-Tropsch synthesis

The oxidation of cobalt during Fischer–Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study, wax coated samples of a Co/Pt/Al2O3 catalyst were taken from a 100-barrel/day slurry bubble column reactor operated at commercially relevant FTS conditions, i.e. 230 °C, 20 bar, (H2 + CO) conversion between 50 and 70%, feed gas composition of ca. 50 vol.% H2 and 25 vol.% CO, PH2O/PH2=1–1.5, PH2O=4–6 bar and quantitatively characterised by X-ray absorption near edge spectroscopy (XANES). The cobalt catalyst samples, carefully removed from the reactor during the course of Fischer–Tropsch synthesis, were protected from air by the FTS wax. It is clear from the XANES measurements that during realistic FTS conditions cobalt crystallites of 6 nm supported on alumina were stable against oxidation to CoO/CoAl2O4 and a gradual reduction of residual cobalt oxide (i.e. following activation in pure hydrogen) was observed. This result is in line with recent thermodynamic analysis of the oxidation and re-reduction of nano-sized cobalt crystallites in water/hydrogen mixtures

In-situ surface oxidation study of a planar Co/Si02/Si(100) model catalyst with nanosized cobalt crystallites under model Fischer-Tropsch synthesis conditions

Density functional theory (DFT) calculations have been performed to determine the interaction energy between a CO probe molecule and all atoms from the first three rows of the periodic table coadsorbed on Rh(100), Pd(100) and Ir(100) metal surfaces. Varying the coverage of CO or the coadsorbed atom proved to have a profound effect on the strength of the interaction energy. The general trend, however, is the same in all cases: the interaction energy becomes more repulsive when moving towards the right along a row of elements, and reaches a maximum somewhere in the middle of a row of elements. The absolute value of the interaction energy between an atom-CO pair ranges from about -0.40 eV (39 kJ mol-1) attraction to +0.70 eV (68 kJ mol-1) repulsion, depending on the coadsorbate, the metal and the coverage. The general trend in interaction energies seems to be a common characteristic for several transition metals