Cobalt-based Fischer-Tropsch Catalysts Research Articles

Manganese as a structural promoter in silica-supported cobalt Fischer-Tropsch catalysts under simulated high conversion conditions

Understanding the deactivation mechanism of cobalt-based Fischer-Tropsch catalysts is of significant practical importance. Herein, we explored the role of manganese as a structural promoter on silica-supported cobalt nanoparticles under simulated high CO conversion conditions, i.e., high relative humidity. The structural changes in cobalt dispersion and oxidation state were followed by in situ Mössbauer emission spectroscopy. Adding manganese oxide to silica-supported cobalt enhanced the dispersion of metallic cobalt in the reduced catalysts. This higher cobalt dispersion, however, led to a stronger tendency of cobalt silicate formation under humid conditions. Without manganese, the cobalt particles sintered, and the larger ones were prone to transformation into cobalt carbide under high conversion conditions. As such, silica is not preferred as a support for practical FTS.

MOFs-assisted synthesis of robust and efficient cobalt-based Fischer–Tropsch catalysts

The supported catalyst featuring highly dispersed active species is a challenge for industrial catalyst even with high loadings. The traditional impregnation method generally results in large particles due to the particle migration and coalescence or Ostwald ripening process. Herein, a Metal-Organic Frameworks (MOFs) strategy was proposed to assist the manufacture of supported CoOx/SiO2 catalyst. The classic organic ligand (e.g., 2-methylimidazoles) was coordinated with cobalt ions dispersed in advance on silica support, which was post-treated as a sacrificial and protective agent in different atmosphere. These operations can effectively restrain the growth of cobalt particles, yielding the high dispersion of Co metal active sites. The prepared catalyst (CoOx/SiO2-A#) exhibits superior activity, C5+ selectivity and stability in Fischer–Tropsch synthesis. It reveals that the high dispersion as well as the increased number of surface-exposed Co sites contribute to superior activity. As demonstrated by TG-MS and Raman characterizations, a large amounts of carbon species in CoOx/SiO2-N catalyst impeded the accessible active sites. The existence of Co2SiO4 verified by TPR and XPS characterizations means the strong metal-support interaction, that contributes robust performance in long-term stability test. This sacrificial MOFs strategy paves the way for the design of highly dispersed and robust FTS catalysts.

Sintering and carbidization under simulated high conversion on a cobalt-based Fischer-Tropsch catalyst; manganese oxide as a structural promotor

The commercial application of cobalt-based Fischer-Tropsch synthesis (FTS) suffers from catalyst deactivation. One of the main deactivation mechanisms under industrial conditions is sintering. In this work, we explored the role of manganese oxide as a structural promoter against sintering in a carbon nanofiber supported cobalt model catalyst. We employed in situ Mössbauer emission spectroscopy to study cobalt sintering in synthesis gas as a function of the steam partial pressure, which mimics high CO conversion during FTS. Steam accelerates the sintering of non-promoted metallic cobalt particles. Model experiments point to a synergistic effect between carbon monoxide and steam on cobalt sintering. In the mangense-promoted case, sintering is significantly reduced, indicative of the structural stabilization of small cobalt particles by manganese oxide. Nevertheless, a fraction of cobalt particles in close interaction with manganese oxide carburized under these conditions, resulting in a lower catalytic activity.

Role of Zeolites in Heat and Mass Transfer in Pelletized Multifunctional Cobalt-Based Fischer–Tropsch Catalysts

Role of Zeolites in Heat and Mass Transfer in Pelletized Multifunctional Cobalt-Based Fischer–Tropsch Catalysts

On the link between CO surface coverage and selectivity to CH4 during CO2 hydrogenation over supported cobalt catalysts

An operando IR study was carried out to investigate the loss of selectivity to higher hydrocarbons observed over cobalt-based Fischer-Tropsch catalysts when CO is replaced with CO2. Our data unambiguously show that the structure and oxidation state of the surface of metallic cobalt remains the same whether CO2 or CO are cofed with H2. The selectivity to CH4 appears to increase monotonously with decreasing surface coverage of adsorbed CO. The switch from Fischer-Tropsch synthesis to methanation using CO2-rich feeds over metallic cobalt can therefore be assigned to surface coverage effects only, assuming that most, if not all, hydrocarbons products are formed on metallic cobalt.

Manganese promotion of a cobalt Fischer-Tropsch catalyst to improve operation at high conversion

The addition of manganese to cobalt-based Fischer-Tropsch catalysts increases activity and significantly improves liquid fuel yield by decreasing CO2 and CH4 selectivity. These effects are particularly noticeable at high CO conversions where the catalyst is exposed to high H2O, and low CO and H2, partial pressures. Under these conditions, manganese-promotion causes a shift in the maximum liquid fuel yield from 61 C-% to 79 C-% (at CO-conversion levels of 78% and 91%, respectively), thus enabling once-through operation. Our CO-TPD and DFT studies show that manganese promotion cannot to be linked to enhanced CO adsorption nor promotion of the C-O bond cleavage (both typically postulated mechanisms). Rather, our DFT-calculations indicate that manganese promotion modelled as a manganese oxide cluster on Co(111) is associated with improved oxygen removal from the catalytically active surface, explaining the reduced selectivity towards CO2 at high conversion and, thus, reduced methane selectivity and improved liquid fuel yield.

Effect of Ru-promotion on the catalytic performance of a cobalt-based Fischer-Tropsch catalyst activated in syngas or H2

This research demonstrated the effect of Ru addition on the structure-performance relationships of Co/SiO2 catalysts reduced in either syngas or H2. The activity results show that the syngas reduced Co-Ru/SiO2 catalyst afforded better product selectivity (high C5+ = 86.8% and low CH4 = 8.45%) but low activity (CO conversion = 15.2%) compared to the H2 reduced Co-Ru/SiO2 catalyst (C5+ = 78.9%, CH4 = 10.5%, CO conversion = 44.8%) at 120 h TOS. Furthermore, the activity of the unpromoted H2-reduced Co/SiO2 catalyst (C5+ = 82%, CH4 = 9.19%, CO conversion = 25%) was comparable to that of syngas catalyst Co-Ru/SiO2 catalyst. Ru improved the reducibility and subsequent dispersion of the Co sites leading to enhanced catalytic activity for the catalyst reduced by H2-reduction. However, the addition of Ru suppressed the CO hydrogenation for the syngas reduced Co-Ru/SiO2 catalyst. This strongly reflects on the superiority of H2 as a reducing agent for Ru-promoted Co-based catalysts. XRD and XPS analysis of the spent catalysts points out at the formation of Co2C species, which suppressed the FT activity and stimulated the formation of olefinic products over the syngas-reduced catalysts. While there is a potential for syngas reduction for unpromoted Co-based catalysts, in small FTS plants, this system in not favorable for Ru-promoted catalysts.

Effect of Co-fed Water on a Co–Pt–Si/γ-Al2O3 Fischer–Tropsch Catalyst Modified with an Atomic Layer Deposited or Molecular Layer Deposition Overcoating

Atomic layer deposition (ALD) and molecular layer deposition (MLD) methods were used to prepare overcoatings on a cobalt-based Fischer–Tropsch catalyst. A Co–Pt–Si/γ-Al2O3 catalyst (21.4 wt % Co, 0.2 wt % Pt, and 1.6 wt % Si) prepared by incipient wetness impregnation was ALD overcoated with 30–40 cycles of trimethylaluminum (TMA) and water, followed by temperature treatment (420 °C) in an inert nitrogen atmosphere. MLD-overcoated samples with corresponding film thicknesses were prepared by using TMA and ethylene glycol, followed by temperature treatment (400 °C) in an oxidative synthetic air atmosphere. The ALD catalyst (40 deposition cycles) had a positive activity effect upon moderate water addition (PH2O/PH2 = 0.42), and compared with a non-overcoated catalyst, it showed resistance to irreversible deactivation after co-fed water conditions. In addition, MLD overcoatings had a positive effect on the catalyst activity upon moderate water addition. However, compared with a non-overcoated catalyst, only the 10-cycle MLD-overcoated catalyst retained increased activity throughout high added water conditions (PH2O/PH2 = 0.71). All catalyst variations exhibited irreversible deactivation under high added water conditions.

Sintering and Carbidisation Under Simulated High Conversion on a Cobalt-Based Fischer-Tropsch Catalyst; Manganese Oxide as a Structural Promotor

Sintering and Carbidisation Under Simulated High Conversion on a Cobalt-Based Fischer-Tropsch Catalyst; Manganese Oxide as a Structural Promotor

Water-induced deactivation of cobalt-based Fischer–Tropsch catalysts

The Fischer–Tropsch product, water, is regularly hypothesized to be the driving force for catalyst deactivation. Cobalt nanoparticles may be oxidized to CoO, form mixed-metal oxides with supports, or sinter to larger particles. This Comment discusses the feasibility of these deactivation pathways, highlighting the importance of in situ characterization.

Cobalt-based Fischer–Tropsch catalysts supported on different types of N-doped carbon nanotubes

N-doped nanocarbons are promising materials for metal-free and supported catalysts. Three types of N-doped carbon nanotubes (N-CNTs) were synthesized by chemical vapor deposition (CVD), by oxidation of the CVD produced N-CNTs with nitric acid, and by post-doping of oxidized undoped CNTs with ammonia. Cobalt catalysts with 20[Formula: see text]wt.% loading supported on N-CNTs were tested in the Fischer–Tropsch synthesis. Oxidized N-CNTs containing pyridone groups demonstrated the best stabilization of cobalt nanoparticles. The catalyst on this support showed the highest selectivity towards [Formula: see text] hydrocarbons. The performance of the catalyst supported on CVD N-CNTs was the worst because of the large variation in cobalt particle size and low reduction degree. The catalysts supported on post-doped CNTs demonstrated the best activity, but high methane selectivity because of the low Co particle size ([Formula: see text][Formula: see text]nm).

Stability of cobalt-based Fischer–Tropsch catalyst supported on oxidized carbon nanotubes

The stability of 20[Formula: see text]wt.% Co/CNT catalyst was tested in the Fischer–Tropsch synthesis and the structural transformations both in the catalyst and support were analyzed. The catalyst showed high conversion and stable selectivity during three weeks of the test, which was attributed to the optimal and stable cobalt particle size of [Formula: see text]13–14[Formula: see text]nm promoted by the support pre-oxidation. XPS, Raman, and nitrogen adsorption data revealed that the carefully chosen catalyst annealing and reduction conditions ensured the preservation of the support structure.

Effect of the cobalt-precursor on the reactivity of cobalt-based Fischer-Tropsch catalysts supported on stabilized alumina

This study investigates the effect of the cobalt precursor (nitrate and acetate) on the catalytic performance in the Fischer-Tropsch synthesis of catalysts supported on stabilized CoAl2O4/Al2O3 pellets. The catalysts are prepared via multi-steps of impregnation of the support with aqueous solutions of cobalt nitrate or acetate followed by calcination in air. The dispersion of the Co3O4 particles obtained via exothermic decomposition of acetate increases if each impregnation is followed by calcination. In fact, halving the calcinations leads to Co3O4 particles even bigger than those formed from cobalt-nitrate. A worsening of the catalytic performances is observed when the Co-acetate catalysts are tested in the FTS. Such a result is explained on the basis of Co0 particles size effect. Pt improves the catalysts reducibility regardless of the Co-precursor. The metallic surface area of the Pt-promoted Co-nitrate catalyst is higher than that of the corresponding Co-acetate catalyst, thus resulting in improved catalytic performances.

Deactivation by sintering of the cobalt catalyst during the Fischer-Tropsch reaction

In the present work, the deactivation by sintering of cobalt-based catalyst during Fischer-Tropsch synthesis at low temperature was studied by numerical simulation. For this purpose, a mathematical model was developed. The obtained simulation results allowed us to highlight and improve the understanding of the deactivation phenomena of cobalt-based Fischer-Tropsch catalysts by sintering. The main results also show that the sintering phenomenon is strongly dependent on the operating conditions, in particular, the temperature, the pressure, and the H2/CO molar ratio, as well as the reaction by-products such as water. The results obtained can, therefore, be used to understand more the sintering mechanism which may be linked to the change in the concentration of the active sites and the reaction rates.

A robust and precious metal-free high performance cobalt Fischer–Tropsch catalyst

Slurry-phase Fischer–Tropsch catalysis is widely used in the production of synthetic transportation fuels, but places severe stresses on the catalyst due to the demanding hydrothermal and mechanical reaction conditions. In this Article we report the synthesis, characterization and catalytic performance of a robust cobalt-based Fischer–Tropsch catalyst. Combining an inert alpha alumina support with an appropriate cobalt addition method produces a material that is easy to reduce without precious metal additives and is mechanically and hydrothermally stable. Unlike many Fischer–Tropsch catalysts, this material is resistant to sintering and shows excellent selectivity and good activity in slurry-phase testing over 1,000 hours. The Fischer–Tropsch reaction is one of the key means of producing synthetic fuels. Here a deposition method to disperse cobalt nanoparticles across an alpha alumina support is shown to produce a highly stable system capable of withstanding demanding conditions while providing excellent activity.

Capturing the interconnectivity of water-induced oxidation and sintering of cobalt nanoparticles during the Fischer-Tropsch synthesis in situ

Supported nano-sized metal crystallites as catalysts in the Fischer-Tropsch synthesis have become a major research focus due to their high mass specific surface area and resulting lower cost. Such small supported cobalt crystallites have been reported to show a very different resistance with regard to deactivation compared to larger cobalt particles. The Fischer-Tropsch product water is reported to have a severe effect on the deactivation of cobalt-based Fischer-Tropsch catalysts. Compared to other water-induced deactivation mechanisms, hydrothermal sintering of cobalt nanoparticles is fairly well established in literature. A previously hypothesised interconnection between oxidation of cobalt nanoparticles and hydrothermal sintering has – for the first time – been captured in situ in the presented study. High concentrations of water induce oxidation of the cobalt nanoparticles increasing their mobility and resulting in crystallite growth via particle migration and coalescence whilst in the oxidised state. A well-defined model catalyst comprising highly dispersed cobalt nanoparticles on a relatively inert exfoliated graphite support in combination with an in situ magnetometer allowed for these observations, which resulted in irreversible deactivation of the catalyst.

The effect of aerosol-deposited ash components on a cobalt-based Fischer–Tropsch catalyst

The effect of ash salts on Co-based Fisher–Tropsch catalysts was studied using an aerosol deposition technique. The major elements in the ash were found to be K, S and Cl. The ash was deposited on a calcined catalyst as dry particles with an average diameter of approx. 350 nm. The loading of ash particles was varied by varying the time of exposure to the particles in a gas stream. Catalyst characterization did not reveal significant differences in cobalt dispersion, reducibility, surface area, pore size, or pore volume between the reference and the catalysts with ash particles deposited. Activity measurements showed that following a short exposure to the mixed ash salts (30 min), there were no significant loss of activity, but a minor change in selectivity of the catalyst . Extended exposure (60 min) led to some activity loss and changes in selectivity. However, extending the exposure time and thus the amount deposited as evidenced by elemental analysis did not lead to a further drop in activity. This behavior is different from that observed with pure potassium salts, and is suggested to be related to the larger size of the aerosol particles deposited. The large aerosol particles used here were probably not penetrating the catalyst bed, and to some extent formed an external layer on the catalyst bed. The ash salts are therefore not able to penetrate to the pore structure and reach the Co active centers, but are mixed with the catalyst and detected in the elemental analysis.

The Effect of Potassium on Cobalt-Based Fischer–Tropsch Catalysts with Different Cobalt Particle Sizes

The effect of K on 20%Co/0.5%Re/γ-Al2O3 Fischer–Tropsch catalysts with two different cobalt particle sizes (small, in the range 6–7 nm and medium size, in the range 12–13 nm) was investigated. The catalyst with the smaller cobalt particle size had a lower catalytic activity and C5+ selectivity while selectivities towards CH4 and CO2 were slightly higher than over the catalyst with larger particles. These effects are ascribed to lower hydrogen concentration on the surface as well as the lower reducibility of smaller cobalt particles. Upon potassium addition all samples showed decreased catalytic activity, reported as Site Time Yield (STY), increased C5+ and CO2 selectivities, and a decrease in CH4 selectivity. There was no difference in the effect of potassium between the sample with small cobalt particles compared to the sample with medium size particles). In both cases the specific activity (STY) fell and the C5+ selectivity increased in a similar fashion.

Products selectivity and reaction stability of cobalt-based Fischer-Tropsch catalysts affected by glow discharge plasma treatment and silica structure

Abstract Fischer-Tropsch performance was evaluated on 15% Co 0.5% Pt based catalysts supported on amorphous SiO2 and SBA-15, and the cobalt precursors were decomposed either by thermal calcination or glow discharge plasma. Plasma assisted decomposition could remarkably improve the dispersion of cobalt species, not only diminished the particle size but also led cobalt crystallites much more homogeneously dispersed; meanwhile, surface acidity was improved and all in the form of Lewis acid sites. The ordered structure of SBA-15 could confine the cobalt particles inside its pore channels, against the occurence of sintering, and the promotion of platinum could maintain the reducibility of cobalt species at relatively high level, even for the plasma treated catalysts with much smaller cobalt particles. As a result, the plasma treated CoPt/SBA-15-P catalyst combined the effects aroused by both plasma treatment and ordered structure of support, showed a best FTS performance among the four investigated samples, including high activity, a highest C5-12 selectivity and a best reaction stability.

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/gcat and constant O2 concentration of 1 vol.%. Increasing the O2 flow fed to the reactor, the specific amount of O2 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 °C) than that needed to fully reduce the calcined catalyst (400 °C). The temperature of the catalyst almost linearly increases with the increase of the O2 feed flow. Indeed, an abrupt increase of the catalyst temperature is observed for high O2 flows, resulting in a decrease of the Co° dispersion (i.e. increase of the average Co° 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 °C is compared to that of the same catalyst reduced ex-situ at the same temperature, passivated at low O2 flow and depassivated in-situ at 300 °C. Interestingly, similar stability and reactivity, expressed both in terms of activity and selectivity, are obtained.