Cobalt-based Fischer Tropsch Synthesis Research Articles

The promotional effects of ZrO2 modification on the activity and selectivity of Co/SiC catalysts for Fischer-Tropsch synthesis

Co/SiC catalysts have exhibited excellent performance in Fischer-Tropsch synthesis reaction. However, few research focuses on investigating the effect of SiC supports surface properties of on catalyst performance. In this study, ZrO2 was utilized to modify the SiC surface, leading to the preparation of a series of Co-ZrO2/SiC catalysts. The physicochemical properties of the catalyst were comprehensively analyzed by using N2 adsorption, XRD, H2-TPR, XPS analyses. Catalytic performance was evaluated using a fixed bed reactor, shedding light on the effect of ZrO2 modified SiC support on cobalt-based Fischer-Tropsch synthesis catalysts. The results indicated that ZrO2 surface modification on SiC resulted in an enhanced reduction degree of Co/SiC catalysts. Additionally, ZrO2 exhibited strong interaction with the amorphous phase on the SiC surface, thereby weakening the interaction between Co and the amorphous phase. This led to an increase in the electron density of cobalt species, consequently improving the selectivity of Co/SiC catalysts towards long-chain hydrocarbons.

Effect of different alkaline earth metals on the adsorption and catalytic behavior of cobalt Fischer–Tropsch synthesis

Alkali earth metals are effective promoters in cobalt-based Fischer–Tropsch synthesis. To compare the similarity and difference of the effect of the same main group of alkali earth metals on Fischer–Tropsch reaction, a series of alkali earth metals-modified alumina was prepared and applied as the supports for Co catalysts. Promotion of Co/Al2O3 catalyst by alkali earth metals achieved a low CH4 selectivity,high C5+ selectivity and C3 o/p due to the lower H*/CO* ratio and enhanced CO dissociation on the surface of catalyst than the unmodified one.

Atomic-scale understanding of the impact of CoO phase in cobalt-catalyzed Fischer-Tropsch synthesis: A combined computational and experimental study

In cobalt-catalyzed Fischer-Tropsch synthesis (FTS), the active metallic cobalt phase always suffers from oxidation under FTS reaction conditions, leading to the coexistence of metallic cobalt and cobalt oxide (CoO) phases. However, there still exist contradictory views on the impact of CoO phase in FTS. Some researchers proposed that the oxidation of metallic Co to CoO causes the deactivation of cobalt-based FTS catalysts, while some held the view that CoO phase could act as the potential active phase in FTS. In this study, by combining theoretical calculations and experiments, we systematically investigated the fundamental FTS reactions including CO adsorption and activation, C-C coupling, and CxHy hydrogenation on CoO phase, and compared them with those on metallic cobalt phase, aiming at identifying the exact role of CoO, understanding how CoO influences the activity and selectivity, and unraveling the intrinsic mechanism that determines the catalytic behaviors of CoO phase in FTS. The results reveal that the CoO(200) crystal facet is the dominantly exposed surface structure for CoO phase under FTS reaction conditions. The d-band center of CoO(200) surface shifts downward in comparison with that of active metallic Co surface, which causes difficulties in adsorbing and activating CO and gives rise to high energy barriers for the dissociation of C-O bond. This determines the inactive nature of CoO phase in FTS reaction. As for the product selectivity, the CoO(200) surface restricts the C-C chain growth due to its higher barriers for C-C coupling, facile desorption ability for light olefins, and lower energy barrier for CH4 formation compared with metallic cobalt, and consequently leads to the loss of selectivity to C5+ hydrocarbons and enhancement of selectivity to light olefins and undesired methane. Moreover, a strong linear relationship between the experimental observed selectivity change and the DFT-calculated energy difference of CoO and metallic cobalt phases was found, validating the DFT-calculated results. This study provides a comprehensive understanding of the nature and intrinsic catalytic behaviors of CoO phase in cobalt-catalyzed FTS.

Enhancing the stability of a cobalt-based Fischer–Tropsch synthesis catalyst using g-C3N4-coated SBA-15 as support

BackgroundThe design and synthesis of catalysts with high stability, activity and selectivity in Fischer–Tropsch synthesis (FTS) have been attracting extensive research attention. Maintaining high catalytic stability and activity is the best strategy for improving the catalyst efficiency and economic benefits. MethodsIn this study, graphitic carbon nitride (g-C3N4)-coated SBA-15 was utilised as a support material to prepare cobalt-based FTS catalysts. The catalysts coated with g-C3N4 were studied via TEM, XPS, XRD, H2-TPR, TG, N2 adsorption–desorption, and tested in a fixed-bed reactor. Significant findingsThe catalysts coated with g-C3N4 were considerably more stable than the Co/SBA-15 catalyst without g-C3N4 coating during a 100 h test period. Furthermore, the CO-conversion performance of the Co/SBA-15 catalyst decreased continuously. Notably, the activity of the Co/SBA-15@0.5C3N4 catalyst with a low g-C3N4 loading was higher than that of the Co/SBA-15@1C3N4 catalyst with a relatively high g-C3N4 loading. g-C3N4 can significantly strengthen the interaction between the cobalt species and the support, thereby increasing the catalyst stability. Therefore, a highly stable catalyst with good activity can be achieved by employing a small amount of g-C3N4 for surface modification. The results highlight the effectiveness of the proposed strategy for preparing highly stable, cobalt-based catalysts with good activity.

Compositional Evolution of Individual CoNPs on Co/TiO2 during CO and Syngas Treatment Resolved through Soft XAS/X-PEEM.

The nanoparticle (NP) redox state is an important parameter in the performance of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Here, the compositional evolution of individual CoNPs (6-24 nm) in terms of the oxide vs metallic state was investigated in situ during CO/syngas treatment using spatially resolved X-ray absorption spectroscopy (XAS)/X-ray photoemission electron microscopy (X-PEEM). It was observed that in the presence of CO, smaller CoNPs (i.e., ≤12 nm in size) remained in the metallic state, whereas NPs ≥ 15 nm became partially oxidized, suggesting that the latter were more readily able to dissociate CO. In contrast, in the presence of syngas, the oxide content of NPs ≥ 15 nm reduced, while it increased in quantity in the smaller NPs; this reoxidation that occurs primarily at the surface proved to be temporary, reforming the reduced state during subsequent UHV annealing. O K-edge measurements revealed that a key parameter mitigating the redox behavior of the CoNPs were proximate oxygen vacancies (Ovac). These results demonstrate the differences in the reducibility and the reactivity of Co NP size on a Co/TiO2 catalyst and the effect Ovac have on these properties, therefore yielding a better understanding of the physicochemical properties of this popular choice of FTS catalysts.

Tailoring Carbon Deposits on a Single-Crystal Cobalt Catalyst for Fischer–Tropsch Synthesis without Further Reduction: The Role of Surface Carbon and Penetrating Carbon

The preparation of Fischer–Tropsch synthesis (FTS) catalysts that exhibit excellent catalytic performance and are suitable for direct use in a fixed-bed reactor without further reduction is crucial in industrial applications. However, only marginal progress has been made with respect to the preparation of cobalt-based FTS catalysts owing to tendency of metallic cobalt to oxidize easily; this tendency necessitates the activation of these catalysts via reduction despite having undergone reduction treatment during their preparation. Herein, this problem has been addressed by tuning the carbon deposits (surface C and penetrating C) on the surface of a single-crystal cobalt catalyst. Screen-like surface C on a catalyst pretreated with 5% CO (p-Co–CO) exhibits diffusion suppression and chemical inertness to oxidizing gas, thus preventing the oxidation of metallic cobalt and resulting simultaneously in high activity and low CH4 selectivity (7.2%) without requiring further reduction. Moreover, the exact role of surface C and penetrating C deposited on cobalt catalysts in the FTS performance is explored. Both surface C and penetrating C enhance the activity of the cobalt catalyst but with opposite effects on the FTS selectivity. Surface C improves the adsorption ability of bridged-type CO and the formation of long-chain hydrocarbons, whereas penetrating C is conducive to adsorbing linear CO and increases undesired CH4 selectivity. This study clarifies the effect of deposited carbon on the FTS reaction and provides insights into the design of high-performance nonconventional FTS catalysts that do not require further reduction.

Hydrogen spillover effects in the Fischer–Tropsch reaction over carbon nanotube supported cobalt catalysts

Support (CNTs) surface defect-induced hydrogen spillover significantly impacted the catalytic activity (turnover frequency, TOF) and methane selectivity evolution in cobalt-based Fischer–Tropsch synthesis.

Enhancement of C5+ selectivity in photothermal Fischer–Tropsch synthesis by an inorganic heteropoly acid catalyst promotor

The C5+ product selectivity of cobalt-based Fischer–Tropsch synthesis catalyst was significantly improved from 65% to 93% under photothermal conditions by using heteropoly acid (H3PW12O40) as a catalyst promotor.

Synthesis of Al-SBA-15 and Its Application in Cobalt Based Fischer–Tropsch Synthesis

Synthesis of Al-SBA-15 and Its Application in Cobalt Based Fischer–Tropsch Synthesis

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.

Theoretically predicted CO adsorption and dissociation on Ru-doped Co(1 0 0) surfaces

CO adsorption and dissociation on Ru-doped Co(100) surfaces at different CO coverage compared to clean surface were investigated based on spin-polarized density functional theory (DFT), in order to explore the effect of the initial step of Ru promoter on CO activation in cobalt-based Fischer-Tropsch synthesis (FTS). The results show that CO adsorption is highly site-dependent, the adsorption strength of CO at different types of adsorption sites is determined by the interaction between the adsorbed CO molecule and Ru-doped Co(100) surfaces. CO dissociation is promoted at hollow sites but inhibited at top and bridge sites above the Ru atoms. In addition, the amount of saturated adsorbed CO is increased while the amount of dissociated CO is decreased on the Ru-doped Co(100) surfaces at high coverage. Combining the experimental results, it is concluded that the adsorption model is preferred than substitution model for CO activation, because the increased CO molecular states in adsorption model is facile for the subsequent H-assisted dissociation on Co surface. The present work suggests that the addition of Ru promoter can improve the CO activation on Co(100) surface and pave the way for experimental research.

Effect of Ba on the catalytic performance of Co-Ru/Al2O3 catalyst for Fischer-Tropsch synthesis

Alkali earth metals and Ru are famous selectivity promoters in cobalt-based Fischer-Tropsch synthesis. Ba has displayed as an effective selectivity promoter, however, the presence of Ba leads to a dropped CO conversion yield. To combine the advantages of Ba and Ru in promoting selectivity, Ba-modified alumina was fabricated and used as the supports for Co and Co-Ru catalysts. Promotion of Co/Al2O3 by a combination of Ru and Ba achieved an increase of CO conversion and a low CH4 selectivity due to the enhanced CO dissociation than unmodified anda single additive modified catalyst. The influences of other alkali earth metal additives were also investigated, which had a similar impact that of Ba on the Co-Ru catalysts for promoting FTS reaction.

The interaction of CO, H2 and ethylene over a typical cobalt-based Fischer-Tropsch synthesis catalyst

Co-feeding ethylene to a Fischer-Tropsch synthesis (FTS) reactor exhibited multiple-effects, depending on the reaction conditions. Co-feeding a small amount resulted in a significant improvement in catalytic performance; however, both CO reactivity and chain-growth probability decreased when increasing ethylene concentration at a high temperature. This may be due to the "H-scavenging effect". Co-feeding CO into the ethylene hydrogenation system suppressed ethylene reactivity, which was seen as evidence of the CO "occupancy effect". This effect was dominant at low reaction temperatures, but weaker at high reaction temperatures. Both the CO "occupancy effect" and the "H-scavenging effect" of ethylene had a significant impact on the chain-growth reaction pathways. A new mechanism called the “ethylene-assisted CO-insertion mechanism” was hypothesised to explain the ethylene promotional effects on FTS. This mechanism may co-exist with both the CO-dissociation type mechanism and the ethylene hydrocracking chain-growth mechanism, and may allow the FTS product distribution to be tuned.

Cobalt-based Fischer-Tropsch synthesis: Effect of the catalyst granule thermal conductivity on the catalytic performance

A series of model Fischer-Tropsch synthesis (FTS) catalyst granules with different thermal conductivity were prepared to study the influence of catalyst granule thermal conductivity on FTS performance. A clear correlation was found between catalyst granule thermal conductivity and FTS activity, CH4 selectivity, C5+ selectivity, mole ratio of olefin to paraffin and branched hydrocarbon selectivity at high CO conversion. This could be due to that the overheating effect occurred within catalyst granule with low thermal conductivity. On the basis of the widely accepted ‘alkyl’ mechanism for FTS, the overheating effect was able to affect the adsorption of H2 and CO molecules, the hydrogenation of carbide species as well as chain growth. In addition, the overheating effect could lead to the catalyst structure reconstruction, sintering and deactivation.

Scanning Nanobeam Diffraction and Energy Dispersive Spectroscopy Characterization of a Model Mn-Promoted Co/Al2O3 Nanosphere Catalyst for Fischer–Tropsch Synthesis

Manganese oxide is an effective promoter for enhancing the activity and selectivity of cobalt-based Fischer–Tropsch synthesis (FTS) catalysts. Previous studies suggest that Co atoms located at the Co–MnO perimeter adsorb CO more strongly and undergo C–O bond cleavage more readily than CO adsorbed on Co sites, which leads to improved C5+ selectivity. Thus, direct characterization of the formation and structure of the active interface between Co nanoparticles and MnO is essential for a full understanding of the FTS promotion effect. Scanning transmission electron microscopic (STEM) methods are well-suited for direct characterization of nanoscale features through imaging, energy dispersive spectroscopy (EDS) elemental mapping, and spatially resolved electron diffraction, a technique known as scanning nanobeam diffraction. However, these techniques require that the catalytic nanoparticles are non-overlapping and well-defined, which is not typical for nanoparticles on porous metal oxide FTS supports. In the present study, we addressed these challenges by synthesizing catalysts composed of cobalt dispersed on nonporous alumina nanospheres (Co/ANS), which yielded isolated nanoparticles that are amenable to elemental and phase mapping by STEM-EDS and scanning nanobeam diffraction. Then, manganese oxide-promoted Co/ANS (Mn–Co/ANS) was characterized using these two techniques, which provided direct evidence for the transformation of a Co–Mn oxide spinel present in the as-prepared catalyst into closely associated Co3O4 and MnO2 nanoparticles in the passivated catalyst after its use for FTS.

Insights into the Promotion with Ru of Co/TiO2 Fischer–Tropsch Catalysts: An In Situ Spectroscopic Study

Although ruthenium is a common promoter for cobalt-based Fischer–Tropsch synthesis (FTS) catalysts, the origin of Ru promotion and promoter concentration effects remains controversial. To gain a fundamental understanding of the Ru promotion effects, we herein performed an in situ spectroscopic study by X-ray absorption spectroscopy (XAS), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS)–Auger of working Co/TiO2 catalysts (ca. 12 wt % Co, pure anatase TiO2 phase) promoted with different amounts of Ru (0.1–1.2 wt %). At typical FTS conditions (220 °C, 20 bar, H2/CO = 2), the activity in terms of both metal-time-yield (MTY) and initial turnover frequency (TOF) was maximized for the catalysts promoted with 0.1–0.2 wt % Ru, for which most of the Ru was alloyed with Co in bimetallic nanoparticles. The analysis of Auger electrons evidenced that the lower (apparent) TOF of accessible Co0 sites in the absence of Ru and at Ru concentrations beyond 0.2 wt % is bridged to the development, under reaction conditions, of FTS-inactive cobalt carbide (CoCx) and cobalt oxide (CoOx) patches, respectively, at the outermost surface of cobalt crystallites.

Metal-organic framework (MOF)-derived catalysts for Fischer-Tropsch synthesis: Recent progress and future perspectives

Fischer-Tropsch Synthesis (FTS) is an important catalytic chemical reaction that converts a mixture of CO and H2 (syngas) derived from biomass, coal or natural gas into ultra-clean fuels or value-added chemicals. However, most traditional catalysts used in the Fischer-Tropsch process are faced with the problems of high deactivation rate triggered by sintering, phase changes and oxidation which hamper their catalytic performance. Metal-organic frameworks (MOFs)-derived materials have been a promising alternative in addressing the catalyst deactivation problems in FTS because of the encapsulation of their metal nanoparticles in carbon matrix and absence of large particle size, among other reasons. Therefore, this review emphasizes the most recent research headway in the investigation of MOFs as precursors to achieve high-performance FTS catalysts. Precisely, the design of iron and cobalt-based FTS catalysts from parent MOFs via MOF-mediated synthesis, the catalytic activity of the MOF-derived materials and the promoter effects under FTS operation were outlined and discussed. We have also evaluated the influence of MOF structures on the FTS performance and compared them with traditional/commercially available catalysts to show the importance of this approach. Finally, the challenges and opportunities to further expedite the extensive research efforts and promote their applications in material design and FT technology were mentioned.

Impact of promoter on the cobalt based Fischer–Tropsch synthesis: a comprehensive kinetic study

A comprehensive kinetic model for cobalt based Fischer–Tropsch synthesis products was developed based on the recently proposed generalized mechanism for monomer formation. The model was evaluated with recent kinetic data from the literature (Co–Re/Al2O3) as well as newly obtained experimental data (Co–Zr/Al2O3). The model results were in good agreement with experimental data. Also estimated model parameters were statistically and physically significant. The model can predict the effect of promoter on the species surface coverage. According to the results, the surface of both catalysts is mainly covered with CO, monomer and carbide. At the same operating condition, carbide surface coverage was significantly higher in the case of Zr promoted catalyst (13.8%) compared to that of Re promoted catalyst (3.3%). Moreover the utilization of Zr promoter resulted in less hydrogenated carbon species (16.3%) which was about 20% in the case of Re promoter. The results also indicated that the product selectivity was mainly governed with monomer, hydrogen and vacant site surface coverage ratios.

Facile preparation of highly thermal conductive ZnAl2O4@Al composites as efficient supports for cobalt-based Fischer-Tropsch synthesis

The development of alternative to silicon carbide (SiC) holds great promise in energy and environmental related catalysis because the harsh preparation process and high cost limit its large-scale application. In this work, the core-shell structured ZnAl2O4@Al ceramic-metal composites (Zn-Al-T) are prepared from low cost metallic Al powder by a facile and environment-friendly approach to replace silicon carbide for Co-based Fischer-Tropsch synthesis (FTS, a typical strong exothermic heterogeneous catalytic reaction). The composition, thermal conductivity, surface chemical properties and pore configuration of the ZnAl2O4@Al composites can be easily modulated by controlling the calcination condition. Among Zn-Al-T composites, the Zn-Al-650 and Zn-Al-750 samples demonstrate optimum coupling of physicochemical properties, i.e., robust chemical stability, meso-macropore structure (wide mesopore and high macroporosity) and high thermal conductivity. Thus, the excellent FT performance (low CH4 selectivity and high selectivity to C5+) was obtained on the Co/Zn-Al-650 and Co/Zn-Al-750 catalysts. The heat transfer experiments were conducted on a special tubular fixed-bed reactor (I.D. = 22 mm) under near-industrial conditions. At similar reaction rates, the effective radial thermal conductivity coefficient (keaff) in the Co/Zn-Al-650 catalyst bed is 3–4 times and 1–2 times that in the Co/Al2O3 and Co/SiC catalyst beds respectively, resulting in a more uniform temperature distribution in Co/Zn-Al-650 catalyst bed. In addition, the ZnAl2O4@Al composite has wide application prospect in strong exothermic/endothermic catalytic system owing to its high thermal conductivity, adjustable physico-chemical properties, facile preparation method and low cost.

Mechanistic pathways for oxygen removal on Pt-doped Co(111) in the Fischer-Tropsch reaction

Water is a significant by-product in the cobalt-based Fischer-Tropsch synthesis as the dissociation of CO on metallic cobalt must eventually lead to the formation of water. Hence, the removal of oxygen in the form of water, could potentially limit the intrinsic activity on a cobalt-based catalyst. In this study, potential mechanisms for the removal of strongly bound surface oxygen by hydrogenation to water on fcc Co(111) and Pt doped fcc Co(111) surfaces are investigated using density functional theory and micro-kinetic modelling to probe the effect that of platinum promotion. We show that the formation of water proceeds primarily via disproportionation of adsorbed OH, 2 OH ↔ H2O + O, rather than the hydrogenation of surface hydoroxyl species, OH + H ↔ H2O, on both surfaces. Doping cobalt with platinum increases the rate of oxygen removal in the form of water by four orders of magnitude, by reducing the barrier of the hydrogenation of surface oxygen, O + H ↔ OH, by 0.48 eV.