Cobalt Crystallites Research Articles

Optimization and evaluation of the distribution of Fischer-Tropsch products over a cobalt-based catalyst utilising design expert software

Modelling biomass to liquid via the Fischer-Tropsch synthesis (FTS) system allows researchers to investigate the most efficient parameters while running the system under optimal conditions. As part of the design of experiments (DOE) procedure, a special data simulation method based on response surface methodology (RSM) is utilized to thoroughly analyse the impact of operating circumstances. The objective of this study was to examine the factors that affect the production of C1, C2–C4, and C5+ in FTS process, and then optimize the critical factors utilising factorial design and response surface techniques. The parameters evaluated were reaction temperature, reaction pressure and the crystallite size of cobalt. The effects of these factors and their potential for synergy were explored simultaneously using multivariate DOE, with the yield of different hydrocarbon composition selectivity's as the measured responses. In the concept generation phase, optimization was based on the literature consulted, which proved to be an effective method for determining the optimization parameters. The detailed conceptual design included the generation of models using statistical methods and response surface models. Finally, the optimized design was validated using catalysts and parameters obtained during the optimization process, and this were compared to the output recorded in the theoretical modelling. The optimized parameters resulted in performance consistency, with the theoretical model for each group of hydrocarbons being validated by actual experiments. The established models were seen to characterize hydrocarbon distributions accurately and repeatedly over a wide range of reaction conditions (200–270 °C, 5–20 Bar, and 3–26 nm) using a cobalt-based catalyst. According to the detailed quantitative models developed, for higher C5+ production, 220 °C, 10 barg and 11 nm (cobalt crystallite) benchmark parameters were set to produce 19.3 % C1, 11.4 % C2–C4 and 69 % C5+ selectivity's. Comparative analysis showed a 1.9 %, 3.9 % and 0.3 % percentage difference between the theoretical output and the actual output of C1, C2–C4 and C5+, respectively.

The water assisted vinylene mechanism for cobalt Fischer-Tropsch synthesis assessed by multi-catalyst modelling of kinetics and deactivation

The paper describes development of a mechanism and a consistent rate expression for Fischer-Tropsch (FT) synthesis over cobalt-based catalysts. The developed mechanism relies on a two-step hydrogen assisted activation of CO. The carbon atom of CO is first hydrogenated by surface hydrogen to formyl; followed by the rate-limiting step whereby the oxygen atom is hydrogenated by adsorbed water. The produced CH* monomer is incorporated into the growing chain giving vinylene intermediate. The vinylene intermediate is either terminated to an olefin by adding hydrogen to the α-carbon atom or propagates by adding hydrogen to the β-carbon position. The resulting expression for CO consumption, the Fischer-Tropsch rate, can respond positively or negatively to the partial pressure of water, in agreement with experimental observations. A special feature is that the chain propagation probability does not depend on the partial pressure of hydrogen. The resulting kinetic model is tested on several cobalt catalysts supported on alumina; spanning from γ-alumina with average pore sizes ranging from 6.1 to 18.3 nm to α-alumina with a wide pore structure; and with cobalt particle sizes from 8 to 19 nm. Water was added sequentially to the syngas feed, causing enhanced deactivation, for testing the water response on activity and selectivity. A deactivation model comprising sintering and cobalt oxidation, and the FT-kinetics, describe the observed CO conversions with great precision for all catalysts. Selectivities are also well described, but with slight deviations at least partly due the effect of deactivation. Trends in some of the kinetic parameters are rationalized in terms of cobalt crystallite and pore sizes.

Nanostructuring Effect of Nano-CeO2 Particles Reinforcing Cobalt Matrix during Electrocodeposition Process.

The electrodeposition method was used to obtain nanostructured layers of Co/nano-CeO2 on 304L stainless steel, from a cobalt electrolyte in which different concentrations of CeO2 nanoparticles (0, 10, 20, and 30 g/L) were dispersed. The electrodeposition was performed at room temperature using three current densities (23, 48, and 72 mA cm−2), and the time was kept constant at 90 min. The influence of current densities and nanoparticle concentrations on the characteristics of the obtained nanostructured layers is also discussed. An X-ray diffractometer (XRD) was used to investigate the phase structure and cobalt crystallite size of the nanostructured layers, and a contact angle (sessile drop method) was used to assess the wettability of the electrodeposited layers. The roughness of the surfaces was also studied. The results show that the nanostructured layers became more hydrophilic with increasing nanoparticle concentration and increasing current density. In the case of pure cobalt deposits, an increase in the current density led to an increase in the size of the cobalt crystallites in the electrodeposited layer, while for the Co/nano-CeO2 nanostructured layers, the size of the crystallites decreased with increasing current density. This confirms the nanostructuring effect of nano-CeO2 electrocodeposited with cobalt.

Activation of Cobalt Foil Catalysts for CO Hydrogenation

CO hydrogenation has been studied on cobalt foils as model catalysts for Fischer–Tropsch (FT) synthesis. The effect of pretreatment (number of calcinations and different reduction times) for cobalt foil catalysts at 220 °C, 1 bar, and H2/CO = 3 has been studied in a microreactor. The foils were examined by scanning electron microscopy (SEM). It was found that the catalytic activity of the cobalt foil increases with the number of pretreatments. The mechanism is likely an increase in the available cobalt surface area from progressively deeper oxidation of the foil, supported by surface roughness detected by SEM. The highest FT activity was obtained using a reduction time of only 5 min (compared to 1 and 30 min). Prolonged reduction caused the sintering of cobalt crystallites, while too short of a reduction time led to incomplete reduction and small crystallites susceptible to low turn-over frequency from structure sensitivity. Larger crystals from longer reduction times gave increased selectivity to heavier components. The paraffin/olefin ratio increased with the increasing number of pretreatments due to olefin hydrogenation favored by enhanced cobalt site density. From the results, it is suggested that olefin hydrogenation is not structure sensitive, and that mass transfer limitations may occur depending on the pretreatment procedure. Produced water did not influence the results for the low conversions experienced in the present study (<6%).

Ammonia as a carrier for hydrogen production by using lanthanum based perovskites

LaNiO3 and LaCoO3 perovskites synthesized by self-combustion were characterised and studied in the ammonia decomposition reaction for obtaining hydrogen. Both the fuel to metal nitrates molar ratio and calcination temperature were found to be crucial to synthesize perovskites by self-combustion. Moreover, generating non-precursor species during synthesis and small metal size were two factors which significantly influenced catalytic activity. Hence, with a citric acid to metal nitrates molar ratio equal to one a LaNiO3 perovskite was obtained with suitable physicochemical properties (specific surface area, lower impurities, and basicity). In addition, a lower calcination temperature (650 °C) resulted in small and well-dispersed Ni0 crystallite size after reduction, which in turn, promoted the catalytic transformation of ammonia into hydrogen. For cobalt perovskites, calcination temperature below 900 °C did not have a significant influence on the size of the metallic cobalt crystallite size. The nickel and cobalt perovskite-derived catalysts, calcined at 650 °C and 750 °C, respectively, yielded excellent H2 production from ammonia decomposition. In particular, at 450 °C almost 100% of the ammonia was converted over the LaNiO3 under study. Furthermore, these materials displayed admirable performance and stability after one day of reaction.

Effect of Crystallite Size Distribution on the Oxidation and Re-reduction of Cobalt in the Fischer–Tropsch Synthesis: A Thermodynamic Analysis

Nano-sized cobalt crystallites may oxidize under realistic Fischer–Tropsch conditions, especially when operating at high conversion where oxygen availability on the surface is large. The extent of deactivation due to oxidation differs for monodispersed catalysts as opposed to a log-normal distribution of cobalt crystallites. The optimum cobalt crystallite size for a Fischer–Tropsch catalyst is dependent on not only the mean particle size, but also the log-normal variance and conversion level.

Co/CeO2 and Ni/CeO2 catalysts for ethanol steam reforming: Effect of the cobalt/nickel dispersion on catalysts properties

The Co/CeO2 and Ni/CeO2 catalysts were obtained by impregnation method with using different active phase precursors, different impregnation solvents and in presence of organic additive in order to improve cobalt/nickel dispersion and metal-support interaction. An addition of citric acid to precursor metal salt solution allowed to obtain their better dispersion and smaller metal particle size of Co/CeO2 and Ni/CeO2 samples. Whereas catalysts obtained from ammonia solution exhibited the largest size of metal particles. The key step for steam reforming of ethanol at 420 °C over both Co/CeO2 and Ni/CeO2 catalysts is ethanol dehydrogenation which occurs preferentially on cobalt/nickel terrace sites. But efficiency to cleavage C─C bond is favored by the edge/steps sites. Because nucleation of carbon requires relative large domains of flat terraces or larger ensemble sizes, the terrace atoms are involved in the catalyst deactivation under SRE conditions. Whereas the increase in fraction of edge and corner, i.e. better dispersion and strong metal-oxide interactions between metal and ceria could enhance the transfer of oxygen and thus improve the oxidation of carbon formed on the catalysts surface under SRE conditions. Catalytic performance of Ni/CeO2 catalysts in SRE reaction depends on the nickel crystallite size. Whereas catalytic performance of Co/CeO2 catalysts in SRE reaction is related to combination of cobalt crystallite size dependence and the oxidation state of the cobalt nanoparticles.

Significance of C3 Olefin to Paraffin Ratio in Cobalt Fischer–Tropsch Synthesis

The ratio between propene and propane (C3 o/p) during Fischer–Tropsch synthesis (FTS) has been analyzed based on both literature reports and experiments for five catalysts. The latter comprise four cobalt catalysts on γ-alumina with variations in pore sizes, and one catalyst on α-alumina. Overall variations include H2/CO feed ratio, residence time, water addition, transients between test conditions, CO conversion, cobalt particle size, promoter (Re), and support material. It was possible to rationalize all data based on secondary hydrogenation of olefins. In fact, it was deduced that olefins are dominating termination products in FTS, estimated to ca. 90% for C3, but that some paraffins most likely are also produced directly. Increased residence time and high H2/CO feed ratio favors olefin hydrogenation, while added water presumably displaces hydrogen on cobalt giving enhanced C3 o/p. High cobalt dispersion favors hydrogenation, as also promoted by Re. Effect of intraparticle diffusion is seen in transient periods; for example, as water is added or depleted. There is frequently positive correlation between C3 o/p and selectivity to longer chains; the latter expressed as C5+ selectivity, as both are sensitive to hydrogen activity. Some modifications, however, are needed due to the accepted volcano plot for C5+ selectivity with cobalt crystallite size. Titania as support shows unexpectedly low C3 o/p; probably due to SMSI (strong-metal-support-interaction).

Cobalt catalyst for steam reforming of ethanol–Insights into the promotional role of potassium

The promotional effect of potassium doping on the ESR performance of the CoZn|α-Al2O3 catalyst was thoroughly investigated in the context of its impact on the selectivity and stability of the catalyst, and discussed in terms of cobalt dispersion, oxidation state, cobalt-alumina interactions, catalyst acidity, and the deactivation resistance of the catalyst. The potassium slightly inhibited the cobalt reducibility (XPS, TPR, STEM/EELS), significantly increased the surface concentration and the strength of Lewis sites (FT-IR), enhancing the selectivity to hydrogen. The surface state of potassium at various stages of the catalyst evolution was examined with a species resolved thermal alkali desorption (SR-TAD) method. These studies revealed a pronounced migration of K from the cobalt nanograins towards the alumina support upon the activation and during the ESR processes. Potassium stabilizes the high dispersion of cobalt crystallites and prevents their detachment from the surface and subsequent encapsulation by the growing carbon deposit.

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.

Cobalt/graphene electrodeposits: Characteristics, tribological behavior, and corrosion properties

Cobalt/graphene composite coatings were electrodeposited from the baths containing 0.1–0.5 g/L graphene particles. The effect of graphene particles on structure, morphology, hardness, tribological behavior, and corrosion of the coatings was studied. The results revealed that the pyramidal morphology of the pure Co film became smaller at low graphene concentrations, and changed to needle-shaped at the high concentrations. The presence of graphene in the composite films resulted in the reduction of the cobalt crystallite size and thereby increased the hardness of the coatings. The hardness of Co/0.2 g/L graphene composite coating was 530 Hv that was about 1.6 times higher than the pure Co film. The composite coatings also had better tribological behavior than the steel substrate and the Co film. The lowest volume loss achieved in a composite coating deposited from a bath with 0.2 g/L graphene particles. The most corrosion resistance samples were those electrodeposited from baths with 0.2–0.4 g/L graphene particles. The corrosion current density of these samples in 3.5 wt% NaCl solution was <1 μA/cm2.

Deactivation of Co-Al2O3/SiO2 Fischer–Trospch Synthesis Catalyst in Industrially Relevant Conditions

The deactivation rate of Co-Al2O3/SiO2 Fischer–Tropsch synthesis catalyst have been experimentally assessed at industrially relevant conditions (T 198–237 °C, P = 60 bar, H2/CO = 1.85, GHSV 2000–7000 h–1). The fresh and spent catalyst was characterized by XRD, XPS, BET, TEM, and SEM–EDX techniques. Decreasing active surface area due to cobalt oxidation by product water and sintering of cobalt crystallites were found to be minor factors for the deactivation. The principal reason of activity loss seems is diffusion limitations arising due to catalyst pores plugging by product wax. The linear dependence between gas flow rate and estimated catalyst lifetime was revealed. High GHSV of fresh synthesis gas and/or high circulation ratio of product gas should be employed to prevent rapid loss of catalyst activity.

The effect of silanol groups on the metal-support interactions in silica-supported cobalt Fischer-Tropsch catalysts. A temperature programmed surface reaction

The effect of H bonded and isolated silanol groups on the metal support interactions of SiO2-supported Co catalysts in Fischer-Tropsch (FT) synthesis are reported. SiO2 supports were pre-treated in order to improve the concentration of isolated SiOH on the surface of the SiO2. Isolated SiOH groups act as an aid to achieving relatively good metal dispersions by creating metal anchoring sites, and enhances smaller metal crystallites deposition on the surface of the SiO2. SiO2 supports were pre-treated in two different ways: (a) pre-treatment by thermal method (HT); (b) pre-treatment by ethylene glycol (EG). The incipient wetness impregnation technique was adopted for the preparation of the catalysts, each catalyst loaded with approximately 10 wt% Co on each of the three supports designated as Co/SiO2 – UT, Co/SiO2 – HT, and Co/SiO2 – EG. Co/SiO2 – UT was used as a reference for this study. A number of characterization methods were applied to study the chemical and physical properties of the supports and the catalysts namely BET, XRD, ICP-OES, FTIR, H2 chemisorption, TEM, TPR, and TPSR. The results from H2 chemisorption, TEM and XRD indicated that the Co/SiO2 – EG showed relatively smaller metal crystallites and a better cobalt crystallite dispersion while TPR, CO TPD, and TPSR studies indicated that Co/SiO2 – EG catalyst exhibited stronger metal-support interaction, unlike the other two catalysts. Catalyst evaluation experiments showed that Co/SiO2-HT catalysts showed better FT activity while the Co/SiO2 – EG catalyst favored light olefins. FT catalyst evaluation results were in agreement with the TPSR studies.

Evolution of cobalt species in glow discharge plasma prepared CoRu/SiO2 catalysts with enhanced Fischer-Tropsch synthesis performance

Glow discharge plasma was applied to substitute thermal calcination during catalyst preparation. Evolution of cobalt particles in cobalt-ruthenium based catalysts at different stages (fresh prepared, reduced, after Fischer-Tropsch synthesis (FTS) reaction) were studied using a series of characterization techniques. Glow discharge plasma can be developed as an efficient technique to decompose metal salt. Cobalt dispersion was greatly enhanced by glow discharge plasma treatment, and higher initial cobalt dispersion was found on the one with lower treating intensity. However, cobalt crystallites generated by plasma treatment below certain intensity had poor thermal stability, they were tended to aggregate into large particles during reduction and reaction processes, and thus severe deactivations were observed during FTS tests. Besides cobalt sintering, mass transfer limitation at high FTS activity due to the pore plugging by long-chain hydrocarbons was also one of the important reasons for deactivation. The CRS-P140 catalyst with higher plasma treating intensity combined relatively high cobalt dispersion, high cobalt reducibility and relatively stable cobalt distribution during reduction or reaction, showed a high FTS activity with less activity loss.

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.

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.

α-alumina as catalyst support in Co Fischer-Tropsch synthesis and the effect of added water; encompassing transient effects

Three catalyst supports containing from 84 to 100% α-alumina prepared by heat treatment of γ-alumina have been impregnated with cobalt and rhenium. The catalysts were tested for Fischer-Tropsch synthesis (FTS) under dry and enhanced water vapor pressure conditions. Both activity and selectivity to higher hydrocarbons respond positive to water added or generated in situ by the reaction. A linear trend between formation of methane and C5+ products was found, but displaced to higher C5+ values compared to catalysts on γ-alumina. Calculation of chain propagation probabilities (αn) for α-alumina supported catalysts discloses that the first step, characterized by α1, increases the most under higher water partial pressure. Moreover, α1 is significantly higher for α- compared to all γ-alumina supports irrespective of pore sizes of the latter. These results are ascribed to suppression of hydrogen coverage on the cobalt surface, linked to more regular cobalt crystallites, accompanied by enhanced water assisted generation of CHx polymerization monomers. Surprisingly, the following α2 probability is comparably low for α-alumina supports, although it increases significantly with water concentration. Linear correlations are found between each pair of parameters α1, α2, α4 and SC5+; giving support to a mechanistic model where all products are interlinked, including methane. Transients observed when water was added or removed from the system are ascribed to pore diffusion. Selectivities in these periods follow closely the general selectivity trends found for different process conditions.

Synthesis and Application of Metallosilicate Supports for Cobalt-Based Fischer–Tropsch Synthesis Catalysts

Metallosilicates (metal = Al, Ti, Zr, V and Ce) and silica supports were synthesized by a one-pot surfactant-free process for use in cobalt-based Fischer–Tropsch catalysts. The in-depth physical, chemical, and textural properties of all supports were determined by several characterization techniques. The introduction of metal precursors to the synthesis gel reduced the pore size of the support by 24–60%, but the use of Al, Ti, and Zr increased the surface area by 5–35%. Cobalt (15 wt %) was loaded on the supports; hydrogen chemisorption and X-ray diffraction revealed that the dispersion and cobalt crystallite sizes in all catalysts were comparable. X-ray photoelectron spectroscopy and temperature-programmed reduction techniques revealed a greater interaction between Co and metallosilicates, necessitating a higher reduction temperature for these catalysts. The Fischer–Tropsch activities of all the catalysts were determined under industrially relevant conditions (220 °C, 1.83 MPa and 2000 mLsyngas/mLcatalys...

Confined small-sized cobalt catalysts stimulate carbon-chain growth reversely by modifying ASF law of Fischer\u2013Tropsch synthesis

Fischer–Tropsch synthesis (FTS) is a promising technology to convert syngas derived from non-petroleum-based resources to valuable chemicals or fuels. Selectively producing target products will bring great economic benefits, but unfortunately it is theoretically limited by Anderson–Schulz–Flory (ASF) law. Herein, we synthesize size-uniformed cobalt nanocrystals embedded into mesoporous SiO2 supports, which is likely the structure of water-melon seeds inside pulps. We successfully tune the selectivity of products from diesel-range hydrocarbons (66.2%) to gasoline-range hydrocarbons (62.4%) by controlling the crystallite sizes of confined cobalt from 7.2 to 11.4 nm, and modify the ASF law. Generally, larger Co crystallites increase carbon-chain growth, producing heavier hydrocarbons. But here, we interestingly observe a reverse phenomenon: the uniformly small-sized cobalt crystallites can strongly adsorb active C* species, and the confined structure will inhibit aggregation of cobalt crystallites and escape of reaction intermediates in FTS, inducing the higher selectivity towards heavier hydrocarbons.

Water as key to activity and selectivity in Co Fischer-Tropsch synthesis: γ-alumina based structure-performance relationships

Thirteen γ-alumina supports with different pore sizes and pore size distributions have been impregnated with cobalt and rhenium. The catalysts were tested for Fischer-Tropsch synthesis (FTS) under dry and enhanced water vapor pressure conditions. It is shown that there is a positive correlation between pore size and cobalt crystallite size of the reduced catalyst after incipient wetness impregnation as long as the pore size distribution is sufficiently narrow. There is a concurrent increase in C5+ with pore diameter due to higher chain propagation probabilities α3+. Higher α values are ascribed to larger cobalt crystallites that promote CO activation resulting in higher surface coverage of CHx monomers. Adding water vapor to the syngas feed has a strong positive effect on selectivity to higher hydrocarbons and activity. Water imposes a significant enhancement of α1 that is attributed to higher relative cobalt surface coverage of H2O(OH−) and CHx relative to hydrogen. Small pores are susceptible to condensation of water during FTS.