Metallic Cobalt Phase Research Articles

Hydrocarbon Formation from Syngas with In-Operando Monitoring of Cobalt- and Manganese-Based (pre)Catalysts Using X-ray Diffraction.

Two-layered metal oxides (LiCoO2 and cobalt-doped K n MnO2, n < 1) were explored as precatalysts for nanoconfined cobalt-based Fischer-Tropsch catalysts for conversion of syngas (CO and H2) to hydrocarbons. Ex situ, in situ, and PDF XRD analyses are presented. Based on in situ XRD analysis, LiCoO2 underwent reduction to predominantly cubic and hexagonal phases of cobalt metal. Reaction with syngas resulted in the generation of carbon, cobalt carbide, and lithium carbonate, in addition to the metallic cobalt phases. In the case of cobalt-doped birnessite, catalyst activation converted the birnessite phase to manganite and the cobalt to elemental cobalt, along with similar lithium and carbon phases. Conversion of syngas to C1 through C7 products was observed. The best conversions were observed for the LiCoO2 precursor catalyst, with generally a low olefin-to-paraffin ratio. While the conversions for the cobalt-doped birnessite precatalyst were generally lower, with lower chain lengths (up to C5), these catalysts gave a strikingly high olefin-to-paraffin ratio: in the best case, greater than 20:1.

A recyclable and durable magnetic cobalt nanoparticle-embedded mesoporous graphene nanohybrid for removal of pollutants from aqueous media

Cobalt nanoparticles (Co NPs) embedded in mesoporous graphene (MG) were prepared via a hydrothermal reaction followed by carbothermal reduction in an argon atmosphere. This cobalt nanoparticle-embedded mesoporous graphene (CoMG) was systematically investigated for structural, morphological, and magnetic properties before and after 10 successive cycles of adsorption and regeneration, using methylene blue (MB) dye as a model pollutant. The CoMG nanohybrid has a superior specific surface area of 807 m2 g−1, with a significant mesopore volume of 1.63 cm3 g−1 that allows fast and high pollutant removal efficiency of ∼84 % within 30 min as well as the maximum adsorption capacity of 178.6 mg g−1 with MB solution of 100 mg L−1, which is well described with pseudo-second-order adsorption kinetics. The consecutive adsorption−regeneration experiments revealed that the CoMG nanohybrid retained more than half of the initial dye removal efficiency even after being reused 10 times. After the tenth cycle, the regenerated CoMG maintained a high specific area of 688 m2 g−1 and a large mesopore volume of 1.51 cm3 g−1, indicating the long-lasting porous structure of the CoMG nanohybrid. The metallic cobalt phase and ferromagnetic nature of the Co NPs in the nanohybrid are retained because of the host MG, which prevents nanoparticle aggregation and serves as a protective environment for the embedded Co NPs in aqueous media during the adsorption−regeneration cycle tests of the adsorbent. The experimental results demonstrate that the CoMG nanohybrid has superior pollutant removal capacity and maintains more than half of the initial efficiency even after 10 cycles of use, making it a cost-efficient and energy-saving material for the removal of organic pollutants from aqueous media and in other environmental applications.

Dual interfacing with metallic cobalt boosts the electron shuttle of CdS-carbide nanoassemblies

Harnessing accelerated interfacial redox, thus boosting charge separation, is of great importance in photocatalytic solar hydrogen generation. In effect, nanoassembling non-noble metallic phases in CdS-based systems and elucidating their role in photocatalysis hold the key to eventually boosting electron shuttle in the field. Here we combine an efficient in-situ exsoluted metallic Co0 nanoparticles on a carbides matrix (CMG) with CdS (CdS@CoCMG) for photogeneration of hydrogen. The metallic cobalt phase exhibits strong binding at the CdS-carbide dual interfaces, forming the accelerated “electron converter” mechanism validated by charge transfer kinetics and achieving two orders of magnitude faster hydrogen production (44.42 mmol g−1 h−1) relative to CdS (0.43 mmol g−1 h−1). We propose that the unique catalyst configuration enable the directional electron-relay photocatalysis via harnessing interfaces between Co0 phase, carbides, and CdS clusters, which eventually boosts the redox process and charge separation of the integrated system, leading to high H2 production rates in the suspension.

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.

Morphological, electronic, and magnetic properties of multicomponent cobalt oxide nanoparticles synthesized by high temperature arc plasma

Many technological applications demand large amount of nanoparticles with well-defined properties, which is feasible only by using large-scale production methods. In this framework, we have performed structural and local geometric investigations of cobalt oxide nanoparticles synthesized by high temperature arc plasma route in helium and in air atmosphere with different arc currents, a competitive and low cost technological approach to synthesize large quantity of different types of nanoparticles. The complex scenario of phase fraction, shape, size distribution and hysteresis loop features of high temperature arc plasma synthesis of nanoparticles can be determined by the arc current and the selected gas. X-ray diffraction patterns reveal a multicomponent phase formation containing cubic cobaltous oxide (CoO), cobaltic oxide (Co3O4) and metallic cobalt phases. The synthesis of different phases is confirmed by x-ray absorption spectroscopy measurements at the Co K-edge. Both extended x-ray absorption fine structure and x-ray absorption near edge structure analyses show the presence of metallic nanoparticles in He ambient at high arc current. Moreover, high-resolution transmission electron microscopy images and magnetic hysteresis loop measurements show that the mean particle size increases and the coercivity decreases with increasing arc current in air ambient due to the intense particle–particle interaction. At variance, in He ambient synthesized samples due to the high quenching rate and the high thermal conductivity, a multi-domain formation in which the nanoparticles’ crystalline fraction decreases and a fluctuating coercivity due to core–shell structure is observed.

Tunable CO Dissociation Assisted by H2 over Cobalt Species: A Mechanistic Study by In‐situ DRIFTS

AbstractFischer‐Tropsch synthesis (FTS) is an important heterogeneous catalytic process that can effectively reduce human dependence on the non‐renewable petroleum resources. Cobalt is a crucial active metal center in most catalysts, which effectively catalyzes syngas into valuable products such as liquid fuels. On this basis, we systematically investigated the adsorption of CO on the surface of different cobalt species and its dissociation characteristics assisted by H2 are studied by the in‐situ DRIFTS. For the metallic cobalt phase, H2‐assisted CO dissociation not only possessed the FTS activity, but also inhibited the CO disproportionation reaction. However, for the CoO phase, the bidentate carbonate species were decomposed into CO2. In addition, CoO presented a high‐chain propagation ability than that of Co under the same operation conditions. Co2O3 and Co3O4 also exhibited similar CO dissociation patterns with the assistance of H2. However, different from CoO phase, formate species also formed under the H2 atmosphere over these two Co species. In summary, CO dissociation would be well tuned via the assistance of H2 as demonstrated by the in‐situ DRIFTS. This work provides a new sight on the rational design of efficient cobalt catalysts toward the FTS processes.

Constructing efficient hcp-Co active sites for Fischer-Tropsch reaction on an activated carbon supported cobalt catalyst via multistep activation processes

A highly efficient activated carbon supported cobalt based catalyst for Fischer-Tropsch synthesis was constructed. The catalyst was prepared by incipient wetness co-impregnation. The obtained catalyst was activated by multistep activation processes, which was composed of reduction of calcined sample in H2, syngas treatment, carburization in CO, and re-reduction in H2 successively. The multistep activation processes derived catalyst (CoZr/AC-RSCR) demonstrated far higher CO conversion and lower CH4 selectivity than those of conventional H2 reduction derived catalyst (CoZr/AC-R). The excellent performance of CoZr/AC-RSCR catalyst resulted from its higher amount of exposed active sites and the dominant hexagonal closed-packed (hcp) metallic cobalt phases. Furthermore, the catalytic performance could be further improved by tuning the content of hcp metallic cobalt phase stacking using the proper and effective treatment conditions of the multistep activation method. Finally, extremely high productivity of the mixture of alcohols and hydrocarbons with carbon number larger than 5 (194.6 g/h·kg-cat.) was acquired over CoZr/AC catalyst.

Effect of Magnesia Addition in Stability of Cobalt Catalysts Supported on Alumina for Hydrogen Generation by Glycerol Steam Reforming

Cobalt catalysts were prepared by wet impregnation of four distinct supports: alumina, magnesia and two mixed supports of alumina with magnesia (10 and 30 wt%), prepared by wet impregnation of magnesia precursor on alumina. Magnesia addition decreased catalyst acidity and reduced spinel phase formation; however, high magnesia content (30 wt%) decreased catalyst reducibility and cobalt dispersion. The catalysts were evaluated on steam reforming of glycerol for 30 h at 500 °C, GHSV of 200,000 h−1 and a glycerol solution 20% v/v in the feed. All catalysts, except the catalyst supported on pure magnesia, presented deactivation during reaction time, because of coke formation and reoxidation of metallic cobalt phase, mainly for the catalyst supported on 30%MgO–Al2O3. The catalyst supported on Al2O3 exhibited the highest mean conversion to gas (42.4%) and hydrogen yield (31.2%) during time on stream. However, the catalyst supported on 10%MgO–Al2O3 presented higher stability in terms of glycerol conversion to gas and hydrogen yield, associated with preferential formation of filamentous coke instead of amorphous coke.

Plasma-Assisted Preparation of CoRu/SiO₂ Catalysts for Enhanced Fischer-Tropsch Synthesis Performance: Effect of Plasma Atmosphere.

CO atmosphere plasma and first H2 then CO plasma were attempted to substitute the traditional "calcination-reduction-carburization" processes for the preparation of metallic cobalt phase with hcp structure. CoRu/SiO₂ catalyst precursor was prepared via incipient wetness impregnation. Characterization and catalytic results showed that CO could be decomposed in glow discharge plasma field, a large fraction of carbon species was deposited as elementary substance on the catalyst surface, while some of the carbon reacted with cobalt and formed cobalt carbide, which could transform into Co0 with hcp structure after reduction. The hcp structured cobalt containing catalysts showed higher initial Fischer-Tropsch synthesis activity than both calcined and air plasma treated samples.

Role of CO in the Water-Induced Formation of Cobalt Oxide in a High Conversion Fischer–Tropsch Environment

A Co/C model catalyst was exposed to increasing partial pressures of water simulating high Fischer–Tropsch conversions at varying concentrations of synthesis gas. The stability of the metallic cobalt phase against oxidation and sintering under such conditions was monitored in an in situ magnetometer. Direct oxidation of cobalt to cobalt oxide with water-derived oxygen through a water splitting mechanism was shown to be kinetically hindered, even at high partial pressures of water. Combined threshold partial pressures of carbon monoxide and water were identified for rapid oxidation of cobalt, above which the removal of adsorbed oxygen species, originating from the dissociation of carbon monoxide on the metallic cobalt surface, by surface produced water was hindered resulting in water-induced oxidation.

The effect of ruthenium promotion of the Co/δ-Al2O3 catalyst on the hydrogen reduction kinetics of cobalt

The effect of ruthenium content on the reductive activation of the Co/δ-Al2O3 catalyst was investigated using thermal analysis and in situ synchrotron radiation X-ray diffraction. Data of thermal analysis and phase transformations can be described by a kinetic scheme consisting of three sequential steps: Co3+ → Co2+ → (Co0Co2+) → Co0. The first step is the generation of several CoO clusters within one Co3O4 crystallite followed by their further growth obeying the Avrami–Erofeev kinetic equation (An1) with dimensional parameter n1 < 1, which may indicate the diffusion control of the growth. The second step is the kinetically controlled sequential process of the metallic cobalt phase nucleation (An2), which is followed by the third step of slow particle growth limited by mass transport according to the Jander model (D). Ruthenium promotion of Co/δ-Al2O3 catalysts significantly accelerates the reduction of cobalt. As the ruthenium content is raised to 1 wt%, the characteristic temperature of metal phase formation decreases by more than 200 °C and Ea for An2 step decreases by 25%. For step D, a joint decrease in activation energy and pre-exponential factor in case of ruthenium promotion corresponds to a weaker diffusion impediment at the final step of cobalt reduction. In the case of unmodified Co/δ-Al2O3, the characteristic temperature of the metal phase formation reaches very high values, the metallic nuclei rapidly coalesce into larger ones, and the further process is inhibited by diffusion of the reactants through the product layer. For ruthenium promoted catalysts, each CoO crystallite generates one metal crystallite; thus, ruthenium enhances the dispersion of the active component.

Magnetic interactions and hysteresis loops study of Co/CoFe2O4 nanoparticles

Co/CoFe2O4 nanoparticles with the mean size of 8.8, 10.8 and 16.9nm were prepared by thermal decomposition of metal salts in the presence of citric acid. The X-ray diffraction patterns and Rietveld refinements confirmed coexistence of Co-ferrite and metallic cobalt phases in the nano-powders. Scanning electron microscope images showed an increase in particles aggregates mean diameter with increasing the annealing temperature. Magnetic hysteresis loops showed a demagnetization jump at low fields, which was attributed to different reversal fields of ferrite and the cobalt phases. Field-dependent behavior of maximum magnetization (Mmax), remanence (Mr), squareness (S) and coercivity (Hc) were studied through minor loops measurements. The calculated S value of the loops showed a maximum, between anisotropy and coercive fields. A sharp increase in Hc of larger particles was observed with increasing the applied field when compared to smaller particles. Henkel plots showed that the samples are interacting. Negative deviation of Henkel plots from linear behavior and negative δm plots revealed the dominant role of dipole–dipole interactions in the nano-aggregates.

Determination of formate decomposition rates and relation to product formation during CO hydrogenation over supported cobalt

The concentration and reactivity of the formate species present at the surface of a supported cobalt catalyst were investigated by in situ diffuse reflectance FT-IR spectroscopy (DRIFTS). The specific concentration of formates under a CO/H2 feed was determined through calibration curves determined from reference samples prepared by impregnation with sodium formate. Two types of formates reactivity were observed upon removal of CO from the feed. The decaying period of the fastest decomposing species (“fast formats”) was identical to that of the carbonyl adsorbed on the metallic cobalt phase, suggesting that the fast formates were produced by spill-over of CO adsorbed on the metal onto the support. The slowest decomposing formates were at least one order of magnitude less reactive than the fast formates. The comparison of the rate of formation of methanol measured under steady-state reaction to that of decomposition of formates indicated that only the fast formates could potentially be an intermediate in the formation of methanol. The rate of formation of methane and propene, the main products observed under our reaction conditions, were yet markedly higher than that of fast formate decomposition. Therefore, the formation of hydrocarbons involved surface intermediates other than the formates observable by DRIFTS.

The influence of layer thickness and post annealing on magnetism of pulsed laser deposited ZnO/Co multilayers

ZnO/Co multilayers with different layers have been deposited on silicate (100) substrate by the pulsed laser deposition method. The as-deposited multilayers present the structure of amorphous phase and a narrow hysteresis loop with high saturation magnetization and low coercivity, showing the typical soft magnetic behavior. The post annealing at the elevated temperature causes the transformation from as-deposited amorphous phase to crystalline metallic cobalt and ZnO phase, as well as the interdiffusion of different layer forming CoO and ZnCo2O4 phases. The presence of crystalline metallic cobalt phase with ferromagnetic (FM) and cobalt oxide (CoO) phase with antiferromagnetic (AFM) is beneficial to the FM–AFM coupling, resulting in the shift of widening of the hysteresis loop. But an excessively high annealing temperatures leads to the formation of large amount of simple or complex oxides with paramagnetic, antiferromagnetic properties, inducing the decrease of magnetic properties significantly.

Wettability of hardmetal surfaces prepared for brazing with various methods

Hardmetals belong to the materials hardly wettable by liquid brazing alloys, so they should not be brazed without removing the surface layer after sintering. In the paper, mechanical and chemical methods of preparing a hardmetal surface for brazing are discussed. Special attention is paid to the electrolytic etching method that gives very good energetic properties to surfaces of hardly wettable materials. Electrolytic etching of hardmetals consists in anodic dissolution of tungsten carbide grains in water solution of alkaline metal hydroxides. The α phase (WC) is removed from surfaces of carbide preforms, leaving the much better wettable β phase (Co-W-C). Cobalt does not show amphoteric properties, so it does not dissolve in bases. With regard to brazing, besides significant development of the surface, an especially profitable feature of this method is isolating the metallic cobalt phase, which allows easier wetting by brazing alloys. On the grounds of surface roughness measurements, parameters of electrolytic etching were determined to remove WC grains from surfaces of carbide performs and to obtain much higher Co fraction on the brazed surface. The presented results are based on measurements of wetting angles on B40 hardmetal surfaces, isolated α and β phases and C45 steel, as well as on EDX cobalt analysis on hardmetal surfaces.

Electronic and magnetic structure of bulk cobalt: The α, β, and ε-phases from density functional theory calculations

The geometric, electronic and magnetic properties of the three metallic cobalt phases: hcp(alpha), fcc(beta), and epsilon(epsilon) have been theoretically studied using periodic density functional calculations with generalized gradient approximation (GGA) and plane wave basis set. These results have been compared with those obtained with GGA+U approach which have shown a noticeable improvement with regard to experimental data. For instance, the cohesive energy values predicted by GGA are overestimated by approximately 25%, whereas GGA+U underestimate them by 14%-17%. On the other hand, magnetic moment values are underestimated in GGA while are overestimated for GGA+U approach by almost the same amount. Besides, the introduction of U parameter gives rise to an electronic redistribution in the d-band structure, which leads to variations in the magnetic properties. Moreover, a higher attention has been paid in the study of the electronic and magnetic properties of the epsilon-phase that has not described previously. These studies show that this phase possesses special properties that could lead to an unusual behavior in magnetic or catalytic applications.

Genesis of supported carbon-coated Co nanoparticles with controlled magnetic properties, prepared by decomposition of chelate complexes

Following procedures formerly developed for the preparation of supported heterogeneous catalysts, carbon-coated cobalt nanoparticles dispersed on porous alumina have been prepared by impregnation of γ-Al2O3 with (NH4)2[Co(EDTA)] and thermal decomposition in inert atmosphere. Below 350 °C, Co(II) ions are complexed in a hexa-coordinated way by the EDTA ligand. The thermal treatment at 400–900 °C leads to the EDTA ligand decomposition and recovering of the support porosity, initially clogged by the impregnated salt. According to X-ray absorption spectroscopy, and due to in situ redox reactions between the organic ligand and Co(II), both oxidic and metallic cobalt phases are formed. Characterisation by transmission electron microscopy, X-ray diffraction and magnetic measurements reveals that an increase in the treatment temperature leads to an increase of the degree of cobalt reduction as well as to a growth of the cobalt metal particles. As a consequence, the samples prepared at 400–700 °C exhibit superparamagnetism and a saturation magnetisation of 1.7–6.5 emu g−1 at room temperature, whilst the sample prepared at 900 °C has a weak coercivity (0.1 kOe) and a saturation magnetisation of 12 emu g−1. Metal particles are homogeneously dispersed on the support and appear to be protected by carbon; its elimination by a heating in H2 at 400 °C is demonstrated to cause sintering of the metal particles. The route investigated here can be of interest for obtaining porous magnetic adsorbents or carriers with high magnetic moments and low coercivities, in which the magnetic nanoparticles are protected from chemical aggression and sintering by their coating.

Characterization of cobalt Fischer–Tropsch catalysts: 2. Rare earth-promoted cobalt-silica gel catalysts prepared by wet impregnation

Rare earth (RE) promoted cobalt-silica gel catalysts for Fischer–Tropsch (F–T) synthesis were investigated systematically by various analytical techniques. The addition of small amounts of praseodymium to cobalt-silica gel catalysts greatly enhances their activity and selectivity in the F–T synthesis. The optimal atomic ratio of Pr/Co for 6% cobalt-silica gel catalysts was found to be between 0.26 and 0.35. Spectroscopic and other experimental results show that impregnated praseodymium readily substitutes the surface silicon atoms to form negatively charged centres on the surface of the support. Due to the presence of praseodymium, the deposited cobalt species can partially react with the support. Consequently, significant amounts of surface cobalt silicates or hydrosilicates can be formed. Most of these species undergo reduction during activation and can be converted to a metallic cobalt phase that remains incorporated at the silica surface.

Physico-Chemical Properties of Cobalt–Ruthenium (10% Co–0.5% Ru) Catalysts Supported on Binary Oxides 8.5%ZrO2/Support (SiO2, Al2O3, TiO2) for Fischer-Tropsch Synthesis

The promotion of Fischer-Tropsch catalysts 10%Co/Al2O3, 10%Co/SiO2, 10%Co/TiO2 by 0.5% Ru and the modification of supports by 8.5 wt% ZrO2 have been studied. The following properties: catalyst specific surface area as well as reducibility and dispersion of metallic phase were studied by different techniques: BET, TPR, and H2 chemisorption. The modification of supports by non-reducible ZrO2, results in a decrease of cobalt oxide reduction on Al2O3 and TiO2 but not on SiO2 supports. Additionally the enhancement of cobalt dispersion was found for all catalysts with ZrO2 modified supports. The impact of Ru promotion is likely due to the stabilization of applied supports, prevention or blockage of interaction between surface Co species and support and an increase in cobalt oxide reducibility to the catalytically active metallic cobalt phase.

XPS Structural Studies of Nano-composite Non-platinum Electrocatalysts for Polymer Electrolyte Fuel Cells

The chemical structure of non-platinum electrocatalysts obtained from cobalt porphyrins (CoTMPP or CoTPP) by pyrolysis is investigated by X-ray Photoelectron Spectroscopy (XPS). The catalysts represent highly dispersed, self-supported nano-composites that demonstrate electrocatalytic performance for oxygen reduction and practically no activity in methanol electro-oxidation. High-resolution Co2p, C1s, N1s and O1s XPS spectra acquired from precursors and electrocatalysts pyrolyzed at various experimental conditions were curve-fit using (a) individual peaks of constrained width and shape as well as (b) experimentally obtained photopeaks from the precursor with additional peaks required for a complete curve fit. Principal Component Analysis (PCA) applied to quantitative results from the curve-fits of both types of spectra facilitates visualization and identification of the chemical species that are formed or destroyed, and simplifies evaluation of critical correlations. As a result of these studies it is established that the catalyst is a nano-composite of highly dispersed pyropolymer with some remaining N x -centers inserted into a graphite-like matrix. Approximately 50% of the metal is distributed as Co2+, associated with N4-centers. The remaining cobalt is present in crystallites of metallic Co. A thin layer of CoO coats these metallic cobalt phases. The developed methodology, described herein, combines model curve-fits and principal component analysis (PCA), resulting in a quantitative and unambiguous understanding of the chemical composition and structure of complex electrocatalysts.