Cobalt Cluster Size Research Articles

Theoretical investigation of cobalt cluster size on adsorption kinetics in Fischer-Tropsch synthesis

In this study, the particles of cobalt with different sizes were simulated by Materials Studio software then the adsorption of H2, CO, and CH4 and its components (CH, CH2, CH3) as Fischer-Tropsch synthesis monomers were investigated using Density-functional theory. The results showed that the suitable sites for the adsorption of CO, H2 and monomers in Fischer-Tropsch synthesis are affected by the size of cobalt clusters. Increasing in the size of the cobalt cluster, the number of suitable sites for adsorption of the species decreases. In general, H2 adsorption energy decreases with increasing cobalt cluster size because larger clusters tend to have a less reactive surface due to increased coordination number and reduced surface charge density. On the other hand, CO adsorption energy tends to increase with cluster size because the larger clusters can accommodate the linear CO molecule more effectively due to their extended surface area. With considering the deformation of adsorbed species, Mulliken charge result, density of states and partial density of states, it could be stated that all species were chemically adsorbed on the surface of clusters, except CH4 that was adsorbed on Co-10 and Co-18 clusters is physically. Kinetic parameters for H2 and CO adsorption were calculated considering the change in cobalt cluster size.

Impact of CO hydrogenation diffusion limitation and cluster size on Fischer-Tropsch synthesis mechanism: An integrated experimental and theoretical study

This research examines how varying cobalt particle sizes and support pore sizes affect CO diffusion in Fischer-Tropsch synthesis (FTS). It explores cobalt catalysts deposited on perlite (P) and its modified version (S) with weight loadings ranging from 5% to 20%. N2 adsorption-desorption, BET analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), hydrogen temperature-programmed desorption (H2-TPD), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FT-IR) analyses were utilized for characterization. The results designate that higher cobalt loading on the modified support (S) decreases CH4 selectivity while enhancing selectivity towards C5+ hydrocarbons. Despite larger cobalt particles in 15CP compared to 15CS, 15CP exhibits higher CH4 selectivity and reduced C5+ selectivity because of CO diffusion limitations and smaller pore size. DFT calculations reveal product selectivity depends on FTS mechanism and cobalt cluster size influences reaction pathways, with smaller clusters favoring CO insertion and larger clusters favoring carbide mechanisms.

Determining the hydrocarbon chain growth pathway in Fischer–Tropsch synthesis through DFT calculations: impact of cobalt cluster size

In the present work, the barrier energies (Eb) of CH4 formation and C–C coupling and the mechanism of Fischer–Tropsch synthesis (FTS) on different cluster sizes of cobalt were investigated.

Loading and promoter effects on the performance of nitrogen functionalized graphene nanosheets supported cobalt Fischer-Tropsch synthesis catalysts

The effects of nitrogen functional groups on graphene surface and also the effects of cobalt loading and ruthenium promoter on the performance of nitrogen functionalized graphene nanosheets (N-GNS) supported cobalt catalysts in Fischer–Tropsch synthesis (FTS) are investigated. A 15 wt% Co/PGNS catalyst, a series of Co/N-GNS (15–30 wt% loading) and ruthenium promoted catalysts (25 wt% Co,0.5 wt% Ru/N-GNS and 25 wt% Co,0.5 wt% Ru/PGNS) were prepared by impregnation method. The purified GNS and functionalized GNS and all catalysts were characterized by Raman spectroscopy, FTIR, SEM, EDS, ICP, BET, TEM, XRD and TPR. The catalysts assessed in FTS in a fixed bed micro-reactor at 220 °C, 1.8 Mpa and H2/CO ratio of 2. Functionalization of GNS shifted the TPR reduction peaks to lower temperature, increased the dispersion of cobalt particles and increased the percentage CO conversion from 70.6% to 74.5%. Increasing the cobalt loading resulted in increasing the average cobalt cluster size, improvements in the reducibility of Co3O4. The maximum FTS activity for N-GNS supported catalyst is achieved at 30 wt % cobalt loading. The C5+ selectivity for the 30 wt % cobalt catalyst was higher than that of the 15 wt % Cobalt catalyst. Addition of 0.5 wt%Ru increased the FTS rate (gCH/(gcat·h)) from 0.377 to 0.412 and the liquid products selectivity from 86.5% to 91.2%.

MAGNETIC NANOMATERIALS AND NANOSTRUCTURES. TRENDS OF DEVELOPMENT

There are analyzed creation and development of magnetic nanomaterials and nanostructures, films with a columnar type of crystal structure, multilayer film structures, nanocomposites, granular alloys and nanowires. The methodics of obtaining, structure, magnetic and magnetoresistive properties of three types of nanowires – multilayered and granular ones and the ones of spin-valve type are discussed. It is shown that multilayer film coatings with very thin (< 1 nm) alternating magnetic and nonmagnetic layers behave like films of granular alloys. It is emphasized that the films of granular Cu–Co alloys were first obtained at the Scientific and Practical Materials Research Center of the National Academy of Sciences of Belarus by the method of electrolytic deposition without subsequent annealing, as is the case with other methods for their preparation. As prepared Cu–Co films are superparamagneties. That is, they demonstrate ferromagnetic below blocking temperature, which is dependent on the size of cobalt clusters in diamagnetic matrix of copper. The granulated nanowires, firstly obtained by us, exhibit similar behavior. Special attention is paid to the analysis of obtaining and properties of multilayered films of the spin-valve type. The trends in the development of materials science are predicted, which will allow creating new materials with a high level of quality and specified properties, what will allow expanding the area of export of such materials and products from them in the future.

Size Dependence Adsorption of Hydrogen on Cobalt Clusters: A DFT Study

Density functional theory was used to investigate the effect of size on the adsorption state of hydrogen atom on small cobalt particles. For this propose, we have performed series of DFT-GGA calculations on various sizes of Co clusters, between 4 and 24 atoms, and a Co FCC (100) slab, with and without hydrogen atom adsorbate. The results showed that the destabilization and the cohesive energies per atom in a metal cluster are represented as linear function of the surface-to-volume ratio of the metal clusters. In addition, the energy of the HOMO–LUMO gap from 4s and 3d valence orbitals of the cobalt atoms in the cluster is decreased with increasing in cobalt cluster size, which is size dependent parameter. We have studied the effect of the size of Co clusters and the infinite Co (100) surface, on the energy of adsorption of hydrogen atom. The calculated Eads for hydrogen atom in our considered cobalt clusters showed decreasing behavior with increasing of the number of atoms in cluster.

Anomalous scaling behaviour of cobalt cluster size distributions on graphite, epitaxial graphene and carbon-rich (6√3 × 6√3)R30°

The influence of the underlying interface on adsorption of cobalt (Co) is investigated by comparing the nucleation and growth of Co at room temperature on three carbon (C) surfaces, i.e. highly oriented pyrolytic graphite (HOPG), epitaxial graphene/SiC(0001) (hereafter abbreviated as EG) and precursor of EG i.e. C-rich (6√3×6√3)R30°/SiC(0001) (hereafter abbreviated as 6√3). On all three surfaces, Co adopts Volmer–Weber growth mode via formation of three-dimensional dome-shaped nanoclusters. Co clusters formed on 6√3 surface are smaller but denser than Co/HOPG or Co/EG. Scaling analysis reveals a critical nucleus size, i*=1 (atom) and the smallest stable cluster (i*+1) would be a dimer. Co/HOPG and Co/EG have the same order of magnitude for their cluster densities and sizes. Scaling analyses however show that the i* for Co/EG (i*=3) is larger than Co/HOPG (i*=0) and in this respect the smallest stable cluster would be tetramer and monomer respectively. This difference is attributed to the influence of an interface situated between graphene and SiC bulk. It appears that EG is more inert than HOPG towards the adsorption of Co and may act as a better substrate to host Co clusters.

Fischer–Tropsch synthesis: Support and cobalt cluster size effects on kinetics over Co/Al 2O 3 and Co/SiO 2 catalysts

The influence of support type and cobalt cluster size (i.e., with average diameters falling within the range of 8–40 nm) on the kinetics of Fischer–Tropsch synthesis (FT) were investigated by kinetic tests employing a CSTR and two Co/γ-Al 2O 3 catalysts having different average pore sizes, and two Co/SiO 2 catalysts prepared on the same support but having different loadings. A kinetic model - r CO = kP CO a P H 2 b / ( 1 + mP H 2 O / P H 2 ) that contains a water effect constant “ m” was used to fit the experimental data obtained with all four catalysts. Kinetic parameters suggest that both support type and average Co particle size impact FT behavior. Cobalt cluster size influenced kinetic parameters such as reaction order, rate constant, and the water effect parameter. In the cluster size range studied, decreasing the average Co cluster diameter by about 30% led to an increase in the intrinsic reaction rate constant k, defined on a per g of catalyst basis, by 62–102% for the γ-Al 2O 3 and SiO 2-supported cobalt catalysts. This increase was due to the higher active Co 0 surface site density as measured by hydrogen chemisorption. Moreover, less inhibition by adsorbed CO and greater H 2 dissociation on catalysts having smaller Co particles was suggested by the higher a and lower b values obtained for the measured reaction orders. Interestingly, irrespective of support type, the catalysts having smaller average Co particles were more sensitive to water. Comparing the catalysts having strong interactions between cobalt and support (Co/Al 2O 3) to the ones with weak interactions (Co/SiO 2), the water effect parameters were found to be positive (indicating a negative influence on CO conversion) and negative (denoting a positive effect on CO conversion), respectively. No clear trend was observed for b values among the different supports, but greater a and a/b values were observed for both Al 2O 3-supported Co catalysts, implying greater inhibition of the FT rate by strongly adsorbed CO on Co/Al 2O 3 relative to Co/SiO 2. For both supports, the order on P CO was always found to be negative (i.e., suggesting an inhibiting effect) and positive for P H 2 for all four catalysts. The order of the reaction on P H 2 was close to 0.5, suggesting that dissociated H 2 is likely involved in the catalytic cycle. Finally, in the limited range of average pore diameters studied (13.5 and 18.2 nm), the average pore size of the Al 2O 3-supported Co catalysts displayed no observable impact on the reaction rate or water effect, suggesting either that the reaction is kinetically controlled, or that the pore size difference was not significant enough to elicit a measurable response.

ChemInform Abstract: Size and Location of Cobalt Clusters in Zeolite NaY: A NMR Study.

ChemInform Abstract: Size and Location of Cobalt Clusters in Zeolite NaY: A NMR Study.

Effect of Silylation of SBA-15 on Its Supported Cobalt Catalysts for Fischer-Tropsch Synthesis

Cobalt catalysts supported on unsilylated and silylated SBA-15 were prepared by incipient wetness impregnation and characterized by elemental analysis, N2 adsorption-desorption, X-ray diffraction, H2 temperature-programmed reduction, H2 temperature-programmed desorption, and oxygen titration. Their catalytic properties for Fischer-Tropsch synthesis were evaluated in a fixed-bed reactor. The results show that the interaction between cobalt and the support was decreased, and the reducibility of the cobalt oxide species was more facile on the silylated Co/SBA-15 catalyst. With increasing (CH3)3SiCl loading, the surface coverage of the trimethylsilyl group increased, and the cobalt cluster size was larger. The higher activity of the silylated 10%Co/SBA-15 catalysts was ascribed to its improved reducibility. The increase in selectivity for C5+ hydrocarbons was attributed to the increase in cobalt cluster size.

Enhanced Fischer–Tropsch activity on Co/P–Al2O3 catalyst: Effect of phosphorous content

Abstract The phosphorous-modified γ-Al2O3 support is used to prepare cobalt-based Fischer–Tropsch (FT) catalysts (Co/P–Al2O3) by slurry impregnation method. Since alumina surface is partially transformed to aluminum phosphate by phosphorous modification, it alters the cobalt cluster size resulting in the changes of FT activity. The appropriate amount of phosphorous loading on alumina surface is around 1–2 wt.% based on the alumina support. The enhanced activity is mainly attributed to the formation of a homogeneously distributed small cobalt clusters and redispersion of cobalt species during the FT reaction.

Co, Ru and K loadings effects on the activity and selectivity of carbon nanotubes supported cobalt catalyst in Fischer–Tropsch synthesis

Abstract An extensive study of Fischer–Tropsch synthesis (FTS) on carbon nanotubes (CNTs) supported cobalt catalysts with different loadings of cobalt, ruthenium and potassium is reported. Up to 30 wt.% of Co, 1 wt.% of Ru and 0.0066 wt.% of K were added to the catalyst by co-impregnation method. The physico-chemical properties, activity and selectivity of the catalysts were assessed. For the 15 wt.%Co/CNT catalyst, most of the metal particles were homogeneously distributed inside the tubes and the rest on the outer surface of the CNTs. Increasing the Co loading to 30 wt.% increased the amount of Co on the outer surface of the CNTs, increased the cobalt cluster sizes and decreased the reduction temperature and dispersion. Increasing the Co loading from 15 to 30 wt.% increased the CO conversion from 48 to 86% and the C5+ selectivity from 70 to 77%. Ruthenium was found to enhance the reducibility of Co3O4 to CoO and that of CoO to Co0, increase the dispersion and decrease the average cobalt cluster size. However, potassium was responsible in shifting the reduction temperatures to higher temperatures. 0.5 wt.%Ru increased the FTS rate of 15 wt.%Co/CNT catalyst by a factor of 1.4 while addition of 0.0066 wt.%K decreased the FTS rate by a factor of 7.5. Both promoters enhanced the selectivity of FTS towards the higher molecular weight hydrocarbons however; the effect of Ru is less pronounced. Potassium increased the olefin to paraffin ratio from 0.73 to 3.5 and the C5+ selectivity from 70 to 87%.

Influence of bimodal pore size distribution of Ru/Co/ZrO 2–Al 2O 3 during Fischer–Tropsch synthesis in fixed-bed and slurry reactor

Cobalt-based catalysts with 20 wt.% Co and 0.5 wt.% Ru supported on the coprecipitated ZrO 2–Al 2O 3 mixed oxide were prepared by three different methods, such as slurry precipitation (SLP), physical-mixing of precipitates (PMP) and conventional impregnation (IMP) to investigate their catalytic performance during Fischer–Tropsch (FT) synthesis. Pore size distribution suggested that the catalysts prepared by SLP and PMP possessed a bimodal pore size distribution data in contrast to the IMP catalyst possessing a unimodal pore size distribution. Information obtained using transmission electron microscopy (TEM) revealed the presence of homogeneously dispersed cobalt clusters in the case of SLP catalyst, whereas heterogeneously dispersed cobalt clusters were observed on IMP and PMP catalysts. Although Ru/Co/ZrO 2–Al 2O 3 catalyst prepared by IMP showed low CO conversion, it showed high C 8+ selectivity due to the existence of large cobalt clusters with high reducibility. In case of Ru/Co/ZrO 2–Al 2O 3 catalyst prepared by SLP method, the studies on cobalt cluster size reveals that the highest activity was obtained at 5 h of aging time. The activity of the catalysts performed in slurry reactor is much higher on SLP catalyst showing a bimodal pore size distribution due to the facile mass-transfer of heavy hydrocarbons. The functionality of the catalysts during FT synthesis was interpreted in terms of their structural aspects and reducibility of cobalt species obtained with different preparation methods and the type of reactor.

Fischer–Tropsch synthesis: The role of pore size for Co/SBA-15 catalysts

Co/SBA-15 (30 wt.%) catalysts with different pore sizes were prepared from the same materials by incipient wetness impregnation and characterized by diffuse reflectance infrared Fourier transform spectroscopy, N 2 adsorption–desorption, X-ray diffractometry, temperature-programmed reduction and H 2 desorption, oxygen titration as well as transmission electron microscopy. The reduction of the catalysts took place in two stages with Co 3O 4 reduction to CoO and then to Co 0. The first stage reduction was facile regardless of the catalyst pore size while the second stage reduction was much easier on the catalysts with larger pore. After the reduction, cobalt particles were found to be distributed on both the exterior and interior surfaces of the support. Compared to the catalyst with smaller pores, larger pore catalysts have more adsorption sites for CO of both the linear and bridge types. The catalysts with larger pore led to larger cobalt cluster size, lower dispersion and higher reducibility. The larger pores gave rise to more adsorbed CO of the bridge-type on FTS. CO conversion increased and then decreased with the pore size in the range studied. The catalysts with larger cobalt cluster size showed higher C 5+ selectivity for the FTS.

Fischer–Tropsch synthesis: A review of water effects on the performances of unsupported and supported Co catalysts

Abstract Fischer–Tropsch synthesis (FTS) process aims at converting synthesis gas to liquid fuels. Due to high activity and long catalyst life, cobalt-based catalyst is currently the catalyst of choice for gas to liquid (GTL) technology. Water is most undesirable byproduct of FTS process. Due to low water–gas-shift (WGS) activity of cobalt-based catalyst, the water concentration rises with time-on-stream (TOS) in FTS. This paper reviews the effects of water on the performances of various cobalt catalysts for FTS. The effects of water on FTS is quite complex and depends on the support and its nature, Co metal loading, its promotion with noble metals, and preparation procedure. Added water up to certain concentrations has positive effects (in terms of higher CO conversion, C 5+ selectivity, olefin selectivity and lower methane and CO 2 selectivity) on unsupported cobalt oxide catalysts. If the effects of support are taken into account, water has positive effect for silica-supported catalysts. The effects are negative for alumina where as for titania support, water has little positive effect. However in general, oxidation of cobalt active site depending on the cluster size and water partial pressure, the removal of transport restrictions via the formation of water-rich intra-pellet liquids, and kinetic effects have been considered as the main responsible factors. The effects are strongly influenced by the cobalt cluster size as well as on pore size of the support. Addition of noble metals at low cobalt loading increases the dispersion of cobalt on the support and hence improves its activity. Higher cobalt dispersion enhances the negative impact of water especially at higher water partial pressures under FTS conditions.

Deactivation behavior of ruthenium promoted Co/γ-Al2O3 catalysts in Fischer–Tropsch synthesis

Abstract Detailed activity study and the deactivation of Ru-Co/γ-Al2O3 catalyst for Fischer–Tropsch (FT) synthesis over 1000 h was investigated considering different deactivation mechanisms. Morphology changes of the catalyst during FT synthesis were studied using XRD, TPR, BET, ICP, carbon determination, H2 chemisorption and re-oxidation techniques. When the P H 2 O / ( P H 2 + P CO ) in the reactor is above 0.75 the deactivation rate is not dependent on the number of the catalyst active sites and is zero order to CO conversion. In this case the main deactivation mechanisms are: cobalt re-oxidation, metal support interactions and aluminates formation. The deactivation of Ru-Co/γ-Al2O3 is related to cobalt cluster size. At lower amounts of P H 2 O / ( P H 2 + P CO ) deactivation can be simulated with a power law expression with a power order of 39.7 and the main deactivation is due to sintering. Regeneration of catalyst at 275 °C recovered the catalyst activity by 69.9% of total activity loss due to the reduction of oxidized cobalts. Catalyst regeneration at 400 °C recovered the activity by 21.9% of total activity loss due to the reduction of refractory forms of oxidized cobalt generated by cobalt–alumina interactions. 7.2% of total activity loss is irreversible and can be assigned to aluminates formation, sintering and coke deposition.

Magnetic structure of free cobalt clusters studied with Stern-Gerlach deflection experiments

We have studied the magnetic properties of free cobalt clusters in two semi-independent Stern-Gerlach deflection experiments at temperatures between 60 and $307\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We find that clusters consisting of 13--200 cobalt atoms exhibit behavior that is entirely consistent with superparamagnetism, though complicated by finite-system fluctuations in cluster temperature. By fitting the data to the Langevin function, we report magnetic moments per atom for each cobalt cluster size and compare the results of our two measurements and all those performed previously. In addition to a gradual decrease in moment per atom with increasing size, there are oscillations that appear to be caused by geometrical shell structure. We discuss our observations in light of the two competing models for Langevin-like magnetization behavior in free clusters, superparamagnetism and adiabatic magnetization, and conclude that the evidence strongly supports the superparamagnetic model.

COBALT LOADING EFFECTS ON THE STRUCTURE AND ACTIVITY FOR FISCHER-TROPSCH AND WATER-GAS SHIFT REACTIONS OF CO/AL2O3 CATALYSTS

An extensive study of Fischer-Tropsch synthesis (FTS) on alumina-supported cobalt catalysts with different amounts of cobalt is reported. Up to 40 wt % of cobalt, is added to the catalysts by impregnation method. The effect of the cobalt loading on the reducibility of the cobalt oxide species, dispersion of the cobalt, average clusters size, water-gas shift (WGS) activity and activity and selectivity of FTS is investigated. Increasing the cobalt loading resulted in increasing the average cobalt cluster size, improvements in the reducibility of Co3O4, decreasing the cobalt surface interaction with the support and decreasing the dispersion of cobalt. The maximum concentration of active surface Co o sites and FTS activity are achieved for the 34 wt% cobalt loading. While the maximum WGS activity is achieved for the 40 wt % cobalt loading. The methane selectivity in the catalyst with 40 wt % of cobalt loading was about 43.7 % less compared to that of the less reducible 8 wt % cobalt catalyst. The C5 + selectivity for the 40 wt % cobalt catalyst was 60.8% higher than that of the 8 wt % Cobalt catalyst.

Monodispersed metal clusters in solid matrices: A new experimental setup

We describe a new experimental setup for the production of samples of metal clusters embedded in matrices stable at ambient conditions. The cluster ions are generated in the gas phase and codeposited fragmentation-free together with the evaporated matrix. Mean cluster sizes range from a few to many thousands of atoms. For small clusters (n<20) mass selection is possible; larger clusters are deposited in narrow size distributions. Matrix materials include metals (Cu and Ag) and oxides such as quartz. The performance of the apparatus as well as sample characterization procedures are described. We show first results on the magnetic properties of different cobalt cluster size distributions (⟨n⟩=15,600,2300,6500) embedded in copper matrices, demonstrating intercluster as well as cluster-lattice interactions.

Catalytic performance of zirconium-modified Co/Al 2O 3 for Fischer–Tropsch synthesis

A series of zirconium-modified Co/Al 2O 3 catalysts were prepared with a two-step impregnation method using the incipient wetness technique. XRD, XPS, TPR, H 2-TPD and oxygen titration were used for the characterization of the catalysts. The catalytic performance was performed in a fixed bed reactor for Fischer–Tropsch synthesis (FTS). The CoAl 2O 4 spinel phase was detected on the prepared catalysts and its content on the catalysts decreased with the increase of zirconium loading, indicating that Zr-added could inhibit CoAl 2O 4 formation. The addition of zirconium to the Co catalyst caused the increase of cobalt cluster size. Zr addition has been shown to improve the activity and C 5+ selectivity of Co/Al 2O 3 catalyst for Fischer–Tropsch synthesis. This could be explained by the increase of active metal cobalt site and reducibility. The increase of zirconium loading on Co/Zr/Al 2O 3 catalyst resulted in the increase of olefin/paraffin ratio in the products.