Supported Cobalt Catalysts Research Articles

Synthesis of Cobalt Nano Particles on Silica Support Using the Strong Electrostatic Adsorption (SEA) Method

Supported cobalt is one of the common catalysts used in Fischer-Tropsch synthesis (FTS). Strong electrostatic adsorption (SEA) was employed to synthesize cobalt nano particles supported on silica. Cobalt nitrate was used as the catalyst precursor and non-porous silica spheres, which were synthesized using the modified Stöber method, were used as a catalyst support. Point of zero charge (PZC) for silica was determined using equilibrium pH at high oxide loading (EpHL) method. The optimum pH was determined by measuring cobalt uptake versus pH. High cobalt uptake at basic pH and low cobalt uptake at acidic pH indicates electrostatic interaction between the cobalt complexes in the precursor solution and the hydroxyl group on the support’s surface. Catalysts prepared at optimum pH were characterized using TPR, XPS and TEM. TPR shows reduction peak at high temperature (587°C) indicating strong interaction between cobalt and silica support. XPS shows presence of Co2+ species on the surface. TEM images of the Co/SiO2 at 5 wt% and 10 wt% cobalt loadings show fairly well-dispersed cobalt oxide nano particles on the spherical silica support with narrow particle size distribution. The findings suggest that SEA was deemed a suitable method to prepare supported cobalt catalysts.

Characteristics and catalytic properties of La-modified ZrO2 supported cobalt catalysts in CO hydrogenation

La-modified ZrO2 with 10 mol% La was prepared by three different methods, namely co-precipitation, impregnation, and mechanical mixing and employed as cobalt catalyst supports. The use of La-modified ZrO2 prepared by mechanical mixing resulted in higher cobalt dispersion and higher CO hydrogenation activity than those supported on pure oxide (La2O3 and ZrO2) and La-modified ZrO2 obtained by the other methods. As revealed by XRD and TEM, an intimate contact and/or incorporation of La atoms in ZrO2 may occur on the La-modified ZrO2 prepared by co-precipitation and impregnation, which induced a stronger interaction between cobalt and the support. On the other hand, the presence of amorphous La2O3 in ZrO2 by mechanical mixing facilitated cobalt reducibility as shown by lower reduction temperature in the TPR profile.

Co-SBA-15 for heterogeneous oxidation of phenol with sulfate radical for wastewater treatment

Mesoporous silica SBA-15 supported cobalt catalysts were prepared by co-condensation during SBA-15 synthesis, using three different Co(II) precursor salts, chloride (Cl), acetate (Ac) and nitrate (N). The physicochemical properties of the catalysts were characterized by several techniques, such as XRD, SEM (EDS), TEM, and FT-IR. It was found that Co 3O 4 was formed in SBA-15 and would effectively activate peroxymonosulfate to produce sulfate radicals for phenol oxidation in heterogeneous solutions. The Co precursors affected the physicochemical property, activity and stability of the catalysts. In heterogeneous reactions, the effects of catalyst loading, peroxymonosulfate concentration and reaction temperature on phenol degradation were investigated. The heterogeneous phenol degradation reactions followed zero order kinetics and the activation energies were 81.4, 67.4 and 67.4 kJ/mol for Co/SBA-15-Cl, Co/SBA-15-Ac, and Co/SBA-15-N, respectively.

Deactivation Behavior of Carbon Nanotubes Supported Cobalt Catalysts in Fischer-Tropsch Synthesis

The effects of electronic properties of inner and outer surfaces of Carbon Nano Tubes (CNTs) on the deactivation of cobalt Fischer-Tropsch (FT) catalysts were studied. The comparative characterization of the fresh and used catalysts by TEM, XRD, TPR, BET and H 2 chemisorption showed that cobalt re-oxidation, cobalt-support interactions and sintering are the main sources of catalyst deactivation. TEM showed that 480 h continuous FT synthesis increased the average particles size of the particles located inside the pores from 7 to 7.4 nm while the average particles size of the particles located outside of the tubes increased from 11.5 to 25 nm. XRD analysis of the used catalyst confirmed cobalt re-oxidation and interaction between cobalt and CNTs and creation of carbide phases. When the %Co conversion and H 2O partial pressure in the reactor are high, 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 and metal support interactions. At lower amounts of the %Co conversion and H 2O partial pressure, deactivation can be simulated with power law expressions with power orders of 11.4 for the particles outside the tubes and 30.2 for the particles inside the tubes and the main deactivation mechanism is sintering. Due to the electron deficiency of the inner sides of the CNTs, the interaction between the cobalt oxides and the support is stronger leading to lower rates of sintering as compared with the particles located on the outer layers of the CNTs. Regeneration recovered the catalyst activity by 54.3% of the total activity loss.

Development of a Kinetic Model for Supercritical Fluids Fischer−Tropsch Synthesis

Kinetic models for Fischer−Tropsch synthesis (FTS) were derived to express the reaction behavior in either conventional reaction media (gas-phase) or nonconventional media (near-critical and supercritical solvent media). These models we developed from experimental data generated from a commercial alumina supported cobalt catalyst (15% Co/Al2O3) in a fixed bed reactor. Two models were developed: the first one assumes ideal gas phase behavior for the gas phase reaction and uses partial pressures to express reactant and product concentrations in rate expressions while the second model accounts for the nonideal behavior of the reaction mixture under elevated pressures via the utilization of fugacity parameters and fugacity coefficients. The fugacity coefficients were estimated from a modified Redlich−Kwong−Soave equation of state. A comparison was conducted between the estimated kinetic parameters from the fugacity-based kinetic model and the partial pressure based kinetic model. The heat of adsorption of car...

Identification of the active species in the working alumina-supported cobalt catalyst under various conditions of Fischer–Tropsch synthesis

An in situ time-resolved XRD/XANES/EXAFS investigation combined with on-line evaluation of catalytic performance indicates considerable structural transformations of the conventional Pt-promoted alumina supported cobalt catalyst under the realistic conditions of Fischer–Tropsch synthesis. Cobalt and platinum are reduced into metallic states after pre-treatment in hydrogen at 623 K and form Co–Pt bimetallic nanoparticles. Depending on catalyst and reaction conditions, the metallic nanoparticles can undergo sintering, carbidisation, and fcc– hcp phase transformations. The observed changes in the catalyst structure significantly affect the performance in the Fischer–Tropsch reaction. Cobalt hcp phase seems more active in Fischer–Tropsch synthesis than cobalt fcc metal phase.

Kinetics of the Bray-Liebhafsky oscillatory reaction perturbed by polymer supported cobalt catalyst

The Bray-Liebhafsky (BL) oscillatory reaction generated in the batch reactor at 62- 68 oC was perturbed by cobalt(II)-nitrate, supported on the macroreticular copolymer of poly-4-vinylpyridine with divinylbenzene (Co-PVPDVB). The kinetic data was analyzed of the complex pathways of the hydrogen peroxide decomposition in the examined BL reaction. The obtained results confirm that the kinetics of the BL reaction in the presence Co-PVPDVB comes partially from the Co-catalyst and partially from the macroreticular copolymer support.

β-Cyclodextrin for design of alumina supported cobalt catalysts efficient in Fischer–Tropsch synthesis

Addition of β-cyclodextrin during catalyst preparation strongly affects the structure and catalytic performance of alumina supported cobalt catalysts for Fischer-Tropsch synthesis. Impregnation of the support with solutions containing β-cyclodextrin leads to higher metal dispersion and spectacularly enhances both reaction rate and heavy hydrocarbons productivity in Fischer-Tropsch synthesis.

Promotion Effects of Platinum and Ruthenium on Carbon Nanotube Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

Carbon nanotube supported nano-size monometallic and noble metal (Pt and Ru) promoted cobalt catalysts were prepared by incipient wetness impregnation (IWI) using solution of cobalt nitrate and characterized by nitrogen adsorption isotherm, X-ray diffraction (XRD), temperature programmed reduction, in situ magnetic method and TEM. Analysis of the magnetization and H2-TPR data suggested promotion with platinum and ruthenium significantly decreased the cobalt species reduction temperature. TEM and XRD results showed that the presence of noble metal promoters had no significant effect on the size of cobalt for carbon naotube as catalytic support. Promotion of cobalt carbon nanotube-supported catalysts with small amounts of Pt and Ru resulted in slight increase in Fischer–Tropsch cobalt time yield. The Pt and Ru promoted cobalt catalyst exhibited carbon monoxide conversion of 37.1 and 31.4, respectively. C5+ hydrocarbon selectivity was attained at 80.0%. The Pt promoted cobalt supported on carbon nanotube yielded better catalytic stability than that of the monometallic cobalt catalyst. Over carbon nanotube supported cobalt catalyst, the addition of noble promoter significantly decreased the cobalt reduction temperature, but had no significant influence on the cobalt particle size.

Gas-to-Liquids Research at Texas A&M University at Qatar

Abstract As the State of Qatar endeavors to become the ‘Gas Capital of the World’, it is imperative to develop cost-effective technologies for gas processing. In this context, gas-to-liquid (GTL) technology represents a major pathway for the production of ultra-clean liquid hydrocarbons and value-added chemicals. The Fischer-Tropsch synthesis (FTS) reaction is the principal route for the GTL process. Professor Bukur is exploring effects of novel and non-traditional activation (pretreatment) procedures, and promoters on performance of supported cobalt catalysts during FTS. This research is conducted in collaboration with a team of experts in both fundamental and applied aspects of catalysis. It may ultimately lead to establishing scientific basis for design of the next generation of supported cobalt catalysts for the GTL conversion process. Another area of research is development of a comprehensive kinetic model for FTS in a slurry reactor. The kinetic model developed in this study, coupled with the appropriate conservation equations and transport properties for a given reactor configuration (fixed bed or slurry bubble column), would be a valuable tool for optimizing product yield, simulating the plant design, and evaluating the economic cost benefits. Dr. Elbashir's research activities are focused on the design of an advanced reactor technology for the FTS to leverage certain advantages over the current commercial technologies while at the same time overcoming several of their major limitations. The pillars of this innovative research approach are based on fundamental studies leading to better understanding of the complex nature of FTS, coupled with applied research work targeting the development of novel catalysts and reactors. All phases of Dr Elbashir's research activity in this field are performed in collaboration with leading scientists of multidisciplinary backgrounds and with the involvement of both graduate and undergraduate students from Qatar. This novel research approach is designed to lead to alternative FTS technologies, enabling Qatar to be on the forefront of technology development in GTL.

Effect of water on the space-time yield of different supported cobalt catalysts during Fischer–Tropsch synthesis

The effect of partial pressure of water on the Fischer–Tropsch (FT) rate of six cobalt-based catalysts supported on three different carrier materials (γ-Al 2O 3, α-Al 2O 3, TiO 2) with varying Co particle sizes was investigated in a fixed-bed reactor by changing space velocity and by external water vapour addition. A typical catalyst pellet size (<100 μm) for industrial slurry-bed FT reactors was used. Water was found to have a positive kinetic effect, at least up to moderate amounts, on the FT rate of all catalysts in the present study, including the γ-Al 2O 3-supported catalyst with pores smaller than ∼10 nm. The reason for the apparent negative effect on the space-time yield at a direct exposure of Co supported on narrow-pore γ-Al 2O 3 to high partial pressures of water is due to a rapid and extensive deactivation. This could be ascribed to formation of hard-to-reduce oxidized cobalt species. The choice of support material was found to have a major effect on the response to changes in partial pressure of water, both with respect to deactivation behaviour and kinetics. However, there is a minor Co-particle size effect on the magnitude of the kinetic effect of water, larger Co particles showing a more positive response. Different extents of mass transfer limitations and/or differences in fugacities of H 2, CO and water among the six catalysts could be ruled out as causes for the observed differences.

Adsorption/Desorption Behavior of Ethanol Steam Reforming Reactants and Intermediates over Supported Cobalt Catalysts

The interactions of reactants and intermediates with the surfaces in ethanol steam reforming over Co catalysts supported on ZrO2 and CeO2 were investigated using Temperature Programmed Desorption, Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and isotopic labeling techniques. Possible mechanistic steps are proposed that lead to acetaldehyde and acetone formation, steam reforming and coking. The role of the support versus active metal (i.e., Co) and the involvement of water in the reaction network are discussed. The interactions of reactants and intermediates with the surfaces were investigated in ethanol steam reforming over Co catalysts supported on ZrO2 and CeO2 were investigated using Temperature Programmed Desorption, Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and isotopic labeling techniques.

Influence of the porous structure of the support on the properties of cobalt catalysts for hydrocarbon synthesis from CO and H2

The dependence of the properties of supported cobalt catalysts for hydrocarbon synthesis from CO and H2 on the porous structure of the supports (aluminas and aluminosilicates) is reported. A correlation between the mean Co0 crystallite size and the pore diameter of the support is established. The specific CO conversion rate falls sharply as the size of Co0 particles decreases to 6 nm and below. The methane and C5+ hydrocarbon selectivities as a function of the Co0 crystallite size pass through an extremum.

The mechanism of the Fischer–Tropsch reaction over supported cobalt catalysts

Natural gas and coal are converted to fuels by the Fischer–Tropsch reaction, i.e., by reacting CO with H 2, the currently accepted mechanism involving surface carbide formation. We have monitored the adsorbed species and their evolution during Fischer–Tropsch reaction on the commercial catalyst Pt,Co/γ-Al 2O 3 (as a standard), on Co/alumina-pillared montmorillonite (Co,Al-EFW) and its beidellite analogue (both are tri-octahedral smectite clays, but beidellite possesses tetrahedrally coordinated Al in the sheets) via optical diffuse reflectance (DRS 1 1 Optical diffuse reflectance spectroscopy. ) and variable temperature in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS 2 2 Diffuse reflectance infrared Fourier transformation spectroscopy. ). We show that in this case over Pt/Co-γ-Al 2O 3, under model conditions, H 2 reacts with CO forming H–C O bridges over [Co-μO 2-Co] units; this transition state moiety also involves adjacent matrix Al–O–CO units. Subsequent stages provide differing oxygenate species via concerted acid–base reactions. The early stages of the Fischer–Tropsch synthesis thus do not involve surface carbide species and surface oxygenates are generated via concerted reaction of support surface Al-OH with the above HCO-bridging binuclear Co 2+ unit.

Activated carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution

A heterogeneous cobalt catalyst was prepared by impregnation of cobalt ion on an activated carbon (AC) and used to activate peroxymonosulphate (PMS) for advanced oxidation of phenol in aqueous solution. The Co/AC catalyst was characterised by several techniques such as XRD, EDS, SEM, and TGA. It was found that Co2O3 was the major form of Co species and was homogeneously distributed on the activated carbon surface. Co/AC exhibited high activity in oxidation of phenol with sulphate radicals and 100% decomposition and 80% TOC removal could be achieved in 60min at the conditions of 500ml phenol solution of 25ppm, 0.1g catalyst and 1g peroxymonosulphate. The catalyst also exhibited stable performance after several rounds of regeneration. Several operational parameters such as catalyst and oxidant amount, temperature on the rate of oxidation were found to influence the phenol oxidation.

Investigation of the Reaction Network in Ethanol Steam Reforming over Supported Cobalt Catalysts

In this study, a network of reactions involved in the ethanol steam reforming (ESR) over supported Co catalysts has been investigated using temperature-programmed reaction (TPRxn) and isotopic labeling techniques. The products and intermediates in the gas phase have been monitored by mass spectrometry (MS) as a function of temperature. Ethanol conversion and product distribution obtained in steady-state reaction experiments have also been presented. The results point to a complex network of reactions that directly impact the selectivity and H2 yield that can be obtained in ethanol steam reforming. The reaction network is governed by the temperature range as well as the type of catalyst used, with catalysts exhibiting higher oxygen mobility favoring complete oxidation of ethanol to CO2 and leading to increased hydrogen yield as well as improved stability.

Effects of pore diameter on particle size, phase, and turnover frequency in mesoporous silica supported cobalt Fischer–Tropsch catalysts

A series of mesoporous silica supported cobalt catalysts for the Fischer–Tropsch reaction with pore diameters from 3 to 22 nm was synthesized and characterized. Detailed X-ray diffraction measurements were used to determine the composition and particle diameters of the metal fraction, analyzed as a three-phase system containing Co fcc, Co hcp and CoO. Catalyst properties were determined at three stages of catalyst history: (1) after the initial calcination step to thermally decompose the catalyst precursor into Co 3O 4, (2) after the hydrogen reduction step to activate the catalyst to Co and (3) after the FT reaction. Small Co hcp particles were detected in all reduced catalysts contributing significantly to the Co surface area. The reaction rate increased with the pore diameter of the silica support. The results indicate the importance of careful catalyst characterization in determining the factors that contribute to reactivity. In addition, the identification of significant quantities of two cobalt metal phases suggests that further study of the intrinsic activity of each phase, as well as the structural features of the supports that determine the distribution of cobalt phases and particle sizes is warranted.

ChemInform Abstract: The Structure and Activity of Titania Supported Cobalt Catalysts.

A series of titania supported cobalt catalysts (0.5-6%) were prepared by incipient wetness impregnation, and were characterized by ESCA, XRD, and hydrogen chemisorption. After calcination at 400 C, a surface CoTiO[sub 3]-like phase was the main species present in the 0.5 and 1% cobalt catalysts. For higher cobalt loadings, discrete Co[sub 3]O[sub 4] particles were formed in addition to surface CoTiO[sub 3]. ESCA indicates that after reduction the cobalt metal particle size (6-13 nm) increases with increasing cobalt loadings, but does not vary with reduction temperature (400-500 C). Hydrogen chemisorption was found to be activated and suppressed. The extent of hydrogen chemisorption suppression increases with increasing reduction temperature and decreasing cobalt particle size. The turnover frequency (based on cobalt dispersion derived from ESCA) for benzene and CO hydrogenation decreases with increasing reduction temperature and decreasing cobalt particle size. The decline in activity correlates with the extent of suppression of H[sub 2] chemisorption. The results were interpreted in terms of a decrease in the fraction of exposed surface cobalt due to site blocking by reduced TiO[sub 3] moieties.

Effect of La 2O 3-dopping on the Al 2O 3 supported cobalt catalyst for Fischer-Tropsch synthesis

Alumina was doped with lanthana, La 2O 3, by either impregnation or co-precipitation. The doped and undoped alumina was then used as supports for the preparation of cobalt catalysts by incipient wetness impregnation. The catalysts prepared were characterized by nitrogen adsorption–desorption, powder X-ray diffraction, hydrogen temperature programmed reduction, hydrogen temperature programmed desorption, O 2-titration and diffuse reflectance infrared Fourier transform spectroscopy, and the catalytic properties for Fischer-Tropsch synthesis (FTS) have been evaluated. The results showed that La 2O 3-dopping impacted the catalytic performance significantly. The catalyst with the La 2O 3-doped support prepared by co-precipitation showed better reducibility, activity and product selectivities. The higher ratio of the octahedral to tetrahedral cobalt species of the co-precipitated catalyst relative to the catalyst doped by impregnation might have caused the enhanced performance. The presence of the hydrocarbonyl species, distinguished by its vibrational feature, is directly related to CO conversion and chain-growth.

Formation of a supported cobalt catalyst for the synthesis of carbon nanofibers on aluminum oxide

Comparative studies of the effect of the physicochemical characteristics of a support (aluminum oxide) on the formation of a supported Co catalyst and its activity in the pyrolysis of alkanes (propane-butane) were performed. The effect of the crystalline modification of alumina on the yield of catalytic filamentous carbon (CFC) ((g CFC)/(g Co)) was studied. The surface morphologies of Co-containing catalysts and synthesized carbon deposits were studied by scanning electron microscopy. It was found that carbon deposits with a well-defined nanofiber structure were synthesized by the pyrolysis of a propane-butane mixture in the presence of hydrogen at 600°C on supported Co catalysts prepared by homogeneous precipitation on macroporous corundum (α-Al2O3). The yield of CFC was no higher than 4 (g CFC)/(g Co). On the Co catalyst prepared by homogeneous precipitation on mesoporous Al2O3, the intense carbonization of the initial support; the formation of cobalt aluminates; and, as a consequence, the deactivation of Co0 as a catalyst of FC synthesis occurred. The dependence of the yield of CFC on the preheating temperature (from 200 to 800°C) of Co catalysts before pyrolysis was studied. It was found that, as the preheating temperature of supported Co/Al2O3 catalysts was increased, the amount of synthesized carbon, including CFC, decreased because of Co0 deactivation due to the interaction with the support and coke formation.