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Effect of Re and Al2O3 Promotion on the Working Stability of Cobalt Catalysts for the Fischer–Tropsch Synthesis
... The dependence of CO chemisorption on the concentration of Al 2 O 3 is of an extreme character with a maximum corresponding to the catalyst with the optimum content of the promoter. The relatively high chemisorption of CO changes the ratio of surface concentrations of hydrogen and carbon monoxide, promoting the formation of C 5+ hydrocarbons.The stability of the developed catalyst and the Co/γ-Al 2 O 3 reference catalyst promoted by Re in FT synthesis (pressure, 0.1 MPa; ratio H 2 : CO = 2; gas hourly space velocity (GHSV), 100 h −1 ) was studied during continuous tests lasting 200-300 h[110]. The prospects of using Co/SiO 2 catalysts promoted by Al 2 O 3 for the selective synthesis of C 5+ hydrocarbons (including long chain products of C 35+ ) were confirmed. ...
... The addition of rhenium to iron or cobalt catalysts promotes higher activity and selectivity towards alcohols and larger chain hydrocarbons, increases particle dispersibility and decreases nanoparticle average sizes while also lowering the reduction temperature of the first metal [30,88,89] . Since the patents of Mauldin, from Exxon [90] , and the Shell Middle Distillate processes [91] , the cobalt catalyst promoted by rhenium has been extensively studied in FTS, and it currently remains a topic of research interest since improvements are still needed in preventing catalyst deactivation and further improving control over selectivity [92][93][94][95][96] . ...
Although rhenium may not be the most common choice of active species in catalysis, it has been reported as a highly active and selective catalyst over a wide range of reactions. Its applications include hydrogenation reactions of great relevance in the field of renewable materials and bio‐derived platform molecules, such as valorisation of lignin, CO 2 , and carboxylic acids. Different from several transition metals, rhenium presents oxidation numbers varying from ‐3 to +7. Such diversity in the coordination chemistry of rhenium is reflected in the variety of known rhenium compounds since this metal can compose stable structures as ligand‐bridged multinuclear and organometallic compounds, as well as inorganic oxides, metal‐organic frameworks, and clusters. The exceptional flexibility in rhenium speciation yields numerous selective catalysts; however, it also makes the characterization of rhenium catalysts challenging, and its influence on the catalytic activity is not trivial. This review will outline the most established rhenium‐based materials used in hydrogenation catalysis and shed some light on the relation of rhenium species to selectivity based on advanced characterization techniques. Finally, our perspectives on the use of rhenium catalysts to produce value‐added products will be given.
The effect exerted by the content of metallic (Co-Al 2 O 3 /SiO 2 catalyst) and acidic (ZSM-5 zeolite in the H-form) components on the properties of bifunctional catalyst for the integrated synthesis of waxy diesel fuel by the Fischer–Tropsch method was studied. Catalysts represented by a composite mixture with a boehmite binder were characterized by XRD, BET and TPR methods. The testing was performed in a flow reactor with a fixed catalyst bed at a pressure of 2.0 MPa, temperature 240 °С and gas hourly space velocity 1000 h –1 . Activity and selectivity of the catalysts as well as the fractional and hydrocarbon composition of the products were investigated in dependence on the ratio of components. It was found that productivity of the synthesis for С 5+ hydrocarbons and selectivity for the С 11 –С 18 diesel fraction products with a high content of isomeric products correlated with the ratio of metallic and acidic components in the catalysts. The composition of the catalyst recommended for the diesel fuel production has the 1.17 ratio of metallic and acidic components.
Fischer-Tropsch synthesis (FTS) is a promising technology for converting renewable energy sources to synthetic liquid fuels, olefins or oxygenates. As one of the metal-based FTS catalysts with excellent intrinsic activity, supported Co-based catalysts have attracted much attention. Their properties depend on their geometric morphology, surface composition and metal-support interaction (MSI). This paper reviews the latest research progress of the supported Co-based catalysts for FTS, including recently-developed functional modification strategies to adjust the cobalt phase, crystallite size, metal dispersion, site density and metal-support interaction to optimize the performance of cobalt-based catalysts. The effects of important materials (including cobalt precursors, supports and promoters) on catalytic performance are discussed and compared. A summary is provided of the modifications made to catalysts with a unique and controllable structure, in order to improve the catalytic activity and focus the product distribution on targeted wax, gasoline, diesel, olefins and oxygenate products. Guidance is provided regarding controlling the structure and functions of Co-based catalysts. Opportunities and perspectives on future research into modification strategies used for Co-based FTS catalyst are also noted.
Mn-doped spinel oxides CoAl2O4 marked as CMA-m (m = Co/Mn atom ratio) herein were synthesized and evaluated for the first time as supports of Co-based Fischer-Tropsch synthesis (FTS) catalysts. HAADF-STEM images showed that Mn doping caused more crystal defects on the surface of CMA-m grains, which may provide more attachment sites for cobalt loading, leading to much smaller Co⁰ particle size of 15Co/CMA-m than that of 15Co/CoAl2O4. Data of H2/CO chemisorption showed that the interaction of highly dispersed Co⁰ with lattice Mn-doped CMA-m grain has a promotive effect on CO adsorption capability. FTS evaluation results demonstrate that 15Co/CMA-20 showed the highest CO conversion, C5+ selectivity and C5+ STY, benefiting from its high Co⁰ dispersion and CO chemisorption capability as a result of suitable Mn doping into the lattice of CoAl2O4, which was confirmed very different from the influence of impregnated Mn oxide over CoAl2O4.
Five different γ-alumina supports were used, one commercial and four prepared in a macro-mesoporosity range of 7–1000 nm by different preparation methods. A simple method was used to obtain a macro-mesoporous alumina support by modification of an initial commercial mesoporous alumina with a pore generating agent. Supports were used to prepare Fischer–Tropsch synthesis (FTS) catalysts. Supports and catalysts were characterized by several techniques and tested in a lab-scale fixed-bed reaction unit in the FTS at 493 K and 2 MPa, with small (<63 μm) and large (500–710 μm) catalyst particle size (PS). Catalytic results showed that with small catalyst PS, the behavior is similar among them. With large catalyst PS, C5+ selectivity considerably decreased and CH4 selectivity increased for all catalysts due to diffusional restrictions. Nevertheless, the effect of diffusion limitations of reactants and products through catalyst pores were lower for catalysts with a higher mesoporosity (20–50 nm), and much lower for the catalyst obtained when the support was modified to add macroporosity between 100 and 1000 nm. These macropores improve the transport of reactants and heavy hydrocarbons produced between the gaseous phase and the active sites. As a consequence, a better performance of the catalyst is attained, reducing the non-desirable effects of a diffusion-limited regime.
Abstract The effect of water on the activity and selectivity for a series of carbon nanofiber supported Fischer–Tropsch cobalt catalysts has been investigated. Fischer–Tropsch synthesis was carried out in a fixed bed reactor at industrially relevant conditions (483 K, 20 bar, H2/CO = 2.1). Three catalysts were prepared either by incipient wetness impregnation or wet impregnation and consisted of 12 or 20 wt% cobalt deposited on carbon nanofibers of the platelet and fishbone structures. Addition of 20 and 33 mol% water to the reactor inlet increased the reaction rates, but also the deactivation rates of all the catalysts. As a result of the high water concentrations, the catalysts were irreversibly deactivated. A positive correlation between the amount of water in the reactor and the selectivity to long-chain hydrocarbons (C5+) was found. Graphical Abstract
The properties of cobalt catalysts for the Fischer-Tropsch synthesis used in XTL processes (the conversion of any organic raw materials into liquid hydrocarbons) are considered. It is shown that the catalysts used for the production of synthetic petroleum are characterized by a relatively low activity in polymerization reactions, and they make it possible to obtain hydrocarbon mixtures with the concentration of solid paraffins no higher than 10%. The catalysts used for conducting the synthesis of middle oil distillates are characterized by high polymerizing activity (alpha of 0.9 or higher) and by the high yields of C5+ hydrocarbons.
The effect of the nature of the support of a Co catalyst on the synthesis of hydrocarbons from CO, H2, and C2H4 was studied in this work. It was found that the introduction of ethylene into synthesis gas resulted in an increase in the
yield of liquid hydrocarbons. In this case, the conversion of C2H4 was complete and the degree of its involvement into the synthesis of C5+ hydrocarbons depended on the concentration of this component in the starting mixture and the nature of the support. Specific
features of the adsorption of CO and C2H4 on the used Co catalysts were determined using a temperature-programmed desorption method.
The effect of ceria promotion on the performance of Co/Al2O3 catalyst was evaluated in a high pressure fixed bed reactor for Fischer–Tropsch (FT) synthesis at close industrial conditions. Ce–Al2O3 supports with a molar ratio of Al/Ce = 8 were prepared by two different methods: one by co-precipitation of cerium and aluminum precursors in water-in-oil microemulsion and the other one by aqueous impregnation of cerium nitrate on commercial alumina. These supports, together with the unmodified alumina carrier, were used to prepare four cobalt-based catalysts. These catalysts were characterized by XRD, SEM, EDX, TEM, N2 adsorption/desorption, TPR and chemisorption techniques. The results show that the presence of CeO2 on the surface of the support favors the reducibility of cobalt oxides with a shift down in reduction temperature of about 70 °C. The catalytic evaluation of the catalysts revealed that cerium addition by impregnation increases the activity and selectivity to C5+ catalyst in FTS. The catalyst synthetized by microemulsion show lower catalytic performance. Nevertheless, the catalytic property of this material can be improved by increasing the crystalline micro-domains size of CeO2.
The stability of a series of Co/Al2O3 and Re-Co/Al2O3 Fischer-Tropsch (FT) catalysts, with varying Co particle size, was measured in a continuous flow, stirred tank reactor operated at 220oC, 2.1MPa and a H2/CO = 2/1 synthesis gas for periods up to 190 h time-on-stream (TOS). Results showed that the stability of the catalysts was dependent upon the Co particle size, degree-of-reduction (DOR) of the catalyst precursor and the CO conversion. At the chosen operating conditions, carbon deposition was the main cause of catalyst deactivation and the initial rate of carbon deposition per active Co site increased with increased Co particle size (dCo = 2 ? 22 nm) when measured at approximately the same CO conversion level. On the 15wt% Co/Al2O3 catalyst the initial rate of carbon deposition increased with CO conversion (CO conversion ? 40%) whereas on the 1.2wt%Re-12wt%Co/Al2O3 catalyst, the initial rate of carbon deposition decreased with increased CO conversion (CO conversions > 60%) due to high concentrations of H2O and CO2 in the reactor.
The impact of the chemical nature of the oxide support on the performance of cobalt Fischer-Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous γ-Al2O3 with various oxides representative of a wide range of Lewis acid-base character, as quantified by UV-vis spectroscopy coupled to alizarine adsorption. Incorporation of cobalt (20 wt%) results in model catalysts with identical porosity and similar Co particle sizes (>10 nm) allowing to study support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt-time-yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence with a maximum at an intermediate acid-base character. As inferred from in situ CO-FTIR, "selective" blockage of few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and "unselective" site-blockage via decoration of Co nanoparticles (strong-metal-support interaction) with acidic, reducible oxides causes the decrease in reaction rate for supports with pronounced alkaline and acidic character, respectively. The extent of secondary isomerization reactions of α-olefin products, of relevance for chain re-insertion processes and product selectivity, correlates also with the support acidity. For a monolayer TiOx/Al2O3 as support, a remarkable C13+ productivity exceeding 0.09 molC•gCo-1•h-1 is achieved, owing to the combination of optimal activity and selectivity. These results provide a unifying view of the support effects over a considerably broad study space and delineate a blueprint towards advanced Fischer-Tropsch catalysts.
The decision to build a Fischer–Tropsch (FT) plant is still fraught with risk because it has to be based on the perceived future price and availability of petroleum crude oil and on local politics. The most expensive section of an FT complex is the production of purified syngas and so its composition should match the overall usage ratio of the FT reactions, which in turn depends on the product selectivity. The kinetics, reactor requirements, control of selectivity and the life of cobalt and iron catalysts are discussed and compared. Control of the FT conditions coupled with appropriate downstream processes results in high yields of gasoline, excellent quality diesel fuel or high value linear α-olefins. The history of the various FT options and of the improvements in FT reactor technologies over the last 50 years is reviewed. It appears that “new” technologies are re-discovered in cycles of 15–30 years and it often takes the same time for the implementation of new concepts.
The composition of the products of hydrocarbon synthesis from carbon monoxide and hydrogen and also the composition and properties of individual fractions (commercial products) obtained by this method are considered. The reaction paths of the formation of different carbon-containing compounds (gaseous, liquid, and solid hydrocarbons) under the Fischer-Tropsch synthesis conditions and synthesis by-products (alcohols and CO2) and the possible directions of their secondary conversions are demonstrated.
The 20% Co-M/CoAlx
Oy
catalytic systems promoted with Ru, Pd, and Re, in which cobalt is not only an active component but also a constituent of the support, were studied in this work. The effects of the addition and concentration of a promoter on the catalytic properties of the resulting system in the synthesis of hydrocarbons from CO and H2 was studied. It was found that the introduction of rhenium into the composition of the 20% Co/CoAlx
Oy
catalyst considerably increased the yield of C5+ hydrocarbons with a minimum methane formation in the synthesis of hydrocarbons from CO and H2.
Cobalt catalysts supported on SIRAL aluminosilicates have been prepared and tested in hydrocarbon synthesis from CO and H2. It has been shown that the state of the cobalt metal on the catalyst surface is determined by both the cobalt content and the support nature, in particular, by the phase composition on its surface. The reaction rate on cobalt crystallites 3–6 nm in size is lower than that on 8–13-nm crystallites. It has been found that promotion with cerium oxide considerably increases the catalyst activity, although the selectivity for C5+ hydrocarbons slightly decreases.
The Fischer-Tropsch synthesis is a practically significant reaction catalyzed by Fe and Co, which are active at different H2: CO ratios of 0.5 and 2, respectively. For this reason, Fe-containing catalysts are preferentially used in coal-to-liquids (CTL) and biomass-to-liquids (BTL) processes, while Co-containing catalysts are preferred for the gas-to-liquids (GTL) technology. With the Fe catalysts, which are remarkable for their high CO2 formation selectivity, it is appropriate to carry out the carbon dioxide reforming of the feedstock.
Pd was examined as a promoter for Fischer–Tropsch synthesis, and its effects on cobalt oxide reduction and product selectivities relative to commonly used promoters (i.e., Pt, Re, and Ru) at atomically equivalent levels were compared. Pd was identified to promote cobalt oxide reduction to even lower temperatures than Pt and Ru. However, Pd addition deleteriously affected product selectivity, and a clear shift to favor light products was observed. XANES analysis of an activated model catalyst revealed that Pd was in the reduced state. Local atomic structure was examined by EXAFS. Unlike Pt, Re, and Ru promoters, where previous investigations by groups such as Dr. Guczi’s and ours have only observed coordination of the promoter with cobalt, Pd displayed both direct coordination to Co as well as other Pd atoms. The results suggest that this feature may be responsible for the measurably higher light gas selectivities observed.
The effect of noble metal promoters (atomic ratio of promoter to Co = 1/170) on the activity and selectivity of a 25%Co/Al2O3 catalyst was studied at a similar CO conversion level of 50% at 493 K, 2.2 MPa and H2/CO = 2.1 using a 1-L continuously stirred tank reactor (CSTR). The results show that all promoted catalysts exhibited markedly higher initial CO conversion rates on a per gram catalyst basis than the unpromoted one, which was ascribed to increased Co site density when the promoters were present. This is because the Re, Ru, Pt and unpromoted Co catalysts give essentially the same initial Co TOF values of 0.092–0.105 s−1 (based on hydrogen-chemisorption). However, the initial Co TOF value for the Pd-Co catalyst was about 40% lower, which might be caused by Pd atoms segregating on the Co surface and partially blocking Co sites.At 50% CO conversion, Re and Ru promoters decreased CH4 selectivity and increased C5+ selectivity by nearly the same extent, whereas the opposite effect was observed for Pd and Pt promoters. The Re and Ru promoters had less of an impact on C2–C4 olefin selectivity (7.5–60%), but suppressed the secondary reaction of 1-C4 olefin (from 14.3 to 9%) compared to the unpromoted one; however, the addition of Pd and Pt promoters resulted in lower olefin selectivity (4.4–55%) but higher 2-C4 olefin selectivity (14.3 to 27–31%). Pt promotion had a negligible effect on C4 olefin isomerization. The selectivity results were reproducible.Both Pt and Pd promoters slightly increased WGS activity, whereas Re and Ru promoters had a negligible effect. The Pd and Pt promoters were observed to slightly enhance oxygenate formation, while Re and Ru slightly decreased it.
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, C5+ selectivity, olefin selectivity and lower methane and CO2 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.
This article provides a comparison between cobalt and iron catalysts for use in the slurry phase Fischer–Tropsch synthesis. The themes covered include the typical catalyst preparation methods and properties, catalyst deactivation behaviour, reaction kinetic considerations and the effect of metal crystallite size on catalytic performance. Finally, the implications of catalyst choice for application in different reactors are discussed, paying specific attention to the maximum production capacity of a single reactor.
Improved Fischer–Tropsch synthesis was carried out over Ru, Ni promoted Co/HZSM-5 catalysts to investigate the influence of promoters on the synthesis of gasoline-range hydrocarbons (GRHs, C5–C12 hydrocarbons). The catalysts were characterized by BET, XRD, TEM, H2-TPR and NH3-TPD to clarify their physical and chemical properties. The catalytic activity for GRH production increased significantly with the addition of Ru or Ni promoter. CO conversion increased in the order Co/HZSM-5 < Ru–Co/HZSM-5 < Ni–Co/HZSM-5. Due to the higher reduction degree and weaker acid sites on the Ru–Co/HZSM-5 catalyst, CO conversion and GRH selectivity increased compared with the Co/HZSM-5 catalyst. Furthermore, the Ru–Co/HZSM-5 catalyst maintained its initial high activity and GRH selectivity after 150 h on stream at an optimum temperature of 540 K. The Ni–Co/HZSM-5 catalyst also showed high catalytic activity and GRH selectivity even at the relatively low temperature of 524 K. For all three catalysts, the content of iso-paraffin in the GRHs increased monotonically, almost linearly, with increase in temperature. For the Ni–Co/HZSM-5 catalyst in particular, the content of iso-paraffin in the GRHs increased very significantly.
A range of cobalt catalysts supported on kieselguhr, silica, alumina, bentonite, zeolite Y, mordenite and ZSM-5 was examined for catalytic activity and product selectivity in the Fischer-Tropsch reaction. These results were correlated with catalyst reducibility and adsorptive properties, as well as support acidity, surface area and structure. It was found that high surface area supports give high cobalt dispersions and tend to produce highly active Fischer-Tropsch catalysts, as long as the reducibility of the cobalt is not hindered by metal-support interactions or ion exchange, or that pore diffusion or blocking effects are not taking place. All supports produced catalysts with similar methane, carbon dioxide and higher hydrocarbon selectivities, with carbon dioxide selectivity being low in all cases. The nature of the higher hydrocarbons depended strongly upon support acidity, with the non-zeolitic, low acidity supports producing the classic straight chained Fischer-Tropsch product. Of the zeolite supported catalysts, the most strongly acidic ZSM-5 supported catalyst produced the most highly branched product, followed by the zeolite Y supported catalyst. The straight chained character of the product from the mordenite supported catalyst was explained by the inaccessibility of the primary Fischer-Tropsch products to the mordenite acid sites, due to the mordenite channel system and ion exchange properties, thereby preventing secondary reactions such as isomerisation from occurring.
The addition of small amounts of Ru to Co/TiO[sub 2] or Co/SiO[sub 2] catalysts (Ru/Co < 0.008 at.) increases their turnover rate and C[sub 5]+ selectivity in the Fischer-Tropsch synthesis. Also, deactivated bimetallic Co-Ru catalysts can be regenerated by a hydrogen treatment at reaction temperatures whereas monometallic Co catalysts cannot. On Co/TiO[sub 2], Ru addition increases turnover rates by a factor of three and C[sub 5]+ selectivities from 84.5 to 91.1%, without an apparent change in cobalt dispersion. Activation energies and reaction kinetics are unaffected by Ru addition. These data suggest that the presence of Ru leads to higher Co site density during reaction without modifying the chemical reactivity of exposed Co surface atoms. Ru appears to inhibit the deactivation of surface Co ensembles. Ru atoms at the surface of Co crystallites increase the rate of removal of carbon and oxygen species during reaction and during regeneration of deactivated Co catalysts. The resulting higher Co site density leads to higher apparent turnover rates and to enhanced readsorption of [alpha]-olefins. As a result, Co-Ru catalysts yield a heavier and more paraffinic product than monometallic Co catalysts with similar initial dispersion. The observed bimetallic interactions require intimate contact between Co and Ru, a state that forms during oxidation of the bimetallic precursors at high temperature. This treatment induces migration of Ru oxide species and leads to mixed Co-Ru oxides. The reducibility and the carbon deposition rates on well-mixed bimetallic Co-Ru catalysts are very different from those of monometallic Co catalysts. Electron microscopy, X-ray absorption spectroscopy, and thermogravimetric studies confirm the intimate mixing required for catalytic rate and C[sub 5]+ selectivity enhancements, for faster reduction of Co oxide precursor, and for inhibition of carbon deposits during reactions of H[sub 2]/CO mixtures. 40 refs., 14 figs., 7 tabs.
Fresh and used, unpromoted and noble metal-promoted 15% Co/Al2O3 catalysts were analyzed by XANES and EXAFS to provide insight into catalyst deactivation. XANES analysis of the catalysts gave evidence of oxidation of a fraction of the cobalt clusters by water produced during the reaction. Comparison of XANES derivative spectra to those of reference materials, as well as linear combination fitting with the reference data, suggest that some form of cobalt aluminate species was formed. Because bulk oxidation of cobalt by water is not permitted thermodynamically under normal Fischer–Tropsch synthesis (FTS) conditions, it is concluded that the smaller clusters interacting with the support deviate from bulk-like cobalt metal behavior and these may undergo oxidation in the presence of water. However, in addition to the evidence for reoxidation, EXAFS indicated that significant cobalt cluster growth took place during the initial deactivation period. Promotion with Ru or Pt allowed for the reduction of cobalt species interacting with the support, yielding a greater number of active sites and, therefore, a higher initial catalyst activity on a per gram catalyst basis. However, these additional smaller cobalt clusters that were reduced in the presence of the noble metal promoter, deviated more from bulk-like cobalt, and were therefore, more unstable and susceptible to both sintering and reoxidation processes. The latter process was likely in part due to the higher water partial pressures produced from the enhanced activity. The rate of deactivation was therefore faster for these promoted catalysts.
The addition of a small amount of AlâOâ to silica-supported cobalt catalysts significantly increased the dispersion of cobalt and Co-metallic surface area, resulting in the remarkable enhancement of the Fischer-Tropsch synthesis (FTS) activity in the slurry-phase reaction. The addition of AlâOâ adjusted the interaction between cobalt and the silica support quite well, realizing the favored dispersion and reduction degree of supported cobalt and leading to high catalytic activity in FTS. The properties of various catalysts were characterized by in situ DRIFT, XRD, TPR, Nâ physisorption, and Hâ chemisorption. 16 refs., 3 figs., 2 tabs.
A series of 10%Co/ITQ-2 model catalysts have been prepared by combining a reverse micellar synthesis with a surface silylated ITQ-2 delaminated zeolite. The catalysts display rather uniform Co0 particle size distributions in the 5–11nm range as ascertained by XRD, H2-chemisorption and (HR)TEM. Additionally, a low dispersed 30%Co/SiO2 reference sample (d(Co0)=141nm) has been prepared by supporting a Co3O4 nanopowder on spherical SiO2. H2-TPR and DR UV–vis spectroscopy reveal that the preparative approach leads to highly reducible catalysts in the d(Co0) range of 5.6–141nm, while the activation energies for the stepwise Co3O4→CoO→Co0 reduction are found to be particle size dependent. Formation of barely reducible surface and bulk Co silicate species is observed for samples with d(Co3O4)⩽5.9nm. Under realistic Fischer–Tropsch synthesis conditions (493K, 2.0MPa) the TOF increases from 1.2×10−3 to 8.6×10−3s−1 when d(Co0) is increased from 5.6 to 10.4nm, and then it remains constant up to a particle size of 141nm. In situ and at work FTIR of adsorbed CO reveal a severe cobalt surface reconstruction towards more open crystal planes and/or defect sites (Co–carbonyl bands in the region of 2000–2025cm−1) and suggest adsorbed C adatoms (surface carbidic species), derived from CO dissociation, as the true restructuring agent. Under FTS conditions, this Co surface reconstruction occurs similarly irrespective of the metal particle size. Moreover, an enhancement in the proportion of Co–SiO2 interfacial Coδ+ sites (Co–CO band at 2060cm−1) takes place particularly in small cobalt nanoparticles (5.6nm) likely as a consequence of nanoparticle flattening, as suggested by TEM after catalysis. These Co–SiO2 interfacial sites are tentatively proposed as responsible for the decreased TOF observed for d(Co0)
The influence of cobalt particle size in the range 3 to 18 nm on the Fischer–Tropsch synthesis intrinsic activity and product distribution was investigated over 26 alumina (γ-Al2O3 and α-Al2O3) supported catalysts. A volcano-like curve was obtained for the γ-Al2O3 based catalysts when the C5+ selectivity was plotted as a function of the particle size. The maximum selectivity was located at 7–8 nm. This is the first time an optimum particle size has been identified. An apparent optimum size was also discovered for the series of α-Al2O3 based catalysts. Interestingly, the C5+ selectivity of the α-Al2O3 based catalysts was higher than the selectivity of the γ-Al2O3 based catalysts at all particle sizes. Thus, the selectivity is related both to the particle size and the support. No relation was observed between the cobalt particle size and the cobalt site-time yield.
The deactivation of Co/SiO2 catalyst for Fischer–Tropsch synthesis (FTS) at different H2 / CO ratios was investigated by XRD, FTIR, BET, XPS, TPR and H2 chemisorption. It was found that the deactivation rate of the catalyst increased with the rise of the H2 / CO ratio. The generation of silicates and/or hydrosilicates species was evidenced by TPR and XPS, and their amounts were monotonously enhanced with increasing H2 / CO ratio, which suggested that the deactivation was caused by the transformation of metallic cobalt into inactive silicates and the high partial pressure of H2 facilitated the formation of the silicates. Moreover, the percentage loss of the surface cobalt was larger than that of bulk cobalt, suggesting that the cobalt silicates and/or hydrosilicates species were formed mainly on the surface of the catalyst or in the small crystallites. For the catalyst run at H2 / CO ratio of 1, it was observed that the sintering also contributed to the catalyst deactivation, but it was a less important factor for the deactivation.
The effect of the measurement conditions on the hydrogen chemisorption on CoRe/γ-Al2O3 has been studied systematically. The measurement temperature is one of the most important factors. In the range from 298 K to 433 K the amount of strongly adsorbed hydrogen increases, and remains stable up to 473 K. The difference observed is significant, the estimated cobalt dispersion is up to 20% higher at the higher temperatures. Different γ-alumina samples were examined, and the behaviour was similar for all the samples as well as for samples with different apparent particle sizes. The most effective evacuation temperature to remove the adsorbed hydrogen on cobalt before hydrogen chemisorption analysis was found to be the temperature used for catalyst reduction.
The impact of water on the deactivation of a 0.5% Pt-promoted 15% Co/Al2O3 catalyst was studied by XAFS. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under an inert gas blanket. Synthesis operating conditions were maintained constant while differing amounts of argon were replaced by added water. Below 25% added water (H2O/CO=1.2; H2O/H2=0.6), the slight negative effect on activity was reversible, and no changes were observed in the EXAFS or XANES spectra. This indicates that the effect of water in this range is most likely kinetic. However, XAFS results strongly suggest that, above 25% water addition, the sudden irreversible loss in activity is due to reaction of the cobalt clusters with the support, forming cobalt aluminate-like species. The XAFS and previously reported activity data indicate that there are two regions for the water effect: at lower H2O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H2O/CO ratios irreversible oxidation of cobalt occurs.
An investigation of the CO hydrogenation of Pt- or Re-promoted 8.7 wt% Co/Al2O3 (1.0 wt% Pt or 1.0 wt% Re) has been carried out at two different conditions: 473 K, 5 bar, H2/CO = 2 and 493 K, 1 bar, H2/CO = 7.3. The addition of Pt or Re significantly increases the CO hydrogenation rate (based on weight of Co), but the selectivity was not changed by the presence of Pt or Re. The results show that the observed increases in the reaction rates are caused by increased reducibility and increased number of surface exposed Co-atoms. Steadystate isotopic transient kinetic analysis (SSITKA) with carbon tracing was used to decouple the effects of the concentration of active surface intermediates and the average site reactivity of intermediates during steady-state CO hydrogenation. The SSITKA results show that the concentration of active surface intermediates leading to CH4 increased as a result of the addition of a noble metal promoter. However, the average site activity was not significantly affected upon Re or Pt addition.
A systematic study of the effect of γ-Al2O3 support variables on Fischer–Tropsch synthesis activity and selectivity was carried out at industrially relevant conditions (T=483 K, P=20 bar, H2/CO=2.1). A total of 13 catalysts were prepared by incipient wetness impregnation of γ-Al2O3 supports of varying pore characteristics and chemical purities with aqueous solutions containing the required amounts of cobalt and rhenium precursor to give nominal loadings of 20 and 0.5 wt%, respectively. The catalysts were analysed for cobalt, rhenium, nitrogen, and sodium and characterised by nitrogen adsorption/desorption, mercury intrusion, X-ray diffraction, hydrogen chemisorption, temperature-programmed reduction, and oxygen titration. The size of the Co3O4 cobalt particles was controlled by the support pore size, with small particles formed in narrow pores and large particles formed in wide pores. The degree of reduction increased with increasing catalyst pore size and Co3O4 particle size. The catalysts contained different amounts of sodium (20–113 ppm) and the site-time yield decreased with increasing sodium content. Positive correlations were found between cobalt particle size and C5+ selectivity and between catalyst pore size and C5+ selectivity. The results indicate that the C5+ selectivity is controlled by the size and appearance of cobalt particles.
The unpromoted and promoted Fischer–Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer–Emmett–Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al2O3 catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.
The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al2O3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H2/CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al2O3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al2O3 supports had very different pore characteristics. The post-calcination Co3O4 crystallite size was predominantly controlled by the γ-Al2O3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co3O4 was reduced in two steps to Co0 with CoO as intermediate species. However, while reduction of Co3O4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts. Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield. The deactivation rates of all the catalysts were also similar up to 100 h on stream. Positive correlations were found between the catalyst pore diameter and the C5+ selectivity and between the cobalt particle size and the C5+ selectivity. Re had a consistent positive, albeit small, effect on the C5+ selectivity.
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