Effect of promotion with ruthenium on the structure and catalytic performance of mesoporous silica (smaller and larger pore) supported cobalt Fischer–Tropsch catalysts

The effects of promotion with ruthenium on the structure of cobalt catalysts and their performance in Fischer–Tropsch synthesis were studied using MCM-41 and SBA-15 as catalytic supports. The catalysts were characterized by N₂ physisorption, H₂-temperature programmed reduction, in situ magnetic measurements, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that monometallic cobalt catalysts supported by smaller pore mesoporous silicas (d_p = 3–4 nm) had much lower activity in Fischer–Tropsch synthesis than their larger pore counterparts (d_p = 5–6 nm). Promotion with ruthenium of smaller pore cobalt catalysts led to a considerable increase in Fischer–Tropsch reaction rate, while the effect of the promotion with ruthenium was less significant with the catalysts supported by larger pore silicas.Characterizations of smaller pore cobalt catalysts revealed strong impact of ruthenium promotion on the repartition of cobalt between reducible Co₃O₄ phase and barely reducible amorphous cobalt silicate in the calcined catalyst precursors. Smaller pore monometallic cobalt catalysts showed high fraction of barely reducible cobalt silicate. Promotion with ruthenium led to a significant increase in the fraction of reducible Co₃O₄ and in decrease in the amount of cobalt silicate. In both calcined monometallic and Ru-promoted cobalt catalysts supported by larger pore silicas, easy reducible Co₃O₄ was the dominant phase. Promotion with ruthenium of larger pore catalysts had smaller influence on cobalt dispersion, fraction of reducible cobalt phases and thus on catalytic performance.     

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

Abstract  

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.     

Reaction performance and characterization of Co/Al2O3 Fischer–Tropsch catalysts promoted with Pt, Pd and Ru

A series of noble metal (Pt, Ru or Pd) promoted Co/Al₂O₃ catalysts were prepared by sequential impregnation method. The catalysts were characterized by XRD, TPR, H₂-TPD and TPSR techniques, and their catalytic performance in Fischer–Tropsch synthesis was investigated in a fixed-bed reactor. The results of activity measurements show that the addition of small amounts of noble metal greatly improved the activity of the Co/Al₂O₃ catalyst. TPR experimental results demonstrate that hydrogen spillover from the noble metal to cobalt oxide clusters facilitated the reduction of cobalt oxide and, thus significantly increased the reducibility of Co/Al₂O₃ catalyst. The presence of noble metal increased the amount of chemisorbed hydrogen and weakened the bond strength of Co–H. TPSR results indicate that CO was adsorbed in a more reactive state on the promoted catalysts.     

Chemisorption of C3 hydrocarbons on cobalt silica supported Fischer–Tropsch catalysts

Chemisorption of propene and propane was studied in a pulse reactor over a series of cobalt silica-supported Fischer–Tropsch catalysts. It was shown that interaction of propene with cobalt metal particles resulted in its rapid autohydrogenation. The reaction consists in a part of the propene being dehydrogenated to surface carbon and CH_x chemisorbed species; hydrogen atoms released in the course of propene dehydrogenation are then involved in hydrogenation of remaining propene molecules to propane at 323–423 K or in propene hydrogenolysis to methane and ethane at temperatures higher than 423 K. The catalyst characterization suggests that propene chemisorption over cobalt catalysts is primarily a function of the density of cobalt surface metal sites. A correlation between propene chemisorption and Fischer–Tropsch reaction rate was observed over a series of cobalt silica-supported catalysts. No propane chemisorption was observed at 323–373 K over cobalt silica-supported catalysts. Propane autohydrogenolysis was found to proceed at higher temperatures, with methane being the major product of this reaction over cobalt catalysts. Hydrogen for propane autohydrogenolysis is probably provided by adsorbed CH_x species formed via propane dehydrogenation. Propene and propane chemisorption is dramatically reduced upon the catalyst exposure to synthesis gas (H₂/CO = 2) at 323–473 K. Our results suggest that cobalt metal particles are probably completely covered by carbon monoxide molecules under the conditions similar to Fischer–Tropsch synthesis and thus, most of cobalt surface sites are not available for propene and propane chemisorption.     

The formation of Co2C species in activated carbon supported cobalt-based catalysts and its impact on Fischer–Tropsch reaction

The cobalt carbide (Co₂C) species was formed in some activated carbon supported cobalt-based (Co/AC) catalysts during the activation of catalysts. It was found that the activity of Fischer–Tropsch reaction over Co-based catalysts decreased due to the formation of cobalt carbide species. Some promoters and pretreatment of activated carbon with steam could restrain the formation of cobalt carbide.     

Partial oxidation of methane to synthesis gas over α-Al2O3-supported bimetallic Pt–Co catalysts

The partial oxidation of methane to synthesis gas was studied at atmospheric pressure and in the temperature range of 550–800°C over -Al₂O₃-supported bimetallic Pt–Co, and monometallic Pt and Co catalysts, respectively. Both methane conversion and CO selectivity over a bimetallic Pt_0.5Co₁ catalyst were higher than those over monometallic Pt_0.5 and Co₁ catalysts. Furthermore, the addition of platinum in Pt–Co bimetallic catalysts effectively improved their resistance to carbon deposition with no coking occurring on Pt_0.5Co₁ during 80 h reaction. The FTIR study of CO adsorption observed only linearly bonded CO on bimetallic Pt–Co catalysts. TPR and XPS showed enhanced formation of a cobalt surface phase (CSP) in bimetallic Pt–Co catalysts. The origins of the good coking resistivity of bimetallic Pt–Co catalysts were discussed.     

Reducibility and CO hydrogenation over Pt and Pt-Co bimetallic catalysts encaged in NaY-zeolite

Temperature programmed techniques (TPR, TPD) and X-ray diffraction (XRD) have been used to study ion migration and location as well as reducibility of platinum and cobalt ions encapsulated in Pt/NaY, Co/NaY and Pt-Co/NaY zeolites prepared by ion exchange. The temperature required to reduce Co²⁺ in NaY was significantly lowered by the presence of Pt and dependent upon the relative locations of Pt and Co ions in zeolite cages. The exact location was controlled by the calcination condition and the metal contents. For bimetallic catalyst with low Pt content (0.5 wt% Pt and 0.9 wt% Co), the TPR results indicated that reduction of Co²⁺ ions in the vicinity of Pt shifted toward lower temperature, while that of Co²⁺ staying alone was not affected. With high Pt loading (4.5 wt% Pt, 0.7 and 2.6 wt% Co), however, most of the Co²⁺ ions were reduced by means of Pt at temperature below 723 K after calcination at 573 K. The temperature for Pt reduction in bimetallic catalysts was somewhat higher than Pt/NaY and increased with Co atomic fraction, indicating that mixed oxide, PtCo _x O _y , might be formed during calcination. After reduction in hydrogen at 723 K, highly dispersed metal particles were formed. These fine particles were most probably confined inside zeolite cages as indicated by the absence of XRD peak for all samples after calcination and reduction. Surface composition of the bimetallic particles may be different for catalysts with similar Pt content but different Co loading. Accordingly, H/Pt ratios of 1.0 and 0.72 for catalysts with low and high Co content, respectively, were shown by hydrogen chemisorption. It was further supported by the increase in TPD peak intensity with Co loading in the high temperature range, which was related to the reoxidation of Co in bimetallic particles by surface hydroxyl groups. Preliminary results on CO hydrogenation demonstrated that activity and methanol selectivity were higher on Pt-Co bimetallic catalysts than either over monometallic Pt or Co catalyst, which was consistent with the Pt enhanced Co reduction and formation of Pt-Co bimetallic particles.     

Ruthenium promotion of Co/SBA-15 catalysts with high cobalt loading for Fischer–Tropsch synthesis

30 wt.%Co/SBA-15 catalysts with different ruthenium contents (0.05–0.5 wt.%) were prepared by incipient wetness impregnation and characterized by diffuse reflectance infrared fourier transform spectroscopy, N₂ adsorption-desorption, X-ray diffractometry, temperature-programmed reduction and H₂ desorption, oxygen titration as well as X-ray photoelectron spectroscopy. The addition of a small amount of Ru promoter to Co/SBA-15 shifted the reduction temperature of both steps (Co₃O₄ → CoO and CoO → Co⁰) to lower temperatures and suppressed the formation of Co²⁺ species. After reduction, ruthenium atoms were encapsulated partially with cobalt cluster. There was no strong electronic interaction between metal cobalt and ruthenium, however, hydrogen spillover from ruthenium to cobalt oxide clusters occurred. With increasing ruthenium content, catalyst reducibility increased and the surface was enriched in cobalt atoms. Moreover, the peak intensities of both the linear and bridge types CO adsorption increased with the increase of ruthenium content, enhancing the catalytic activity on Fischer–Tropsch synthesis.     

Characterization of Silica-Supported Cobalt Catalysts Prepared by Decomposition of Nitrates Using Dielectric-Barrier Discharge Plasma

Abstract  

Low temperature decomposition of precursors usually leads to higher cobalt dispersion. In this study, we present a method to decompose cobalt precursors by using dielectric-barrier discharge (DBD) plasma without requiring a thermal calcination process. Cobalt (Co) catalysts prepared by DBD plasma were characterized by a range of techniques. The results indicate that the DBD decomposition method can not only reduce the decomposition time but also achieve an increased Co dispersion, small Co₃O₄ cluster size and uniform distribution compared to traditional calcination method. It was observed that the DBD-treated catalysts performed well in Fischer–Tropsch synthesis and were favorable for heavy hydrocarbon formation.     

Fischer–Tropsch synthesis on mono- and bimetallic Co and Fe catalysts supported on carbon nanotubes

An extensive study of Fischer–Tropsch synthesis (FTS) on carbon nanotubes (CNTs)-supported bimetallic cobalt/iron catalysts is reported. Up to 4 wt.% of iron is added to the 10 wt.% Co/CNT catalyst by co-impregnation. The physico-chemical properties, FTS activity and selectivity of the bimetallic catalysts were analyzed and compared with those of 10 wt.% monometallic cobalt and iron catalysts at similar operating conditions (H₂/CO = 2:1 molar ratio, P = 2 MPa and T = 220 °C). The metal particles were distributed inside the tubes and the rest on the outer surface of the CNTs. For iron loadings higher than 2 wt.%, Co–Fe alloy was revealed by X-ray diffraction (XRD) techniques. 0.5 wt.% of Fe enhanced the reducibility and dispersion of the cobalt catalyst by 19 and 32.8%, respectively. Among the catalysts studied, cobalt catalyst with 0.5% Fe showed the highest FTS reaction rate and percentage CO conversion. The monometallic iron catalyst showed the minimum FTS and maximum water–gas shift (WGS) rates. The monometallic cobalt catalyst exhibited high selectivity (85.1%) toward C₅+ liquid hydrocarbons, while addition of small amounts of iron did not significantly change the product selectivity. Monometallic iron catalyst showed the lowest selectivity for 46.7% to C₅+ hydrocarbons. The olefin to paraffin ratio in the FTS products increased with the addition of iron, and monometallic iron catalyst exhibited maximum olefin to paraffin ratio of 1.95. The bimetallic Co–Fe/CNT catalysts proved to be attractive in terms of alcohol formation. The introduction of 4 wt.% iron in the cobalt catalyst increased the alcohol selectivity from 2.3 to 26.3%. The Co–Fe alloys appear to be responsible for the high selectivity toward alcohol formation.     

Mesoporous silica–alumina as support for Pt and Pt–Mo sulfide catalysts: Effect of Pt loading on activity and selectivity in HDS and HDN of model compounds

The potential of mesoporous silica–alumina (MSA) material as support for the preparation of sulfided Pt and Pt–Mo catalysts of varying Pt loadings was studied. The catalysts were characterized by their texture, hydrogen adsorption, transmission electron microscopy, temperature programmed reduction (TPR) and by activity in simultaneous hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyridine. Sulfided Pt/MSA catalysts with 1.3 and 2 wt.% Pt showed almost the same HDS and higher HDN activities per weight amounts as conventional CoMo and NiMo/Al₂O₃, respectively. The addition of Pt to sulfided Mo/MSA led to promotion in HDS and HDN with an optimal promoter content close to 0.5 wt.%. The results of TPR showed strong positive effect of Pt on reducibility of the MoS₂ phase which obviously reflects in higher activity of the promoted catalysts. The activity of the MSA-supported Pt–Mo catalyst containing 0.5 wt.% Pt was significantly higher than the activity of alumina-supported Pt–Mo catalyst. Generally, Pt–Mo/MSA catalysts promoted by 0.3–2.3 wt.% Pt showed lower HDS and much higher HDN activities as compared to weight amounts of CoMo and NiMo/Al₂O₃. It is proposed that thiophene HDS and pyridine hydrogenation proceed over Pt/MSA and the majority of Pt–Mo/MSA catalysts on the same type of catalytic sites, which are associated with sulfided Pt and MoS₂ phases. On the contrary, piperidine hydrogenolysis takes place on different sites, most likely on metallic Pt fraction or sites created by abstraction of sulfur from MoS₂ in the presence of Pt.     

Calcination of Co-Based Fischer–Tropsch Synthesis Catalysts

The calcination of Co-based slurry-phase Fischer–Tropsch synthesis catalysts was investigated. Fischer–Tropsch synthesis is part of the gas-to-liquids (GTL) process that produces gas oil and naphtha from natural gas. For the GTL process, the preparation of highly active Co-based catalysts is of utmost importance. This paper shows that the conditions during the calcination of impregnated cobalt precursors have a significant influence on the performance of the final catalyst. The options of calcination in rotary kilns, furnaces and fluidized-bed reactors were considered. It was found that the catalyst performance is strongly dependent on the heating rate and the air-space velocity during the preferred option of fluidized bed calcination. The postulation that Co₃O₄ is not the preferred oxide phase of the calcined intermediate catalyst is supported by a temperature-programmed reduction (TPR) study.     

Strong dependence on pressure of the performance of a Co/SiO2 catalyst in Fischer–Tropsch slurry reactor synthesis

A 10 wt% Co/SiO₂ catalyst was prepared by the incipient wet-impregnation method and tested in Fischer–Tropsch synthesis in a slurry reactor under conditions approaching industrial practice. The catalyst precursor was investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and photoelectron spectroscopy (XPS). The XPS and XRD techniques revealed the presence of a crystalline Co₃O₄ spinel-type phase, while-in addition-TEM and XPS analyses pointed to the formation of another amorphous Co₃O₄ spinel phase, both species interacting weakly with the silica substrate. The influence of total pressure on the conversion, selectivity and stability of the catalyst was studied. Upon increasing the overall pressure from 20 to 40 bar, not only activity increased but also the catalyst are not deactivating. These results are explained in terms of an increase of gases solubility in the solvent, this increment of CO concentration in the liquid phase favours carbonyl species formation and the cobalt particles segregation that implies an increase in the metal surface area.     

Effect of CO2 in the feed stream on the deactivation of Co/γ-Al2O3 Fischer–Tropsch catalyst

The influence of CO₂ on the deactivation of Co/γ-Al₂O₃ Fischer–Tropsch (FT) catalyst in CO hydrogenation has been investigated. The presence of CO₂ in the feed stream reveals a negative effect on catalyst stability and in the formation of heavy hydrocarbons. The CO₂ acts as a mild oxidizing agent on cobalt metal during Fischer–Tropsch synthesis. During FT synthesis on Co/γ-Al₂O₃ of 70 h, the CO conversion and C₅₊ selectivity in the presence of CO₂ decreased more significantly than in the absence of CO₂. CO₂ is found to be responsible for the partial oxidation of surface cobalt metal at FT synthesis environment with the co-existence of generated water.     

Characterization of Fischer–Tropsch Cobalt-Based Catalytic Systems (Co/SiO<Subscript>2</Subscript> and Co/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) by X-ray Diffraction and Magnetic Measurements

Results of the characterization of six Co-based Fischer–Tropsch (FT) catalysts, with 15% Co loading and supported on SiO₂ and Al₂O₃, are presented. Room temperature X-ray diffraction (XRD), temperature and magnetic field (H) variation of the magnetization (M), and low-temperature (5 K) electron magnetic resonance (EMR) are used for determining the electronic states (Co⁰, CoO, Co₃O₄, Co²⁺) of cobalt. Performance of these catalysts for FT synthesis is tested at reaction temperature of 240 °C and pressure of 20 bars. Under these conditions, 15% Co/SiO₂ catalysts yield higher CO and syngas conversions with higher methane selectivity than 15% Co/Al₂O₃ catalysts. Conversely the Al₂O₃ supported catalysts gave much higher selectivity towards olefins than Co/SiO₂. These results yield the correlation that the presence of Co₃O₄ yield higher methane selectivity whereas the presence of Co²⁺ species yields lower methane selectivity but higher olefin selectivity. The activities and selectivities are found to be stable for 55 h on-stream.     

Cobalt supported on carbon nanotubes — A promising novel Fischer–Tropsch synthesis catalyst

An extensive study of Fischer–Tropsch synthesis (FTS) on carbon nanotubes (CNT) supported and γ–alumina-supported cobalt catalysts with different amounts of cobalt are reported. Up to 40 wt.% of cobalt is added to the supports by the impregnation method. The effect of the support on the reducibility of the cobalt oxide species, dispersion of the cobalt, average cobalt clusters size, water–gas shift (WGS) activity and activity and selectivity of FTS is investigated. Using carbon nanotubes as cobalt catalyst support was found to cause the reduction temperature of cobalt oxide species to shift to lower temperatures. The strong metal-support interactions are reduced to a large extent and the reducibility of the catalysts improved significantly. CNT aided in well dispersion of metal clusters and average cobalt clusters size decreased. Results are presented showing that the hydrocarbon yield obtained by inventive CNT supported cobalt catalyst is surprisingly much larger than that obtained from cobalt on alumina supports. The maximum concentration of active surface Co° sites and FTS activity for alumina and CNT supported catalysts are achieved at 34 wt.% and 40 wt.% cobalt loading respectively. CNT caused a slight decrease in the FTS product distribution to lower molecular weight hydrocarbons.     

An experimental investigation of Fischer–Tropsch synthesis over washcoated metallic structured supports

In principle, the application of monolithic catalysts to the Fischer–Tropsch synthesis can solve many of the problems related to the classical Fischer–Tropsch reactors, in particular concerning the necessity to operate with short diffusion distances and low pressure drops, preferably according to the ideal plug-flow behavior, while still maintaining a reasonable inventory of catalytic material in the reactor volume.The preparation of prototype cobalt-based catalysts, washcoated onto metallic structured supports with different geometries, is described herein, together with the evaluation of the catalytic properties of such systems in the Fischer–Tropsch synthesis at industrially relevant process conditions (220–235 °C, 20 bar, 2.1  mol_H2/mol_CO,  5000 cm³(STP)_CO+H2/h/g_cat). Comparative tests with the same catalyst in the powdered form were also carried out at the same process conditions.It was found that the structured catalysts maintained the activity and the selectivity of the original powdered catalyst, provided that the washcoat thickness is sufficiently thin.     

Optimization of the pretreatment procedure in the design of cobalt silica supported Fischer–Tropsch catalysts

The effect of cobalt precursor, catalyst pretreatment and promotion with ruthenium and rhenium on the formation of cobalt metal nanoparticles and catalytic performance of supported Fischer–Tropsch (FT) catalysts was studied using a combination of techniques (DSC–TGA, UV–vis spectroscopy, XPS, XRD, EXAFS–XANES, in situ magnetization measurements, propene chemisorption and catalytic measurements). The cobalt promoted and unpromoted catalysts were prepared by aqueous co-impregnation using cobalt nitrate or acetate, ruthenium nitrosyl nitrate or perrhenic acid. In both unpromoted and Ru and Re-promoted cobalt catalysts after impregnation and drying, cobalt is present mainly in octahedrally coordinated complexes. The repartition of cobalt species between Co₃O₄ and cobalt silicate depends essentially on the exothermicity of precursor decomposition. Cobalt nitrate precursor, with an endothermic decomposition, favors Co₃O₄ crystallites. Lower temperature of cobalt nitrate decomposition and catalyst calcination generally leads to higher dispersion of supported cobalt oxide. Cobalt acetate precursor, with an exothermic decomposition, favors cobalt silicate. By optimizing the conditions of cobalt acetate decomposition, the fraction of cobalt silicate can be decreased favoring a more reducible Co₃O₄ phase. For the catalysts prepared from cobalt nitrate, promotion with ruthenium increases the cobalt dispersion, while maintaining high reducibility. For the catalyst prepared via low temperature decomposition of cobalt acetate, addition of ruthenium increases the fraction of Co₃O₄ crystalline phase and decreases the concentration of barely reducible cobalt silicate. The Fischer–Tropsch reaction rates over unpromoted and promoted cobalt catalysts were found to be primarily a function of the number of cobalt metal sites, which are generated by the reduction of Co₃O₄ crystallites.     

Structure and catalytic performance of Pt-promoted alumina-supported cobalt catalysts under realistic conditions of Fischer–Tropsch synthesis

The structure of alumina-supported cobalt catalysts promoted with platinum and their catalytic performance in Fischer–Tropsch synthesis were investigated under realistic reaction conditions (P = 20 bar, T = 493 K) using in situ time-resolved X-ray diffraction with simultaneous analysis of reaction products. The catalysts were prepared via incipient wetness impregnation and characterized by a wide range of ex situ techniques. Direct in situ measurements were indicative of considerable versatility of alumina-supported cobalt catalysts during Fischer–Tropsch synthesis. Cobalt sintering occurred at the first hours of the reaction and resulted in a significant drop of the catalytic activity. In addition to sintering, partially oxidized catalysts containing smaller cobalt particles (mean particle size <5 nm) were slowly reducing during Fischer–Tropsch reaction. Treatment of cobalt catalysts in pure carbon monoxide led to selective transformation of cobalt metallic phases to Co₂C cobalt carbide. Cobalt carbidization followed by hydrogenation selectively led to cobalt hcp metallic phase, which seems to be more active in Fischer–Tropsch synthesis than cobalt fcc phase. Cobalt oxidation by water was not significant in the catalysts with metal particles larger than 5 nm even at high water concentrations.     

Comparative Study on Catalytic Performances for Low-temperature CO Oxidation of Cu–Ce–O and Cu–Co–Ce–O Catalysts

Two series of Cu–Ce–O and Cu–Co–Ce–O catalysts were prepared by co-precipitation method. The prepared catalysts were characterized by XRD, IR, TPR, XPS, BET and ICP-AES. The catalytic activities of the catalysts for low-temperature CO oxidation were evaluated through a microreactor-GC system. TPR results indicate that the addition of cobalt to the Cu–Ce–O can increase the dispersion of copper oxide, and the interaction between cobalt and copper can enhance the reducibility of each other. XPS analysis show that Ce⁴⁺, Cu²⁺, along with Co₃O₄, are present on the surface of Cu_0.4Co_0.6Ce₄ catalyst. The Co/Cu atomic ratio and the calcination temperature have significant effect on the activities of the catalysts. Compared with Cu₁Ce₄ catalyst, the Cu_0.4Co_0.6Ce₄ catalyst has better activity and thermal stability.