Fischer–Tropsch synthesis. Effect of CO pretreatment on a ruthenium promoted Co/TiO2

The effect of pretreatment, using hydrogen or carbon monoxide, on the activity and selectivity of a ruthenium promoted cobalt catalyst (Ru(0.20 wt%)/Co(10 wt%)/TiO₂) during Fischer–Tropsch (FT) synthesis was studied in a continuous-stirred tank reactor (CSTR). The hydrogen reduced catalyst exhibited a high initial synthesis gas conversion (72.5%) and reached steady state after 40 h on stream, after which the catalyst deactivated slightly with time on stream. The carbon monoxide reduced catalyst reached steady state quickly and showed a lower activity and a good stability. Methane selectivity on the carbon monoxide reduced catalyst was 15–20% (carbon base), much higher than that on the hydrogen reduced catalyst (5–10%). Carbon monoxide regeneration increased the activity on the hydrogen reduced catalyst; however, it did not have significant effect on the carbon monoxide reduced catalyst.     

Fischer–Tropsch synthesis over cobalt dispersed on carbon nanotubes-based supports and activated carbon

Carbon nanotubes (CNTs) and the ones grown on MgO and alumina are used as supports for cobalt catalyst in Fischer–Tropsch (FT) synthesis. Carbon nanotubes were synthesized by chemical vapor deposition of methane on 5.0 wt.% iron on MgO or alumina at 950 °C. The carbon nanotubes were characterized by SEM and TEM microscopy and Raman spectroscopy. Cobalt nitrate was impregnated onto the supports by impregnation, and the samples were dried and reduced in-situ at 400 °C for 12 h, and then FT synthesis was carried out in a fixed-bed reactor. The catalysts were characterized by BET surface area measurement, TPR and TPD. The effect of carbon nanotubes as cobalt support on CO conversion, product selectivity, and olefin to paraffin ratio of FT synthesis was investigated and compared with activated carbon as well as Al₂O₃, as a traditional support. The results revealed that the activity of the Co/CNT catalyst was improved by 22%, compared to the conventional Co/alumina catalysts. Also the cobalt supported on CNTs grown on MgO (Co/CNT–MgO) shows the highest selectivity to C₅₊ as the most desired FTS products. The C₅₊ selectivity enhancement was about 37, 34, 17, and 77% as compared to the Co/CNT, Co/alumina, Co/CNTs-alumina, and Co/activated carbon, respectively. Also the olefin/paraffin ratio on the Co/CNTs-MgO catalyst is about 7.7 times higher than the conventional cobalt catalysts. It seems that the degree of reduction of cobalt is higher when supported on CNTs than on alumina. This leads to higher FTS activity. Also, the particle size distribution of the cobalt is affected by the CNT–MgO support leading to higher C₅₊ selectivity.     

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.     

The interaction of cobalt species with alumina on Co/Al2O3 catalysts prepared by atomic layer deposition

Alumina supported cobalt catalysts were prepared by atomic layer deposition (ALD) of cobalt acetylacetonate precursors (Co(acac)₂ and Co(acac)₃). The main modes of interaction between the acetylacetonate precursors and the support were found to be the exchange reaction between the alumina OH-groups and the acac-ligands of the precursor and dissociative adsorption on coordinatively unsaturated Al³⁺ sites. The amount of precursor that could adsorb on the support was determined by steric hindrance. Samples were prepared using 1–5 reaction cycles, i.e. subsequent precursor addition (Co(acac)₂) and calcination, resulting in catalysts containing ca. 3–10 wt.% Co. Samples were also prepared where the last calcination step was omitted, i.e. uncalcined catalysts. Calcination at 450 °C decreased the reducibility of the Co(acac)₂/Al₂O₃ catalysts due to formation of a cobalt oxide phase strongly interacting with the support and aluminate type surface species. The reducibility increased with metal loading on both calcined and uncalcined catalysts; however the reducibility of the calcined catalysts remained lower than of the uncalcined ones. The dispersion was found to be lower on the calcined catalysts. The cobalt particle sizes on the calcined samples was ca. 8 nm and on the uncalcined 4–5 nm, for cobalt loadings of ca. 6–10 wt.%. Catalytic activity was tested by gas phase hydrogenation of toluene in temperature programmed mode (30–150 °C).     

Fischer–Tropsch Synthesis over Ordered Mesoporous Carbon Supported Cobalt Catalysts: The Role of Amount of Carbon Precursor in Catalytic Performance

Abstract  

Ordered mesoporous carbon supported cobalt-based catalysts (Co/MC) were synthesized via incipient wetness impregnation with different amounts of furfuryl alcohol (FA) as carbon precursor. The characterizations of obtained Co/MC were subjected to N₂ adsorption, XRD, XPS, TEM, H₂-TPR, H₂-TPD and H₂-TPSR. The results indicate that the reducibility and dispersion of Co active species vary significantly due to the difference of FA amount. By simply tuning the FA content from 25 to 100 wt%, the reduction temperature of deriving metallic Co shifts gradually to lower. The catalytic performance of Co/MC was evaluated for Fischer–Tropsch (FT) synthesis. The observed FT activity exhibits a volcano-type curve with the amount of FA due to the effect of both reducibility and dispersion of active species. As the precursor concentration overweighs 50 wt%, the ability of CO to dissociate over the active surface and the selectivity to the C₅₊ products level off after experiencing an initial increase. Substantially, the catalysts with higher concentration of FA render the larger crystallites having an average size of more than 6 nm, which facilitates the CO hydrogenation by way of carbon chain propagation. It seems that the sample with FA content of 50 wt% is optimum in terms of FT activity and C₅₊ selectivity.     

Hydroformylation of olefins by Au/Co3O4 catalysts

The hydroformylation of olefins over supported gold catalysts in an autoclave reactor under mild conditions (100–140 °C, 3–5 MPa) has been studied. Over Au/AC (activated carbon), Au/PVP (polyvinylpyrrolidone), Au/Al₂O₃, Au/TiO₂, Au/Fe₂O₃, Au/ZnO, Au/CeO₂ and Co₃O₄, 1-olefin mainly remained unchanged and the major products were isomerized olefins or hydrogenated paraffin. In contrast, Au nanoparticles deposited on Co₃O₄ led to remarkably high catalytic activities in hydroformylation reaction with selectivities above 85% to desired aldehydes. The hydroformylation of olefins proceeds preferentially at temperatures below 140 °C, above which the reactions of olefins gradually shifted to isomerization and then to hydrogenation. It appeared that the activity and selectivity of hydroformylation reaction strongly depend on the molecular structure of olefins, which could be ascribed to steric constraints as internal olefins are relatively inappropriate to form alkyl group and subsequent acyl group by insertion of CO. The Au/Co₃O₄ catalyst can be recycled by simple decantation with slight decrease in catalytic activity along with an increase in recycle times, which is a great advantage over homogeneous catalysts. The role of gold nanoparticles can be assumed to dissociate hydrogen molecule into atomic species which reduce Co₃O₄ to Co metal under mild reaction conditions.     

NOx storage/reduction catalysts based in cobalt/copper hydrotalcites

The use of materials based on hydrotalcites as NO_x storage/reduction (NSR) catalysts has been investigated, examining their activity at low temperature and their resistance to poisons such as H₂O and SO₂. The results obtained show that catalysts derived from Mg/Al hydrotalcites containing copper or cobalt is active at low temperatures, specially the samples containing 10 or 15% of Co. The addition of 1 wt% of transition metals with redox properties such as Pt, Pd, V and Ru to the hydrotalcite increases its activity because the combination of the redox properties of these metals and the acid-base properties of the hydrotalcite. The best results were obtained with the catalyst derived from a hydrotalcite with a molar ratio Co/Mg/Al = 15/60/25 and containing 1 wt% V. This material shows a higher activity, at low temperatures and in the presence of H₂O and SO₂, than a Pt–Ba/Al₂O₃ reference catalyst.     

Synthesis and catalytic properties of eggshell cobalt catalysts for the Fischer-Tropsch synthesis

CO diffusional restrictions decrease C₅₊ synthesis rates and selectivity within large (1–3 mm) catalyst pellets often required in Fischer-Tropsch (FT) synthesis reactors. Eggshell catalysts, where Co is located preferentially near outer pellet surfaces, reduce the severity of these transport restrictions and lead to higher synthesis rates and C₅₊ selectivity. Maximum C₅₊ selectivities occur on catalysts with intermediate shell thickness, within which transport restrictions limit the removal of reactive olefins but not the arrival of reactants at catalytic sites. A new synthetic technique leads to sharp distributions of active sites near outer pellet surfaces by controlling the rate of imbibition of cobalt nitrate melts. Also, slow reduction of the impregnated salt leads to moderate Co dispersions (0.05–0.10) even at high local Co loadings present within shell regions.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

Preparation of Pdshell–Aucore/SiO2 catalyst and catalytic activity for acetylene hydrogenation

Various Pd shell thicknesses (0.12–1.5 nm) were synthesized on preformed Au particles of 5 nm size by seeded growth technique (15–80 at% Pd) using sodium citrate and tannic acid. The sols were characterized by UV–vis spectroscopy, TEM and high-resolution TEM (HRTEM) measurements. HRTEM confirmed the pseudomorphic growth of the Pd shell on the Au core. The Au/Pd core/shell particles were fixed onto SiO₂ (Aerosil 200) support by PDDA polycation. The catalytic activity in acetylene hydrogenation and selectivity of competition between acetylene and propene were tested after O₂ and H₂ pretreatment. The samples even in “as prepared” state hydrogenated acetylene. The thin Pd layer (15–30 at% Pd) on Au provided higher hydrogenation activity than the thick Pd shell. However, thermal treatment of the samples in H₂ stream causing Au/Pd intermixing shifted the activity maximum to higher Pd concentration (68–80 at% Pd). Comparison of the TOF (1/s) and selectivity values allowed us to conclude that the homogenized particle with 68–80 at% Pd shows better hydrogenation activity and selectivity than the thin Pd shell (15–30 at% Pd) on the Au core.     

New carbon composites containing ultrafine metal particles. 2. Synthesis by pyrolisis of pitch-organometallic blends

Air-stable new carbon composites containing uniformly dispersed ultrafine Fe, Co, Ni, Ru, Pd, Rh, or Ag particles were obtained by thermal degradation of coal pitches mixed homogeneously with 1–10% of poly(vinyl ferrocene), cobaltocene, nickelocene, (acenaphthylene) Ru₃(CO)₇, PdCl₂(COD), RhCp(COD), and AgC₆H₄CH₂NMe₂, respectively, at 400–1200°C in Ar. Carbonization yields are 45–55% and the size of metal particles varied from 5 to 65 nm depending upon the treatment temperature and identity of the metal. The carbon composites containing Fe particles showed high Vicat hardness and good electrical conductivity. Pd- and Rh-dispersed materials exhibited good catalysis in hydrogenation of 1-hexene. The composite containing ultrafine Ag particles showed excellent bacteriostatic activity forEscherichia coli, Pseudomonas aeruginosa, andBatchillus subtilis.     

Effect of Ru addition to Co/SiO2/HZSM-5 catalysts on Fischer–Tropsch synthesis of gasoline-range hydrocarbons

Microporous HZSM-5 zeolite and mesoporous SiO₂ supported Ru–Co catalysts of various Ru adding amounts were prepared and evaluated for Fischer–Tropsch synthesis (FTS) of gasoline-range hydrocarbons (C₅–C₁₂). The tailor-made Ru–Co/SiO₂/HZSM-5 catalysts possessed both micro- and mesopores, which accelerated hydrocracking/hydroisomerization of long-chain products and provided quick mass transfer channels respectively during FTS. In the same time, Ru increased Co reduction degree by hydrogen spillover, thus CO conversion of 62.8% and gasoline-range hydrocarbon selectivity of 47%, including more than 14% isoparaffins, were achieved simultaneously when Ru content was optimized at 1 wt% in Ru–Co/SiO₂/HZSM-5 catalyst.     

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.     

Electroreduction of oxygen on carbon-supported gold catalysts

The electrochemical reduction of oxygen was studied on Au/C catalysts (20 and 30 wt%) in 0.5 M H₂SO₄ and 0.1 M KOH solutions using the rotating disk electrode (RDE) method. The thickness of the Au/C–Nafion layers was varied between 1.5 and 10 μm. The specific activity of Au was independent of catalyst loading in both solutions, indicating that the transport of reactants through the catalyst layer does not limit the process of oxygen reduction under these conditions. The mass activity of 20 wt% Au/C catalysts was higher due to smaller particle size. The number of electrons involved in the reaction and the Tafel slopes were found; the values of these parameters are similar to that of bulk polycrystalline gold and indicate that the mechanism of O₂ reduction is not affected by carbon support or the catalyst configuration.     

Highly active Fischer–Tropsch synthesis Co/SiO2 catalysts prepared from microwave irradiation

Studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as energy-diffusive X-ray spectroscopy (EDX), a highly active Co/SiO₂ catalyst which was prepared through microwave irradiation showed more uniform and better dispersion of the Co particles within the catalyst pellets, compared to that prepared by the conventional heating method. The Fischer–Tropsch (FT) synthesis activity of the microwave-irradiated catalyst was greater than that of the conventional heating one. Not only did the catalytic activity depend on the particle distribution, but also the irradiative time. The longer irradiative time increased the catalytic activity. It was shown that the irradiative time of 14 min was an optimum. It was found that cobalt distributed uniformly inside the whole catalyst pellet prepared by the microwave irradiation. On the contrary, cobalt concentration at the outer surface, with agglomerated form, was greater than that at the center or inner part of the catalyst pellet made by the conventional heating method. Higher dispersion state of the supported cobalt, as also proved by XRD, realized at the microwave irradiated catalyst determined its improved FT activity.     

Catalytic behavior of hybrid Co/SiO2-(medium-pore) zeolite catalysts during the one-stage conversion of syngas to gasoline

The catalytic properties of 10-MR (membered ring) zeolites (ZSM-5, MCM-22, IM-5, ITQ-2, all with a similar Si/Al ratio of ca. 15) in hybrid Co/SiO₂-zeolite catalysts for the direct conversion of syngas to mainly high-octane gasoline-range hydrocarbons has been studied under typical Fischer-Tropsch (FT) conditions: 250 °C, 2.0 MPa, and H₂/CO = 2. Special emphasis has been given to the deactivation behavior and the characterization of the amount and nature of the carbonaceous deposits formed by a combination of techniques (elemental analysis, TGA (thermogravimetric analyses), GC–MS, and DR (diffuse reflectance) UV–vis spectroscopy). The presence of the medium-pore zeolite increases the gasoline yield by about 20–50%, depending on the particular zeolite, and enhances the formation of branched products with respect to the base Co/SiO₂ catalyst, which is explained by the promotion of isomerization and cracking of long-chain (C₁₃₊) n-paraffins formed on the FT component. The initial zeolite activity is mostly determined by the surface acidity rather than by the total amount of Brønsted acid sites, pointing out to the existence of limitations for the diffusion of the long-chain n-paraffins through the 10-MR channels under FT conditions. Thus, ITQ-2 bearing the largest surface area presents the highest initial yield of branched gasoline-range products, followed by ZSM-5, IM-5, and MCM-22. All zeolites experience a loss of activity with TOS, particularly during the initial reaction stages. This deactivation is governed by the morphological and structural properties of the zeolite, which finally determine the amount and location of the coke species, and not by the acidity.     

Strontium carbonate supported cobalt catalyst for dry reforming of methane under pressure

Catalytic performance of Co–SrO catalyst for dry reforming of methane was investigated at 1 MPa, 1023 K. The catalyst prepared by oxalate co-precipitation method or citric acid method showed a steady activity for dry reforming of methane under pressure. The importance and stability of cobalt metal with strontium carbonate were suggested for the Co–SrO catalyst, and thus it should be denoted as Co–SrCO₃. In addition, cobalt supported on strontium carbonate prepared by impregnation method (Co/SrCO₃) showed the comparable activity with high tolerance to oxidative atmosphere under reaction conditions.     

Ethylene epoxidation in low-temperature AC corona discharge over Ag catalyst: Effect of promoter

In this work, the epoxidation of ethylene using a low-temperature corona discharge system was investigated with various reported catalytically active catalysts: Ag/α-Al₂O₃, Cs–Ag/α-Al₂O₃, Cu–Ag/α-Al₂O₃, and Au–Ag/α-Al₂O₃. It was experimentally found that the investigated catalysts could improve the ethylene conversion and the ethylene oxide (EO) yield and selectivity for the corona discharge system, particularly 1 wt.% Cs–12.5 wt.% Ag/α-Al₂O₃ and 0.2 wt.% Au–12.5 wt.% Ag/α-Al₂O₃. The power consumption per EO molecule produced in the corona discharge system, combined with the superior bimetallic catalysts, was much lower than that of the sole corona discharge system and that of the corona discharge system combined with the monometallic Ag catalyst.     

Cathode catalysts for fuel cell development: A theoretical study based on band structure calculations for tungsten nitride and cobalt tungsten nitrides

Band structure calculations were performed for tungsten nitride, cobalt tungsten nitrides, and platinum slabs. The major requirements for the development of a superior cathode catalyst are: (1) that the Fermi level of the cathode catalyst is close to the energy level of the lowest unoccupied molecular orbital of O₂, the lowest unoccupied atomic orbital of an oxygen atom, and the lowest unoccupied atomic orbital of a hydrogen atom so that they can readily interact with one another; and (2) that the cathode catalysts have smaller ΔE value which represent the difference between the Fermi level and the peak position of the density of states of the O_p orbital of O₂ adsorbed on the catalyst. The active site structures of cobalt tungsten nitrides for activation of the oxygen reduction reaction were found to have the surface structure of Co–O–Co, which lowered the unoccupied orbital of the oxygen atom to approximately that of the Fermi level. However, this structure concomitantly lowered the Fermi level, which resulted in an increase in ΔE. Consequently, the optimal cathode catalyst regarding the surface conformation contains a Co–O–Co structure that is dispersed on the surface of the cobalt tungsten nitride. The cobalt tungsten oxynitride exhibited a catalytic activity for the oxygen reduction reaction. A linear dependence is observed between the ΔE and the oxygen reduction reaction offset potentials of the tungsten nitride, cobalt tungsten nitride, cobalt tungsten oxynitride, and platinum.     

Low-temperature reforming of ethanol over CeO2-supported Ni-Rh bimetallic catalysts for hydrogen production

A new series of Ni-Rh bimetallic catalysts with different Ni and Rh loadings on a high-surface-area CeO₂ was developed for the reforming of bio-ethanol at low-temperature (below 450 °C) to produce H₂-rich gas for on-site or on-board fuel cell applications. Oxidative steam reforming of ethanol (OSRE) over a Ni-Rh/CeO₂ catalyst containing 5 wt% Ni and 1 wt% Rh was found to be more efficient offering about 100% ethanol conversion at 375 °C with high H₂ and CO₂ selectivity and low CO selectivity compared to the steam reforming of ethanol (SRE) reaction which required a higher temperature of about 450 °C to achieve 100% ethanol conversion. The high temperature SRE reaction favors the formation of large amount of CO, which would make the downsteam CO cleanup more complicated for polymer electrolyte membrane fuel cell (PEMFC). The presence of O₂ in the feed gas was found to greatly enhance the conversion of ethanol to produce H₂ and CO₂ as major products. Increase in Ni content above 5 wt% in the catalyst formulation decreased the H₂ selectivity while the selectivity of undesirable CH₄ and acetaldehyde increased. The 1 wt% Rh/CeO₂ catalyst was twice as active as 10 wt% Ni/CO₂ catalyst in terms of ethanol conversion and acetaldehyde selectivity and this indicated that Rh was more effective in C–C bond cleavage than Ni. The reaction was found to proceed through the formation of acetaldehyde intermediate, which subsequently underwent decomposition to produce a mixture of CO and CH₄ or reforming with H₂O and O₂ to produce CO, CO₂ and H₂. The role of Rh is mainly to cleave the C–C and C–H bonds of ethanol to produce H₂ and COx while Ni addition helps converting CO into CO₂ and H₂ by WGS reaction under the conditions employed.