Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

The oxidation of supported cobalt based slurry bed Fischer–Tropsch catalysts by means of water was studied. Water is one of the Fischer–Tropsch reaction products and can probably cause oxidation and deactivation of a reduced cobalt catalyst. Model experiments using Mössbauer emission spectroscopy and thermogravimetry as well as realistic Fischer–Tropsch synthesis runs were performed. It was demonstrated that Mössbauer emission spectroscopy can successfully be applied to the investigation of high cobalt loading Fischer–Tropsch catalysts. Strong indications were found that oxidation of reduced cobalt catalysts occurs under realistic Fischer–Tropsch conditions. Mössbauer emission spectroscopy and thermogravimetry results showed that the oxidation depends on the P_H2/P_H2O ratio, and that oxidation proceeds to less than complete extents under certain conditions. The formation of both reducible and less reducible cobalt oxide species was observed, and the relative ratio between these species depends on the severity of the oxidation conditions.     

Selective catalytic reduction of NO by C3H8 over CoOx/Al2O3: An investigation of structure–activity relationships

A series of CoO_x/Al₂O₃ catalysts was prepared, characterized, and applied for the selective catalytic reduction (SCR) of NO by C₃H₈. The results of XRD, UV–vis, IR, Far-IR and ESR characterizations of the catalysts suggest that the predominant oxidation state of cobalt species is +2 for the catalysts with low cobalt loading (≤2 mol%) and for the catalysts with 4 mol% cobalt loading prepared by sol–gel and co-precipitation. Co₃O₄ crystallites or agglomerates are the predominant species in the catalysts with high cobalt loading prepared by incipient wetness impregnation and solid dispersion. An optimized CoO_x/Al₂O₃ catalyst shows high activity in SCR of NO by C₃H₈ (100% conversion of NO at 723 K, GHSV: 10,000 h⁻¹). The activity of the selective catalytic reduction of NO by C₃H₈ increases with the increase of cobalt–alumina interactions in the catalysts. The influences of cobalt loading and catalyst preparation method on the catalytic performance suggest that tiny CoAl₂O₄ crystallites highly dispersed on alumina are responsible for the efficient catalytic reduction of NO, whereas Co₃O₄ crystallites catalyze the combustion of C₃H₈ only.     

Performance and characterization of Ru/Al2O3 and Ru/SiO2 catalysts modified with Mn for Fischer–Tropsch synthesis

Mn effect and characterization on γ-Al₂O₃-, -Al₂O₃- and SiO₂-supported Ru catalysts were investigated for Fischer–Tropsch synthesis under pressurized conditions. In the slurry phase Fischer–Tropsch reaction, γ-Al₂O₃ catalysts showed higher performance on CO conversion and C₅⁺ selectivity than -Al₂O₃ and SiO₂ catalysts. Moreover, Ru/Mn/γ-Al₂O₃ exhibited high resistance to catalyst deactivation and other catalysts were deactivated during the reaction. From characterization results on XRD, TPR, TEM, XPS and pore distribution, Ru particles were clearly observed over the catalysts, and γ-Al₂O₃ catalysts showed a moderate pore and particle size such as 8 nm, where -Al₂O₃ and SiO₂ showed highly dispersed ruthenium particles. The addition of Mn to γ-Al₂O₃ enhanced the removal of chloride from RuCl₃, which can lead to the formation of metallic Ru with moderate particle size, which would be an active site for Fischer–Tropsch reaction. Concomitantly, manganese chloride is formed. These schemes can be assigned to the stable nature of Ru/Mn/γ-Al₂O₃ catalyst.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).     

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.     

Silica supported cobalt Fischer–Tropsch catalysts: effect of pore diameter of support

The influence of the effect of average pore diameter of silica support on the physical and chemical properties of supported cobalt catalysts and their performance in the Fischer–Tropsch synthesis was investigated. Silicas with different mean pore diameter (20, 40, 60, 100 and 150 Å) were impregnated with cobalt nitrate to produce catalysts containing 20 wt.% cobalt. The metal crystallite size and degree of reduction was found to increase with increasing pore diameter of the support for supports with an average pore diameter larger or equal to 40 Å, and hence the dispersion decreased. In impregnated catalysts, the metal crystallites seems to appear in clusters on the support. With increasing average pore diameter, the size of these clusters increases. In the Fischer–Tropsch synthesis, the 100 Å supported catalyst proved to be the most active and selective catalyst for hydrocarbon formation. The C₅₊ and methane selectivity passed through a maximum and minimum at the 100 Å supported catalyst, respectively, which can be explained quantitatively using the reactant transport model proposed by Iglesia et al.     

Catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides for methane combustion: Influence of catalyst pretreatment temperature and oxygen concentration in the reaction mixture

The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration (λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co₃O₄/CeO₂ and Co₃O₄/CeO₂–ZrO₂ containing 30 wt.% of Co₃O₄. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples).

Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g⁻¹ h⁻¹). The catalytic performance of Co₃O₄/CeO₂ and Co₃O₄/CeO₂–ZrO₂ were compared with those of two pure Co₃O₄ oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co₃O₄/CeO₂ and Co₃O₄/CeO₂–ZrO₂ as well as to their reducibility. Particular attention was paid to the thermal stability of the Co₃O₄ phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co₃O₄ heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co₃O₄ decomposition into CoO was observed.

Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co₃O₄ and by promoting the reduction at low temperature.     

Modified Co3O4/ZrO2 catalysts for VOC emissions abatement

The catalytic activity study of cobalt oxides dispersed on different supports evidenced first the highest performances of zirconia based catalysts in the reaction of toluene oxidation. The influence of the presence of ethylenediamine (en) during the preparation of Co/ZrO₂ and the ZrO₂ support modification by Y₂O₃ were then studied and compared with reference catalyst prepared conventionally by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. Addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase on the catalytic activity of the activated solids in the toluene deep oxidation as compared with the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion but also in terms of Co–support interaction. The generated cobalt species were reducible at much lower temperatures and then were more active in the toluene total oxidation. Finally an efficient catalyst for VOC oxidation was produced combining the modifications of ZrO₂ by yttrium and of the precursor.     

Structural changes and activation treatment in a Co/SiO2 catalyst for Fischer–Tropsch synthesis

We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10 wt.% Co/SiO₂ catalyst prepared by wet impregnation was separately activated under H₂, CO or a H₂/CO mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H₂/CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer–Tropsch synthesis after the H₂/CO activation.     

Fischer–Tropsch synthesis on monolithic catalysts of different materials

Monolithic structures made of cordierite, γ-Al₂O₃ and steel have been prepared as catalysts and tested for Fischer–Tropsch activity. The monoliths made of cordierite and steel were washcoated with a 20 wt.% Co–1 wt.% Re/γ-Al₂O₃ Fischer–Tropsch catalyst whereas the γ-Al₂O₃ monoliths were made by direct impregnation with an aqueous solution of the Co and Re salts resulting in a loading of 12 wt.% Co and 0.5 wt.% Re. The activity and selectivity of the different monoliths were compared with the corresponding powder catalysts.

Higher washcoat loadings resulted in decreased C₅₊ selectivity and olefin/paraffin ratios due to increased transport limitations. The impregnated γ-Al₂O₃ monoliths also showed similar C₅₊ selectivities as powder catalysts of small particle size (38–53 μm). Lower activities were observed with the steel monoliths probably due to experimental problems.     

Comparison of preparation methods for carbon nanotubes supported iron Fischer–Tropsch catalysts

Carbon nanotubes supported iron catalysts were prepared by incipient wetness, deposition/precipitation using K₂CO₃, and deposition/precipitation using urea. The incipient wetness method and the deposition/precipitation technique using urea yielded highly dispersed Fe³⁺ on the carbon nanotubes support. The deposition/precipitation technique using K₂CO₃ also yielded larger Fe₂O₃-crystallites. After reduction the three catalysts had similar metal surface areas. Nevertheless, the activity of these catalysts in the Fischer–Tropsch synthesis differed significantly with the catalyst prepared by incipient wetness being the most active one. It is speculated that the differences in the performance of the catalysts might be attributed to the different crystallite size distributions, which would result in a variation in the amount of the different phases present in the catalyst under reaction conditions. The selectivity in the Fischer–Tropsch synthesis over the three catalysts seems to be independent of the method of preparation.     

CO and CO2 hydrogenation study on supported cobalt Fischer–Tropsch synthesis catalysts

The conversion of CO/H₂, CO₂/H₂ and (CO+CO₂)/H₂ mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO₂ hydrogenation is slow. For the cases of only CO or only CO₂ hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an of about 0.80; in contrast, the CO₂ hydrogenation products contained about 70% or more of methane. Thus, CO₂ and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO₂ even though the H₂O/H₂ ratio is at least two times larger for the conversion of CO₂. Since the catalyst ages more slowly in the presence of the higher H₂O/H₂ conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO.     

Additional effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts

The influence of the ZrO₂ support modification by Y₂O₃ and the presence of ethylenediamine (“en”) during the preparation of Co/ZrO₂ were studied and compared with a reference catalyst conventionally prepared by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. The effect of the en/Co molar ratio (x = 1–3) was studied. Activation of cobalt species was followed by differential thermal and thermogravimetric analyses (DTA/TG) analyses and by specific surface area measurements which evidence the complete cobalt precursor decomposition at 450 °C, whatever the support composition and the en/Co molar ratio. The addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase in the catalytic activity of the activated solids for the toluene deep oxidation as compared to the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion (X-ray diffraction (XRD)) and also in terms of Co-support interaction (H₂-temperature-programmed reduction (TPR)). The generated cobalt species were reducible at much lower temperatures and were more active in the toluene total oxidation. Finally, an efficient catalyst was produced combining the modifications of the support by yttrium oxide and of the precursor (use of ethylenediamine).     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.     

Promotion effect of residual K on the decomposition of N2O over cobalt–cerium mixed oxide catalyst

A series of cobalt–cerium mixed oxide catalysts (Co₃O₄–CeO₂) with a Ce/Co molar ratio of 0.05 were prepared by co-precipitation (with K₂CO₃ and KOH as the respective precipitant), impregnation, citrate, and direct evaporation methods and then tested for the catalytic decomposition of N₂O. XRD, BET, XPS, O₂-TPD and H₂-TPR methods were used to characterize the catalysts. Catalysts with a trace amount of residual K exhibited higher catalytic activities than those without. The presence of appropriate amount of K in Co₃O₄–CeO₂ may improve the redox property of Co₃O₄, which is important for the decomposition of N₂O. When the amount of K was constant, the surface area became the most important factor for the reaction. The co-precipitation-prepared catalyst with K₂CO₃ as precipitant exhibited the best catalytic performance because of the presence of ca. 2 mol% residual K and the high surface area. We also discussed the rate-determining step of the N₂O decomposition reaction over these Co₃O₄–CeO₂ catalysts.     

Co3O4/CeO2 composite oxides for methane emissions abatement: Relationship between Co3O4–CeO2 interaction and catalytic activity

Co₃O₄/CeO₂ composite oxides with different cobalt loading (5, 15, 30, 50, 70 wt.% as Co₃O₄) were prepared by co-precipitation method and investigated for the oxidation of methane under stoichiometric conditions. Pure oxides, Co₃O₄ and CeO₂ were used as reference. Characterization studies by X-ray diffraction (XRD), BET, temperature programmed reduction/oxidation (TPR/TPO) and X-ray photoelectron spectroscopy (XPS) were carried out.

An improvement of the catalytic activity and thermal stability of the composite oxides was observed with respect to pure Co₃O₄ in correspondence of Co₃O₄–CeO₂ containing 30% by weight of Co₃O₄. The combined effect of cobalt oxide and ceria, at this composition, strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co₃O₄ phase and promoting the efficiency of the Co³⁺–Co²⁺ redox couple. The presence in the sample Co₃O₄(30 wt.%)–CeO₂ of a high relative amount of Ce³⁺/(Ce⁴⁺ + Ce³⁺) as detected by XPS confirms the enhanced oxygen mobility.

The catalysts stability under reaction conditions was investigated by XRD and XPS analysis of the used samples, paying particular attention to the Co₃O₄ phase decomposition. Methane oxidation tests were performed over fresh (as prepared) and thermal aged samples (after ageing at 750 °C for 7 h, in furnace). The resistance to water vapour poisoning was evaluated for pure Co₃O₄ and Co₃O₄(30 wt.%)–CeO₂, performing the tests in the presence of 5 vol.% H₂O. A methane oxidation test upon hydrothermal ageing (flowing at 600 °C for 16 h a mixture 5 vol.% H₂O + 5 vol.%O₂ in He) of the Co₃O₄(30 wt.%)–CeO₂ sample was also performed. All the results confirm the superiority of this composite oxide.     

Steam reforming of ethanol over Co3O4/CeO2 catalysts prepared by different methods

In this paper, Co₃O₄/CeO₂ catalysts for steam reforming of ethanol (SRE) were prepared by co-precipitation and impregnation methods. The catalysts prepared by co-precipitation were very active and selective for SRE. Over 10%Co₃O₄/CeO₂ catalyst, ethanol conversion was close to 100% and hydrogen selectivity was about 70% at 450 °C. The catalysts were characterized by X-ray diffraction, temperature-programmed reduction (TPR) and BET surface area measurements. The preparation method influenced the interaction between cobalt and CeO₂ evidently. The incorporation of Co ions into CeO₂ crystal lattice resulted in weaker interaction between cobalt and ceria on catalyst surface. In comparison with catalysts prepared by impregnation, more cobalt ions entered into CeO₂ lattice, and resulted in weaker interaction between active phase and ceria on surface of Co₃O₄/CeO₂ prepared by co-precipitation. Thus, cobalt oxides was easier to be reduced to metal cobalt which was the key active component for SRE. Meanwhile, the incorporation of Co ions into CeO₂ crystal lattice was beneficial for resistance to carbon deposition.     

Novel utilization of MCM-22 molecular sieves as supports of cobalt catalysts in the Fischer–Tropsch synthesis

Cobalt catalysts (2–10 wt% Co) supported on silica-rich MCM-22 zeolites have been prepared by impregnation with aqueous Co(NO₃)₂ solutions. The catalysts are characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption, solid state nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The catalytic properties for the Fischer–Tropsch synthesis (FTS) at 280 °C, 12.5 bar and H₂/CO = 2 are evaluated. The catalysts supported on MCM-22 exhibit the highest selectivity to long-chain (C₅₊) hydrocarbons when MCM-22 supports are synthesized with the appropriate Si/Al ratio.     

Preparation and structural characterization of Co/Al2O3 catalysts for the ozonation of pyruvic acid

A series of Co/Al₂O₃ catalysts were prepared by the incipient wetness impregnation method using γ-Al₂O₃ support and (CH₃COO)₂Co·4H₂O solutions, followed by calcination at 500–800 °C. Characterization of catalysts was accomplished by several techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), physisorption of nitrogen, mercury and helium-based pycnometries, Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and pH of zero charge (PZC). Impregnation of support produced a moderate decrease of its surface area and pore volume and also led to minor changes of its PZC. Depending on preparation conditions (i.e., calcination atmosphere and temperature and metal loading), one or more of the following Co-containing compounds were identified: CoO, Co₃O₄ and CoAl₂O₄. The support and prepared Co/Al₂O₃ catalysts were tested to catalyze the ozonation of aqueous pyruvic acid at pH 2.5. Pyruvic acid was shown refractory towards single ozonation but the use of γ-Al₂O₃ and Co/Al₂O₃ catalysts resulted in 56–96% pyruvic acid conversion and 41–78% decrease in DOC after 2 h of ozonation of phosphate-buffered solutions. In the absence of the buffer, conversion rate was enhanced likely as a result of pH increase during the course of the process thus giving rise to the indirect way of ozonation through hydroxyl radicals. Acetic acid was found as the main by-product of pyruvic acid ozonation. Depending on the catalyst used, yield of acetic acid varied from 32 to 49%, values noticeably lower that that obtained from the control non-catalytic ozonation experiment (73%). Differences in catalytic activity amongst the various Co/Al₂O₃ catalysts investigated were attributed to the different Co active phases deposited on the γ-Al₂O₃ surface. The following sequence of increasing activity can be inferred from experimental results: CoO, CoAl₂O₄ and Co₃O₄. All the Co/Al₂O₃ catalysts prepared showed good stability as the percentage of cobalt leached out was rather low.     

Hydrodesulfurization over supported monometallic, bimetallic and promoted carbide and nitride catalysts

The preparation of alumina-supported β-Mo₂C, MoC_1−x (x≈0.5), γ-Mo₂N, Co–Mo₂C, Ni₂Mo₃N, Co₃Mo₃N and Co₃Mo₃C catalysts is described and their hydrodesulfurization (HDS) catalytic properties are compared to conventional sulfide catalysts having similar metal loadings. Alumina-supported β-Mo₂C and γ-Mo₂N catalysts (Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃, respectively) are significantly more active than sulfided MoO₃/Al₂O₃ catalysts, and X-ray diffraction, pulsed chemisorption and flow reactor studies of the Mo₂C/Al₂O₃ catalysts indicate that they exhibit strong resistance to deep sulfidation. A model is presented for the active surface of Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃ catalysts in which a thin layer of sulfided Mo exposing a high density of sites forms at the surface of the alumina-supported β-Mo₂C and γ-Mo₂N particles under HDS conditions. Cobalt promoted catalysts, Co–Mo₂C/Al₂O₃, have been found to be substantially more active than conventional sulfided Co–MoO₃/Al₂O₃ catalysts, while requiring less Co to achieve optimal HDS activity than is observed for the sulfide catalysts. Alumina-supported bimetallic nitride and carbide catalysts (Ni₂Mo₃N/Al₂O₃, Co₃Mo₃N/Al₂O₃, Co₃Mo₃C/Al₂O₃), while significantly more active for thiophene HDS than unpromoted Mo nitride and carbide catalysts, are less active than conventional sulfided Ni–Mo and Co–Mo catalysts prepared from the same oxidic precursors.