Cobalt Alumina Catalysts Research Articles

Selective hydrogenation of stearic acid to stearyl alcohol over cobalt alumina catalysts

The selective hydrogenation of stearic acid to produce stearyl alcohol at 300 °C and 50 bar pressure was studied over a series of cobalt alumina catalysts with different cobalt loadings. Catalysts were prepared by wet impregnation and characterised by X-ray diffraction, H2 temperature programmed reduction, H2 chemisorption, N2 adsorption-desorption and Fourier-transform infrared spectroscopy. Stearyl alcohol was observed to be the main reaction product, although heptadecane, octadecane and stearyl stearate were also produced in lesser amounts. The product selectivity was influenced by the availability of Lewis acid sites on the support which were found to be responsible for ester formation. Increasing the cobalt loading diminished the number of acid sites on the support surface whilst increasing the number of cobalt metal sites and thus enhanced the relative rate of hydrogenation leading to improved selectivity to stearyl alcohol.

Cobalt–Alumina Coprecipitated Catalysts for Green Diesel Production

Cobalt–alumina catalysts with high Co loading (35–70 wt %) were synthesized by coprecipitation. The influence of activation (reduction) temperature and the Co loading on their efficiency for green diesel production via selective deoxygenation (SDO) of solvent-free sunflower oil (SO) and waste cooking oil (WCO) has been investigated. The change of reduction temperature and the cobalt content caused remarkable changes in the physicochemical properties and catalytic behavior of the catalysts. A catalyst containing 55 wt % Co, activated at 500 °C, proved the most efficient. Its high efficiency has been attributed to its high metallic cobalt surface, the synergy between Co0/CoO phases, and its relatively enhanced moderate and strong acidity. The catalysts studied exhibited relatively lower performance in the SDO of WCO than that in the SDO of SO. This has been attributed to their deactivation caused by extended coking and concomitant pore blockage.

Effect of Ce Doping of a Co/Al2O3 Catalyst on Hydrogen Production via Propane Steam Reforming

We synthesized cerium-doped cobalt-alumina (CoxCey/Al2O3) catalysts for the propane steam reforming (PSR) reaction. Adding cerium introduces oxygen vacancies, and the oxygen transfer capacity of the Ce promoter favors CO to CO2 conversion during PSR, inhibiting coke deposition and promoting hydrogen production. The best PSR activity was achieved at 700 °C using the Co0.85Ce0.15/Al2O3 catalyst, which showed 100% propane (C3H8) conversion and about 75% H2 selectivity, and 6% CO, 5% CO2, and 4% CH4 were obtained. In contrast, the H2 selectivity of the base catalyst, Co/Al2O3, is 64%. The origin of the difference in activity was the lower C3H8 gas desorption temperature of the Co0.85Ce0.15/Al2O3 catalyst compared to that of the Co/Al2O3 catalyst; thus, the PSR occurred at low temperatures. Furthermore, more CO was adsorbed on the Co0.85Ce0.15/Al2O3 catalyst, and subsequently, desorbed as CO2. The activation energy for water desorption from the Co0.85Ce0.15/Al2O3 catalyst was 266.96 kJ/mol, higher than that from Co/Al2O3. Furthermore, the water introduced during the reaction probably reacted with CO on the Co0.85Ce0.15/Al2O3 catalyst, increasing CO2 generation. Finally, we propose a mechanism involving the Co0.85Ce0.15/Al2O3 catalyst, wherein propane is reformed on CoxCey sites, forming H2, and CO, followed by the conversion of CO to CO2 by water on CeO2 sites.

Promoted porous Co3O4-Al2O3 catalysts for ammonia decomposition

Transition metal catalysts have been considerably used for NH3 decomposition because of the potential application in CO x -free H2 generation for fuel cells. However, most transition metal catalysts prepared via traditional synthetic approaches performed the inferior stability due to the agglomeration of active components. Here, we adopted an efficient method, aerosol-assisted self-assembly approach (AASA), to prepare the optimized cobalt-alumina (Co3O4-Al2O3) catalysts. The Co3O4-Al2O3 catalysts exhibited excellent catalytic performance in the NH3 decomposition reaction, which can reach 100% conversion at 600 °C and maintain stable for 72 h at a gaseous hourly space velocity (GHSV) of 18000 cm3 gcat−1 h−1 . The catalysts were characterized by various techniques including transmission electron microscope (TEM), scanning electron microscope (SEM), nitrogen sorption, temperature-programmed reduction by hydrogen (H2-TPR), ex - situ / in - situ Raman and ex - situ / in - situ X-ray diffraction (XRD) to obtain the information about the structure and property of the catalysts. H2-TPR and in - situ XRD results show that there is strong interaction between the cobalt and alumina species, which influences the redox properties of the catalysts. It is found that even a low content of alumina (10 at%) is able to stabilize the catalysts due to the adequate dispersion and rational interaction between different components, which ensures the high activity and superior stability of the cobalt-alumina catalysts.

Ruthenium promoted cobalt–alumina catalysts for the synthesis of high-molecular-weight solid hydrocarbons from CO and hydrogen

The effect of the ruthenium promotion of Fischer–Tropsch (FT) cobalt–alumina catalysts on the temperature of catalyst activation reduction and catalytic properties in the FT process is studied. The addition of 0.2–1 wt % of ruthenium reduces the temperature of reduction activation from 500 to 330–350°C while preserving the catalytic activity and selectivity toward C5+ products in FT synthesis. FT ruthenium-promoted Co–Al catalysts are more selective toward higher hydrocarbons; the experimental value of parameter αASF of the distribution of paraffinic products for ruthenium-promoted catalysts is 0.93–0.94, allowing us to estimate the selectivity toward C20+ synthetic waxes to be 48 wt %, and the selectivity toward C35+ waxes to be 23 wt %. Ruthenium-promoted catalysts also exhibit high selectivity toward olefins.

Hydrogen spillover in the Fischer–Tropsch synthesis: An analysis of gold as a promoter for cobalt–alumina catalysts

This study investigated the operation of gold as a potential substitute for the platinum promoter in the Co/Al2O3 Fischer–Tropsch catalyst. Au–Co/Al2O3 was tested in conjunction with a model Hybrid Au–Co sample (comprised of a mechanical mixture of Au/Al2O3+Co/Al2O3) to investigate hydrogen spillover which has been demonstrated to play a vital role in the reduction promotion mechanism. TPR, TGA and in situ XRD provided evidence to support the improved reducibility of supported cobalt oxide crystallites in Au–Co/Al2O3. However, no improvement in the reducibility of the Hybrid Au–Co catalyst was observed, in contrast to previous studies on noble metal reduction promoters. It was hypothesized that even though gold-to-cobalt spillover occurred during reduction in Au–Co/Al2O3, the great separation between the gold and cobalt crystallites, combined with gold’s much lower affinity for hydrogen activation adversely affected the efficiency of the spillover process in the hybrid sample. Nevertheless, Au–Co/Al2O3 had an improved mass-based activity, and a turnover frequency comparable to a platinum promoted sample, which highlighted the potential of gold as a reduction promoter for Co/Al2O3 catalysts.

Hydrogen spillover in the Fischer–Tropsch synthesis: An analysis of platinum as a promoter for cobalt–alumina catalysts

Hydrogen spillover has been invoked to explain the functioning of platinum as a promoter in cobalt-based Fischer–Tropsch catalysts. In this study, the operation of Pt was investigated using a model hybrid catalyst (i.e. mixture of Pt/Al2O3+Co/Al2O3) which allowed for decoupling of hydrogen spillover effects from those requiring direct Pt–Co contact. Pt improved the reducibility of the hybrid catalyst despite physical separation of Pt from Co.TPR, TGA, in situ XRD and quasi in situ XPS confirmed that supported Co3O4 reduced via formation of CoO as a stable intermediate. Pt had only a slight effect on Co3O4→CoO reduction, but greatly improved the CoO→Co0 reduction which was severely hindered by interaction with the alumina support. The catalysing effect of Pt on the reduction was attributed to H2 dissociation and its subsequent spillover occurring more readily than direct H2 activation by the cobalt oxides. During the Fischer–Tropsch reaction, the role of spillover hydrogen was again invoked to explain the higher TOF and enhanced selectivity towards CH4 and paraffins. Spillover from Pt was proposed to induce a hydrogen-rich microenvironment that resulted in a ‘cleansing’ effect on the catalyst surface and an increase in the selectivity of hydrogenated products.

Facile synthesis of highly ordered mesoporous cobalt–alumina catalysts and their application in liquid phase selective oxidation of styrene

We illustrate the preparation of highly ordered mesoporous cobalt–alumina catalysts with highly homogeneously dispersed Co(ii) species via an evaporation-induced triconstituent cooperative co-assembly method.

SHS-produced cobalt-alumina catalysts for dry reforming of methane

We report on the catalytic activity of SHS-produced cobalt-alumina catalysts for dry (carbon dioxide) reforming of methane. The catalysts were prepared from Al-Co3O4 powder mixtures at several processing conditions. XRD data, Fourier transform IR spectra, and SEM-EDS data suggest that the catalysts consisted predominantly of spinel and mixed oxides but with a strongly distorted atomic structure. Catalytic tests for dry reforming of methane to produce syngas were carried out at temperatures up to 900°C and chromatographic analysis showed that carbon dioxide conversion approached 88% whereas conversion of methane reached 100%. The hydrogen to carbon monoxide ratio in the syngas produced varied from about 1.1 to over 2, depending on material and testing temperature.

Fischer–Tropsch Synthesis: Higher Oxygenate Selectivity of Cobalt Catalysts Supported on Hydrothermal Carbons

The performance of carbon-supported cobalt catalysts was compared with that of Co/γ-Al2O3 reference catalysts for the Fischer–Tropsch synthesis (FTS) reaction. The carbon support (CS) was prepared using a hydrothermal method that formed mostly spherical ∼300–800 nm carbons that were first carbonized at 900 °C and then partially graphitized at 1900 °C. The FTS study was conducted using a continuously stirred tank reactor, and the cobalt catalysts were promoted with Pt (0.2% Pt–10% Co) to facilitate the reduction of cobalt oxides. Catalysts were prepared by an evaporative method (Co/CS-IWI) and by a chemical vapor deposition technique (Co/CS-CVD). The CVD technique led to a higher CO conversion (26.5%) relative to the conventional evaporative (IWI) method (7.4%) at the same temperature (220 °C) and space velocity (1.5 NL/gcath). Remarkably, the Co/CS-CVD displayed a high oxygenate selectivity (∼10%) in comparison with cobalt alumina catalysts (i.e., including one having similar Pt and Co loadings, as well as a conventional cobalt alumina catalyst with a higher Co loading of 25% Co) at similar conversion levels. The difference in the CO conversion on a per gram catalyst basis observed between Co/CS-IWI and Co/CS-CVD catalysts was due to the smaller average Co particle size and more uniform distribution resulting from the CVD method.

Fischer–Tropsch synthesis: Effect of catalyst particle (sieve) size range on activity, selectivity, and aging of a Pt promoted Co/Al2O3 catalyst

Abstract The effect of cobalt–alumina catalyst particle (sieve) size range on the performance of a traditional cobalt catalyst (platinum promoted cobalt/alumina) was investigated during Fischer–Tropsch synthesis (FTS) using a continuously stirred tank reactor (CSTR). In this study, the catalyst was sieved into four different size ranges: 20–63, 63–106, 106–180 and 180–355 μm. CO conversion varied with varying sieve size range under similar reaction conditions. With increasing catalyst sieve size, the CO conversion decreased, except for the smallest sieve size range (20–63 μm). These results are consistent with the view that, for the larger sieve size range, losses in CO conversion beyond catalyst aging may be explained by filling of the interior of the catalyst particle with heavy wax, thereby blocking available catalytically active sites. The adverse selectivities are directly the result of significant deactivation, since in FT synthesis, selectivity is a function of CO conversion. The loss in CO conversion for the 20–63 μm range catalyst was higher compared to the other three catalysts; due to the presence of heavy wax (Polywax 3000), the results suggest that the small particles tended to flocculate to larger clusters or were small enough to move at the speed of the liquid.

Effect of process conditions on the product distribution of Fischer–Tropsch synthesis over a Re-promoted cobalt-alumina catalyst using a stirred tank slurry reactor

The effects of process conditions on Fischer–Tropsch synthesis (FTS) product distribution were studied using a 1-L stirred tank slurry reactor and a 0.48%Re–25%Co/Al2O3 catalyst. It was found that the chain growth probability of C1 intermediate (α1) has the most dominant effect on CH4 and C5+ selectivity. α1 was found to be highly dependent on process conditions. Relatively constant values of C2+ growth probabilities with reactor residence time, as well as other process variables, suggest that 1-olefin readsorption has a minor effect on product selectivities. A low value of α1 and its different response to variations in process conditions, compared to higher chain growth probabilities, seems to support a hypothesis that a higher-than-expected yield of methane is caused by at least two separate methane formation pathways. Understanding these pathways and ways to suppress excess methane formation is a key factor in obtaining higher C5+ selectivity.

Fischer–Tropsch Synthesis: Effect of Start-Up Solvent in a Slurry Reactor

The effect of start-up solvent on the performance of cobalt-based catalysts and potassium-promoted precipitated iron catalysts was investigated during Fischer–Tropsch (FT) synthesis by using a continuously stirred tank reactor. In this study, four different molecular weight start-up solvents were tested: Polywax-3000 ( $$\overline{MW}$$ (average molecular weight) = 3,000), Polywax-2000 ( $$\overline{MW}$$ = 2,000), Polywax-500 ( $$\overline{MW}$$ = 500) and C-30 oil ( $$\overline{MW}$$ = 420). The conversion and selectivities (methane, C5+ and CO2) were essentially the same for all the start-up solvents tested for the potassium promoted precipitated iron catalysts suggesting that there was no significant effect of startup solvent for the iron catalyst tested. However, with the cobalt catalyst, conversion varied with solvent, with the conversion increasing with decreasing molecular weight of the solvent. This is considered to be due to the particle size of the cobalt alumina catalysts being large relative to that of iron that was used. Under synthesis conditions, the iron catalyst attrits to form a measurable fraction of 1–3 micron sized particles in the lower range of the particle size distribution, whereas the alumina support retains essentially the same, larger size during synthesis. Thus, the decrease in conversion with time is likely to be a result of pore filling with solvent into the interior of the catalyst, which increases with increasing molecular weight of the start-up solvent. In obtaining conversion and aging data for FT catalysts, wax accumulation needs to be considered.

Ammoxidation of ethylene to acetonitrile over chromium or cobalt alumina catalysts prepared by sol–gel method

A series of Cr/Al2O3 and Co/Al2O3 catalysts were tested in the selective ammoxidation of ethylene to acetonitrile. Catalysts were prepared either by sol–gel method or by impregnation with chromium or cobalt acetylacetonate salts. Physicochemical properties of catalysts were accomplished by several techniques such as chemical analysis, physisorption of N2, X-ray diffraction (XRD), 27Al MAS NMR, UV–Visible diffuse reflectance (DRS) and Raman spectroscopy and temperature programmed reduction of H2 (H2–TPR). Textural analysis reveals that mesoporous materials with pronounced surface areas were obtained using sol–gel procedure while impregnation of the support produces a moderate decrease of its surface area and pore volume. XRD analysis confirms the presence of highly dispersed metal species which reside essentially on the surface and measure less than 4 nm. Furthermore, 27Al MAS NMR shows that for xerogels, part of metal species occupies sites on/in A12O3 in close vicinity of octahedral 27Al. This, apparently, is not the case for aerogels. For Cr/Al2O3 catalysts, isolated Cr6+, mono and polychromate species were identified using DRS, Raman Spectroscopy and H2–TPR which seem to play a key role in the ammoxidation of ethylene. Furthermore, for cobalt doped catalysts, CoAl2O4 was identified as active phase on the basis of DRS and H2–TPR results. From the supercritical drying, it results generally better catalysts than catalysts calcined by ordinary procedure which leads to inactive agglomerated Co3O4 and CoO–Al2O3 phase.

Fischer–Tropsch synthesis: Deactivation of promoted and unpromoted cobalt–alumina catalysts

The loss in activity of Pt-promoted and unpromoted 25 wt% Co–Al2O3 catalysts has been compared under identical conditions except for adjustment of the space velocity to give the same initial CO-conversion. Both catalysts underwent a 200 h period of rapid, initial decline in CO conversion and then a slower, linear decline during the next 1000 h. Pt-promotion did not alter the cobalt dispersion (or average particle size) from that of the unpromoted catalyst but did increase the amount of cobalt that was reduced. When compared not by time-on-stream, but by the moles of Co converted per unit weight of catalyst, both the Pt-promoted and unpromoted catalysts decline in activity at the same rate.

04/00153 Fischer-Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts: Das, T. K. et al. Fuel, 2003, 82, (7), 805–815

04/00153 Fischer-Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts: Das, T. K. et al. Fuel, 2003, 82, (7), 805–815

Fischer–Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts☆

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/Al 2O 3 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.

Decomposition of Chlorinated Hydrocarbons on Iron-Group Metals

The decomposition of 1,2-dichloroethane and chlorobenzene on nickel–alumina, cobalt–alumina, and iron–alumina catalysts at 400–600°C was studied. Thermodynamic calculations demonstrated that the susceptibility of metals to chlorination under exposure to HCl increases in the order Ni < Co < Fe. The addition of hydrogen to the reaction mixture was found to dramatically decrease the rate of carbon deposition in the decomposition of 1,2-dichloroethane because of the intense hydrogenation of intermediates that are graphite precursors. Two fundamentally different reaction paths were found in the degradation of 1,2-dichloroethane: decomposition via a carbide-cycle mechanism with the formation of carbon as the main product (under conditions of a deficiency of hydrogen) and 1,2-dichloroethane hydrodechlorination accompanied by methanation of the formed carbon (under conditions of an excess of hydrogen). The degradation of chlorobenzene diluted with hydrogen in a molar ratio of 1 : 50 was not accompanied by carbon formation on the catalyst. A comparison between the selectivity for reaction products on nickel–alumina and cobalt–alumina catalysts indicated that the former catalyst is more active in the rupture of C–C bonds and in the methanation reaction of deposited carbon, whereas the latter is more favorable for hydrodechlorination. The optimum conditions and thermal regime for efficient and stable operation of the catalysts were found.

The Effect of Preparation Variables on the CO Hydrogenation Activity of Coprecipitated Co/Al2O3 Catalysts

Studies have been conducted on the effect of preparation variables on the activity of coprecipitated cobalt–alumina catalysts to be used for the production of C1–C4 hydrocarbons by CO hydrogenation. The preparation parameters considered were the precipitation pH, the precipitation agent, the metal loading, the reduction temperature and the reduction period. It was found that changing pH precipitation with a constant final pH between 11·0 and 12·5 and the use of NaOH together with nitrates as precursors yielded better catalysts with maximal metal surface area. The optimum cobalt loading for the selective production of C1–C4 hydrocarbons is around 35 wt%. Optimum activity and selectivity are obtained by applying an 8-h reduction scheme at 648 K under 100 cm3 min−1 hydrogen. Calcination prior to reduction has a detrimental effect on metal surface area and hence on catalytic activity. © 1997 SCI.

The Effect of Preparation Variables on the CO Hydrogenation Activity of Coprecipitated Co/Al2O3 Catalysts

Studies have been conducted on the effect of preparation variables on the activity of coprecipitated cobalt–alumina catalysts to be used for the production of C1–C4 hydrocarbons by CO hydrogenation. The preparation parameters considered were the precipitation pH, the precipitation agent, the metal loading, the reduction temperature and the reduction period. It was found that changing pH precipitation with a constant final pH between 11·0 and 12·5 and the use of NaOH together with nitrates as precursors yielded better catalysts with maximal metal surface area. The optimum cobalt loading for the selective production of C1–C4 hydrocarbons is around 35 wt%. Optimum activity and selectivity are obtained by applying an 8-h reduction scheme at 648 K under 100 cm3 min−1 hydrogen. Calcination prior to reduction has a detrimental effect on metal surface area and hence on catalytic activity. © 1997 SCI.