The effect of cobalt on the reactants adsorption and activity of fused iron catalyst for ammonia synthesis

Modern adsorption study facilities as well as scanning electron microscopy, X-ray diffraction, Mössbauer spectroscopy and X-ray fluorescence spectroscopy were used to investigate the effect of cobalt on the adsorption of nitrogen, hydrogen and ammonia on the surface of cobalt modified iron fused catalysts. Adsorption studies were carried out at the temperature range specific for ammonia synthesis (385C). Activity tests were carried out under 10 MPa in the 350–450 C temperature range. Investigations were performed on the traditional multipromoted iron catalyst and on the series of catalysts prepared with addition of cobalt. Introduction of cobalt changed considerably the sample behaviour during activation and ammonia synthesis. Addition of cobalt promoted the iron catalyst for ammonia synthesis. The most active sample was that containing approximately 5.5 wt% Co. Cobalt changed the adsorption behaviour of the catalyst. Chemisorption of nitrogen is much higher for cobalt catalysts. Growth of nitrogen chemisorption and decrease of ammonia adsorption resulted in the growth of catalytic activity of cobalt catalysts in ammonia synthesis.     

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.     

Silica-supported rhodium-cobalt catalysts for Fischer–Tropsch synthesis

The effects of the addition of Ag, Au, or Rh to a 15 wt% Co/SiO₂ catalyst on the Fischer–Tropsch (FT) synthesis were studied. Both Au and Rh showed a promoting effect on the FT activity, whereas the addition of Ag decreased the activity. The addition of a small amount of Rh (0.1–0.5 wt%) increased the CO conversion by 50% without affecting the selectivity. It was found that Rh catalyzed the reduction of cobalt oxides, but it did not change the number of surface cobalt atoms. It is proposed that the higher activity of Rh-promoted catalysts is due to the hydrogen spillover from Rh to Co during FT synthesis.     

Cobalt Catalyst Doped with Cerium and Barium Obtained by Co-Precipitation Method for Ammonia Synthesis Process

Abstract  

Unsupported cobalt catalysts promoted with barium (symbol Co/Ba), cerium (Co/Ce) or both (Co/Ce/Ba) were synthesized and tested in ammonia synthesis at 6.3 MPa. The Ba-free Co and Co/Ce oxide forms of the catalysts were prepared by precipitation/co-precipitation and a subsequent calcination at 500 °C. The Co and Co/Ce powders were impregnated with an aqueous solution of barium nitrite. Nitrogen physisorption and H₂ chemisorption measurements revealed that cerium and barium play the role of structural promoters, which hinder the sintering of cobalt oxide during calcination and stabilize the surface of cobalt under reduction conditions. It seems that barium also modifies the surface of the active phase, i.e., cobalt. The kinetic studies of NH₃ synthesis have shown that the co-promoted material (Co/Ce/Ba) is about 2–3 times more active than the system doped with barium (Co/Ba) and more than ten times as active as that with Ce. At 400 °C and at low conversion (1% NH₃), the ammonia synthesis rate (TOF) over Co/Ce/Ba proved to be almost 60% as high as that obtained for the commercial iron catalyst (KMI, H. Tops?e) commonly used in ammonia plants all over the world. Moreover, at the same temperature and a high ammonia concentration (8%) the co-promoted cobalt catalyst is over two times more active than the fused iron catalyst. Another asset of the cobalt catalyst is its high thermal stability.     

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.     

Morphology and deactivation behaviour of Co–Ru/γ‐Al2O3 Fischer–Tropsch synthesis catalyst

Deactivation of Co–Ru/γ‐Al₂O₃ Fischer–Tropsch (FT) synthesis catalyst along the catalytic bed over 850 h of time‐on‐stream (TOS) was investigated. Catalytic bed was divided into four parts and structural changes of the spent catalysts collected from each catalytic bed after FT synthesis were studied using BET, ICP, XRD, TPR, carbon determination, H₂ chemisorption and oxygen titration techniques. Rapid deactivation was observed during first 200 h of FT synthesis. In this case, the deactivation rate was not dependent on the number of the catalyst active sites. It was zero order to CO conversion and independent of the size of active sites. Beyond the TOS of 200 h, the deactivation could be simulated with a power law expression: . The physical properties of the catalyst charged in 1st half of the reactor did not change significantly. Interaction of cobalt with alumina and formation of mixed oxides of the form xCoO·yAl₂O₃ and CoAl₂O₄ was increased along the catalytic bed. Percentage reducibility and dispersion decreased by 2.4–25.5% and 0.5–8.8% for the catalyst in the beds 1 and 4, respectively. Particle diameter increased by 0.8–6.1% for the catalyst in the beds 1 and 4 respectively suggesting higher rate of sintering at last catalytic bed. The amount of coke formation in the 4th catalytic bed was 6 times more than that of in bed 1.     

Co/TiO2-SiO2催化剂上肉桂醛选择性加氢制备肉桂醇

采用等体积浸渍法制备了系列Co/TiO₂-SiO₂催化剂,用于肉桂醛选择性加氢制备肉桂醇反应体系。系统考察了钴含量、焙烧温度、还原温度、稀土助剂等参数变化对钴催化剂选择性加氢性能的影响。结果表明,钴催化剂的活性和选择性与其表面钴的晶粒度有一定关系,较大尺寸的钴物种对肉桂醛加氢有利。当Co含量为15%、焙烧温度和还原温度均为823 K时,催化剂表现出良好的加氢性能。稀土助剂La和Ce的引入能改善Co /TiO₂-SiO₂催化剂表面活性组分钴的分散度,提高了钴催化剂的加氢性能。     

Effect of calcination behaviors on precipitated iron–manganese Fischer–Tropsch synthesis catalyst

Calcination behaviors play an important role in Fischer–Tropsch (FT) performance over a slurry iron–manganese catalyst. The present study was undertaken to investigate the effects of calcination behaviors (calcination temperature, heating rate and calcination atmosphere) on the textural properties, reduction/carburization behavior, bulk phase structure and FT synthesis performances over precipitated Fe–Mn catalysts. N₂ physisorption, X-ray photoelectron spectroscopy (XPS), H₂ thermal gravimetric analysis (TGA) and M?ssbauer effect spectroscopy (MES) were used to characterize the catalyst. It is found that increasing calcination temperature and heating rate lead to low surface area and high enrichment of Mn on the catalyst surface. High calcination temperature also increased the crystallite size of α-Fe₂O₃ and suppressed the reduction/carburization of the catalysts in H₂ and syngas. Low calcination temperature and low heating rate promoted the further carburization of the catalyst and increased the activity during FT process. High calcination temperature and low heating rate restrained the formation of CH₄, increases C₅⁺ selectivity and improved the selectivity to light olefins. In addition, calcination in argon could improve the carburization and increase FT activity of the catalyst. The present iron–manganese catalyst with lower calcination temperature, lower heating rate and calcined in argon is optimized for its FT performances.     

Active state of model cobalt foil catalyst studied by SEM, TPR/TPO, XPS and TG

Four states of the cobalt foil catalyst, corresponding to different redox treatment and activity, were defined: oxidised, reduced, active and deactivated. They were investigated by scanning electron microscopy (SEM), temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and thermogravimetric (TG) methods and in the hydrogenation of ethylene used as a test reaction. Particular emphasis was laid on the study of the active state, achieved after the catalyst reduction at moderate temperatures. It was shown that the catalyst preactivated by a series of redox cycles is built of a cobalt oxide layer of a characteristic size and dispersion, which is stuck to the metallic bulk. Reduction at a moderate temperature, prolonged even to several hours, converts only a small fraction of the oxide layer into metallic Co. XPS, TPR and TPO methods distinguished various states of oxygen and cobalt on the surface of the activated or partially activated samples. The results were interpreted in terms of the mechanism of autocatalytic reduction. The deactivation was associated with the structural reconstruction of the surface, taking place either in the reaction mixture during the hydrogenation of ethylene or in hydrogen atmosphere. Formation of the inactive carbon deposit was experimentally excluded.     

Cobalt species in promoted cobalt alumina-supported Fischer–Tropsch catalysts

The structure of cobalt species at different stages of the genesis of monometallic and Pt-promoted cobalt alumina-supported Fischer–Tropsch catalysts was studied using X-ray diffraction, UV–visible spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, and DSC–TGA thermal analysis. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate and dihydrogen hexachloroplatinate. Both variation of catalyst calcination temperature between 473 and 773 K and promotion with 0.1 wt% of Pt had no significant affect on the size of supported Co₃O₄ crystallites. The size of cobalt oxide particles in the calcined catalysts seems to be influenced primarily by the pore diameter of the support. Cobalt reducibility was relatively low in monometallic cobalt alumina-supported catalysts and decreased as a function of catalyst calcination temperature. The effect was probably due to the formation of mixed surface compounds between Co₃O₄ and Al₂O₃ at higher calcination temperatures, which hinder cobalt reduction. Promotion with platinum spectacularly increased the rate of cobalt reduction; the promotion seemed to reduce the activation energy of the formation of cobalt metallic phases. Analysis of the magnetization data suggests that the presence of Pt led to the reduction of smaller cobalt oxide particles, which could not be reduced at the same conditions in the cobalt monometallic catalysts. Promotion of cobalt alumina-supported catalysts with small amounts of Pt resulted in a significant increase in Fischer–Tropsch cobalt time yield. The efficient control of cobalt reducibility through catalyst calcination and promotion seems to be one of the key issues in the design of efficient cobalt alumina-supported Fischer–Tropsch catalysts.     

助剂对活性炭负载钴催化剂氨合成活性的影响

采用四槽高压连续流动反应器研究了添加助剂Ba、K和Sm对活性炭负载钴催化剂氨合成活性的影响,结果发现,添加助剂Ba、K和Sm可以提高催化剂的氨合成活性,其中,Ba的促进效果最好,Ba与Co物质的量比为0.3时,催化活性最高。在Ba-Co/AC催化剂中,助剂Sm的加入降低了催化剂的氨合成活性,而少量K助剂(K与Co物质的量比为0.25～0.5)可以提高其催化性能,在10 MPa、10 000 h^-1和450 ℃条件下,双助剂催化剂的氨合成活性可达120 mmol·(g·h）^-1,进一步增加K的量,其氨合成活性下降。     

Carbon-supported cobalt–iron catalysts for ammonia synthesis

The cobalt, iron and Co–Fe catalysts deposited on carbon were prepared, characterised (XRD, H₂ TPD) and studied in ammonia synthesis at 90 bar (H₂:N₂ = 3:1). Partly graphitised carbon material obtained via high temperature treatment (1900 °C) of commercial activated carbon was used as a support for the active metals (10 wt.%) and barium or potassium were used as promoters. XRD studies of unpromoted materials have shown that cobalt (5–20% in Co + Fe) dissolves in the iron phase (alloy formation); the average sizes of crystallites (20–30 nm) are roughly independent of the metal kind (Co, Fe, Co–Fe). The effect of Ba and that of K on the catalyst performance proved to be strongly dependent on the choice of an active phase (Co or Fe or Co–Fe). In the case of Co/C, the promotional effect of barium was extremely large. Furthermore, the Ba–Co/C system was found to be less inhibited by the ammonia product than Ba–Fe/C. At low temperature (400 °C) and at high conversion (8% NH₃ in the gas), the surface-based reaction rate (TOF) for Ba–Co/C is about six times higher than that for Ba–Fe/C.     

还原温度对Co/ZnO催化剂的F-T合成反应性能影响

采用沉淀-浸渍法制备Co/ZnO催化剂,研究还原温度对Co/ZnO催化剂F-T合成反应性能影响。结果表明,催化剂Co/ZnO适宜在较低温度还原,Co负载质量分数5%和10%的催化剂最佳还原温度分别为260 ℃和250 ℃。还原温度与催化剂活性之间的关系取决于Co/ZnO催化剂上Co的分布状态,氧化态Co以约50 nm的颗粒存在于ZnO表面,容易被氢还原。低温还原的Co基催化剂是浆态床F-T合成的良好催化剂。     

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/Al₂O₃ catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.     

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.     

Structure and catalytic properties of Co/ porcelain in the synthesis of 2,3‐dihydrofuran

The cyclodehydration of 1,4‐butanediol over cobalt catalysts in the liquid phase is used for the production of 2,3‐dihydrofuran. The catalyst preparation parameters considered were the metal loading, precipitation pH and reduction temperature of cobalt salt. It was found that the use of Co(NO₃)₂ together with Na₂CO₃ in a 1:1 ratio yielded better catalysts. Under the conditions used in this study the optimum cobalt loading for the selective production of 2,3‐dihydrofuran is in the range 15–50 wt%. The optimum reduction temperature of Co/porcelain catalyst depends on cobalt loading. The optimum reduction temperatures for 15 and 50 wt% cobalt loading are 773 and 723 K (reduction time 20 min), respectively. © 2001 Society of Chemical Industry     

Effect of manganese on a potassium-promoted iron-based Fischer-Tropsch synthesis catalyst

The effects of manganese promoter on the reduction–carburization behavior, surface basicity, bulk phase structure and their correlation with Fischer-Tropsch synthesis (FTS) performances have been emphatically studied over a series of spray-dried Fe–Mn–K catalysts with a wide range of Mn incorporation amount. The catalysts were characterized by means of H₂ and CO temperature-programmed reduction (TPR), CO₂ temperature-programmed desorption (TPD), Mössbauer spectroscopy etc.. The results indicated that small amount of Mn promoter can promote the reduction of the catalyst in H₂. However, FeO phase formed during reduction is stabilized by MnO phase with the further increase of Mn content, making FeO phase difficult to be reduced in H₂. The addition of Mn promoter can stabilize the Fe²⁺ and Fe³⁺ ions, and suppresses the reduction and carburization of the catalyst in syngas and CO. Mn promoter can also enhance the amount of the basic sites and weaken the strength of the basic sites, which possibly come from the reason that the Mn–K interaction is strengthened with the addition of Mn promoter. The change of surface basicity can modify the selectivity of hydrocarbons and olefins, and the change of bulk structure phase derived from the addition of Mn promoter will affect the catalyst activity and run stability. The synergetic effects of the two main factors result in an optimized amount of Mn promoter for the highest catalyst activity and heavy hydrocarbon selectivity in slurry FTS reaction of Fe–Mn–K 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.     

氨合成催化剂母体相组成对还原性能的影响

采用Shimadzu DT-40型热分析仪,研究了不同原始铁氧化物组成对氨合成熔铁催化剂的还原性能的影响。发现催化剂的还原性能和活性与母体相组成的关系具有相似的规律性。当母体中两种物相共存时,催化剂的还原过程是按物相分阶段依次进行的,其结果使还原速度变慢,还原温度升高,亦使还原后催化剂的活性降低。氨合成催化剂的还原性能与其活性具有一致性,愈易还原的催化剂,其活性亦愈高。研究结果表明,在熔铁催化剂中,Fe_(1-x)O基氨合成催化剂还原速度最快、还原温度最低,其活性亦最高。     

Kinetic study of Fischer–Tropsch process on titania-supported cobalt–manganese catalyst

An active cobalt–manganese catalyst was prepared by co-precipitation method, and was also tested for hydrogenation of carbon monoxide to light olefins. The catalyst was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area techniques. The kinetic experiments on a well-characterized Co–Mn/TiO₂ catalyst were performed in a fixed-bed micro-reactor, and were also conducted in a temperature range of 190–280 °C, pressure range of 1–10 bar, H₂/CO feed ratio (mol/mol) range of 1–3 and a space velocity range of 2700–5200 h^?1. Two kinetic expressions based on Langmuir–Hinshelwood–Houngen–Watson (LHHW) mechanism were observed to fit the experimental data accurately for Fischer–Tropsch synthesis reaction. The kinetic parameters were estimated with non-linear regression method. Activation energies obtained were 35.131 and 44.613 kJ/mol for optimal kinetics models.