Kinetics of the water-gas shift reaction in Fischer-Tropsch synthesis over a nano-structured iron catalyst

Based on formate and direct oxidation mechanisms, three Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic models of the water-gasshift (WGS) reaction over a nano-structured iron catalyst under Fischer-Tropsch synthesis (FTS) reaction conditions were derived and compared with those over the conventional catalyst. The conventional and nanostructured Fe/Cu/La/Si catalysts were prepared by co-precipitation of Fe and Cu nitrates in aqueous media and water-oil micro-emulsion, respectively. The WGS kinetic data were measured by experiments over a wide range of reaction conditions and comparisons were also made for various rate equations. WGS rate expressions based on the formate mechanism with the assumption that the formation of formate is rate determining step were found to be the best.     

Fischer-Tropsch synthesis by nano-structured iron catalyst

Effects of nanoscale iron oxide particles on textural structure, reduction, carburization and catalytic behavior of precipitated iron catalyst in Fischer-Tropsch synthesis (FTS) are investigated. Nanostructured iron catalysts were prepared by microemulsion method in two series. Firstly, Fe2O3, CuO and La2O3 nanoparticles were prepared separately and were mixed to attain Fe/Cu/La nanostructured catalyst (sep-nano catalyst); Secondly nanostructured catalyst was prepared by co-precipitation in a water-in-oil microemulsion method (mix-nano catalyst). Also, conventional iron catalyst was prepared with common co-precipitation method. Structural characterizations of the catalysts were performed by TEM, XRD, H2 and CO-TPR tests. Particle size of iron oxides for sep-nano and mix-nano catalysts, which were determined by XRD pattern (Scherrer equation) and TEM images was about 20 and 21.6 nm, respectively. Catalyst evaluation was conducted in a fixed-bed stainless steel reactor and compared with conventional iron catalyst. The results revealed that FTS reaction increased while WGS reaction and olefin/paraffin ratio decreased in nanostructured iron catalysts.     

费托合成反应中纳米铁催化剂上La和Ba助剂的协同效应(英文)

Effect of promoters such as Barium and Lanthanum has been investigated on the conventional nanostructured iron catalyst in Fischer-Tropsch synthesis (FTS). The nanosized iron-based catalysts were prepared by a microemulsion method with the general formulation of 100Fe/4Cu, 100Fe/4Cu/2La, 100Fe/4Cu/ 1La/1Ba, and 100Fe/4Cu/2Ba. The phase, structure, and morphology of the catalysts were characterized by X-ray diffraction, N2 adsorption, transmission electron microscopy, temperature-programmed desorption of CO2, and temperature- programmed reduction. The results indicated that the addition of promoters could improve the activity of Fe catalysts for FTS and WGS (water-gas shift reaction) and lower the gas fraction at the outlet. In addition, both Ba- and La-promoted Fe catalysts exhibit the highest activity due to the synergetic effect.     

Particle size effects in Fischer-Tropsch synthesis by Co catalyst supported on carbon nanotubes

The effect of Co particle size on the Fischer-Tropsch synthesis (FTS) activity of carbon nanotube (CNT)-supported Co catalysts was investigated. Microemulsion (using water-to-surfactant molar ratios of 2 to12) and impregnation techniques were used to prepare catalysts with different Co particle sizes. Kinetic studies were performed to understand the effect of Co particle size on catalytic activity. Size-dependent kinetic parameters were developed using a thermodynamic method, to evaluate the structural sensitivity of the CNT-supported Co catalysts. The size-independent FTS reaction rate constant and size-independent adsorption parameter increased with increasing reac-tion temperature. The Polani parameter also depended on catalyst particle size, because of changes in the catalyst surface coverage.     

Deactivation studies of nano-structured iron catalyst in Fischer-Tropsch synthesis

A nano-structured iron catalyst for syngas conversion to hydrocarbons in Fischer-Tropsch synthesis (FTS) was prepared by micro-emulsion method. Compositions of bulk iron phase and phase transformations of carbonaceous species during catalyst deactivation in FTS reaction were characterized by temperature-programmed surface reaction with hydrogen (TPSR-H2), and XRD techniques. Many carbonaceous species on surface and bulk of the nano-structured iron catalysts were completely identified by combined TPSR-H2 and XRD spectra and which were compared with those recorded on conventional co-precipitated iron catalyst. The results reveal that the catalyst deactivation results from the formation of inactive carbide phases and surface carbonaceous species like graphite, and it will be increased when the particle size of iron oxides was reduced in FTS iron catalyst.     

Fe/SiO_2纳米复合物上合成气制低碳烯烃催化性能研究

分别采用一步合成法和常规共沉淀法制备了Fe/SiO2催化剂,通过N2物理吸附、X射线衍射、透射电镜、傅里叶变换红外光谱和程序升温还原等方法对催化剂进行了表征,并在固定床反应器中对其费托合成制低碳烯烃的催化性能进行了评价。结果表明,与共沉淀铁基催化剂不同,采用一步合成法制备的纳米复合物主要由Fe3O4相构成,形貌呈规则球形,平均粒径为30 nm,尺寸分布窄,更容易还原。一步合成法制得的Fe/SiO2催化剂对费托合成反应具有较高的活性和低碳烯烃选择性、较低的甲烷选择性和良好的稳定性。     

碳纳米管负载的钴催化剂在费托合成反应中的粒子尺寸效应（英文）

The effect of Co particle size on the Fischer-Tropsch synthesis(FTS) activity of carbon nanotube(CNT)-supported Co catalysts was investigated. Microemulsion(using water-to-surfactant molar ratios of 2 to12) and impregnation techniques were used to prepare catalysts with different Co particle sizes. Kinetic studies were performed to understand the effect of Co particle size on catalytic activity. Size-dependent kinetic parameters were developed using a thermodynamic method, to evaluate the structural sensitivity of the CNT-supported Co catalysts. The size-independent FTS reaction rate constant and size-independent adsorption parameter increased with increasing reaction temperature. The Polani parameter also depended on catalyst particle size, because of changes in the catalyst surface coverage.     

硅对铁基费-托合成催化剂的影响

用共沉淀法制备了一系列不同硅含量的铁基催化剂,采用N2吸附和原位X射线衍射对催化剂进行了表征,在固定床反应器中考察了催化剂的费-托合成反应活性、选择性和稳定性.结果表明,含硅的催化剂具有较大的比表面积和较小的平均孔径,在CO还原及费-托合成反应中生成的碳化铁物种的稳定性比不含硅的催化剂高.在费-托合成反应中,不含硅的催化剂具有较高的初始活性,但易失活;含硅的催化剂具有较低的初始活性,但稳定性较高.Fe7C3是活性最高的碳化铁物种.随着硅含量的增加,催化剂的费-托合成反应更易生成低碳数产物.     

费托合成 Fe基催化剂中铁物相与活性的关系

 采用连续共沉淀和喷雾干燥相结合的方法制备了微球形 Fe 基催化剂, 采用 N2 吸附-脱附、X 射线衍射和穆斯堡尔谱等手段, 考察了催化剂在不同还原条件下铁物相的转变, 并在浆态床反应器中评价了催化剂的费-托合成 (FTS) 反应性能. 结果表明, Fe 基催化剂在合成气气氛下首先从α-Fe2O3 转变为 Fe3O4, 然后转变为铁碳化物 (FexC); 还原压力的增大有利于 α-Fe2O3 向 Fe3O4 的转变, 而抑制 Fe3O4 向 FexC 的转变; 还原空速的增加则促进 Fe3O4 转变为 FexC. 催化剂的 FTS 反应活性随着催化剂中 Fe3O4 含量的增加而逐渐下降, 而随着 FexC 含量的增加而逐渐上升.     

Water–gas shift kinetics over lanthanum-promoted iron catalyst in Fischer–Tropsch synthesis: thermodynamic analysis of nanoparticle size effect

The kinetic parameters of water–gas shift (WGS) reaction in the Fischer–Tropsch synthesis (FTS) on lanthanum-promoted iron catalyst are analyzed by size-dependent thermodynamic method. A Langmuir–Hinshelwood kinetic equation is considered for the catalysts activity evaluation. A series of unsupported iron catalysts with different particle sizes are prepared via microemulsion method. These results show that the iron particle size has considerable effects on reactants adsorption and WGS kinetic parameters and WGS activity pass from a maximum by increasing the catalyst particle size. Finally, the analysis of data showed that by increasing the iron particle from 14 to 41 nm, the WGS activation energies and heats of adsorption of carbon monoxide and water on catalysts increased from 68 to 83, 22 to 28 and 75 to 94 kJ/mol, respectively.     

Effect of nano-particle size on product distribution and kinetic parameters of Fe/Cu/La catalyst in Fischer-Tropsch synthesis

Effects of nano-particle size on hydrocarbon production rates and distributions for precipitated Fe/Cu/La catalysts in Fischer-Tropsch synthesis were investigated. Nano-structured iron catalyst was prepared by micro-emulsion method. The concept of two superimposed Anderson-Schulz-Flory (ASF) distributions has been applied for the representation of the effects of reaction conditions and nano-particles size on kinetics parameters and product distributions. These results reveal that by reducing the particle size of catalyst, the break in ASF distributions was decreased. Also useful different kinetics equations for synthesis of C_3 to C_9 and C_(10) to C_(22) were determined by using α_1 and α_2 chain growth probabilities.     

La2O3对费-托合成沉淀铁催化剂的影响

制备了一系列不同La2O3含量(100Fe/2.8Si/nLa,n=0,0.5,1,2,4,原子比)的沉淀铁费-托合成催化剂,通过原位X射线衍射、一氧化碳程序升温还原和N2吸附-脱附等手段对催化剂进行了表征.催化剂费-托反应评价在固定床反应器中进行.结果表明,少量La2O3助剂(La/Fe≤0.01,原子比)的加入显著降低了Fe2O3颗粒大小,增加了催化剂的比表面积和分散度,有利于碳化铁的形成,从而导致费-托合成催化活性的提高.随着La2O3含量的进一步增加(La/Fe≥0.02),催化剂表面大量La2O3的覆盖和LaFeO3化合物的形成降低了铁碳化物的形成,反应活性反而降低.因此,最适宜的La2O3含量为La/Fe=0.01.La2O3的加入提高了甲烷的选择性,抑制了C5+碳氢化合物的形成.     

F-T组分改性KCuZrO2催化剂上CO加氢合成异丁醇的性能研究

采用共沉淀法分别制备了不同F-T组分(Fe、Co、Ni)改性的KCuZrO_2催化剂,并用于催化CO加氢合成异丁醇。通过BET、XRD、TEM、XPS、H_2-TPR、CO-TPD以及in-situ DRIFTS对催化剂进行了表征。结果显示,F-T组分的加入促进了乙醇和丙醇的形成,但是对异丁醇选择性影响不同。结果表明,Fe促进了催化剂中各组分的分散,活性组分Cu在催化剂表面发生了富集,提高了H_2/CO活化吸附;另外,KFeCuZrO_2的催化剂表面含有较多的C1物种,有利于乙醇和丙醇进一步发生β-加成反应得到异丁醇,而Co和Ni改性的催化剂上缺少足够的C1物种,因此,异丁醇的选择性并未明显增加。Co的引入对催化剂结构以及Cu的分散影响不大,但是Co改性后催化剂性能有所下降,其原因是催化剂发生了失活; Ni添加后催化剂比表面积有所减小,且催化剂表面Cu/Zr物质的量比也降低到0. 19,催化剂粒径增大,Cu-Zr之间相互作用减弱,异丁醇选择性降低。     

结构助剂SiO2、Al2O3对铁基催化剂浆态床F-T合成性能的影响

采用连续共沉淀与喷雾干燥成型技术相结合的方法制备了微球状Fe/Cu/K/SiO2和Fe/Cu/K/Al2O3催化剂，研究SiO2和Al2O3作为结构助剂对铁基催化剂吸附行为、炭化行为及F-T合成反应性能的影响。表征结果表明,与Al2O3相比较，SiO2抑制了H2的吸附，但促进了CO的吸附，有利于催化剂的炭化。催化剂在260℃、1.5MPa、H2/CO=0.67和2000h-1下的浆态床F-T合成反应评价表明，Fe/Cu/K/SiO2催化剂具有较高的F-T合成活性、高的水煤气变换反应（WGS）活性，且其烃产物选择性明显向高炭数方向偏移，而Fe/Cu/K/Al2O3催化剂则表现出较低的F-T合成活性、低的水煤气变换反应（WGS）活性和高的轻质烃选择性。但Fe/Cu/K/Al2O3催化剂比Fe/Cu/K/SiO2催化剂具有更好的运行稳定性。     

F-T合成Fe/Cu/K/Al2O3催化剂的结构性质、还原及碳化行为

采用连续共沉淀与喷雾干燥成型技术相结合的方法制备了微球状Fe/Cu/K/Al2O3催化剂, 结合TG、N2物理吸附、XRD、H2-TPR、CO-TPR、Mossbauer谱等表征手段, 研究焙烧温度对Fischer-Tropshc (F-T)合成铁基催化剂的结构性质、还原行为和碳化行为的影响. 结果表明, 较高的焙烧温度有利于碳酸盐的分解和结晶水的脱除, 促进了催化剂的还原. 随着焙烧温度的进一步升高, 催化剂的比表面积减小, 平均孔径增大, α-Fe2O3晶粒的粒径增大, 催化剂中金属与载体的相互作用增强, 从而削弱了CuO、K2O助剂的作用, 严重抑制了催化剂的还原和碳化.     

Effect of reduction temperature on a spray-dried iron-based catalyst for slurry Fischer–Tropsch synthesis

An industrial iron-based catalyst (100Fe/5Cu/6K/16SiO₂, by weight) was characterized after reduction at different temperatures and after Fischer–Tropsch synthesis (FTS) in a stirred tank slurry reactor (STSR). The BET surface area and pore volume of the catalyst decreases with increasing reduction temperature, and the contrary trend was found for pore size. The iron phase compositions of catalysts reduced with syngas were strongly dependent on pretreatment conditions employed. Pretreatment with syngas at lower temperature prevents iron catalyst activation. Carburization was intensified with the increase in reduction temperature. The formation of iron carbides in reduced catalyst was necessary for obtaining stable high FTS activity. The relationship between the amount of CO₂ in tail gas during activation and the Fe³⁺ (spm) content in the reduced catalyst was observed. The rapid carburization at high reduction temperature resulted in the formation of a superparamagnetic Fe³⁺ core and an iron carbide layer of the reduced catalyst. FTS activity decreased with the increase in the reduction temperature, but the stability distinctly improved. It was found that the working catalyst loss in the heavier waxy products resulted in higher deactivation rate of the catalyst reduced at lower temperature. With the increase in the reduction temperature, the product distribution shifted towards the lower molecular weight products.     

Study of an iron-based Fischer–Tropsch synthesis catalyst incorporated with SiO2

The effect of adding SiO₂ to a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst was investigated using N₂ physical adsorption, H₂ differential thermogravimetric analysis, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer spectroscopy. The FTS performances of the catalysts with or without SiO₂ were compared in a fixed bed reactor. The characterization results indicated that SiO₂ facilitates the high dispersion of Fe₂O₃ and significantly influences the Fe/Cu and Fe/K contacts, which play an important role in the surface basicity, reduction and carburization behaviors, as well as the FTS performances. The incorporation of SiO₂ enhances the Fe/Cu contact, further enlarges the H₂ adsorption and promotes the reduction of Fe₂O₃ → FeO_x, while the transformation of FeO_x → Fe is suppressed probably due to the strong Fe–SiO₂ interaction. SiO₂ indirectly weakens the surface basicity and severely suppresses the carburization and CO adsorption of the catalyst. In the FTS reaction, it was found that SiO₂ decreases the FTS initial activity but improves the catalyst stability. Due to the lower surface basicity than the catalyst without SiO₂, the catalyst incorporated with SiO₂ has higher selectivity to light hydrocarbons and methane and decreased selectivity to the olefins and heavy hydrocarbons.     

Effect of niobium promoter on iron-based catalyst for Fischer-Tropsch reaction

Niobium-promoted Fe/CNTs catalysts were prepared using a wet impregnation method.Samples were characterized by nitrogen adsorption,H2-TPR,TPD,XRD and TEM.The Fischer-Tropsch Synthesis(FTS) was carried out in a fixed-bed microreactor at 220 ℃,1 atm and H2/CO=2 for 5 h.Addition of niobium into Fe/CNTs increased the dispersion,decreased the average size of iron oxide nanoparticles and the catalyst reducibility.Niobium-promoted Fe catalyst resulted in appreciable increase in the selectivity of C5+ hydrocarbons and suppressed methane formation.These effects were more pronounced for the 0.04%Nb/Fe/CNTs catalyst,compared to those observed from other niobium compositions.The 0.04%Nb/Fe/CNTs catalyst enhanced the C5+ hydrocarbons selectivity by a factor of 67.5% and reduced the methane selectivity by a factor of 59.2%.     

Activation pressure studies with an iron-based catalyst for slurry Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) was carried out with an industrial iron-based catalyst (100Fe/5Cu/6K/16SiO2, by weight) under the baseline conditions in a stirred tank slurry reactor (STSR). The effects of activation pressure on the catalyst activity and selectivity were investigated. It was found that iron phase compositions, textural properties, and FTS performances of the catalysts were strongly dependent on activation pressure. The high activation pressure retards the carburization. Møssbauer effect spectroscopy (MES) results indicated that the contents of the iron carbides clearly decrease with the increase of activation pressure, especially for the activation pressure increasing from 1.0 MPa to 1.5 MPa, and the reverse trend is observed for superparamagnetic Fe3+ (spm). The higher content of Fe3+ (spm) results in the higher amount of CO2 in tail gas when the catalyst is reduced at higher pressure. The catalyst activity decreases with the increase of activation pressure. The high quantity of iron carbides is necessary to obtain high FTS activity. However, the activity of the catalyst activated in syngas can not be predicted solely from the fraction of the carbides. It is concluded that activation with syngas at the lower pressure would be the most desirable for the better activity and stability on the iron-based catalyst.     

A new kinetic model for direct CO_2 hydrogenation to higher hydrocarbons on a precipitated iron catalyst:Effect of catalyst particle size

The kinetic of the direct CO_2 hydrogenation to higher hydrocarbons via Fischer–Tropsch synthesis(FTS)and reverse water-gas shift reaction(RWGS) mechanisms over a series of precipitated Fe/Cu/K catalysts with various particle sizes was studied in a well mixed, continuous spinning basket reactor. The iron catalysts promoted with copper and potassium were prepared via precipitation technique in various alcohol/water mixtures to achieve a series of catalyst particle sizes between 38 and 14 nm. A new kinetic model for direct CO_2 hydrogenation was developed with combination of kinetic model for FTS reaction and RWGS equilibrium condition. For estimate of structure sensitivity of indirect CO_2 hydrogenation to higher hydrocarbons, the kinetic parameters of developed model are evaluated for a series of iron catalysts with various particle sizes. For kinetic study a wide range of syngas conversions have been obtained by varying experimental conditions. The results show that the new developed model fits favorably with experimental data. The values of activation energies for indirect CO_2 hydrogenation reaction are fall within the narrow range of 23–16 kJ/mol.