Stability of nanocrystals: thermodynamic analysis of oxidation and re-reduction of cobalt in water/hydrogen mixtures

The stability of nanosized materials differs significantly from the stability of bulk materials. In this study a thermodynamic analysis on the simultaneous oxidation and re-reduction of small metallic cobalt crystallites in the presence of water and hydrogen as a function of the crystallite diameter was performed as a model for catalyst deactivation in the Fischer-Tropsch synthesis. It is shown that spherical cobalt crystallites with a diameter less than 4.4 nm are likely to be oxidized under realistic Fischer-Tropsch synthesis conditions (p(H)(2)(O)/p(H)(2) < 1.5, T = 493 K).     

X-ray absorption spectroscopy of Mn/Co/TiO2 Fischer-Tropsch catalysts: relationships between preparation method, molecular structure, and catalyst performance

The effects of the addition of manganese to a series of TiO(2)-supported cobalt Fischer-Tropsch (FT) catalysts prepared by different methods were studied by a combination of X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and in situ X-ray absorption fine structure (XAFS) spectroscopy at the Co and Mn K-edges. After calcination, the catalysts were generally composed of large Co(3)O(4) clusters in the range 15-35 nm and a MnO(2)-type phase, which existed either dispersed on the TiO(2) surface or covering the Co(3)O(4) particles. Manganese was also found to coexist with the Co(3)O(4) in the form of Co(3-x)Mn(x)O(4) solutions, as revealed by XRD and XAFS. Characterization of the catalysts after H(2) reduction at 350 degrees C by XAFS and TEM showed mostly the formation of very small Co(0) particles (around 2-6 nm), indicating that the cobalt phase tends to redisperse during the reduction process from Co(3)O(4) to Co(0). The presence of manganese was found to hamper the cobalt reducibility, with this effect being more severe when Co(3-x)Mn(x)O(4) solutions were initially present in the catalyst precursors. Moreover, the presence of manganese generally led to the formation of larger cobalt agglomerates ( approximately 8-15 nm) upon reduction, probably as a consequence of the decrease in cobalt reducibility. The XAFS results revealed that all reduced catalysts contained manganese entirely in a Mn(2+) state, and two well-distinguished compounds could be identified: (1) a highly dispersed Ti(2)MnO(4)-type phase located at the TiO(2) surface and (2) a less dispersed MnO phase being in the proximity of the cobalt particles. Furthermore, the MnO was also found to exist partially mixed with a CoO phase in the form of rock-salt Mn(1-x)Co(x)O-type solid solutions. The existence of the later solutions was further confirmed by scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) for a Mn-rich sample. Finally, the cobalt active site composition in the catalysts after reduction at 300 and 350 degrees C was linked to the catalytic performances obtained under reaction conditions of 220 degrees C, 1 bar, and H(2)/CO = 2. The catalysts with larger Co(0) particles ( approximately >5 nm) and lower Co reduction extents displayed a higher intrinsic hydrogenation activity and a longer catalyst lifetime. Interestingly, the MnO and Mn(1-x)Co(x)O species effectively promoted these larger Co(0) particles by increasing the C(5+) selectivity and decreasing the CH(4) production, while they did not significantly influence the selectivity of the catalysts containing very small Co(0) particles.     

Water Oxidation Catalyzed by Cobalt(II) Adsorbed on Silica Nanoparticles

A novel, highly efficient, and stable water oxidation catalyst was prepared by a pH-controlled adsorption of Co(II) on ～10 nm diameter silica nanoparticles. A lower limit of ～300 s(-1) per cobalt atom for the catalyst turnover frequency in oxygen evolution was estimated, which attests to a very high catalytic activity. Electron microscopy revealed that cobalt is adsorbed on the SiO(2) nanoparticle surfaces as small (1-2 nm) clusters of Co(OH)(2). This catalyst is optically transparent over the entire UV-vis range and is thus suitable for mechanistic investigations by time-resolved spectroscopic techniques.     

SMAI法制备的Co/SiO2催化剂及La3+促进Co/SiO2催化剂的比较

实验证明,钴基催化剂是非常有效的F-T合成催化剂. 由于钴基催化剂对形成长链烷烃具有高活性和高选择性,故它尤其适用于天然气间接转换为液态燃料和蜡的过程[1～5]. F-T合成用钴基催化剂由四个主要成分组成: 主金属(Co)、第二过渡金属、氧化物助剂(碱金属、稀土金属或过渡金属氧化物)及大比表面积氧化物载体(氧化硅或氧化铝)[5]. La对钴基催化剂的促进效果因其被加入到催化剂前体中的方式和顺序以及载体的性质和金属钴的状态等参数的变化而有所不同. 为了评价La对Co/SiO2催化CO加氢作用的促进效果,本文对溶剂化金属原子浸渍法(SMAI)制备的Co/SiO2和Co/La-SiO2进行了对比研究. 在保持某些参数(如载体的性质和金属钴的价态等)不变的情况下,评价了La的助催化效果,取得了一些有益的结果.     

用不同前驱体制备的Co/SiO2催化剂的结构及其CO2/CH4重整反应性能

 分别采用硝酸钴、醋酸钴、硫酸钴和氯化钴为前驱体制备了Co／SiO2催化剂,用XRD,TPR,SEM和H2－TPD等实验技术考察了钴盐前驱体对催化剂结构和二氧化碳重整甲烷反应性能的影响,重点考察了硝酸钴和醋酸钴的作用．结果表明,由醋酸钴制备的Co／SiO2催化剂有最佳的催化活性和稳定性,它在钴物种的存在状态、金属－载体相互作用、钴金属晶粒度及抗烧结、抗积炭能力等方面,均与由硝酸钴制备的Co／SiO2催化剂存在显著的差别．Co／SiO2催化剂的反应活性和稳定性分别与其金属分散度和抗烧结、抗积炭能力密切相关．     

多级孔ZSM-5负载的钴催化剂的费-托合成催化性能

采用水蒸气辅助转晶(SAC)法合成了粒径均一(180 nm)的纳米ZSM-5颗粒,颗粒间堆积形成大量的开放介孔,与ZSM-5的微孔共同形成多级孔结构。以该材料为载体采用满孔浸渍法制备了负载量为15%(质量分数)的钴催化剂。采用XRD、SEM、TEM、N_2物理吸附-脱附等表征技术对多级孔ZSM-5载体及其负载催化剂的形貌和结构进行了表征,并对催化剂的费-托合成催化性能进行了测试。结果表明,相比于大颗粒的ZSM-5和商业ZSM-5,多级孔ZSM-5负载的钴催化剂的费-托合成活性最高,CH_4选择性最低,C_(5-20)产物的选择性高达68.9%,这归因于多级孔ZSM-5的介孔孔道有效地促进反应过程中产物的传质扩散以及ZSM-5微孔骨架上的酸中心促进了长链烃产物的二次加氢裂解。     

助剂对Co／HMS催化剂结构和F-T合成性能的影响

详细研究了钍、锆、锗及铈助剂对钴质量分数为15％的Co／HMS催化剂结构、 F-T合成CO转化率、CO2选择性及烃分布的影响,结果表明：钍能适当提高F-T合成 活性,且低温下具有较强的链增长能力;锆、锗、铈降低了催化剂CO转化率,催化 剂加氢能力变强,导致低碳烃增加较快,汽柴油馏分段减低,相应的链增长能力降 低,并以锰和铈较为明显;XRD,TPR及TG表征表明：锆和铈可提高催化剂Co还原度 ,但F-T合成反应时金属Co易披氧化,反应中金属Co量明显减少,CO转化率降低, 并以铈最为显著;Th助催化剂Co还原度稍有减低,Co分散度高于Co／HMS,且反应 中金属Co较为稳定,Co转化率得以提高;添加Mn助剂后,催化剂难以还原,反应中 活性相金属Co量较小,CO转化率较低．     

Decontamination of gaseous acetaldehyde over CoOx-loaded SiO2 xerogels under ambient, dark conditions

A series of CoO(x)-doped silica xerogels with various Co(2+) loadings (Co/Si = 0, 1, 2, 4, 6, and 10 mol %) has been prepared. All xerogels exhibit large (800-1050 m(2)/g) surface areas. Narrow pore size distributions with pore size maxima around 3 nm are characteristic for Co/Si = 1, 2, 4, 6, 10 samples. As-prepared CoO(x)/SiO(2) xerogels show high catalytic activity in the air oxidation of gaseous acetaldehyde at room temperature. Carbon dioxide and trace amounts of methane are the only products detected in the gas phase. Acetic acid, a less volatile product, resides on the surface of the xerogels but can slowly desorb. The formation of CO(2) begins after an induction period. The beginning of CO(2) production coincides with the conversion of Co(2+) incorporated in the SiO(2) framework into Co(3+). Thermogravimetry/gas chromatography/mass spectrometry analysis, UV-vis and FTIR spectroscopies, as well as kinetic measurements are employed for CoO(x)/SiO(2) catalyst characterization. A possible mechanism of the reaction is discussed.     

Fischer-Tropsch synthesis over ruthenium-promoted Co/Al2O3 catalyst with different reduction procedures

The effect of reduction procedure on catalyst properties, activity and products selectivity of ruthenium-promoted Co/γ-Al2O3 catalyst in Fischer-Tropsch synthesis (FTS) was investigated. Catalyst samples were reduced with different reduction gas compositions and passivated before being characterized by TPR and XRD techniques. Different activity and product selectivity analyses were also performed. These results showed that the catalyst dispersion, particle size, and the degree of reduction changed with different reduction gas compositions, which were resulted from the water partial pressures in reduction process that give varying degrees of interaction with the support. It has been suggested that the FTS activity of cobalt catalyst was directly dependent on the catalyst reducibility. A reduction gas with a molar ratio of H2/He = 1 was used to prevent the formation of Co-support compound during catalyst reduction.     

Microemulsion-based synthesis of CeO(2) powders with high surface area and high-temperature stabilities

Pure ceria powders, CeO(2), were synthesized in heptane-microemulsified aqueous solutions of CeCl(3) or Ce(NO(3))(3) stabilized by AOT (sodium bis(2-ethylhexyl) sulfosuccinate), DDAB (di-n-didodecyldimethylammonium bromide), or DDAB + Brij 35 surfactant mixtures. Micellar DTAB (n-dodecyltrimethylammonium bromide) and vesicular DDAB systems were also used as media for generating CeO(2). Characterization of the powders by X-ray powder diffractometry, laser-Raman spectroscopy, and Fourier transform infrared spectroscopy revealed that in the presence of surfactants almost-agglomerate-free nanosized crystallites (6-13 nm) of anionic vacancy-free cubic CeO(2) were produced. In the absence of surfactants 21-nm-sized crystallites were formed, comparing with the 85-nm-sized crystallites when cubic CeO(2) was created via thermal decomposition of cerium oxalate. Surface characterization, by X-ray photoelectron spectroscopy, N(2) sorptiometry, and high-resolution electron microscopy showed AOT- or (DDAB + Brij 35)-stabilized microemulsions to assist in formation of crystallites exposing surfaces of large specific areas (up to ca. 250 m(2)/g) but of low stability to high-temperature calcination (28-13 m(2)/g at 800 degrees C). In contrast, the double-chained DDAB was found to generate cubic CeO(2) crystallites of lower initial surface areas (144 (microemulsion) to 125 (vesicles) m(2)/g)) but of higher thermal stability (55-45 m(2)/g at 800 degrees C). Hence, the latter cerias could be considered as appropriate components for total oxidation (combustion) catalysts.     

Combined in situ XRD and in situ XANES studies on the reduction behavior of a rhenium promoted cobalt catalyst

A 10% Co-4% Re/(2% Zr/SiO(2)) catalyst was prepared by co-impregnation using a silica support modified by 2% Zr. The catalyst was characterized by temperature programmed reduction (TPR), in situ XRD and in situ XANES analysis where it was simultaneously exposed to H(2) using a temperature programmed ramp. The results showed the two step reduction of large crystalline Co(3)O(4) with CoO as an intermediate. TPR results showed that the reduction of highly dispersed Co(3)O(4) was facilitated by reduced rhenium by a H(2)-spillover mechanism. In situ XRD results showed the presence of both, Co-hcp and Co-fcc phases in the reduced catalyst at 400 °C. However, the Co-hcp phase was more abundant, which is thought to be the more active phase as compared to the Co-fcc phase for CO hydrogenation. CO hydrogenation at 270 °C and 5 bar pressure produces no detectable change in the phases during the time of experiment. In situ XANES results showed a decrease in the metallic cobalt in the presence of H(2)/CO, which can be attributed due to oxidation of the catalyst by reaction under these conditions.     

A zirconium modified Co/SiO2 Fischer-Tropsch catalyst prepared by dielectric-barrier discharge plasma

Co/SiO₂ and zirconium promoted Co/Zr/SiO₂ catalysts were prepared using dielectric-barrier discharge (DBD) plasma instead of the conventional thermal calcination method. Fischer-Tropsch Synthesis (FTS) performances of the catalyst were evaluated in a fixed bed reactor. The results indicated that the catalyst treated by DBD plasma shows the higher FTS activity and yield of heavy hydrocarbons as compared with that treated by the conventional thermal calcination method. Increase in CO conversion was unnoticeable on the Co/SiO₂ catalyst, but significant on the Co/Zr/SiO₂ catalyst, both prepared by DBD plasma. On the other hand, heavy hydrocarbon selectivity and chain growth probability (α value) were enhanced on all the catalysts prepared by the DBD plasma. In order to study the effect of the DBD plasma treatment on the FTS performance, the catalysts were characterized by N₂-physisorption, H₂-temperature programed reduction (H₂-TPR), H₂-temperature-programmed desorption (H₂-TPD) and oxygen titration, transmission electron microscope (TEM) and X-ray diffraction (XRD). It was proved that, compared with the traditional calcination method, DBD plasma not only could shorten the precursor decomposition time, but also could achieve better cobalt dispersion, smaller Co₃O₄ cluster size and more uniform cobalt distribution. However, cobalt reducibility was hindered to some extent in the Co/SiO₂ catalyst prepared by DBD plasma, while the zirconium additive prevented significantly the decrease in cobalt reducibility and increased cobalt dispersion as well as the FTS performance.     

Study of the elementary processes involved in the selective oxidation of methane over MoOx/SiO2

Isolated molybdate species supported on silica are reported to have the highest specific activity and selectivity for the direct oxidation of methane to formaldehyde. The present investigation was undertaken to understand the elementary redox processes involved in the formation of formaldehyde over such species. A MoO(x)/SiO(2) catalyst was prepared with a Mo loading of 0.44 Mo/nm(2). On the basis of evidence from extended X-ray absorption fine structure (EXAFS) and Raman spectroscopy, the Mo atoms in this catalyst are present as isolated, pentacoordinated molybdate species containing a single Mo=O bond. Isotopic labeling experiments in combination with in-situ Raman spectroscopy were used to examine the reducibility of the dispersed molybdate species and the exchange of O atoms between the gas phase and the catalyst. It was established that treatment of MoO(x)/SiO(2) at 873 K under pure methane reduces the dispersed molybdate species to only a limited extent and results mainly in the deposition of amorphous carbon. During CH(4) oxidation to formaldehyde, the catalyst undergoes only a very small degree of reduction and typically only approximately 50-500 ppm of Mo(VI) is reduced to Mo(IV). Reactions carried out using CH(4) and (18)O(2) show that there is extensive scrambling of O atoms between the species in the gas phase and the catalyst. Additional experiments revealed that H(2)O formed in the reaction is the principal species responsible for the exchange of O atoms between the gas phase and the SiO(2) support. Low concentrations of H(2)O were observed to enhance the activity of MoO(x)/SiO(2) for CH(4) oxidation to formaldehyde. A mechanism for the oxidation of CH(4) over MoO(x)/SiO(2) was formulated in light of the observations made here and is discussed in the light of previous studies. It is proposed that peroxides are produced by the reaction of O(2) with a small concentration of reduced molybdate species and that the reaction of CH(4) with these peroxide species leads to the formation of formaldehyde. The proposed mechanism also accounts for the positive effects of low concentrations of H(2)O on the rate of formaldehyde formation.     

Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H2), are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 approximately 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H2 treatment at low temperatures (100 approximately 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (>300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 approximately 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (>300 degrees C) is still higher than that directly pretreated with H2. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.     

Cobalt catalysts supported on silica nanotubes for Fischer-Tropsch synthesis

Silica nanotubes(SNT) have been synthesized using carbon nanotubes(CNT) as a template.Silica-coated carbon nanotubes(SNT-CNT) and SNT were loaded with a cobalt catalyst for use in Fischer-Tropsch synthesis(FTS).The catalysts were prepared by incipient wetness impregnation and characterized by N2 physisorption,X-ray diffraction(XRD),hydrogen temperature programmed reduction(H2-TPR) and transmission electron microscopy(TEM).FTS performance was evaluated in a fixed-bed reactor at 493 K and 1.0 MPa.Co/CNT and Co/SNT catalysts showed higher activity than Co/SNT-CNT in FTS because of the smaller cobalt particle size,higher dispersion and stronger reducibility.The results also showed that structure of the support affects the product selectivity in FTS.The synergistic effects of cobalt particle size,catalytic activity and diffusion limitations as a consequence of its small average pore size lead to medium selectivity to C5+ hydrocarbons and CH4 over Co/SNT-CNT.On the other hand,the Co/CNT showed higher CH4 selectivity and lower C5+ selectivity than Co/SNT,due to its smaller average pore size and cobalt particle size.     

Purification Influence of Synthesis Gas Derived from Methanol Cracking on the Performance of Cobalt Catalyst in Fischer-Tropsch Synthesis

1. Introduction Fischer-Tropsch synthesis (FTS) has gained in- dustrial attention for converting synthesis gas to high- boiling points waxes that can be further converted to sulfur-free motor fuels by hydrogenation or hydroc- racking as alternative resour…     

Metallic single-crystal CoSi nanowires via chemical vapor deposition of single-source precursor

We report the synthesis, structural characterization, and electrical transport properties of free-standing single-crystal CoSi nanowires synthesized via a single-source precursor route. Nanowires with diameters of 10-150 nm and lengths of greater than 10 mum were synthesized through the chemical vapor deposition of Co(SiCl(3))(CO)(4) onto silicon substrates that were covered with 1-2 nm thick SiO(2). Transmission electron microscopy confirms the single-crystal structure of the cubic CoSi. X-ray absorption and emission spectroscopy confirm the chemical identity and show the expected metallic nature of CoSi, which is further verified by room-temperature and low-temperature electrical transport measurements of nanowire devices. The average resistivity of CoSi nanowires is found to be about 510 muOmega cm. Our general and rational nanowire synthesis approach will lead to a broad class of silicide nanowires, including those metallic materials that serve as high-quality building blocks for nanoelectronics and magnetic semiconducting Fe(1-x)Co(x)Si suitable for silicon-based spintronics.     

Thermal decomposition and cobalt species transformation of carbon nanotubes supported cobalt catalyst for Fischer-Tropsch synthesis

The effect of calcination condition on the cobalt species and Fischer-Tropsch synthesis (FTS) was studied. It was found that higher calcination temperature resulted in decreased FTS activities because CNTs were consumed by oxidation in air at temperature higher than 230°C. Cobalt species went through transformation from Co3O4 to metallic Co in Ar by autoreduction at temperature over 500°C. The autoreduction route might be Co3O4→CoO→Co or Co3O4→Co2C→Co. Reduction at temperature higher than 500°C also resulted in decreased FTS activities due to the methanation of CNTs in hydrogen.     

Effect of the pore size of Co/SBA-15 isomorphically substituted with zirconium on its catalytic performance in Fischer-Tropsch synthesis

Cobalt catalysts supported on a series of mesoporous SBA-15 materials isomorphically substituted with zirconium (Zr/Si atomic ratio = 1/20) with different pore sizes (5.7 nm, 7.8 nm, 11.6 nm, 17.6 nm) have been synthesized. The catalysts were characterized by transmission electron microscopy, ²⁹Si solid state magic angle spinning (MAS) NMR, N₂ adsorption-desorption measurements, X-ray powder diffraction, X-ray photoelectron spectroscopy, H₂-temperature programmed reduction, H₂-temperature programmed desorption and O₂ titrations. The results indicated that larger pore size led to weaker interactions between cobalt and the supports which lowered the temperature of both reduction steps (Co₃O₄→CoO and CoO→Co⁰). The catalytic performances of the catalysts in Fischer-Tropsch synthesis (FTS) were tested in a fixed bed reactor. It was found that the FTS catalytic activity and product selectivity depended strongly on the pore size of the catalysts. The catalyst with a pore size of 7.8 nm showed the best FTS activity, and the catalyst with a pore size of 17.6 nm showed the highest selectivity to C₁₂–C₂₀ and C₂₀₊ hydrocarbons.     

Silylation of a Co/SiO2 catalyst. Characterization and exploitation of the CO hydrogenation reaction

Several silylated- and nonsilylated Co/SiO2 catalysts have been prepared by reaction of the surface silanol groups with hexamethyldisilazane (HMDS). These samples have been characterized by means of N2 adsorption isotherms, solid-state nuclear magnetic resonance (29Si and 1H), X-ray photoelectron spectroscopy, thermogravimetric analysis, and diffuse reflectance IR spectroscopy. We have focused on the study of the silylated surface stability at high temperatures and in different atmospheres. The characterization techniques have shown that silica silylation after cobalt impregnation leads to a silylated SiO2 surface composed of hydrophobic Si-(CH3)3 species highly stable up to 600-650 K in both oxidizing and reducing atmospheres. However, X-ray diffraction and temperature-programmed reduction have shown that the hydrophobic nature of the silica surface does not affect the metal dispersion and its reducibility. The materials prepared in this way have been tested as catalysts for the Fischer-Tropsch synthesis reaction. The CO conversion reaction rate increased over the silylated catalyst, probably as a consequence of the higher number of available active sites because water adsorption over the catalyst surface is impeded. However, catalyst deactivation was not affected by the hydrophobic nature of the support, suggesting that carbon deposition is the more probable mechanism of cobalt-based catalyst deactivation during the Fischer-Tropsch synthesis.