P. A. Chernavskii

Glow‐Discharge Plasma‐Assisted Design of Cobalt Catalysts for Fischer–Tropsch Synthesis

In the last twenty years, remarkable advances in nanosciences and nanotechnology have given an impulse to the design of heterogeneous catalysts. Bell emphasized in 2003 the role of nanoparticle size in catalyst performance, and Schl#gl and Abd Hamid proposed in 2004 that the synthesis of nanosized catalysts may require multidimensional structural control. Glow-discharge (luminous) plasma is obtained by applying a potential difference between two electrodes placed in a gas. The plasma provides energy for decomposition of metal precursors. Several active catalysts have been developed by using glow discharge. The glow-discharge activation process is simple, quick, audio-visual, and easy to control. It does not require the high temperatures and significant amounts of compressed gases which are typically used in conventional catalyst pretreatments. The increasing interest in Fischer–Tropsch (FT) synthesis has been due to the growing demand for clean fuels and utilization of abundant natural gas, coal, and biomass-derived synthesis gas. Cobalt catalysts are preferred for FT synthesis due to their high productivity, high selectivity for heavy hydrocarbons, high stability, and low activity in the water-gas shift reaction. The catalytic performance of cobalt catalysts in FT synthesis appears to be strongly affected by the size of the cobalt metal particles. Conventional cobalt FT catalysts are prepared by aqueous impregnation of supports (silica, alumina, titania, etc.) with solutions of cobalt salts. After decomposition of the supported cobalt salts by calcination in an oxidizing atmosphere, the catalysts are reduced in hydrogen to generate cobalt metal sites. The present work focuses on the effects of pretreatment with glow-discharge plasma on cobalt dispersion and reducibility in alumina-supported catalysts and their performance in FT synthesis. Details of catalyst preparation are given in the Experimental Section. Cobalt and platinum contents in catalysts were 15 wt% and 0.1 wt%, respectively. The conventionally calcined catalysts are denoted Co(Pt)-Al2O3-T, where T indicates the temperature of the calcination pretreatment and Pt indicates promotion with Pt. The monometallic and Pt-promoted catalysts that were prepared using glowdischarge plasma (shortened to: plasma-assisted catalysts) are designated Co-Al2O3-PNH and CoPt-Al2O3-PNH respectively (Table 1).

The nature of cobalt species in carbon nanotubes and their catalytic performance in Fischer–Tropsch reaction

The nature of cobalt species in the catalysts supported by multi-wall carbon nanotubes and their catalytic performance in Fischer–Tropsch synthesis were investigated using nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, in situ magnetic method, X-ray absorption and temperature programmed reduction. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate assisted by sonochemical process followed by calcination in nitrogen. The characterization techniques uncovered that acid pretreatment oxidized the carbon nanotube surface and removed impurities. Small cobalt oxide particles of 8–10 nm diameter and irregular shape anchored to the outer surface of carbon nanotubes were detected in the calcined samples by several characterization techniques. The catalysts displayed high cobalt reducibility, which was slightly affected by the pretreatment with nitric acid and nanotube outer diameter. Cobalt catalysts supported on carbon nanotubes exhibited high catalytic activity in Fischer–Tropsch synthesis. Pretreatment with nitric acid leads to a 25% increase in hydrocarbon yield, while carbon nanotube diameter does not seem to significantly affect the Fischer–Tropsch performance of the resulting catalysts.

Oxidation of metal nanoparticles: Experiment and model

Studies of oxygen adsorption on a metal surface are reviewed. The relationship between oxygen adsorption and the initial stage of oxidation is considered. Current theories of low-temperature oxidation and experimental data obtained during recent decades are reviewed. Thermodynamic aspects of low-temperature oxidation of metal nanoparticles and their relationship with the physical properties of metal nanoparticles are discussed. An original method for investigating the oxidation kinetics of ferromagnetic nanoparticles is described. The technique is based on continuous measurements of magnetization in the course of oxidation under controlled conditions. A stochastic model of oxidation is presented, which is based on the atomistic concepts of Cabrera-Mott theory. Oxidation is regarded as a random sequence of elementary acts (oxygen adsorption, oxygen ionization, metal ionization, metal ion migration, etc.).

Genesis of active sites in silica supported cobalt Fischer-Tropsch catalysts: effect of cobalt precursor and support texture

Optimization of cobalt deposition and textural properties of catalytic support represent two different but complementary approaches to enhance the performance of cobalt silica-supported Fischer-Tropsch catalysts. It is shown that low temperature decomposition of supported cobalt complexes involves formation of small Co3O4 crystallites. The exothermic effect due to the decomposition of cobalt precursor seems to be a major factor responsible of cobalt silicate formation. Gentle decomposition of cobalt precursor generally leads to Fischer-Tropsch catalysts with higher activity and selectivity to higher hydrocarbons. It was found that the sizes of supported cobalt particles prepared using cobalt nitrate depended on silica pore sizes. Narrow pore size distribution in periodic mesoporous silicas allows relatively high cobalt dispersion to be maintained at high cobalt loading. This leads to higher density of active sites and more active Fischer-Tropsch catalysts.

The influence of oxide-oxide interaction on the catalytic properties of Co/Al2O3 in CO hydrogenation

The influence of oxide–oxide interaction on the catalytic properties of cobalt in CO hydrogenation is investigated on the example of a Co/Al2O3 catalyst. Oxide–oxide interaction was prevented by modification of the alumina surface with magnesia. It has been shown that oxide–oxide interaction affects catalytic activity and the amount of carbon deposited on the catalyst surface.

Experimental Setup for Investigating Topochemical Transformations of Ferromagnetic Nanoparticles

An experimental setup controlling the topochemical transformations in synthesis of ferromagnetic metal nanoparticles is described. The setup is based on a vibrating magnetometer. The range of operating temperatures in the reaction zone is 300–870 K. The required sensitivity is maintained by a magnetic field of up to 0.6 T. Gases (or gas mixtures) may be blown through the reactor at a flow rate as high as 150 cm3/min. The experimental results illustrating the capabilities of the setup are presented.

Size Distribution of Cobalt Particles in Catalysts for the Fischer-Tropsch Synthesis

The effect of the size distribution of metal particles on the process kinetics was studied for the oxidation and reduction of cobalt nanoparticles (6–10 nm) in the Co/SiO2, Co/Al2O3, and Co/ZrO2 systems in both isothermic and temperature-programmed regimes in the temperature range from 280 to 500 K. The average size of the cobalt particles was estimated by measurements of the coercive force and residual magnetization using a vibration magnetometer. It was found that the average particle size increases upon cobalt oxidation and decreases upon cobalt reduction due to changes in the fraction of nonsuperparamagnetic particles with sizes of at least 6.5 nm.

Effect of the ZrO2 Pore Structure on the Reduction of a Supported Cobalt Oxide in Catalysts for Fischer–Tropsch Synthesis

Processes occurring in the preparation of the Co/ZrO2 + 6% Y2O3 catalyst are studied by temperature-programmed reduction (TPR). The effects of the concentration of Co, the porosity of the support, and the calcination temperature on catalyst reduction were studied. As was shown by continuous magnetization measurements in the course of TPR, metallic cobalt appeared on the microporous support in two temperature ranges irrespective of the precalcination temperature and the concentration of supported cobalt. These factors affect the reduction rate but do not change the maximum temperatures of the corresponding peaks. It is suggested that the first maximum of the Co formation rate is due to the reduction of CoO particles on the surface of the support and within macropores, whereas the second maximum is due to the reduction of CoO particles located within support micropores. Only one temperature range of CoO reduction was found in the macroporous ZrO2 + 6% Y2O3. This effect is likely due to mass transfer in support micropores.

Mechanistic Aspects of the Activation of Silica-Supported Iron Catalysts for Fischer–Tropsch Synthesis in Carbon Monoxide and Syngas

The mechanism of activation of silica‐supported iron catalysts for Fischer–Tropsch synthesis was investigated in syngas or carbon monoxide under transient and isothermal conditions using the in situ magnetic method. The catalyst activation proceeds in two steps and involves reduction of hematite into magnetite and magnetite carbidisation into Hägg carbide. Smaller supported iron particles exhibit higher rates of hematite reduction and magnetite carbidisation than the larger counterparts. The reduction of hematite to magnetite proceeds with similar rates in syngas and pure carbon monoxide, while magnetite can be carbidised more rapidly in carbon monoxide. The concentration of iron carbide was approximately 3 times higher after activation in CO relative to the activation in syngas.

Dimensional Effects in the Carbidization of Supported Iron Nanoparticles

Fe-based catalysts are of both practical and academic interest for the synthesis of hydrocarbons from syngas (mixtures of carbon monoxide and hydrogen) by using the Fischer–Tropsch (FT) reaction. Iron catalysts are much cheaper than cobalt catalysts and they can be used in both low-temperature and hightemperature FT syntheses. In addition, Fe-based catalysts can be used to convert syngas with lower H2/CO ratios, which is usually produced from the gasification of coal and renewable feedstock, such as biomass. Nanoparticle size is of major importance in FT synthesis. Indeed, it has been shown that the reaction turnover frequency (TOF) and hydrocarbon selectivity depend on the metal-particle size for cobalt and ruthenium catalysts. For Fe-based catalysts, information regarding the effect of iron-nanoparticle size on catalytic performance is scarcer. However, Mabaso et al. have shown that the TOF decreases and that the selectivity for methane grows if the Fe particles are smaller than 7–9 nm. In more-recent work, a notable decrease in the TOF is observed for nanoparticles that are smaller than 6 nm; the light-hydrocarbon selectivity also grows along with a decrease in the particle size. It is known that Fe-based catalysts can undergo a number of phase transformations during catalyst preparation and activation, as well as during the catalytic reactions. Catalyst activation usually involves a transition from hematite (Fe2O3) into magnetite (Fe3O4), w stite (FeO), and metallic iron in the presence of hydrogen or syngas; subsequent transformation depends on the composition of the gas mixture (hydrogen, syngas, CO), the total pressure, catalyst reduction, and the reaction temperature. The presence of carbon monoxide in the reducing gas mixture also leads to the formation of iron carbides. Several iron carbides have been described in the literature: FeC, hexagonal e-Fe2C, pseudo-hexagonal e’-Fe2.2C, monoclinic H gg c-Fe5C2 carbide, and orthorhombic q-Fe3C cementite. All of these carbides were detected in iron catalysts during Fischer–Tropsch syntheses. The carbide composition depends on the H2/CO ratio, the reaction pressure, the size of the iron particles, the extent of iron reduction, and the temperature. In a large number of publications, the FT activity of the iron catalysts has been associated to the presence of iron carbides. In this context, the influence of iron-nanoparticle size on the kinetics of iron-carbide formation and iron-carbide hydrogenation seem to be very important for the design of efficient catalysts and for tuning their catalytic performance in FT synthesis. At the same time, no information is available regarding the effect of iron-particle size on the formation and stability of iron carbides under FT conditions. Herein, we focus on the study of nanoparticle-size effects on the kinetics of iron carburization and on the stability of iron carbides in Fe/SiO2 catalysts in pure carbon monoxide and in syngas under FT conditions. The details of catalyst preparation and activation are given in the Experimental Section. In agreement with our previous report, after reduction at 500 8C, the degree of iron reduction was close to 100 % for all three samples. The Supporting Information, Figure S1, shows histograms of the iron-particle size in the studied catalysts. Clearly, the average particle size increases from 11Fe/Q-15 to 5.7Fe/Q-50 and then to 11Fe/Q50. The average particle size in samples 11Fe/Q-50 and 5.7Fe/ Q-50 was 25 and 15 nm, respectively, and that in 11Fe/Q-15 was 9 nm (Table 1). Note that the particle-size distribution in these three catalysts may overlap to some extent. The Supporting Information, Figure S2, shows the temperature dependence of the relative magnetization (s) of catalysts 5.7Fe/ Q-50, 11Fe/Q-50, and 11Fe/Q-15, which were pretreated with H2/CO (1:1) at 3308C for 1 h, as measured during the temperature ramping in ultrahigh-purity Ar. As expected, the magnetization decreases with temperature, which corresponds to thermal disordering of the ferromagnetic domains. Interestingly, the magnetization is constant at a given temperature and does not vary with time. This result suggests that treatment in H2/CO (1:1) at 330 8C for 1 h lead to the highest possible extent of iron carbidization under these conditions and that the catalyst composition does not change during the thermal treatment in argon. Another interesting observation is that the thermomagnetic curves (see the Supporting Information, Figure S2 a) are about the same for both catalysts 5.7Fe/Q-50 and 11Fe/Q-50. The Supporting Information, Figure S2, also shows the temperature dependence of ds/dT; its minima correspond to the Curie temperatures for the individual iron carbides that are present in catalysts 5.7Fe/Q-50 and 11Fe/Q-50. Within the range 20–4008C, two Curie temperatures can be distinguished from these minima: 2408C (513 K) and 3678C (640 K). In agreement with the report of Xu and Bartholomew, these [a] Prof. P. A. Chernavskii, Dr. G. V. Pankina Department of Chemistry M. V. Lomonosov Moscow State University, Moscow (Russia) E-mail : chern@kge.msu.ru [b] Dr. V. I. Zaikovskii Boreskov Institute of Catalysis Siberian Branch of Russian Academy of Sciences Novosibirsk (Russia) [c] Dr. A. Y. Khodakov Unit de Catalyse et de Chimie du Solide USTL-ENSCL-EC LIlle, B t. C3, Cite Scientifique 59655 Villeneuve d’Ascq (France) Fax: (+ 33) 3-20-43-65-61 E-mail : andrei.khodakov@univ-lille1.fr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200960.