A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H2O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differe...

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

https://science.sciencemag.org/content/sci/316/5832/1726.full.pdf

The Structure of Ferrihydrite, a Nanocrystalline Material

Magnetic Relaxation in Fine‐Particle Systems

Nowadays, magnetic hyperthermia constitutes a complementary approach to cancer treatment. The use of magnetic particles as heating mediators, proposed in the 1950s, provides a novel strategy for improving tumor treatment and, consequently, patient's quality of life. This review reports a broad overview about several aspects of magnetic hyperthermia addressing new perspectives and the progress on relevant features such as the ad hoc preparation of magnetic nanoparticles, physical modeling of magnetic heating, methods to determine the heat dissipation power of magnetic colloids including the development of experimental apparatus and the influence of biological matrices on the heating efficiency.

Fundamentals and advances in magnetic hyperthermia

Nanotechnology has spurred efforts to design and produce nanoscale components for incorporation into devices. Magnetic nanoparticles are an important class of functional materials, possessing unique magnetic properties due to their reduced size (below 100 nm) with potential for use in devices with reduced dimensions. Recent advances in processing by chemical synthesis and the characterisation of magnetic nanoparticles are the focus of this review. Emphasis has been placed on the various solution chemistry techniques used to synthesise particles, including: precipitation, borohydride reduction, hydrothermal, reverse micelles, polyol, sol–gel, thermolysis, photolysis, sonolysis, multisynthesis processing and electrochemical techniques. The challenges and methods for examining the structural, morphological, and magnetic properties of these materials are described.

Chemically prepared magnetic nanoparticles

Nanocrystalline α–Fe, Fe3C, and Fe7C3, particles with narrow size distributions were produced by CO2 laser pyrolysis of vapor mixtures of Fe(CO)5 and C2H4. Details of the synthesis procedure are discussed. Mossbauer spectroscopy and x-ray diffraction were used to identify the structural phases and the former was used also to study the magnetism of the nanoparticles. All the nanoparticles were observed to be ferromagnetic in this size range. If excess C2H4 appears in the reactant gas mixture, several monolayers of pyrolytic carbon were observed to form on the particle surface, as deduced from transmission electron microscopy and Raman scattering studies. Results of thermo-gravimetric analysis/mass spectroscopy studies of this carbon coating indicate it is gasified in hydrogen at temperatures T ∼ 250 °C.

Nanocrystalline α-Fe, Fe3C, and Fe7C3 produced by CO2 laser pyrolysis

Mossbauer spectroscopy, XAFS spectroscopy, and a number of complementary techniques have been used to investigate the structure and size dispersion of a variety of ultrafine iron-based DCL catalysts. In the as-prepared state, iron that was chemically incorporated into the coal exhibited an FeOOH structure, while iron catalysts prepared separately had the Fe 2 O 3 structure. The Mossbauer spectra exhibited pronounced superparamagnetic relaxation effects. The relaxation spectra could be analyzed at several temperatures to determine size distributions for the catalyst particles. The resulting size distributions were in the nanometer range and agreed reasonably well with size information obtained by SQUID magnetometry, STEM, and XRD

Structure and dispersion of iron-based catalysts for direct coal liquefaction

Mossbauer spectroscopy indicates that a 24 hour pretreatment in CO at 260°C and 8 atm. in a tetralin solvent almost completely converts ultrafrne iron oxide (about 3 nm) to iron carbide. However, pretreatment in hydrogen under the same conditions resulted in reduction of about 33% of the iron to metallic Fe; the remainder was Fe3O4Exposure of the CO pretreated catalyst to a 1:1 H2/CO synthesis gas resulted in the gradual reoxidation of the carbides to Fe304During the first 2 hours of exposure of the H2 pretreated sample to synthesis gas, the metallic Fe was converted to iron carbides. Further exposure of the H2 pretreatment sample to synthesis gas did not result in a composition change of the catalyst. Therefore, it is concluded that iron carbides with different oxidation characteristics were formed in these two cases.

Fischer-tropsch synthesis: mossbauer studies of pretreated ultrafine iron oxide catalysts

A significant enhancement of the liquefaction yields and desirable products from two lignites has been achieved by incorporating iron in the lignites by an ion-exchange process. The total conversion of iron ion-exchanged Hagel and Beulah lignites was found to increase by up to 25% and the oil yield by 10% relative to the untreated lignites. The ion-exchanged lignites were prepared by stirring a slurry mixture of the lignite and ferric acetate [Fe(OOCCH 3 ) 3 ] in a 10-L fermenter. The ion-exchange process, in which iron was exchanged primarily for calcium, yielded a highly dispersed catalytic iron species for coal liquefaction

B. Ganguly

Papers

Nanocrystalline α-Fe, Fe3C, and Fe7C3 produced by CO2 laser pyrolysis

Structure and dispersion of iron-based catalysts for direct coal liquefaction

Fischer-tropsch synthesis: mossbauer studies of pretreated ultrafine iron oxide catalysts

Liquefaction of lignite containing cation-exchanged iron

Determination of the Particle-Size Distribution of Iron Oxide Catalysts from Superparamagnetic Mössbauer Relaxation Spectra