A.M. van der Kraan

Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

The oxidation of supported cobalt based slurry bed Fischer–Tropsch catalysts by means of water was studied. Water is one of the Fischer–Tropsch reaction products and can probably cause oxidation and deactivation of a reduced cobalt catalyst. Model experiments using Mossbauer emission spectroscopy and thermogravimetry as well as realistic Fischer–Tropsch synthesis runs were performed. It was demonstrated that Mossbauer emission spectroscopy can successfully be applied to the investigation of high cobalt loading Fischer–Tropsch catalysts. Strong indications were found that oxidation of reduced cobalt catalysts occurs under realistic Fischer–Tropsch conditions. Mossbauer emission spectroscopy and thermogravimetry results showed that the oxidation depends on the PH2/PH2O ratio, and that oxidation proceeds to less than complete extents under certain conditions. The formation of both reducible and less reducible cobalt oxide species was observed, and the relative ratio between these species depends on the severity of the oxidation conditions.

Study of the critical behaviour of the magnetization and electrical resistivity in cubic La(Fe, Si)13 compounds

Abstract The magnetic properties of the cubic NaZn 13 type pseudobinary compounds La(T x Si 1- x ) 13 were studied for T = Fe, Co and Ni in the temperature range 4.2–300 K. 57 Fe Mossbauer spectroscopy was performed on LaFe 11 Si 2 and 57 Fe-doped LaCo 11 Si 2 . The compounds with T = Fe or Co are ferromagnetic, while the compound LaNi 11 Si 2 is a Pauli paramagnet. The critical behaviour around the Curie temperature T c was studied in La(Fe x Si 1- x ) 13 by measuring the temperature dependences on the zero-field susceptibility and the electrical resistivity. The critical exponent γ in the expression χ ∝ ( T - T c ) -γ was found to be close to 1.38 corresponding to an isotropic Heisenberg ferromagnet. The anomalous critical behaviour shown by electrical resistivity can be explained in terms of lattice softening associated with the Invar effect.

On the time-dependent behavior of iron catalysts in Fischer-Tropsch synthesis

Iron, cobalt, and nickel behave differently during the Fischer-Tropsch synthesis. Iron catalytic activity is initially low and increases slowly to a maximum while the catalytic activity of Co and Ni is essentially constant throughout the process. Three explanations denoted as the carbide model, competition model, and the slow activation model, have been previously advanced to explain this behavior. A brief survey of these explanations is included herein, and experiments and described whose results should define the catalytic behavior. Failure of the slow activation model and the carbide model in explaining two of the four experiments is taken as sufficient reason to reject these two models. The different rates of C diffusion into the metals will satisfactorily explain the difference between Fe and the other Fischer-Tropsch catalysts. Results of competition between the carbidation rate and the Fischer-Tropsch rate for the different catalysts leads to the assumption that the competition model most closely describes catalytic behavior. (BLM)

Magnetic properties of the ternary carbides Tm2Fe17Cx

Abstract We have investigated the crystallographic and magnetic properties of the ternary carbides Tm2Fe17Cx by means of X-ray diffraction, 57Fe Mossbauer spectroscopy, 169Tm Mossbauer spectroscopy and magnetic measurements. It is shown that small amounts of carbon raise the Curie temperature in Tm2Fe17Cx from below room temperature to about 500 K, at the same time increasing the average Fe moment. Important conclusions regarding the rare-earth sublattice anisotropy were derived from the quadrupole splitting of the 169Tm Mossbauer spectra and from the strong concentration dependence of the spin reorientation temperature in Tm2Fe17Cx.

Preparation and properties of supported cobalt catalysts for Fischer-Tropsch synthesis

Abstract Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.

Magnetic properties of amorphous rare-earth—Iron alloys

Abstract The magnetic properties of various amorphous alloys of the type R 1- x Fe x ( R = Sm , Gd , Tb , Dy , Ho , Er , Tm , Lu ) have been determined in the concentration range 0.3 ⩽ x ⩽ 0.5. With the exception of the Gd and Lu alloys pronounced thermomagnetic history effects were observed in the temperature dependence of the magnetization. These effects are due to a strong temperature dependence of the coercive force ( H c ) which is found to obey an exponential law of the form H c ∞ exp(-α T ). 57 Fe Mossbauer spectra were obtained on the alloys of a composition close to 40 at% Fe. From the combined results of the Mossbauer spectroscopy and magnetic measurements it is derived that the Fe moment decreases if one passes through the rare-earth series. It is postulated that this decrease is due to small differences in the compositional short-range order in the amorphous alloys caused by the heat of mixing becoming more negative in the same sense.

Characterization of surface phases in bimetallic FeRhSiO2 catalysts by in situ Mössbauer spectroscopy at cryogenic temperatures

Abstract Mossbauer spectra of FeRh SiO 2 catalysts (5 wt% metal, sol Fe Rh atomic ratio = 1) have been recorded in situ at liquid-helium, liquid-nitrogen, and room temperatures. After reduction in H 2 at 725 K, 80% of the iron in the catalyst is still in an Fe 3+ surface phase, while 20% of the iron is Fe 0 alloyed with at least 1.5 times as much Rh. Fischer-Tropsch synthesis in CO + 3.3 H 2 at 525 K converts a part of the surface Fe 3+ into an Fe 2+ compound. Exposure of this catalyst to air at room temperature results in oxidation of all Fe 2+ and nearly all of the Fe 0 to an Fe 3+ state. The results illustrate the information contained in the temperature dependence of the resonant absorption areas of the Mossbauer spectra and the need for low-temperature spectra in quantitative analysis.

Superparamagnetism in submicroscopic α-FeOOH particles observed by the Mössbauer effect

Mossbauer hyperfine spectra of ultrafine α-FeOOH particles measured at 77°K and 295°K show drastic changes such that the hyperfine field decreases before its collapse with decreasing particle size. These observations are similar to those obtained in superparamagnetic α Fe2O3.

Thermodynamics and Long-Range Order of Interstitials in an h.c.p. Lattice: Nitrogen in ε-Fe2N1-z

Abstract The thermodynamics of nitrogen in the e-Fe 2 N 1 − z phase were evaluated. To this end absorption isotherms for nitrogen in e-Fe 2 N 1 − z , depicting the dependence of the nitrogen content on the applied chemical potential of nitrogen, were determined in the temperature range 673–823 K by equilibrating iron foils in gaseous ammonia/hydrogen mixtures. The absorption isotherms could be described very well by a Gorsky-Bragg-Williams approximation for long-range order (LRO) of nitrogen atoms on the hexagonal nitrogen sublattice, formed by the octahedral interstices of the h.c.p. sublattice of iron atoms. The actual occurrence of LRO was evidenced by diffraction analysis. Mossbauer spectra were interpreted as composed of spectra of iron atoms surrounded by 1–3 nitrogen atoms. The relative amounts of the iron environments as determined from the Mossbauer spectra agreed very well with the corresponding probabilities predicted by the LRO model.

57Fe and 169Tm Mössbauer effect and magnetic properties of Tm2Fe15M2 (M ≡ Al, Ga, Si)

Abstract The Curie temperatures Tc and spin reorientation temperatures TSR were found to increase strongly when aluminium, gallium or silicon was substituted for iron in Tm2Fe17. By means of 57Fe Mossbauer spectroscopy we were able to show that there is no preferential substitution of these elements into the 4f or dumbbell site. The 169Tm Mossbauer effect was studied in Tm2Fe15Si2. We found that silicon substitution has led to a marked shift of the second-order crystal field parameter A02 to more negative values, the corresponding increase in K1 explaining the shift of TSR towards higher temperatures.