Agnieszka Pattek-Janczyk

Photodecomposition of dyes on Fe-C-TiO2 photocatalysts under UV radiation supported by photo-Fenton process.

Fe-C-TiO(2) photocatalysts were prepared by mechanical mixing of commercial anatase TiO(2) precursor with FeC(2)O(4) and heating at 500-800 degrees C under argon flow. These photocatalysts were tested for dyes decomposition: Methylene Blue (MB), Reactive Black (RB) and Acid Red (AR). The preliminary adsorption of dyes on the photocatalysts surface was performed. Modification of anatase by FeC(2)O(4) caused reducing of zeta potential of the photocatalyst surface from +12 to -7mV and decreasing of their adsorption ability towards RB and AR, which were negatively charged, -46.8 and -39.7, respectively. Therefore, unmodified TiO(2) showed the highest degree of RB and AR decompositions in the combination of dyes adsorption and UV irradiation. Methylene Blue, which had zeta potential of +4.3 in the aqueous solution was poorly adsorbed on all the tested photocatalysts and also slowly decomposed under UV irradiation. The high rate of dyes decomposition was noted on Fe-C-TiO(2) photocatalysts under UV irradiation with addition of H(2)O(2). It was observed, that at lower temperatures of heat treatment such as 500 degrees C higher content of carbon is remained in the sample, blocking the built in of iron into the TiO(2) lattice. This iron is reactive in the photo-Fenton process resulting in high production of OH radicals and also high activity of the photocatalyst. At higher temperatures of heat treatment, less active FeTiO(3) phase is formed, therefore Fe-C-TiO(2) sample prepared at 800 degrees C showed low photocatalytic activity for dyes decomposition. Fe-C-TiO(2) photocatalysts are active under visible light irradiation, however, the efficiency of a dye decomposition is lower than under UV light. In a dark Fenton process there is observed an insignificant generation of OH radicals and very little decomposition of a dye, what suggests the powerful of photo-Fenton process in the dyes decomposition.

Double electron exchange in Fe1−xO : A Mössbauer study

Abstract Mossbauer spectroscopy investigations have been performed at room temperature upon Fe 1−x O samples with 0.05 ⩽ x ⩽ 0.094 which have been previously characterized by X-ray diffraction and thermomagnetic analysis and the composition of which have been determined by thermogravimetric oxidation to Fe 2 O 3 . A model allowing to calculate the amount of Fe 3+ ions has been proposed; it assumes a fast electron exchange over iron atoms in octahedral sites between Fe 2+ and Fe 3+ . The amount of Fe 3+ ions calculated from the Mossbauer data agrees very well with those obtained directly from the thermogravimetric analysis.

Kinetics of activation of the industrial and model fused iron catalysts for ammonia synthesis

Abstract The paper summarizes our results published for many years and also adds new information. Kinetic data concerning the reduction of the oxidized forms of model as well as industrial catalysts used in ammonia synthesis were obtained in dry and in wet atmosphere containing 1% water vapour. The data for the industrial catalyst have been reassessed using three kinetic models. Modifications applied to the classical Seth-Ross model of the shrinking-core type resulted in the best fitting of this equation to the experimental data. The failure of the crackling core model to describe the kinetic data in a quantitative way is tentatively explained. The reduction of model catalysts containing enhanced amounts of wustite and/or potassium proceeds initially linearly with time. The effect of promoters, and especially of potassium, is discussed in more detail. The magnetite-alumina subsystem is responsible for the retardation effect of the water vapour on the reduction rate. A hypothesis is formulated that — in the presence of wustite or potassium — the inhibitive, Al-rich, hydrated surface layer is not effective in hindering the progress of the reduction process.

Wustite phase transformations in iron catalysts for ammonia synthesis

Abstract The behaviour of wustite (ferrous oxide) during the catalyst preparation and its activation process has been studied. The Mossbauer spectroscopy and X-ray diffraction have been used to follow wustite transformations. Inside the wustite phase two different wustite structures were found – one attributed to the nonstoichiometric wustite and the second one, to the compound close to the “stoichiometric” FeO. The relative contents of both wustites depended on the cooling rate of the melted precursor. At the initial stage of activation, at low temperatures, nonstoichiometric ferrous oxide was transformed into magnetite and a compound closer to the stoichiometric FeO. The last one decomposes further into magnetite and metallic iron.

Crystal structure and magnetic properties of tris(2-hydroxymethyl-4-oxo-4H-pyran- 5-olato-κ2O5,O4)iron(III)

Tris(2-hydroxymethyl-4-oxo-4H-pyran-5-olato-κ2O5,O4)iron(III) [Fe(ka)3], has been characterised by magnetic susceptibility measurements Mössbauer and EPR spectroscopy. The crystal structure of [Fe(ka)3] has been determined by powder X-ray diffraction analysis. Magnetic susceptibility and EPR measurements indicated a paramagnetic high-spin iron centre. Mössbauer spectra revealed the presence of magnetic hyperfine interactions that are temperature-independent down to 4.2 K. The interionic Fe3+ distance of 7.31 Å suggests spin-spin relaxation as the origin of these interactions.

Effect of preliminary heating on the activation of model iron catalyst for ammonia synthesis.: I. Mössbauer studies of the catalyst reduction

Abstract The oxidized form of a model, fused iron catalyst for ammonia synthesis, containing large amounts of wustite (ca. 40 wt.-%), was activated by reduction. The reduction of catalyst samples with different thermal history and different grain size was continuously monitored by thermogravimetry as well as periodically by Mossbauer spectrometry. Prior to the reduction, the samples were annealed in nitrogen atmosphere. The longer the time of primary annealing was, the faster was the reduction process. Some conclusions could be drawn about the nature of the acceleration—the induction period seemed to be shorter, whereas the reduction mechanism seemed to remain unchanged. The wustite phase disproportionates during the initial annealing, forming metallic iron and magnetite as the final products. Magnetite orginating from the decomposition of wustite showed a faster reduction rate than the primary magnetite.

SUBSTITUTION OF Fe3+ FOR Al3+ CATIONS IN LAYERED DOUBLE HYDROXIDE [LiAl2(OH)6]2 CO3·nH2O

Synthesis of the Li-Al-Fe layered double hydroxides was performed by the coprecipitation method at constant pH (11.0±0.2) and temperature (40±2°C). Structural features of the as-synthesized samples were investigated by X-ray diffraction (XRD), infrared (IR) spectroscopy, scanning electron microscopy and Mössbauer spectroscopy. The samples consisted of well crystallized [LiFexAl2-x(OH)6]2CO3·nH2O phases with strict ordering of M+ and M3+ cations in the sheets. However, only a proportion of Al3+ could be substituted by Fe3+ ions. The excess Fe3+ cations formed a separate ferrihydrite phase. Incorporation of Fe into the hydrotalcite-like structure resulted in an increase in the a lattice parameter determined by XRD. In addition, a shift of IR absorption bands, ascribed to the stretching vibrations of interlayer CO32− anions as well as the transitional motions of oxygen in the layers, to lower frequencies was observed. The presence of Fe3+ in the octahedral sheets caused a splitting of the band assigned to the stretching vibrations of the layer OH groups. Mössbauer experiments revealed that Fe exists in the synthesized samples in two different chemical environments. A proportion of the Fe3+ cations is incorporated as isolated ions in the [LiFexAl2-x(OH)6]2CO3.nH2O crystal structure. However, Fe3+ ions forming the ferrihydrite phase are dominant in the Fe-rich materials.

Effect of preliminary heating on the activation of model iron catalyst for ammonia synthesis.: Part II. Structural changes leading up to the nucleation process

Abstract Microscopic and microprobe studies of significantly active, fused model iron catalysts with increased amounts of wustite are reported. The catalysts were examined in untreated form and after annealing. Two types of magnetite (isometric and lamellar) were found. A definite influence of the annealing on the relative content of both magnetites was observed. In the annealed sample a third kind of magnetite (enriched in iron) was found. The creation of iron nuclei in the catalyst precursor is discussed and an suitable model is proposed.

Study on the preparation of KCFeAl2O3 catalysts for ammonia synthesis at mild conditions followed by Mössbauer spectroscopy

Abstract The state of iron in novel KCFe Al 2 O 3 catalysts for ammonia synthesis at mild conditions was studied by Mossbauer spectroscopy and X-ray diffraction. The activities of the catalysts were found to be comparable or even higher than the activity of the commercial contact. Two catalysts differing in iron loading were examined after every stage of preparation. The high (ca. 33 wt.%) and low (ca. 11 wt.%) Fe content resulted in a different behavior of iron during the catalyst preparation and different iron forms were found in the active contacts. In the catalyst with high iron loading the difference in Fe dispersion was observed while the low loading contact contained mainly Fe(III) species. Both highly dispersed iron and Fe(III) species were influenced by potassium, this being concluded from the Mossbauer parameters calculated for these forms.