Jan Šubrt

Transformation of brookite-type TiO2 nanocrystals to rutile: correlation between microstructure and photoactivity

Nanometric particles of pure brookite TiO2 were synthesized by modified thermolysis of reactant solutions containing titania powder, HCl, urea and PEG 10000. Unique flower-like brookite agglomerates with an average diameter of ∼400–450 nm composed of single brookite nanocrystals of ∼4–5 nm were obtained at 105 °C. The brookite → rutile transformation has been studied and TiO2 mixtures with variable amount of anatase, brookite and rutile polymorphs at different temperatures (from 200 to 800 °C) were obtained. High resolution transmission electron microscopy (HRTEM), electron diffraction pattern and BET/BJH analyses were used to characterize the phase assemblages, crystallite size and pore volume of the pure-phase brookite and TiO2 mixtures. In order to understand the metastable–stable TiO2 phase transformation X-ray powder diffraction (XRD) was performed. The photoactivity of pure brookite and TiO2 powders with different compositions of the brookite–anatase–rutile and anatase–rutile polymorphs obtained during the transitions was examined by photocatalyzed degradation of 4-chlorophenols in aqueous solution. The titania sample having the highest catalytic activity was obtained at 500 °C, contained 3.2% brookite, 42.9% anatase and 53.9% rutile and is referred to as TiO[B])/500 .

Magnesium incorporated bentonite clay for defluoridation of drinking water.

Low cost bentonite clay was chemically modified using magnesium chloride in order to enhance its fluoride removal capacity. The magnesium incorporated bentonite (MB) was characterized by using XRD and SEM techniques. Batch adsorption experiments were conducted to study and optimize various operational parameters such as adsorbent dose, contact time, pH, effect of co-ions and initial fluoride concentration. It was observed that the MB works effectively over wide range of pH and showed a maximum fluoride removal capacity of 2.26 mgg(-1) at an initial fluoride concentration of 5 mg L(-1), which is much better than the unmodified bentonite. The experimental data fitted well into Langmuir adsorption isotherm and follows pseudo-first-order kinetics. Thermodynamic study suggests that fluoride adsorption on MB is reasonably spontaneous and an endothermic process. MB showed significantly high fluoride removal in synthetic water as compared to field water. Desorption study of MB suggest that almost all the loaded fluoride was desorbed ( approximately 97%) using 1M NaOH solution however maximum fluoride removal decreases from 95.47 to 73 (%) after regeneration. From the experimental results, it may be inferred that chemical modification enhances the fluoride removal efficiency of bentonite and it works as an effective adsorbent for defluoridation of water.

STUDY OF NATURAL AND ION EXCHANGED VERMICULITE BY EMANATION THERMAL ANALYSIS, TG, DTA AND XRD

Emanation thermal analysis (ETA), thermogravimetry, DTA and XRD were used in thermal characterization of natural vermiculite (Santa Olalla, Huelva, Spain) and of Na+- and - exchanged vermiculite samples during heating in air in the range 25-1100°C. A good agreement between the results of these methods was found. Changes in the radon release rate measured by ETA, which reflected the decrease and collapse of the interlayer space after the release of water as well as the formation of new crystalline phases were evaluated using a mathematical model. The model used for the evaluation was found suitable for the quantitative characterization of microstructure changes during in situ conditions of heating of vermiculite samples.

Study of nano-structured ceria for catalyticCO oxidation

Mesoporous, nano-structured ceria has been synthesized using a low-cost biopolymer chitosan as a template, and the same material has been studied for its CO oxidation activity. The ceria thus prepared shows low crystallite size (6.5 nm), high surface area (BET-SA 144.2 m2 g−1) and mesopores (32.4 A) without ordered structure. Different ceria samples have been synthesized by manipulating the chitosan : cerium ratio and synthesis temperature. These ceria samples have been characterized using XRD, BET-SA, SEM, FTIR, UV-DRS, PL and TEM techniques. The ceria samples show fluorite structure with cubic symmetry and increased lattice volume. The nano-structured ceria thus synthesized lowers the CO oxidation temperature by 30–60 °C as compared to the commercial ceria. The improved catalytic activity primarily appears to be the effect of improved surface area and pore characteristics of nano-structured ceria. Also the expansion in lattice cell volume leads to the improvement in redox properties of ceria, thereby facilitating the formation of surface oxygen vacancies. Such mesoporous ceria without ordered structure could be useful as an oxidation catalyst as well as a support for various catalysts. The platinum incorporated mesoporous ceria also shows excellent CO oxidation.

Synthesis of acicular hematite catalysts with tailored porosity

The topotactic thermal decomposition of a synthetic acicular goethite sample into hematite has been studied by controlled rate thermal analysis (CRTA). This method allows a precise, simultaneous and independent control of both the partial pressure of water vapour generated from the decomposition of goethite and the decomposition rate of this precursor. It has been demonstrated that the CRTA method allows obtaining acicular particles of α‐Fe2O3 with tailored porosity oriented along the c lattice axis (the longest axis of the particle). This finding would be of great potential interest from the point of view of the use of this material as a catalyst.

Fe2O3–SiO2 composites obtained by sol–gel synthesis

The present work deals with the synthesis of Fe2O3–SiO2 composites, obtained by embedding iron oxide particles in a silica matrix, through sol–gel method based on hydrolysis and condensation of a silicon alkoxide and thermal treatment. Methods employed in characterisation of the materials obtained at different iron concentration and at suitable thermal treatments are infrared spectroscopy, X-ray diffraction analysis and SEM analysis. In purpose to evaluate magnetic properties of these composites, static magnetic and Mossbauer spectra measurements were carried out.

Magnetic properties of magnetite prepared by ball-milling of hematite with iron

Magnetic properties of magnetite powder prepared by ball-milling of stoichiometric mixture of hematite and iron in an inert atmosphere are reported. Hysteresis loops, isothermal remanence acquisition curves and temperature dependence of magnetic susceptibility measurements are used to characterise this material and to examine the effects of heating in air and in an argon atmosphere. Ball-milling of hematite with iron during periods ranging from 30 min up to almost 5 h yields magnetite which exhibits high magnetic hardness, characterised by coercive force three times higher than that typical for single-domain natural magnetites. However, the magnetite produced is unstable upon heating in air, reoxidising almost completely to hematite. Heating in an argon atmosphere causes enhancement of typical magnetic parameters, but decreases the magnetic hardness.

Laser photolysis of liquid benzene and hexafluorobenzene: Graphitic and polymeric carbon formation at ambient temperature

Abstract ArF laser-induced liquid-phase photolysis of benzene and hexafluorobenzene afford similar products which are polyaromatic hydrocarbons or fluorocarbons with the preponderance of biphenyl or decafluorobiphenyl, graphite, and polymeric H-containing or F-containing carbon. The remarkable feature is that the graphite formation occurs at ambient temperature of the irradiated liquids; photolytic graphitization modes must therefore be different from those occurring in thermally-induced graphitization at temperatures above 2500 °C.

The use of constant rate thermal analysis (CRTA) for controlling the texture of hematite obtained from the thermal decomposition of goethite

Constant rate thermal analysis (CRTA) has been used for studying the decomposition of three synthetic needle-shaped goethite samples. This method controls the reaction temperature such that the reaction rate and partial pressure of water vapour are kept constant at a selected value. The effects of the pressure of water vapour generated during the dehydration of goethite and of the decomposition rate on the porosity of the resulting hematite have been studied. The effect of the particle size of the precursors on the texture of the final products has also been analysed. By this method, acicular particles of α-Fe 2 O 3 with controlled porosity oriented along the c-lattice axis (the long axis of the particle) have been prepared.

Emanation Thermal Analysis: Ready to fulfill the future needs of materials characterization

The paper reviews the actual state of the development and use of emanation thermal analysis (ETA). Examples of its recent applications are presented. The advantages of ETA in the microstructure characterization of materials under in situ conditions of their heat treatment are outlined.