Tomáš Grygar

TiO2-graphene oxide nanocomposite as advanced photocatalytic materials

BackgroundGraphene oxide composites with photocatalysts may exhibit better properties than pure photocatalysts via improvement of their textural and electronic properties.ResultsTiO2-Graphene Oxide (TiO2 - GO) nanocomposite was prepared by thermal hydrolysis of suspension with graphene oxide (GO) nanosheets and titania peroxo-complex. The characterization of graphene oxide nanosheets was provided by using an atomic force microscope and Raman spectroscopy. The prepared nanocomposites samples were characterized by Brunauer–Emmett–Teller surface area and Barrett–Joiner–Halenda porosity, X-ray Diffraction, Infrared Spectroscopy, Raman Spectroscopy and Transmission Electron Microscopy. UV/VIS diffuse reflectance spectroscopy was employed to estimate band-gap energies. From the TiO2 - GO samples, a 300 μm thin layer on a piece of glass 10×15 cm was created. The photocatalytic activity of the prepared layers was assessed from the kinetics of the photocatalytic degradation of butane in the gas phase.ConclusionsThe best photocatalytic activity under UV was observed for sample denoted TiGO_100 (k = 0.03012 h-1), while sample labeled TiGO_075 (k = 0.00774 h-1) demonstrated the best activity under visible light.

MnOx/C composites as electrode materials. I. Synthesis, XRD and cyclic voltammetric investigation

MnOx/C composite material was obtained by precipitation on suspended carbon black particles from dissolved KMnO4. Doped composites were obtained by addition of Ca, Mg, Ni and other ions to the reaction mixture. The resulting solids are XRD amorphous or very poorly crystalline, but their chemical composition, Mn valence and electrochemical behaviour are very similar to those reported for birnessite, 7-A layered manganate(III,IV). We tested the electrochemical reduction/oxidation of the composite in 2 M KOH using cyclic voltammetry with three kinds of electrodes: microparticles immobilised on a carbon electrode (voltammetry of microparticles, vmp), carbon paste electrodes (cpee) and PTFE-bonded electrodes (tbe). Each technique has been used for its own merit: vmp is the best pilot technique, cpee enables fast quantification of the charge/mass relations, and tbe is the most reliable, but the slowest and the most laborious one. All the employed techniques yield directly comparable results. The composite can be discharged/recharged several tens of times at approximately 1 e−/Mn efficiency.

Iron oxide mineralogy in late Miocene red beds from La Gloria, Spain: rock-magnetic, voltammetric and Vis spectroscopy analyses

Abstract Free ferric oxides of a red bed series were analyzed by rock-magnetic techniques (IRM component analysis) and by two less traditional methods: visible spectroscopy and voltammetry. All three methods have low limits of detection, making them suited for this type of analysis. The red bed samples studied contained clay minerals, quartz, and calcium carbonate as major constituents. Free Fe oxides occurred at a concentration of 0.3–2.1%, i.e. in the majority of the samples below the detection limit of X-ray powder diffraction. The combination of the employed analytical techniques enabled to characterize the mineralogy of free ferric oxides and to estimate the ratio of goethite and to hematite. This ratio changes substantially within the section that probably indicates paleoclimatic changes.

Cerium dioxide as a new reactive sorbent for fast degradation of parathion methyl and some other organophosphates

Abstract Cerium dioxide was used for the first time as reactive sorbent for the degradation of the organophosphate pesticides parathion methyl, chlorpyrifos, dichlofenthion, fenchlorphos, and prothiofos, as well as of some chemical warfare agents-nerve gases soman and O -ethyl S -[2-(diisopropylamino) ethyl] methylphosphonothioate (VX). CeO 2 specimens were prepared by calcination of basic cerous carbonate obtained by precipitation from an aqueous solution. The CeO 2 samples containing certain amounts (1 wt.%-5 wt.%) of the neighboring lanthanides (La, Pr, Nd) were prepared in a similar way from pure lanthanide salts. It was shown that ceria accelerated markedly the decomposition of parathion methyl causing the cleavage of the P-O-aryl bond in the pesticide molecule. A similar reaction mechanism was proposed for the degradation of other organophosphate pesticides and nerve agents. The degradation times (reaction half-times) were in an order of minutes in the presence of CeO 2 , compared to hours or days under common environmental conditions. The reaction in suitable organic solvents allowed conversions of about 90% for parathion methyl loading of 20 mg pesticide/g CeO 2 within 2 h with a reactant half-life in the order of 0.1 min. The key parameter governing the degradation efficiency of CeO 2 was the temperature during calcination. At optimum calcination temperature (about 773.15 K), the produced ceria retained a sufficiently high surface area, and attained an optimum degree of crystallinity (related to a number of crystal defects, and thus potential reactive sites). The presence of other lanthanides somewhat decreased the reaction rate, but this effect was not detrimental and permitted the possible use of chemically impure ceria as a reactive sorbent. A fast organophosphate degradation was demonstrated not only in non-polar solvents (such as heptane), but also in polar aprotic solvents (acetonitrile, acetone) that are miscible with water. This opens new possibilities for designing more versatile decontamination strategies. The cleavage of phosphate ester bonds is of a great importance not only for the degradation of dangerous chemicals (chemical weapons, pesticides), but also for interactions of ceria (especially the nano-sized one) in biologically relevant systems.

The pH-dependent leaching of inorganic contaminants from secondary lead smelter fly ash

The leaching behaviour of fly ash (FA) from a secondary Pb smelter was assessed using the pH-static leaching experiment according to prEN 14997 (pH range 3-11) coupled with mineralogical investigation of the leached FA by XRD and Rietveld analyses and thermodynamic modelling using PHREEQC-2. The procedure was performed on fresh FA and FA washed at a cumulative L/S ratio of 60l/kg to remove readily soluble salts. For both fresh and washed FA, high amounts of inorganic contaminants were released under acidic conditions, exhibiting L-shaped leaching patterns: up to 300g Pb/kg, 4.5g Cd/kg, 4g Zn/kg, 1.05g As/kg and 70mg Sb/kg. The washing of soluble salts significantly decreased the leachability of Cd, Zn, As and Sb and increased the release of Pb, especially under acidic conditions. The leaching of fresh FA removed part of primary caracolite and all the KPb(2)Cl(5) and NaCl. The Pb release was controlled by the precipitation of anglesite and PbSO(3) under acidic conditions and of laurionite and carbonates (hydrocerussite and phosgenite) under alkaline conditions. In contrast, the washed FA was composed mainly of anglesite and PbSO(3), both phases being the main solubility-controlling phases for Pb over the whole studied pH range.

Sol–gel processing and magnetic properties of nickel zinc ferrite thick films

Abstract Nickel zinc ferrite Ni 0.36 Zn 0.64 Fe 2 O 4 (NZF) thick films have been synthesised using a dip coating sol–gel process. The coating sol has been formed from NZF powders dispersed in the NZF raw sol. A suitable processing temperature for the preparation of NZF composite films with reasonable magnetic properties has been found to be 400°C. The magnetisation of NZF films increased and coercive force decreased with the processing temperature. Typical values of magnetisation and coercive force of the NZF films have been found to be Ms=110 emu/cm 3 and Hc=20 Oe, respectively. The sol–gel method combined with dispersion of ceramic NZF particles in starting sols has been proved to be useful for producing thick nickel zinc ferrite films.

The electrochemical dissolution of iron(III) and chromium(III) oxides and ferrites under conditions of abrasive stripping voltammetry

Abstract Electrochemical dissolution of iron(III) and chromium(III) oxides can be described as an irreversible reaction of homogeneous, continuous sets of particles or as a mixture of such sets. Each set can be characterized by the rate constant at reference potential, charge-transfer coefficient and heterogeneity factory γ within a certain potential range in which not mass transport but electron transfer controls the overall reaction rate. This condition is easy to maintain in the case of chronoamperometry but not in the case of linear sweep voltammetry at a scan rate of 1 mV s −1 or more. However, the commonly used relationship between the potential of a voltammetric peak and the kinetic characteristics of the charge-transfer is valid. The commonly used exponential shape of chronoamperometric curve is a special case for certain size distributions of particles. The influence of the heterogeneity factor γ on the shape of voltammetric peak is discussed. According to the results obtained, abrasive stripping voltammetry is a more effective tool for studying solid compounds than the carbon paste method, because of negligible disturbance from dissolved species and a correspondingly simpler voltammetric curve, as demonstrated here in the case of CuFe 2 O 4 .

Voltammetric analysis of iron oxide pigments

Eighteen earthy and four pure synthetic pigments containing alpha-Fe2O3 (hematite), alpha-FeOOH (goethite) and poorly crystalline Fe and Mn oxide species were analyzed by voltammetry of microparticles. Three natural samples were subjected to an interlaboratory test to evaluate the reproducibility of the voltammetric peak potentials and peak shapes. The results confirmed that linear-sweep voltammetry is able to distinguish between poorly crystalline, ferrihydrite-like oxides and well-crystalline hematite and goethite and to detect XRD-amorphous Mn(III,IV) oxides via the peak occurrence. Voltammetry is further able to distinguish between pigments containing well-crystalline goethite (according to its structural features) and hematite (according to its particle size). The microsamples of primers from two baroque paintings were also analyzed by XRD and voltammetry and shown to be analogous to common clayey ochres.

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.

Cadmium, lead and zinc leaching from smelter fly ash in simple organic acids—Simulators of rhizospheric soil solutions

Emissions from base-metal smelters are responsible for high contamination of the surrounding soils. Fly ash from a secondary Pb smelter was submitted to a batch leaching procedure (0.5-168 h) in 500 microM solutions of acetic, citric, or oxalic acids to simulate the release of toxic metals (Cd, Pb, Zn) in rhizosphere-like environments. Organic acids increased dissolution of fly ash by a factor of 1.3. Cadmium and Pb formed mobile chloro- and sulphate-complexes, whereas Zn partly present in a citrate (Zn-citrate(-)) complex is expected to be less mobile due to sorption onto the positively charged surfaces of hydrous ferric oxides (HFO) and organic matter (OM) in acidic soil.