A study of the oxidation kinetics of CuFeO2 in the Cu-Fe-O system

The oxidation kinetics of CuFeO₂ in the Cu-Fe-O system have been studied between 500 and 900° C in an atmosphere containing 1 vol% oxygen in a nitrogen stream using thermogravimetric analysis (TGA). It was found that addition of Fe₂O₃ to the CuFeO₂ caused a decrease in the oxidation rate while addition of CuO caused an increase. On increasing the concentration of Fe₂O₃ the activation energy was found to increase from ~ 18 kcal mol^–1 to ~ 45 kcal mol^–1 and the exponent n in Avrami's equationf=1-exp (–kt ⁿ) was also observed to increase, from 1.3 to 2.3. On adding CuO to the CuFeO₂ in the Cu-Fe-O system the activation energy decreased from ~ 18 kcal mol^–1 to ~ 8 kcal mol^–1. The variation in both values indicates changes in the oxidation mechanisms. The microstructural changes associated with oxidation have been studied using optical microscopy. A model has been proposed to explain the results.     

Kinetics of the oxidation of CuFeO2 in Cu-Fe-O system

The oxidation kinetics of CuFeO₂ in Cu-Fe-O system have been studied between temperature range 500° C and 900° C in an ambient atmosphere containing constant oxygen partial pressure in the flowing nitrogen. Thermogravimetric analysis (TGA) technique was used as the main tool. The oxidation rate was found to decrease and the activation energy calculated from the oxidation rate constants for various composition increased from 8 to 45 k cal/mole on increasing the Fe₂O₃ content in the system. The value of exponentn in Avrami’s equation=1 ? exp (?kt ⁿ ) also increased from 1·3 to 2·3. The microstructural changes associated with the oxidation have been studied using optical microscopy and correlated with results obtained from thermogravimetric analysis.     

A structural,thermogravimetric and magnetic study of the WOx-TiO2 system

TiO₂ unit cell parameter determinations, thermogravimetric measurements and magnetic susceptibility data (over the temperature range 78 to 300 K) were used to investigate samples of titanium dioxide containing a small percentage (up to 5% molar ratio) of tungsten dioxide. The samples were prepared by heating pellets of mixture of the appropriate dioxides in an evacuated silica tube at 1223 K. The results show that tungsten is incorporated in the rutile structure in the 4+ oxidation state and causes an expansion of the TiO₂ unit cell volume. The expansion is anisotropic: whilea increases with the tungsten content,c decreases. The magnetic data as well as the distortion of the TiO₂ unit cell point out to the existence of metal-metal interactions among the tungsten ions that are not randomly incorporated in the TiO₂ matrix.     

A facile novel synthesis of delafossite CuFeO2 powders

P-type transparent conducting oxides copper delafossite CuFeO₂ powders have been successfully synthesized by using hydrothermal method. The influence of synthesis conditions on the crystal structure, morphology, optical and magnetic properties was systematically studied using X-ray diffraction, scanning electron microscope (SEM), UV–Vis–NIR scanning spectrophotometer and vibrating sample magnetometer. The results indicated the precipitated precursor at pH value of 12 using 1 M NaOH as a base in the presence of hydrazine hydrate as a reducing agent hydrothermally treated at 280 °C for 96 h was transformed to pure rhombohedral 3R delafossite phase. SEM observations of these powders confirmed their hexagonal like structure. The transmittance of the sample was around 65 %. The optical band gap of delafossite–CuFeO₂ sample prepared was 3.5 eV as a visible transparent material. Furthermore, magnetic properties behavior was identified and paramagnetism property was found at a room temperature. The saturation magnetization and coercive force for the pure sample were 1.2 emu/g and 58.35 Oe, respectively.     

A structural,thermogravimetric and ESR study of the RhO x -TiO2 system

Titanium dioxide samples containing a few per cent of rhodium oxide, heated in air at 1273 K, have been investigated in order to study the formation of solid solutions. The results, monitored by X-ray diffraction, thermogravimetry in a hydrogen stream and ESR spectroscopy, show that rhodium is present both as separate phase (Rh₂O₃) and in solid solution in the rutile structure. The incorporated rhodium is mainly present in the 3 + oxidation state, only a small fraction being present as Rh(II). Rh₂O₃ and the rhodium species in solid solution are reduced by hydrogen to metal in different temperature ranges. By combining thermogravimetric and analytical data, the average oxidation state of rhodium,n, has been evaluated. The variation ofn with the rhodium content has been accounted for on the basis of a strong metal-support interaction developing with the reduction. This interaction affects the metal dispersion.     

Diffusional effects for the oxidation of SiC powders in thermogravimetric analysis experiments

The oxidation behavior of SiC powders was studied using a thermogravimetric analysis (TGA). The effects of temperature (910–1010 °C), particle size (120 nm, 1.5 μm), and the initial reactant mass on the oxidation behavior of SiC were investigated. Kinetics analysis showed that intra-particle diffusion and chemical reaction controls the oxidation rate. Moreover, a new kinetics model was proposed to describe the oxidation rate of where all diffusion steps (bulk, inter- and intra-diffusion) and the chemical reaction may affect the overall reaction rate. The model was validated through a comparison with the experimental results obtained from the oxidation of SiC powders in TGA experiments. It was found that during the experiments, inter diffusion must also be taken account to describe adequately the oxidation rate. Numerical analysis indicated that inter-particle diffusion has a significant effect on the oxidation rate, especially for larger system.     

Electromagnon Excitation of the Triangular Lattice Antiferromagnet CuFeO2 in High Magnetic Fields

In this paper, we report results of high field ESR measurements of the triangular lattice antiferromagnet CuFeO₂. From polarization measurements for the ESR signals, we show an appearance of the electromagnon excitation in the field-induced 1/5 plateau phase of this compound.     

Chemical solution deposition and transport properties of epitaxial CuFeO2 thin films

In this paper, chemical solution deposition is used to prepare CuFeO₂ thin films on Al₂O₃ (001) substrates by nitrate-based precursors. The derived films are characterized by X-ray diffraction and field-emission scanning electron microscopy. The derived thin film is epitaxial growth. The lowest resistivity is 0.58 Ω cm at 300 K, which is comparable to the single crystals, indicating the chemical solution deposition is an effective route to obtain the CuFeO₂ films. The high- and low-temperature transport mechanisms are attributed to thermal activation and Mott variable range hopping type, respectively. The result will provide a feasible route to prepare epitaxial CuFeO₂ thin films using chemical solution deposition.     

Enhanced photocatalytic performance of CuFeO2-ZnO heterostructures for methylene blue degradation under sunlight

One of the strategies to overcome the drawbacks of fast charge recombination of a photocatalyst is to develop semiconductor heterostructures. Herein, we report a two-step precipitation-hydrothermal process to create CuFeO₂-ZnO heterostructures with different weight percentages of CuFeO₂ (0.5, 1, 5, and 10%). Though X-ray diffraction detected the presence of CuFeO₂ on ZnO above 5%, Raman spectroscopy could reveal the presence of CuFeO₂ phase as low as 0.5 wt%. For all of the compositions, the bandgap of ZnO did not vary (3.15 eV) on forming heterostructures with CuFeO₂. The oxidation of methylene blue under sunlight was used to determine the photocatalytic performance of the heterostructures. In comparison to pure ZnO and CuFeO₂, CuFeO₂-ZnO heterostructures exhibited a better photocatalytic efficiency. Overall, 5 wt% CuFeO₂ on ZnO showed 100% degradation with a rate constant of 0.272?±?0.002 min^?1, which is 16 times faster than ZnO. Time-resolved photoluminescence analysis indicated a higher lifespan of charge carriers in the 5wt% CuFeO₂-ZnO heterostructure (32.3 ns) than that of CuFeO₂ (0.85 ns) and ZnO (27.6 ns). The Mott–Schottky flat band potentials of ZnO and CuFeO₂ was determined to be -0.82 and 1.17 eV, respectively, revealing the presence of Type I heterostructures. The heterostructures also showed outstanding recyclability, with a degradation rate of 97% even after four cycles. The current study shows the significance of forming p-type CuFeO₂ and n-type ZnO heterostructures for enhanced photocatalysis.

     

A comparative study of the advanced oxidation of 2,4-dichlorophenol

Advanced oxidation processes (AOPs) using UV, UV/H2O2, Fenton and photo-Fenton treatment were investigated at laboratory scale for aqueous solutions of 2,4-dichlorophenol (DCP). The effects on degradation of different reactant concentrations, irradiation time, temperature and pH were assessed. DCP removal, TOC mineralization, dechlorination and change in oxidation state were monitored. UV photolysis was less efficient for total DCP degradation than other AOPs. In contrast, photo-Fenton reaction in acidic conditions led to a higher DCP degradation in a short time. Sixty minutes of treatment were sufficient for 100% DCP removal with 75 mg l(-1) H2O2 and 10 mg l(-1) Fe(II) initial concentrations. In these conditions, a first-order degradation constant for DCP of 0.057 min(-1) was obtained.     

Thermal stability of carbon-MoSi2-carbon composites by thermogravimetric analysis

A thermal stability of carbon-carbon composites with increase of oxidation resistant filler, MoSi₂, have been studied by thermogravimetric analysis during the graphitization process. In this work, the initial decomposition temperature, temperature of maximum rate of weight loss, integral procedure decomposition temperature, and decomposition temperature range for the degradation temperatures, and the activation energy based on Horowitz-Metzger calculating method were characterized in a thermal stability study. It has been found that 12–20 wt% filler on the basis of the resin matrix leads to an improvement of degradation temperature and to an effectively increase of activation energy of the composites. This is probably due to the effect of the inherent MoSi₂ properties, resulted from a brittle-to-ductile transition for increasing the interfacial adhesion between fiber and matrix, and a mobile diffusion barrier formation against oxygen attack, in the vicinity of 900–1000°C.     

Phase equilibrium of the Cu-Fe-O system under Ar,CO2 and Ar+0.5% O2 atmospheres during CuFeO2 single-crystal growth

Phase relations of CuFeO₂ during crystal growth under Ar, CO₂ and Ar+0.5% O₂ surrounding atmospheres have been investigated using scanning electron microscopy together with electron probe microanalysis and X-ray powder diffraction. CuFeO₂ was found to partially decompose into Fe₂O₃ and Cu₂O before melting and the crystal growth process was by means of Cu₂O-solvent movement. Under Ar and CO₂ atmospheres, Cu₂O was found to be further reduced to metallic copper, and under CO₂ more severe reduction of copper was observed. No copper reduction could be found under an Ar+0.5% O₂ surrounding atmosphere.     

A comparative study of catalytic partial oxidation of methane over CeO2 supported metallic catalysts

In the present study, the catalytic partial oxidation of methane (CPOM) over various active metals supported on CeO2 (M/CeO2, M = Ir, Ni, Pd, Pt, Rh and Ru) has been investigated. The catalysts were characterized by X-ray diffraction (XRD), BET surface area, H2-temperature programmed reduction (H2-TPR), CO chemisorption and transmission electron microscope (TEM) analysis. Ir/CeO2 catalysts showed higher BET surface area, higher metal dispersion, small active metal nano-particles (approximately 3 nm) than compared to other M/CeO2 catalysts. The catalytic tests were carried out in a fixed R(mix) ratio of 2 (CH4/O2) in a fixed-bed reactor, operating isothermally at atmospheric pressure. From time-on-stream analysis at 700 degrees C for 12 h, a high and stable catalytic activity has been observed for Ir/CeO2 catalysts. TEM analysis of the spent catalysts showed that the decrease in the catalytic activity of Ni/CeO2 and Pd/CeO2 catalysts is due to carbon formation whereas no carbon formation has been observed for Ir/CeO2 catalysts.     

A study of the oxidation products of some LiRF4 phases

The effects of oxidation in both normal and moist air upon powdered single crystals of the scheelite-type compounds LiYF₄, LiErF₄, -YLF (LiY_0.434 Er_0.5 Tm_0.055 Ho_0.011 F₄) and -LuLF (LiLu_0.434 Er_0.5 Tm_0.05 Ho_0.011 F₄) have been studied by X-ray powder diffraction and differential thermal analysis. For all the materials the oxidation rate is enhanced in the presence of moisture. The oxidation process results in the formation of rare-earth oxyfluoride phases, LiYF₄, forming yttrium oxyfluoride (with a rhombohedral phase predominating) and LiErF₄ forming erbium oxyfluoride (with a tetragonal phase predominating). The LiYF₄ compound oxidizes more rapidly than LiErF₄ both in normal and moist air. The more complex -YLF compound oxidizes to form a mixture containing significant quantities of both the rhombohedral and tetragonal rare-earth oxyfluoride phases. The -LuLF compound oxidizes to form the tetragonal rare-earth oxyfluoride phase together with a very small amount of the rhombohedral variety. The melting reactions of oxidized samples are very broad thermal events which are coupled with the presence of a strong LiF/LiRF₄-type eutectic component. These observations are rationalized in terms of the appearance of excess LiF due to selective oxidation of the trifluoride component.     

A study of the activity of the Cu-Co-Fe catalyst in oxidation of carbon

The paper addresses the thermal stability of fruit-stone activated carbon impregnated with the Cu-Co-Fe oxide catalyst. The nature and concentration of the oxygen-containing surface groups are determined. The kinetic investigation of the process of oxidation of fruit-stone activated carbon reveals a high catalytic activity of the Cu-Co-Fe oxide catalyst. It is shown that the nitric-acid treatment of fruit-stone activated carbon leads to an increase in the concentration of carboxyl groups which are the binding centers for Cu-Co-Fe oxide system with subsequent formation of the active center for the low-temperature oxidation. This gives rise to a great number of active centers of oxidation in the surface layer of fruit-stone activated carbon, resulting in a decrease of the temperature of the oxidation reaction of activated carbon by almost 100°C.     

A kinetic study on the degradation of p-nitroaniline by Fenton oxidation process

A detailed kinetic model was developed for the degradation of p-nitroaniline (PNA) by Fenton oxidation. Batch experiments were carried out to investigate the role of pH, hydrogen peroxide and Fe(2+) levels, PNA concentration and the temperature. The kinetic rate constants, k(ap), for PNA degradation at different reaction conditions were determined. The test results show that the decomposition of PNA proceeded rapidly only at pH value of 3.0. Increasing the dosage of H(2)O(2) and Fe(2+) enhanced the k(ap) of PNA degradation. However, higher levels of H(2)O(2) also inhibited the reaction kinetics. The k(ap) of PNA degradation decreased with the increase of initial PNA concentration, but increased with the increase of temperature. Based on the rate constants obtained at different temperatures, the empirical Arrhenius expression of PNA degradation was derived. The derived activation energy for PNA degradation by Fenton oxidation is 53.96 kJ mol(-1).     

Effect of heat treatment on the mechanical properties of North American jack pine: thermogravimetric study

Heat treatment improves dimensional stability of wood, reduces its decay, and darkens its color. However, mechanical properties of wood can deteriorate during the heat treatment. The effect of heat-treatment conditions (maximum treatment temperature, heating rate, exposure time at the maximum heat-treatment temperature, and the gas humidity) on the mechanical properties of North American jack pine (Pinus banksiana) was studied using thermogravimetric analyzer. This type of study permits the identification of the best treatment conditions which will minimize reduction of mechanical properties of jack pine. The results showed that the degree of change in bending strength, hardness, screw withdrawal strength, and dimensional stability of jack pine during heat treatment depends strongly on the treatment conditions used. Therefore, great care should be taken to select the treatment conditions. Thermogravimetric analysis can be used as a first step for selection.     

A kinetic study of the oxidation of cerium ions in an X zeolite

The oxidation of cerium ions by oxygen in 13 X type molecular sieve C₂₂ H₁₄ Na₆ X has been studied.The kinetic study of oxygen adsorption and CO chemisorption measurements confirm the cerium ions localization in the sodalite units.Results obtained are in good agreement with a model implying the formation of Ce⁴⁺-O-Ce⁴⁺ bridges in the sodalite cages.A diffusion step seems to be rate limiting in the oxidation process of the zeolite.