Thermal Dehydroxylation Research Articles

Mechanochemical synthesis of lanthanum chromate by grinding of constituent oxides : F. Saito et al. (Tohoku University, Sendai, Japan.) Powder Technol., Vol. 122, No 2–3, 2002, 145–149

Alkaline earth-doped LaCrO3 is a satisfactorily conducting ceramic material for the interconnector of Solid Oxide Fuel Cells (SOFC). For the synthesis of highly sinterable LaCrO3 powders spraydrying of adequate precursors like sols based on alkoxides has been used. Lanthanum chromium hydroxide sol was prepared by hydrolysis of synthesized methoxide complexes. The chemical reactions during hydrolysis and peptization of the precursors are described. The crystalline phase formation during heat treatment of spray-dried amorphous gel consisting of hydroxides and contaminant chloride ligands was characterized by thermal analysis methods (TG, DTG, DTA) and X-ray diffraction analysis. The progressive oxidation of Cr(III) by thermal dehydroxylation transforms the powder into fine crystallized La(III) Cr(V)hydroxo-oxychloride at 450 °C. A thermal treatment permits complete chlorine removal forming sinterable crystalline LaCrO3 perovskite powder at temperatures above 720 °C. The same sol-gel process also can be applied to chlorine-free nitrate-containing alkoxide sols. The properties of sinterable lanthanum chromite powders obtained from these sols are compared with powders made by direct spray-drying of lanthanum-chromium nitrate solutions.

Thermal analysis of K( x)/La 2O 3, active catalysts for the abatement of diesel exhaust contaminants

Potassium loaded lanthana is a promising catalyst to be used for the abatement of diesel exhaust pollutants. In this paper we have combined several thermal techniques to study relevant processes that take place during the soot combustion reaction. Temperature programmed oxidation (TPO) experiments show that with potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700 °C, these catalysts mixed with soot give maximum combustion rates between 350 and 400 °C. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) indicate that the reaction coexists with the thermal decomposition of bulk hydroxide species. For the La 2O 3 solid, the La(OH) 3→LaO(OH) and LaO(OH)→La 2O 3 processes occur at ca. 360 and 500 °C, respectively, whereas the presence of K in the K( x)/La 2O 3 catalysts provokes a shift of these endothermic peaks to higher temperatures. In all the studied solids, oxycarbonates decompose in the 550–800 °C temperature range. On the other hand, microbalance results show that the bulk carbonate formation depends on both the potassium content and the calcination temperature. The High Frequency CO 2 Pulses technique is useful to study the dynamics of the CO 2 adsorption–desorption process and to characterize the surface basicity of the solids. When both La 2O 3 and K/La 2O 3 solids are calcined at 700 °C, a strong decrease on the CO 2–surface interaction takes place, which correlates with a small decrease in catalytic activity and with an increase in the K/La surface ratio. These effects may be originated, at least in part, by a decrease in oxygen vacancies concentration and a thermal dehydroxylation of the catalysts.

Repeatedly performed solid-state substitution of silicon for framework aluminium in Y zeolite using crystalline (NH4)2[SiF6] as dealumination agent

Mixtures of NH4Na-Y zeolite and crystalline (NH4)2[SiF6] (applied in molar amounts corresponding to 0.38% of the framework aluminium) were subjected in covered but not hermetically closed dishes to heat treatment at temperatures below the decomposition point of the complex salt and subsequently washed with water or buffer solution. This process was twice repeated without any further intermediate treatment. The products were characterized by XRD, IR spectroscopy, without and with pyridine as probe molecule, 29Si MAS NMR spectroscopy, ammonia TPE (thermal deammoniation) and wet chemical analysis. They proved to be highly crystalline with generally low concentrations of lattice defects (vacancies) due to desilication and/or non-substitutive dealumination. The products obtained after two subsequent dealumination steps exhibited X-ray crystallinities of 88–93% at dealumination degrees up to 70%. After the third successive dealumination step the X-ray crystallinity was found to amount to 80–88% and the dealumination degree to about 80%. With increasing dealumination degree the thermal and hydrothermal stability of the structure as well as the resistance of HF and LF bridging hydroxyls to thermal dehydroxylation gradually increased and the LF hydroxyls became accessible to pyridine.

Thermal transformations of selected transition metals oxyhydroxides

Abstract The results of investigation of thermal dehydroxylation of freshly precipitated iron(III) oxyhydroxide, α-FeOOH (goethite) and titanium(IV) oxyhydroxide, TiO(OH)2 are reported. The measurements have been carried out by thermogravimetric method in static air atmosphere at different heating rates. The crystal structure of intermediate products has been identified by X-ray powder diffractometry. The JMAEK model A3 has been used as the adequate equation representing the dehydroxylation process of precipitated goethite. The third order model F3 has been used for description the rate of dehydroxylation reaction of titanium(IV) oxyhydroxide. The products of thermal dehydroxylation of mentioned oxyhydroxides can be used as precursors for preparation of ferrites or titanates.

Voltammetric Identification of Pedogenic Iron Oxides in Paleosol and Loess

Voltammetry of immobilized microparticles was used to detect low concentrations of goethite and hematite in paleosol and loess samples. The total content of Fe oxides in the natural samples was below 2 wt% and below the detection limit of X-ray powder diffraction. Goethite was distinguished from hematite due to the different electrochemical reactivity of the product of thermal dehydroxylation of goethite (very fine crystalline hematite). The voltammetric analysis was tested using synthetic samples of ferrihydrite, goethite, hematite, magnetite, or Mg-ferrite with SiO2 and a natural sample of smectite with an admixture of goethite. The detection is most sensitive for goethite and hematite (detection limit about 0.1 wt%), and approximately an order of magnitude better than X-ray powder diffraction analysis.

Infrared spectroscopy of goethite dehydroxylation: III. FT-IR microscopy of in situ study of the thermal transformation of goethite to hematite

Fourier transform infrared microscopy has been used to investigate in situ dehydroxylation of goethite to form hematite. The characterisation was based on the behaviour of hydroxyl units, which were observed in the hydroxyl stretching and hydroxyl deformation and water bending regions, and the Fe–O vibrations of the newly formed hematite during the thermal dehydroxylation process. Two hydroxyl stretching modes ( ν 1 and ν 2), and three bending ( ν bending-1, 2, 3) and two deformation ( ν deformation-1, 2) modes were observed for goethite. The characteristic vibration at 916 cm −1 was observed together with the residuals of the ν 1 and ν 2 bands in hematite spectrum. The structural transformation between goethite and hematite through thermal dehydroxylation was interpreted in order to provide criteria that can be used for the characterisation of thermally activated bauxite and their conversion to activated alumina phases.

Spectroscopic and thermal contribution to the structural characterization of vandenbrandeite

The vibrational (IR and Raman) spectrum of Vandenbrandeite, Cu[UO2(OH)4], has been recorded and discussed on the basis of structural considerations through the use of empirical expressions. As the H-bonding in the lattice is poorly known, the characteristics of this bond has been analyzed with the aid of H/D isotope exchange, thermal assays and electron microscopy. Samples are practically amorphous to XRD up to ∼700°C whereas crystals morphologically different by SEM microscopy and well characterized by XRD technique are observed from ∼800°C. Vibrational spectroscopy reveals that the atomic arrangement in the structure remains largely unaffected as the dehydration proceeds, suggesting a topochemically controlled process. EPR spectroscopy suggests a disordered distribution of Cu(II) ions as temperature increases. Finally, thermal dehydroxylation and paragenesis of the copper-uranyl oxide hydrate are correlated.

Nanoscale Magnesium Hydroxide and Magnesium Oxide Powders: Control over Size, Shape, and Structure via Hydrothermal Synthesis

Mg(OH)2 nanocrystallines with rod-, tube-, needle-, or lamella-like morphologies have been synthesized by a hydrothermal reaction using different magnesium precursors and solvents as the reactants. The products appeared to have narrow size distributions with a monodisperse nature. Subsequent thermal decomposition at 450 °C gave nanosized MgO, which preserved well the morphological features of the Mg(OH)2 samples. The specific surface areas of the MgO samples were determined by the BET technique, which gave a feature of high surface area generally larger than 100 m2/g. The channels formed in the thermal dehydroxylation process may account for this feature of the MgO nanocrystallines.

IR-Spectroscopic Study of Changes in Silica Surface Structure on Thermal Dehydroxylation and Rehydroxylation in Water Vapors

Amount, properties, state of silica siloxane, silanol surface structures can be expedientlyregulated by varying the regime of thermal dehydroxylation and the amount of chemisorbed water. The obtained data can be used in the development of the procedures for the synthesis and stabilization of nanostructures and layers of chemical modifiers on the silica surface.

Kinetics of thermal transformations of precipitated magnetite and goethite

The aim of the present study was to determine the kinetic equations for the thermal transformations of precipitated iron oxides and hydroxides, namely for the process of thermal dehydroxylation of goethite and consecutive of hematite crystal structure growth as well as for the oxidation of magnetite to maghemite and its thermal transformation into crystalline hematite. The investigations have been carried out using thermogravimetry (TG/DTG/DTA), X-ray powder diffractometry (XRD) and high temperature powder diffractometry (HT-XRD). This presentation contains the continuation of our earlier works.

Surface modification of silica with ultraviolet laser radiation

A frequency-doubled copper vapour laser was used to irradiate the surface of silica with ultraviolet light at fluences of between 0.1 and 0.5 J/cm2 per pulse for several hundred pulses. Analysis of the surface composition using time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) shows that the relative surface-hydroxyl (OH) concentration decreases with increasing laser irradiance. This dehydroxylation results in silica surfaces with final-state hydroxyl concentrations similar to those obtained through the thermal treatment of silica at around 1000 °C. The mechanism for the dehydroxylation reported here, however, is more likely to be photolytic than thermal. Laser dehydroxylation allows for the selective and rapid dehydroxylation of silica offering many practical advantages over thermal dehydroxylation. The modified surface has significantly increased hydrophobicity compared to the untreated silica surface.

Factors Influencing the Texture and Stability of Maghemite Obtained from the Thermal Decomposition of Lepidocrocite

The thermal decomposition of a series of lepidocrocite samples has been monitored by means of the Constant Rate Thermal Analysis (CRTA). This method allows controlling the reaction temperature in such a way that both the reaction rate and the partial pressure of the water vapor generated in the reaction are maintained constant at values that can be arbitrarily selected by the user. The final products obtained from the thermal dehydroxylation of the lepidocrocite precursors under residual pressures of water vapor ranging from 5 × 10-5 mbar to 10 mbar have been characterized by TEM, XRD, and Mosbauer spectroscopy. It has been concluded that they preserve the shape and size of the corresponding precursors. The precise control of the partial pressure of the water vapor self-generated in the above reaction allows tailoring the internal porosity of the maghemite−hematite mixture obtained as final product. It has been shown that the larger the particle size of the starting lepidocrocite is, the larger the porosi...

Nanocrystalline NiO and NiO-Ni(OH) 2 composite powders prepared by thermal and mechanical dehydroxylation of nickel hydroxide

Nanocrystalline NiO and nanocomposite NiO-Ni(OH) 2, in the form of ultrafine powders, were prepared from Ni(OH) 2 by thermal and mechanical dehydroxylation respectively. The powders were characterized by X-ray diffraction, differential scanning calorimetry, secondary electron imaging and transmission electron microscopy. An orientation relationship was observed such that the (0001) planes in the original hexagonal hydroxide structure were parallel to (111) planes in the cubic NiO following thermal dehydroxylation. It is proposed that the crystallographic transformation required for the dehydroxylation process involves the glide of Shockley partial dislocations. It is further suggested that the effect of mechanical grinding is to cause extensive faulting through the glide of Shockley partial dislocations, which lowers the activation energy for dehydroxylation so that the process proceeds at ambient temperature.

Re-examination of the kinetics of the thermal dehydroxylation of goethite

The kinetics of thermal dehydroxylation of aluminuous goethites [1] synthesised from a ferrous salt has been re-examined using the general reaction order kinetic law. The utilised data processing was based on the procedures employed by dissolution kinetics. Recalculation of the activation energies EA of the dehydroxylation yielded the values 130, 132, 128, and 123 kJ mol−1 for pure goethite, goethite with 10, 20, and 30 mol% Al substitution, respectively. The values of EA are in a good agreement with those given for goethite in literature. The EA values are linearly related with the chemically bound excess H2O/OH− in the crystal lattice that is apparently influenced by Al substitution.

Mechanism of thermal decomposition of magnesium hydroxide

The kinetic characteristics (the rates of decomposition J and activation energies E a) of thermal dehydroxylation of Mg(OH) 2, reported in a number of available publications, have been analysed with the supposition of dissociative evaporation of Mg(OH) 2 in the form of MgO and H 2O molecules. Despite the considerable differences in the measurement conditions, the experimental values of J and E a have been found to be in agreement with theoretical calculations. The calculations took into account the self-cooling effect, the depressing effect of external water vapor on the decomposition rate, and partial transfer of the energy released in the condensation of MgO(g) to the reactant. The agreement between the calculated and experimental data confirms the proposed mechanism of decomposition.

Effect of microstructure on thermal dehydroxylation of silica gels

Dehydroxylation of gels is important for the preparation of high quality glasses and for the application of the porous gel materials at high temperatures. In this report, the effect of changing the microstructure of silica aerogels on the thermal dehydroxylation of the gels in vacuum is presented. Surface area, density and IR spectra of the aerogels were determined after heat treatment to 1100°C in vacuum. The two different microstructures have been obtained by aging one set of gels in a water/ethanol solution to promote coarsening of the wet gel prior to supercritical drying. Network coarsening causes a larger pore size and less concave neck curvature of this type of aerogels. Coarsening of the network was shown to facilitate dehydroxylation of the aerogels.

Sol-gel synthesis of lanthanum chromite powder

Alkaline earth-doped LaCrO 3 is a satisfactorily conducting ceramic material for the interconnector of Solid Oxide Fuel Cells (SOFC). For the synthesis of highly sinterable LaCrO 3 powders spraydrying of adequate precursors like sols based on alkoxides has been used. Lanthanum chromium hydroxide sol was prepared by hydrolysis of synthesized methoxide complexes. The chemical reactions during hydrolysis and peptization of the precursors are described. The crystalline phase formation during heat treatment of spray-dried amorphous gel consisting of hydroxides and contaminant chloride ligands was characterized by thermal analysis methods (TG, DTG, DTA) and X-ray diffraction analysis. The progressive oxidation of Cr(III) by thermal dehydroxylation transforms the powder into fine crystallized La(III) Cr(V)hydroxo-oxychloride at 450 °C. A thermal treatment permits complete chlorine removal forming sinterable crystalline LaCrO 3 perovskite powder at temperatures above 720 °C. The same sol-gel process also can be applied to chlorine-free nitrate-containing alkoxide sols. The properties of sinterable lanthanum chromite powders obtained from these sols are compared with powders made by direct spray-drying of lanthanum-chromium nitrate solutions.

Accommodation of the actual solid-state process in the kinetic model function. Part 2. Applicability of the empirical kinetic model function to diffusion-controlled reactions

The significance of the empirical kinetic model function for diffusion-controlled reactions is discussed in connection with the fractal nature of the reaction geometry and the macroscopic character of the thermoanalytical curves for solid-state reactions. The mathematical properties of such empirical kinetic model functions based on the geometrical fractal were investigated numerically in order to evaluate their practical usefulness as a possible diagnostic tool for distinguishing the most appropriate kinetic model function. The procedure of distinguishing the kinetic model function is extended, including such empirical kinetic model functions based on the geometrical fractal, and applied to the kinetic analysis of the thermal dehydroxylation of synthetic brochantite Cu 4(OH) 6SO 4.

Kinetics of Thermal Dehydroxylation and Carbonation of Magnesium Hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH)2 were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X‐ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting‐sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting‐sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH)2 underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO3 precipitated on the surface of disrupted Mg(OH)2 crystals acting as a kinetic barrier to both the outward diffusion of H2O and the inward diffusion of CO2.

Kinetics of thermal dehydroxylation of aluminous goethite

The kinetics of dehydroxylation of synthetic aluminous goethite was studied using isothermal and non-isothermal thermogravimetry. The complete isothermal dehydroxylation can be described by the Johnson-Mehl equation with up to three linear regions in plots of lnln [1/(1−y)]vs. Int Kinetics for the initial stage of dehydroxylation changed from diffusion to first-order through the temperature range 190 to 260°C. The rate of dehydroxylation was reduced by Al-substitution and increased with temperature. Activation energy for dehydroxylation, calculated from the time to achieve a given dehydroxylation extent, varied depending on the extent of dehydroxylation and Al-substitution. Non-stoichiometric OH existed in goethite and some remained in hematite after the complete crystallographic transition.