Solid-solid interactions in pure and Li2O-doped manganese and magnesium mixed oxides system

The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li₂O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn₂O₃ and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H₂O₂ decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg₂MnO₄, Mg₆MnO₈, MgMnO₄ besides unreacted portions of MgO, Mn₂O₃ and Mn₃O₄). Li₂O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn₂O₃ (Bixbyite) with subsequent increase in the lattice parameter 'a' of MgO to an extent proportional to the amount of Li₂O added. This finding might suggest that the doping process enhanced the dissolution of Mn₂O₃ in MgO forming solid solution. This treatment led also to the formation of Li₂MnO₃. Furthermore, the doping with 3 and 6 mol% Li₂O conducted at 800°C resulted in the conversion of Mn₂O₃ into Mn₃O₄, a process that took place at 1000°C in absence of Li₂O. The produced Li₂MnO₃ phase remained stable by heating at up to 1000°C. Furthermore, Li₂O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn₂O₃ in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C (k _20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li₂O-doping of the samples having the formula 0.1 Mn₂O₃/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k _20°C due to Li₂O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date.     

Solubility of Native Gold in H—O—S Fluids at 100–400°C and High H2S Content

Reaction of MgS, water, and air in sealed gold capsules at 100 to 400°C and0.15 GPa is used to generate an aqueous fluid with very high (20.6 m) H₂Scontent and to remobilize significant quantities of native gold as gold sulfides.A combination of X-ray photoelectron and Auger electron spectroscopy (XPS,AES), analytical scanning-electron microscopy (SEM—EDX),electron-micro-probe analysis (EPMA), and calculated solution properties shows that the goldsulfides precipitated during quenching and later perforation of the capsulesrepresent native gold dissolved as Au(I)-bisulfide under the experimental conditions.The equilibrium constant (logK) for the reaction:Au(s) H₂S(aq) + HS^– = Au(HS)₂ ^– + 1/2H₂(g)ranges from –3.96 ± 0.40 at 115°C to –1.06 ± 0.32 at 400°C; it is in goodagreement with literature values for 25°C and 300–350°C, and varies inverselywith absolute temperature T[–H ⁰ ₁/(2.303R)= –2644 ± 33K; r = 1.00]. Themaximum solubility of native gold in this study (29.4 g/kg at 200°C) issignificantly greater than that from published studies on Au(I)-bisulfides and maystimulate interest in developing bisulfides as gold-complexing agents in goldextraction technology.     

Hybridization thermodynamics of DNA bound to gold nanoparticles

Isothermal Titration Calorimetry (ITC) was used to study the thermodynamics of hybridization on DNA-functionalized colloidal gold nanoparticles. When compared to the thermodynamics of hybridization of DNA that is free in solution, the differences in the values of the Gibbs free energy of reaction, Δ_rG°, the enthalpy, Δ_rH°, and entropy, Δ_rS°, were small. The change in Δ_rG° between the free and bound states was always positive but with statistical significance outside the 95% confidence interval, implying the free DNA is slightly more stable than when in the bound state. Additionally, ITC was also able to reveal information about the binding stoichiometry of the hybridization reactions on the DNA-functionalized gold nanoparticles, and indicates that there is a significant fraction of the DNA on gold nanoparticle surface that is unavailable for DNA hybridization. Furthermore, the fraction of available DNA is dependent on the spacer group on the DNA that is used to span the gold surface from that to the probe DNA.     

Anodic Dissolution of Gold in Cyanide Solutions Containing Lead Ions: Effect of the Solution pH

The effect of the electrode potential on the gold dissolution rate in alkali–cyanide solutions with and without 10^–5 M of hydroxy compounds of lead is studied. With the compounds, the process rate passes through a maximum, whose potential E _m shifts in the negative direction and whose height drops with increasing pH. The pH dependence of E _m is linear, with the slope dE _m/dpH = –71 ± 5 mV, and correlates with that of the potential at which lead adatoms start to undergo desorption from the gold surface in alkali solutions. Without the compounds, the gold dissolution rate in alkali–cyanide solutions is independent of the solution pH at E < 0. Thus, the effect of the solution pH in this potential range is connected not with a direct participation of hydroxide ions in the anodic process but is of a secondary nature caused by the dependence of the region of adsorption of catalytically active lead adatoms on the hydroxide ion content in solution.     

Kinetics of the electrodissolution of metallic uranium

The kinetics of the anodic dissolution of metallic uranium in 1, 2, 3, and 4 mol^.l^-1 HNO₃ solutions at 30 °C were studied by potentiostatic polarization. The dissolved uranium was determined by polarography and the anodic dissolution rates by the initial rate method. It was observed that the dissolution rate increases with the applied potential, but is independent of the HNO₃ concentration, because it is a zero order reaction. A mechanism for the anodic reaction was proposed based on the adsorption theory.     

Effect of dioctyl sulfide on the kinetics of oxidative dissolution of gold nanoparticles in triton N-42 reverse micelles

The effect of additions of hydrophobic dioctyl sulfide (L) on the kinetics of dissolution of gold nanoparticles in the interaction with a dispersed aqueous hydrochloric solution of H₂O₂ in Triton N-42 reverse micelles (decane was the dispersion medium) was studied spectrophotometrically. The process consists of a two-stage oxidation Au⁰ → AuCl₂⁻ → AuCl₄⁻ at the surface of gold particles; the first stage occurs in two ways: a spontaneous reaction and an autocatalytic reaction involving AuCl₄⁻ ions. With small additions of L (c _L < c _Au), only spontaneous oxidation of Au(0) to Au(I) takes place because Au(I) is completely bound in an inert complex AuLCl. When unbound L is exhausted, the newly formed AuLCl is accumulated in micellar shells, changes the properties of the medium inside the micelles, and affects the rate constant of the autocatalytic reaction, which increases with increasing c _L. At high concentrations of L, the coagulation of particles occurs instead of their dissolution, because of the deterioration of the protective properties of micellar shells as a result of the ingression and accumulation of dioctyl sulfide molecules on account of selective adsorption on gold particles. The rate constants of all stages of dissolution and coagulation are determined.     

A kinetic model for the ultrasound catalyzed hydrolysis of solventless TEOS-water mixtures and the role of the initial additions of ethanol

The acid and ultrasound catalyzed hydrolysis of solventless TEOS-water mixtures are studied, as a function of the initial additions of ethanol to the mixtures, by means of flux calorimetry measurements. A device was specially designed for this purpose. Under acid conditions, our proposed method has been able to resolve hydrolysis from other condensation reactions, by detecting the exothermal hydrolysis reaction heat. The process has been explained by a dissolution and reaction mechanism. Ultrasound forces the dissolution process to start the reaction. The alcohol produced in the reaction helps the dissolution process to further enhance the hydrolysis. Initial amounts of pure ethanol added to the mixtures shorten the start time of the reaction, due to an additional effect of dissolution, and diminish the reaction rate, as a result of the solvent dilution effect. Our dissolution and reaction mechanism modeling describes the main points arising from the experimental data and yields k _H =0.24 M^-1 min^-1 for the second-order hydrolysis rate constant at 39°C.     

Effect of KBr Additions on NiS Anodic Dissolution in Sulfate Electrolytes

Electrochemical behavior of nickel sulfide (NiS) in 0.5 M solutions of Na₂SO₄, CuSO₄at pH 2.0-2.2 and T = 90°C was studied. A possibility was shown for depassivation and efficient dissolution of nickel sulfides upon addition of KBr to the electrolyte.     

The kinetics and mechanism of dissolution of gold in solutions of thiocarbamide with an oxidizer

The kinetics and mechanism of dissolution of gold in solutions of thiocarbamide (T) in the presence of Fe₂(SO₄)₃ as an oxidizer were studied. The dependences of the rate of dissolution of gold on the concentration ratio between iron(III) and T and pH were determined, and optimum solution compositions for the dissolution of gold were found. The compositions of gold(I) complexes formed in the boundary double layer ([Au{(NH₂)₂C=S}₂]⁺) and in the bulk ([Au{(NH₂)₂C=S}₃]⁺) were determined. The diffusion and kinetic components of the overall reaction of gold dissolution in solutions of T in the presence of the oxidizer were obtained by the rotating disc method. The first-order rate constants at 278–333 K, k _Au = 3.5 × 10^?5?2.73 × 10^?4 s^?1, and the activation energies at 278–295 K (E _a = 13.4 kJ/mol, which is evidence that dissolution value characteristic of kinetically controlled reactions) were determined for the dissolution of gold in solutions of T. The composition of the adsorption sulfide-containing film on the surface of gold was studied by Auger electron spectroscopy. The film, which inhibited gold dissolution, consisted of gold(I) hydrosulfide (AuHS) and sulfide (Au₂S). The solubility products of these compounds and their solubilities in aqueous solutions were calculated.     

Dissolution kinetics of MgO in aqueous,acidic media

The method of the rotating disc was used to study the kinetics of MgO dissolution. Single crystals of MgO with the orientation {100}, {101}, and {111} were dissolved in N₂-saturated HClO₄-NaClO₄ solutions of constant ionic strength (I=1.0 mol kg^–1). Chemical, mixed and diffusional control of the reaction leading to dissolution has been found between 25 to 90°C and 0.5<pH<3. The dissolution at 90°C andpH3 occurred diffusionally controlled with respect to H⁺ ions. Independent measurements of the limiting current densities for the cathodic reduction of H⁺ ions were carried out in solutions of the same composition and confirmed this kind of control. In the chemical control region the dissolution rates were proportional to the Mg²⁺ ion densities of the single crystal surfaces investigated. The dissolution rates relative to {111}, at 40°C andpH=1, were found to be: 1.0±0.1 (1.0) {111}; 1.3±0.2 (1.2) {101} and 1.7±0.2 (1.7) {100}. Figures in parentheses refer to the relative Mg²⁺ ion densities of the surface. Reproducible results of high quality can only be obtained when fresh dislocations are removed by chemical polishing.In a limitedpH range these results were formally consistent with the complex formation model of metal oxide dissolution. For substances with large surfaces this model applies frequently. However, for single crystals no methods are known which allow the independent determination of the amount of surface OH-groups and the surface complex formation constants. Although the overall dissolution behaviour of MgO, which is relevant for industrial applications, is now rather well known, not enough information for unambiguous mechanistic conclusions is available.Herrn O. Univ.-Prof. Dr.K. Komarek zum 60. Geburtstag gewidmet.     

Studies on recrystallised aluminium trihydroxide precipitates: The energetics of dissolution by sodium hydroxide solutions

Summary Aluminium hydroxide gels were recrystallised in high pH solution for 2–1000 hr to give a series of gibbsite powders of different surface areas, 1`11/2, P11/8, P11/48, P11/360 and P11/X; their average platelet lengths were 30, 60, 130, 260 and 450 nm. The dissolution equilibria of these powders in sodium hydroxide solutions of C=1-8 M were studied at 20° to 80°C.The equilibrium constants (for formation of diaquo-tetrahydroxoaluminate anion) increased with temperature and with increasing surface area; the K value for P11/2 was about four times that of P 11/X at 20 °C and about three times (that for P 11/X) at 80 °C. The dissolutions were endothermic; however, the value over this temperature range increased from –29.5 kJ mol<sup>–1</sup> (for P11/X) to –24.7 kJ (for P 11/2). This increase was related to the appreciable surface energies of the high surface-area materials.The (– G°) values increased with increasing temperature and in turn with increasing surface area; the (– G°) values for 1`11/2 dissolution at any temperature were about 4 kJ mol^–1 higher than those for P11/X dissolution.With 1 figure and 1 table     

Characterization of Alumina Gel Catalysts by Emanation Thermal Analysis (ETA)

Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NO _x by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C₄H₉O)₃. The wet gels were dried at 90°C (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N₂ adsorption. The ETA curves show the ²²⁰Rn-release rate, E, of the samples, previously labelled with ²²⁸Th and ²²⁴Ra, during heating. The decrease in E of the xerogel at temperatures higher than 400°C indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500°C. On the other hand, constant E of the aerogels and modified xerogels above 450°C suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500°C of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined.     

The anodic behavior of gold in phosphoric acid at elevated temperatures

Summary The anodic dissolution of gold in concentrated phosphoric acid was investigated at 145–200°C. Gold dissolves in a narrow potential region (150 mV) about 300 mV positive to the oxygen potential in the same electrolyte. At higher potentials it passivates. At very high overvoltages dissolution, which increased rapidly with temperature, was also observed in the transpassive region. The product of the dissolution is finely divided gold, probably formed by immediate decomposition of the oxydation product. At 200°C, at which temperature pyrophosphoric acid is formed, the gold dissolution also yields a clear yellow solution of gold pyrophosphate

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Zusammenfassung Die anodische Auflösung von Gold in konzentrierter Phosphorsäure wurde im Temperaturbereich von 145–200° C untersucht. Gold geht in Lösung in einem engen Potentialintervall (150 mV) etwa 300 mV positiver als das Sauerstoffpotential im gleichen Elektrolyten. Bei höheren Potentialen wird es passiv. Bei sehr hohen Überspannungen wurde auch im transpassiven Bereich Auflösung beobachtet, deren Geschwindigkeit mit der Temperatur stark ansteigt. Bei der Auflösung entsteht fein verteiltes Gold, vermutlich durch unmittelbar folgende Zersetzung des Oxydationsproduktes. Bei 200° C, wo Pyrophosphorsäure gebildet wird, ergibt die Goldauflösung auch eine klare gelbe Lösung von Goldpyrophosphat.

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Dedicated to Prof. Dr. M. v. Stackelberg on his 70th birthday.     

Kinetics of Structure Formation of YBa2Cu3O7-y

The kinetics of the synthesis of YBa₂Cu₃O_7-y (YBCO) was studied in situ on an X-ray diffractometer using the time vs temperature dependences of the concentration of the phase. The highest rate of YBCO synthesis was observed at 750-780°C. The effective activation energy was minimal at 700°C, E _a = 20-30 kJ/mole. The structural nonhomogeneity parameters and the dimensions of coherent scattering regions were determined for the YBCO product.     

The kinetics of zinc dissolution in nitric acid

The dissolution of zinc in 0.48–1.49M HNO₃ was studied at 15–25°C, by following simultaneously the concentration changes of the reactants (Zn and HNO₃), intermediate (HNO₂) and product (Zn²⁺) with time. Explicit mechanisms were suggested for the dissolution of zinc in nitric acid. The kinetics of the dissolution process show that it is of the first-order with respect to [Zn] and [HNO₂]. The data obtained show that the dissolution process is diffusion-controlled. The mechanism of zinc dissolution is compared with the mechanism of copper dissolution.

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Die Kinetik der Auflösung von Zink in Salpetersäure

Zusammenfassung Die Auflösung von Zink in 0.48–1.49M HNO₃ wurde bei 15–25°C mittels gleichzeitiger Verfolgung der Konzentrationsänderungen der Reaktanden (Zn und HNO₃), des intermediären HNO₂ und des Produkts Zn²⁺ untersucht. Es wird ein Mechanismus vorgeschlagen. Die Kinetik der Auflösung ist erster Ordnung bezüglich [Zn] und [HNO₂]. Die Daten zeigen, daß der Auflösungsvorgang diffusionskontrolliert ist. Der Mechanismus der Auflösung von Zink wird mit dem der Kupferauflösung verglichen.

     

Phase equilibrium and intermediate phases in the Eu–Sb system

Rapid heating rate thermal analysis, X-ray diffraction, fluorescence spectrometry, and differential dissolution method were used to study the high-temperature phase equilibrium in the Eu–Sb system within the composition range between 37 and 96 at% Sb. The techniques were effective in determination of the vapor–solid–liquid equilibrium since intermediate phases except Eu₄Sb₃ evaporated incongruently after melting. A thermal procedure was developed to determine the liquidus and solidus lines of the T−x diagram. Six stable phases were identified: two phases, EuSb₂ and Eu₄Sb₃, melt congruently at 1045±10 °C and 1600±15 °C, the Eu₂Sb₃, Eu₁₁Sb₁₀, Eu₅Sb₄, and Eu₅Sb₃ phases melt incongruently at 850±8 °C, 950±10 °C, 1350±15 °C, and 1445±15 °C, respectively. The exact composition shifting of Sb-rich decomposable phases towards Eu₄Sb₃, the most refractory compound, was determined. The topology of the Eu–Sb phase diagram was considered together with that of the Yb–Sb system.     

Effects of colloidal and dissolved silica on the dissolution of UO2 nuclear fuel in groundwater leaching tests

The influence of aqueous silica of two different physical forms (dissolved ions and SiO₂ colloid) on the dissolution of UO₂ nuclear fuel material was investigated at 95 °C temperature in autoclaves. It was tested that SiO₂ colloids can contribute to the surface degradation or act as carrier for uranium ions during a near field geochemical dissolution process. In the presence of colloids, well-crystallized secondary phases containing U and Si were formed on the surfaces, the latter attacked by the treatment. This was not the case when dissolved Si was used. SiO₂ colloids were partly found in their original form on the surfaces after 1000 hours at 95 °C. A surface charge model suggests that this different effects are due to the development of electrostatic interactions between the UO₂ and SiO₂ surfaces.This revised version was published online in November 2005 with corrections to the Cover Date.     

The Mechanism of Selective NO<Subscript>x</Subscript> Reduction by Hydrocarbons in Excess Oxygen on Oxide Catalysts: III. Adsorption Properties of the Commercial STK Catalyst

According to X-ray diffraction data, the STK catalyst is a mixture of Fe₂O₃ and Cr₂O₃. The temperature-programmed reduction spectrum exhibited two reduction peaks: one, with T _max = 250°C, corresponds to the reduction process Cr₂O₃ → CrO and the other, with T _max = 360°C, corresponds to the reduction Fe₂O₃ → Fe₃O₄. The results of thermal desorption measurements suggest that the individual adsorption of oxygen on the surface of the STK catalyst is low; in this case (according to IR-spectroscopic data), an atomic form is the main species. Surface nitrite-nitrate complexes are formed upon the adsorption of NO. Nitrite and nitrate complexes desorbed at maximum rates at 105 and 160°C, respectively. Unlike the NTK-10-1 catalyst, the NO species, which desorbed at high temperatures (250–400°C), was absent from the surface of STK. Propane adsorbed at room temperature to form surface compounds containing an acetate group. The interaction of propane with the surface of the STK catalyst at reaction temperatures resulted in strong surface reduction.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 550–558.Original Russian Text Copyright © 2005 by Tret’yakov, Burdeinaya, Zakorchevnaya, Matyshak, Korchak.     

Anodic dissolution of gold in cyanide solutions containing ions of thallium(I)

The potential dependence of the gold dissolution rate in alkali-cyanide solutions with and without 1.5 × 10^-5 M T1NO₃ is studied using the technique of the electrode surface renewal prior to taking measurements. The thallium ions accelerate the gold dissolution at potentials below 0.2 V and inhibit it at more positive potentials. The effect depends on the time the electrode is in contact with solution before the measurements, the potential scan rate, and concentrations of KCN and KOH. Possible approaches to interpreting the discovered phenomena are suggested.     

Dissolution of sintered thorium dioxide in phosphoric acid using autoclave and microwave methods with detection by gamma spectrometry

A quantitative and fast method of dissolution of refractory thoria (ThO₂) was developed for the determination of thorium (Th) in a given sample. The dissolution of sintered ThO₂ powder, microspheres and pellets using 88% phosphoric acid was investigated. The conditions of quantitative dissolution of ThO₂ microspheres were optimized by conventional heating in autoclave and also by microwave heating. 100 mg of sintered ThO₂ microspheres were dissolved in 8 g of phosphoric acid in an autoclave, and heating at 170 °C for 3 h, in comparison to 5 g of phosphoric acid by microwave heating (375 W) at 220 °C for 1 h. Dissolution studies on the powder form of sintered ThO₂ were also performed. 1 g of sintered ThO₂ powder could be dissolved in 6.5 g of phosphoric acid in autoclave heating at 170 °C for 1 h. Strong complexing of (PO₄)³⁻ with Th⁴⁺ may be the influencing factor for quantitative dissolution of ThO₂.