Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: I. Role of phase transitions in 5 M H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

Low-temperature oxidation of U(IV) with xenon difluoride, accompanied by formation of the UO ₂ ²⁺ ion in an electronically excited state, was examined in relation to the aggregation state and phase composition of 5 M aqueous H₂SO₄ solution. The reaction was found to proceed at a measurable rate in a supercooled liquid solution at T > 190 K and in a polycrystalline sample at T > 150 K. The chemiluminescence emitted by *(UO ₂ ²⁺ ) grows in intensity by several (up to 6) orders of magnitude during the exothermic phase transitions. The increase in the chemiluminescence intensity during phase transitions (crystallization of liquid, in particular, supercooled 5 M H₂SO₄ solutions) correlates with formation of the crystalline phase. Presumably, one of the factors accelerating the low-temperature reaction of U(IV) with XeF₂ during crystallization of the solution (along with concentration of the reactants in the intercrystallite spaces) is the catalytic activity exhibited by the freshly formed surface of the H₂SO₄ crystal hydrates.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: III. Effect of solvent deuteration

The effect of the solvent deuteration on the reaction of U(IV) with XeF₂ in solutions of H₂SO₄ in H₂O and D₂SO₄ in D₂O, occurring in the course of heating these solutions after their quick cooling (10–15 K s^?1) to 77 K, was examined. The effect is manifested in essentially different temperature dependences of the chemiluminescence accompanying the oxidation of U(IV) with XeF₂. The results obtained are interpreted within the framework of the concept that the low-temperature reaction of U(IV) with XeF₂ is catalyzed by the juvenile surface of crystal hydrates of sulfuric and deuterosulfuric acids, formed in the course of low-temperature phase transitions. The isotope effect is due to different kinetics of the phase transitions in solutions of H₂SO₄ in H₂O and D₂SO₄ in D₂O. In the course of crystallization of supercooled H₂SO₄ solutions, depending on the mode of temperature variation, either one or two eutectics appreciably differing in the physical properties are formed. In the course of crystallization of supercooled solutions of D₂SO₄ in D₂O, one stable eutectic is formed in each sample, and its characteristics depend on the thermal history of the sample. The eutectics melt in the course of heating of the frozen solution at temperatures differing by only 3–4 K.     

Formation of excited uranyl ion in oxidation of U(IV) with xenon difluoride in aqueous solutions of sulfuric and deuterated sulfuric acids

Kinetic features of the chemiluminescence accompanying oxidation of U(IV) with xenon difluoride in solutions of 0.2 M H₂SO₄ in H₂O and 0.2 M of D₂SO₄ in D₂O at temperatures within 283–313 K and concentrations (M) 10^?6 ≤ [U(IV)] ≤ 10^?4 and 10^?4 ≤ [XeF₂] ≤ 10^?3 were examined. The shape of the kinetic curve depends on the concentration of the reactants and the solution temperature. At [U(IV)] = 10^?6 and [XeF₂] = 10^?4 M, the curve exhibits a maximum, more prominent at low temperatures. At relatively high concentration ([U(IV)] = 10^?4 and [XeF₂] = 10^?3 M), self-acceleration of the reaction is observed: The mechanism of oxidation of U(IV) turns into a branched-chain mode owing to a higher concentration in the solution of radicals ^·OH, HO ₂ ^· , and XeF ₂ ^· , whose additional amounts are yielded by hydrolytic reduction of XeF₂. A kinetic isotope effect of the solvent k _H/k _D was revealed, which attains a maximum of 1.8 at 283 K. The activation energies of the oxidation of U(IV) with xenon difluoride in H₂SO₄ and D₂SO₄ solutions were estimated at E _a,H = 12 and E _a,D = 13 kcal mol^?1, respectively. The occurrence of the isotope effect is an indirect evidence of participation in the reaction of the OH (or OD) groups. The rate of hydrolytic reduction of XeF₂ in deuterated solvent (0.2 M D₂SO₄ in D₂O) in its photochemical stage is several times lower, and the luminescence accompanying the reaction is by an order of magnitude smaller than that in 0.2 M H₂SO₄.     

Luminescence properties of uranyl in POCl<Subscript>3</Subscript>-SnCl<Subscript>4</Subscript> solvent

The spectral-luminescence properties of uranyl in aprotic binary solvent POCl₃-SnCl₄ were studied. The uranyl luminescence lifetime τ in the POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ system does not exceed 20 μs. There is no concentration quenching of uranyl up to [UO ₂ ²⁺ ] = 0.14 M. When anhydrous UO₃ is dissolved, τ increases with increasing water content of the initial solvent,. In the solutions containing uranyl perchlorate, τ decreases with increasing uranyl and SnCl₄ concentrations. This effect is caused by products of ClO ₄ ^? decomposition scavenged by SnCl₄.     

Molybdenum dissolution in mixtures of H<Subscript>2</Subscript>O<Subscript>2</Subscript> and concentrated HNO<Subscript>3</Subscript> and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> in the presence of tungsten

The oxidation of molybdenum and tungsten in hydrogen peroxide solutions and the effect of H₂O₂ on the selectivity of Mo and W dissolution in mixtures of concentrated nitric and sulfuric acids have been studied.     

Etching behavior of CdTe in aqueous H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HI-C<Subscript>6</Subscript>H<Subscript>8</Subscript>O<Subscript>7</Subscript> solutions

This paper examines the dissolution behavior of the (111)A, (111)B, (110), and (100) surfaces of CdTe single crystals in aqueous H₂O₂-HI-C₆H₈O₇ (citric acid) solutions. We have determined the dissolution rate of the crystals as a function of temperature and solution concentration, located the composition regions of polishing and selective etchants, and studied the microstructure and roughness of surfaces polished with optimized etchants. The etching behavior of CdTe is shown to depend on its crystallographic orientation.     

Graphite intercalation in the graphite-H<Subscript>2</Subscript>SO<Subscript>4</Subscript>-R (R = H<Subscript>2</Subscript>O,C<Subscript>2</Subscript>H<Subscript>5</Subscript>OH,C<Subscript>2</Subscript>H<Subscript>5</Subscript>COOH) systems

The electrochemical behavior of graphite in polar solvent-H₂SO₄ electrolytes is studied in a wide range of H₂SO₄ concentrations. The results demonstrate that, with decreasing H₂SO₄ concentration, the charging curves become smoother and shift to higher potentials, the stage index increases, and intercalation compounds are more difficult to obtain. At H₂SO₄ concentrations of 50% and lower, graphite polarization is accompanied by a significant overoxidation, as evidenced by the anomalously small intercalate layer thicknesses: 7.75–7.85 Å. Anodic polarization of graphite in electrolytes consisting of H₂SO₄ and a polar solvent (H₂O and C₂H₅OH) follows the same mechanism as in the case of the formation of graphite bisulfate. In going from water to C₂H₅OH, a less polar solvent, the intercalation threshold increases from 30 to 70% H₂SO₄. It is shown using a set of characterization techniques that, in the graphite-H₂SO₄-R (R = H₂O, C₂H₅OH) systems, the solvent is not intercalated into graphite. Stage I–III ternary graphite intercalation compounds (TGICs) are synthesized for the first time in the graphite-H₂SO₄-C₂H₅COOH system: stage I TGICs at H₂SO₄ concentrations above 70%, stage II in the range 30–70% H₂SO₄, and stage III at H₂SO₄ concentrations down to 10%. The intercalate layer thickness in the TGICs is 7.94 Å. The mechanism of TGIC formation in this system is shown to differ from those in mixtures of H₂SO₄ and other organic acids. Thermal analysis in combination with spectroscopic analysis of gaseous products provides clear evidence for intercalation of propionic acid into the TGIC and indicates that the thermal stability of this compound is lower than that of graphite bisulfate.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 162–169.Original Russian Text Copyright © 2005 by Shornikova, Sorokina, Maksimova, Avdeev.     

Study on titania nanotube arrays prepared by titanium anodization in NH<Subscript>4</Subscript>F/H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

In this paper, the formation of titania nanotube arrays was investigated in NH₄F/H₂SO₄ electrolyte. Under optimized electrolyte conditions, the titania nanotube arrays with an inner diameter of about 120 nm and a length of about 300 nm was obtained. During the formation process, the variety of current was observed. The current–time curve implied that the evolvement process of titania nanotube arrays include three stages. The stability of titania nanotube arrays at elevated temperatures was studied. The as-prepared titania nanotube arrays is amorphous, crystallized in anatase and rutile as the rising of the temperature. The samples were characterized by ESEM and XRD.     

Stability limits of graphite intercalation compounds in the systems graphite-HNO<Subscript>3</Subscript>(H<Subscript>2</Subscript>SO<Subscript>4</Subscript>)-H<Subscript>2</Subscript>O-KMnO<Subscript>4</Subscript>

The formation of binary graphite intercalation compounds (GICs) with nitric and sulfuric acids in the presence of a strong oxidant has been studied by x-ray diffraction and potentiometry in a wide range of acid concentrations. The redox potential of the oxidizing solution and the intercalation ability of the acid are shown to influence the stage number (the number of graphite layers between two successive intercalate layers) of the forming GIC and the concentration ranges of GIC formation. The (\(E_{H_2 } \))-H ₀ (redox potential of the oxidizing solution-Hammett function of the acid) stability fields of graphite nitrate and graphite bisulfate are presented. Our results are the first to demonstrate that KMnO₄ extends the concentration ranges of GIC formation and reduces the threshold acid concentration for the synthesis of binary GICs (to 40%).     

Effect of Cl<Superscript>−</Superscript>on the corrosive wear of AISI 321 stainless steel in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

The effect of Cl⁻ on the corrosive wear behaviour of AISI 321 stainless steel in H₂SO₄ solution was studied via the corrosive wear rate, the load bearing capacity of passive film and the relationship between pitting and corrosive wear. There is a critical load at natural potential, below which the corrosive wear rate is slightly lowered by Cl⁻, while above which is increased. At natural potential there are more pits at low load than that at a higher one in the wear tracks and the pits are also deeper. The load bearing capacity is lowered by Cl⁻ at passive region and then the corrosive wear rate increased.     

High performance films obtained from PVA/Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>/H<Subscript>2</Subscript>O and PVA/CH<Subscript>3</Subscript>COONa/H<Subscript>2</Subscript>O systems

The atactic poly(vinyl alcohol) (a-PVA) aqueous solutions with Na₂SO₄ or CH₃COONa were cast to prepare films and then the Na₂SO₄ or CH₃COONa in the films was removed. Both films prepared by removing Na₂SO₄ or CH₃COONa in water had a water-resistance property. The degree of crystallization of the films increased with an increase of the contents of Na₂SO₄ and CH₃COONa in the solutions up to 0.05 and 0.1 wt%, respectively. However, the melting temperature (226–228°C) was independent of the content of Na₂SO₄ and CH₃COONa in the solutions. The draw ratio and tensile modulus of the films prepared from the solutions with 0.01 wt% Na₂SO₄ and 0.1 wt% CH₃COONa were 1.3–1.6 times more than that of the films obtained from an aqueous solution. Namely, in case of the films obtained from a-PVA/H₂O/Na₂SO₄ and a-PVA/H₂O/CH₃COONa systems, both the drawability and mechanical properties as well as the degree of crystallization were higher than those for the film obtained from an aqueous a-PVA solution.     

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H₂C₂O₄ was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H₂C₂O₄]²[H⁺]^−1.6, k = 182 mol^−1.4 l^1.4 s⁻¹. The activation energy in the range 25–45°C is 26 kJ mol⁻¹. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H⁺ ion.     

Formation of excited uranyl ion in oxidation of U(IV) with xenon trioxide in aqueous H2SO4 and HClO4 solutions: III. Kinetic mode of self-accelerated reaction

Self-acceleration was revealed for chemiluminescent radical-chain oxidation of U(IV) with xenon trioxide in aqueous HClO₄ solutions in a reactor made of Teflon (instead of conventionally used glass). In oxidant excess, U(IV) oxidation with XeO₃ follows the first-order kinetics only at low (under 10^?6 M) concentrations of U(IV) in solution. Self-acceleration is observed at relatively high concentrations of the reactants, when the contribution of the heterogeneous decay of the radicals to chain termination substantially decreases and that of the degenerate branching reactions to the U(IV) oxidation mechanism increases. Oxidation of U(IV) with XeO₃ demonstrates a critical phenomenon, uncommon for liquid-phase radical-chain processes: a minor increase in the concentrations of the reactants is responsible for transition from a steady-state mode to self-acceleration of the redox process.     

Etching behavior of GaAs,GaSb, InAs,and InSb in aqueous H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HBr-ethylene glycol solutions

The chemical interaction of GaAs, GaSb, InAs, and InSb single crystals with H₂O₂-HBr-ethylene glycol bromine-releasing etchants has been studied under reproducible hydrodynamic conditions. We have located boundaries between regions of polishing and nonpolishing solutions, assessed the surface condition of the crystals using microstructural analysis and surface profiling, and optimized the compositions of the polishing solutions and dynamic chemical polishing conditions for the materials studied.     

Corrosion resistance of amorphous Fe<Subscript>82</Subscript>P<Subscript>16</Subscript>Si<Subscript>2</Subscript> in 0.1 M Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A soft-magnetic amorphous Fe-P-Si alloy prepared using ferrophosphorus waste was tested for corrosion in 0.1 M Na₂SO₄. In a nonequilibrium state, the Fe₈₂P₁₆Si₂ alloy interacts with the medium, but annealing and relaxation reduce the interaction, without influencing the magnetic properties of the alloy. The corrosion resistance of the alloy is comparable to that of Finemet (Fe-Si-B-Nb-Cu) materials.     

Degradation of bisphenol A by combining ozone with UV and H<Subscript>2</Subscript>O<Subscript>2</Subscript> in aqueous solutions: mechanism and optimization

A laboratory-scale study on the abatement of bisphenol A (BPA) was performed by combining O₃ with H₂O₂ and UV (O₃/H₂O₂/UV), an ozone-based advanced oxidation processes technique (AOP). This work aimed to (1) evaluate the removal of BPA with O₃/H₂O₂/UV, and to compare the degradation efficiency with other ozone-based AOPs (such as O₃ alone, O₃/H₂O₂, and O₃/UV), (2) structurally optimize BPA abatement by using a central composite design (CCD) for experimental design purposes and/or a response surface methodology to find the optimum, and (3) identify the degradation pathways, and main intermediate products, formed during BPA abatement with O₃/H₂O₂/UV. The degradation pathways of BPA degradation were revealed by O₃/H₂O₂/UV on the basis of evidences of intermediate generation. The effect of initial pH, ozone, and H₂O₂ dose during BPA abatement was studied in detail. By increasing each of these three parameters, an enhancement of the BPA degradation efficiency is mostly observed. BPA can be degraded completely when a sufficiently high ozone dose is applied. However, excess H₂O₂, as a scavenger of HO·, has a negative effect on BPA abatement, resulting in a decrease in the BPA’s degradation efficiency. For example, the removal decreased from 64 to 58% by enhancing the H₂O₂ initial dose from 0.5 to 0.75 mmol/L (at an initial pH and ozone dose of, respectively, 7 and 0.1 mg/L). The results confirmed that combining ozone with H₂O₂ and UV was a more efficient method than the other three ozone-based AOPs on the removal of BPA. Therefore, this method could be further applied for the treatment of real wastewaters containing BPA and other micropollutants.     

Enhanced stability of Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> during aqueous oxidation reaction via SiH<Subscript>4</Subscript> treatment

γ-Al₂O₃ is widely present as a support in catalytic application. However, the transformation of γ-Al₂O₃ into the undesired hydrated boehmite (γ-AlOOH) under the aqueous reaction conditions usually results in an irreversible inactivation of supported Al₂O₃ catalysts. Toward suppressing the hydration of γ-Al₂O₃ in the process of catalytic reactions, herein we have devised a new strategy by exploring the SiH₄ treatment for successful preparation of Si–Pd/Al₂O₃ catalysts. SiH₄ treatment enables a significant improvement in the poor stability of Pd/Al₂O₃ for selective oxidation of toluene because SiH₄ could effectively anchor palladium nanoparticles and inhibit the formation of boehmite by reacting with unsaturated aluminum sites to reduce the intensity of Lewis acid sites on the surface of γ-Al₂O₃. Pd/Al₂O₃ pretreated with SiH₄, that is Si–Pd/Al₂O₃, provides high catalytic activity and stability in the aqueous oxidation of toluene even at high temperature. Moreover, Si–Pd/Al₂O₃ is reusable without obvious loss of the catalytic activity and selectivity, compared to Pd/Al₂O₃.     

Reaction of Np(VI) with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in carbonate solutions

The kinetics and stoichiometry of the reaction of Np(VI) with H₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as Δ[Np(VI)]/Δ[H₂O₂] ≈ 2. In Na₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5–0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol⁻¹.     

Reduction of U(VI) in POCl<Subscript>3</Subscript>-lewis acid solutions

Features of the behavior of uranyl ions in POC1₃-MCl _x -²³⁵UO ₂₊ ² solutions (M = Ti, Si, Zr, Sn, Sb) were considered. Irreversible accumulation of U(IV) in the course of synthesis of POCl₃-SnCl₄-²³⁵UO ₂₊ ² solutions prepared from water-containing U(VI) compounds, excluding UO₂(ClO₄)₂ · 5H₂O, was found. The reaction rate increases with increasing uranyl concentration, k _l[U(IV)] ～ (1.6±0.2) × 10^?6 s^?1 (T = 380 K). The U(IV) accumulation was also observed on heating (T = 360–380 K) POCl₃-SnCl₄-²³⁵UO ₂₊ ² and POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions hermetically sealed in glass cells and on irradiating them by the light of a xenon lamp. In POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions prepared from UO₂(C1O₄)₂ · 5H₂O, U(IV) disappears within several days after stopping the irradiation. The reduction of U(VI) is caused by formation of uranyl dichlorophosphate complexes and by deactivation of uranyl excitation with chlorine-containing agents.