Catalytic reduction of U(VI) in H2SO4 solutions with hydrazine and formic acid in the presence of bimetallic platinum-ruthenium catalysts

The structure of bimetallic platinum-ruthenium catalysts was studied by X-ray phase analysis and X-ray absorption spectroscopy. The effects of strong electronic interaction of Pt and Ru atoms in formation of the binary catalyst were revealed. The kinetics of catalytic reduction of U(VI) with hydrazine and formic acid in sulfuric acid solutions on 5% (Pt-Ru)/SiO₂ was studied. The mechanisms of these reactions with platinum and bimetallic platinum-ruthenium catalysts are similar. A catalytic synergistic effect is observed in heterogeneouscatalytic reactions of U(VI) reduction with hydrazine and formic acid in H₂SO₄ solutions.     

Catalytic Reduction of U(VI) with Hydrazine and Formic Acid in HNO3 Solutions

Catalytic reduction of 0.75 M U(VI) with hydrazine in HNO₃ solutions was studied under various conditions. At 58°C up to 0.55 M U(IV) is accumulated within 2 h in solutions containing 1-1.5 M N₂H₅ ⁺ and 2 M HNO₃ in the presence of 1% Pt/SiO₂ (S : L = 1 : 10). The reduction is decelerated with decreasing N₂H₅ ⁺ concentration to 0.75 M or with increasing HNO3 concentration to 3-4 M. 1.2 mol of U(IV) is formed per mole of oxidized hydrazine. Uranium(VI) reduction with formic acid in the presence of 1% Pt/SiO₂ and 1% Pt/VP-1An anion exchanger was studied. There exists a threshold N₂H₅ ⁺ concentration below which U(VI) is not reduced. The reduction in solutions containing 1-2 M HCOOH, 1-2 M HNO₃, and 0.1 M N₂H₅ ⁺ is faster than in solutions free of formic acid and containing 1-1.5 M N₂H₅ ⁺.     

Reactivity of platinum nanoaggregates on various types of supports in catalytic decomposition of hydrazine in acid solutions

Platinized catalysts on various types of supports were tested in the catalytic decomposition of hydrazine in HClO₄ and HNO₃ solutions, where the process follows different pathways. In HClO₄, the activity of the catalysts supported on a Termoksid ceramic material is higher than that of the catalysts supported on amorphous silica gel. In nitric acid solutions, the trend is reverse. Peptization of the ceramic supports in acid solutions increasing in the order 75% TiO₂-25% SnO₂ < 75% TiO₂-25% ZrO₂ ? TiO₂ < ZrO₂ was observed. In perchloric acid solutions, the catalyst specific activity in the hydrazine adsorption-dissociative decomposition increases with decreasing size of platinum crystallites on the support. In nitric acid solutions, where the hydrazine decomposition proceeds as its catalytic oxidation with nitric acid, the catalyst specific activity decreases with a decrease in the size of the catalyst crystallites, i.e., the catalyst centers located on large crystallites are more active. The results obtained were attributed to the energetic heterogeneity of the surface Pt atoms and various mechanisms of the hydrazine catalytic decomposition in various media.     

Catalytic reduction of U(VI) with hydrazine on palladium catalysts in acid solutions

The stability of finely dispersed palladium supported on silica gel with respect to various acids was studied. It was shown that palladium catalysts can be used in moderately acidic media under reducing conditions. In nitric acid solutions within a wide range of experimental conditions, the palladium catalysts do not initiate reduction of U(VI) with hydrazine. The catalytic properties of palladium catalysts differing in the size of nanocrystallites of the active metal were examined in the reduction of U(VI) with hydrazine in sulfuric acid solutions. The specific activity of Pd/SiO₂ catalysts is determined solely by the size of metal nanocrystals and is independent of the metal content on the support. The negative size effect is observed, i.e., the surface Pd atoms located on large crystallites exhibit higher catalytic activity. The results obtained were interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the mechanism of U(VI) catalytic reduction with hydrazine in the sulfuric acid solutions.     

Catalytic reduction of U(VI) with formic acid in acid solutions on palladium catalysts

The kinetics of catalytic reduction of U(VI) with formic acid in H₂SO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO₂ leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by catalytic reduction with formic acid.     

Heterogeneous Catalytic Decomposition of Hydrazine in Nitric Acid Solutions

The kinetics and stoichiometry of catalytic decomposition of hydrazine in 0.25-8 M HNO₃ in the presence of 1% Pt/SiO₂ were studied. The main reaction products are N₂, N₂O, and ammonium nitrate whose yield decreases with increasing [HNO₃]. It was found that catalytic decomposition of hydrazine in HNO₃ is a result of three different processes: heterogeneous catalytic disproportionation of hydrazine at the catalyst surface, oxidation of hydrazine with catalytically generated HNO₂, and heterogeneous catalytic oxidation of hydrazine with HNO₃. The contribution of these processes to the overall reaction is mainly governed by HNO₃ concentration and temperature. The role of platinum in the catalysis of hydrazine oxidation in nitric acid media and possible mechanisms of catalytic processes are discussed.     

Effect of O2, H2O2, and HNO2 on the stability of Np(III–VII) in aqueous solutions

Published data on reactions of Np ions with O₂, H₂O₂, HNO₂, and HNO₃ in solutions of various compositions in a wide pH range are considered. O₂ oxidizes Np(III) in acid solution and Np(IV) and Np(V) in alkaline solutions. H₂O₂ exhibits dual behavior. In weakly acidic solutions, it converts Np(III) and (IV) to Np(V), in 0.75?C1 M NaHCO₃ it oxidizes Np(V) to Np(VI), whereas in dilute HClO₄ and HNO₃ and in carbonate and alkali solutions it reduces Np(VI), and in alkali solutions it reduces Np(VII). The first step of reduction in most cases is the formation of the Np(VI) peroxide complex, and the next step is the intramolecular charge transfer. In concentrated HNO₃ solutions, H₂O₂ converts Np(V) to Np(IV) and Np(VI) and then reduces Np(VI). Some radiation-, photo-, and sonochemical reactions occur via formation of excimers, i.e., of dimers arising from excited and unexcited Np ions. The excimer decomposes into two ions with higher and lower oxidation states. In reduction reactions, the excimer eliminates H₂O₂ (in addition to the H₂O₂ arising as primary product of water radiolysis). In HNO₃ solutions, oxidation of Np ions occurs only in the presence of HNO₂ arising as reaction product or upon radiolysis, photolysis, or sonolysis. The active species are NO ₂ ^? , NO₂, and NO⁺ present in equilibrium with HNO₂.     

Reduction of Np(V) with formic acid in acidic solutions in the presence of palladium catalysts

The kinetics of catalytic reduction of Np(V) with formic acid in HClO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the Pd content and size of Pd nanocrystals was studied. The reaction is a structure-insenitive catalytic process, i.e., the size effect is absent. An increase in the percentage of Pd on SiO₂ leads to a decrease in the activity of the catalysis centers due to a considerable increase in the contribution of the side reaction catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The effect of the S:L ratio, concentrations of HCOOH and HClO₄, and temperature on the rate of catalytic reduction of Np(V) in the presence of palladium catalysts was examined. The suggested mechanism of the catalytic reduction of Np(V) with formic acid in the presence of Pd/SiO₂ involves a slow step of decomposition of the protonated species NpO₂H, formed by the reaction of the NpO ₂ ⁺ ion with chemisorbed hydrogen atoms Pd(H).     

Catalytic Reduction of Np(VI,V) with Formic Acid in Perchloric Acid Solutions

Neptunium(VI) is successively reduced with formic acid to Np(V) and Np(IV) in perchloric acid solutions in the presence of 1% Pt/SiO₂ catalyst. The kinetic features of Np(VI,V) reduction with formic acid in 0.1-4.0 M HClO₄ in the presence of 0.01-0.1 g ml^-1 of 1% Pt/SiO₂ at [HCOOH] = 0.001-1.0 M and T = 40-70°C were studied. The rate-determining steps of reduction of Np(VI) to Np(V) and Np(V) to Np(IV) are diffusion and decomposition of the activated complex adsorbed on the catalyst surface, respectively. The mechanisms of both processes are discussed.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: II. Effect of solvent and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> concentration

The effect of solvent (aqueous solutions of HClO₄ and H₂SO₄) on the low-temperature (T < 273 K) reaction of U(IV) with XeF₂, accompanied by formation of uranyl ion in the electronically excited state *(UO ₂ ²⁺ ), was examined. In the course of heating of 0.2–8.31 M HClO₄ solutions after their quick cooling to 77 K, the chemiluminescent reaction occurs at a detectable rate only at T > 220 K. In the similar experiments performed with 0.05–0.5 M H₂SO₄ as solvent, the emission appears at a considerably lower temperature, 165–175 K. The chemiluminescence (CL) intensity increases in the course of exothermic phase transitions occurring when the temperature of H₂SO₄ solutions (at [H₂SO₄] > 0.05 M) is changed. The increase in the CL intensity in the course of a phase transition is associated with the appearance in a multicomponent frozen system of a juvenile surface of H₂SO₄ crystal hydrates. The lack of CL at T < 220 K in HClO₄ solutions is associated with the fact that heating of the samples after their quick cooling to 77 K is not accompanied by formation of crystalline phases exhibiting catalytic properties toward the oxidation of U(IV) with XeF₂.     

Catalytic reduction of Np(V) with hydrazine in nitric acid solutions in the presence of ruthenium catalysts

The reduction of Np(V) with hydrazine in HNO₃ solution in the presence of Ru/SiO₂ catalysts was studied by spectrophotometry. The back oxidation of Np(IV) with nitric acid, also catalyzed by ruthenium, occurs concurrently with the Np(V) reduction with hydrazine. The contribution of each process is determined by the HNO₃ concentration. The mechanism of the ruthenium-catalyzed reduction of Np(V) with hydrazine was suggested on the basis of the kinetic data. The effect of the size of Ru nanoparticles on the activation energy of the catalytic reduction of Np(V), characterizing the structural sensitivity of the heterogeneous-catalytic reaction (positive size effect), was revealed.     

Catalytic Reduction of Pu(IV) with Formic Acid in Nitric Acid Solutions

Pu(IV) is reduced to Pu(III) in nitric acid solutions with formic acid in the presence of urea and 1% Pt/SiO₂ catalyst. The kinetics of reduction were studied in 0.3-2.3 M HNO₃ containing 0.2-1 M HCOOH, 0.1-0.5 M (NH₂)₂CO, and 0.01-0.1 g ml^{- 1} of 1% Pt/SiO₂ at 30-60°C. At HNO₃ concentration higher than 2 M, the Pu(IV) reduction is reversible because of catalytic decomposition of urea. The reduction mechanism is discussed.     

Kinetics of the reaction of Np(VI) with iminodiacetic acid

The stability of Np(VI) in 5–200 mM iminodiacetic acid (H₂IDA) solutions at 23.5–55°С was studied by spectrophotometry. In a solution with pH 2 and excess Np(VI), 1 mol of H₂IDA reduces 2 mol of Np(VI) to Np(V). In 1 and 0.5 M HClO₄ solutions containing 200 mM H₂IDA and 1 mM Np(VI), no more than 36 and 65% of Np(VI), respectively, is reduced at 44.5°С. Complete reduction of Np(VI) is observed in solutions containing 0.2 M HClO₄ and less. In the examined ranges of H2IDA concentrations and temperatures, Np(VI) is consumed in accordance with the first-order rate law. The reduction mechanism involves formation of a Np(VI) iminodiacetate complex, which is followed by intramolecular charge transfer. The generated radical reduces Np(VI). The activation energy is 107 ± 3 kJ mol^–1.     

Composites reinforced with reused tyres: Surface oxidant treatment to improve the interfacial compatibility

Reused tyres whose surfaces were treated with various chemical acids, such as H₂SO₄, HNO₃ and HClO₄, were used as reinforcement material in HDPE-reused tyre composites. Their mechanical properties (e.g. tensile strength, Young’s modulus, toughness and elongation at break) were studied to evaluate how surface treatments may improve compatibility between the two components. The effect of chemical modifications on the surface of reused tyres was monitored by FTIR, the determination of mechanical properties and SEM. The importance of rubber treatment can be assessed by comparing the results of treated and untreated composites with those for neat HDPE. Reused tyre rubber, added to the HDPE in small quantities, acts as a filler, improving the stiffness and providing a more brittle behaviour. Nevertheless, a rubber content above 10%, using either untreated rubber or rubber treated with HClO₄, gives lower values of Young’s modulus than neat HDPE. Elongation and toughness values are also lower. Treatment with H₂SO₄ and HNO₃ improves the ability of rubber to interact with the HDPE, increasing the material’s stiffness. The treatment with H₂SO₄ was the most effective, whereas HClO₄ did not improve the material’s properties.     

Autocatalytic dissolution of Pb in HNO3

The reaction between Pb and HNO₃ has been investigated using the thermometric technique. Weight-loss measurements on the reaction were also obtained. As the HNO₃ concentration is increased from 5×10^–2 to 4mol l^–1, the corrosion rate increases. This is shown thermometrically by a substantial increase in the maximum temperature attained, T _m, as well as a decrease in the time, t, required for reaching T _m. Dissolution of Pb in HNO₃ is proposed to take place according to an autocatalytic mechanism. Passivation sets were detected in solutions 11 mol l^–1 HNO₃. A parallel indication between the thermometric technique and weight-loss measurements was obtained. The rate-determining step of the autocatalytic process involves HNO₂ in dissolution of Pb in HNO₃. This is supported by the results of addition of hydrazine to the solution. This additive raises the maximum measured temperature, without affecting the corresponding time necessary to reach it. The effect of addition of NaNO₂, NaNO₃, NaCl, Na₂SO₄, NaH₂PO₄ and NaClO₄ on the reaction number, RN, of Pb in 4 mol l^–1 HNO₃ was examined. Only NaNO₂ accelerates the dissolution reaction while the other salts show as inhibition effect. It was found that these additives inhibit dissolution due to the displacement of some cathodic depolarizing components, as NO₂, from the active sites on the metal surface. The effect of addition of HCl, H₂SO₄, HClO₄ and H₃PO₄ on the reaction number, RN, of Pb in 4 mol l^–1 HNO₃ was also investigated. The observed acceleration and retardation of the dissolution of Pb was found to be dependent on both the concentration and nature of anions of the extra acids added.     

Specific Features of Electrochemical Oxidation of Pu(III) in a Diaphragmless Cell in Hydrazine-Containing Nitric Acid Solutions

Oxidation of Pu(III) in a diaphragmless cell with a Ti cathode and a Pt anode in 1.0–3.9 M HNO₃ solutions containing 4.5 × 10^?2–1.8 × 10^?1 M hydrazine was studied. The final solutions contain, along with Pu(IV), also a small amount of Pu(VI), increasing with an increase in the current density. Methods allowing minimization of the Pu(VI) amount in the solutions after the electrolysis were suggested. The possible process mechanism involving reactions of Pu ions at the electrodes and in the bulk of the solution with HNO₂ was considered.

     

Extraction of metal ions from aqueous solutions of mineral acids with crown ether-ionic liquid systems

The extraction of U(VI), Sr, and Cs from solutions of mineral acids (HNO₃, HCl) with a crown ether, cis-syn-cis-dicyclohexyl-18-crown-6 (DCH18C6-A), dissolved in ionic liquids (ILs), 1-butyl-3-methyl-imidazolium derivatives (bmimPF₆, bmimBF₄, bmimTf₂N), was studied. The best physicochemical characteristics (solubility in the aqueous phase, viscosity, hydrophobicity, etc.) are exhibited by a solution of DCH18C6-A in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (bmimTf₂N). The metal distribution ratios (D _M) in the extraction with a 0.01 M DCH18C6-A solution are 80 for CsNO₃, 78 for CsCl, and 162 for Sr(NO₃)₂. With an increase in the HNO₃ and HCl concentrations, D _M decreases, and with 1 M acids it does not exceed 1. The U(VI) extraction from nitric acid solutions with a 1 M solution of DCH18C6-A in bmimTf₂N initially increases with an increase in the aqueous phase acidity, with D _U(VI) reaching 5.4 in 4 M HNO₃, but then decreases in the interval 4–8 M HNO₃, whereas in the extraction with a 1 M solution of tributyl phosphate (TBP) in bmimTf₂N D _U(VI) monotonically increases with an increase in the HNO₃ concentration to 8 M. From hydrochloric acid solutions, U(VI) is extracted with solutions of DCH18C6-A in bmimTf₂N with the D _U(VI) values characteristic of solutions of DCH18C6-A in nonpolar organic diluents. On the whole, ILs as solvents exhibit unusual properties in the extraction of alkali and alkaline-earth elements from neutral solutions and of U(VI) with TBP from concentrated nitric acid solutions.     

Investigation of effect of protonic acid media on the optical and thermal properties of chemically synthesized poly(o-toluidine)

Conducting polymer, Poly(o-toluidine) was synthesized by chemical oxidative polymerization method using different inorganic acids such as HCl, H₂SO₄, HClO₄, HNO₃, H₃PO₄ and H₃BO₃ as protonic acid media. Synthesized polymers were characterized by UV-Visible and FT-IR spectroscopy. Conducting emeraldine salt phase of the polymer has been confirmed with the help of spectroscopic analysis. Thermal stability of these polymers were investigated by thermogravimetric (TG/SDTA) analysis. Increase in conductivity with increase in temperature was observed in all the samples.     

Effect of thermal and chemical treatments on carbon and silica contents in rice husk

Thermal treatment of rice husk has been conducted up to 1000° C in air, oxygen, argon and non-oxidizing atmospheres. Chemical treatments consisted of HCI, H₂SO₄, HNO₃, NaOH and NH₄OH. Purity, particle size distribution and SEM micrographs of chemically treated samples are presented. Carbon and SiO₂ contents in rice husk, coked at different temperatures and time, have been determined to show that a C : SiO₂ ratio of 2:1, required for the production of solar grade silicon, can be achieved at low temperature. SiO₂ of 99% purity can be obtained by acid leaching. The results have been interpreted in terms of two types of bonding of silicon, a rigid structural framework, and selective sites for the preferential attack of acid and alkali in rice husk.     

Reduction of Pu(IV) and Np(VI) with hydroxylamine in solutions of low acidity with high uranium content

Decomposition of hydroxylamine in HNO₃ solutions containing 350 to 920 g l^?1 U(VI) was studied. In the absence of fission and corrosion products (Zr, Pd, Tc, Mo, Fe, etc.), hydroxylamine is stable for no less than 6 h at [HNO₃] < 1 M and 60°C. In the presence of these products, the stability of hydroxylamine appreciably decreases. The reduction of Pu(IV) and Np(VI) with hydroxylamine in aqueous 0.33 and 0.5 M HNO₃ solutions containing 850 g l^?1 U(VI) and fission and corrosion products at 60°C was studied. Np(VI) is rapidly reduced to Np(V), after which Np(V) is partially reduced to Np(IV). The rate of the latter reaction in such solutions is considerably higher than the rate of the Np(V) reduction with hydroxylamine in HNO₃ solutions without U(VI). At [HNO₃] = 0.33 M, the use of hydroxylamine results in the conversion of Pu to Pu(III) and of Np to a Np(IV,V) mixture, whereas at [HNO₃] = 0.5 M the final products are Pu(IV) and Np(V).