A. V. Anan’ev

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) 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.

Bimetallic Pd-M (M = Co, Ni, Zn, Ag) nanoparticles containing transition metals: Synthesis, characterization, and catalytic performance

The reductive thermolysis of Pd(OOCMe)4M(OH2) (M = NiII, CoII, ZnII) and Pd(OOCMe)4Ag2(HOOCMe)4 molecular complexes results in the generation of bimetallic Pd-based Pd-M (M = Co, Ni, Zn, Ag) nanoparticles. The composition and morphology of nanoparticles and the electron state of metal atoms were characterized using electron microscopy, elemental ICP analysis, X-ray diffraction, and XAFS (XANES/EXAFS) techniques. The catalytic performance of nanoparticles was studied using the example of reactions of catalytic hydrazine decomposition and U(VI) reduction to U(IV) by hydrazine and formic acid. The catalytic performance of Pd-Ni nanoparticles is superior to that of the standard supported Pd/SiO2 catalyst containing a similar amount of Pd atoms, while Pd-Co, Pd-Zn, and Pd-Ag nanoparticles do not catalyze the studied reactions.

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.

Catalytic decomposition of organic anions in alkaline radioactive waste: III. Oxidation of oxalate and glycolate

Decomposition of oxalate and glycolate ions in alkaline solutions under the action of O3, H2O2, and Na2S2O8 was studied spectrophotometrically and titrimetrically. At 20°C, ozone slowly decomposes oxalate in 0.05 M NaOH. In 1 M NaOH, heating at 90°C is required to oxidize oxalate with ozone. Glycolate is readily oxidized with ozone at 20°C in 0.05–1 M NaOH, predominantly into oxalate. Hydrogen peroxide is ineffective reagent for oxalate and glycolate decomposition. Persulfate oxidizes oxalate ion in 0.5–5 M NaOH at 90°C. The reaction of persulfate with glycolate proceeds at 50°C and higher temperatures and is characterized by an induction period, which shortens with increase in concentration of S2O82−, OH−, and temperature, or in the presence of AgNO3 and K4Fe(CN)6. Oxidation involves thermal dissociation of persulfate ions into radical ions followed by a chain reaction.

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate 63T2−1 of 63Cu nuclei in CuO2 layers is measured in the normal and superconducting states of the compound YBa2Cu3O6.9 (Tconset=94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to Tconset=68 K (Φ=7×1018 cm−2) and Tconset<4 K (Φ=12×1018 cm−2). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction 63T2G−1, which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χs(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x2−y2 symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-Tc cuprates.

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.

Disproportionation of Pu(V) in aqueous HCOOH solutions

The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH-H2O system was studied by spectrophotometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO4 changes on adding HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at 830.6 nm, corresponding to an f-f transition in the Pu22+ aqua ion, a new band arises with the maximum shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1: 1). The Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO4 solution, which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately proportional to [Pu(V)]2 and grows with an increase in [HCOOH]. The reaction products affect the reaction rate: Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cationcation complex. The disproportionation occurs via formation of a Pu(V)-Pu(V) cation-cation complex whose thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).

Kinetics of Pu(V) Disproportionation in CH3COOH-CH3COOLi Aqueous Solutions

The behavior of Pu(IV–VI) in CH3COOH-CH3COOLi solutions was studied by spectrophotometry. The Pu(VI) absorption spectrum changes essentially with an increase in the CH3COOLi concentration. Owing to formation of Pu(VI) acetate complexes, the maximum of the main absorption band is shifted from 830.6 (in HClO4 solution) to 845 nm, with the band intensity decreasing by a factor of approximately 8. The Pu(V) and Pu(IV) absorption spectra at low concentrations of acetate ions vary insignificantly relative to the spectra in noncomplexing media. With an increase in the acetate concentration in the system to 1–3 mM, the effect of Pu(V) complexation on its absorption spectrum becomes noticeable (the absorption intensity considerably decreases), whereas the Pu(IV) absorption spectra remain essentially unchanged. Solutions containing 1–2 mM Pu(V) and 0.2–0.5 M CH3COOLi remain unchanged at 18–25°C for 2 days. In solutions with [CH3COOLi] = 1–3 M, Pu(V) disproportionates with the formation of soluble Pu(VI) complexes and a suspension of Pu(IV) hydroxide. Introduction of CH3COOH to a concentration of 0.1–1.0 M prevents the formation of a suspension of Pu(IV) hydroxide, but only up to a temperature of 45°C. The Pu(V) loss follows a second-order rate law, with the reaction products, Pu(IV) and Pu(VI), accelerating the Pu(V) consumption. The reaction rate at a constant concentration of acetate ions is proportional to [H+]. The reaction order with respect to Ac− ions is close to 1.6. The activation energy of the Pu(V) disproportionation in the range 19–45°C is estimated at 74.5 kJ mol−1. It is assumed that the disproportionation mechanism involves the formation of dimers from Pu(V) acetate complexes and aqua ions, their protonation, and decomposition with the transformation into Pu(IV) and Pu(VI).

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 NpO2+ ion with chemisorbed hydrogen atoms Pd(H).