Characterisation of nanocrystalline magnesium oxide by X-ray absorption spectroscopy.

There are numerous methods of preparing nanocrystalline materials. Magnesium oxide is an ideal model system on which to probe the relation of the preparative route and the microstructure. Using X-ray absorption spectroscopy (XAS) we show that the sol-gel route can be used to prepare highly crystalline material provided there is careful control of the calcination conditions. In the present work this is achieved by calcining at high temperatures (at least 800 degrees C). However, this results in grain growth that can be prevented by the addition of a pinning agent, SiO(2), during the preparation of the sol. The pinned samples maintain a particle size of 11 nm even after calcining at 1000 degrees C. Ball-milling is a common method of preparing nanocrystalline oxides, however the present work shows that this produces a significant fraction of amorphous material, the fraction increasing with decreasing grain size (e.g. approximately 30 % for a grain size of 23 nm).     

Mono and bis[N-aryl(benzyl)]amides of phosphorylacetic acids. One-pot synthesis and extraction of actinides from nitric acid solutions

A one-pot synthetic route to phosphorylacetic acid N-aryl(alkylaryl)amides, including those containing two phosphorylmethylcarbamoyl moieties attached to the arene framework, has been developed. The method is based on reactions of amines with the corresponding acid chlorides generated in situ with the use of phosphorus trichloride as a mild chlorinating agent. The compositions and structures of the compounds obtained and their extraction ability toward Am^III were determined. Suggestions were made about the compositions of the extracted complexes with phosphorylacetic acid N-aryl(alkylaryl)amides. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1387–1394, August, 2006.     

Effect of preparative conditions on the surface characteristics of mixed zirconium and titanium oxides

The surface characteristics of mixed zirconium and titanium oxides prepared from different starting materials are investigated. One mode of preparation entailed the use of zirconium sulfate and titanium oxysulfate as starting materials and ammonium hydroxide as precipitating agent. The produced oxides were washed to different extents to obtain samples with different sulfate content. A second preparative mode used zirconium oxychloride and titanous chloride as starting materials also with ammonium hydroxide as precipitating agent. The oxidation of the titanous to the titanic form for these oxides was carried out by means of oxygen gas. Resulting samples were heat treated at 400 °C and 600 °C, and textural characteristics determined from the adsorption of N₂ at 77 K, complemented by infrared and thermal studies. The samples precipitated from the oxychloride and chloride salts of zirconium and titanium, as well as those precipitated from the sulfate and oxysulfate salts and washed free of the sulfate ions displayed quite similar textural characteristics. The unheated samples and those heat-treated at 400 °C were mesoporous, with some microporosity, and relatively large surface areas in the order of 200–300 m²/g. Heat treatment to 600 °C led to a relative decrease in surface area, in the order of 100 m²/g, and to the disappearance of microporosity. The mixed zirconium and titanium oxides with a sulfate content of ≈17% displayed significantly lower surface areas, smaller than 10 m²/g, with a prevalence of micro and mesoporosity. Infrared and thermal studies indicated the presence of differently bounded sulfato groups, which seem to have a blocking effect on the pores, resulting in the observed smaller surface areas.     

Gradient expansion for a spiral phase in a spin-fermion model

Starting from a doped spin-fermion model for high-temperature superconductors, we derive an effective continuum theory for the spin degrees of freedom by means of a gradient expansion around a spiral spin configuration. By integrating out the fermions in a path-integral representation, we obtain an effective spin-action. An incommensurate, planar spiral configuration for the spin-background is assumed. The long-wavelength limit is obtained by expanding the effective action in powers of a short distance cutoff. The occurring infinite series can be summed to all orders of the coupling constant by exploiting the constraint that the order parameter lives on a circleS ₁. It is shown that the low-energy limit of the effective action can be mapped onto a O(2) nonlinear model and an additional term due to parity breaking.     

Microcalorimetric Studies on the Product Inhibition of Hydrolysis of L-arginine Catalyzed by Bovine Liver Arginase

A new thermokinetic reduced extent method for the product inhibition of single substrate enzyme-catalyzed reactions is proposed and compared with the traditional initial rate method in this paper. The arginase-catalyzed hydrolysis of L-arginine to L-ornithine and urea was studied at 37°C in 40 mM sodium barbiturate-HCl buffer solution (pH=9.4). Michaelis constant (K _m) for arginine and maximum velocity (V _m) of the reaction were determined by initial method and thermokinetic method. The activation of exogenous manganese to this reaction was also studied. The product inhibition constant (K _P), which cannot be obtained directly from the initial rate method, was determined by thermokinetic without adding L-ornithine to the reaction system. When the concentration of Mn²⁺ in cell is 0.1 mM, the enzyme gets its full activity. Incubation arginase with appropriate concentration of Mn²⁺resulted in increased Vmax and a higher sensitivity of the enzyme to product with no change in the K _m for arginine. We suggest that the exogenous manganese ions in solution have just recovered the activity of arginase, which was lost in dissolving and dilution, but no effect on the mechanism of the reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of Stark broadening of Mn I and Mn II spectral lines in plasmas used for Laser-Induced Breakdown Spectroscopy

We present measurements of the Stark broadening of several Mn lines in the conditions of typical laser-induced plasmas. Single-and double-pulse Laser-Induced Breakdown spectroscopy (LIBS) configurations are studied on a series of Fe–Mn alloy samples with Mn concentration ranging from 6% to 30%. The effects of self-absorption on the measured line broadenings are discussed in detail. In particular, the experimental results evidence that self-absorption is much higher in laser-induced plasmas generated with double pulses, compared to the case of single pulse. After measurement of the electron density, the Stark coefficients of several neutral and ionic Mn lines are derived through the measure of the broadening in conditions of optically thin plasma. The results obtained for singly ionized Mn lines are compared with the theoretical and experimental data present in the literature. For the first time, experimental measurements of the Stark coefficient for several neutral Mn lines are also presented.     

Catalytic Properties of Heteropolyacid-Modified ZrO2 in the Partial Oxidation of Lower Alkanes by Nitric Oxide

The partial oxidation of C₃ and C₄ hydrocarbons by nitric oxide, in particular, their oxidation to alcohols, occurs on zirconium dioxide modified by various heteropolyacids (HPA). The catalyst activity depends on the amount of deposited HPA, while the reaction selectivity relative to the partial oxidation products correlates with the acid properties of the surface, in particular, with the concentration of weak acid sites.     

Effect of rare earth oxides (CeO2, La2O3) on properties of Pd/Al2O3 catalysts for reduction of nitrogen oxides by methane

We have established that the thermal stability of supported Pd/Al₂O₃ catalysts is increased after they are modified by rare earth oxides (La₂O₃, Ce₂O₃). We have observed the effect of thermal activation of an aluminopalladium catalyst modified by lanthanum oxide. This effect is apparent in the increase of the specific catalytic activity in the reaction of high-temperature reduction of nitrogen oxides by methane after heat treatment of the catalyst at 850 °C. We have used X-ray photoelectron spectroscopy (XPS) to show that the reason for the thermal activation effect is stabilization of palladium in the Pd¹⁺ state. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 44–48, January–February, 2006.     

锂离子二次电池正极材料镍酸锂的量子化学研究

利用周期性体系的Hartree-Fock方法计算了以LiC6／LiNiO2锂离子二次电池的平均电压，结果与实验值相差 15％。计算表明，NiO2中嵌入一个Li原子变成LiNiO2后，负电荷主要从Li转移到O上，转移到Ni上的负电荷仅约20％，讨论了其对Jahn-Tell效应的影响。以Li0.5NiO2作为嵌锂中间物的代表，研究了锂离子的可能迁移路径。通过对NiO2和LiNiO2的电子态密度的计算，研究了NiO2在嵌锂过程中的能带变化及其对电极的电化学性质的影响。     

Preparation of Magnesia-Silica Oxides: Effect of Mg/Si Ratio and Sulfate on Acidity

Mg−Si binary oxides were prepared by the sol-gel method, using magnesium diethoxide and tetraethoxysilane as starting materials. Mg−Si mixed oxides were prepared at pH3 and 9 at nominal concentrations of 1.3, 4.0, and 6.7 mole% MgO. Density and strength of the acid sites were determined by pyridine thermal programmed desorption. Comparable Lewis acidity was found in pH3 and 9 preparations. However, when the samples were sulfated with sulfuric acid, strong Bronsted acidity was developed, reaching a maximum at 6.7 mole% MgO. The strong acidity of sulfated samples was corroborated using the isopropanol decomposition reaction as catalytic test. Similar conversions are obtained at 100°C on sulfated samples, whereas 260°C are required for non-sulfated ones.