Structure of a Platinum–Alumina Catalyst Prepared from the Carbonyl Cluster H2[Pt3(CO)6]5

The state of a platinum carbonyl cluster in an initial aqueous acetone solution and its transformations on the surface of aluminum oxide in the course of catalyst preparation were studied by EXAFS spectroscopy. It was found that water enters the polynuclear framework of the dissolved cluster (the Pt–O distance is 2.55 Å, where O is the oxygen atom of water). Structural changes in the supported cluster in the course of catalyst preparation exhibited a strong interaction of platinum with alumina (the Pt–O distance is 1.92–1.95 Å), beginning at the step of H₂[Pt₃(CO)₆]₅ adsorption. This interaction was retained upon the subsequent high-temperature treatments of the catalyst. The structures of samples prepared from platinum carbonyl and chloroplatinic acid were significantly different. In the former case, a surface prototype was formed from the initial cluster; in the latter case, the sample consisted of platinum metal clusters of a considerable size.     

Oxidation kinetics of tetravalent uranium monofluoride complex by nitrous acid in HNO3 medium

The kinetics of oxidation of uranium(IV) monofluoride complex by nitrous acid in nitric acid solution have been studied. The experiments were carried out at constant ionic strength of 2M (HNO₃ and NaNO₃) and temperature in the range of 18–47 °C. The rate of reaction was determined spectrophotometrically at a wavelength of 621 nm, at which the molar extinction coefficients of UF³⁺ and UF ₂ ²⁺ are the same. It was shown that reaction orders for [HNO₂] and [HNO₃] are equal to 0.12 and 0.39, respectively. The values of activation parameters H and S are determined to be 83 kJ mol^–1 and 75 J (mol·K)^–1, respectively. The rate order of the reaction studied has a weak direct dependence on [H⁺] in contrary to the strong and reverse dependence in the absence of fluoride ions. In conclusion, fluoride ions may strongly stabilize the U(IV) in nitric acid solutions.     

Formation of higher chloride complexes of Np(IV) and Pu(IV) in water-stable room-temperature ionic liquid [BuMeIm][Tf2N

A UV/vis/near-IR spectroscopic study shows that in [BuMeIm][(CF3SO2)2N] hydrophobic room-temperature ionic liquid solutions, [BuMeIm]2[AnCl6] complexes, where BuMeIm+ is 1-n-butyl-3-methylimidazolium and An(IV) is Np(IV) or Pu(IV), have an octahedral An(IV) environment similar to that observed in solid complexes. Water has no influence on the absorption spectra of AnCl6(2-) complexes, indicating their stability to hydrolysis in ionic liquid. Adding [BuMeIm]Cl modifies the UV/vis/near-IR absorption spectra of An(IV) in the ionic liquid and causes solids to precipitate. The solid-state reflectance spectra of the precipitates reveal considerable differences from the corresponding An(IV) hexachloro complexes. A voltammetric study indicates that AnCl6(2-) complexes are electrochemically inert in [BuMeIm][(CF3SO2)2N] at the glassy carbon working electrode. By contrast, quasi-reversible electrochemical reduction An(IV)/An(III) and An(IV) oxidation are observed in ionic liquids in the presence of [BuMeIm]Cl. The oxidation wave of noncoordinated chloride ions interferes with the An(IV) oxidation waves. The spectroscopic and voltammetric data clearly indicate the formation of nonoctahedral actinide(IV) chloride complexes with a Cl-/An(IV) ratio exceeding 6/1 in [BuMeIm][(CF3SO2)2N] in excess chloride ions.     

Transport of polarized neutrons through magnetic noncoplanar layered systems

The transport of neutrons through noncoplanar systems is shown to have a partly unidirectional character. To this end, the transmission of a three-layer magnetic film with a noncoplanar magnetization in the absence of applied magnetic field and the transmission of a two-layer magnetic film with a noncollinear magnetization in the presence of a magnetic field are calculated. The calculations demonstrate that, if the losses in the films are neglected, the transmissions in two opposite directions differ only for processes with and without spin flip. If these losses are taken into account, the total transmissions of a nonpolarized neutron beam in two opposite directions are also different. The consequences of these specific features of noncoplanar systems are discussed.     

Neutron standing waves in layered systems

A theory of propagation of neutron waves in multilayer magnetic and nonmagnetic systems is presented. It is shown how a system forms the wave function of a neutron, how the wave function formed can be controlled, and how this function can be used to investigate materials. The results of experimental investigations of the neutron reflection from multilayer systems with simultaneous measurement of different types of secondary radiation are reported.     

Reflection of neutrons from a helical system

Analytical expressions have been derived for the reflection and transmission of neutrons from magnetic mirrors with helical magnetization. The reflection curves with and without neutron spin flip are presented. The resonance properties of helical systems have been analyzed.     

Effect of surface fields on luminescence of ZnS and ZnO crystals

It is shown that changes in luminescence characteristics of zinc sulfide and oxide in the presence of external effects that alter the surface charge are associated with a change in the parameters of surface barriers and can be explained within the framework of a model that takes into account separation of recombining partners in the Schottky barriers. We suggest a barrier mechanism for the effect of the surface region of a crystal on experimentally measured luminescence parameters and show that errors occuring in the measurement of these parameters can reach 10% and greater, depending on the depth of penetration of light. Moscow State University of Railways, 15 Obraztsov Str., Moscow, 103055, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 804–808, November–December, 1999.