Metal-cluster compounds: Model systems for nanosized metal particles

Meta-cluster compounds can be exploited advantageously to study the evolution, with increasing size of the molecules of the physical properties of metal clusters from molecular to bulk-metal behavior. The metal-cluster molecules are well-defined, stoichiometric, chemical compounds. The molecules consist of a metal core of a variable number of atoms, surrounded by a shell of ligand atoms or molecules. Depending on the compound, the type of metal atom may be varied, whereas the core size can be changed from a few up to several thousands of atoms. Accordingly, these materials provide excellent model systems for monodisperse metal particles, embedded in a dielectric matrix, and can be investigated by the well-known experimental techniques of solid-state physics. © 1998 John Wiley & Sons, Ltd.     

Low-temperature wave conversion of energetic compounds in filled condensed systems

The features of flameless low-temperature wave conversion (LWC) of some energetic materials in filled systems were studied experimentally and analyzed theoretically. The results attest to an relay-race mechanism of process propagation in the initial composition. An important role of the filling material in stabilization of the LWC wave of the energetic component is revealed. In the presence of mono- or oligomeric binders, flameless LWC of the energetic component affords strong highly porous composite materials. The possibility of using this method for preparing new high-performance grinding materials and catalysts containing nanosized particles of transition metals and their derivatives is demonstrated.     

Thermostimulated transformations in nanosized Bi-MoO3 systems

Transformations in Bi-MoO₃ nanosized systems are studied by optical spectroscopy, microscopy, and gravimetry. The contact potential difference for the Bi and MoO₃ films and the photovoltage of the Bi-MoO₃ systems are measured, depending on the thickness of Bi (d = 3–92 nm) and MoO₃ films (d = 5–40 nm) and the temperature (373–673 K) and time of heat treatment. An energy band diagram of the Bi-MoO₃ systems is constructed. A model of the thermal transformation of MoO₃ films in Bi-MoO₃ systems is proposed that involves the redistribution of equilibrium charge carriers on a contact, the formation of a ([(V_a)⁺⁺e]) center during the preparation of a MoO₃ film, the transformation of this center into a ([e(V_a)⁺⁺e]) center during the formation of Bi-MoO₃ systems, and the thermal transition of an electron to the level of a ([(V_a)⁺⁺e]) center to form a ([e(V_a)⁺⁺e]) center.     

Fine tuning the production of nanosized beta-carotene particles using spinning disk processing

Nanoparticles of trans-beta-carotene are accessible using spinning disk processing (SDP), by varying the reaction conditions and the choice of surfactant, macrocyclic amphiphiles, sulfonato-calix[4,5,6,8]arenes, and alpha,beta-cyclodextrins. SDP ensures rapid mixing and fast kinetics, and nanoparticles of the carotene formed in the presence of the calixarenes are stable with respect to extraction of the carotene into an organic solvent, unlike in the presence of the cyclodextrins. Insight into the supramolecular structure of the carotene nanoparticles has also been established. The mean particle sizes (dynamic light scattering, DLS) have been optimized at 40(2) and 56(1) nm and 71.4(6) and 82(1) nm, respectively, for each sulfonato-calix[5,6 and 4,8]arene, whereas the cyclodextrins form nanoparticles with a mean diameter of 71(1) and 68.5(6) nm, respectively. Zeta-potential studies show stability of all the colloidal dispersions at pH > 4 with values below -30 mV. UV-visible spectroscopy shows a blue shift indicative of H-aggregates of the carotene within the nanoparticles. The surface area derived from BET studies is 39.12 m(2)/g corresponding to particles of 76.7(5) nm in diameter, in agreement with sizes obtained from DLS and TEM measurements.     

Determination of admixtures of high-melting metals in rare-earth metals and their compounds

The study considers the possibility of using inductively coupled plasma mass spectrometry (ICP MS) for the determination of admixtures of Zr, Nb, Mo, Hf, Ta, W, and Te in rare-earth metals and their compounds using preliminary extraction or extraction-chromatographic preconcentration. The admixture elements have been separated by extraction with trioctylphosphine oxide in dichloroethane from hydrochloric acid media and back extraction with oxalic acid. Chemical separation provided a 1–4-order reduction of the lower quantification limit for these elements using ICP MS.     

Peculiarities of cellulose nitrate combustion in ballasted systems

Russian Chemical Bulletin - The dependence of the combustion parameters of cellulose nitrate (nitrocellulose, NC) mixtures with magnesium oxide on the content of the energetic component was studied...     

Kinetics of photostimulated transformations in nanosized bismuth films

Optical spectroscopy, gravimetric analysis, and microscopy studies show that irradiating bismuth films (d = 3–55 nm) with light (λ = 360 nm and I = 1.8 × 10¹⁵–7.0 × 10¹⁵ quantum cm^?2 s^?1) leads to major changes in their absorption and reflectance spectra and in film mass. The kinetic curves of the degree of photochemical transformation of bismuth films are shown to obey linear, inverse logarithmic, cubic, and logarithmic laws. The contact potential difference (CPD) of Bi and Bi₂O₃ films and the photo-electromotive force (emf) of Bi-Bi₂O₃ systems are measured. An energy band diagram of Bi-Bi₂O₃ systems is constructed. A model that includes the stages of generation, recombination, and redistribution of nonequilibrium charge carriers in the contact field of a Bi-Bi₂O₃ system, oxygen adsorption, diffusion of cation vacancies, and Bi₂O₃ formation is proposed.     

Kinetics of thermal transformations in nanosized chromium films

The transformations in nanosized chromium layers at different layer thicknesses (d = 14–154 nm) and thermal treatment temperatures (T = 673–873 K) were studied by optical spectroscopy, microscopy, and gravimetry. The kinetic curves of conversion at different chromium film thicknesses and treatment temperatures are well approximated by the linear, inverse logarithmic, cubic, and logarithmic functions. The contact potential difference for Cr and Cr₂O₃ films and photo-emf for Cr-Cr₂O₃ systems were measured. An energy band diagram of Cr-Cr₂O₃ systems was constructed. A model of thermal transformation was constructed for Cr films that included the stages of oxygen adsorption, charge carrier redistribution in the contact field of Cr-Cr₂O₃, and chromium(III) oxide formation.     

Surface forms of unsaturated hydrocarbons and their transformations on group VIII metals

IR-spectroscopic and kinetic studies of the surface forms of unsaturated hydrocarbons and their catalytic transformations on group VIII metals have been carried out. Data on the nature and properties of surface compounds in hydrogenation and hydrocracking reactions have been obtained, yielding the sequence of selectivities in these processes.

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- VIII . , .

     

Kinetic regularities of thermal transformations in nanosized bismuth films

Transformations in a nanosized bismuth layer are studied by means of optical spectroscopy, microscopy, and gravimetry, depending on the thickness (d = 3–120 nm), thermal treatment temperature (T = 373–673 K) and time (τ] = 0.05–2500 min). It is established that, depending on the initial thickness of the bismuth films and the thermal treatment temperature, the kinetic curves of the degree of transformation are satisfactorily described within linear, inverse logarithmic, cubic, and logarithmic laws. The contact potential difference for the Bi, Bi₂O₃ films and the photo-electromotive force for the Bi-Bi₂O₃ systems is measured. An energy-band diagram for the Bi-Bi₂O₃ systems is constructed. A model for the thermal transformation of Bi films that includes the stage of oxygen adsorption, the redistribution of charge carriers in the Bi-Bi₂O₃ contact field, and the formation of bismuth(III) oxide is proposed.     

Synthesis of nanosized TiO2 particles in reverse micelle systems and their photocatalytic activity for degradation of toluene in gas phase

Nanosized pure TiO₂ particles with high crystallinity and large surface area were prepared by hydrolysis of tetrabutyl titanate in water/Triton X-100/isooctane reverse micelle solutions as reaction media followed by hydrothermal treatment to improve crystallinity. The prepared TiO₂ nanoparticles were characterized by XRD, BET, TGA, FT-IR and TEM. The size of ultrafine particles was controlled by changing the water content of the reverse micelle solution. The TiO₂ particles showed monodispersity, large surface area and high degrees of crystallinity and thermostability. The photocatalytic activity of the TiO₂ particles was evaluated by decomposition of toluene in the gas phase. The activity of the TiO₂ nanoparticles was higher than that of commercially available anatase fine particles, such as ST-01, which is one of the most active photocatalysts for degradation of organic compounds in the gas phase.     

Study of structural transformations of equiatomic compounds of rare earth metals with copper

Electrical resistivity, magnetic susceptibility, X-ray diffraction and differential thermal analysis have been performed over a wide temperature range on binary compounds RCu. Melting points or decomposition temperatures for RCu (R=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Dy) phases and a number of FeB-CsCl structural transitions are reported, and heats and entropies of transformation are evaluated. Large hysteresis effects are observed for the structural transformations at different temperatures.In LaCu and CeCu no structural transformation was detected, while TbCu and DyCu failed to reveal any high-temperature thermal effect of transformation. In EuCu a nonreproducible thermal effect was evidenced after thermal cycling.

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Zusammenfassung Messungen der elektrischen Leitfähigkeit und der magnetischen Susceptibilität sowie röntgendiffraktometrische und differentialthermoanalytische Untersuchungen von binären Verbindungen des Typs RCu wurden in einem weiten Temperaturbereich ausgeführt. Schmelzpunkte bzw. Zersetzungstemperaturen von RCu-Phasen (R=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb und Dy) und eine anzahl von Phasenübergängen im System FeB-CsCl sind zusammen mit den bestimmten Entropiewerten der Phasenübergänge angegeben. Die Temperaturabhängigkeit der Phasenübergänge weist eine ausgeprägte Hysterese auf. In den Systemen LaCu und CeCu konnten keine Phasenübergänge festgestellt werden, während sich bei TbCu und DyCu kein auf einen Phsenübergang hinweisender thermischer Effekt im Hochtemperaturbereich zeigt. Bei EuCu wurde ein nicht-reproduzierbarer thermischer Effekt nach einem thermischen Kreisprozeß nachgewiesen.

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, , RCu. RCu (=La, , Pr, Nd,Sm, Eu, Gd, Tb Dy) , FeB-CsCl, . . LaCu , TbCu DyCu - . EuCu .

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The authors are grateful to Prof. A. Iandelli for his interest and for helpful discussion.

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This work was supported by the Italian Consiglio Nazionale delle Ricerche.     

Molecular meccano for light-sensitive and light-emitting nanosized systems based on unsaturated and macrocyclic compounds

The review presents the results of the development of an universal approach to the molecular design of light-sensitive and light-emitting nanosized systems with desired properties based on unsaturated and macrocyclic compounds. Within the same class of compounds, various nanosized systems were constructed using a limited number of structural fragments. These nanosized systems are susceptible to all main types of photoprocesses, such as fluorescence, photodissociation, photoisomerization, photocycloaddition, photoelectrocyclization, excimer formation, charge-transfer complex formation, the formation of the twisted intramolecular charge-transfer state (TICT state), and the electron transfer. The use of photostructural transformations for controlling the complexation and mechanical movements in molecular devices and machines is discussed. The prospects of application of the new strategy are exemplified by the design of the previously unknown types of molecular switches, materials for optical chemosensors, optical data recording and storage media, photoswitchable molecular devices, and photocontrolled molecular machines. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1299–1323, July, 2008.     

Low-temperature ortho-para conversion of hydrogen on films of rare-earth metals and their copper alloys

Kinetic data for the ortho-para conversion of H₂ at 77 k rear earth metals (REM) and their Cu alloy films are given. Conversion on the surface of Sc, Y, La, Yb and Lu has been shown previously to follow a chemical mechanism, for the others it is magnetic. When alloyed with Cu, the specific catalytic activity (K_sp) of Sc, Y, La, Yb and Lu sharply increases. A sharp increase of K_sp is also observed on Cu alloys of Ce, Sm, Nd, Pr, Eu and Gd and the chemical conversion mechanism becomes predominant. On Tb, Dy, Er, Ho and Tm, K_sp rises only slightly and the magnetic mechanism preserves.

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- H₂ 77 P3M . , Sc, Y, La, Yb Lu , . Cu () Sc, Y, La, Yb, Lu, Ce, Sm, Nd, Pr, Eu Gd, . Ha Tb, Dy, Er, Ho Tm .

     

Ranges of oxidation states ford-transition metals in their complex compounds

The range of oxidation states for a given element is determined by its highest and lowest oxidation states; the range is numerically equal to the difference between these highest and lowest values, and ford-transition metals may be assumed to be a value from 0 to 10. The oxidation states known at the present time for elements of the first twelve groups of the periodic table are used to plot diagrams reflecting the three-dimensional nature of the periodic table. The ranges of oxidation states are minimum for elements of the 1st, 2nd, 3rd, and 12th groups, and maximum for elements of the 6th, 7th, and 8th groups. The oxidation states above 2+ are stabilized by complexation with donor ligands, while zero and negative oxidation states are observed only in complexes with donor-acceptor ligands. The oxidation state 1+ occupies an intermediate position in this scheme. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prosp. Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 33, No. 5, pp. 284–290, September–October, 1997.     

The band structures of some transition metals and their NaCl-type compounds

The band structures of Group IVB (Ti,Zr,Hf),VB (V,Nb,Ta) and VIB (Cr,Mo,W) transition metals and some of their carbides and nitrides (TiN,ZrN,HfN,VC,NbC,TaC,VN,NbN,TaN) with NaCl-type (Bl-type) structure have been calculated by using the tight-binding method within the Extended Hiickel approximation (EHT).The energy bands,densities of states and crystal orbital overlap populations are given.The relationship between the bonding properties and the superconducting transition temperatures (Tc) of them is discussed.The influences of various kinds of metallic atoms and changes of bond lengths on Tc are also discussed.     

Synthesis of nanosized TiO2 particles via ultrasonic irradiation and their photocatalytic activity

Summary The photocatalytic degradation of p-nitrophenol has been studied on TiO₂ nano- particles prepared by hydrolysis of titanium tetraisopropoxide using ultrasonication technique coupled with sol-gel method. The titania particles prepared with ultrasonic irradiation shows the higher activity on the photocatalytic decomposition of p-nitrophenol compared to those prepared without ultrasonic irradiation.     

Radiation-thermal transformations and distillation of lipids and disperse systems on their basis

The radiation-thermal transformations of rape oil under conditions of postradiation distillation, radiation heating, and electron-beam distillation with auxiliary heating were studied. Diesel hydrocarbons were predominant in the condensate distilled off upon combined heating. The concentration of oxygen atoms decreased as a result of radiation-induced decarboxylation and dehydration processes. The irradiation of oil as a constituent of disperse systems facilitated purposeful deoxygenation, a decrease in the viscosity, and an increase in the yield of the gasoline fraction.     

High-pressure structures and phase transformations in elemental metals

At ambient conditions the great majority of the metallic elements have simple crystal structures, such as face-centred or body-centred cubic, or hexagonal close-packed. However, when subjected to very high pressures, many of the same elements undergo phase transitions to low-symmetry and surprisingly complex structures, an increasing number of which are being found to be incommensurate. The present critical review describes the high-pressure behaviour of each of the group 1 to 16 metallic elements in detail, summarising previous work and giving the best present understanding of the structures and transitions at ambient temperature. The principal results and emerging systematics are then summarised and discussed.