Initial Precursor Concentration Research Articles

Supersensitivity of Transition-Metal Nanoparticle Formation to Initial Precursor Concentration and Reaction Temperature: Understanding Its Origins

A supersensitivity of the formation and stabilization of transition-metal nanoclusters to the initial nanocluster concentration and temperature synthesis conditions is reported, then probed, herein for the specific case of prototype Ir(O)n nanoclusters prepared from the organometallic precursor [Bu4N]5Na3[(1,5-COD)Ir x P2W15Nb3O62] by reduction with H2 in propylene carbonate solvent. Fully isolable, redissolvable, near-monodisperse (i.e., < or = +/- 15% size distribution) and thus excellent Ir(O)n nanoclusters are formed using low temperature (22 degrees C) and moderate precursor concentration (1.2 mM) in propylene carbonate solvent. However, inferior, polydisperse (+/- 40% size distribution), non-redispersable nanoclusters are formed at the seemingly only moderately different conditions of 38 degrees C higher temperature (i.e., 60 degrees C) and 5-fold lower precursor concentration (0.24 mM). Investigation of this supersensitivity to the nanocluster synthesis conditions reveals that it derives from the dissociation of (1,5-COD)Ir(solvent)2+ from the P2W15Nb3O62(9-) polyanionic ligand/ stabilizer, subsequently resulting in a too fast, kinetically uncontrolled reduction of the quickly reduced (1,5-COD)Ir(solvent)2+ as the cause of the inferior synthesis of polydisperse, non-isolable, non-redissolvable nanoclusters. The results are significant in that they illustrate that understanding the mechanism of nanocluster formation, and then performing the nanocluster synthesis under kinetically carefully controlled, understood conditions, is necessary for the formation of superior nanoclusters in this, and by implication probably many other, cases.

Synthesis of 3-D Hierarchical Dendrites of Lead Chalcogenides in Large Scale via Microwave-Assistant Method

Hierarchical superstructures of lead chalcogenides (PbS, PbSe, and PbTe) have been fabricated in large scale via microwave-assistant method. The FESEM and HRTEM images showed that dendritic and six-armed PbS superstructures were constructed with prism-shaped branches which grew epitaxially from the backbone of PbS. Detailed experiments revealed that PbS microcrystals with different morphologies (cubes, six-pods, bugle-like dendrites, and six-armed dendrites) could be simply fabricated by adjusting the reaction conditions (initial concentration of precursors or sulfur source). The results implied that the formation of 3-D hierarchical structures was controlled by the kinetic conditions. In addition, the dendrites of PbSe and PbTe have also been fabricated by the assistance of appropriate reductive reagent in a similar method.

Al-Containing Porous Titanium Dioxide Networks: Sol−Gel Synthesis within Agarose Gel Template and Photocatalytic Activity

Al-containing titania porous networks have been produced by conducting sol−gel chemistry within the porous structures of agarose gels. The use of the agarose template gives inorganic materials with a homogeneous macroporous structure. The amount of aluminum added to the titania system was studied from 0 to 10 wt % (where wt % corresponds to the (Al2O3 × 100)/(Al2O3 + TiO2) expected from the initial precursor concentrations), and the morphology and crystallization behavior (as a function of both wt % of Al2O3 and heating temperature) were characterized along with a study of the photocatalytic efficiency of the materials. The 4 wt % sample was found to have the highest photocatalytic activity observed for the photodecomposition of aqueous methylene blue. For low doping amounts, 0.5 and 1 wt % Al2O3, the rutile phase of titania was not observed in the samples heated to 780 °C. Above 4 wt % Al2O3, the temperature required for the anatase to rutile phase transformation increased with increasing amounts of aluminum.

A solvothermal route to size- and shape-controlled CdSe and CdTe nanocrystals

The size- and shape-controlled CdSe and CdTe nanocrystals, which exhibit obvious quantum confinement effect, have been synthesized by a solvothermal route. It is found that initial precursor concentrations are key factors in controlling the shape of the resulting nanocrystals. Moreover, the obtained nanocrystals are all of zinc blende structure, regardless of their sizes and shapes. A possible mechanism for the formation and growth of the nanocrystals is put forward. It is inferred that the adhesion and subsequent recrystallization of nanocrystals with an assistance of remaining monomers should be a major reason for formation and growth of the elongated nanocrystals.

Intensive pyrolysis of unsaturated organic precursors over acidified oxide surfaces

Carbon-like composite materials fabricated by non-catalytic heterogeneous pyrolysis (NCHP) of organic precursors over porous oxide supports are used in a variety of fields. The intensification of NCHP procedures for fabrication on a commercial scale is technically problematic. Direct intensification of operation parameters cannot be considered as the most effective method. For instance, an increase in support bed temperature often takes the pyrolysis process into the homogeneous reaction field, and greater gas space velocities and initial organic precursor concentrations lead to a rise in operating costs because of considerable losses of raw materials and increased power requirements. This paper describes an attempt to intensify non-catalytic heterogeneous pyrolysis procedures by optimizing the acid–base properties of the reaction agents (supports, precursors). A procedure of controlled acidification of the alumina support followed by thermal degradation of surface base salt was applied in order to cover the support surfaces with additional Lewis acid sites functioning as efficient host centres for precursor molecules. Unsaturated organic compounds, powerful Lewis bases, were used as the precursor agents. A probable elementary mechanism for non-catalytic heterogeneous pyrolysis of unsaturated organic precursors over an oxide surface reinforced by Lewis acids is proposed. The main characteristics of the composite hydrophobic materials produced by the NCHP technique using pre-acidified alumina samples—their chemical compositions, surface affinities to water, thermal stabilities in air, dynamic VOC adsorption and desorption capacities are presented and discussed. The elaborated composites are non-flammable in air until 600 °C, whereas the thermal stability limit of activated carbons is situated in the 250–300 °C range. As compared to activated carbons, these materials manifest really stable surface activities in VOC adsorption–desorption cycles.

Solvothermal synthesis of CdO and CuO nanocrystals

Nanocrystals of CdO have been obtained by the decomposition of the cupferron complex in the presence of tri- n-octylphosphine oxide (TOPO) under solvothermal conditions. The precursor:TOPO ratio plays an important role in determining the size of the nanocrystals. The nanocrystals have been characterized by electron microscopy, absorption spectroscopy and fluorescence spectroscopy, besides X-ray diffraction. The CdO nanocrystals are single crystalline and show evidence for quantum confinement. CuO nanocrystals could also be prepared by the decomposition of the cupferronate under solvothermal conditions, the particle size being controlled by the initial precursor concentration.

Functional Nanostructured Silica Powders Derived from Colloidal Suspensions by Sol Spraying

The preparation of silica powders derived from sols with a variety of primary particle sizes from 4 to 100 nm and the influences of preparation conditions such as drying temperature and initial concentration of precursor on the final powders morphology were investigated for the first time. The prepared silica powders have spherical morphology, and the final diameter (between 0.1 and 2 um) was controlled by changing the concentration of starting colloidal suspension. The preparation of silica based luminescent powders was investigated by adding europium ion as the dopant. The Eu-doped silica powders showed a visible-luminescence with wavelength of 613 nm, a red emission, under excitation of 394 nm light source. The effects of Eu-doping concentration, operation temperature and primary particle size of silica sols on the luminescence were also investigated.

Free tropospheric ozone production following entrainment of urban plumes into deep convection

It is shown that rapid vertical transport of air from urban plumes through deep convective clouds can cause substantial enhancement of the rate of O3 production in the free troposphere. Simulation of convective redistribution and subsequent photochemistry of an urban plume from Oklahoma City during the 1985 PRESTORM campaign shows enhancement of O3 production in the free tropospheric cloud outflow layer by a factor of almost 4. In contrast, simulation of convective transport of an urban plume from Manaus, Brazil, into a pristine free troposphere during GTE/ABLE 2B (1987), followed by a photochemical simulation, showed enhancement of O3 production by a factor of 35. The reasons for the different enhancements are (1) intensity of cloud vertical motion; (2) initial boundary layer O3 precursor concentrations; and (3) initial amount of background free tropospheric NOx. Convective transport of ozone precursors to the middle and upper troposphere allows the resulting O3 to spread over large geographic regions, rather than being confined to the lower troposphere where loss processes are much more rapid. Conversely, as air with lower NO descends and replaces more polluted air, there is greater O3 production efficiency per molecule of NO in the boundary layer following convective transport. As a result, over 30% more ozone could be produced in the entire tropospheric column in the first 24 hours following convective transport of urban plumes.

Biochemical characterization and seasonal changes in the concentration of testosterone-metabolizing enzymes in the European great tit (Parus major) brain

Homogenates of diencephala obtained from brains of European great tits (Parus major) were incubated in the presence of tritiated testosterone (T) as precursor, and four metabolites produced from this steroid were formally identified and quantified. Conversion into 5 beta-dihydrotestosterone (5 beta-DHT) constituted the major metabolic pathway of T. Smaller amounts of 5 alpha-dihydrotestosterone (5 alpha-DHT), 5 beta-androstane-3 alpha, 17 beta-diol (5 beta-DIOL), and estradiol (E2) were also produced. The metabolism of T was time-dependent, and it varied as a function of the initial precursor concentration. The kinetics of 5 beta- and 5 alpha-reductases, as well as aromatase, followed the Michaelis-Menten model. It was found that 5 beta-reductase has a low apparent affinity for T, but is present in large concentrations. In contrast, the apparent affinity for T and the concentration of aromatase were approximately 3.9 times higher and 130 times smaller, respectively, than those of 5 beta-reductase. Intermediate values were found for 5 alpha-reductase. The validated assay was used to measure seasonal changes in the in vitro metabolism of T in the anterior (AH) and posterior (PH) hypothalamus and the cerebellum (CER) of free-living juvenile and adult male great tits. The production of 5 beta-DHT was low during the winter period in the PH of adult males, whereas the 5 beta-DIOL production was low in both parts of the hypothalamus at this time of the year. During autumn the production of these metabolites showed a transitory decrease in both parts of the hypothalamus of the juveniles. The production of 5 beta-reduced metabolites by the CER was high at all times of the year. In juveniles, the CER production of 5 beta-DHT did not change seasonally, whereas 5 beta-DIOL production peaked during summer. In the CER of adults, maximum production of both metabolites occurred during summer. Generally, less T was converted into 5 beta-reduced metabolites by the PH than by either the AH or the CER. E2 production was observed only in the AH and PH. With one exception (summer; AH), E2 production was high in both parts of the hypothalamus of adults throughout the year. In both AH and PH of juveniles, E2 production was low during summer. In these birds, it increased between summer and autumn in both parts of the hypothalamus, and also between autumn and winter in the PH. The production of 5 alpha-DHT did not change as a function of the season, the age of the birds, or the brain region.(ABSTRACT TRUNCATED AT 400 WORDS)

High Temperature Reactions of Benzyl Radicals

AbstractThe thermal decomposition of benzyl radicals has been investigated behind reflected shock waves by monitoring the formation of hydrogen atoms. The results reported here have been obtained by using a shock tube in conjunction with atomic resonance absorption spectrometry (ARAS). The temperatures ranged between 1450 and 1700 K at total pressures around 2 bar. The test gas mixtures consisted of argon with relative concentrations of a few ppm benzyliodide, C7H7I. From the measurements of the temporal H‐atom concentrations, in connection with thermochemical restraints, it was deduced that the benzyl decay is preceded by isomerization of benzyl. Due to the low initial concentration of the radical precursor, it was possible to evaluate a first order rate coefficient for the initiation reaction:magnified imageThe heat of formation of the benzyl radical is discussed. A value of ΔH0f, 298 = 215.5 kJ mol−1 fits best to the experimental results.

Oscillations of an exothermic reaction in a closed system - I. Approximate (exponential) representation of Arrhenius temperature-dependence

In a closed system in which matter is conserved the only true steady state is the final chemical equilibrium state. This means that potential oscillatory behaviour in models appropriate to such systems is not as easily open to analysis as in flow systems. Oscillatory trains cannot last indefinitely and they often begin and end well before the equilibrium point. Here we study such transitory behaviour via the ‘pool chemical approximation’. We study the simple prototype P→ A rate = k 0 p ; E 0 = Q 0 = 0, A → B + heat rate = k 1 a , k 1 = A 1 exp (─ E 1 / RT ), in which only the step A → B evolves heat and is in any way responsive to temperature. Heat losses obey newtonian cooling: heat release provides thermal feedback. If the initial concentration of precursor exceeds a certain value, oscillations may be seen. Analytic expressions can be found for the major features (such as duration of pre-oscillatory incubation periods, post-oscillatory decays, etc.). Primitive treatments of these problems ignore the consumption of the precursor. The significance and validity of this ‘pool chemical’ assumption are analysed qualitatively and quantitatively. Properly applied, it is an approximation of the greatest utility.