Reactant Size Research Articles

Some metallic nanoparticles’ ignition temperature by thermodynamic consideration

Self-propagating high-temperature synthesis (SHS) provides an attractive alternative route for the preparation of various compounds. Considering the diffusion reaction at the solid interface, the size-dependent activation energy and ignition temperature of the compounds synthesized by SHS can be used to help in modeling. Therefore, a quantitative model for size-dependent ignition temperature is established by considering energetic and physical characteristics. To the author’s knowledge, this is the first theoretical model to explore the effects of size on ignition temperature for SHS applications, with size ranging from nano- to microscale. An assumption of ideal SHS is made in the deduction of the model. As the size of reactants decreases, the ignition temperature decreases. This is because increasing the contact area and reducing the diffusion barrier between the reactants must be used specifically to calculate the reactants at the nanometer scale. The model predictions are still in good agreement with the experimental and theoretical observations for metallic aluminides and titanium carbide systems.

Mechanism and Kinetics of Methylating C6–C12 Methylbenzenes with Methanol and Dimethyl Ether in H-MFI Zeolites

This study uses periodic density functional theory (DFT) to determine the reaction mechanism and effects of reactant size for all 20 arene (C6–C12) methylation reactions using CH3OH and CH3OCH3 as ...

Spectroscopic characterization of LiFePO4 as cathode material for Li-ion battery prepared in the pulse thermo-acoustic reactor

Lithium iron phosphate (LiFePO4) is a cathode material for the rechargeable-lithium batteries. In this paper is presented a novel method of fabrication carbon-coated LiFePO4 in a pilot reactor built according to the principles of the thermo-acoustic burner of Helmholtz-type. Crystalline powder with a high percentage of LiFePO4 was synthesized by incomplete combustion, i.e. in the reductive atmosphere, and calcined at 700?C for 6 h. The obtained samples were characterized by X-ray diffraction, IR and Raman spectroscopy. The aim of this study was to demonstrate the production of the high-quality lithium-ion cathode material by the incomplete combustion. The synthesis of LiFePO4 is completed during calcination and an ordered structure is attained. Fast synthesis in the reactor (less than 2 s) is achieved due to the reduction in the size of reactant's particles and a huge number of collisions owing to their strong turbulent flow associated with explosive combustion.

Orthogonally clickable hyperbranched polymers: effect of reactant size and polarity on core-functionalization of peripherally jacketed HBPs

Using an orthogonally clickable strategy, the accessibility of internal allyl groups in jacketed HBPs, bearing either PEG or docosyl peripheral segments, was shown to depend both on the size and relative polarity of the reactant thiol.

Mass transfer and kinetic study on BEA zeolite-catalysed oil hydroesterification

In the present study, we have assessed the effect of mass transfer on the both steps of the zeolite catalysed oil hydroesterification to produce biodiesel (hydrolysis of the feed soybean oil producing free fatty acid and free fatty acid esterification). Both reactions were conducted over BEA zeolite in a batch reactor. Experimental results indicate the absence of internal and external mass transfer limitations on the overall rate of either reaction. A reversible, second order reaction was invoked in order to estimate the reaction rate constant, pre-exponential factor and activation energy of the esterification and hydrolysis reactions. Activation energy of 25.9 kJ/mol (soybean oil hydrolysis), 38.5 kJ/mol (ethyl acetate hydrolysis), 24.8 kJ/mol (oleic acid esterification) and 46.7 kJ/mol (acetic acid esterification) were calculated over H-BEA zeolite, illustrating the importance of reactant size and the fact that reactions take place on the pour mouth of the mesopouros zeolite in a case of large molecules reaction. Experimental results suggested that the relative hydrophobicity of the catalyst is a key parameter, which influences whether the reaction is under kinetic or diffusion control.

Bio-hydrogen production by APR of C2-C6 polyols on Pt/Al2O3: Dependence of H2 productivity on metal content

The effect of the platinum loading on the production of hydrogen by aqueous-phase reforming of ethylene glycol, glycerol, xylitol and sorbitol was studied on Pt/Al2O3 catalysts containing 0.30, 0.57, 1.50 and 2.77 Pt%. Catalytic runs were performed at a space velocity of 1.2h−1, 498K, 29.3bar and using a polyol(1.0%)/water feed. The total polyol conversion and the polyol conversion to gaseous products increased with surface Pt concentration (Pts, μmol Pt/gcat). Similarly, the yield and the productivity to hydrogen (Pr, molH2/gh) increased continuously with Pts in all the cases, but the Pr values diminished with the polyol chain length. Coke formation depended on Pts and the polyol size; the amount of carbon formed on the catalyst increased indeed with Pts and diminished with the polyol chain length. The metal fraction was severely sintered during the APR reaction, irrespective of the reactant size. Nevertheless, the magnitude of the Pt dispersion drop was not dependent on the amount of platinum on Pt/Al2O3 catalysts.

Extraction of lead from galena concentrates using fluosilicic acid and peroxide

A study on lead extraction from lead concentrate had been conducted. Galena is usually associated with other sulfide ores such as sphalerite.The lead concentrate was able to be extracted and purified into its metal through a leaching process using a selective solvent of fluosilicic acid (H 2 SiF 6 ). Parameters used in this process include ratio (dose) of reactant (H 2 SiF 6 :H 2 O 2 ), temperature (without heating; 30; 50; 70; 80; and 90°C) and particle size (-100+150#, -150+200#, -200+325, dan -325#).The best extraction was achieved using the particle size of -325 mesh. The amount of extracted lead was increased due to the rise of temperatureand dose of fluosilicic. The amount of peroxide addition was determined by its optimum influence on the lead extraction because its excess would produce PbSO 4 . The influence of H 2 SiF 6 and H 2 O 2 doses was calculated using ANOVA.

Association equilibria for proteins interacted with crowders of short-range attraction in crowded environment

Based on a very simple coarse-grained colloidal model, here we implement an effective hard-sphere theory and numerical simulation to capture the general features of the association equilibria for globular proteins in crowded environment. We measure the activity coefficient, i.e., the deviation from ideal behavior of protein solution, and the crowding factor, i.e., the contribution of crowders to the association equilibria, for proteins in macromolecular crowding. The results show that the association balance in macromolecular crowding depends sensitively on the magnitude of protein–crowder attraction and the relative size of reactant to crowding agent. Since our coarse-grained model is irrelevant to the microscopic details of the molecules, it can be applied to the control of the association equilibria of many globular proteins such as bovine serum albumin, crystallin and lysozyme.

SHS of Eu2+-doped β-SiAlON phosphors: Impacts of N2 pressure and Si particle size

Self-propagation high-temperature synthesis (SHS) method was developed as a highly-efficient route for the preparation of Eu-doped β-SiAlON green phosphors. The as-synthesized products with composition Si5.5Al0.5O0.5N7.5:0.035Eu2+ were featured with high phase purity, good uniformity and equiaxed grain morphology. Strong green light emission spectra covering the range of 450–650nm were obtained under the excitation of UV or blue light. The impacts of the nitrogen pressure in the reaction system and average particle size of Si reactant, which were two of the most vital parameters in the SHS reaction, were systematically investigated. Based on the experimental results, these two factors were confirmed to have striking influences on the phase composition, microstructure and photoluminescence properties of resultant phosphors. In the SHS for β-SiAlON:Eu2+ phosphors, there demonstrated that the selection of N2 pressure of 4MPa and the utilization of Si reactants with fine particle size would bring about remarkable enhancement in the photoluminescence properties.

Influence of grain size, water, and deformation on dolomite reaction rim formation

Solid-solid mineral reaction rates are influenced by the microfabrics of reactant phases and concurrent deformation. To investigate this interplay in carbonate systems, we performed annealing and deformation experiments on polycrystalline and single-crystal calcite and magnesite, forming dolomite (Dol) and magnesio-calcite (Mg-Cal). At a fixed temperature of T = 750 °C and confining pressure of P = 400 MPa, samples were either annealed for 29 h, or deformed in triaxial compression or torsion for 18 h using a Paterson-type gas deformation apparatus. At the contact interface of the starting reactants, Dol reaction rims and polycrystalline Mg-Cal layers were formed. The widths of the layers were in the ranges 4–117 and 30–147 μm, respectively, depending on the microstructure of starting materials and experimental conditions. Annealing experiments with polycrystalline reactants in contact with each other resulted in a ~22-fold increase in Dol rim thickness compared to a contact between two single crystals and a larger Mg-Cal layer width by a factor of 5 (cf. Helpa et al. 2014). This suggests that the microstructure of magnesite controls migration of the reaction front. For polycrystalline starting materials, axial stress accelerated Mg-Cal growth rates but not Dol growth rates. Highly strained torsion samples showed Dol formation along grain boundaries in Mg-Cal as well as in the polycrystalline calcite reactant. A reduction of Dol rim thickness between polycrystalline reactants deformed in torsion is possibly caused by concurrent grain coarsening of polycrystalline magnesite. Dol and Mg-Cal growth kinetics between single crystals were unaffected by the addition of ~0.3 wt% water. The experiments demonstrate that Dol reaction kinetics strongly correlate with magnesite reactant grain sizes, while Mg-Cal growth depends on the calcite reactant grain sizes. The dolomite-forming mineral reaction kinetics are not significantly affected by concurrent deformation. In contrast, deformation enhances Mg-Cal formation, especially at small calcite grain sizes that promote efficient grain boundary diffusion. Therefore, the fastest reactions forming Dol and Mg-Cal in nature are expected to occur in very fine-grained reactants. Concurrent deformation may drastically enhance reaction kinetics if grain size reduction of the reactants occurs by, for example, cataclasis or dynamic recrystallization.

Shape selectivity of beta and MCM-49 zeolites in liquid-phase alkylation of benzene with ethylene

NH3, pyridine and 2,4,6-trimethyl-pyridine were selected as the base probes to characterize the different pore acidity of H-beta and H-MCM-49 zeolites, revealing the essence in their catalytic performance from the view of shape selectivity in different pore systems. Based on the size of base probes, reactants, product, transition states and pore channels, the difference in catalytic performance over H-beta and H-MCM-49 was investigated. Easy diffusion of reactants into and long diffusion distance for ethylbenzene escaping out of confined space within beta are the main reason for superior activity and poorer ethylbenzene (EB) selectivity. As for H-MCM-49, acid sites located in 10 MR channels contributed little to the formation of EB due to transition state selectivity; Acid sites located in 12 MR cups on the outer surface with little restriction on the diffusion of reactant/product toward/from the active centers were the major active centers for EB formation, which could be detected by 2,4,6-trimethyl-pyridine; There was diffusion limitation on reactant/product in 12 MR supercages through 10 MR windows to influence activity/selectivity. The enhancement in activity and selectivity in liquid-phase alkylation of benzene with ethylene over MWW zeolites should be mainly focused on transforming confined 12 MR supercages into 12 MR cups with improved openness.

Processing factors influencing the free carbon contents in boron carbide powder by rapid carbothermal reduction

High quality boron carbide powder without free carbon is desired for many applications. In this study, the factors that influence free carbon content in boron carbide powders synthesized by rapid carbothermal reduction reaction were evaluated. The dominant factors affecting free carbon contents in boron carbide powder were reaction temperature, precursor homogeneity, the particle size of reactants, and excess boron reactant amount. The reaction temperature at 1850°C was sufficient to synthesize boron carbide with low free carbon content. Depending on process conditions, precursor homogeneity was also affected by the calcination temperature and time. Smaller particle size of reactants contributed to less carbon content and more uniformity in synthesized boron carbide. Excess boric acid effectively compensated for B2O3 volatilization. In the optimal sample, using 80mol% excess nano boric acid and calcined at 500°C, the free carbon in the synthesized boron carbide was negligible (0.048wt.%).

Microstructure and strength of fused high alumina materials with 2.5 wt% zirconia and 2.5 wt% titania additions for refractory applications

High alumina alumina–zirconia–titania (AZT) materials have a high potential to resist thermal shock due to complex microcracking. In this study, therefore, the microstructure, phase composition, porosity and strength were investigated dependent on the cooling rate of fused raw materials, the sintering temperature and a thermal shock treatment. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, dilatometry, true density, porosity and strength analysis were conducted. By slow cooling of fused AZT, stabilized aluminum titanate formed in the raw material whereas by fast cooling the formation or stabilization was prevented. The formation of new aluminum titanate during sintering occurred above 1300°C. Stabilization by solid solutions with e.g. titania—forming at sintering temperatures above 1450°C—was superior to the stabilizing effect of crack growth retarding additives like zirconia. After sintering at 1450°C the fast cooled material contained unstabilized aluminum titanate whereas the titanate in the slowly cooled AZT was stabilized. But the strength losses with thermal shock were comparable and both effects beneficial: the presence of stabilized aluminum titanate and the decomposition of the titanate. The additives deposited in the fast cooled material between and inside of the alumina grains. In the slowly cooled material they deposited mainly inter-granular. Consequently, in the fast cooled material the alumina grains cracked, leading to lower strengths. AZT may reach its full potential to resist thermal shock for reactant particle sizes below 63μm, inter-granular deposition areas without exception and an open porosity below 20%. Sintering temperatures above 1300°C were beneficial if the raw material contained unconverted reactants of the aluminum titanate formation.

Multiscale Modeling of a Proton Exchange Membrane Fuel Cell: Effect of Electric Double Layer Structure

Proton Exchange Membrane Fuel Cells (PEMFCs) are very promising to generate electric energy for transport and electronic devices because they produce minimal pollutant emissions and can operate at low temperature [1]⁠. Despite of enormous progresses made to date, this technology has to reach all technical requirements in order to be competitive.The presence of solvated cations as charge-transporting species has a direct influence on the potential profile at interfaces involved in PEMFCs. This leads to a shift in the electric potential acting on the reactants and modifies their activation energies. In this way, the rate of mass and charge transfer processes are determined by the electric double layer (EDL) morphology and structure. For instance, a simple modification in the medium permittivity close to the surface results in appreciable increase of the apparent reaction rate constant [2].The importance of EDL structure on transfer mechanisms has led to several researches based on mean field theories and molecular simulations [3-4]. However, an integration of EDL structure simulations with PEMFC reaction model is still lacking. This will contribute to understand and cuantify the effect of electrolyte and solvent compositions on the reaction kinetics.In order to simulate the EDL structure and its modifications with respect to applied potential and solution composition, it is required a proper molecular model that resembles the size, polar and chemical characteristics of reactants, electrolyte and solvent species involved in PEMFCs, which is proposed to carry out in this work by means of molecular dynamics (MD) simulations.The results achieved by molecular simulations would enable to evaluate the modification of reaction rates by the EDL structure, under an applied potential in PEMFC. In order to accomplish this, it is employed the Stillinger-Weber interaction potential of water and platinum [7], coupled with electrostatic interactions for the molecular simulations of EDL.

Morphology Controlled Lifetime and Selectivity in Zeolite Catalysis

Zeolites are widely used in the chemical industry for their catalytic properties and their ability to control the size and shape of both reactant and product molecules. In this presentation we will show unexpected catalytic properties of ZSM-57 and SUZ-4 during conversion of methanol to hydrocarbons (MTH). We have previously shown how analysis of anisotropic peak broadening in XRPD data [1] can reveal not only the average shape of the crystallites, but also how the zeolite channels are orientated relative to the morphology [2]. The same method was used to analyze the ZSM-57 and SUZ-4 samples revealing nano-sheet and -needle morphologies respectively. Electron imaging and diffraction furthermore revealed that the sheets of ZSM-57 are five-fold twinned with the 10-ring channel system perpendicular to the sheet. The 8-ring channel system would form a closed pentagon shape in defect free crystals. Based on these observations we will explain the remarkable MTH lifetime and selectivity displayed by the ZSM-57 and SUZ-4 samples respectively [3].

Two Decades of Research on Nano‐Energetic Materials

Two Decades of Research on Nano‐Energetic Materials

The confined space inside carbon nanotubes can dictate the stereo- and regioselectivity of Diels–Alder reactions

Chemical reactions inside carbon nanotubes can yield unusual outcomes. Molecular dynamics simulations show that within the confined space of carbon nanotubes, the 1,4-exo adduct of a Diels-Alder cycloaddition may be produced instead of the 9,10-adduct, which is favoured in bulk. The likely product is highly dependent on the nanotube radius and reactant size.

Studies of Sedimentation Rates of Non-Transition Alkaline- Earth Metal Carbonates in Aqueous Medium

Rate data of sedimentation are useful in designing sedimentation equipment and in modeling operations such as filtration, fluidization, two-phase flow operations, and in environmental engineering. This study was thus embarked on to obtain empirical data on the gravitational sedimentation rates of magnesium and calcium carbonates (non-transition alkaline-earth metal carbonates) by determiningthe effect of reactant concentration and size of settling vessel of these carbonates, in chemical precipitation,by a modified approach of the 'isolation method'.The results showed that the sedimentation behaviour varied with the ionic concentrations of the chemical species involved in precipitation. Initial settling characteristics also showed that the nature of the precipitates and the medium in which the precipitation occurred affected the sedimentation rates of the metal carbonates.When the metal ion concentration was varied,-0.87 and -1.1 were obtained as sedimentation orders and 9.92 x 10 -4 cm s -1 M -1 and 6.82 x 10 -4 cm s -1 M -1 as sedimentation rate constants for magnesium carbonate and calcium carbonate respectively. On the other hand,when the carbonate ion concentration was varied, -0.79 and -1.8 were obtained as sedimentation orders and 9.87 x 10 -4 cm s -1 M -1 and 6.92 x 10 -5 cm s -1 M -1 as sedimentation rate constants for magnesium carbonate and calcium carbonate respectively.Sedimentation rate constants of MgCO3 and CaCO3 also showed close dependence on their densities (r 2 = 0.9963). The results of the sedimentation rates measured in cylinders of varying sizes showed that the sedimentation rate of the precipitates correlated positively with the internal diameter of the cylinders.

Hierarchical high-silica zeolites as superior base catalysts

For more than four decades, the design of zeolite base catalysts has relied on the application of aluminium-rich frameworks exchanged with alkali metal cations (preferably Cs+). However, moderate activity associated with access and diffusion limitations, and high manufacturing costs associated with high caesium content (typically over 30%) have hampered their industrial implementation so far. Herein, we have discovered that high-silica USY zeolites outperform their Al-rich counterparts in a variety of base-catalysed reactions of relevance in the fine chemical industry, as well as in the upgrading of biofuels. The benefits of this class of materials are amplified upon the alleviation of diffusion constraints through the introduction of a network of intracrystalline mesopores by post-synthetic modification. For example, the resulting cation-free hierarchical USY provides an up to 30-fold Knoevenagel condensation activity compared to the benchmark Cs–X, and similar observations were made upon application in liquid-phase (nitro)aldol reactions. Moreover, in the gas-phase aldol condensation of propanal, high-silica zeolites provide superior activity, selectivity, and lifetime compared to caesium-containing zeolites and even a strong solid base such as MgO. We decouple the complex interplay between mesoporosity and intrinsic zeolitic properties such as crystallinity, and quantify the increase in catalyst effectiveness upon hierarchical structuring as a function of reactant size. The obtained results are a major step to resolve the drawbacks of zeolites catalysis and thereby revitalise their potential for industrial application.

Effects of On-Time on Microstructure and Corrosion Resistance of Pulse-Micro-Arc Oxidation Coatings Formed on Titanium Alloy

Titanium dioxide (TiO2) has widely been applied in various fields of industry and technology. To improve the corrosion resistance of TiO2, many manufacturing processes have been researched. MAO is a new method of the TiO2 manufacturing process, of which titanium dioxide is coated on titanium substrate. According to electrochemistry researches, there is a more superior level for Pulse-MAO than traditional MAO. Parameters of on-time were significant in the Pulse-MAO process. The results show that on-time affects reactant shape, morphologies, thickness of films, and corrosion resistance. When on-time increased, the size of reactants was bigger and the shape of reactants was changed from sphere to lump. Meanwhile, the corrosion resistance and thickness of the film increased.