Studies of the Hydrolysis and Polymerization of Silicon Alkoxides in Basic Alcohol Solutions

1988 ◽  
Vol 121 ◽  
Author(s):  
G. H. Bogush ◽  
G. L. Dickstein ◽  
P. Lee ◽  
K.C. ◽  
C. F. Zukoski
Keyword(s):  
Aqueous Ethanol ◽  
Average Particle Size ◽  
Reaction Medium ◽  
Particle Growth ◽  
Ionic Species ◽  
Average Particle ◽  
Silicon Alkoxides ◽  
Long Time ◽  
Time Region ◽  
Kinetics Of

ABSTRACTWe report here an experimental technqiue based on changes in reaction medium volume which we have used to observe the kinetics of hydolysis and polymerization of TEOS in aqueous ethanol solutions containing NH3. Our results show that there are three distinct reaction regions. By combining these results with determinations of change in reaction medium conductivity and average particle size, we are able to link the short and long time regions to TEOS hydrolysis and particle growth respectively. The intermediate reaction time region is not associated with hydrolysis, loss of ionic species due to condensation or to particle growth.

Kinetics of synthesizing process for obtaining iron nanopowder by chemical-metallurgical method under isothermal conditions

2021 ◽  
pp. 72-77
Author(s):  
Tien Hiep Nguyen ◽  
◽  
Van Minh Nguyen ◽  
Keyword(s):  
Hydrogen Reduction ◽  
Average Particle Size ◽  
Chemical Deposition ◽  
Reduction Process ◽  
Nitrogen Adsorption ◽  
Specific Rate ◽  
Average Particle ◽  
Isothermal Conditions ◽  
Electron Microscopes ◽  
Kinetics Of

In this work the kinetics of synthesizing process of metallic iron nanopowder by hydrogen reduction from α-FeOOH hydroxide under isothermal conditions were studied. α-FeOOH nanopowder was prepared in advance by chemical deposition from aqueous solutions of iron nitrate Fe(NO3)3 (10 wt. %) and alkali NaOH (10 wt. %) at room temperature, pH = 11, under the condition of continuous stirring. The hydrogen reduction process of α-FeOOH nanopowder under isothermal conditions was carried out in a tube furnace in the temperature range from 390 to 470 °C. The study of the crystal structure and composition of the powders was performed by X-ray phase analysis. The specific surface area S of the samples was measured using BET method by low-temperature nitrogen adsorption. The average particle size D of powders was determined via the measured S value. The size characteristics and morphology of the particles were investigated by transmission and scanning electron microscopes. The calculation of the kinetic parameters of the hydrogen reduction process of α-FeOOH under isothermal conditions was carried out by the Gray-Weddington model and Arrhenius equation. It is shown that the rate constant of reduction at 470 °C is approximately 2.2 times higher than in the case at 390 °C. The effective activation energy of synthesizing process of iron nanopowder by hydrogen reduction from α-FeOOH was ~38 kJ/mol, which indicates a mixed reaction mode. In this case, the kinetics overall process is limited by both the kinetics of the chemical reaction and the kinetics of diffusion, respectively, an expedient way to accelerate the process by increasing the temperature or eliminate the diffusion layer of the reduction product by intensive mixing. It is show that Fe nanoparticles obtained by hydrogen reduction of its hydroxide at 410 °C, corresponding to the maximum specific rate of the reduction process, are mainly irregular in shape, evenly distributed, the size of which ranges from several dozens to 100 nm with an average value of 75 nm.

Study on the kinetics of process for obtaining cobalt nanopowder by hydrogen reduction under isothermal conditions

2021 ◽  
Vol 1 (1) ◽  
pp. 49-56
Author(s):  
Hieр Nguyen Tien
Keyword(s):  
Specific Surface ◽  
Surface Area ◽  
Specific Surface Area ◽  
Hydrogen Reduction ◽  
Average Particle Size ◽  
Chemical Deposition ◽  
Average Particle ◽  
Isothermal Conditions ◽  
Electron Microscopes ◽  
Kinetics Of

The kinetics of metallic cobalt nanopowder synthesizing by hydrogen reduction from Co(OH)2 nanopowder under isothermal conditions were studied. Co(OH)2 nanopowder was prepared in advance by chemical deposition from aqueous solutions of Co(NO3)2 cobalt nitrate (10 wt.%) and NaOH alkali (10 wt.%) at room temperature, pH = 9 under continuous stirring. The hydrogen reduction of Co(OH)2 nanopowder under isothermal conditions was carried out in a tube furnace in the temperature range from 270 to 310 °C. The crystal structure and composition of powders was studied by X-ray phase analysis. The specific surface area of samples was measured using the BET method by low-temperature nitrogen adsorption. The average particle size of powders was determined by the measured specific surface area. Particles size characteristics and morphology were investigated by transmission and scanning electron microscopes. Kinetic parameters of Co(OH)2 hydrogen reduction under isothermal conditions were calculated using the Gray–Weddington model and Arrhenius equation. It was found that the rate constant of reduction at t = 310 °C is approximately 1.93 times higher than at 270 °C, so the process accelerates by 1.58 times for 40 min of reduction. The activation energy of cobalt nanopowder synthesizing from Co(OH)2 by hydrogen reduction is ~40 kJ/mol, which indicates a mixed reaction mode. It was shown that cobalt nanoparticles obtained by the hydrogen reduction of its hydroxide at 280 °C are aggregates of equiaxed particles up to 100 nm in size where individual particles are connected to several neighboring particles by contact isthmuses.

Luminescence kinetics of semiconductor doped glasses in the long time region

1992 ◽  
Vol 71 (2) ◽  
pp. 942-945 ◽  
Author(s):  
S. Burkitbaev ◽  
M. Bertolotti ◽  
E. Fazio ◽  
A. Ferrari ◽  
G. Liakhou ◽  
...  
Keyword(s):  
Luminescence Kinetics ◽  
Long Time ◽  
Doped Glasses ◽  
Time Region ◽  
Kinetics Of

Nickel-Containing Systems in Reaction of Partial Oxidation of Hydrocarbons

2015 ◽  
Vol 670 ◽  
pp. 33-38 ◽  
Author(s):  
Sergey Galanov ◽  
Olga I. Sidorova ◽  
Vera A. Batyreva
Keyword(s):  
High Temperature ◽  
Average Particle Size ◽  
Reaction Medium ◽  
Oxide Phase ◽  
Average Particle ◽  
Catalytic Layer ◽  
Temperature Synthesis ◽  
High Temperature Synthesis ◽  
Catalytically Active ◽  
Catalyst Systems

Nickel dispersion, which provides total catalytically active metal surface, is significant for partial catalytic oxidation of natural gas to obtain high purpose products yield and productivity in syngas. The interaction of reaction medium under high reaction temperatures during 20-25 hours promotes the increase of total square of active Ni component for block catalysts obtained with self-propagating high-temperature synthesis, which allows achieving productivity in syngas 7.1·103cm3(syngas)/cm3(catalyst)·hour. It is observed that for catalyst systems obtained with precipitation, chemical composition of oxide phase influences the particles size of metal nickel. For granular catalysts obtained through precipitation after 25-hour exploitation, average particle size (according to CSR) metal nickel is 3-4.5 times smaller than Ni in catalysts obtained with self-propagating high-temperature synthesis. This allows achieving productivity in syngas 8.1·103cm3/сm3·hour, when there is average temperature decrease over the catalytic layer by ~100°С in comparison with blocks acquired through self-propagating high-temperature synthesis.

Kinetic study and modeling of the solid-state reaction Y2BaCuO5 + 3BaCuO2 + 2CuO ⇉ 2YBa2Cu3O6.5−x + xO2

1990 ◽  
Vol 5 (10) ◽  
pp. 2056-2065 ◽  
Author(s):  
Nae-Lih Wu ◽  
Ta-Chin Wei ◽  
Shau-Y Hou ◽  
S-Yen Wong
Keyword(s):  
Particle Size ◽  
Solid State ◽  
Solid State Reaction ◽  
Size Dependence ◽  
Average Particle Size ◽  
Average Particle ◽  
Fractional Conversion ◽  
X Ray ◽  
Unreacted Core ◽  
Kinetics Of

The kinetics of the solid-state reaction Y2BaCuO5 + 3BaCuO2 + 2CuO ⇉ 2YBa2Cu3O6.5−x + xO2 was studied by using x-ray diffractometric and thermogravimetric analyses. Both analyses established that the reaction was well described by the kinetic equation: 1 − 3(1 − F)2/3 + 2(1 − F) = k0 exp(− E/RT)t, where F is the fractional conversion of a calcined powder, E is 520 kcal/molc and, for a rcactant mixture with an average particle size of 3 μm, k0 is 2.03 ⊠ 1092 min−1. An unreacted-core shrinking model was proposed to obtain the particle-size dependence of the reaction, and predicted that the pre-exponential constant k0 changed with reactant particle size by k0 = 2.03 ⊠ 1092(3/d)2 exp(4/d − 4/3), where d is the average reactant particle size in μm.

Molten Salt Synthesis of Anisotropy BaBi4Ti4O15 Powders in K2SO4-Na2SO4 Flux

2011 ◽  
Vol 687 ◽  
pp. 333-338 ◽  
Author(s):  
Yong Jun Gu ◽  
Jin Liang Huang ◽  
Li Hua Li ◽  
Ke Zhang ◽  
Xiao Wang ◽  
...  
Keyword(s):  
Activation Energy ◽  
Particle Size ◽  
Molten Salt ◽  
Process Parameters ◽  
Average Particle Size ◽  
Particle Growth ◽  
Molten Salt Synthesis ◽  
Average Particle ◽  
Synthesis Conditions ◽  
Arrhenius Dependence

Anisotropic BaBi4Ti4O15 powder was synthesized by a molten salt synthesis (MSS) method in K2SO4-Na2SO4flux and the effects of different process parameters such as calcining temperature, and ratio of salt to reactant (R) on the phase formation and morphology of anisotropic BaBi4Ti4O15particles were also investigated. The as-synthesized powder calcined at 850-950°C exhibits a single tetragonal BaBi4Ti4O15phase. The morphology of BaBi4Ti4O15powder could be adjusted by changing the synthesis conditions. The average particle size (APS) of BaBi4Ti4O15powder increased with R changing from 0.8 to 1.0, while it decreased with further increasing of R to 1.2. In addition, the APS increased with increasing calcining temperature and it showed an Arrhenius dependence on the temperature. The corresponding apparent activation energy for particle growth is 31.9kJ/mol for calcining temperature of 850-1000°C.

scholarly journals Kinetics of Heavy Metal Ion Adsorption on to, and Proton Release from, Electrolytic Manganese Dioxide

1998 ◽  
Vol 16 (1) ◽  
pp. 39-50 ◽  
Author(s):  
Madhav P. Dahal ◽  
Geoffrey A. Lawrance ◽  
Marcel Maeder
Keyword(s):  
Kinetic Model ◽  
Metal Ions ◽  
Manganese Dioxide ◽  
Metal Ion ◽  
Average Particle Size ◽  
Transition Metal Ions ◽  
Average Particle ◽  
Electrolytic Manganese Dioxide ◽  
Electrolytic Manganese ◽  
Kinetics Of

The kinetics of adsorption of lead(II), as well as of copper (II), zinc(II) and thallium(I), on commercial electrolytic manganese dioxide (EMD) powder of different size fractions has been followed by a pH-stat method involving the computer-controlled neutralisation of released protons from the EMD at a fixed pH with hydroxide ion. Base uptake versus time profiles in all cases indicated a rapid initial change followed by a slower process. These can be interpreted in terms of either a solution kinetic model or an adsorption kinetic model. In the former analysis, the adsorption-time profile can be interpreted in terms of two, or at most three, exponentials. The contribution of the first fast step increases with decreasing average particle size (or increasing surface area), consistent with its interpretation as being associated with the replacement of accessible protons on the exposed surface of the EMD by metal ions. The slower rate processes can be considered to be associated with adsorption in less accessible clefts or pores in the structure. In the diffusion model, the experimental data were fitted to the equation of diffusion in a sphere, fitting again requiring a first initial step followed by a slower step, with one diffusion coefficient having typically an upper limit of 10-9 and the other a lower limit of 10-11 cm2/s. For the complete reaction, the moles of protons released per mole of metal ion adsorbed were also measured and found to be relatively insensitive with respect to pH, metal ion concentration, electrolyte concentration and EMD size fraction. At pH 4, the H+: Mn+ ratio ranged from 1.4 to 2.1 (±0.1) for divalent transition metal ions, consistent with dominant chelation of the divalent metal ion to two adjacent oxygen sites on the surface; for thallium(I), a low ratio (0.5) indicated a different mode of surface attachment.

The concentration quenching of photoluminescence in Eu3+-doped La2O3

2013 ◽  
Vol 37 (1) ◽  
pp. 47-54 ◽  
Author(s):  
V. Đorđević ◽  
Ž. Antić ◽  
M. G. Nikolić ◽  
M. D. Dramićanin
Keyword(s):  
Room Temperature ◽  
Hexagonal Phase ◽  
Average Particle Size ◽  
Average Particle ◽  
X Ray Diffraction ◽  
X Ray ◽  
Maximum Lifetime ◽  
Transmission Electron ◽  
Lifetime Value ◽  
Kinetics Of

Abstract This work explores the influence of dopant concentration on photoluminescent emission and kinetics of Eu3+-doped (0.2−10 at.%) nanocrystalline lanthanumoxide powders. The X-ray diffraction analysis confirmed that all samples crystallize in La2O3 hexagonal phase with space group P3¯ m1. Transmission electron microscopy showed particles with non-uniform shape and diverse size distribution with an average particle size of (95 ± 5) nm. The room temperature photoluminescence spectra of all samples contain characteristic Eu3+ luminescence lines with the most pronounced red 5D0 →7F2 emission at about 626 nm. The maximum intensity of red emission is observed for the sample containing 5at.% of Eu3+ ions. The emission kinetics was recorded in the temperature range from 10K to 300 K. The maximum lifetime value of 0.98 ms obtained for the sample with 0.5at.% Eu3+ at room temperature increases up to 1.3ms at 10 K.

scholarly journals Kinetics of Iron Bioleaching using Isolated Leptospirillum Ferriphilum: Effect of Temperature

2019 ◽  
Vol 8 (12) ◽  
pp. 76-81
Keyword(s):  
Average Particle Size ◽  
Effect Of Temperature ◽  
Average Particle ◽  
Optimum Temperature ◽  
Leptospirillum Ferriphilum ◽  
Ash Layer ◽  
Initial Ph ◽  
Kinetics Of ◽  
Rate Controlling Step ◽  
Leaching Efficiency

This study was designed to investigate the effect of temperature on iron bioleaching kinetics using Leptospirillum ferriphilum. The bacteria were isolated and subjected to molecular characterization technique for confirming L. ferriphilum. Using the isolate, bioleaching data were collected in the temperature range of 298–318 K at an initial pH of 1.5 and 5% pulp density with an average particle size being 300 µm. The results of experiments concluded that leaching efficiency increases with temperature and maximum of 93.85% were observed after 20 days at 313 K. The bioleaching kinetics indicated that the maximum rate (rate constant: 0.1452 d1 ) was found in the experiment conducted at the optimum temperature, and the rate-controlling step was “diffusion through ash layer.” The activation energy was calculated to be 37.59 kJ/mol. From the thermodynamic study of the bioleaching system, ∆H˚ and ∆S˚ were found to be 0.7399 × 10−3 and 28.512 J/mol, respectively

scholarly journals The influence of alkaline extraction on some keratin hydrolysates properties

2020 ◽  
Author(s):  
Mariana Daniela Berechet ◽  
Demetra Simion ◽  
Maria Stanca ◽  
Ciprian Chelaru ◽  
Cosmin-Andrei Alexe ◽  
...  
Keyword(s):  
Average Particle Size ◽  
Control Sample ◽  
Reaction Medium ◽  
Wheat Plant ◽  
Average Particle ◽  
High Concentration ◽  
Fixed Temperature ◽  
Fibrous Protein ◽  
The Fourier Transform ◽  
Biological Performance

Keratin is a fibrous protein abundant in nature, being the component of wool, hair, hooves, horns, feathers, and claws. Keratin is one of the most valuable natural biopolymers due to its chemical versatility and biological performance. At the molecular level, keratin is distinguished from other biopolymers by its high concentration of cysteine-containing sulfur. Two keratin hydrolysate batches were obtained in alkaline medium, at a constant concentration of 8% NaOH and 75°C (KerNa875), 85°C (KerNa885), and 95°C (KerNa895), and at a fixed temperature of 99°C and different concentrations of NaOH, i.e. 3% (KerNa399), 5% (KerNa599), and 8% (KerNa899), respectively. Physical-chemical analyses showed that the protein content ranging between 83.60% for KerNa875 and 88.88% for KerNa399, while the total nitrogen was found 13.83% and 14.67% in the case of KerNa875 and KerNa399, respectively. Dynamic light scattering analysis showed that the particle sizes decreased with the increased concentration in the reaction medium. The average particle size was between 1352 nm and 1771 nm for the samples obtained at a temperature of 99°C and with lower values between 463.3 nm and 571.6 nm for the samples obtained with 8% NaOH. The Fourier transform infrared (FT-IR) spectra evidenced the specific bands of keratin-specific proteins and sulfur compounds. Experiments were also performed to evaluate the antioxidant activity and the growth of Tamino and Mirastar wheat plants by applying the treatments with 3% and 5% concentrations of KerNa899 on wheat seeds. These experiments showed an improvement in the wheat plant growth during 10 days of observation compared to control sample. The results recommend the potential use for keratin hydrolysates in the medical, pharmaceutical, cosmetics fields, and also as fertilizers in agriculture.

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