Kyun Young Park

Vapor-Phase Synthesis and Characterization of Ultrafine Iron Powders

ABSTRACT Formation of ultrafine iron powders by vapor-phase reduction of ferrous chloride with hydrogen was studied in a tubular reactor made of quartz, 3.5 cm in diameter and 1.5 m in length. Effects on the particle-size distribution of produced iron powders were investigated of various operating variables including the temperature in reaction zone, the concentration of FeCl2, and the flow rate of dilution argon gas. Ultrafine iron powders ranging from 40 to 88 nm in average particle size were produced. The geometric standard deviation was about 1.4. Particles were linked to each other to form a chain, probably due to magnetic interaction. By electron diffraction, all the particles are found to be single crystal. Over the reaction zone temperature ranging from 800 to 950°C, the average size of primary particles decreased with temperature, probably due to enhanced nucleation rate. No significant change in the particle size was observed for a reduction of reactor residence time by one-third. The coercive f...

Photocatalytic reduction of carbon dioxide using Co3O4 nanoparticles under visible light irradiation

Photocatalytic reduction of carbon dioxide under visible light irradiation was carried out with Co3O4 powders suspended in water. A Pyrex glass batch reactor of 10mL in volume was used with a 21 W LED lamp of 510 to 620 nm in wave length as light source, and the reaction time was held at 4 h. The major products were formic acid and formaldehyde; the production rates were 4.53 μmol g−1h−1 and 0.62 μmol g−1h−1 for formic acid and formaldehyde, respectively. Carbon monoxide and methane were detected in trace amounts. The occurrence of the photo-reduction with Co3O4 is against the expectation from the valence band edge of Co3O4 in the literature. Possible causes for the contradictory result are discussed.

Comparison of Titania Particles Between Oxidation of Titanium Tetrachloride and Thermal Decomposition of Titanium Tetraisopropoxide

Thermal decomposition of TTIP was compared with oxidation of TiCl 4 in morphology and primary particle size of produced TiO 2 particles in a tubular reactor 2.7 cm in diameter and 54 cm in length under equal rate constants. The reactor temperature was varied from 850 to 1000°C for TiCl 4 oxidation and from 492 to 579°C for TTIP decomposition. The lower and upper limits of decomposition temperature for TTIP were determined so that the rate constants become equal, at corresponding limits, between TiCl 4 oxidation and TTIP decomposition. In order to maintain constant concentration with variation of reactor temperature, the flow rate of dilution gas was adjusted to compensate for the volume change of gas with temperature. The precursor concentration at the reaction condition was in the range of 1.09 2 10 m 6 to 1.09 2 10 m 5 mol/L, and the residence time of 3.1 to 10.8 s was based on the reactor set temperature. Particles from TTIP were spherical, while those from TiCl 4 were polyhedral. A considerable fraction of the precursor admitted to the reactor was consumed on the tube wall by surface reaction to form a zone coated with TiO 2 . The loss of precursor to the wall was greater with TiCl 4 oxidation. The particle size was, however, larger by 20% with TiCl 4 oxidation. By replacing the straight reaction tube with a concentric tube, the loss could be reduced, thereby increasing the amount of TiCl 4 available for particle formation significantly; the particle size was similar, however. With the straight tube a mixture of TiCl 4 and oxygen entered the reactor and the reaction occurred over the gradual increase from 650°C to a reactor set temperature of 900°C. With the concentric tube, the reactants had been preheated separately and then brought into contact right at the set temperature. The difference in the history of temperature for reaction may have brought about a difference in nucleation rate and consequently yielded particles of similar size. By analyses of BET surface area, X-ray diffraction patterns, and thermogravimetric data, TiO 2 particles from both routes were nearly nonporous, showed anatase peaks in majority, and contained no appreciable volatiles.

In situ deposition of iron nanoparticles on transmission electron microscopy grid in furnace aerosol reactor

In the current work, a device was proposed for the first time to deposit the particles in situ on a transmission electron micrography grid within furnace aerosol reactors. The device was successfully tested on iron particles produced by thermal decomposition of Fe(CO) 5 at 600 °C in a quartz tube heated by an electric heater. The particle depositions were made at four different spatial locations in the axial direction and investigated by transmission electron microscopy. At the reactor inlet, chain agglomerates of 2–3-nm particles were observed. At 19 cm from the inlet, the particles within the agglomerate structures fully coalesced by sintering, and at 32 cm (reactor outlet), polyhedral particles of about 100 nm in diameter emerged from the sintered body.

Effect of temperature on particle size for vapor-phase synthesis of ultrafine iron particles

For the vapor-phase synthesis of iron particles from FeCl2 at temperatures ranging from 800 to 950‡C the reason is sought why the model based on the classical nucleation theory brought an increase of particle size with temperature increase, in reverse to experimental observation. The nucleation rate according to the classical theory should decrease with a temperature increase, due to the decrease of super-saturation ratio resulting from the increase of vapor pressure. The decrease of nucleation rate ultimately leads to an increase of particle size. Yang and Qiu’s nucleation theory was applied in place of the classical theory. However, the same result as with the classical theory was obtained : the nucleation rate decreased with the temperature increase. Finally, an Arrhenius-type nucleation rate equation was introduced. The preexponential factor and the activation energy for nucleation were determined to be 1348.2 sec-1 and 159.1 KJ/mol, respectively. With these values put into Park et al.’s model, good agreement was obtained in temperature dependence of particle size between model prediction and experimental data.

Computer simulation of H2S and CO2 absorption processesabsorption processes

A new computer program has been developed for the simulation of chemical absorption of H2S and CO2 using MEA, DEA or hot potassium carbonate. The program can calculate either tray or packed columns, The material and energy balances, and equilibrium relationship are solved using the Naphtali-Sandholm method and the stage to stage method complementarily. The packed column is divided into a number of sections. Each section is treated as a tray, but with a different method of efficiency calculation. The enhancement factor was incorporated to reflect the enhancement of absorption rate which is an inherent nature of chemical absorption. Using this program named as AGRES, 20 sample problems were solved for absorption and stripping and the results were compared with those of other competing programs of AMSIM, PROCESS. ASPEN PLUS and DESIGN II. In the calculation of ideal stages, all the programs gave similar results. In the calculation of real stages, however, only AMSIM and AGRES were effective. AMSIM could not calculate packed columns and tray columns having more than 22 stages. While, AGRES could overcome this limitation of AMSIM, providing a broader application.

A discrete-sectional model for particle growth in aerosol reactor: Application to titania particles

A one-dimensional discrete-sectional model has been developed to simulate particle growth in aerosol reactors. Two sets of differential equations for volume and surface area, respectively, were solved simultaneously to determine the size distributions of agglomerates and primary particles. The surface area equations were derived in such a way that the coagulation integrals calculated for the volume equations could be used for the surface area equations as well, which is new in this model. The model was applied to a production of TiO2 particles by oxidation of titanium tetrachloride. Model predictions were compared with experimental data and those of a two-dimensional sectional model. Good agreement was shown in calculated particle size distributions between the present model and the two-dimensional model, which is more rigorous but demands a large amount of computer time and memory. Compared to experimental data, the primary particle size calculated by the model was more sensitive to the variation of reactor temperature.

Prospective application of carbon-silica derived from SiC-Si sludge as a support for Fe catalysts

A unique carbon-silica (30 wt%) material was prepared by H2O activation at 700 °C for 8 h with the carbon derived from SiC-Si sludge and the in-situ hydrolysis of the SiCl4 trapped in the pores of the carbon into silica. The BET surface area of the carbon-silica was 1,750 m2/g and the pore volume by QSDFT was 1.13 cm3/g, 40% of which stemmed from micropores smaller than 2 nm with 60% from mesopores between 2 nm and 50 nm. The activated carbon-silica was loaded with Fe by means of chemical vapor infiltration (CVI) and incipient wetness impregnation (IWI). A hydrogen temperature-programmed reduction test showed that the activated carbon-silica is a prospective support material for Fe catalysts and that the dispersion of Fe in the carbon-silica is higher with CVI than with IWI.

Separation of triethoxysilane from tetraethoxysilane by batch distillation in a packed column

A batch distillation of a crude tetraethoxysilane containing 8mol% triethoxysilane was performed in a glass packed column, 2.54 cm in diameter and 1m in height. Two distillate rates, 3.0mL/min and 6.0mL/min, were used and the reflux ratio was varied up to 3.0. Experimental data were compared with predicted values by Pro/II, a process simulator widely used in the chemical industry. The differential condensation of the vapor in the packed column due to heat losses from the vapor to the column internals and to the surroundings affected the separation efficiency seriously so that a considerable discrepancy was observed between experimental data and prediction by Pro/II in which such heat-loss effects are unaccountable. A model was developed to explain the effect of the differential condensation. For a larger distillation unit scaled up by 100 times where the heat-loss effect is regarded to be minimal, Pro/II simulations were performed to produce 99.9% TEOS with varying reflux ratio, number of stage, and feed composition.

Control of particle size distribution of ultrafine iron particles in the gas phase reaction

Experiments on the synthesis of ultrafine iron particles have been made for the control of particle size distribution using the gas phase reduction of ferrous chloride with hydrogen. The previous studies were focused on the control of particle size of ultrafine particles with the variation of the partial pressure of reactants, residence time of feed, and reaction temperature. However, it is also very important to control the size distribution of ultrafine particles. In this study, the control of particle size distribution was investigated from the standpoint of nucleation. The variation of evaporating condition at the same evaporation rate of ferrous chloride, and of the temperature gradient of reactants between preheating zone and reaction zone were adopted as experimental variables. Ultrafine iron particles having uniform size distribution could be produced under the control of evaporating condition such as the change of the surface area at constant evaporating temperature. As the temperature gradient decreased, particle size distribution became uniform and average particle sizes were also decreased.