Self-oscillations of methane oxidation rate over Pd/Al2O3 catalysts: Role of Pd particle size

Oscillations of the methane oxidation rate were studied under methane-rich conditions on Pd/Al₂O₃ catalysts differing in Pd particle size. It was demonstrated that the temperature interval where oscillations occur narrows from 300–360 °C for the catalyst with Pd particle aggregates from 50–100 nm to 345–355 °C for the catalyst with isolated Pd particles of ~ 5 nm in size. At the same time, the period of oscillations showed ~ 6-fold increase. Structural transformations of Pd in the oscillation cycle were similar to those observed on bulk Pd used as a catalyst in the same reaction.     

Role of the effective electrical conductivity of nanosuspensions in the generation of TiO2 agglomerates with electrospray

Suspensions with varying volume fraction of TiO₂ nanoparticles and ionic strength were electrosprayed to obtain agglomerates of different characteristics, which were then deposited to produce films with tailored morphology, thickness, and porosity. The role of the nanoparticle volume fraction in both the effective electrical conductivity of TiO₂ nanosuspensions and the control of the size of agglomerates produced by electrospray was investigated. A simple modified equation for the effective electrical conductivity of TiO₂ nanoparticle suspensions was derived. The equation, which accounted for nanoparticles' diffuse ionic layer and their agglomeration in a liquid, showed that the effective electrical conductivity is not only a function of the liquid and particle conductivities, and the particle volume fraction but also a function of both the thickness of the adsorbed ionic layer on the particles and the particle size. Gradual increase of particle volume fraction resulted in an increase in the suspension's effective electrical conductivity, when the initial liquid conductivity was in the range of 10^?4–10^?3 S m^?1. When the liquid conductivity was in the range of 10^?3–10^?2 S m^?1; however, addition of particles did not have any significant effect on the effective electrical conductivity. Control over the size of the TiO₂ nanoparticle agglomerates was achieved by electrospraying suspensions with liquid electrical conductivity of the order of 10^?3 S m^?1 and by varying the particle volume fraction. Electrospray deposition of suspensions with TiO₂ volume fraction=0.04% resulted in a more compact film with lower porosity and showed better water-splitting performance.     

A miniature disk electrostatic aerosol classifier (mini-disk EAC) for personal nanoparticle sizers

We have developed a miniature disk electrostatic aerosol classifier (mini-disk EAC) for use in electrical mobility-based personal nanoparticle instrumentation for measurement of personal exposures to nanoaerosols. The prototype consists of two parallel disk electrodes separated by an electrically insulating spacer, to create the particle classification zone. The aerosol enters and exits the classification zone along the bottom disk electrode. An additional, particle-free sheath flow is used to improve the measurement resolution. The transmission measurement of the mini-disk EAC for DMA-classified particles shows that particle losses due to diffusion and electrical image forces were low. The particle penetration at 10 nm diameter (the designed lower size limit for the classifier) was 67% when the prototype was operated at the aerosol and sheath flow rates of 0.5 and 1.0 l min^?1, respectively. The performance of the mini-disk EAC was experimentally characterized using the particle cutoff curves that describe their penetration through the classifier as a function of applied voltage across the two disk electrodes. Based on the measurement of particle penetration at different aerosol and sheath flows, it was found that the aerosol and sheath flow rates of 0.5 and 1.5 l min^?1 were optimal for classifier operation. Finally, a semi-empirical model was also developed to describe the transfer function of the mini-disk EAC for non-diffusive particles.     

Preparation of stable cross-linked enzyme aggregates (CLEAs) of NADH-dependent nitrate reductase and its use for silver nanoparticle synthesis from silver nitrate

The NADH-dependent nitrate reductase from Fusarium oxysporum cell extract was directly immobilized as cross linked enzyme aggregates (CLEAs) and investigated for the synthesis of silver nanoparticles by a reduction of silver nitrate. The effects of precipitant type and cross-linking on activity recovery of enzyme in CLEAs were studied. After aggregation of enzyme with ammonium sulfate followed by cross-linking formed aggregates for 4 h with 8 mM glutaraldehyde, 93% activity recovery was achieved in CLEAs with enhanced thermal stability at 50 °C and 40 °C. Scanning electron microscopy analysis showed that immobilized NADH-dependent nitrate reductase was of spherical structure. CLEAs showed 90% catalytic yield even after 4 cycles of repeated use in silver nanoparticle synthesis at pH 7.2 and temperature 35 °C.     

Characterization of silica-coated silver nanoparticles prepared by a reverse micelle and hydrolysis–condensation process

Described herein is the synthesis of individually silica-coated silver nanoparticles using a reverse micelle method followed by hydrolysis and condensation of tetraethoxysilane (TEOS). The size of a silica-coated silver nanoparticle can be controlled by changing the reaction time and the concentration of TEOS. By maintaining the size of a silver nanoparticle as a core particle at around 7 nm, the size of a silica-coated silver nanoparticle increased from 13 to 28 nm as the reaction time increased from 1 to 9 h due to an increase in silica thickness. The size of silica-coated silver nanoparticles also increased from 15 to 22 nm as the TEOS concentration increased from 7.8 to 40 mM. The size of a silica-coated silver nanoparticle can be accurately predicted using the rate of the hydrolysis reaction for TEOS. Neither the dispersion nor the film of silica-coated silver nanoparticles exhibited any peak shifting during surface plasmon resonance (SPR) at around 410 nm, whereas, without silica coating, the SPR peak of Ag film shifted to 466 nm.     

Onset coarsening-coalescence kinetics of γ-type related Al2O3 nanoparticles: Implications to their assembly in a laser ablation process

An onset coarsening-coalescence event based on the incubation time of cylindrical mesopore formation and a significant decrease of specific surface area by 50% and 70% relative to the dry pressed samples was determined by N₂ adsorption–desorption hysteresis isotherm for two Al₂O₃ powders having 50 and 10 nm in diameter respectively on an average and with γ-type related structures, i.e. γ- and its distortion derivatives δ- and/or θ-types with {1 0 0}/{1 1 1} facets and twinning according to transmission electron microscopy. In the temperature range of 1100–1400 °C, both powders underwent onset coarsening-coalescence before reconstructive transformation to form the stable α-type. The apparent activation energy for such a rapid coarsening-coalescence event was estimated as 241 ± 18 and 119 ± 19 kJ/mol, for 50 and 10 nm-sized particles, respectively indicating easier surface diffusion and particle movement for the latter. The size dependence of surface relaxation and onset coarsening-coalescence of the γ-type related Al₂O₃ nanoparticles agrees with their recrystallization–repacking upon electron irradiation and accounts for their assembly into nano chain aggregates or a close packed manner under the radiant heating effect in a dynamic laser ablation process.     

Effect of operating parameters on PVP/tadalafil solid dispersions prepared using supercritical anti-solvent process

Supercritical anti-solvent (SAS) process was employed to produce tadalafil solid dispersion sub-micron particles. Three independent variables for the SAS process (temperature, pressure, and drug concentration) were varied in order to investigate the effects on particle size and morphology of PVP/tadalafil solid dispersion (drug to polymer ratio 1:4). The mean particle size decreased with decreasing temperature (50 → 40 °C) and concentration (15 → 5 mg/mL) and increasing pressure (90 → 150 bar). Depending on the experimental variable, the mean particle size varied from 200 nm to 900 nm, and the dominant experimental variable was determined to be the drug concentration. Moreover, at a concentration of 15 mg/mL with any other process conditions, tadalafil tended to partially aggregate in crystalline form with irregular particle shapes. The results of in vitro dissolution experiments showed good correlation with mean particle size and crystallinity of the SAS-processed particles, in that the highest drug concentration showed the least dissolution rate and vice versa. Therefore, among the three variables studied, the drug concentration is the major factor that produces sub-micron particles in the SAS process.     

Micronization of ketoprofen by the rapid expansion of supercritical solution process

The particle size of the pharmaceutical substances is important for their bioavailability (the percentage of the drug absorbed compared to its initial dosage). The absorption rate can be increased by reducing particle size of the drug particles. This study was conducted to investigate the effects of the extraction pressure (140–220 bar), extraction temperature (308–338 K), nozzle length (2–15 mm), effective nozzle diameter (450–1700 μm), and collection distance (1–10 cm) on the size and morphology of the precipitated ketoprofen particles. The characterization (size and morphology) of the particles was investigated using scanning electron microscopy (SEM). The average particle size of the original material was 115.42 μm, while the average particle size of the micronized particles is between 0.35 and 7.03 μm near to quisi-spherical, needle and irregular shape depending upon the experimental conditions.     

Reynolds number dependence of gas-phase turbulence in particle-laden flows: Effects of particle inertia and particle loading

A gas-particle flow experiment at a low particle loading (m = 0.4) in a vertical downward pipe is conducted at three different Reynolds numbers (Re = 6000, 10,000, and 13,000) to investigate the Re influence on the gas-phase turbulence modulation. The mean and fluctuating velocity data of both phases are acquired using a two-component LDV/PDA system. Two particles of varying degrees of inertia (i.e. high-density 70 µm glass beads and low-density 60 µm cenospheres) are used as the model particles to examine the effect of particle inertia on the trend in the turbulence modulation as a function of Re. An experiment at a higher particle loading (m = 4.0) using the glass beads is also conducted to examine the effect of particle concentration. In the presence of high inertia particles (St_T > 500) at a low particle loading, the gas-phase turbulence intensity in the pipe core is increased with increasing Re resulting in turbulence enhancement relative to the unladen flow. The turbulence enhancement is attributed to 1) a modification of the turbulence production by the Reynolds stress due to interparticle collision and/or 2) a reduction in the fluctuating drag force due to a change in the radial profile of the particle concentration. In contrast, the gas-phase turbulence intensity in the presence of low inertia particles (St_T < 500) is found to decrease with increasing Re similar to the trend in the unladen flow. Lastly, the turbulence enhancement at high Re is not observed at a high particle loading where the turbulent energy dissipation by the fluctuating drag force is dominant.     

Creating new materials with melamine resins

Acid catalyzed condensation of hexa-methoxy methyl melamine (HMMM) in aqueous phase leads to new functional particles and up to now unknown lamellar mesoscopic gels. The investigation with transmission electron microscopy (TEM) shows that the polymer formation starts with nonspherical nanoparticles. Dynamic light scattering experiments reveal a particle size of about 60–100 nm. Atomic force microscopy (AFM) measurements disclose nonuniform flat particles with an aspect ratio of about 0.3. These nanoparticle dispersions form thermoreversible gels. Molecular modeling investigations indicate energy minimized layer-by-layer condensation of the melamine resin molecules. The next step in growth is the nucleation of the nanoparticles via the narrow sides. This forms nonperfect lamellar layers. This time, we get a thermoreversible gel which is fluid at 80 °C and gets fixed at 20 °C. Out of these platelet structures as precursors, a mesoporous, nonthermoreversible gel with essentially lamellar sides and pore sizes about 10 μm is formed. Scanning electron microscopy (SEM) studies show very uniform wall and plate sizes with a directed three-dimensional structure.     

Plasma-assisted synthesis of carbon encapsulated magnetic nanoparticles with controlled sizes correlated to smooth variation of magnetic properties

This paper reports rapid, continuous and carbon-nanotube free synthesis of carbon encapsulated magnetic nanoparticles by thermal-plasma expansion technique, which combines the typical advantages of high-temperature plasma assisted synthesis method with efficient particle-size control. Core nanocrystals were encapsulated with few layers of graphitized carbon, which could provide protection against both oxidation and intense chemical treatment. The average iron/iron-carbide nanoparticle diameter (7.7, 9 and 10 nm) and the width of the size distribution increased with pressure in the sample collection chamber, as a result of the decreasing quenching rate of the plasma jet. This also resulted in the smaller particles remaining frozen predominantly in the high-temperature γ-Fe phases, part of which was oxidized subsequently and eliminated preferentially during the purification process. All samples could be correlated with smooth variation of magnetic properties; saturation magnetization, remnant magnetization and coercive-field enhancing with increasing chamber pressure or average particle size. The low pressure synthesized sample with smallest average particle size approached super-paramagnetic behavior (saturation magnetization = 51.8 emu/g, ratio of remnant to saturation magnetization = 4.9 and coercive field = 52 Oe), which may be ideal for biomedical applications. High-pressure samples on the other hand have a higher saturation magnetization (76.3 emu/g) and coercive fields (123 Oe).     

Rheology and colloidal structure of silver nanoparticles dispersed in diethylene glycol

Rheological behavior of agglomerated silver nanoparticles (~ 40 nm) suspended in diethylene glycol over a wide range of volumetric solids concentrations (? = 0.11–4.38%) was studied. The nanoparticle suspensions generally exhibited a yield pseudoplastic behavior. Bingham plastic, Herschel–Bulkley and Casson models were used to evaluate the shear stress-shear rate dependency. Analyzing the effect of silver concentrations on the yield stress and viscosity of the suspensions followed an exponential form, revealing an increase in the degree of interparticle interactions with increasing solid concentrations. Fractal dimension (D_f) was estimated from the suspension yield stress and ? dependence, and was determined as D_f = 1.51–1.62 for the flocculated nanoparticle suspensions. This suggested that the suspension structure was probably dominated by the diffusion-limited cluster–cluster aggregation (DLCA) due mostly to the strong attractions involved in the interparticle potentials. Maximum solids concentration of the suspensions was determined to be ?_m = 11%.     

Platinum nanoparticles as active sites for C–С bond activation in high-silica zeolites

Density functional theory (DFT) was applied to investigate the structure and reactivity of Pt₆ particles encapsulated in silicalite as well as in sodium and hydrogen ZSM-5 zeolites. Incorporation of a metal particle in silicalite or sodium zeolite leads to the formation of a negatively charged metal cluster with stabilization energies close to 9 and 16 kcal/mol, respectively. Interaction of a platinum cluster with hydrogen zeolite and extra-framework aluminum species is characterized by energies as high as 45–50 kcal/mol and yields an electron-deficient metal particle. This positively charged metal cluster stabilized in the zeolite channel can function as active site in alkane transformations. The formation of the active site is accompanied by suppression of both Brønsted and Lewis acidities. An alternative mechanism of alkane cracking and isomerization avoiding the direct participation of acid sites has been proposed.     

Preparation of porous silver particles using ammonium formate and its formation mechanism

Here we report the preparation of two different types of macroporous silver particles (round and coral) by simple chemical reduction using ammonium formate. We also discuss the chemical mechanism of silver particle and macroporous silver particle formation. The synthesized round type and coral-type porous silver particles were 20–50 μm and 30–150 μm in size and their pores were 100–200 nm and 1–2 μm across, respectively. They were characterized by particle distribution analysis, X-ray diffraction, and scanning electron microscopy.     

Formation and stabilization of submicron particles via rapid expansion processes

Submicron particles were produced by rapid expansion of supercritical solution into air (RESS) or an aqueous surfactant solution (RESSAS) to minimize particle growth and to prevent particle agglomeration. Thereby the effect of process conditions on the size of the particles precipitated was investigated. The obtained product was evaluated by measuring particle size by 3-wavelength extinction measurements, dynamic light scattering, specific surface areas by nitrogen gas adsorption, melting behaviour by differential scanning calorimetry, particle morphology by X-ray diffraction, scanning electron micrographs (SEM), and drug loading by high performance liquid chromatography.Prior to the particle formation experiments, the melting temperature of Salicylic acid under CO₂ pressure and the solubility of Salicylic acid in CO₂ were measured. The size of Salicylic acid particles produced via RESS decreased from 230 to 130 nm as the pre-expansion temperature decreased from 388 to 328 K and the specific surface area of the micronized particles was found to be up to 60 times higher than that of the unprocessed material. RESSAS experiments demonstrate that in 1 wt.% Tween 80 solutions Salicylic acid concentrations of 4.6 g/dm³ could be stabilized with particle diameters in the range of 180 nm. Additional experiments show that Ibuprofen nanoparticles with an average size of 80 nm and a drug concentration of 2.4 g/dm³ could be stabilized in 1 wt.% Tween^® 80 solutions. The use of a SDS solution instead of Tween^® 80 results in a stable aqueous suspension of phytosterol nanoparticles, where the average particle size is 50 nm at a drug concentration of 5.6 g/dm³.     

Micronization of salicylic acid and taxol (paclitaxel) by rapid expansion of supercritical fluids (RESS)

The particle sizes of the pharmaceutical substances are important for their bioavailability. The bioavailability can be improved by reducing the particle size of the drug. In this study, salicylic acid and taxol were micronized by the rapid expansion of supercritical fluids (RESS). Supercritical CO₂ and CO₂ + ethanol mixture were used as solvent. Experiments were carried out to investigate the effect of extraction temperature (318–333 K) and pressure (15–25 MPa), pre-expansion temperature (353–413 K), expansion chamber temperature (273–293 K), spray distance (6–13 cm), co-solvent concentration (ethanol, 1, 2, 3, v/v, %) and nozzle configuration (capillary and orifice nozzle) on the size and morphology of the precipitated salicylic acid particles. For taxol, the effects of extraction pressure (25, 30, 35 MPa) and co-solvent concentration (ethanol, 2, 5, 7, v/v, %) were investigated. The characterization of the particles was determined by scanning electron microscopy (SEM), optical microscopy, and LC–MS analysis.The particle size of the original salicylic acid particles was L/D: 171/29–34/14 μm/μm. Depending upon the different experimental conditions, smaller particles (L/D: 15.73/4.06 μm/μm) were obtained. The particle size of taxol like white crystal powders was reduced from 0.6–17 μm to 0.3–1.7 μm The results showed that the size of the precipitated salicylic acid and taxol particles were smaller than that of original particles and RESS parameters affect the particle size.     

Oxidation treatments for SiC particles and its compatibility with glass

Different thermal treatments were performed to produce a protective coating on the surface of SiC particles in order to allow their incorporation in a glass matrix. These oxidation treatments were carried out in air at different temperatures ranging from 800 °C to 1500 °C and different times at 1200 °C (10 min–48 h). The oxidation kinetics followed the Deal–Grove model and the thickness of the protective coating increased with temperature and SiC particle size. Protected SiC particles with different particle sizes were incorporated in a borosilicate glass. With small particles sizes foam glasses were obtained, whereas particles with higher grain size, i.e., higher coating thickness, were stable in the glass matrix and a smooth glass was obtained.     

Computational fluid dynamics simulations of submicrometer and micrometer particle deposition in the nasal passages of a Sprague-Dawley rat

Computational fluid dynamics (CFD) simulations were conducted in a model of the complete nasal passages of an adult male Sprague-Dawley rat to predict regional deposition patterns of inhaled particles in the size range of 1 nm to 10 μm. Steady-state inspiratory airflow rates of 185, 369, and 738 ml/min (equal to 50%, 100%, and 200% of the estimated minute volume during resting breathing) were simulated using Fluent?. The Lagrangian particle tracking method was used to calculate trajectories of individual particles that were passively released from the nostrils. Computational predictions of total nasal deposition compared well with experimental data from the literature when deposition fractions were plotted against the Stokes and Peclet numbers for micro- and nanoparticles, respectively. Regional deposition was assessed by computing deposition efficiency curves for major nasal epithelial cell types. For micrometer particles, maximum olfactory deposition was 27% and occurred at the lowest flow rate with a particle diameter of 7 μm. Maximum deposition on mucus-coated non-olfactory epithelium was 27% for 3.25 μm particles at the highest flow rate. For submicrometer particles, olfactory deposition reached a maximum of 20% with a particle size of 5 nm at the highest flow rate, whereas deposition on mucus-coated non-olfactory epithelium reached a peak of approximately 60% for 1–4 nm particles at all flow rates. These simulations show that regional particle deposition patterns are highly dependent on particle size and flow rate, indicating the importance of accurate quantification of deposition in the rat for extrapolation of results to humans.     

From nanometric zirconia powder to transparent polycrystal

Coprecipitated zirconia-yttria (8 mol%) gel subjected to hydrothermal treatment at 240 °C resulted in the solid solution powder of 8 nm particle sizes and specific surface area 132.7 m²/g. Uniaxial compaction followed by cold isostatic pressing under 300 MPa resulted in samples of the extremely small and narrow pore size distribution. Such samples start to shrink at about 200 °C which is related to the desorption of water layers surrounding particles. The state of closed porosity is achieved at 1150 °C. Pore closing was performed in air or oxygen atmosphere. Hot isostatic pressing at 1150 °C for 2 h under 250 MPa argon pressure led to transparent materials. Some pores remained in the material whose preliminary pore closure was performed in air. The samples initially sintered in oxygen atmosphere show no porosity and higher light transmittance than the former ones.