Precursor particles

The particle generation rate was calculated by a step mechanism, namely formation of primary precursor particles by homogeneous nucleation (JLQ.) followed by coagulation to latex particles (8-9). This homogeneous nucleation mechanism is often referred to as the HUFT mechanism for its originators Hansen, Ugelstad, Fitch, and Tsai. 

Particle Generation Rate. The particle generation rate was calculated from the concentration of k-fold precursor particles assuming Muller coagulation kinetics 2.) as well as including propagation terms. 

Number of k-fold Precursor Particles. Dynamic differential equations were written for the concentration of the k-fold precursors to account for birth and death by coagulation, growth by propagation, and the formation of primary precursors by homogeneous nucleation. There... 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

Besides supported (transition) metal catalysts, structure sensitivity can also be observed with bare (oxidic) support materials, too. In 2003, Hinrichsen et al. [39] investigated methanol synthesis at 30 bar and 300 °C over differently prepared zinc oxides, namely by precipitation, coprecipitation with alumina, and thermolysis of zinc siloxide precursor. Particle sizes, as determined by N2 physisorpt-ion and XRD, varied from 261 nm for a commercial material to 7.0 nm for the thermolytically obtained material. Plotting the areal rates against BET surface areas (Figure 3) reveals enhanced activity for the low surface area zinc... 

Effects of the initial pH on the selective deposition of the precursor particles were investigated. For the Ru, Rh, and Pd precipitates, their yields reached 100% at maximum, while the maximum yield of the Ir precipitate was somewhat lower than 100% and that of the Pt precipitate was still lower around 90%. The difference in the... 

Figure 8. Changes in the yield of the precursor particles of Pt in the presence (a) and absence (b) of the a-Fe203 ellipsoid (A) support.

These results suggest that the surfaces of the a-Fc203 ellipsoid support play the role of nucleation centers of the precursor particles. The large precursor particles of 20—50nm were formed by aging for 72 h in the absence of any support. The size distribution was relatively narrow and each particle consisted of much smaller particles of 2—3 nm. As a consequence, the support plays an important role in the formation of the well-dispersed precursor nanoclusters. 

The precursor particles of Pt, Pt02 H20, were tried to be deposited on hematite (a-Fe203) supports (a) polycrystalline ellipsoid (A), (b) monocrystalline ellipsoid (B), (c) monocrystalline pseudocube, and (d) monocrystalline platelet. Also, the precursor particles of Pt were tried to be formed on other supports other than a-Fe203 (a) a-FeOOH, (b) P-FeOOH, (c) Zr02 (A) with rough surfaces, (d) Zr02 (B) with smooth surfaces, and (e) Ti02 (anatase). The mean sizes and yield of the precursor particles are summarized in Table 2 with the specific surface area of the supports. 

The reverse cholesterol pathway is mediated by HDL. HDL is formed from precursor particles originating from the intestine and the liver. In addition, surface... 

Nover, L., Scharf, K. D., and Neumann, D. (1989). Cytoplasmic heat shock granules are formed from precursor particles and are associated with a specific set of mRNAs. Mol. Cell Biol. 9, 1298-1308. 

The internal surface area of a porous inorganic membrane is often significantly affected by the heat treatment temperature. Leenaars, Keizer and Burggraaf (1984) have shown that, even if the crystallite size of the membrane precursor particles remains essentially the same (from the X-ray line-broadening measurements), the surfaee area of a transition-phase alumina membrane decreases with increasing calcination temperature. Con-... 

Figure 4.12 Histogram from automated SEM-EDS particle analysis of 362 zeolite precursor particles. Particles were reacted from silica and alumina sources slowly combined and aged at (a) room temperature and (b) 75°C.

For condensable precursors, particle formation may occur by homogeneous or heterogeneous nucleation. It is generally accepted that heterogeneous processes are most likely in the atmosphere, because of the large number of nuclei present. 

Fig. 1.3.18 SF.M images of CuO precursor particles (a) and uniform polycrystalline spheres of Cu20 (b) prepared therefrom by phase transformation. (From Ref. 36.)...

Infrared spectra suggested that a sulfate ion coordinates to two titanium atoms as a bidentate in particles. The maximum particle size was found at Aerosol OT mole fraction of 0.35 in the mixtures. The particle size increased linearly with increasing the concentration of sulfuric acid at any Wo, but with increasing Wo the effect was the opposite at any sulfuric acid concentration. These effects on the particle size can be explained qualitatively in relation with the extent of number of sulfate ions per micelle droplet. These precursor particles yield amorphous and nanosized TiO particles, reduced by 15% in volume by washing of ammonia water. The Ti02 particles transformed from amorphous to anatase form at 400°C and from anatase form to rutile form about at 800°C. In Triton X-100-n-hexanol-cyclohexane systems, however, spherical and amorphous titanium hydroxide precursor were precipitated by hydrolysis of TiCl4 (30). When the precursor particles were calcinated,... 

Feeney et al. proposed a concept of precursor particle in which ultrafine particles named as precursor particles, of a diameter of a few nanometers, are first precipitated and then aggregate to become stable primary particles having a diameter of a... 

Nd2Ni04+5 powder was prepared by nitrate-citrate route as described by Courty, et a1. (1973). Stoichiometric amounts of neodymium and nickel oxides were dissolved in diluted nitric acid. After addition of a large excess of citric acid, the solution was dehydrated and heated until self-combustion of the precipitate to obtain submicronic precursor particles (Boehm, 2005). The final annealing was performed at 1000°C for 12 hours to obtain a single crystalline phase. The particles were then ball milled to obtain an average grain size (d0 5) of about 0.8 pm. A terpineol-based slurry was prepared from this powder and this was deposited on the electrolyte by screen printing and then sintered at 1100°C for three hours in air (Lalanne, 2008). 

Although typical catalyst preparation procedures vary slightly from one laboratory to another, the "conventional approach" is to deposit a metal salt on a support, convert this salt to the oxide, and then reduce to the metallic state. When two metals are simultaneously so treated and reduced, bimetallic clusters may form. However, it cannot be assumed that bimetallic clusters are produced since monometallic separate particles may predominate. In fact, it is perhaps the unusual case where bimetallics do form since there are many possible paths, both thermodynamic and kinetic, that can lead to (1) separate monometallic s, (2) one metal not reduced, (3) thermal segregation of bimetallic precursor particle, and/or (4) volatilization or migration of one metal. 

The growth of the disulfide nanotubes from the trisulfide precursors starts with the reduction of the trisulfide precursors to a nanoparticulate form on heating in a gas flow. As the temperature reaches the decomposition temperature of the trisulfide, a few layers of the disulfide are formed at the periphery of the trisulfide particle. The growth then proceeds from inside outwards as the trisulfide at the core decomposes to form the disulfide layers that starts projecting outwards. This mechanism is supported by the presence of the precursor particle at the tip of some of the nanostructures, the diameter of such particles... 

Krahl, J., Baum, K., Hackbarth, U., Jeberien, H.-E., Munack, A., Schtitt, C., Schroder, O., Walter, N., Btinger, J., Muller, M. M., and Weigel, A. 2001. Gaseous Compounds, Ozone Precursors, Particle Number and Particle Size Distributions, and Mutagenic Effects Due to Biodiesel. Trans. ASAE, 44,179-191. 

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