Silica particles electron micrograph

Electron micrographs (scanning and transmission) showed that tungsten carbide is well dispersed on the surface of each support as nanosized particles (20 - 50 nm) as typified by the images in Figs. 3 (a b). However, BET surface area decreased in the order alumina > silica > titania > zirconia. With highest surface area obtained for each support being 240,133,18 and 9 m g respectively. 

We measured the dispersion of Pt (impregnated from a chloroplatinic acid precursor, calcined at 450 C and reduced at 500 C) on a series of Nd203-loaded silica-aluminas (Fig. 8). We find, unexpectedly, that dispersion increases with increasing rare earth oxide loading up to about 18% Nd203, where it plateaus at between 40 and 50%, compared to 10% with unmodified Si-Al. This compares with dispersions of -60-80% measured on similarly Pt-loaded transitional AI2O3 catalysts. Transmission electron micrographs confirmed the decrease in particle size with rare earth content on Si-Al. 

Fig. 3. Electron micrograph of 2.5% (w/w) platinum/silica catalyst. Prepared by impregnation with chloroplatinic acid, reduced in hydrogen at 210°C. Micrograph obtained by thin sectioning. The black dots are platinum particles. (X 100,000). Reproduced with permission from T. A. Dorling and R. L. Moss, J. Calal. 7, 378 (1967) R. L. Moss, Platinum Metals Rev. 11 (4), 1 (1967), and British Crown Copyright.

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

Figure 10. Electron micrograph of composite silica-polystyrene latex system,SPL(-), prepared by using bare silica particles as the seed.

Transmission electron microscopy (T.E.M.). electron micrographs of the silica particles were produced using an Hitachi HU11B apparatus. Particle size distributions were obtained from these using a Carl Zeiss particle size analyser. 

FIGURE 13.8 Scanning electron micrograph showing nonreticulated porous structures of standard spherical silica particles. 

Figure 7.1 Transmission electron micrographs of rhodium particles supported on silica spheres (from Datye and Long [7]). 

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

Fig. 2.1.7 Transmission electron micrograph of StOber silica particles.

Fig. 2.1.8 Transmission electron micrograph showing the internal structure of Stober silica particles. (From Ref. 51.)...

Fig. 2.1.12 Scanning electron micrograph, showing the ordered arrangement of silica particles after a slow sedimentation process. (From Ref. 26.)...

Crosslinked polyacrylamide latexes encapsulating microparticles of silica and alumina have also been prepared by this method [179], Three steps are involved a) formation of a stable colloidal dispersion of the inorganic particles in an aqueous solution containing acrylamide, crosslinker, dispersant, and initiator b) HIPE preparation with this aqueous solution as the dispersed phase and c) polymerisation. The latex particles are polyhedral in shape, shown clearly by excellent scanning electron micrographs, and have sizes of between 1 and 5 pm. 

Figure 25-5 Scanning electron micrographs of silica chromatography particles, (a) Aggregate of spherical particles with 50% porosity and a surface area of 150 m2/g. (b) Spongelike structure with 70% porosity and a surface area of 300 m2/g. Pores are the entryways into the interior of the particles. In both cases, the nominal pore size is 10 nm, but the distribution of pore sizes is greater in the spongelike structure. The spongelike structure also dissolves more readily in base. [From Hewlett-Packard Co. and R. Majors, LCGC May 1997, p. S8.J...

It is assumed that the small Pd particles are well-formed single crystals. This assumption is supported by electron-micrographic studies of deBoer and Coenen (12), who found that 50 100-A particles of nickel supported on silica had straight edges, indicating that the particles were either cubic or octahedral. 

Figure 8.1 Transmission electron micrograph of a PDMS network containing 34.4 wt % in situ precipitated silica particles.25 The average particle diameter is 150 A. Reproduced by permission of John Wiley and Sons.

Fig. 5.1. Scanning electron micrographs of continuous-bed columns prepared from ODS-silica particles packed in a capillary. Prepared from (a) 75 pm capillary packed with silica particles by sintering in the presence of NaHC03 [9], and (b) large pore (left) and small-pore (right) ODS particles by a sol-gel method [10]. Reproduced from refs. 9 and 10, with permission.

Figure 1.4 in Chapter 1 shows a scanning electron micrograph (SEM) of typical irregular and spherical silica particles for comparison. 

The tobermorite obtained in the hydration of tricalcium silicate (Ca3Si02), / -dicalcium silicate (/ -Ca2Si04), portland cement, and concrete is a colloid, with a specific surface area of the order of 300 sq. meters per gram. To give an idea of how the elementary particles of tobermorite look, Figure 7 is an electron micrograph of a few particles (obtained by L. E. Copeland and Edith G. Schulz at the Portland Cement Association Research and Development Laboratories). These particles look like fibers, but if you watch them closely, you see that they are very thin sheets, rolled up as one would roll up a sheet of paper. At the lower end the sheets are partly unrolled. When one prepares tobermorite by the reaction of lime and silica, one usually obtains crumpled sheets, which are not rolled up. The electron microscopists tell us that the sheets are very thin, of the order of a single unit cell in thickness. 

The structure of these pyrogenic silicas has been discussed by Barby [5], particularly with reference to their specific surface area. It was concluded that the initially condensed particles are only about 1 rnn in diameter and that these are so closely packed (high coordination number) to secondary particles of 10 to 30 nm that only a small amount of nitrogen can penetrate the micropores between them. Thus the secondary particles are the ones that are commonly identified in electron micrographs and which determine the specific surface area. They are the primary particles in the voluminous aggregate structure and have a low coordination number of about 3 (see Fig. 8.3). Because of the low level of impurities this type of silica is often used as catalyst support in fundamental studies. 

Figure 3. Scanning electron micrographs of catalyst microspheres before and after leaching with acids, (a) Commercial MCM powder made with ca. 50% colloidal silica 22 nm particle size, (b) Same powder after acid leaching the active metal oxide components. Continued on next page.

Figure 1 Scanning transmission electron micrograph of a Pd-silica composite formed via an organically modified gel. The average particle size is 3.8nm, and the distribution range is 2.8-5.2nm. (Reprinted with permission from Ref. 10. 1991 American Chemical Society)...

Figure 11. Electron micrograph of a PDMS network containing in situ precipitated silica particles. (Reproduced with permission from reference 55. Copyright 1984 Wiley.)...

Fig. 23. Transmission electron micrograph showing silica particles associated with phospholipid vesicles. Bar = 100 nm.

A fascinating example of a support effect is seen in the case of silica-supported Ru-Cu and Os-Cu catalysts prepared by Sinfelt and co-workers. From examinations of electron micrographs, these authors claim that a proportion, sometimes a high proportion, of the metal particles is present as... 

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