Silica particles, high surface area

High Surface Area Silica Particles as a New Vehicle for Ligand Immobilization on the Quartz Crystal Microbalance... 

This type of surface modification, if adequate functional groups are used, can be useful in incorporating silica particles into polymeric matrices (e.g., into rubber for tires) or in increasing the hydrolytic stability of high-surface-area silica (e.g., that used for membranes). Surface modification of silica is a very important principle and is widely commercialized. 

CAS 112945-52-5 EINECS/ELINCS 231-545-4 Synonyms Amorphous silica dust Cl 77711 Colloidal silica Colloidal silicon dioxide Eossil flour Eumed silica Fumed silicon dioxide Pigment white 27 Silica, amorphous Silica, amorphous fumed Silica, pyrogenic Silicic anhydride Silicon dioxide Definition High surface area aggregate particles of silica, with min. 89.5% SiOj content Empirical OjSi Formula SiOj... 

Definition High surface area aggregate particles of silica, with min. 89.5% Si02 content Empirical 02Si Formula Si02... 

Fillers, especially high surface area silicas, are commercially available with such hydrophobic surface coatings. They are expensive and seem to be mainly used in sealant and adhesive applications rather than in polymer composites, where they produce low particle matrix adhesion similar to fatty acid and other cheaper additives. 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

Figure 4.1. Supported catalyst, consisting of small particles on a high surface area carrier such as silica or alumina, along with two simplified model systems, which in general offer much better opportunities for characterization at the molecular level.

Pt particles remain highly dispersed in the reaction mixture during mesostructure formation. All measurements including XRD, SAXS, and TEM indicate a well-ordered silica structure. N2 physisorption measurement indicated high surface areas (523-661 m g ) and meso-sized pores (112-113 A) for the silica supports produced in the presence of different Pt particles. 

Supported metal catalysts are used in a large number of commercially important processes for chemical and pharmaceutical production, pollution control and abatement, and energy production. In order to maximize catalytic activity it is necessary in most cases to synthesize small metal crystallites, typically less than about 1 to 10 nm, anchored to a thermally stable, high-surface-area support such as alumina, silica, or carbon. The efficiency of metal utilization is commonly defined as dispersion, which is the fraction of metal atoms at the surface of a metal particle (and thus available to interact with adsorbing reaction intermediates), divided by the total number of metal atoms. Metal dispersion and crystallite size are inversely proportional nanoparticles about 1 nm in diameter or smaller have dispersions of 100%, that is, every metal atom on the support is available for catalytic reaction, whereas particles of diameter 10 nm have dispersions of about 10%, with 90% of the metal unavailable for the reaction. 

Wade and Hackerman (302) measured the heats of immersion in water of both anatase and rutile as a function of particle size and outgassing temperature. Apart from the distinct influence of the particle size, a maximum in the heat of immersion was observed after outgassing at 300 to 350°, indicating a rehydroxylation reaction. This is similar to the behavior of silica. Whereas, with silica, the decrease at higher evacuation temperatures is caused by the slowness of the reopening of siloxane bonds (see Section III,A,2), it is very probably caused by a decrease in surface area in the case of TiOj. The maximum in the heat of immersion curves was distinct only with samples of high surface area. Stbber et al. (225) observed a decrease in the surface area of fine particle size anatase already at 450°. 

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