Silica encapsulation

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

The acidic conditions of standard SBA-15 synthesis [35] cause the precipitation of metal nanoparticles without silica encapsulation, or the formation of amorphous silica due to the presence of the polymer used for nanoparticle synthesis. Therefore, the SBA-15 framework was synthesized under neutral condition using sodium fluoride as a hydrolysis catalyst and tetramethylorthosilicate (TMOS) as the silica precursor. Pt particles with different sizes were dispersed in the aqueous template polymer solution sodium fluoride and TMOS were added to the reaction mixture. The slurry aged at 313 K for a day, followed by an additional day at 373 K. Pt(X)/SBA-15-NE (X = 1.7, 2.9, 3.6, and 7.1nm) catalysts were obtained by ex-situ calcination (see Section 3.2). TEM images of the ordered... 

Figure 10 Size-dependence of the melting point and diffusion coefficient of silica-encapsulated gold particles. The dotted curve is calculated by the equation of Buffat and Borel. The bulk melting temperature of An is indicated by the double arrow as (oo). The solid curve (right-hand side axis) is a calculated An self-diffusion coefficient. (From Ref. 146.)...

The oxidation of various other alkenes using the optimal heterogeneous system (10% PEO-10% PPO) was compared to a typical homogeneous reaction using MeOH as solvent (data reported in Table 13). Compared to the homogeneous standard, the polyether tethered-silica-encapsulated catalysts show somewhat lower activity but much better selectivity. 

Miwa and Yamamoto (31) described a simple and rapid method with high accuracy and reliability for the determination of C8 0-C22 6 fatty acids, which occur in esterified forms in dietary fats and oils and in living cells [the biological effects of routinely consumed fats and oils are of wide interest because of their impact on human health and nutrition (28,29), in particular, the ratio of cu-3 polyunsaturated fatty acid to w-6 polyunsaturated fatty acids (w-3/cu-6) seems to be associated with atherosclerosis and breast and colon cancers (30)]. They report improved separation of 29 saturated and mono- and polyunsaturated fatty acids (C8-C22), including cis-trans isomers and double-bond positional isomers, as hydrazides formed by direct derivatization with 2-nitrophenylhydrazine hydrochloride (2-NPH HC1) of saponified samples without extraction. The column consisted of a J sphere ODS-M 80 column (particle size 4 /xm, 250 X 4.6-mm ID), packed closely with spherical silica encapsulated to reach a carbon content of about 14% with end-capped octadecyl-bonded-spherical silica (ODS), maintained at 50°C. The solvent system was acetonitrile-water (86 14, v/v) maintained at pH 4-5 by adding 0.1 M hydrochloric acid with a flow rate of 2.0 ml/min. Separation was performed within only 22 min by a simple isocratic elution (Fig. 6). The resolution of double-bond positional isomers, such as y-linolenic ( >-6) and a-linolenic acid ( >-3) hydrazides and w-9, >-12, and >-15 eicosenoic acid hydrazides was achieved by use of this column. 

M. I. Goller and B. Vincent, Silica encapsulation of PDMS droplets, Colloid Surf. A 142, 281-285 (1998). 

A dielectric oxide layer (eg. silica) is useful as a shell material because of the stability it lends to the core and its optical transparency. The classic method of Stober for solution deposition of silica are adaptable for coating of nanocrystals with silica shells.111 This method relies on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticlcs including hematite and magnetite111 Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparlides is that due to Mulvaney and co-workers.114 In this method, the surface of the nanoparticles is functionalized with aminopropyltrimethylsilane. a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Fig. 13 shows the TEM images... 

A limitation in using bismuth vanadate is the care needed in dispersing the pigment. As with other silica-encapsulated pigments, even partial shearing of the encapsulant will noticeably diminish thermal stability and durability. 

Ultramarine blues, reds and violets are acid sensitive. As such, their thermal and weathering properties are adversely affected in acidic media. Silica-encapsulated versions offer marked improvement in thermal stability and some improvement in weathering properties. The treated versions allow broad polymer usage, even in engineering plastics such as polycarbonate, and provide sufficient durability for automotive interior applications. Weatherability is highly dependent upon the degree of exposure to acid or alkaline conditions. 

Cerium sulfide pigments are produced from hydrated cerium oxide or oxalate and calcined in an oxygen-free, sulfide environment. They are silica-encapsulated to minimize water-reactivity and to improve heat stability and chemical resistance properties. Because of their low relative value-in-use, they are used primarily in engineering plastics and in particular the polyamides where high-performance organic colorant alternatives and other inorganic pigment alternatives are few. 

The PVC Emulsion types (containing up to 2.5% emulsifier) and those containing filler require a higher pigment loading because of their opacity. Since the emulsifiers are mostly alkaline, it is recommended to use silica-encapsulated metallic pigments to ensure consistent quality of the final product. 

FIGURE 84 (A) TEM image of silica encapsulated Y202S Yb,Er particles (B) Emission and... 

Kiistner B, Gellner M, Schiitz M, Schdppler F, Marx A, Strdbel P, Adam P, Schmuck C, Schliicker S (2009) SERS labels for red laser excitation silica-encapsulated SAMs on tunable gold/silver nanoshells. Angew Chem Inter Ed 48 1950... 

An example is washcoating of platinum and silica using a sol containing both a silica and a platinum precursor [58,59]. After calcination a catalyst containing platinum on silica-washcoated monolith is obtained. Since the platinum precursor is homogeneously mixed with silica, encapsulation of platinum may occur, which reduces the effectiveness of platinum as compared to that of a two-step preparation. In this respect, the method may be more suitable for less expensive active phases. 

Figure 1. Silica Encapsulated Metal Hydride Results...

NOVEL SILICA ENCAPSULATED METALLIC NANOPARTICLES ON ALUMINA... 

Novel silica encapsulated metallic nanoparticles on alumina... 

Dick, K., Dhanasekaran, T., Zhang, Z., and D. Meisel. 2002. Size-dependent melting of silica-encapsulated gold nanoparticles. Journal of the American Chemical Society 124 2312-2317. 

In the 1970s, after the expiration of Iler s dense silica patent, major pigment manufacturers used the silica encapsulation technology and added R-960 counterparts to their product lines. 

Aluminum pigments, silica encapsulated, (PCR-type) PVC, powder coating, thermoplastic resins, thermosetting resins Improved chemical resistance and outdoor durability... 

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