Shell type particles

Particulate Development Core and Shell Type Particles... 

One approach for preparing carbon supported, non-noble metal core, Pt shell type particles is depicted in Fig. 9.11. First, a bulk alloy of Co(Ni)Au(Pd) is formed on the carbon support by reduction of the metal salt precursors. Surface segregation of the noble metal is achieved by hydrogen treatment at temperatures between 600 and 850°C. After this, a Cu monolayer is deposited at underpotential (Cu UPD) and displaced by Pt atoms. °... 

Preparation of magnetic field sensitive core-shell type particles... 

However, there may be good reasons why a catalyst should not consist of particles that are too small, as we saw in the beginning of this chapter, e.g. to avoid pressure gradients in the reactor. Based on an analysis such as the above, one can decide whether it makes sense to use support particles that contain a homogeneous distribution of the catalytic phase. With expensive noble metals, one might perhaps decide to use an egg-shell type of arrangement, where the noble metal is only present on the outside of the particles. 

Similarly, Pd, Ag, and Pd-Ag nanoclusters on alumina have been prepared by the polyol method [230]. Dend-rimer encapsulated metal nanoclusters can be obtained by the thermal degradation of the organic dendrimers [368]. If salts of different metals are reduced one after the other in the presence of a support, core-shell type metallic particles are produced. In this case the presence of the support is vital for the success of the preparation. For example, the stepwise reduction of Cu and Pt salts in the presence of a conductive carbon support (Vulcan XC 72) generates copper nanoparticles (6-8 nm) that are coated with smaller particles of Pt (1-2 nm). This system has been found to be a powerful electrocatalyst which exhibits improved CO tolerance combined with high electrocatalytic efficiency. For details see Section 3.7 [53,369]. 

We have extended the seed-mediated technique for the synthesis of bimetallic nanoparticles, having core-shell type structure appending photoreduction of metal ions. It has been proved that the deposition of a less noble metal (M) as shell on a preformed nobler nanoparticle core (M ) seems to be very effective by UV activation. Using this seed-mediated method we were able to synthesize Aucore Agsheii particles. First for the preparation of gold seeds (S), TX-lOO (10 M) and HAuC (5.0 x 10 %) were taken in a quartz cuvette so that the final concentration of Au(III) ion remained 5.0 x 10 M. Then the... 

Tamura M, Ichinohe S, Tamura A, Ikeda Y, Nagasaki Y (2011) In vivo and in vitro characteristics of core-shell-type nanogel particles optimization of core cross-linking density and surface PEG density in PEGylated nanogels. Acta Biomaterialia. doi 10.1016/j. actbio.2011.05.027... 

Steam reforming is a heterogeneously catalyzed process, with nickel catalyst deposited throughout a preformed porous support. It is empirically observed in the industry, that conversion is proportional to the geometric surface area of the catalyst particles, rather than the internal pore area. This suggests that the particle behaves as an egg-shell type, as if all the catalytic activity were confined to a thin layer at the external surface. It has been demonstrated by conventional reaction-diffusion particle modelling that this behaviour is due to... 

FBE reactor (Originally AKZO) Billiton Research b.v. Fluidized bed of metal particles in a tube-and-shell type geometry Continuous via withdrawal of grown particles Yes V V V... 

Maye et al. studied gold nanoparticles supported on carbon black for ORR in both acidic and alkaline media.210 The gold nanoparticles were of a core-shell type where the particle consisted of a gold nanocrystal core of 1-6 nm in diameter that was surrounded by an organic monolayer shell.214 While the Au/C catalyst was found to be active for ORR, its activity was much lower than that of Pt/C, PtRu/C and AuPt/C. The electron transfer in 0.5 M H2S04 was reported as 2.9 0.2, indicating a mixed reduction pathway.210... 

This technique was successfully used to monitor the deposition of catalyst within a support pellet and clearly distinguish the amount of penetration there was into the pellet. Therefore, one can quickly obtain a time required to produce a catalyst pellet that is of the egg-shell type or completely saturated. Moreover, it is possible to very finely resolve the thickness of the catalyst penetration into the particle, resulting in good control of the finished catalyst. 

Composite core-shell type microspheres were prepared by in situ heterogeneous polymerization on monodispersed seed latex particles suspended in an aqueous magnetite dispersion stabilized with sodium oleate (58). 

In the literature a difference is made between matrix encapsulation and true encapsulation. In matrix encapsulation the resulting particles are more correctly described as aggregates of actives in a matrix material. A significant portion of the active is lying on the surface of the particles. True encapsulation is used for processes leading to core-shell-type products. However, this distinction of true and matrix is prone to argumentation. 

Formation of composite and sandwich-type semiconductors constitutes an interesting development (Fig. 100) [576,605-609]. A composite semiconductor, like a cherry and its stone, contains one material as its core and a second material as its shell. CdS particles coated by Cd(OH)2, CdSe coated by ZnS, and HgS coated by CdS are examples of recently prepared composite semiconductors... 

Like many polymers, the fluidity or flow (rheological) properties of denrimers are of interest (Box 14.1). Dendrimers behave like soft, spherical particles, surrounded by a hard surface shell and hence are often described as core-shell type bodies. Hence the interior of dendrimers can be deformed according to the type of linkage and building blocks and my contain voids suitable for encapsulating guest species.2... 

PdAu/Ti02178 and PdAu/Fe203179 (Pd Au = 2) have been prepared by co-impregnation of the chloride precursors. After calcination at 673 K, both samples exhibit metal particles with a bimodal distribution of size (4-10 nm and 30-70 nm). These particles, at least the largest ones, are core-shell type alloy with a Pd-rich shell. 

Recently, core-shell type microgels, which contain a hydrophobic core and a hydrophilic thermosensitive shell, have become attractive for scientists because such systems can combine the properties characteristic of both the core and the shell [53], We have prepared core-shell microgel particles consisting of a poly(styrene) core onto which a shell of polyCA-isopropylacrylamide) (PS-PNIPA) has been affixed in a seeded emulsion polymerization [54-56], In this case, the ends of the crosslinked PNIPA chains are fixed to a solid core, which defines a solid boundary of the network. In this respect, these core-shell latex particles present crosslinked polymer brushes on defined spherical surfaces. The solvent quality can be changed from good solvent conditions at room temperature to poor solvent conditions at a temperature... 

A configuration in the independent-particle model may be of either the closed-shell or open-shell type. In the former the N electrons occupy all the orbitals of the lowest-energy sets with the same symmetry and principal quantum number , called shells. In the latter some orbitals with particular values of the projection quantum numbers are unoccupied. The... 

Bimetallic nanomaterials Currently used monometallic SERS tags such as gold and silver NPs have a main drawback in that the metal surface is unprotected. The assays are therefore unreliable as components of the analyte can adsorb on the metal surface, leading to the possibility of replacing the label species. One way to overcome this problem is to encapsulate the metal particle/label molecule in a protective shell. Bimetallic nanostructures, with two different metals in a single particle, such as core-shell-type nanomaterials are gaining importance as Raman tags [1]. 

The first question that must be addressed concerns the stability of the aqueous complex (hereafter referred to as the aquaion). The stability, though usually thought of in a temporal sense, is related, albeit indirectly, to the real space structure at the level of the partial radial distribution functions ga r). This measures the probability of finding a /3-type particle at a distance r from an a-type particle placed at the origin. To understand gap r) quantitatively, consider an a-type particle at the origin and ask what is the average number of /3-type particles that occupy a spherical shell of radius r and thickness dr at an instant of time. That number is given by... 

Particles derived from the evaporation of solution droplets are spheroidal. Shape (primarily in surface features), density, and size control of particles can be achieved by the appropriate selection of the compound, the concentration of the solution, the size of the droplet generated, and the conditions for the evaporation of the droplets. Fast evaporation rates tend to produce less solid and rough-surface particles, but this is tempered by the chemical properties of the compound. Smooth, spherical particles call for compounds with high solubility and slow evaporation rates. These requirements were used by Vanderpool and Rubow [48] to produce solid, smooth spheres of up to 70 pm in diameter. The different types of particles that can be produced from the evaporation of solution droplets include solid spheres with surfaces that are smooth, cracked, or wrinkled hollow spheres, shells, and spheroidal particles that have a wrinkled surface like raisins porous-type particles that are perforated with holes, and single crystals and particles composed of several crystals, which may be angular or spheroidal in shape. 

We argue that the above features of star dynamics are generic for soft systems of the core-shell type for which stars serve as prototype. Support for this comes from the dynamic light scattering (DLS) investigation of large block copolymer micelles, where all three relaxation modes, i.e., cooperative, structural and selfdiffusion are observed [188]. In particular, the star model discussed above applies to core-shell particles with a small spherical core relative to the chain (shell) dimensions. For a surface number density a = f / (47i r ) the polymer layer thickness under good solvent conditions is L ... 

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