Catalysis core-shell

An important class of materials that originates from the precursor core-shell particles is hollow capsules. Hollow capsules (or shells ) can be routinely produced upon removal of the core material using chemical and physical methods. Much of the research conducted in the production of uniform-size hollow capsules arises from their scientific and technological interest. Hollow capsules are widely utilized for the encapsulation and controlled release of various substances (e.g., drugs, cosmetics, dyes, and inks), in catalysis and acoustic insulation, in the development of piezoelectric transducers and low-dielectric-constant materials, and for the manufacture of advanced materials [14],... 

The benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

Sequential hydroformylation/reductive amination of dendritic perallylated polyglycerols with various amines in a one-pot procedure to give dendritic polyamines in high yields (73-99%). Furthermore, the use of protected amines provides reactive core-shell-type architectures after deprotection. These soluble but membrane filterable multifunctional dendritic polyamines are of high interest as reagents in synthesis or as supports in homogeneous catalysis as well as nonviral vectors for DNA-transfection (Scheme 18) [65]. 

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

Figure 3.3.14 Experimental ORR activity of dealloyed Pt-Cu and Pt-Ni core-shell nanoparticle ORR catalysts compared to a pure-Pt nanoparticle catalyst. All three catalyst particles are supported on a high surface area carbon material indicated by the suffix 1C. The shift of the j-E curve of the core-shell catalysts indicates the onset of oxygen reduction catalysis at a more anodic electrode potential (equivalent to a lower overpotential) and hence represents improved ORR reactivity compared to pure Pt.

To inspect and compare the activation overvoltage of the three catalysts in more detail, so-called Tafel plots are used, which plot the cell voltage as a function of the logarithm of the current density. Figure 3.3.16B shows the Tafel plots derived from Figure 3.3.15A. At a cell voltage of 0.9 V, where the overall reaction rate is limited by the chemical surface catalysis, the dealloyed core-shell catalysts perform three... 

The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a multitude of phases containing the active metallic components.22 Of these many phases, a characteristic one is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catch-all term. A historical overview of the properties of alloys in connection with catalysis has been published by Ponec.23 At present, a... 

A very important concept pioneered in the 1970s is that of catalysis using two different metals such as Au and Pd in the same NP [15]. This idea has been beautifully developed by Toshima s group who used PVP to stabUize core-shell bimetallic Au-PdNPs, i.e. for instance NPs in which the core is Au whereas Pd atoms are located on the shell (Fig. 1.6) [16]. Subsequent to co-reduction, this structure is controlled by the order of reduction potentials of both ions and the coordination abilities of both atoms to PVP. The location of Au in the core and Pd on the shell was demonstrated by EXAFS, and it was shown that such heterobimetallic... 

Au-cored PdNPs are more active in catalysis than simple PVP-stabilized PdNPs. Thus, the Au core enhances the catalytic properties of PdNPs at the PdNP surface [5f,g]. Conversely, design strategies can lead to the opposite core-shell structure (Pd core, Au shell), and specific catalytic properties were obtained for methylacrylate hydrogenation [16]. 

Several approaches have recently been developed that directly apply natural architectures for artificial chanical reactions, some of which are detailed in different chapters of this book. Although not classified as homogeneous catalysis, the reduction of metal salts inside nanoreactors could be the first step on the way to reactivity with the corresponding metal coUoids or nanoparticles in e.g. hydrogenation reactions. A variety of carrier systems have been studied lately, including virus capsids, polymeric micelles, miniemulsions and hollow core-shell particles, as nanoreactors and hosts for the synthesis and encapsnlation of well-defined, stable nanoparticles. ... 

Here we have reviewed our recent studies on metallic nanoparticles encapsulated in spherical polyelectrolyte brushes and thermosensitive core-shell microgels, respectively. Both polymeric particles present excellent carrier systems for applications in catalysis. The composite systems of metallic nanoparticles and polymeric carrier particles allow us to do green chemistry and conduct chemical reactions in a very efficient way. Moreover, in the case of using microgels as the carrier system, the reactivity of composite particles can be adjusted by the volume transition within the thermosensitive networks. Hence, the present chapter gives clear indications on how carrier systems for metallic nanoparticles should be designed to adjust their catalytic activity. 

Inorganic nanoshell-coated organic polystyrene beads with well-defined nanostructures are attractive because of their applications in the fields of SERS, catalysis, biochemistry, and chemical sensors. The core-shell type composite materials are in the frontier of advanced research, in which the shell component determines the surface properties and the core component indirectly induces the other properties of the system. Bimetallic nanosheUs on functionalized polystyrene beads have been fabricated through a layer-by-layer deposition pathway involving the electrostatic interaction of the polystyrene moiety. 

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