Yolk-shell

In particular, the Pd Si02 yolk-shell nanocatalyst activated by PhICl2 showed high reactivity and superior stability to the other Pd-based catalysts for hydroalkoxylation reactions even at 25 °C (14CC14938). 

Shi et al7 also designed a yolk-shell-type porous organic network with gold nanoparticles inside the cavities, which can be used as an efficient nanoreactor for the catalytic decomposition of tyclohetyl hydroperoxide. 

S. Shi, C. Chen, M. Wang, J. Ma, H. Ma and J. Xu, Designing a yolk-shell type porous organic network using a phenyl modified template, Chem. Commun., 2014, 50(65), 9079-9082. 

Besides the well-known allotropic species, diamond, graphite, and amorphous carbon, other forms are possible and were investigated, from fullerenes (Ceo being the most studied [15]) to single- or multiwalled carbon nanombes (SWCNT and MWCNT) [16], to the more recent graphene [17-19] and graphene oxide (GO) [20-22], Moreover, nowadays we can meet carbon nanodots [23,24], carbon nanoleaves [25], yolk-shell nanoparticles [26], nanorings [27], three-dimensional porous carbon materials [28], nanorods [29], or composite [30] and porous carbon [31] materials. 

A single Au nanoparticle inside the pNIPAM shell catalytic system, a so-called hybrid yolk-shell nanostructure, was synthesised as shown in Fig. 13.9 (Wu et al, 2012). This system showed catalytic activity in the reduction of 4-nitrophenol and nitrobenzene with NaBH, in which the selectivity of hydrogenation depended on temperature. The reduction of 4-nitrophenol was much faster at lower temperature, whereas nitrobenzene reacted faster at higher temperature. Both compounds are of similar size thus the changes in pNIPAM core conformation from a swollen to a shrunken state could not affect the diffusion of the reactants through the pNIPAM core. However, 4-nitrophenol is more hydrophilic than nitrobenzene. The interaction of the reactants with pNIPAM in its hydrophilic and hydrophobic state could be a reason for the temperature dependence of the selectivity of catalysis. 

Wu, S., Dzubiella, I, Kaiser, J., Drechsler, M., Guo, X., Ballauff, M. and Lu, Y. (2012). Thermosensitive Au-PNIPA yolk-shell nanoparticles with tunable selectivity for catalysis. Angewandte Chemie International Edition, 51,2229-2233. 

Fig. 5.1 Schematic representations of three types of stable nanocatalysts (a) core-shell and (b) yolk-shell nanostructures, (c) Supported catalysts covered with a shell layer...

Metal/Metal Oxide Yolk-Shell Nanocatalysts... 

The yolk-shell nanostructure is a modified form of the core-shell configuration. The yolk-shell structure is distinguished from the core-shell structure in that the former has a hollow inner space between the core and the shell layers. Compared to... 

Fig. 5.9 Plot of ln(Ct/Co) versus time for Au Si02 yolk-shell nanocatalysts with different core sizes in / -nitrophenol reduction reaction. Adapted from ref. [49]...

The Co Si02 yolk-shell structure was obtained by thermal reduction of CoO Si02 core-shell particles via volume contraction of CoO to Co. The Co Si02 yolk-shell nanocatalysts exhibited high activity and reusability for phenoxycarbonylation of iodobenzene. In addition, the magnetic property of the cobalt cores permitted facile separation of the catalysts from the products. 

An alternative method to the yolk-shell structure is the use of a sacrificial intermediate layer, which is etched away to generate hollow spaces. This general scheme was realized by Schiith and his co-workers using the Au Zr02 yolk-shell NPs as an example (Fig. 5.11) [55]. They first synthesized 15-17 nm Au NPs using sodium citrate as the reductant, followed by coating a silica layer on the individual Au... 

Fig. 5.11 Schematic representation and TEM images (left dark field middle and right bright field) of the products obtained after each step during the synthesis of Aut ZtOj yolk-shell NPs. Adapted from ref. [55]...

The construction of core- or yolk-shell nanoarchitectures relies on the encapsulation of an individual metal NP with a shell layer. As demonstrated in the above examples, these catalysts were effective as nanoscale model catalysts that show thermal and chemical stabilities, as well as enhanced catalytic activity and selectivity. Recent works demonstrated that the principles acquired from these model catalysts are applicable to more complex model catalytic systems, and even to industrial supported catalysts. 

In this review, recent progress in the synthesis of core-shell-stractured NPs for catalytic applications has been presented. Since the first realization of the nanoreactor concept, various versions of core-shell-structured nanocatalysts have emerged within a very short period of time, driven by advances in nanochemistry. Core-shell and yolk-shell nanocatalysts have shown enhanced thermal and chemical stability, thus preventing aggregation and sintering of small metal NPs. In core-shell (yolk-shell) structures where the shell layer was composed of metal oxides that show metal-support interaction, enhanced catalytic activity was observed. Furthermore, recent examples demonstrated that reactant size selectivity could be achieved by making shell layers with a crystalline, microporous MOF material. 

Yin Y, Rioux RM, Erdonmez CK, Hughes S, Somoijai GA, Alivisatos AP (2(X)4) Formation of hollow nanocrystals through the nanoscale KirkendaU effect. Science 304 711-714 Park JC, Song HJ (2010) Metal silica yolk-shell nanostructures as versatile bifunctional... 

Park JC, Bang JU, Lee J, Ko CH, Song H (2010) Ni Si02 yolk-shell nanoreactor catalysts high temperature stability and recyclahUity. J Mater Chem 20 1239-1246... 

Park JC, Lee HJ, Jung HS, Kim M, Kim HJ, Park KH, Song H (2011) Gram-scale synthesis of magnetically separable and recyclable Co Si02 yolk-shell nanocatalysts for phenoxycaibon-ylation reactions. ChemCatChem 3 755-760... 

Guttel R, Paul M, Schtith F (2010) Ex-post size control of high-temperature—stable yolk-shell Au, Zr02 catalysts. Chem Commun 46 895-897... 

Hollow inorganic particles have various potential applications such as in coatings catalysis, adsorption, and drug dehvery system (DDS) due to their imique structural features [90]. The porous shell provides diffusion pathways to their interior voids, which is particularly important for catalysis and drug dehvery. Fiuther-more, the yolk-shell structured particles containing functional cores inside the hollow interior are useful as nanoreactors for various catalytic reactions and multifunctional carriers for drug dehvery [91]. 

Yang, Y., Liu, J., Li, X., Liu, X., and Yang, Q. (2011) Organosilane-assisted transformation from core-shell to yolk-shell nanocomposites. Chem. Mater., 23, 3676-3684. 

Liu, J., Yang, H.Q., Kleitz, F., Chen, Z.G., Yang, T., Strounina, E., Lu, G.Q., and Qiao, S.Z. (2012) Yolk-shell hybrid materials with a periodic mesoporous organosilica shell ideal nanoreactors for selective alcohol oxidation. Adv. Funct Mater., 22, 591-599. 

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