Encapsulated Metal Nanoparticles Research Articles

Polymer Nanoreactor with “Mobility‐Recalling” Domains for On/Off Switchable Catalysis

AbstractHow to furnish catalysts with switchable catalytic ability is a challenge in self‐controlled catalysis. This issue was addressed by constructing a unique polymer nanoreactor containing “mobility‐recalling” switchable domains that acted as a molecular switch for providing access to the encapsulated metal nanoparticles. This polymer reactor revealed poor reactivity at relatively low temperatures because of the “frozen” molecular chains in the switchable domains inhibiting the access of substrate to the encapsulated metal nanoparticles (i.e., catalytic “off” status). In contrast, the polymer reactor demonstrated significant reactivity at relatively high temperatures, resulting from the dramatically increased mobility of the molecular chains, which allowed access to the encapsulated nickel nanoparticles (i.e., catalytic “on” status). Switching between the catalytic off and on statuses could be repeated owing to the recallable mobility of molecular chains in the switchable domains. This protocol opens up the opportunity to develop smart nanoreactors for controlled chemical processes.

Advances in Porous Organic Catalysis

Porous organic polymers (POPs) have become one of a frontier of the research in recent years. POPs including amorphous (e.g. CMP, HCP, PIM, etc.) and crystalline (e.g. COF) porous organic polymers. Due to their inherent porosity, large specific surface area, light Weight and easy functionalization at the molecular level, POPs have recently received significant attention for potential applications in gas storage/separation, organic photoelectric, sensoring and heterogeneous catalysis. Here, this review focus on recent developments of POPs in heterogeneous catalysis. Currently, the research on the application of POPs for heterogeneous catalysis is classified into three sections: (a) bottom-up embedding metal-ligand catalyst into POPs for heterogeneous catalysis; (b) the encapsulation of metal nanoparticles into POPs for heterogeneous catalysis; (c) bottom-up embedding organocatalyst into POPs for heterogeneous organocatalysis. Benefiting from its structural superiority, these functional POPs exhibit excellent catalytic activity. Reference to the development of homogeneous catalysis, the application of functional POPs for heterogeneous catalysis will also have more room for development.

An autonomic and “off–on–off”-switchable polymer microreactor

An autonomic “off–on–off” polymer microreactor containing the switchable domains of molecular chains is reported. The switching of molecular chains acted as a molecular switch for regulating access to the encapsulated metal nanoparticles.

Catalytic polymer reactor with “self-sorting” domains for hierarchical catalysis

Hierarchical catalysis was realized by using a polymer reactor containing self-sorting domains, which acted as a switch for providing sequenced access to the encapsulated metal nanoparticles.

Hollow metal-organic framework nanospheres via emulsion-based interfacial synthesis and their application in size-selective catalysis.

Metal-organic frameworks (MOFs) represent an emerging class of crystalline materials with well-defined pore structures and hold great potentials in a wide range of important applications. The functionality of MOFs can be further extended by integration with other functional materials, e.g., encapsulating metal nanoparticles, to form hybrid materials with novel properties. In spite of various synthetic approaches that have been developed recently, a facile method to prepare hierarchical hollow MOF nanostructures still remains a challenge. Here we describe a facile emulsion-based interfacial reaction method for the large-scale synthesis of hollow zeolitic imidazolate framework 8 (ZIF-8) nanospheres with controllable shell thickness. We further demonstrate that functional metal nanoparticles such as Pd nanocubes can be encapsulated during the emulsification process and used for heterogeneous catalysis. The inherently porous structure of ZIF-8 shells enables encapsulated catalysts to show size-selective hydrogenation reactions.

A Catalytic and Shape‐Memory Polymer Reactor

An originally designed catalytic and shape‐memory polymer reactor is reported. This reactor is made of a unique shape‐switchable polymer composed of a thermosensitive control layer and an inert substrate layer. With the inert substrate layer made of poly(acrylamide), the thermosensitive control layer consists of nickel nanoparticles and a smart polymer composite of poly(1‐vinylimidazole) (PVIm) and poly(acrylic acid) (PAAc) that exhibit switchable domains. The self‐healing and dissociation between PVIm and PAAc induce convex/concave‐switchable shapes in the resulting reactor, which cause tunable access to the encapsulated metal nanoparticles. In this way, this reactor demonstrates tunable catalytic ability. Unlike reported smart polymer reactors exhibiting tunable catalysis usually due to the thermal phase transition of poly(N‐isopropylacrylamide) (PNIPAm), this novel reactor adopts the shape‐switchable strategy for tunable catalysis. This novel design suggests a new protocol for the development of smart catalytic reactors, which opens new opportunities for controlled chemical processes.

キラルBINOL基を有するポリチオフェンナノチューブの合成と金ナノ粒子の包接

We report the preparation of chiral polymer nanotubes bearing terthiophene and BINOL units [(S)-TBOH-PNTs] and the encapsulation of metal nanoparticles (NPs) therein. The monomer, (S)-TBOH, was synthesized by coventional methods and electrochemically polymerized using porous alumina with a pore diameter of 200 nm on a platinum electrode. The electrolyte solution contained 7mM monomer [(S)-TBOH] and 0.1M Bu4NPF6 in CH2Cl2. A voltage of 1.00 V was applied for polymerization. The porous alumina was removed by dissolving in NaOH. The UV-vis spectrum of the PNTs in C2HCl2 exhibited a broad absorption peak at 425 nm which is ascribed to the polythiophene. The CD spectra of (S)-TBOH-PNTs and (R)-TBOH-PNTs, show intense Cotton effects and a mirror-image relationship. The PNTs and the encapsulated Au NPs in PNTs (Au NPs@(S)-TBOH-PNTs) were characterized by SEM, TEM and EDX analysis.

Self-switchable catalysis by a nature-inspired polymer nanoreactor containing Pt nanoparticles

The proposed nanoreactor was made of Pt nanoparticles and a unique polymer composite of PVI and PTFMA. The self-healing and dissociation of the PVI–PTFMA interaction regulated access to the encapsulated metal nanoparticles, thereby causing self-switchable catalytic ability.

Colloidal stability evolution and completely reversible aggregation of gold nanoparticles functionalized with rationally designed free radical initiators

A series of molecular adsorbates having various chain lengths of terminal poly(ethylene glycol methyl ether) (PEG) moieties, thiol head groups, and intervening free radical initiator moieties was used to functionalize the surface of gold nanoparticles (AuNPs). The bulky PEG groups stabilized the functionalized AuNPs by providing steric hindrance against AuNP aggregation, such aggregation being a major problem in the modification and manipulation of metal nanoparticles. UV–vis spectroscopy was used to evaluate the stability of the adsorbate-functionalized AuNPs as a function of AuNP size (∼15, 40, and 90 nm in diameter) and PEG chain length (Mn 350, 750, and 2,000). The longer PEG chains (Mn 750 and 2,000) afforded stability to AuNPs with smaller gold cores (∼15 and 40 nm in diameter) for up to several days without any marked aggregation. In contrast, the adsorbate-functionalized AuNPs with the largest gold cores (∼90 nm) were noticeably less stable than those with the smaller gold cores. Importantly, the adsorbate-functionalized AuNPs could be isolated in solvent-free “dried” form and readily dispersed in aqueous buffer solution (both acidic and basic) and various organic solvents (protic and aprotic). This isolation–redispersion (i.e., aggregation/deaggregation) process was completely reversible. The chemisorption of the PEG-terminated initiator on the surface of the AuNPs was verified by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). As a whole, the strategy reported here affords colloidally stable, free radical initiator-functionalized AuNPs and offers a promising general method for encapsulating metal nanoparticles within polymer shells.

In Situ Monitoring the Role of Working Metal Catalyst Nanoparticles for Ultrahigh Purity Single‐Walled Carbon Nanotubes

The high‐end applications of single‐walled carbon nanotubes (SWCNTs) are hindered by the existence of large amount of impurities, especially the graphene layers encapsulating metal nanoparticles (metal@C NPs). The role of working metal catalysts during chemical vapor deposition (CVD) growth and post purifications by oxidation are not yet fully understood. Herein, the in situ monitoring the role of working metal catalyst NPs for ultrahigh purity SWCNTs by CVD growth and CO2 purifications is carried out in an online thermogravimetric reactor attached with a mass spectrometer. The growth of SWCNTs almost stops after the initial 2 min, then, the mass increase of the samples mainly originates from the metal@CNP formation. Therefore, high‐purity SWCNTs (98.5 wt%) with few metal@CNPs can be available by 2 min CVD growth. Furthermore, CO2 oxidation of the SWCNTs is also investigated in a thermogravimetric reactor. The oxidation of graphene layers surrounding the metal NPs and the SWCNTs occurs during distinct temperature ranges, which is further demonstrated by the significant differences among their oxidation activation energies. Ultrahigh purity of SWNCT with a carbon content of 99.5 wt% can be available by a CO2‐assited purification method. The in situ study of the CVD growth and CO2 oxidation of SWCNTs provides the real time information on the working catalyst during reaction and the reactivity information of metal@CNPs and SWCNTs under an oxidizing atmosphere. The success for the preparation of high‐purity SWCNT lies in the efficient growth of SWCNTs with a low amount of nanocarbon impurities and partial oxidation of metal@CNPs by catalytic CO2 oxidation with proper operation parameters.

Combustion synthesis: Novel routes to novel molecular nanomaterials

Combustion synthesis in a mode of electrothermal explosion was used to synthesize 1D carbon nanostructures by magnesium reduction of various carbonates [Li2CO3, Na2CO3, CaCO3, FeCO3, (NH4)2CO3] either in argon or in air, at an ambient pressure of 0.1 or 1.0 MPa. At the same time, several possible catalysts were tried, such as Fe, Co, Ni, Pd, Nd, Ta, Ti, Nb, W, and NiO powders. The Na2CO3 system proved to be the most promising for fabrication of fibrous products. On the other hand, our synthesis of carbon encapsulated metal nanoparticles opens up possibilities for new biomedical applications.

MOFs as Multifunctional Catalysts: Synthesis of Secondary Arylamines, Quinolines, Pyrroles, and Arylpyrrolidines over Bifunctional MIL‐101

AbstractBifunctional MIL‐101 MOFs containing Lewis acid Cr3+ sites and Pd or Pt hydrogenation/reduction centers, either as isolated metal complexes or in the form of encapsulated metal nanoparticles, have shown to be highly active catalysts for the one‐pot nitroarene reduction and reductive amination of carbonyl compounds. This preparation procedure has been successfully applied to the synthesis of secondary arylamines, quinolines, pyrrols, and 3‐arylpyrrolidines. In all the cases, the MOFs have shown superior performances with respect to commercially available Pd and Pt metal catalysts under the same conditions.

A novel high efficiency method for the synthesis of graphite encapsulated metal (GEM) nanoparticles

Graphite encapsulated metal (GEM) nanoparticle is a composite material with a core-shell structure; it can retain its novel properties even under severe environments. In recent years, many researchers have dedicated their efforts to improving the synthesizing efficiency of GEM by the arc-discharge method; although some progress has been made, the results remain unsatisfactory. To solve the problem, and under the guidance of the two-step mechanism model, it is clear that the carbon-to-metal ratio within the coalescence region of the arc-discharge must be effectively controlled. Thus, an injection of liquid ethanol was used as the carbon source to replace the synthetic diamond powder; the results showed that the encapsulation efficiency of ferromagnetic GEM was dramatically increased, i.e., 70–80% for Ni-GEM and Co-GEM, and for Fe-GEM it was quadrupled to about 50%. Furthermore, high encapsulation efficiency also prevents the common problem of spontaneous combustion of the as-made Fe-GEM powders. Some evidence as well as observations, indicates that the ethanol jet provides a large quantity of carbon in the arc-discharge area in a relatively short time. Although the detailed mechanism of this phenomenon is not well-understood, it shows that the production rate could be increased by a factor of ten or higher than it was previously. In this study, a simple and novel liquid ethanol injecting method has been developed; experiments have also demonstrated that this is an effective method for the synthesis of ferromagnetic GEM nanoparticles.

Entrapment of metal nanoparticles within nanocages of mesoporous silicas aided by co-surfactants

We demonstrated an efficient approach for in situ encapsulation of metal nanoparticles (NPs) into nanocages of mesoporous silicas using a co-surfactant as the transportation medium. By using this approach, colloidal Pt and Au NPs were successfully encapsulated into the nanocages of cagelike mesoporous silicas (CMS) and FDU-12 using co-surfactants butanol and mesitylene (TMB) as the transportation medium, respectively. The prepared Pt/CMS catalysts are active and highly selective in the hydrogenation of chloronitrobenzene (CNB) to the corresponding chloroaniline (CAN). The catalyst prepared by the in situ method is superior to the catalyst prepared by the traditional solution impregnation method. This work provides a new approach for the synthesis of the mesoporous materials with encapsulated metal NPs.

A new crucible design of the arc-discharge method for the synthesis of graphite encapsulated metal (GEM) nanoparticles

Graphite encapsulated metal (GEM) nanocrystal is a core-shell structured nanomaterial which is capable of surviving in severe environments, such as strong-acid or strongly-oxidizing conditions. Many researches have proven GEM to be useful for its potential multiple applications. However, there is a serious problem: its low production rate hinders future research on possible applications. Although the tungsten arc-discharge method produces more GEM nanoparticles than all the other methods do, the production rate remains unsatisfactory. Based on a previous study, it is known that the temperature gradient in the molten liquid metal pool during arcing is inadequate, i.e., the bottom is cooler than its surroundings, thus preventing the occurrence of convection and the homogeneous evaporation process. In this study, a new crucible design, with a thermal insulating alumina piece at the bottom of the graphite crucible, effectively modifies the temperature gradient inside the molten liquid pool. As a result, the as-made production rate approximately quadruples. This work demonstrates, as attested by experimental evidence, how an alumina piece can effectively change the temperature profile in the crucible and facilitate the synthesis of GEM in a tungsten arc-discharge system.

Synthesis of carbon encapsulated metal (Ni and Cu) nano particles and applications for chiral catalysts

Carbon-encapsulated Ni and Cu nanoparticles with a core/shell structure were synthesized by levitational gas condensation (LGC). Methane (CH4) gas was used to coat the surface of the Ni and Cu nanoparticles. The Ni particles had a core diameter of 10 nm, and were covered by 2–3 nm thin carbon layers. In the case of Cu, the particles had a core diameter of 30 nm, and multi-shells with a thickness of 2–3 nm. Biginelli reaction in the presence of l-proline and Ni encapsulated nanoparticles was carried out to change the ratio between stereoisomers in favor of the S-enantiomer for 3,4–dihydropyrimidine (DHPM) with an excess of about 18%.

ChemInform Abstract: Surface‐Protected Etching of Mesoporous Oxide Shells for the Stabilization of Metal Nanocatalysts

AbstractReview: 170 refs.

Surface‐Protected Etching of Mesoporous Oxide Shells for the Stabilization of Metal Nanocatalysts

AbstractNanoparticles of transition metals, particularly noble metals, are widely used in catalysis. However, enhancing their stability during catalytic reactions has been a challenge that has limited the full use of the benefits associated with their small size. In this Feature Article, a general “encapsulation and etching” strategy for the fabrication of nanocatalyst systems is introduced in which catalyst nanoparticles are protected within porous shells. The novelty of this approach lies in the use of chemical etching to assist the creation of mesopores in a protective oxide shell to promote efficient mass transfer to encapsulated metal nanoparticles. The etching process allows for the direct transformation of dense silica coatings into porous shells so that chemical species can reach the catalyst surface to participate in reactions while the shells act as physical barriers against aggregation of the catalyst particles. By using the surface‐protected etching process, both yolk–shell and core–satellite type nanoreactors are synthesized and their utilization in liquid‐ and gas‐phase catalysis is demonstrated. The thermal and chemical stability of the metallic cores during catalytic reactions is also investigated, and further work is carried out to enhance recyclability via the introduction of superparamagnetic components into the nanoreactor framework.

Morphology and Structure of Aerosol Carbon-Encapsulated Metal Nanoparticles from Various Ambient Metal−Carbon Spark Discharges

The morphology and structure of aerosol carbon encapsulated metal nanoparticles (CEMNs) of various transition metals (anode; Ti, Cu, Zn, Mo, Pd, W, Pt, or Au) formed by ambient spark discharge at the same electrical operating specifications were analyzed. CEMNs were produced with aggregated carbon particles, and their yields and sizes varied according to the metal-to-carbon fraction of each discharge relating to the ionization potential of the electrode material. Each encapsulated metal had natural crystallinity for all discharges, but carbon graphitization for the Mo-C and W-C configurations, which have relatively small differences in melting temperature between the materials, was particularly weak. An empty zone in the carbon shell was also detected in the CEMNs because of the difference in density between the molten and solid phases of the core metal during encapsulation.

Encapsulation of palladium nanoparticles by multiwall carbon nanotubes‐graft‐poly(citric acid) hybrid materials

AbstractCitric acid was polymerized onto the surface of functionalized multiwall carbon nanotubes (MWCNT‐COOH) and MWCNT‐graft‐poly(citric acid) (MWCNT‐g‐PCA) hybrid materials were obtained. Due to the grafted poly(citric acid) branches, MWCNT‐g‐PCA hybrid materials not only were soluble in water but also were able to trap water soluble metal ions. Reduction of trapped metal ions in the polymeric shell of MWCNT‐g‐PCA hybrid materials by reducing agents such as sodium borohydride led to encapsulated metal nanoparticles on the surface of MWCNT. Herein palladium nanoparticles were encapsulated and transported by MWCNT‐g‐PCA hybrid materials (MWCNT‐g‐PCA‐EPN) and their application as nanocatalyst toward Heck reaction in different conditions was investigated. The catalytic activity of palladium ions supported by MWCNT‐g‐PCA hybrid materials (MWCNT‐g‐PCA‐PdCl2) toward Heck reactions is much more than for MWCNT‐g‐PCA‐EPN. Structure, characteristics and catalytic activity of synthesized systems was investigated using spectroscopy and microscopy methods. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010