Metal-Organic Framework-Derived Honeycomb-Like Open Porous Nanostructures as Precious-Metal-Free Catalysts for Highly Efficient Oxygen Electroreduction.

Honeycomb-like porous carbon nanostructures are rationally constructed from a metal-organic framework composite. The unique architecture with uniformly distributed high-density active sites significantly enhances the electrocatalytic performance by increasing the accessible active sites and enhancing mass transport of the gas and electrolyte, rendering the resulting catalyst adequate in reaching the desired catalytic performance afforded by Pt for the oxygen reduction reaction.

Toward Homogenization of Heterogeneous Metal Nanoparticle Catalysts with Enhanced Catalytic Performance: Soluble Porous Organic Cage as a Stabilizer and Homogenizer.

Organic molecular cage (CC3-R) with intrinsically porous skeleton is used as a support for immobilizing Rh nanoparticles (NPs) in an ultrasmall size of ∼1.1 nm for the first time. The CC3-R with the unique characteristic of high solubility can be utilized to homogenize the heterogeneous catalyst in solution. The obtained homogenized heterogeneous catalyst Rh/CC3-R-homo exhibits significantly enhanced catalytic performance toward various liquid-phase catalytic reactions, as compared with the heterogeneous counterpart Rh/CC3-R-hetero. Moreover, Rh/CC3-R-homo shows excellent durability and recyclability. The advantage of combining homogeneous and heterogeneous catalysts is likely to be beneficial for many applications.

Surfactant-free Pd nanoparticles immobilized to a metal-organic framework with size- and location-dependent catalytic selectivity.

Surfactant-free Pd nanoparticles, immobilized to a metal-organic framework (MIL-101), have been used for the first time as highly active and durable catalysts in water for biomass refining (hydrodeoxygenation of vanillin, a typical compound of lignin) with metal nanoparticle size- and location-dependent catalytic activity and selectivity.

From ionic-liquid@metal-organic framework composites to heteroatom-decorated large-surface area carbons: superior CO2 and H2 uptake.

For the first time, high surface area uniformly nitrogen (N)- and boron-nitrogen (BN)-decorated nanoporous carbons have been successfully fabricated by impregnation of ionic liquids (ILs) within a metal-organic framework (MOF), MIL-100(Al), followed by carbonization, which exhibit remarkable CO2 and H2 adsorption capacities.

Metal-organic framework-immobilized polyhedral metal nanocrystals: reduction at solid-gas interface, metal segregation, core-shell structure, and high catalytic activity.

For the first time, this work presents surfactant-free monometallic and bimetallic polyhedral metal nanocrystals (MNCs) immobilized to a metal-organic framework (MIL-101) by CO-directed reduction of metal precursors at the solid-gas interface. With this novel method, Pt cubes and Pd tetrahedra were formed by CO preferential bindings on their (100) and (111) facets, respectively. PtPd bimetallic nanocrystals showed metal segregation, leading to Pd-rich core and Pt-rich shell. Core-shell Pt@Pd nanocrystals were immobilized to MIL-101 by seed-mediated two-step reduction, representing the first example of core-shell MNCs formed using only gas-phase reducing agents. These MOF-supported MNCs exhibited high catalytic activities for CO oxidation.

Novel formation of Ag/Au bimetallic nanoparticles by physical mixture of monometallic nanoparticles in dispersions and their application to catalysts for aerobic glucose oxidation.

Ag/Au bimetallic nanoparticles (BNPs) with a size less than 2 nm were prepared by physical mixture of colloidal dispersions of Ag and Au nanoparticles (NPs). This provides an example of fabrication of BNPs with self-organization by the reaction between metal NPs. Although Ag/Au BNPs having different structures and compositions are one of the most widely studied bimetallic systems in the literature due to their wide range of uses such as in catalysis, electronics, plasmonics, optical sensing, and surface-enhanced Raman scattering, we first prepared such BNPs by physical mixture and characterized them by UV-vis spectroscopy, SERS, XPS, TEM, and EDS in HR-STEM. The present fabrication method has the advantage of avoiding the unfavorable formation of AgCl precipitates in the reaction process which are always produced when Ag(+) ions are used as a starting material in combination with a HAuCl4 precursor. These Ag/Au BNPs showed high catalytic activities for aerobic glucose oxidation, and the highest activity of 11,510 mol of glucose·h(-1)·mol of metal(-1) was observed for the BNPs with a Ag/Au atomic ratio of 1/4; the activity value is about 2 times higher than that of Au NPs with nearly the same particle size. XPS and DFT calculation results show that the negatively charged Au atoms due to the electron charge transfer effects from neighboring Ag atoms and poly(N-vinyl-2-pyrrolidone) act as catalytically active sites and play an important role in the aerobic glucose oxidation.

Electron microscopy study of gold nanoparticles deposited on transition metal oxides.

Many researchers have investigated the catalytic performance of gold nanoparticles (GNPs) supported on metal oxides for various catalytic reactions of industrial importance. These studies have consistently shown that the catalytic activity and selectivity depend on the size of GNPs, the kind of metal oxide supports, and the gold/metal oxide interface structure. Although researchers have proposed several structural models for the catalytically active sites and have identified the specific electronic structures of GNPs induced by the quantum effect, recent experimental and theoretical studies indicate that the perimeter around GNPs in contact with the metal oxide supports acts as an active site in many reactions. Thus, it is of immense importance to investigate the detailed structures of the perimeters and the contact interfaces of gold/metal oxide systems by using electron microscopy at an atomic scale. This Account describes our investigation, at the atomic scale using electron microscopy, of GNPs deposited on metal oxides. In particular, high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are valuable tools to observe local atomic structures, as has been successfully demonstrated for various nanoparticles, surfaces, and material interfaces. TEM can be applied to real powder catalysts as received without making special specimens, in contrast to what is typically necessary to observe bulk materials. For precise structure analyses at an atomic scale, model catalysts prepared by using well-defined single-crystalline substrates are also adopted for TEM observations. Moreover, aberration-corrected TEM, which has high spatial resolution under 0.1 nm, is a promising tool to observe the interface structure between GNPs and metal oxide supports including oxygen atoms at the interfaces. The oxygen atoms in particular play an important role in the behavior of gold/metal oxide interfaces, because they may participate in catalytic reaction steps. Detailed information about the interfacial structures between GNPs and metal oxides provides valuable structure models for theoretical calculations which can elucidate the local electronic structure effective for activating a reactant molecule. Based on our observations with HRTEM and HAADF-STEM, we report the detailed structure of gold/metal oxide interfaces.

A new type of molybdenum oxide crystal encapsulated inside a single-walled carbon nanotube.

The crystal structure of a new type of molybdenum oxide crystal encapsulated in a single-walled carbon nanotube (CNT) was examined via diffraction and spectroscopic techniques using both X-rays and electron beams. This new type of molybdenum oxide crystal has a chemical bonding state of MoO3, as confirmed by X-ray absorption spectroscopy, and the MoO3 units exhibit axial symmetry, as clarified by electron diffraction from bundled and individual CNTs encapsulating the crystal. To obtain three-dimensional information on the structure, a cross-sectional sample was prepared using a conventional dimple and ion-mill method. High-resolution transmission electron microscopy images exhibit ring-like shapes that originated from the arrangement of the MoO3 units inside the CNTs, as observed along the tube axis. The units are spaced 0.36 nm from each other in a ring arrangement and the distance between each ring is 0.391 nm.

Base-free direct oxidation of 1-octanol to octanoic acid and its octyl ester over supported gold catalysts.

The choice of a suitable support for gold nanoparticles (Au NPs) enabled the direct oxidation of unreactive aliphatic alcohol, 1-octanol, to octanoic acid and octyl octanoate in the absence of a base. Under optimized conditions, Au NPs supported on NiO (Au/NiO) exhibited remarkably high catalytic activities and excellent selectivities to octanoic acid (e.g., 97 %) at full conversion. In contrast to Au/NiO, Au/CeO2 selectively produced octyl octanoate as a major product in a base-free aqueous solution with a maximum selectivity of 82 % under optimized conditions.

Study of surface reaction of spinel Li4Ti5O12 during the first lithium insertion and extraction processes using atomic force microscopy and analytical transmission electron microscopy.

Spinel lithium titanate (Li(4)Ti(5)O(12), LTO) is a promising anode material for a lithium ion battery because of its excellent properties such as high rate charge-discharge capability and life cycle stability, which were understood from the viewpoint of bulk properties such as small lattice volume changes by lithium insertion. However, the detailed surface reaction of lithium insertion and extraction has not yet been studied despite its importance to understand the mechanism of an electrochemical reaction. In this paper, we apply both atomic force microscopy (AFM) and transmission electron microscopy (TEM) to investigate the changes in the atomic and electronic structures of the Li(4)Ti(5)O(12) surface during the charge-discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li(4)Ti(5)O(12) surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li(4)Ti(5)O(12) interface. The TEM observation clarified that cubic rock-salt crystal layers with a half lattice size of the original spinel structure are epitaxially formed after the first charge-discharge cycle. Electron energy loss spectroscopy (EELS) observation revealed that the formed surface layer should be α-Li(2)TiO(3). Although the transformation of Li(4)Ti(5)O(12) to Li(7)Ti(5)O(12) is well-known as the lithium insertion reaction of the bulk phase, the generation of surface product layers should be inevitable in real charge-discharge processes and may play an effective role in the stable electrode performance as a solid-electrolyte interphase (SEI).

Intrinsic catalytic structure of gold nanoparticles supported on TiO2.

Despite the fragility of TiO(2) under electron irradiation, the intrinsic structure of Au/TiO(2) catalysts can be observed by environmental transmission electron microscopy. Under reaction conditions (CO/air 100 Pa), the major {111} and {100} facets of the gold nanoparticles are exposed and the particles display a polygonal interface with the TiO(2) support bounded by sharp edges parallel to the 〈110〉 directions.

A one-pot protocol for synthesis of non-noble metal-based core-shell nanoparticles under ambient conditions: toward highly active and cost-effective catalysts for hydrolytic dehydrogenation of NH3BH3.

A one-pot synthesis of non-noble transition metal-based core-shell nanoparticles (NPs) has been developed under ambient conditions. The obtained Cu@M (M = Co, Fe, Ni) NPs exhibit superior catalytic activity for hydrolytic dehydrogenation of NH(3)BH(3), compared to the alloy and monometallic counterparts.

Synergistic catalysis of metal-organic framework-immobilized Au-Pd nanoparticles in dehydrogenation of formic acid for chemical hydrogen storage.

Bimetallic Au-Pd nanoparticles (NPs) were successfully immobilized in the metal-organic frameworks (MOFs) MIL-101 and ethylenediamine (ED)-grafted MIL-101 (ED-MIL-101) using a simple liquid impregnation method. The resulting composites, Au-Pd/MIL-101 and Au-Pd/ED-MIL-101, represent the first highly active MOF-immobilized metal catalysts for the complete conversion of formic acid to high-quality hydrogen at a convenient temperature for chemical hydrogen storage. Au-Pd NPs with strong bimetallic synergistic effects have a much higher catalytic activity and a higher tolerance with respect to CO poisoning than monometallic Au and Pd counterparts.

From metal-organic framework to nanoporous carbon: toward a very high surface area and hydrogen uptake.

In this work, with a zeolite-type metal-organic framework as both a precursor and a template and furfuryl alcohol as a second precursor, nanoporous carbon material has been prepared with an unexpectedly high surface area (3405 m(2)/g, BET method) and considerable hydrogen storage capacity (2.77 wt % at 77 K and 1 atm) as well as good electrochemical properties as an electrode material for electric double layer capacitors. The pore structure and surface area of the resultant carbon materials can be tuned simply by changing the calcination temperature.

Photodeposition of Ag2S quantum dots and application to photoelectrochemical cells for hydrogen production under simulated sunlight.

UV light irradiation of TiO(2) (λ > 320 nm) in a mixed solution of AgNO(3) and S(8) has led to the formation of Ag(2)S quantum dots (QDs) on TiO(2), while Ag nanoparticles (NPs) are photodeposited without S(8). Photoelectrochemical measurements indicated that the Ag(2)S photodeposition proceeds via the preferential reduction of Ag(+) ions to Ag(0), followed by the chemical reaction with S(8). The application of this in situ photodeposition technique to mesoporous (mp) TiO(2) nanocrystalline films coated on fluorine-doped SnO(2) (FTO) electrodes enables formation of Ag(2)S QDs (Ag(2)S/mp-TiO(2)/FTO). Ag(2)S/mp-TiO(2)/FTO has the interband transition absorption in the whole visible region, while in the spectrum of Ag/mp-TiO(2)/FTO, a localized surface plasmon resonance absorption of Ag NPs is present centered at 490 nm. Ag(2)S QD-sensitized photoelectrochemical cells using the Ag(2)S/mp-TiO(2)/FTO and Ag/mp-TiO(2)/FTO photoanodes were fabricated. Under illumination of one sun, the Ag(2)S photoanode cell yielded H(2) at a rate of 0.8 mL·h(-1) with a total conversion efficiency of 0.29%, whereas the Ag/mp-TiO(2)/FTO photoanode is inactive.

Synergistic catalysis of Au@Ag core-shell nanoparticles stabilized on metal-organic framework.

For the first time, this work presents Au@Ag core-shell nanoparticles (NPs) immobilized on a metal-organic framework (MOF) by a sequential deposition-reduction method. The small-size Au@Ag NPs reveal the restriction effects of the pore/surface structure in the MOF. The modulation of the Au/Ag ratio can tune the composition and a reversed Au/Ag deposition sequence changes the structure of Au-Ag NPs, while a posttreatment process transforms the core-shell NPs to a AuAg alloy. Catalytic studies show a strong bimetallic synergistic effect of core-shell structured Au@Ag NPs, which have much higher catalytic activities than alloy and monometallic NPs.

Facile synthesis and catalytic activity of MoS(2)/TiO(2) by a photodeposition-based technique and its oxidized derivative MoO(3)/TiO(2) with a unique photochromism.

UV-light irradiation to TiO(2) in an aqueous ethanol solution of (NH(4))(2)MoS(4) under deaerated conditions has yielded molybdenum(IV) sulfide nanoparticles on a TiO(2) surface (MoS(2)/TiO(2)) to be transformed into molybdenum(VI) oxide species highly dispersed at a molecular level by a subsequent heating at 773K in air (m-MoO(3)/TiO(2)). In HCOOH aqueous solutions, the MoS(2)/TiO(2) system exhibits a high level of photocatalytic activity for H(2) generation, while the m-MoO(3)/TiO(2) system shows unique photochromism.

Formation of electro-conductive titanium oxide fine particles by pulsed UV laser irradiation.

Nano-submicron particles of sub-stoichiometric titanium oxide (TiOx) were synthesized by irradiation of TiO(2) particles dispersed in liquid with a Nd:YAG pulsed UV laser, and their physicochemical and electrochemical properties were examined. After laser irradiation for 1 h, spherical oxide particles of up to ca. 300 nm in diameter were formed regardless of the liquid used, however the reduction of TiO(2) largely depended on the liquid: acetonitrile most strongly promoted the reduction of TiO(2) by UV laser irradiation. The mean valence of titanium in TiOx synthesized in acetonitrile was ca. 3.5, which is comparable to that of the most-reduced Magnéli phase, Ti(4)O(7). While the electrical conductivity of as-washed TiOx was significantly low, annealing at 900 degrees C in hydrogen dramatically improved conductivity. The oxidation resistance of TiOx was examined by cyclic voltammetry to a high potential (1.5 V) using a MEA under PEMFC operating conditions. TiOx showed a much lower anodic corrosion current at >1.0 V than XC-72R carbon, which suggests that TiOx may exhibit superior oxidation resistance as a catalyst support material at high potentials.