Diffusion-Limited Formation of Nonequilibrium Intermetallic Nanophase for Selective Dehydrogenation.

Nonequilibrium intermetallic phases in the nanoscale were realized by diffusion-controlled solid-state transformation, forming SiO2 supported NPs with Pd core and a CsCl type Pd1M1 shell, where M is Sn or Sb. The core-shell geometry is identified from scanning transmission electron microscopy and infrared spectroscopy and the crystal structure is confirmed from in situ synchrotron X-ray diffraction and X-ray absorption spectroscopy. The highly symmetric Pd1M1 intermetallic phase has not been reported previously and contains catalytic ensembles with high selectivity toward dehydrogenation of propane. The kinetically limited solid-state reaction is generally applicable to nanoparticle synthesis and could produce materials with desired structures and properties beyond conventional structural limits.

Investigation of Electron Momentum Density in Carbon Nanotubes Using Transmission Electron Microscopy.

Valence Compton profiles (CPs) of multiwall (MWCNTs) and single-wall carbon nanotubes (SWCNTs) were obtained by recording electron energy-loss spectra at large momentum transfer in the transmission electron microscope, a technique known as electron Compton scattering from solids (ECOSS). The experimental MWCNT/SWCNT results were compared with that of graphite. Differences between the valence CPs of MWCNTs and SWCNTs were observed, and the SWCNT CPs indicate a greater delocalization of the ground-state charge density compared to graphite. The results clearly demonstrate the feasibility and potential of the ECOSS technique as a complementary tool for studying the electronic structure of materials with nanoscale spatial resolution.

Electrocatalytic Water Oxidation at Quinone-on-Carbon: A Model System Study.

Nanocarbon can promote robust and efficient electrocatalytic water oxidation through active surface oxygen moieties. The recent mechanistic understandings (e.g., active sites) of metal-free carbon catalysts in oxygen evolution reaction (OER), however, are still rife with controversies. In this work, we describe a facile protocol in which eight kinds of aromatic molecules with designated single oxygen species were used as model structures to investigate the explicit roles of each common oxygen group in OER at a molecular level. These model structures were decorated onto typical nanocarbon surfaces like onion-like carbons (OLC) or multiwalled carbon nanotubes (MWCNT) to build aromatic molecule-modified carbon systems. We show that edge (including zigzag and armchair) quinones in a conjugated π network are the true active centers, and the roles of ether and carboxyl groups are excluded in the OER process. The plausible rate-determining step could be singled out by H/D kinetic isotope effects. The turnover frequency per C═O (∼0.323 s-1 at η = 340 mV) in 0.1 M KOH and the optimized current density (10 mA/cm2 at 1.58 V vs RHE) of quinone-modified carbon systems are comparable to those of promising metal-based catalysts.

Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon-Nanotube-Based Electrocatalyst.

Ammonia is synthesized directly from water and N2 at room temperature and atmospheric pressure in a flow electrochemical cell operating in gas phase (half-cell for the NH3 synthesis). Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this half-cell. A rate of ammonia formation of 2.2×10-3  gNH3 m-2 h-1 was obtained at room temperature and atmospheric pressure in a flow of N2 , with stable behavior for at least 60 h of reaction, under an applied potential of -2.0 V. This value is higher than the rate of ammonia formation obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with a total Faraday efficiency as high as 95.1 % was obtained. Data also indicate that the active sites in NH3 electrocatalytic synthesis may be associated to specific carbon sites formed at the interface between iron particles and CNT and able to activate N2 , making it more reactive towards hydrogenation.

Facile Synthesis of Au Nanoparticles Embedded in an Ultrathin Hollow Graphene Nanoshell with Robust Catalytic Performance.

Au nanoparticles (NPs) uniformly embedded into an ultrathin hollow graphene nanoshell (Au@HGN) are synthesized using a facile template-based procedure. The obtained Au@HGN catalyst exhibits robust and stable catalytic performance in the reduction of 4-nitrophenol to 4-aminophenol, compared with that of traditional Au/TiO2 and previously reported Au- and Ag-based catalysts.

Calibration of the basic strength of the nitrogen groups on the nanostructured carbon materials.

The basicity of the graphitic, pyridine, and pyrrole nitrogen groups on the graphene and single-walled carbon nanotubes is evaluated and compared in terms of both Brønsted base and Lewis base. It turns out that the pyridine group is the most strong basic site, while the graphitic nitrogen does not bring any improvements over the undoped one.

Communication: Investigation of the electron momentum density distribution of nanodiamonds by electron energy-loss spectroscopy.

The electron momentum distribution of detonation nanodiamonds (DND) was investigated by recording electron energy-loss spectra at large momentum transfer in the transmission electron microscope (TEM), which is known as electron Compton scattering from solid (ECOSS). Compton profile of diamond film obtained by ECOSS was found in good agreement with prior photon experimental measurement and theoretical calculation that for bulk diamond. Compared to the diamond film, the valence Compton profile of DND was found to be narrower, which indicates a more delocalization of the ground-state charge density for the latter. Combining with other TEM characterizations such as high-resolution transmission electron spectroscopy, diffraction, and energy dispersive X-ray spectroscopy measurements, ECOSS was shown to be a great potential technique to study ground-state electronic properties of nanomaterials.

Oxidative Dehydrogenation on Nanocarbon: Intrinsic Catalytic Activity and Structure-Function Relationships.

Physical and chemical insights into the nature and quantity of the active sites and the intrinsic catalytic activity of nanocarbon materials in alkane oxidative dehydrogenation (ODH) reactions are reported using a novel in situ chemical titration process. A study on the structure-function relationship reveals that the active sites are identical both in nature and function on various nanocarbon catalysts. Additionally, the quantity of the active sites could be used as a metric to normalize the reaction rates, and thus to evaluate the intrinsic activity of nanocarbon catalysts. The morphology of the nanocarbon catalysts at the microscopic scale exhibits a minor influence on their intrinsic ODH catalytic activity. The number of active sites calculated from the titration process indicates the number of catalytic centers that are active (that is, working) under the reaction conditions.

Stabilization of Palladium Nanoparticles on Nanodiamond-Graphene Core-Shell Supports for CO Oxidation.

Nanodiamond-graphene core-shell materials have several unique properties compared with purely sp(2) -bonded nanocarbons and perform remarkably well as metal-free catalysts. In this work, we report that palladium nanoparticles supported on nanodiamond-graphene core-shell materials (Pd/ND@G) exhibit superior catalytic activity in CO oxidation compared to Pd NPs supported on an sp(2) -bonded onion-like carbon (Pd/OLC) material. Characterization revealed that the Pd NPs in Pd/ND@G have a special morphology with reduced crystallinity and are more stable towards sintering at high temperature than the Pd NPs in Pd/OLC. The electronic structure of Pd is changed in Pd/ND@G, resulting in weak CO chemisorption on the Pd NPs. Our work indicates that strong metal-support interactions can be achieved on a non-reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high-performance nanostructured catalyst.

TiO2 /Cu2 O Core/Ultrathin Shell Nanorods as Efficient and Stable Photocatalysts for Water Reduction.

P-type Cu2 O has been long considered as an attractive photocatalyst for photocatalytic water reduction, but few successful examples has been reported. Here, we report the synthesis of TiO2 (core)/Cu2 O (ultrathin film shell) nanorods by a redox reaction between Cu(2+) and in-situ generated Ti(3+) when Cu(2+) -exchanged H-titanate nanotubes are calcined in air. Owing to the strong TiO2 -Cu2 O interfacial interaction, TiO2 (core)/Cu2 O (ultrathin film shell) nanorods are highly active and stable in photocatalytic water reduction. The TiO2 core and Cu2 O ultrathin film shell respectively act as the photosensitizer and cocatalyst, and both the photoexcited electrons in the conduction band and the holes in the valence band of TiO2 respectively transfer to the conduction band and valence band of the Cu2 O ultrathin film shell. Our results unambiguously show that Cu2 O itself can act as the highly active and stable cocatalyst for photocatalytic water reduction.

The nucleophilicity of the oxygen functional groups on carbon materials: a DFT analysis.

The nucleophilicity of the five different oxygen functional groups (carboxyl, diketone, ketone, lactone, and quinone) on the graphene are evaluated by DFT calculations. Two different methods are employed to obtain three different reactivity descriptors, namely, sk(-), wk(-), and sk(-)/sk(+). The calculations indicate that the quinone group is the most nucleophilic site among the investigated ones. The diketone and ketone are much less active than the quinone group toward an electrophilic attack. This judgment is further supported by Ag(+)-binding calculations, which mimics an electrophilic attack. In addition, the carboxyl and lactone groups at a zigzag-terminated edge show nucleophilic ability that is comparable to that of diketones. The current work lays the basis for future studies on the redox property of carbon catalysis and its implications in catalytic reactions.

Hybrid nanocarbon as a catalyst for direct dehydrogenation of propane: formation of an active and selective core-shell sp2/sp3 nanocomposite structure.

Hybrid nanocarbon, comprised of a diamond core and a graphitic shell with a variable sp(2)-/sp(3)-carbon ratio, is controllably obtained through sequential annealing treatment (550-1300 °C) of nanodiamond. The formation of sp(2) carbon increases with annealing temperature and the nanodiamond surface is reconstructed from amorphous into a well-ordered, onion-like carbon structure via an intermediate composite structure--a diamond core covered by a defective, curved graphene outer shell. Direct dehydrogenation of propane shows that the sp(2)-/sp(3)-nanocomposite exhibits superior catalytic performance to that of individual nanodiamond and graphitic nanocarbon. The optimum catalytic activity of the diamond/graphene composite depends on the maximum structural defectiveness and high chemical reactivity of the ketone groups. Ketone-type functional groups anchored on the defects/vacancies are active for propene formation; nevertheless, once the oxygen functional groups are desorbed, the defects/vacancies alone might be active sites responsible for the C-H bond activation of propane.

One-pot synthesis of (Au nanorod)-(metal sulfide) core-shell nanostructures with enhanced gas-sensing property.

(Au nanorod)-(metal sulfide) core-shell structures are synthesized by employing Au nanorods as the starting reactants and metal thiobenzoates as the metal sulfide precursors in the presence of Ag(+) ions. The gas-sensing property of the prepared (Au nanorod)-CdS sensor exhibit a significantly enhanced response and higher sensitivity in comparison with a CdS sensor.

Exploring meso-/microporous composite molecular sieves with core-shell structures.

A series of core-shell-structured composite molecular sieves comprising zeolite single crystals (i.e., ZSM-5) as a core and ordered mesoporous silica as a shell were synthesized by means of a surfactant-directed sol-gel process in basic medium by using cetyltrimethylammonium bromide (CTAB) as a template and tetraethylorthosilicate (TEOS) as silica precursor. Through this coating method, uniform mesoporous silica shells closely grow around the anisotropic zeolite single crystals, the shell thickness of which can easily be tuned in the range of 15-100 nm by changing the ratio of TEOS/zeolite. The obtained composite molecular sieves have compact meso-/micropore junctions that form a hierarchical pore structure from ordered mesopore channels (2.4-3.0 nm in diameter) to zeolite micropores (≈0.51 nm). The short-time kinetic diffusion efficiency of benzene molecules within pristine ZSM-5 (≈7.88×10(-19)  m(2) s(-1)) is almost retainable after covering with 75 nm-thick mesoporous silica shells (≈7.25×10(-19)  m(2) s(-1)), which reflects the greatly opened junctions between closely connected mesopores (shell) and micropores (core). The core-shell composite shows greatly enhanced adsorption capacity (≈1.35 mmol g(-1)) for large molecules such as 1,3,5-triisopropylbenzene relative to that of pristine ZSM-5 (≈0.4 mmol g(-1)) owing to the mesoporous silica shells. When Al species are introduced during the coating process, the core-shell composite molecular sieves demonstrate a graded acidity distribution from weak acidity of mesopores (predominant Lewis acid sites) to accessible strong acidity of zeolite cores (Lewis and Brønsted acid sites). The probe catalytic cracking reaction of n-dodecane shows the superiority of the unique core-shell structure over pristine ZSM-5. Insight into the core-shell composite structure with hierarchical pore and graded acidity distribution show great potential for petroleum catalytic processes.

Highly selective hydrogenation of phenol and derivatives over a Pd@carbon nitride catalyst in aqueous media.

Cyclohexanone is an important intermediate in the manufacture of polyamides in chemical industry, but direct selective hydrogenation of phenol to cyclohexanone under mild conditions is a challenge. We report here a catalyst made of Pd nanoparticles supported on a mesoporous graphitic carbon nitride, Pd@mpg-C(3)N(4), which was shown to be highly active and promoted the selective formation of cyclohexanone under atmospheric pressure of hydrogen in aqueous media without additives. Conversion of 99% and a selectivity higher than 99% were achieved within 2 h at 65 °C. The reaction can be accelerated at higher temperature, but even at room temperature, 99% conversion and 96% selectivity could still be obtained. The generality of the Pd@mpg-C(3)N(4) catalyst for this reaction was demonstrated by selective hydrogenation of other hydroxylated aromatic compounds with similar performance.

Covalent triazine framework as catalytic support for liquid phase reaction.

An important goal in the preparation of highly active supported metal particles is the enhancement of the metal support interaction, providing a more stable catalyst, especially for liquid phase reactions as the leaching and reconstruction of the active phase causes deactivation. In this work, a covalent triazine framework (CTF) as support for Pd nanoparticles is compared to activated carbon (AC), the typical support used in liquid phase reactions. The results indicate that the presence of the N-heterocyclic moieties on the surface of the frameworks is beneficial for improving the stability of Pd nanoparticles during the liquid phase glycerol oxidation. Pd/CTF showed better activity and in particular better stability when compared to Pd supported on activated carbon (AC).

Using supported Au nanoparticles as starting material for preparing uniform Au/Pd bimetallic catalysts.

One of the best methods for producing bulk homogeneous (composition) supported bimetallic AuPd clusters involves the immobilization of a protected Au seed followed by the addition of Pd. This paper investigates the importance of this gold seed in controlling the resulting bimetallic AuPd clusters structures, sizes and catalytic activities by investigating three different gold seeds. Uniform Au-Pd alloy were obtained when a steric/electrostatic protecting group, poly(vinyl alcohol) (PVA), was used to form the gold clusters on activated carbon (AC). In contrast Au/AC precursors prepared using Au nanoparticles with only electrostatic stabilization (tetrakis(hydroxypropyl)phosphonium chloride (THPC)), or no stabilization (magnetron sputtering) produced inhomogeneous alloys and segregation of the gold and palladium. The uniform alloyed catalyst (Pd@Au(PVA)/AC) is the most active and selective catalyst, while the inhomogenous catalysts are less active and selective. Further study of the PVA protected Au clusters revealed that the amount of PVA used is also critical for the preparation of uniform alloyed catalyst, their stability, and their catalytic activity.

Soot structure and reactivity analysis by Raman microspectroscopy, temperature-programmed oxidation, and high-resolution transmission electron microscopy.

Raman microspectroscopy (RM), temperature-programmed oxidation (TPO), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS) were combined to get comprehensive information on the relationship between structure and reactivity of soot in samples of spark discharge (GfG), heavy duty engine diesel (EURO VI and IV) soot, and graphite powder upon oxidation by oxygen at increasing temperatures. GfG soot and graphite powder represent the higher and lower reactivity limits. Raman microspectroscopic analysis was conducted by determination of spectral parameters using a five band fitting procedure (G, D1-D4) as well as by evaluation of the dispersive character of the D mode. The analysis of spectral parameters shows a higher degree of disorder and a higher amount of molecular carbon for untreated GfG soot samples than for samples of untreated EURO VI and EURO IV soot. The structural analysis based on the dispersive character of the D mode revealed substantial differences in ordering descending from graphite powder, EURO IV, VI to GfG soot. HRTEM images and EELS analysis of EURO IV and VI samples indicated a different morphology and a higher structural order as compared to GfG soot in full agreement with the Raman analysis. These findings are also confirmed by the reactivity of soot during oxidation (TPO), where GfG soot was found to be the most reactive and EURO IV and VI soot samples exhibited a moderate reactivity.

Behaviour of TEM metal grids during in-situ heating experiments.

The stability of Ni, Cu, Mo and Au transmission electron microscope (TEM) grids coated with ultra-thin amorphous carbon (alpha-C) or silicon monoxide film is examined by in-situ heating up to a temperature in the range 500-850 degrees C in a transmission electron microscope. It is demonstrated that some grids can generate nano-particles either due to the surface diffusion of metal atoms on amorphous film or due to the metal evaporation/redeposition. The emergence of nano-particles can complicate experimental observations, particularly in in-situ heating studies of dynamic behaviours of nano-materials in TEM. The most widely used Cu grid covered with amorphous carbon is unstable, and numerous Cu nano-particles start to form once the heating temperature reaches 600 degrees C. In the case of Ni grid covered with alpha-C film, a large number of Ni nano-crystals occur immediately when the temperature approaches 600 degrees C, accompanied by the graphitization of amorphous carbon. In contrast, both Mo and Au grids covered with alpha-C film exhibit good stability at elevated temperature, for instance, up to 680 and 850 degrees C for Mo and Au, respectively, and any other metal nano-particles are detected. Cu grid covered Si monoxide thin film is stable up to 550 degrees C, but Si nano-crystals appear under intensive electron beam. The generated nano-particles are well characterized by spectroscopic techniques (EDXS/EELS) and high-resolution TEM. The mechanism of nano-particle formation is addressed based on the interactions between the metal grid and the amorphous carbon film and on the sublimation of metal.

Combined study of the ground and excited states in the transformation of nanodiamonds into carbon onions by electron energy-loss spectroscopy.

The electron momentum density and sp2/sp3 ratio of carbon materials in the thermal transformation of detonation nanodiamonds (ND) into carbon nano-onions are systematically studied by electron energy-loss spectroscopy (EELS). Electron energy-loss near-edge structures of the carbon K-ionization in the electron energy-loss spectroscopy are measured to determine the sp2 content of the ND-derived samples. We use the method developed by Titantah and Lamoen, which is based on the ability to isolate the π* spectrum and has been shown to give reliable and accurate results. Compton profiles (CPs) of the ND-derived carbon materials are obtained by performing EELS on the electron Compton scattering region. The amplitude of the CPs at zero momentum increases with increasing annealing temperature above 500 °C. The dramatic changes occur in the temperature range of 900-1300 °C, which indicates that the graphitization process mainly occurs in this annealing temperature region. Our results complement the previous work on the thermal transformation of ND-derived carbon onions and provide deeper insight into the evolution of the electronic properties in the graphitization process.