Morphology-dependent nanocatalysis on metal oxides

The design and fabrication of solid nanomaterials are the key issues in heterogeneous catalysis to achieve desired performance.Traditionally,the main theme is to reduce the size of the catalyst particles as small as possible for maximizing the number of active sites.In recent years,the rapid advancement in materials science has enabled us to fabricate catalyst particles with tunable morphology.Consequently,both size modulation and morphology control of the catalyst particles can be achieved independently or synergistically to optimize their catalytic properties.In particular,morphology control of solid catalyst particles at the nanometer level can selectively expose the reactive crystal facets,and thus drastically promote their catalytic performance.In this review,we summarize our recent work on the morphology impact of Co3O4,CeO2 and Fe2O3 nanomaterials in catalytic reactions,together with related literature on morphology-dependent nanocatalysis of metal oxides,to demonstrate the importance of tuning the shape of oxide-nanocatalysts for prompting their activity,selectivity and stability,which is a rapidly growing topic in heterogeneous catalysis.The fundamental understanding of the active sites in morphology-tunable oxides that are enclosed by reactive crystal facets is expected to direct the development of highly efficient nanocatalysts.     

金属氧化物纳米催化的形貌效应

纳米结构催化剂的设计与制备是多相催化的核心问题之一．提高催化活性的传统方法是减小催化剂粒子的尺寸以暴露更多的表面活性位,即纳米催化中的尺寸效应,但这种方法往往带有一定程度的经验性和随机性．近年来,随着纳米材料科学的快速发展,在溶液体系中通过自下而上的合成技术已经可以在纳米尺度上有效调变固体催化剂粒子的形貌．通过纳米催化材料的形貌可控合成,可选择性地暴露高活性或特定能量晶面,从而大幅度提升催化反应活性、选择性和稳定性,也就是纳米催化中的形貌效应,这也是当前纳米催化研究的热点之一．本文以作者近年来研究的C0304、CeO2和Fe2O3为重点,总结了纳米结构金属氧化物在多相催化反应中的形貌效应,分析了氧化物暴露品面的化学性质对催化性能的作用机制．这种基于形貌效应的纳米催化不仅加深了在纳米尺度甚至原子层次上对催化剂构一效关系的认知,而且对设计和开发实用高效催化剂也具有重要的理论价值．     

Mesosynthesis of ZnO-silica composites for methanol nanocatalysis

Methanol catalysis meets chemistry under confined conditions. Methanol is regarded as one of the most important future energy sources. ZnO/Cu composite materials are very effective in heterogeneous catalysis for methanol production due to the so-called strong metal-support interaction effect (SMSI). Therefore, materials of superior structural design potentially representing model systems for heterogeneous catalysis are highly desired. Ultimately, such materials could help to understand the interaction between copper and zinc oxide in more detail than currently possible. We report the preparation of nanocrystalline, size-selected ZnO inside the pore system of ordered mesoporous silica materials. A new, liquid precursor for ZnO is introduced. It is seen that the spatial confinement significantly influences the chemical properties of the precursor as well as determines a hierarchical architecture of the final ZnO/SiO(2) nanocomposites. Finally, the ability of the materials to act as model systems in methanol preparation is investigated. The materials are characterized by a variety of techniques including electron microscopy, X-ray scattering, solid-state NMR, EPR, EXAFS, and Raman spectroscopy, and physisorption analysis.     

Morphology-dependent electrochemistry of FeOOH nanostructures

Four kinds of iron hydroxide (FeOOH) structures with the morphologies of bulk, nano-sheet, nano-sphere, and nano-rod were synthesized using solvothermal processes. During synthesis different reagents were added to tune the morphology of FeOOH structures. These structures were characterized using TEM and SEM as well as from their Raman and XPS spectra. Voltammetric response of these structures as well as redox probes and endocrine disrupting compounds (EDCs) on these structures based electrodes was investigated. The morphology-dependent electrochemistry of these FeOOH structures was found. The highest redox activity of FeOOH was achieved on the FeOOH nano-rod structure based electrode, which was the best interface as well for the electrochemistry of both redox probes and EDCs. On such an interface, the highest magnitudes of both diethylstilbestrol (DES) and bisphenol A (BPA) were obtained.     

Morphology-dependent switching of polymer-stabilized cholesteric gratings

The use of a fingerprint texture of a cholesteric liquid crystal is demonstrated as a template to direct the formation of periodically ordered micro-size polymer walls. The morphology-property correlation of polymer-stabilized cholesteric gratings (PSCGs) was established for mesogenic and non-mesogenic reactive monomers. These PSCGs are suitable for laser beam steering, control of fibre optic signal and dynamic focus lenses.     

Morphology-dependent switching of polymer-stabilized cholesteric gratings

The use of a fingerprint texture of a cholesteric liquid crystal is demonstrated as a template to direct the formation of periodically ordered micro-size polymer walls. The morphology-property correlation of polymer-stabilized cholesteric gratings (PSCGs) was established for mesogenic and non-mesogenic reactive monomers. These PSCGs are suitable for laser beam steering, control of fibre optic signal and dynamic focus lenses.     

Metal-cluster compounds: Model systems for nanosized metal particles

Meta-cluster compounds can be exploited advantageously to study the evolution, with increasing size of the molecules of the physical properties of metal clusters from molecular to bulk-metal behavior. The metal-cluster molecules are well-defined, stoichiometric, chemical compounds. The molecules consist of a metal core of a variable number of atoms, surrounded by a shell of ligand atoms or molecules. Depending on the compound, the type of metal atom may be varied, whereas the core size can be changed from a few up to several thousands of atoms. Accordingly, these materials provide excellent model systems for monodisperse metal particles, embedded in a dielectric matrix, and can be investigated by the well-known experimental techniques of solid-state physics. © 1998 John Wiley & Sons, Ltd.     

New trends in sustainable nanocatalysis: Emerging use of earth abundant metals

Noble metals are well-known to afford highly active, selective and durable catalysts, and have thus been at the core of the development of greener processes. In recent years, however, growing concerns about their scarcity, cost and toxicity has triggered research efforts towards the development of earth-abundant catalysts. In this Current Opinion, recent examples of the use in catalysis of pure earth-abundant metals, earth-abundant metals with minute quantities of noble metals, or earth-abundant metals activated by light are presented. This highlight article showcases the current trends in sustainable organic transformations, catalyzed by nanomaterials.     

Effect of colloidal nanocatalysis on the metallic nanoparticle shape: the Suzuki reaction

Dominantly tetrahedral shaped poly(vinylpyrrolidone)-platinum (PVP-Pt) nanoparticles are shown to catalyze the Suzuki reaction between phenylboronic acid and iodobenzene but are not as active as the spherical palladium nanoparticles studied previously. The dominantly tetrahedral PVP-Pt nanoparticles (55 +/- 4% regular tetrahedral, 22 +/- 2% distorted tetrahedral, and 23 +/- 2% spherical nanoparticles) are synthesized by using the hydrogen reduction method. The transmission electron microscopy (TEM) results show that a transformation of shape from tetrahedral to spherical Pt nanoparticles takes place 3 h into the first cycle of the reaction. After the first cycle, the spherical nanoparticles have a similar size distribution to that of the tetrahedral nanoparticles before the reaction and the observed shape distribution is 18 +/-6% regular tetrahedral, 28 +/- 5% distorted tetrahedral, and 54 +/- 5% spherical nanoparticles. After the second cycle of the Suzuki reaction, the shape distribution is 13 +/- 5% regular tetrahedral, 24 +/- 5% distorted tetrahedral, and 63 +/- 7% spherical nanoparticles. After the second cycle, the transformed spherical nanoparticles continue to grow, and this could be due to the strong capping action of the higher molecular weight PVP (M(w) = 360 000), which makes the nanoparticles more resistant to aggregation and precipitation, unlike the Pd nanoparticles capped with the lower molecular weight PVP (M(w) = 40 000) used previously. The transformation in shape also occurs when the nanoparticles are refluxed in the presence of the solvent, sodium acetate, and iodobenzene and results in spherical nanoparticles with a similar size distribution to that of the tetrahedral nanoparticles before any perturbations. However, in the presence of phenylboronic acid, the regular tetrahedral nanoparticles remain dominant (51 +/- 6%) and maintain their size. These results support our previous studies in which we proposed that phenylboronic acid binds to the nanoparticle surface and thus acts as a capping agent for the particle and reacts with the iodobenzene. Recycling the nanoparticles results in a drastic reduction of the catalytic activity, and this must be due to the transformation of shape from the dominantly tetrahedral to the larger dominantly spherical nanoparticles. This also supports results in the literature that show that spherical platinum nanoparticles do not catalyze this reaction.     

Ignition of highly reactive metal particles

It was shown that, with increasing reaction rate constant above a certain limit, the bulk ignition mechanism changes to a surface mechanism. In this case, surface layers of a particle undergo local heating and a reaction front forms and propagates into the particle. The time of the frontal burnout of the particle is shorter than the time of its bulk burnout, during which the temperature field relaxes instantaneously.     

Morphology-dependent voltage sensitivity of a gold nanostructure

Gold nanostructures of various morphologies, including nanospheres, nanorods, nanoprisms, and thin films, were immobilized on ITO-coated coverslips in order to investigate the response of their scattering to potential. Shifts in the plasmon band obtained by potential-modulated spectroscopic imaging indicated that the voltage sensitivity of the gold nanostructure is dependent on its morphology, with nanospheres exhibiting the lowest sensitivity and ultrathin gold films exhibiting the highest. The effects of potential on gold nanoparticles are in qualitative agreement with Mie and Gans' theories in which the shift of the gold plasma frequency is due to the charging-discharging of the nanoparticles.     

Morphology-dependent stimulated Raman scattering (MDSRS)

A novel non-linear Raman spectroscopic technique, which can be used to characterize the chemical composition of the interface region of volatile aerosol particles, is described. The technique is morphology-dependent stimulated Raman scattering (MDSRS). We discuss the phenomenology behind this new optical probe, and present experimental results which show that this technique can be used as quantitative spectroscopic tool. We outline the potential application of MDSRS to in situ gas—aerosol kinetics and dynamics studies.     

Recent progress in copper nanocatalysis for sustainable transformations

Copper nanoparticles (CuNPs) have been deeply studied as catalyst for organic synthesis. Various new Cu nanocatalysts are reviewed for different types of organic reactions, such as C–C bond formation (including Mizoroki–Heck, Suzuki–Miyaura, Glaser-Hay coupling), C–N bond formation (including Chan-Lam, Buchwald–Hartwig, Ullmann and Goldberg coupling, alkyne–azide cycloaddition etc.), C–O bond formation and multi-step reactions with C–X (C, N, O) bond formation. Most CuNP-catalyzed protocols possess merits of mild reaction conditions, high catalytic efficiency, good functional group tolerance, lower cost, clean reaction profiles and reusable copper catalyst. The application of these CuNPs in organic synthesis holds potential for significant impact on advancing organic synthesis and promoting further development of organic copper chemistry.     

Morphology-dependent electrochemistry and electrocatalytical activity of cytochrome c

The morphology-dependent electrochemistry and electrocatalytical activity of cytochrome c (cyt. c) were investigated at pyramidal, rodlike, and spherical gold nanostructures directly electrodeposited onto sputtered gold surfaces. Direct, reversible electron transfer of cyt. c, for the first time, was realized at nanorod-like and nanopyramidal gold surfaces without any mediators or promoters, while no redox reaction was observed at the nanospherical gold electrode. The electrochemical properties of cyt. c vary with the shape of gold nanostructures with respect to the reversibility of electrode reactions, kinetic parameters, the formal potentials (E0'), and charge-transport resistance (Rct), suggesting shape-dependent mechanisms for the electrode reactions of cyt. c. The experimental results manifest that cyt. c was stably immobilized on the nanostructured gold electrodes with different conformational changes of the heme microenvironment. Consequently, not only the electroactivity, but also the inherent biological activity of the immobilized cyt. c strongly depended on the shape of the electrode surfaces. The facilitated electron transfer combined with the intrinsic catalytical activity of cyt. c substantially constructed a third-generation H2O2 biosensor with high selectivity, quick response time, large linear range, and good sensitivity. The electrocatalytical activity of the immobilized cyt. c toward H2O2 was also found to be morphology dependent, and the linear range of H2O2 detection could be tuned by means of employing the nanostructured gold surfaces with different shapes.     

Plasmon resonance broadening in spheroidal metal particles

An analytical expression for the surface scattering contribution to the width of plasmon resonances in spheroidally shaped metal particles is derived. The result is compared with analogous quantum-mechanical expressions.     

Changing catalytic activity during colloidal platinum nanocatalysis due to shape changes: electron-transfer reaction

The shape distribution of the catalytic nanoparticles and the activation energy of the electron-transfer reaction between hexacyanoferrate (III) and thiosulfate ions were determined at different times during the course of the reaction. The activation energy is found to increase during the reaction when dominantly tetrahedral nanoparticles are used, decreases slightly when dominantly cubic nanoparticles are used, and remains almost unchanged when spherical nanoparticles are used. Corresponding changes in the shape of the tetrahedral and cubic, but not spherical, shape is observed. This is consistent with the changes in the activation energy that are observed. The shape distribution and activation energy of dominantly spherical nanoparticles is found to remain stable during the course of the reaction.     

Nanodispersed metal and metal oxide particles in polymeric matrices from polyacrylonitrile precursors

Two ways of the colloid formation of metals and metal oxides in the polyacrylonitrile (PAN) and its copolymer (1.5 wt % of itaconic acid) (PAN-I) have been studied. The thermal decomposition of W, Mo and Cr carbonyl complexes with PAN, prepared by the interaction of PAN nitrile groups with VI B group metal hexacarbonyls has been investigated. The thermolysis under air leads to a formation of metal oxide particles. For the Cr-containing PAN, the presence of dispersed Cr₂O₃ with a size less than 3 nm was estimated by ESR. Co-containing polymers were prepared by mixing Co₂(CO)₈ with PAN-I in DMF. It was found that Co₂(CO)₈ interacts with DMF giving salt [Co(DMF)₆]²⁺[Co(CO)₄]-₂. By ferromagnetic resonance and SAXS, colloid size depends on thermolysis conditions and loading of the Co complex and varies from 1 to 10 nm.     

Systems of small metal particles: Optical properties and their structure dependences

Experiments on small particles usually require samples containing large numbers of particles. The properties of such samples are determined both by the properties of the individual particle and by collective effects, if particles are packed closely together. Collective optical effects strongly depend on the topography of the samples. It is shown that they can be classified according to the effective local electromagnetic field. Recent experiments and calculations are presented for optical extinction spectra in the spectral region of plasmon polariton excitations, which clearly show the different behaviour of effective medium-like samples and of samples containing particle aggregates.     

Early stages of some small metal particles: A pulse radiolysis study

Recent advances in the pulsing system of the 4 MeV van de Graaff accelerator ELBENA are described, which now allow the generation of stable timed pulses with nanosecond duration. The reaction of tetrachloroplatinate with the hydrated electron, the t-butanol radical and the methanol radical results, via the Cl⁻→H₂O ligand exchange, in different complexes which absorb at 310 and 360 nm, respectively, or in a chain reaction.