Small Metal Particles Research Articles

Effective Prandtl Number Model Influences on the $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ γ Al 2 O 3 – $${\hbox {H}}_2 {\hbox {O}}$$ H 2 O and $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ γ Al 2 O 3 – $${\hbox {C}}_2 {\hbox {H}}_6 {\hbox {O}}_2 $$ C 2 H 6 O 2 Nanofluids Spray Along a Stretching Cylinder

The flow of common fluids (water, oils and ethylene glycol etc.) is diluted by adding different small particles of metals, and their oxides are more powerful to reduce the scientific issues related to quicker heat transfer. According to this indication, we have contemplated finite film of $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ – $${\hbox {H}}_2 {\hbox {O}}$$ and $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ – $${\hbox {C}}_2 {\hbox {H}}_6 {\hbox {O}}_2 $$ nanoliquid sprayed on an extending cylinder. In this scenario, uniform magnetic field $$B_0$$ and constant reference temperature are employed on the stream of thin film nanofluid. The impact of effective Prandtl number, viscosity and thermal conductivity is derived from the experimental data (Sheikhzadeh et al. in J Appl Fluid Mech 10:209–219, 2017; Lee et al. in J Heat transf 121:280–289, 1992; Wang et al. in J Thermo Phys Heat Transf 13:474–480, 1999; Hamilton and Crosser in Ind Eng Chem Fundam 1:187–191, 1962; Maiga et al. in Super Lattices Microstruct 35:543–55, 2004; Hayat et al. in J Mol Liq. https://doi.org/10.1016/j.molliq.2018.06.029 , 2018). The model problem is excellently converted into a set of proper self-comparable forms with the assistance of possible transformations. Analytical results of velocity and thermal profile are computed using homotopy analysis method for both $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ – $${\hbox {H}}_2 {\hbox {O}}$$ and $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ – $${\hbox {C}}_2 {\hbox {H}}_6 {\hbox {O}}_2 $$ nanoliquid. Furthermore, during coating analysis, rate of spray, pressure distribution, skin friction coefficient (surface drag force) $$C_{\mathrm{f}} $$ and Nusselt number (the rate of heat transfer) Nu for both nanofluids are also intended. The impact of additional ingrained quantities like magnetic parameter M, volume fraction of nanoparticles $$\varphi $$ , Grashof number Gr, fluid thickness parameter $$\beta $$ , Prandtl number Pr and Reynolds number Re is portrayed numerically and graphically for both alumina particles. The key observation indicates that the temperature of $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ – $${\hbox {C}}_2 {\hbox {H}}_6 {\hbox {O}}_2$$ nanoliquid leading on $$\gamma {\hbox {Al}}_2 {\hbox {O}}_3$$ – $${\hbox {H}}_2 {\hbox {O}}$$ nanoliquid during the study. Due to greater viscosity and thermal conductivity, $${\hbox {C}}_2 {\hbox {H}}_6 {\hbox {O}}_2$$ -based nanofluid is observed as upgraded common base fluid assimilated to $${\hbox {H}}_2$$ O.

Preparation of carbon supported Pt-Ni alloy nanoparticle catalyst with high metal loading using cation exchange resin and its application for hydrogen production

Formic acid has been attracting interest as a promising hydrogen carrier because hydrogen can be produced over the catalysts under mild condition with low energy input. Though the catalytic activity for hydrogen production from formic acid is enhanced by increasing the metal loading, the trade-off between high metal loading and small metal particle size practically occurred with carbon catalyst prepared by impregnation method. To overcome this challenge, we herein utilized a noble catalyst preparation method which uses an ion-exchange resin as an initial carbon material. Five compositions of carbon supported Pt-Ni alloy catalysts were successfully prepared at high metal loading (25–50 wt%) with small metal particle sizes (2.3–3.8 nm observed by TEM). The metal particles were embedded in the carbon, which was contributed to small particle size of the loaded metal. The hydrogen production from formic acid was investigated over the prepared catalysts at 100–200 °C. It was elucidated that the turnover frequencies of Pt50Ni50/C and Pt75Ni25/C were higher than that of Pt/C at above 175 °C, suggesting that the decrease of Pt content without losing their catalytic activity could be achieved by Pt50Ni50/C and Pt75Ni25/C due to alloy formation. To further enhance the catalytic activity by increasing the active metal surface area, supercritical water treatment at 400 °C for 1 h was utilized. The metal surface area of catalysts was noticeably increased due to the gasification of the carbon support nearby metal particles. Formic acid conversion, likewise, drastically increased after the treatment.

Porous Nickel Nano‐Foam by Femtosecond Laser Structuring for Supercapacitor Application

AbstractA focused femtosecond laser beam was scanned across a nickel electrode in a line pattern with different line distances to generate a large electrochemical surface area for charge storage. During the laser structuring process, small metal particles were generated and sintered to a porous foam‐like structure, the so‐called laser‐induced nano‐foam (LINF), which strongly adheres to the substrate surface. The structuring was carried out in argon atmosphere, in order to prevent oxidation of the LINF structure during the structuring process. The topography of the LINF was investigated by scanning electron microscopy and laser scanning microscopy. The electrochemical surface area of the electrodes was determined by cyclic voltammetry based on the charging of the double‐layer. The total capacity of the nickel LINF electrodes was measured by galvanostatic charge‐discharge to test their capability for supercapacitor applications. The surface area enlargement and therefore the total capacity increases with decreasing line distance. The LINF structure provides a surface area enlargement up to a factor of 1600 and a total capacity up to 2 C cm−2.

A STM study of Ni-Rh bimetallic particles on reducible CeO2(111)

Ni-Rh particles were prepared on well-ordered CeOx(111) thin films by physical vapor deposition method under ultrahigh vacuum conditions. CeOx(111) (1.5 < x < 2) thin films with controlled oxidation states were grown on Ru(0001) to examine the effect of support structures on the growth and sintering of Ni-Rh particles. Scanning tunneling microscopy studies reveal that pure Ni and Rh grow small particles with 0.3 ML coverage at room temperature on CeO2(111), suggesting a strong metal-ceria interaction. Smaller metal particles are formed on a reduced CeO1.75(111) surface. The degree of Ce reduction in ceria can also affect the electronic properties of deposited metals. Charge transfer from the metal to ceria was observed upon deposition of both Rh and Ni on CeO2. However, Ni and Rh remain a metallic state on CeO1.75. Bimetallic Ni-Rh particles were prepared by deposition of 0.3 ML Rh followed by 0.3 ML Ni on CeO2 at 300 K. A large fraction of Ni deposits on the pre-dosed Rh particles on CeO2 to produce bimetallic Ni-Rh particles. However, a significant amount Ni was observed to form additional pure Ni particles. The formation of Ni-Rh bimetallic particles can be promoted by increasing the particle density upon deposition of the first metal, which can act as the nucleation sites for the second metal. Ni-Rh bimetallic particles exhibit a significantly smaller size compared to that of pure Ni and Rh upon heating. Addition of Rh can inhibit the sintering of Ni and enhance its thermal stability on ceria. Our study has provided morphological and size information for the understanding of the activity of ceria-supported Ni-Rh catalysts.

Understanding the Sn Loading Impact on the Performance of Mesoporous Carbon/Sn‐Based Nanocomposites in Li‐Ion Batteries

AbstractHerein, we report a systematic study to understand the influence of the amount of tin metal precursor salt on the formation of carbon/tin hybrid materials and their performances as anodes in Li‐ion batteries. Small Sn metallic particles (ca. 5 nm) covered by a SnO2 layer were uniformly dispersed in a mesoporous carbon for a low loading of tin; whereas, for higher Sn loadings, the formation of Sn‐based particles aggregates (ca. 200 nm) is promoted as well. By increasing the Sn loading from 20 to 80 %, the irreversible capacity was successfully reduced and the reversible capacity improved. This could be related to the decrease of the C/Sn hybrids specific surface area and the increase of the Sn active species. For long‐term cycling, capacity fading was observed, particularly for high Sn loadings assigned to the Sn nanoparticles placed outside the carbon network, which upon lithiation witness large volume expansion, leading to severe particle growth and agglomeration. Therefore, similar reversible capacities at long cycling are reached, no matter the Sn loading. For optimal electrochemical performances, it appears that a balance between the amount of Sn and uniform small Sn‐based particles dispersion within carbon matrix must be assured to design high‐performance anodes for Li‐ion batteries.

Using Ex-situ TEM to Understand the Effect of Different Oxidative Environments on Small Metal Particle Morphology

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Nonmetal to Metal Transition and Ultrafast Charge Carrier Dynamics of Zn Clusters on p-Si(100) by fs-XUV Photoemission Spectroscopy.

Understanding the electronic structure and charge carrier dynamics of supported clusters is important due to their many potential applications in photochemistry and catalysis. In this investigation, photoemission spectroscopy, in conjunction with femtosecond extreme ultraviolet (XUV) laser pulses, is used to investigate the electronic structure and ultrafast charge carrier dynamics at a Si(100) surface decorated with Zn clusters. Static photoemission spectroscopy is used to investigate the changes in the electronic structure as the dimensionality of the Zn is increased from small clusters composed of a very few atoms to metallic Zn particles. Furthermore, femtosecond optical-pump XUV-probe photoemission spectroscopy is employed to induce a charge transfer from the p-Si(100) substrate to the Zn clusters and to measure in real time the charge trapping at the Zn cluster as well as the subsequent charge relaxation. The ultrafast charge carrier dynamics are also investigated for small clusters and metallic Zn particles. Significant transient charging of the Zn clusters after excitation of the Si(100) substrate by 800 nm light is observed for Zn coverages greater than 0.12 ML Zn, which coincides with the formation of a Schottky barrier at the interface between the Zn particle and the p-Si(100) substrate. The transient signals show that the charge trapping time at the Zn cluster varies with the cluster size, which is rationalized based on the electronic structure of the cluster as well as the band energy alignment at the Zn cluster-Si(100) junction.

Single-atom catalysis: a new field that learns from tradition

Abstract Much of industrial chemical processing (in the petrochemicals industry, for example), and a great deal of laboratory chemical synthesis, involves catalysts that both lower the energy barrier to reaction and may help steer a reaction along a particular path. Traditionally, catalysts have come in two classes: heterogeneous, typically meaning that the catalyst is an extended solid; and homogeneous, where the catalyst is a small molecule that shares a solvent with the reactants. In heterogeneous catalysis, the reaction generally takes place on a surface, involving molecules attached there by covalent bonds. Homogeneous catalysts are often organometallic compounds, in which a metal atom or small cluster of atoms supplies the active site for reaction. In recent years, these distinctions have become somewhat blurred thanks to the advent of single-atom catalysis, where the catalytic site consists of a single atom (as in many homogeneous catalysts) attached to or embedded in a surface. The emergence of this field might be regarded as the logical conclusion of the use of ‘supported metal clusters’—small metal particles of nanometer scale and below, containing perhaps hundreds, tens or just a few atoms. It has became clear that such clusters can sometimes provide greater product selectivity and activity than macro-sized particles or powders of the same metal, partly because the active sites might be atoms at particular locations (such as edges and corners) in the nanoscale particles. By reducing their scale down to the level of single atoms, one can optimize these properties. At the same time, the potential uniformity of the atoms’ environments makes such catalysts more amenable to rational design and modeling to understand mechanism. This field represents an appealing blend of fundamental chemistry and physics—from the quantum-mechanical level upwards—and applied research aimed at producing many of the products vital to society, such as fuels and materials. Researchers in China have been strongly active in this field in recent years (see, for example, refs [1–5]). Jean Marie Basset of the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, is one of the leading practitioners in the area, and National Science Review spoke to him about the development and prospects of the field.

Investigation of nano-sized debris released from CoCrMo secondary interfaces in total hip replacements: Digestion of the flakes.

The in vivo release of wear debris and corrosion products from the metallic interfaces of total hip replacements is associated with a wide spectrum of adverse body reactions and systemic manifestations. The origin of debris and the electrochemical conditions at the sites of material loss both play a role in determining the physicochemical characteristics of the particles, and thus influence their in vivo reactivity. Debris retrieved from revised CoCrMo tapers and cement-stem interfaces consists of heterogeneous flakes that comprise mechanically mixed metal particles, corrosion products and organic material. Detailed investigation of the size and composition of the metal debris contained within these composites requires the digestion of the flakes to release the small metal particles. Here, we compare alkaline and enzymatic digestion methods that both aim to fragment the flakes and reveal their smallest building blocks. The characterization of debris cleaned with both methods revealed crystalline Cr oxide nanoparticles and clusters. Comparison between the treatments showed that the alkaline method is more efficient in fragmenting the flakes and provided cleaner and generally smaller nanoparticles than exhibited in debris released with the enzymatic treatment. The provision of cleaner nanoparticles from the alkaline method also allows the physicochemical properties of the particles to be more clearly identified. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 424-434, 2019.

Catalytic Nanoparticles Prepared By Atomic Layer Deposition

Heterogeneous catalysts normally consist of small metal particles dispersed on a high surface area porous support. Traditional methods, such as wet-chemical processing, can produce metal particle catalysts as small as several nanometers, but these methods cannot precisely control the size of the catalytic nanoparticles and disperse them homogeneously within the porous substrates. In addition, heterogeneous catalysts cannot selectively convert specific molecules in the reactant mixture to catalyze only desired reactions. Novel approaches are required to synthesize stable metal nanoparticles catalysts with tightly controlled sizes to further advance the knowledge of their unique size-dependent catalytic behavior. Recently, atomic layer deposition (ALD) has been used to prepare highly active, highly dispersed metal nanoparticles. In this presentation, we will introduce some examples of nanostructured catalysts prepared by ALD, such as Pt nanoparticles, Ni nanoparticles, Pt-Co bimetallic nanoparticles, and CeO2 doped TiO2 nanoparticles.

Enhancement of the Local Electromagnetic Field over Planar “Particles” Formed on the Surface of a Polar Crystal

Small “particles” of an open surface were formed on a SiC polar crystal with openings in the opaque metal mask covering the sample. The dimensions of the holes were about surface phonon polariton wavelength. Such a sample was irradiated with an electromagnetic wave ( λ = 10.68μm) at a frequency close to the lattice resonance of SiC. A significant enhancement in the field amplitude of surface phonon polariton waves was detected over such “particles” compared to the amplitude over an infinite open surface of SiC. Such a phenomenon, observed by us in the IR band, is similar to plasmon resonance on small metal particles in the visible band, but the lateral resolution of the ASNOM used (no worse than 30 nm at 10 μm) makes the obtained field distribution more detailed. The maps of the local field amplitude and phase obtained on SiC surface with ASNOM are in a good quantitative agreement with simulations using the Green’s function.

A single source method to generate Ru-Ni-MgO catalysts for methane dry reforming and the kinetic effect of Ru on carbon deposition and gasification

A single precursor RuxNiyMg1-x-y(OH)(OCH3) derived from solvothermal synthesis was used to generate Ru-Ni-MgO catalysts for methane reforming with CO2. Calcination-reduction pretreatment of precursors could easily cause segregation of Ru and formation of both large and small metallic particles as RuO2 has limited solubility in NiO-MgO solid solution. Uniform small Ru-Ni alloy particles could only be produced within their limited alloying composition range through direct reduction pretreatment of the precursor. Catalysts derived from calcination-reduction exhibited low initial activity that increased with time-on-stream, whereas catalysts derived from direct reduction demonstrated high and steady activity. Over spent catalysts, Ru was found to have changed the type of deposited carbon from a recalcitrant graphitic one that could only be gasified by O2 to a soft type that can be facilely gasified by CO2. Kinetic studies showed that Ru increased the activation barrier for the rate determining CH4 dissociation step and thereby slows down the carbon deposition rate. A first order reaction dependence for CH4 pressure variation and zeroth for CO2 pressure change for pristine Ni- and Ru-catalysts was identified, while a first order and a deviation from zeroth order for CH4 and CO2 pressure variation were observed on bimetallic Ru-Ni catalyst. Such a deviation is associated with the oxyphilic nature of Ru that is enriched in the alloy surface under reforming conditions. The effects of Ru on carbon gasification over spent catalysts were investigated using a modified CO2-TPO measurement based on an extrapolated Wigner–Polanyi equation for carbon gasification. Ru was found to accelerate carbon gasification by increasing the pre-exponential factor for CO2 oxidation of carbon, albeit a disfavored elevated activation barrier was obtained, thus showing a strong compensation effect. Carbon gasification is favored in high concentration of CO2 and at high temperatures for Ru-Ni catalyst.

Bimetallic Ni-Co/SBA-15 catalysts prepared by urea co-precipitation for dry reforming of methane

A series of NixCo(10-x)/SBA-15 catalysts with constant metal loading (10 wt%) and different Co/Ni mass ratios (0/10–10/0) were prepared by urea co-precipitation and applied to the dry reforming of methane (DRM). The physicochemical properties of the catalysts were characterized in detail by various techniques (N2 physisorption, H2 chemisorption, XRD, TPR, ICP-OES, TGA, HRTEM and XPS), revealing that Ni and Co particles were embedded into the channel of SBA-15 and Ni was alloyed with Co. In addition, the bimetallic catalysts with a Co/Ni ratio (RCo/Ni) lower than 1 had relatively high metal dispersion (ca. 17–20%) and small metal particle sizes (ca. 4–5 nm). The DRM activity and stability of NixCo(10-x)/SBA-15 in terms of coke- and sintering- resistance were systematically examined and compared, which showed that the activity and stability of the catalysts depended strongly on the metal particle size, indicating the structure-sensitive character of the DRM reaction. Compared to monometallic Ni10/SBA-15 and Co10/SBA-15, bimetallic NixCo(10-x)/SBA-15 with RCo/Ni < 1 possessed improved activity and stability, which was mainly attributed to the small metal particle size, the synergetic effect of Co and Ni as well as the confinement effect of SBA-15 mesoporous channels. In particular, Ni9Co1/SBA-15 exhibited the highest activity, stability and H2/CO molar ratio (close to 1) during 50 h of operation at a gas hourly space velocity of 72000 mL/(gcat h) under 973 and 1073 K. Moreover, no coke deposition occurred on the catalyst, with only slight metal particle size growth to around 5 nm, indicating excellent coke- and sintering-resistance.

Smart Pd Catalyst with Improved Thermal Stability Supported on High-Surface-Area LaFeO3 Prepared by Atomic Layer Deposition.

The concept of self-regenerating or "smart" catalysts, developed to mitigate the problem of supported metal particle coarsening in high-temperature applications, involves redispersing large metal particles by incorporating them into a perovskite-structured support under oxidizing conditions and then exsolving them as small metal particles under reducing conditions. Unfortunately, the redispersion process does not appear to work in practice because the surface areas of the perovskite supports are too low and the diffusion lengths for the metal ions within the bulk perovskite too short. Here, we demonstrate reversible activation upon redox cycling for CH4 oxidation and CO oxidation on Pd supported on high-surface-area LaFeO3, prepared as a thin conformal coating on a porous MgAl2O4 support using atomic layer deposition. The LaFeO3 film, less than 1.5 nm thick, was shown to be initially stable to at least 900 °C. The activated catalysts exhibit stable catalytic performance for methane oxidation after high-temperature treatment.

Enhanced catalytic performance of Ni/SBA-15 towards CO2 methanation via P123-assisted method

This study focused on the enhancement of catalytic performance of Ni/SBA-15 towards CO2 methanation via P123(PEG-PPG-PEG triblock copolymer)-assisted impregnation method. The physical and chemical properties of the catalysts were characterized using XRD, BET, TEM, and FTIR, meanwhile the catalytic performance of catalysts towards CO2 methanation were evaluated using stainless steel fixed bed reactor. The presence of coke on the surface of catalysts was characterized using TGA analysis. XRD, BET and TEM results revealed that the dispersion of Ni particles on the surface of SBA-15 were improved with P123 (Ni/SBA-15(P123)) as compared to without P123 (Ni/SBA-15). FTIR analysis revealed that P123 enhanced the formation of metal-support interaction (Si-O-Ni) through the substitution of O-H with O-Ni. Ni/SBA-15(P123) exhibited higher catalytic activity with 90.3% CO2 conversion and 97.5% CH4 selectivity, meanwhile Ni/SBA-15 showed lower catalytic activity with 81.6% CO2 conversion and 97.1% CH4 selectivity at temperature of 623 K. Apart from higher catalytic activity, Ni/SBA-15(P123) also showed better stability than that of Ni/SBA-15 whereby no carbon formation detected in TGA analysis for 48 h time on stream. The promising performance of Ni/SBA-15(P123) associated to its smaller metal particles, stronger metal-support interaction and more homogeneous metal dispersion, which altered the properties of catalyst towards an excellent catalytic performance. This study provides new perspective on the beneficial effect of P123-assisted impregnation method in the enhancement of catalytic performance of Ni/SBA-15 towards CO2 methanation.

Hydrogenation of Nitro and Unsaturated Organic Compounds over Catalysts Containing Nanosized Palladium Particles

The activity of palladium catalysts prepared on the basis of ZnO modified with polyethylene glycol (1 wt % Pd–PEG/ZnO) with molecular weights of 4000 and 6000 in the hydrogenation reaction of a series of nitro and unsaturated organic compounds has been studied. The catalysts are characterized by the formation of small metal particles with a size from 3 to 8 nm which uniformly coat the support surface. The results obtained have been compared to the catalytic properties of palladium-containing nanodiamonds and activated carbon under similar conditions.

Two-step photochemical inorganic approach to the synthesis of Ag-CeO2 nanoheterostructures and their photocatalytic activity on tributyltin degradation

Abstract Herein, we report a simple, sustainable and low-cost approach to design Ag-CeO 2 nanoheterostructures in pure aqueous and ethanol containing aqueous solutions via photochemical UV-light driven process with no capping agents nor stabilizers required. To this end, photochemically synthesized CeO 2 nanoparticles were applied as photoactive compounds in order to generate formation of metallic silver nanoparticles. Irradiation of deaerated CeO 2 suspensions in the presence of Ag + resulted in the rise of a strong surface plasmon resonance band with a maximum at 393–422 nm in the absorption spectra of the solutions, indicating formation of small metallic silver particles. Faster formation of Ag nanoparticles with the lower amount of silver precursor being required was observed when ethanol was introduced to the reaction solution before the irradiation. This implies that oxidative reactions can be strongly suppressed in deaerated ethanol containing solutions with respect to the pure aqueous media. Not only was the overall efficiency of the process remarkably increased by the use of alcohol, but also smaller and more uniform silver nanoparticles with a size comparable to that of ceria nanoparticles (around 15 nm) were formed when compared to those synthesized without radical scavengers as revealed by TEM analysis. The proposed photochemical approach enables the production of silver-semiconductor system without employing organic stabilizers, thus resulting in formation of nanoparticles with “clean”, highly reactive metal surface. The as-synthesized silver-ceria nanoheterostructures demonstrated enhanced visible light driven photocatalytic activity on tributyltin (TBT) degradation if compared to pure ceria nanoparticles.

DNA-Nanotechnology-Enabled Chiral Plasmonics: From Static to Dynamic.

The development of DNA nanotechnology, especially the advent of DNA origami, has made DNA ideally suited to construct nanostructures with unprecedented complexity and arbitrariness. As a fully addressable platform, DNA origami can be used to organize discrete entities in space through DNA hybridization with nanometer accuracy. Among a variety of functionalized particles, metal nanoparticles such as gold nanoparticles (AuNPs) feature an important pathway to endow DNA-origami-assembled nanostructures with tailored optical functionalities. When metal particles are placed in close proximity, their particle plasmons, i.e., collective oscillations of conduction electrons, can be coupled together, giving rise to a wealth of interesting optical phenomena. Nevertheless, characterization methods that can read out the optical responses from plasmonic nanostructures composed of small metal particles, and especially can optically distinguish in situ their minute conformation changes, are very few. Circular dichroism (CD) spectroscopy has proven to be a successful means to overcome these challenges because of its high sensitivity in discrimination of three-dimensional conformation changes. In this Account, we discuss a variety of static and dynamic chiral plasmonic nanostructures enabled by DNA nanotechnology. In the category of static plasmonic systems, we first show chiral plasmonic nanostructures based on spherical AuNPs, including plasmonic helices, toroids, and tetramers. To enhance the CD responses, anisotropic gold nanorods with larger extinction coefficients are utilized to create chiral plasmonic crosses and helical superstructures. Next, we highlight the inevitable evolution from static to dynamic plasmonic systems along with the fast development of this interdisciplinary field. Several dynamic plasmonic systems are reviewed according to their working mechanisms. We first elucidate a reconfigurable plasmonic cross structure that can execute DNA-regulated conformational changes on the nanoscale. Hosted by a reconfigurable DNA origami template, the plasmonic cross can be switched between a chiral locked state and an achiral relaxed state through toehold-mediated strand displacement reactions. This reconfigurable nanostructure can also be modified in response to light stimuli, leading to a noninvasive, waste-free, and all-optically controlled system. Taking one step further, we show that selective manipulations of individual structural species coexisting in one ensemble can be achieved using pH tuning of reconfigurable plasmonic nanostructures in a programmable manner. Finally, we describe an alternative to achieving dynamic plasmonic systems by driving AuNPs directly on origami. Such plasmonic walkers, inspired by the biological molecular motors in living cells, can generate dynamic CD responses when carrying out directional, progressive, and reverse nanoscale walking on DNA origami. We envision that the combination of DNA nanotechnology and plasmonics will open an avenue toward a new generation of functional plasmonic systems with tailored optical properties and useful applications, including polarization conversion devices, biomolecular sensing, surface-enhanced Raman and fluorescence spectroscopy, and diffraction-limited optics.

Towards Controlling the Electrocatalytic Hydrogenation of Oxygenated Hydrocarbons through Particle Size Effects

Metal-catalyzed hydrogenations are regarded as non-sensitive reactions in the gas phase. However, varying proportions of high- and low-coordination surface atoms may lead to differences in the selectivity when hydrogenation is performed in aqueous phase especially under electrochemical condition where H2 evolution reaction (HER) appeared to be a major competitive route. The aim of this work was to test this hypothesis by performing the electrocatalytic hydrogenation (ECH) of aromatic hydrocarbons on Pt/C and Pd/C catalysts with varying metal particle sizes. Phenol and benzaldehyde, representative model compounds for bio-oils derived from lignocellulosic biomass, were chosen as target compounds. ECH Experiments were performed at ambient conditions using H-cells wherein cathodic potential was varied systematically. The results of these experiments were compared with catalytic thermal hydrogenation, where the reduction equivalents are provided by H2 dissociation. In the studied range (from -0.6 V to -0.8 V vs Ag/AgCl), the rates of both ECH and HER increase with increasing cathodic potentials. These increases, however, depended on the metal loading of the cathode. As an exemplary result, the figure shows that the rates of phenol ECH reached a maximum at the Pt dispersion of 26%, which corresponded to a Pt loading of 5 wt.% on carbon. However, the rates normalized per active site showed a different picture. TOF values increased as the dispersion of the metal decreased. In stark contrast, the rates of the H2 evolution reaction (HER) were proportional to the dispersion. Hence, the main conclusion of this work is that the intrinsic rates of ECH of polar molecules are higher on extended metal surfaces than in the corners and edges exhibited by small metal particles. The intrinsic rates of HER, on the other hand, are favored by small particles under the applied reaction conditions. The combination of these two opposite trends rendered the catalyst with Pt particles with the average size of 4.5 nm (5 wt.% Pt on C in this work) as the most active catalyst per gram. However, the catalyst with average Pt particle size of 10 nm shows the highest intrinsic activity as well as faradaic efficiency (30%) Thermal catalytic hydrogenation (TCH) also followed the negative particle size effect to the activity trend, thus, coincide with ECH experiments. This strongly suggests the particle size effect noted in the electrochemical experiments is primarily related to the aqueous environment of the reaction instead of presence of electric potential or competing HER. It is worth highlighting that ECH rates were higher than TCH rates at cathodic potentials higher than -0.7 V vs Ag/AgCl for all tested substrates, which is ascribed to higher coverages of reactive H. This work contributes to the understanding of metal catalysis at low temperatures in condensed phases. It also shows directions for the development of highly active electrocatalysts for the conversion of polar molecules. In specific, we show that as the dispersion of the metal phase increases, the compromise between increasing concentrations of available active sites and their decreasing reactivity leads to optimum particle sizes. Figure caption. Left: Rates of phenol hydrogenation on Pt/C along with dispersion at varying cathodic potentials (vs Ag/AgCl). Right: Turnover frequency (TOF) for phenol hydrogenation on Pt/C along with dispersion at varying cathodic potentials (vs Ag/AgCl). The points labeled as "thermal" correspond to hydrogenation performed with H2 gas. Figure 1

Identification of active sites for hydrogenation over Ru/SBA-15 using in situ Fourier-transform infrared spectroscopy

The active sites for hydrogenation over Ru/SBA-15 catalysts were identified using in situ Fourier-transform infrared spectroscopy. The amount of active sites was proportional to the interfacial circumference of the Ru particles. In contrast, the rate of hydrogen spillover from Ru to the support was inversely proportional to the size of the Ru metal particles. Consequently, a catalyst with small Ru metal particles has a high rate of hydrogen spillover but a low density of active sites, whereas one with large Ru particles has a low rate of hydrogen spillover but a high density of active sites. The formation of these active sites is probably an intermediate step in hydrogen spillover.