Growth Of Metal Particles Research Articles

Mononuclear Ruthenium Hydride Species versus Ruthenium Nanoparticles: The Effect of Silane Functionalities on Silica Surfaces

Aggregation is not a fate! Replacing silica surface silanols with silanes allows [Ru(cod)(cot)] to be grafted by a covalent RuSi bond, which avoids the growth of metal particles during treatment under H2 at 300 °C. This affords supported mononuclear Ru hydride species, which display catalytic properties different from those of supported Ru particles (2 nm), selectively hydrogenating alkenes over aromatic compounds. cod=cyclooctadiene, cot=cyclooctatriene.

Surface modification of antioxidated nanocopper particles’ preparation in polyol process

A simple and efficient chemical process, which is called two step reduction route in polyol process, for producing copper nanoparticles with satisfied antioxidation property, was described here. This process’s reaction mechanism was demonstrated by analysing as prepared copper particles’ XRD patterns, SEM images and size distribution plots. The analysis results shows that all the preparation subprocesses such as reduction of copper ion, nucleation of copper particles and growth of metallic copper particles can be controlled through using polyol process. This two step reduction method can not only compulsively disconnect nucleation and nucleation growth, but also weaken conventional eruptible nucleation, which made it convenient for delaying the nuclei process and for controlling copper particles size. In addition, protective mechanisms of adopted surfactant in this process also were analysed here. When a quantitative surfactant was added into the reaction solution at the end of the reduction reaction, the electrolyte polymer covered on the new generated copper particles, some of it could still adhere on the particles’ surface after lavation, which made this agent play a role not only in particles’ dispersion, but also in particles’ antioxidation.

Improvement in the Durability of Pt Electrocatalysts by Coverage with Silica Layers

Carbon nanotube (CNT)-supported Pt metal nanoparticles were covered with silica layers by utilizing the successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt hydroxide. The CNT-supported Pt metal particles covered with silica layers (denoted as SiO2/Pt/CNT) were used as the electrocatalysts. SiO2/Pt/CNT electrocatalysts showed a high stability for the repeated potential cycling experiment, whereas Pt/CNT electrocatalysts were deactivated seriously for the experiment because of the growth of Pt metal particles in size. The silica layers in SiO2/Pt/CNT prevent the dissolution of Pt metal particles as well as the migration and agglomeration of Pt metal particles on the supports, which results in the improvement of the stability of Pt/CNT electrocatalysts.

In-situ X-ray Absorption Spectroscopy of Supported Ru Catalysts in the Aqueous Phase

The size of supported Ru metal particles on various supports was monitored by in situ X-ray absorption spectroscopy under aqueous phase conditions typical of biomass conversion reactions. In particular, Ru/γ-Al2O3, Ru/C, Ru/TiO2, and Ru/SiO2 were evaluated at the Ru K edge at 473 K in neutral water and 0.4 M NaOH. The as-prepared samples exposed to air contained oxidized Ru that was subsequently reduced by H2-saturated water solution. Significant growth of the metal particles was observed on the Ru/γ-Al2O3 and Ru/SiO2 samples during the aqueous treatments, whereas the Ru/TiO2 and Ru/C samples were quite stable under the conditions used here. Results from post-treatment X-ray diffraction and surface area analysis revealed major structural changes of the alumina and silica supports. The structural stability of the carbon and titania account for the lack of metal particle growth on those supports. Because a reference Pt/C catalyst revealed metal particle growth under the same conditions, the results for the ...

Hydrogen production by steam reforming of LNG over Ni/Al 2O 3–ZrO 2 catalysts: Effect of Al 2O 3–ZrO 2 supports prepared by a grafting method

Al 2O 3–ZrO 2 supports with various zirconium contents were prepared by grafting a zirconium precursor onto the surface of γ-Al 2O 3. Ni(20 wt%)/Al 2O 3–ZrO 2 catalysts were then prepared by an impregnation method, and were applied to the hydrogen production by steam reforming of LNG. The effect of Al 2O 3–ZrO 2 supports on the performance of the Ni(20 wt%)/Al 2O 3–ZrO 2 catalysts was investigated. Al 2O 3–ZrO 2 prepared by a grafting method served as an efficient support for the nickel catalyst in the steam reforming of LNG. ZrO 2 inhibited the incorporation of nickel species into the lattice of Al 2O 3 and prevented the growth of metallic nickel particles during the reduction step. The crystalline structures and catalytic activities of the Ni(20 wt%)/ZrO 2–Al 2O 3 catalysts were strongly influenced by the amount of zirconium grafted. LNG conversion and hydrogen yield showed volcano-shaped curves with respect to zirconium content. Among the catalysts tested, the Ni(20 wt%)/ZrO 2–Al 2O 3 (Zr/Al = 0.17) catalyst showed the best catalytic performance in terms of both LNG conversion and hydrogen yield. The well-developed and pure tetragonal phase of ZrO 2–Al 2O 3 (Zr/Al = 0.17) played an important role in the adsorption of steam and the subsequent spillover of steam from the support to the active nickel. The high reducibility of Ni(20 wt%)/ZrO 2–Al 2O 3 (Zr/Al = 0.17) was also responsible for the enhanced performance of the catalyst.

Preparation of carbon supported binary Pt–M alloy catalysts (M = first row transition metals) by low/medium temperature methods

Carbon supported Pt alloyed with first row transition elements (Pt–M/C) is being used as improved cathode catalyst for low temperature fuel cells. These catalysts have been usually prepared by deposition of the non-precious metal onto pre-formed carbon supported platinum, followed by alloying at temperatures of the order or above 700 °C. As the thermal treatment at high temperature gives rise to an undesired metal particle growth, synthetic methods based on the simultaneous deposition of Pt and M on the carbon substrate, followed by thermal treatment at lower temperature have been developed. In this paper the formation of Pt–M/C by low/intermediate temperature methods is reviewed. Moreover, to investigate the effect of the conditions used in the synthesis on the Pt:M atomic ratio, the degree of alloying and the particle size, carbon supported Pt–Co electrocatalysts with nominal Pt:Co atomic ratio 75:25 were prepared by a low temperature chemical reduction of the precursors with sodium borohydride at two different temperatures and NaBH 4 concentrations. The physical characterization of these electrocatalysts was performed by energy dispersive X-ray analysis and X-ray diffraction.

One Pot Synthesis of Carbon/Ni Nanoparticle Monolithic Composites by Nanocasting and Their Catalytic Activity for 4-Nitrophenol Reduction

Background: There is considerable interest in using transition metal catalysts in place of rare and expensive precious metal catalysts in fixed bed catalysis. The use of hierarchically porous materials potentially provides a method of improving mass transport and decreasing sintering and metal particle growth that leads to decreased catalytic activity. Methods: Composites consisting of nickel nanoparticles incorporated in a hierarchically porous carbon monolith (NC/Ni), with varying Ni loading were synthesized by a one-pot, nanocasting pathway in which the carbon and nickel were produced in a single step, and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption for surface area determination, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman spectroscopy. Catalysis of the reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) was carried out to evaluate the catalytic activity. Results: TEM and SEM images of the composites show well dispersed nanoparticles in the carbon matrix with small nanoparticles of about 4-12 nm located within the mesopores and larger nanoparticles (20-60 nm) located in the macropores. The nanoparticles were unevenly distributed on the mm scale within the monoliths. The surface area and the mesopore volume of the carbon decreased with increasing metal loading. XRD suggested the nanoparticles were metals at zero oxidation state but XPS showed the presence of Ni(OH)2. Raman spectra showed a significant increase in carbon ordering with increasing metal loading. The composites were found to be catalytically active for the reduction of p-NP and the intermediate p-quinoimine (p-QI) was identified by GC-MS. Conclusions: The one-pot nanocasting method is able to produce catalytically active hierarchically porous carbon monoliths incorporating nickel nanoparticles. However, the size and distribution of the nanoparticles is not yet well controlled. Keywords: Nanocasting, monolith, nickel, mesoporous carbon, 4-nitrophenol, reduction, p-quinoimine.

Metal Particle Growth during Glucose Hydrogenation over Ru/SiO2 Evaluated by X-ray Absorption Spectroscopy and Electron Microscopy

Biorenewable resources such as carbohydrates are considered alternative feedstocks for oxygenated chemicals. This work investigates the stability of silica-supported Ru catalysts in the aqueous phase conversion of glucose to sorbitol. In situ X-ray absorption spectroscopy at the Ru K edge revealed that air-exposed silica-supported Ru was in an oxidized state but was subsequently reduced in aqueous solutions saturated with 40 bar H(2) at 373 K. Furthermore, exposure to aqueous phase conditions resulted in the sintering of Ru particles on the silica surface. However, the presence of glucose in the aqueous phase stabilized the growth of the Ru particles. Batchwise hydrogenation of glucose at 373 K and 80 bar H(2) over a Ru/SiO(2) (2.67 wt %) catalyst is nearly 100% selective to sugar alcohol with an average turnover frequency of 0.21 +/- 0.04 s(-1). The hydrogenation reaction was not mass transfer limited according to the Madon-Boudart criterion.

Atom beam sputtering setup for growth of metal particles in silica

A fast atom source has been installed for the synthesis of metal particles in insulating matrix. The source delivers current density of 30μA/cm2 on the target placed at a distance of 10cm and with variable energy ranging from 0.8 to 2.0keV. The area of the beam is approximately 50mm diameter. Using the setup, particles of Ag and Cu have been grown in silica matrix. The sputtered species were deposited on quartz and silicon substrate. These composite films were also prepared by co-evaporation of SiO2 and metal. The samples were annealed at different temperatures and growth of metal particles in silica matrix was investigated by the optical absorption spectrometry. Distinct surface plasma resonance peaks have been observed both from Ag and Cu particles. Mie’s theory is used to estimate the particle size. Rutherford backscattering spectrometry was used to quantify the concentration of metal fraction in the silica matrix of the composite thin films.

A Structural Study on Self-regenerative Catalyst for Automotive Emissions Control by XAFS and DANES

A mechanism of how the Palladium-composite Perovskite catalyst for automotive emissions control keeps its activity long-lived is investigated by using XAFS and DANES (Diffraction Anomalous Near Edge Structure), a family of an anomalous X-ray diffraction techniques. The conventional catalysts are finely divided precious metal particles dispersed on a solid support. Since they suffers from heat in the car, their activity deteriorates because of the growth and agglomeration of the particles. Recently, however, the Palladium-composite Perovskite has been found to have retention of high catalyst activity during long-term use and ageing. In order to investigate the mechanism of such an ideal property in the atomic revel, X-ray diffraction and absorption techniques were employed. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic palladium particles.

03aB01 Electric field effect on the growth of metallic and polar crystal particles(NCCG-36)

03aB01 Electric field effect on the growth of metallic and polar crystal particles(NCCG-36)

Mechanism of Gold Metal Ion Reduction, Nanoparticle Growth and Size Control in Aqueous Amphiphilic Block Copolymer Solutions at Ambient Conditions

Spontaneous formation and efficient stabilization of gold nanoparticles with an average diameter of 7 approximately 20 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) were achieved in air-saturated aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer solutions at ambient temperature in the absence of any other reducing agent. The particle formation mechanism is considered here on the basis of the block copolymer concentration dependence of absorption spectra, the time dependence (kinetics) of AuCl4- reduction, and the block copolymer concentration dependence of particle size. The effects of block copolymer characteristics such as molecular weight (MW), PEO block length, PPO block length, and critical micelle concentration (cmc) are explored by examining several PEO-PPO-PEO block copolymers. Our observations suggest that the formation of gold nanoparticles from AuCl4- comprises three main steps: (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters and reduction of metal ions on the surface of these gold clusters, and (3) growth of metal particles stabilized by block copolymers. While both PEO and PPO blocks contribute to the AuCl4- reduction (step 1), the PEO contribution appears to be dominant. In step 2, the adsorption of block copolymers on the surface of gold clusters takes place because of the amphiphilic character of the block copolymer (hydrophobicity of PPO). The much higher efficiency of particle formation attained in the PEO-PPO-PEO block copolymer systems as compared to PEO homopolymer systems can be attributed to the adsorption and growth processes (steps 2 and 3) facilitated by the block copolymers. The size of the gold nanoparticles produced is dictated by the above mechanism; the size increases with increasing reaction activity induced by the block copolymer overall molecular weight and is limited by adsorption due to the amphiphilic character of the block copolymers.

A kinetic model of CH 4 decomposition and filamentous carbon formation on supported Co catalysts

A kinetic model of CH 4 decomposition on supported Co catalysts, that describes the stable catalyst activity associated with filamentous carbon formation and the catalyst deactivation associated with the formation of encapsulating carbon, is presented. The rate of carbon nucleation at the tailing face of the supported metal particle, a precursor for the growth of filamentous carbon, is described by a cluster nucleation model. The deactivation or stable activity, observed after an initial rate increase that is associated with carbon nucleation, is explained by the competition between the rate of carbon diffusion through the metal particle and filamentous carbon growth, versus the rate of encapsulating carbon formation on the leading face of the particle. The site density profiles through the metal particle at different reaction times are also calculated, and the effect of metal particle size on the initial CH 4 decomposition rate is quantified.

Stresses and adhesion of thin metallic coatings on oxide substrates

The growth of thin metallic coatings on oxide substrates prepared by vapour deposition of gold or copper on alumina or magnesia was investigated by transmission electron microscopy. The stresses developed during the growth were followed by the bending method. The evolution of stresses is related to the growth mechanisms. The stresses are compressive for discontinuous deposits and change to tensile as the deposit becomes continuous. The compressive stresses are interpreted as due to the increase of the free energy of the metal–oxide system during the growth of metal particles. The tensile stresses result from the deposit lattice mismatch with the substrate. During the growth partial recrystallization and annealing of defects occur which modify the tensile behaviour of continuous coatings. The adhesion of deposits (Au and Cu) on oxide substrates (Al2O3 and MgO) was evaluated by the pull-off method as a function of deposition temperature. Au/Al2O3, Cu/Al2O3 and Au/MgO systems exhibit identical behaviour, while Cu/MgO shows a large dependence on deposition temperature compared to the others. This behaviour is interpreted as due to the formation of chemical bonds between the metal (Cu) and the oxide (MgO), leading to an interfacial oxide.

Freely Dispersible Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 Core−Shell Nanoparticles: One-Step Synthesis, Characterization, Spectroscopy, and Optical Limiting Properties

We report a one-step route for the synthesis of Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 particles in nanometer dimensions, with controllable shell thickness. This scalable procedure leads to stable and freely dispersible particles, and bulk nanocomposite materials have been made this way. The procedure leads to particles of various morphologies, with a crystalline core in the size range of 30−60 nm diameter and an amorphous shell of ∼3 nm thickness in a typical synthesis. The core diameter and shell thickness (in the range of 1−10 nm) can be varied, leading to different absorption maxima. The material has been characterized with microscopic, diffraction, and spectroscopic techniques. The metal particle growth occurs by the carbamic acid reduction route followed by hydrolysis of the metal oxide precursor, resulting in the oxide cover. The particles could be precipitated and redispersed. The shell, upon thermal treatment, gets converted to crystalline oxides. Cyclic voltammetric studies confirm the core−shel...

The Use of Mesoporous Systems for Preparation of One-Dimensional Ordered Magnetic Nanowires

ABSTRACTHere we report the synthesis and investigation of iron and iron oxide nanowire arrays using mesoporous silica as a host material. In the present work a novel variant of synthesis of ordered magnetic nanowires in the mesoporous silica matrix was suggested. The method is based on the incorporation of a hydrophobic metal compound into the hydrophobic part of silica-surfactant composite. The amount of iron intercalated into the mesoporous matrix was measured by chemical analysis. In all samples it corresponds well to with the molar ratio SiO2: Fe = 9:1. To provide crystallinity of nanowires additional thermal treatment was performed. Thus prepared nanocomposites were characterized by TEM, ED, SAXS, SANS, BET and magnetic measurements. The anisotropy parameters of nanowires were determined using two non-correlated methods: temperature dependence of magnetic susceptibility and small angle polarized neutron scattering. It was found that the particle length increases with the increasing of the decomposition temperature of the metal complex. Obviously it deals with crystallization and growth of metal particles inside the pores at a constant diameter of a single particle. For iron containing sample annealed at 375 °C (form factor of nanowire is about 40), the coercive force at room temperature was found to be 145 Oe at saturation magnetization of 1.2 emu/g, which is not far from modern information storage. It was shown that particles shape and size are in good agreement with that of the pores. Particles are uniform and well ordered in the silica matrix. Thus, the suggested method leads to one-dimensional anisotropic nanostructures which could find an application as high-density data storage magnetic media.

Self‐Regeneration of a Pd‐Perovskite Catalyst for Automotive Emissions Control.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Al2O3-films onNi3Al(111):a template for nanostructured cluster growth

In scanning tunnelling microscope images of thin Al2O3-films grown on Ni3Al(111) at 1000 K twosuper-lattices with periodicities of 2.6 and 4.5 nm, respectively,can be identified. These well-ordered nanostructures can be usedas nucleation centres for metal particle growth. It can be shownthat both nanostructures act as a template for the fabrication ofordered assemblies of metal clusters by mere physical vapourdeposition. The degree of ordering of these nanostructures islargely dependent on the metal deposited. Here we report on thegrowth of Cu, Ag, Au, Mn, and V clusters on the Al2O3-films. The best results as far as ordering of theclusters is concerned was reached for V deposition at 550 K, whichresulted in a nearly perfect hexagonal array of clusters with aspacing of 2.6 nm.

Self-regeneration of a Pd-perovskite catalyst for automotive emissions control.

Catalytic converters are widely used to reduce the amounts of nitrogen oxides, carbon monoxide and unburned hydrocarbons in automotive emissions. The catalysts are finely divided precious-metal particles dispersed on a solid support. During vehicle use, the converter is exposed to heat, which causes the metal particles to agglomerate and grow, and their overall surface area to decrease. As a result, catalyst activity deteriorates. The problem has been exacerbated in recent years by the trend to install catalytic converters closer to the engine, which ensures immediate activation of the catalyst on engine start-up, but also places demanding requirements on the catalyst's heat resistance. Conventional catalyst systems thus incorporate a sufficient excess of precious metal to guarantee continuous catalytic activity for vehicle use over 50,000 miles (80,000 km). Here we use X-ray diffraction and absorption to show that LaFe(0.57)Co(0.38)Pd(0.05)O(3), one of the perovskite-based catalysts investigated for catalytic converter applications since the early 1970s, retains its high metal dispersion owing to structural responses to the fluctuations in exhaust-gas composition that occur in state-of-the-art petrol engines. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium (Pd) reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic Pd particles, and hence explains the retention of high catalyst activity during long-term use and ageing.

Laser-induced growth of Ag nanoparticles from aqueous solutions.

Laser irradiation of aqueous silver cations in the presence of a reducing agent induces growth of metallic silver particles. The picture shows a schematic illustration of a microscope setup that offers diffraction-limited spatial control of Ag deposition on a glass surface (RA=reducing agent, for example citrate).