Showing papers on "Bimetallic strip published in 2002"

Formation and characterization of Au–Ag bimetallic nanoparticles in water-in-oil microemulsions

Abstract: Gold–silver bimetallic nanoparticles with varying mole fractions were synthesized in water-in-oil microemulsions of water/Aerosol OT/isooctane by the co-reduction of HAuCl4 and AgNO3 with hydrazine at 25 °C. TEM analysis revealed that the bimetallic nanoparticles were essentially monodisperse and had a mean diameter of 4–22 nm, increasing with an increase in the molar ratio of water to Aerosol OT (ω0) and Ag content. The UV/VIS absorption spectra of their solutions exhibited only one plasmon absorption and the absorption maximum of the plasmon band red-shifted almost linearly from 400 to 520 nm with increasing Au ∶ Ag molar ratio, revealing the formation of an alloy. Although the characteristic peaks for Au and Ag were too close to distinguish, the XRD analysis showed that the characteristic peaks for a Au–Ag bimetallic systems became broader and accordingly suggested the formation of bimetallic nanoparticles. The EDX analysis confirmed directly the formation of Au–Ag bimetallic nanoparticles. It showed that the composition of Au–Ag bimetallic nanoparticles was in good agreement with that of the feed solution but the outer layer of the particles was enriched in Ag. The HRTEM study indicated the resultant Au–Ag bimetallic nanoparticles contained single and multiple twins as well as stacking faults, and no mismatch was present. In addition, it was found that the formation rate of Au nanoparticles was much faster than that of Ag nanoparticles. This satisfactorily accounted for the composition distribution within a Au–Ag bimetallic nanoparticle.

Density functional study of structural and electronic properties of bimetallic silver–gold clusters: Comparison with pure gold and silver clusters

Abstract: Bimetallic silver–gold clusters offer an excellent opportunity to study changes in metallic versus “ionic” properties involving charge transfer as a function of the size and the composition, particularly when compared to pure silver and gold clusters. We have determined structures, ionization potentials, and vertical detachment energies for neutral and charged bimetallic AgmAun [3⩽(m+n)⩽5] clusters. Calculated VDE values compare well with available experimental data. In the stable structures of these clusters Au atoms assume positions which favor the charge transfer from Ag atoms. Heteronuclear bonding is usually preferred to homonuclear bonding in clusters with equal numbers of hetero atoms. In fact, stable structures of neutral Ag2Au2, Ag3Au3, and Ag4Au4 clusters are characterized by the maximum number of hetero bonds and peripheral positions of Au atoms. Bimetallic tetramer as well as hexamer are planar and have common structural properties with corresponding one-component systems, while Ag4Au4 and Ag8...

Ni–Ru bimetallic catalysts for the CO2 reforming of methane

Abstract: This paper reports the study on the effect of addition of various amounts of Ru to supported Ni catalysts towards the CO2 reforming of methane. Catalytic activity results have shown that in the case of silica supported samples addition of Ru strongly enhances the catalytic performance of the Ni sample, both in terms of activity and stability. The influence of Ru addition is instead much less remarkable on H-ZSM5 zeolite supported samples. On the basis of FT-IR spectra of adsorbed CO, H2 and O2 chemisorption and temperature-programmed reduction measurements, it has been proposed that the different trend observed on the two series of catalysts is related to different metal–metal and metal-support interactions occurring on the catalysts. In particular, the strong improvement in the activity and stability observed on silica supported Ni–Ru bimetallic samples has been attributed to an increase in the metallic dispersion of Ni as a consequence of the formation of Ni–Ru clusters with the surface mainly covered by Ni.

Synthesis and structure of colloidal bimetallic nanocrystals: The non-alloying system Ag/Co

Abstract: Monodisperse bimetallic Ag/Co composite nanocrystals have been prepared using colloid chemistry methods. Transmission electron microscopy showed well isolated Ag/Co particles centered around 12 nm ...

Melting of Bimetallic Cu−Ni Nanoclusters

Abstract: The thermal evolution of bimetallic Cu−Ni 343- and 1000-atom nanoclusters of compositions (Cu0.25Ni0.75) and (Cu0.5Ni0.5) is examined with molecular dynamics simulations using the Sutton-Chen many-body potential function. A heating curve is constructed starting from 0 K up to 1400 K, and the melting characteristics are determined on the basis of the variations of the potential energy and heat capacity with temperature. The shape evolution of the nanoclusters is analyzed in terms of deformation parameters. It is found that the bimetallic clusters melt in two stages. The first transition, corresponding to surface melting of the external Cu layers whereas the Ni core remains solid, is located at 400−500 K depending on the overall composition and cluster size. The second transition corresponds to homogeneous melting and takes place in the range of 700 to 900 K depending on the nanocluster size and composition. Thus, the melting temperature is much lower than those of bulk Cu and Ni, and the heat capacity show...

Au-core/Pt-shell bimetallic cluster-loaded TiO2. 1. Adsorption of organosulfur compound

Abstract: Au-core/Pt-shell bimetallic clusters have been loaded on the surface of TiO2 (Au(x wt %)−Pt(y wt %)−TiO2(Pt/Au atomic ratio, z)) in a highly dispersed state by a two-step method consisting of the A...

Structure of Pt-Co/Al2O3 and Pt-Co/NaY bimetallic catalysts: characterization by in situ EXAFS, TPR, XPS and by activity in Co (carbon monoxide) hydrogenation

Abstract: Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al2O3 bimetallic catalysts. The EXAFS experiments were carried out at the Pt LIII and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H2 results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co2+ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al2O3 give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.

Monolayer-protected bimetal cluster synthesis by core metal galvanic exchange reaction

Abstract: Bimetallic monolayer-protected nanoparticles have been synthesized by the core metal galvanic exchange reaction of dodecylthiolate monolayer-protected metal (Ag, Pd, Cu) clusters with the more noble metal metal thiolate complexes AuI[SCH2(C6H4)C(CH3)3] and PdII[S(CH2)11CH3)2]. The bimetal nanoparticles produced are stable and can be isolated without core aggregation or decomposition. These new materials have been examined by UV−vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and elemental analysis. Their optical properties reflect bimetal cluster formation by time-dependent shifts in the surface plasmon resonance absorbance. Transmission electron microscopy results suggest that the core metal replacement can also effect a change in nanoparticle core size. Formation of bimetallic nanoparticles appears to stabilized the less stable member of the metal pair.

Influence of oxidizing and reducing treatments on the metal–metal interactions and on the activity for nitrate reduction of a Pt-Cu bimetallic catalyst

Abstract: Bimetallic Pt-Cu/γAl 2 O 3 catalyst has been prepared by controlled deposition of copper onto platinum particles according to a redox reaction between hydrogen, pre-adsorbed on metallic platinum, and Cu 2+ ions in water. This catalyst has been submitted to either oxidizing or reducing treatments, at ambient temperature or at 400 °C. The interaction between copper and platinum in the bimetallic catalysts was estimated from temperature-programmed reduction (TPR) profiles, determined after various pre-treatments, and compared to that of a fresh catalyst. It was demonstrated that reducing and oxidizing pre-treatments noticeably affect the interactions between platinum and copper and consequently the catalytic activity for nitrate reduction in water.

New catalysts for clean technology

Abstract: The preparation, characterisation and catalytic performance of both bimetallic nanoparticles and chiral homogeneous catalysts anchored along the interior walls of mesoporous silica (MCM-41) are described. The bimetallic nanoparticles are prepared from Ru 6Pd 6 and Ru 6CSn carbonyl clusters and the resulting MCM derivatives are characterised by a number of methods, including EXAFS, FTIR and STEM. The solvent-free hydrogenation of naphthalene and cyclic polyenes using these catalysts are described, with the Ru 6CSn based catalyst giving a 90% selectivity for the corresponding monoenes while the Ru 6Pd 6 produces the fully hydrogenated product in high yield. The homogeneous catalysts 1-[-(R)-1′2-bis(diphenylphosphino)ferrocenyl] ethyl-N-N′-dimethylethylenediamine palladium dichloride was heterogenised by anchoring along the inside walls of MCM-41. The catalysed reaction between cinnamylacetate and benzylamine was studied and the catalyst produced 51% of the branched chain product possibly with 100%ee. © 2002 Published by Elsevier Science B.V.

Characterization of bimetallic rhodium-germanium catalysts prepared by surface redox reaction

Abstract: The preparation of bimetallic rhodium-germanium/silica and rhodium-germanium/alumina catalysts was investigated by controlled surface reaction. Their catalytic performances were measured for two gas phase reactions (toluene hydrogenation at 323 K and cyclohexane dehydrogenation at 543 K) and for a liquid phase reaction (citral hydrogenation at 343 K). Elemental analysis of bimetallic catalysts showed that germanium can be deposited by redox reaction between hydrogen activated on a parent monometallic rhodium catalyst and germanium tetrachloride dissolved in water (catalytic reduction method). EDX microanalysis of rhodium-germanium/silica catalysts indicated that rhodium and germanium were deposited in close contact on the silica support. However, on alumina-supported catalysts, germanium deposition occurred also separately on the support. For the different test reactions, the catalytic properties of rhodium were strongly altered by the addition of germanium. On alumina-supported catalysts, interesting catalytic effects were observed in citral hydrogenation when not only close contact exists between both metals but when, in addition, the second metal was deposited on the support in the close vicinity of rhodium.

Synthesis of Bimetallic Colloids with Tailored Intermetallic Separation

Abstract: The synthesis of bimetallic colloid concentric spheres is described through an example involving Au and Ag spaced by amorphous SiO2. The procedure involves a careful combination of existing methods for synthesis of Au nanoparticles, silica coating, and deposition of silver on silica. The choice of materials is motivated by their strong plasmon absorption bands, which allow us to follow the synthetic steps spectrophotometrically. TEM images demonstrate the concentric geometry of the particles prepared.

Structures and Magnetism of Cyanide-Bridged Bimetallic Compounds: Design of Complex-Based Magnetic Materials

Abstract: This account concerns the synthesis, structures and magnetism of cyanide-bridged bimetallic network compounds. The study aims at the development of molecular-based magnetic materials. Different network structures, from one-dimensional to three-dimensional, are synthesized by the reaction of [M(CN)6]n- with a cationic constituent such as [Ni-(diamine)2]2+, [Ni(triamine)]2+, [Mn(salen)]+, [Mn(en)]2+, etc., in the absence or presence of an appropriate counter ion. Magnetostructural studies for those bimetallic compounds have clarified fundamental factors contributing to magnetic ordering in bulk, magnetic nature (ferromagnetism or metamagnetism) and magnetic phase-transition temperature (Tc). The strategies for gaining high Tc magnetic material and modulating magnetic nature by a change in network structure are described.

Synthesis of Bulk and Alumina-Supported Bimetallic Carbide and Nitride Catalysts

Abstract: A new synthesis for the preparation of the bimetallic carbide, Co3Mo3C, as well as syntheses for the preparation of alumina-supported Ni2Mo3N, Co3Mo3N, and Co3Mo3C are described. The syntheses utilize the temperature-programmed reduction (TPR) method in which solid-state precursors are converted to the desired products in the presence of gas-phase reactants. Bulk and alumina-supported Co3Mo3C is prepared in a two-step synthesis in which oxide precursors are converted to Co3Mo3N, which is subsequently carburized to give the bimetallic carbide. Alumina-supported Ni2Mo3N and Co3Mo3N are prepared from supported oxide precursors via nitridation in flowing NH3, while the carburization of bulk and alumina-supported Co3Mo3N utilizes a CH4/H2 mixture to yield the bimetallic carbide products. Elemental analysis of a 22.5 wt % Co3Mo3C/Al2O3 catalyst suggests incomplete replacement of N with C under the synthesis conditions employed. The alumina-supported carbide and nitride materials have high surface areas and O2 c...

From Molecular Clusters to Metal Nanoparticles

Abstract: Different methods to prepare supported metal nanoparticles of uniform size are discussed (i) Supported ruthenium particles were generated from Ru and Ru-Fe bimetallic molecular metal carbonyl cluster precursors (MCC) (ii) Gold nanoparticle formation in the supercage of Y zeolite was studied on Au/NaY, Au/HY and Au-Fe/HY system (iii) Palladium nanoparticles were grown in liquid phase then deposited on an SiO2 support or they were grown on the support surface in a solid-liquid interfacial layer The particle size control was more efficient in the latter two cases than in the preparation starting from MCC

Atomic Scale Characterization of Supported Pd−Cu/γ-Al2O3 Bimetallic Catalysts

Abstract: The reduction behavior of a Pd−Cu/γ-Al2O3 catalyst precursor (containing 2% Pd and 1% Cu) is studied by atomic scale Z-contrast imaging, electron energy-loss spectroscopy (EELS), and X-ray energy dispersive spectroscopy (EDS) techniques available in a scanning transmission electron microscope (STEM). We found that the alloying behavior of the bimetallic nanoparticles strongly depends on the reduction temperature of the catalyst precursor materials. When the precursor is reduced at 523 or 773 K, individual metallic nanoparticles are formed with a composition varying from pure metallic Pd to Pd−Cu bimetallic alloys. Detailed spectroscopic analyses of the individual nanoparticles show that Pd is preferentially segregated onto the surfaces of the bimetallic Pd−Cu nanoparticles. At higher reduction temperatures, e.g., at 1073 K, however, all the nanoparticles are found to be bimetallic Pd−Cu alloys with either Pd- or Cu-rich surfaces.

Lewis acid-base interactions between metal atoms and their applications for the synthesis of bimetallic cluster complexes.

Abstract: A series of new palladium−ruthenium cluster complexes have been prepared by adding Pd(PBut3) fragments to the ruthenium−ruthenium bonds of ruthenium carbonyl complexes Reaction of Pd(PBut3)2 with