Showing papers on "Noble metal published in 2003"

Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts

Abstract: The catalytic performance of supported noble metal catalysts for the steam reforming (SR) of ethanol has been investigated in the temperature range of 600–850 °C with respect to the nature of the active metallic phase (Rh, Ru, Pt, Pd), the nature of the support (Al2O3, MgO, TiO2) and the metal loading (0–5 wt.%). It is found that for low-loaded catalysts, Rh is significantly more active and selective toward hydrogen formation compared to Ru, Pt and Pd, which show a similar behavior. The catalytic performance of Rh and, particularly, Ru is significantly improved with increasing metal loading, leading to higher ethanol conversions and hydrogen selectivities at given reaction temperatures. The catalytic activity and selectivity of high-loaded Ru catalysts is comparable to that of Rh and, therefore, ruthenium was further investigated as a less costly alternative. It was found that, under certain reaction conditions, the 5% Ru/Al2O3 catalyst is able to completely convert ethanol with selectivities toward hydrogen above 95%, the only byproduct being methane. Long-term tests conducted under severe conditions showed that the catalyst is acceptably stable and could be a good candidate for the production of hydrogen by steam reforming of ethanol for fuel cell applications.

Facile In Situ Synthesis of Noble Metal Nanoparticles in Porous Cellulose Fibers

Abstract: In situ synthesis of noble metal (Ag, Au, Pt, Pd) nanoparticles was carried out under ambient conditions in porous cellulose fibers as nanoreactors. Particles of less than 10 nm were readily prepared using the described approach, and monodisperse nanoparticles were obtained under an optimized concentration of the metal precursor solution. The nanoporous structure and the high oxygen (ether and hydroxyl) density of the cellulose fiber constitute an effective nanoreactor for in situ synthesis of metal nanoparticles. The nanopore is essential for incorporation of metal ion and reductant into cellulose fibers as well as for removal of unnecessary byproducts from fibers. This was endorsed by negligible adsorption of metal ion onto nonporous films of poly(vinyl alcohol) and starch. The ether oxygen and the hydroxyl group not only anchor metal ions tightly in cellulose fibers via ion−dipole interactions, but they also stabilize metal nanoparticles by strong bonding interaction with their surface atoms. The prepa...

Single-Phase and Gram-Scale Routes toward Nearly Monodisperse Au and Other Noble Metal Nanocrystals

Abstract: Single-phase approaches are introduced for the synthesis of nearly monodisperse Au and other noble metal nanocrystals. The new approaches possess all the advantages of the popular Brust method. With weak ligands or surfactants for the metal ions, the control of the size and size distribution of the nanocrystals in synthesis in the size range between 1 and 15 nm was achieved via maintaining balanced nucleation and growth by tuning the activities of the metal precursors and reducing reagents. Because only weak ligands are employed in the new synthetic schemes, surface modification and functionalization of the resulting nanocrystals can be readily carried out.

General Model for Water Monomer Adsorption on Close-Packed Transition and Noble Metal Surfaces

Abstract: Ab initio density functional theory has been used to investigate the adsorption of H2O on several close-packed transition and noble metal surfaces. A remarkably common binding mechanism has been identified. On every surface H2O binds preferentially at an atop adsorption site with the molecular dipole plane nearly parallel to the surface. This binding mode favors interaction of the H2O 1b(1) delocalized molecular orbital with surface wave functions.

Gold goes nano--from small clusters to low-dimensional assemblies.

Abstract: Bulk gold is a noble metal and unlike its lighter congeners of Group 11, copper and silver, was for a long time not considered important in catalysis and surface science in general. Although it was known that atomic gold in the gas phase can be very reactive, it was not until 1987 that the interest in gold nanostructures was really awakened through the report by Haruta et al. on CO oxidation on supported gold catalysts such as Au/TiO2 or Au/Fe2O3 at low temperatures.[1] Since then nanocrystalline gold compounds have been the subject of intense research and have found far-reaching applications in areas such as catalysis, sensors, molecular electronics, or as bioconjugate probes for amplification tags in gene analysis, antibody or antigen detection, DNA sequencing, or gene mapping.[1, 2] For example, Wang et al. recently succeeded in attaching 1.4 nm gold clusters to the sphere of the cowpea mosaic virus (CPMV) of a diameter of 30 nm (Figure 1).[3] While most studies have so far concentrated on pure gold nanoclusters of different structure and size, the recent discovery of the extremely stable mixed icosahedral gold clusters W@Au12 and Mo@Au12 (Figure 2A) by Wang and coworkers[4]—as predicted earlier by theoretical studies by Pyykk; and Runeberg[5]—opens up the field to new nanosized gold materials, in which the central atom “impurity” plays a vital role in fine-tuning electronic properties.

Preparation of gold, platinum, palladium and silver nanoparticles by the reduction of their salts with a weak reductant–potassium bitartrate

Abstract: Gold and platinum nanoparticles were synthesized by the reduction of their salts with potassium bitartrate as the reductant and poly(N-vinyl-2-pyrrolidone) (PVP), polyethylene glycol (PEG) (for Pt nanoparticles) or 3,3′-thiodipropionic acid (TDPC) as the protective agent. In the presence of PVP or TDPC, the preparation of palladium and silver nanoparticles were also realized by the reduction of their salts with potassium bitartrate under alkaline conditions. The effect of the concentration of the protective agents on the particle size and morphology were investigated in detail. At a constant concentration of Au and Pt ions, the average diameter of Au or Pt nanoparticles tends to decrease with an increase in the concentration ratios of the protective agents to AuCl4− or PtCl62−. Except for PVP passivated Au nanoparticles, smaller Au and Pt particles with a sharp size distribution could be obtained at higher concentration ratios of the protective agents to the metal ions, as determined by TEM measurements. All the noble metal colloidal particles have very high stability.

The synthesis of metal oxide nanowires by directly heating metal samples in appropriate oxygen atmospheres

Abstract: ZnO, MgO, and GeO2 nanowires were successfully synthesized by simply heating the desired metal powder to a temperature above its melting point in a flow of mixed gases (20% O2, 80% Ar, with the total flow rate of 120 sccm). Transmission electron microscopy observations show that as-synthesized products are exclusively nanowires, structurally uniform and single crystalline. The same technique was used to fabricate arrays of ZnO nanowires on silicon substrates, which would be of particular interest for direct integration in the current silicon-technology-based optoelectronic devices. Based on our experimental results, a metal self-catalytic growth mechanism was proposed and described conceptually. Because of the absence of impurities such as transition metal or noble metal throughout the whole growth process, the intrinsic properties of the resulting metal oxide nanowires could be expressed and utilized. And with in-depth understanding of the growth mechanism, our method could be efficient and controllable in extension to many other low-melting-point metals, such as Al, In, and Sn, for the synthesis of corresponding metal oxide nanostructures.

Supported bimetallic cluster catalysts

Abstract: We review synthesis, structural characterization, and catalysis by supported bimetallic clusters. Initial steps in the preparations include (a) adsorption of molecular metal clusters with preformed metal−metal bonds; (b) synthesis in zeolite cages; and (c) adsorption of a metal complex on a supported complex or cluster of another metal. Chemisorption capacities of supported oxophilic metal−noble metal clusters for hydrogen and CO are typically less than those of supported clusters of just the noble metal. Catalytic properties distinguishing supported bimetallic clusters from monometallics include high selectivities for oxygenates in CO hydrogenation and different selectivities for CO vs CC bond hydrogenation. Advantages of precise synthesis with organometallic precursors (smallness and uniformity of the supported clusters with bonds between the two metals) are offset by the expensive preparations and difficulty of catalyst regeneration. Applications of supported bimetallic clusters are most likely in spec...

Why is a noble metal catalytically active? The role of the O-Ag interaction in the function of silver as an oxidation catalyst.

Abstract: Extensive density-functional theory calculations, and taking into account temperature and pressure, afford a comprehensive picture of the behavior and interaction of oxygen and Ag(111), and provides valuable insight into the function of silver as an oxidation catalyst. The obtained phase diagram reveals the most stable species present in a given environment and thus identifies (and excludes) possibly active oxygen species. In particular, for the conditions of ethylene epoxidation, a thin oxidelike structure is most stable, suggesting that such atomic O species are actuating the catalysis, in contrast to hitherto proposed molecularlike species.

Cryogenic co oxidation on TiO2-Supported gold nanoclusters precovered with atomic oxygen

Abstract: Bulk gold has long been regarded as a noble metal, having very low chemical and catalytic activity. However, metal oxide-supported gold particles, particularly those that are less than 5 nm in diameter, have been found to have remarkable catalytic properties. In this study we show that impinging gas-phase CO molecules react readily with oxygen adatoms preadsorbed on Au/TiO2(110) to produce CO2 even under conditions in which the sample is cryogenically cooled. Gold particle size seems to have little effect on the CO oxidation reaction when oxygen adatoms are preadsorbed. We also show that as the oxygen adatom coverage increases, the rate of CO oxidation decreases on Au/TiO2 at cryogenic temperatures.

A CuO-CeO2 mixed-oxide catalyst for CO clean-up by selective oxidation in hydrogen-rich mixtures

Abstract: A CuO-CeO2 mixed-oxide catalyst was shown experimentally to be highly active and selective for the oxidation of CO in hydrogen-rich mixtures, and an attractive alternative to the noble metal catalysts presently used for CO clean-up in hydrogen mixtures for proton-exchange membrane fuel cells (PEMFC) Although the presence of H2O and CO2 in the feed decreased the activity and increased the reaction temperature considerably to achieve a given CO conversion with a reactor, the selectivity profile with respect to the conversion remained virtually the same The effect of H2O and CO2 on the reaction was found to increase the required energy for reduction of the active copper species in the redox cycles undergone during the reaction The catalyst showed a slow, reversible deactivation, but the activity was restored on heating the catalyst at 300 °C, even under an inert flow At space velocities above 42 g h m-3, the catalyst reduced the CO content to less than 10 ppm in the temperature range 166-176 °C for a feed of 1% CO, 1% O2, 50% H2, 20% H2O, 135% CO2 and balance He Hence, with this catalyst it is feasible to clean up the CO in a single-stage reactor with relatively small excess oxygen, which is in contrast to the typical multistage reactor systems using noble metal catalysts

Ceria-based mixed-metal oxide structure, including method of making and use

Abstract: A homogeneous ceria-based mixed-metal oxide, useful as a catalyst support, a co-catalyst and/or a getter has a relatively large surface area per weight, typically exceeding 150 m 2 /g, a structure of nanocrystallites having diameters of less than 4 nm, and including pores larger than the nanocrystallites and having diameters in the range of 4 to about 9 nm. The ratio of pore volumes, V P , to skeletal structure volumes, V S , is typically less than about 2.5, and the surface area per unit volume of the oxide material is greater than 320 m 2 /cm 3 , for low internal mass transfer resistance and large effective surface area for reaction activity. The mixed metal oxide is ceria-based, includes Zr and or Hf, and is made by a novel co-precipitation process. A highly dispersed catalyst metal, typically a noble metal such as Pt, may be loaded on to the mixed metal oxide support from a catalyst metal-containing solution following a selected acid surface treatment of the oxide support. Appropriate ratioing of the Ce and other metal constituents of the oxide support contribute to it retaining in a cubic phase and enhancing catalytic performance. Rhenium is preferably further loaded on to the mixed-metal oxide support and passivated, to increase the activity of the catalyst. The metal-loaded mixed-metal oxide catalyst is applied particularly in water gas shift reactions as associated with fuel processing systems, as for fuel cells.

Laser-Induced Growth and Deposition of Noble-Metal Nanoparticles for Surface-Enhanced Raman Scattering

Abstract: We describe laser-induced growth and deposition of gold and silver nanoparticles for use as surface-enhanced Raman scattering (SERS) substrates. The nanoparticles form at a glass surface in contact with an aqueous solution with metal ions and a reducing agent if the interface is illuminated with visible laser light. The technique thus allows for positioning of metal nanoparticles in devices with limited accessibility for optical sensing by SERS.

Chemical Vapor Deposition of Iridium, Platinum, Rhodium and Palladium

Abstract: This article reviews the progress in the chemical vapor deposition of iridium, platinum, rhodium and palladium metals. In the course of the last decade the number of articles on CVD of this group of metals has increased significantly. A wide variety of metal organic complexes have been investigated as potential precursors and appreciable results have been obtained. However, some aspects such as low deposition rates and impurity incorporation into the films still remain as concerns in this area. The representative results on CVD of these metals are presented according to the type of metal organic complexes used.

Electrochemical behaviour of pseudomorphic overlayers: Pd on Au(1 1 1)

Abstract: The electrochemical properties of palladium adlayers on Au(1 1 1) are compared with those of massive Pd(1 1 1). Electro-deposited Pd forms a pseudomorphic overlayer on the unreconstructed Au(1 1 1) surface following the stacking sequence of the substrate. The altered electrochemical behaviour of the overlayer can be ascribed to changes in the electronic structure due to lateral strain. A gradual change to bulk properties is observed with increasing Pd coverage. Besides hydrogen adsorption, sulphate adsorption and surface oxidation, formic acid oxidation was chosen as a test reaction. The influence of lateral strain on the reaction kinetics of formic acid oxidation is discussed.

NOx reduction compositions for use in FCC processes

Abstract: Compositions for reduction of gas phase reduced nitrogen species and NOX generated during a partial or incomplete combustion catalytic cracking process, preferably, a fluid catalytic cracking process, are disclosed. The compositions comprise (i) an acidic metal oxide containing substantially no zeolite, (ii) an alkali metal, alkaline earth metal, and mixtures thereof, (iii) an oxygen storage component, and (iv) a noble metal component, preferably rhodium or iridium, and mixtures thereof, are disclosed. Preferably, the compositions are used as separate additives particles circulated along with the circulating FCC catalyst inventory. Reduced emissions of gas phase reduced nitrogen species and NOX in an effluent off gas of a partial or incomplete combustion FCC regenerator provide for an overall NOX reduction as the effluent gas stream is passed from the FCC regenerator to a CO boiler, whereby as CO is oxidized to CO2 a lesser amount of the reduced nitrogen species is oxidized to NOX.

Gasification of Alkylphenols with Supported Noble Metal Catalysts in Supercritical Water

Abstract: Gasification of alkylphenols as lignin model compounds was examined in the presence of supported noble metal catalysts in supercritical water without hydrogen donor at 673 K. The activity of the catalyst was in the order of Ru/γ-alumina > Ru/carbon, Rh/carbon > Pt/γ-alumina, Pd/carbon, and Pd/γ-alumina. The effect of water density with the Ru/γ-alumina catalyst was examined in detail. The main gas products were methane, carbon dioxide, and hydrogen. The yield of gases and the ratio of methane increased with increasing water density. The gasification of the isomers of propylphenols was also examined with the Ru/γ-alumina catalyst. The reactivities of o- and p-propylphenols were relatively higher than those of m-propylphenols.