Showing papers on "Noble metal published in 2005"

Platinum Monolayer on Nonnoble Metal−Noble Metal Core−Shell Nanoparticle Electrocatalysts for O2 Reduction

Abstract: We synthesized a new class of O2 electrocatalysts with a high activity and very low noble metal content. They consist of Pt monolayers deposited on the surfaces of carbon-supported nonnoble metal−noble metal core−shell nanoparticles. These core−shell nanoparticles were formed by segregating the atoms of the noble metal on to the nanoparticles' surfaces at elevated temperatures. A Pt monolayer was deposited by galvanic displacement of a Cu monolayer deposited at underpotentials. The mass activity of all the three Pt monolayer electrocatalysts investigated, viz., Pt/Au/Ni, Pt/Pd/Co, and Pt/Pt/Co, is more than order of magnitude higher than that of a state-of-the-art commercial Pt/C electrocatalyst. Geometric effects in the Pt monolayer and the effects of PtOH coverage, revealed by electrochemical data, X-ray diffraction, and X-ray absorption spectroscopy data, appear to be the source of the enhanced catalytic activity. Our results demonstrated that high-activity electrocatalysts can be devised that contain ...

Optical properties of Pd-Ag and Pt-Ag nanoboxes synthesized via galvanic replacement reactions.

Abstract: Silver nanocubes dispersed in water were transformed into Pd−Ag or Pt−Ag nanoboxes by adding either Na2PdCl4 or Na2PtCl4 By controlling the amount of noble metal salt added, and therefore the molar ratio of Na2PdCl4 or Na2PtCl4 to Ag, we could tune the surface plasmon resonance peak of the nanostructures across the entire visible spectrum, from 440 to 730 nm Replacement of Ag with Pd resulted in the formation of a nanobox composed of a Pd−Ag alloy single crystal, but the nanobox formed after replacement of Ag with Pt was instead composed of distinct Pt nanoparticles DDA calculations suggest that both nanoboxes absorb light strongly, with Qabs/Qsca ≈ 5 After galvanic replacement, Pd−Ag and Pt−Ag nanostructures remain SERS active, suggesting their use as a SERS probe for studying the dependence of interfacial chemistry on composition

Steam reforming of model compounds and fast pyrolysis bio-oil on supported noble metal catalysts

Abstract: The production of hydrogen by steam reforming of bio-oils obtained from the fast pyrolysis of biomass requires the development of efficient catalysts able to cope with the complex chemical nature of the reactant. The present work focuses on the use of noble metal-based catalysts for the steam reforming of a few model compounds and that of an actual bio-oil. The steam reforming of the model compounds was investigated in the temperature range 650–950 °C over Pt, Pd and Rh supported on alumina and a ceria–zirconia sample. The model compounds used were acetic acid, phenol, acetone and ethanol. The nature of the support appeared to play a significant role in the activity of these catalysts. The use of ceria–zirconia, a redox mixed oxide, lead to higher H 2 yields as compared to the case of the alumina-supported catalysts. The supported Rh and Pt catalysts were the most active for the steam reforming of these compounds, while Pd-based catalysts poorly performed. The activity of the promising Pt and Rh catalysts was also investigated for the steam reforming of a bio-oil obtained from beech wood fast pyrolysis. Temperatures close to, or higher than, 800 °C were required to achieve significant conversions to CO x and H 2 (e.g., H 2 yields around 70%). The ceria–zirconia materials showed a higher activity than the corresponding alumina samples. A Pt/ceria–zirconia sample used for over 9 h showed essentially constant activity, while extensive carbonaceous deposits were observed on the quartz reactor walls from early time on stream. In the present case, no benefit was observed by adding a small amount of O 2 to the steam/bio-oil feed (auto-thermal reforming, ATR), probably partly due to the already high concentration of oxygen in the bio-oil composition.

Electrodeposition of Noble Metal Nanoparticles on Carbon Nanotubes

Abstract: Noble metal nanoparticles can be electrodeposited on carbon nanotubes under potential control. The nanotube sidewalls serve both as the electrodeposition template and as the wire electrically connecting the deposited nanoparticles.

Influence of particle agglomeration on the catalytic activity of carbon-supported Pt nanoparticles in CO monolayer oxidation

Abstract: Fuel cell electrocatalysts usually feature high noble metal contents, and these favour particle agglomeration. In this paper a variety of synthetic approaches (wet chemical deposition, electrodeposition and electrodeposition on chemically preformed Pt nuclei) is employed to shed light on the influence of nanoparticle agglomeration on their electrocatalytic properties. Pt loading on model glassy carbon (GC) support is increased systematically from 1.8 to 10.6 μg Pt cm−2 and changes in the catalyst structure are followed by transmission electron microscopy. At low metal loadings (≤5.4 μg Pt cm−2) isolated single crystalline Pt nanoparticles are formed on the support surface by wet chemical deposition from H2PtCl4 precursor. An increase in the metal loading results, first, in a systematic increase of the average diameter of isolated Pt nanoparticles and, second, in coalescence of nanoparticles and formation of particle agglomerates. This behaviour is in line with the previous observations on carbon-supported noble metal fuel cell electrocatalysts. The catalytic activity of Pt/GC electrodes is tested in CO monolayer oxidation. In agreement with the previous studies (F. Maillard, M. Eikerling, O. V. Cherstiouk, S. Schreier, E. Savinova and U. Stimming, Faraday Discuss., 2004, 125, 357), we find that the reaction is strongly size sensitive, exhibiting an increase of the reaction overpotential as the particle size decreases below ca. 3 nm. At larger particle sizes the dependence levels off, the catalytic activity of particles with diameters above 3 nm approaching that of polycrystalline Pt. Meanwhile, Pt agglomerates show remarkably enhanced catalytic activity in comparison to either isolated Pt nanopraticles or polycrystalline Pt foil, catalysing CO monolayer oxidation at ca. 90 mV lower overpotential. Enhanced catalytic activity of Pt agglomerates is ascribed to high concentration of surface defects. CO stripping voltammograms from Pt/GC electrodes, comprising Pt agglomerates along with isolated single crystalline Pt nanoparticles from 2 to 6 nm size, feature double voltammetric peaks, the more negative corresponding to CO oxidation on Pt agglomerates, while the more positive to CO oxidation on isolated Pt nanoparticles. It is shown that CO stripping voltammetry provides a fingerprint of the particle size distribution and the extent of particle agglomeration in carbon-supported Pt catalysts.

Coating noble metal nanocrystals (Ag, Au, Pd, and Pt) on polystyrene spheres via ultrasound irradiation.

Abstract: The method of ultrasound irradiation is used for anchoring metallic nanocrystals (Ag, Au, Pd, and Pt) onto the surface of polystyrene spheres. In former studies, almost all the sonochemically prepared, coated metallic nanomaterials were formed as amorphous nanoparticles (Pol, V. G.; et al. Langmuir 2002, 18, 3352; Pol, V. G.; et al. Chem. Mater. 2003, 15, 1111; Zhong, Z. Y.; et al. Chem. Mater. 1999, 11 (9), 2350; Pol, V. G.; et al. Chem. Mater. 2003, 15, 1378), which were coated on various substrates (silica spheres, carbon spherules, titania, and alumina). On the other hand, the noble metal nanoparticles deposited on polystyrene spheres via ultrasound irradiation yielded nanocrystalline Ag, Au, Pd, and Pt particles on the surface of polystyrene as as-synthesized materials. The sonochemical mechanism is proposed based on chemical interactions between the particles.

Reaction performance and characterization of Co/Al2O3 Fischer–Tropsch catalysts promoted with Pt, Pd and Ru

Abstract: A series of noble metal (Pt, Ru or Pd) promoted Co/Al2O3 catalysts were prepared by sequential impregnation method. The catalysts were characterized by XRD, TPR, H2-TPD and TPSR techniques, and their catalytic performance in Fischer–Tropsch synthesis was investigated in a fixed-bed reactor. The results of activity measurements show that the addition of small amounts of noble metal greatly improved the activity of the Co/Al2O3 catalyst. TPR experimental results demonstrate that hydrogen spillover from the noble metal to cobalt oxide clusters facilitated the reduction of cobalt oxide and, thus significantly increased the reducibility of Co/Al2O3 catalyst. The presence of noble metal increased the amount of chemisorbed hydrogen and weakened the bond strength of Co–H. TPSR results indicate that CO was adsorbed in a more reactive state on the promoted catalysts.

Reforming of methane with carbon dioxide over supported bimetallic catalysts containing Ni and noble metal: I. Characterization and activity of SiO2 supported Ni–Rh catalysts

Abstract: The activity and stability of silica supported monometallic Ni, Rh and bimetallic Ni–Rh catalysts have been studied towards the carbon dioxide reforming of methane. The catalysts were prepared by incipient wetness impregnation with different contents of Rh and Ni and they were characterized by H2 chemisorption, TP R H 2 , XRD and FT-IR methods. SiO2 supported monometallic Ni, Rh and bimetallic Ni–Rh catalysts are comparably good catalysts for carbon dioxide reforming of methane and Rh-rich catalysts are resistant to deactivation and carbon formation. Temperature-programmed hydrogen assisted decomposition of bimetallic Ni–Rh/SiO2 catalyst precursors leads to formation of Ni–Rh alloys. Segregation of metals leads to the formation of Ni-rich surface alloy. TG-DTA-MS, TPS R H 2 , TPO, TPS R C O 2 , TOC and SEM methods were used in order to characterize the carbonaceous deposits. At least two types of carbon deposit with different reactivity have been detected on the surface of bimetallic catalysts. The results suggest a significant metal–metal interaction in the bimetallic systems.

Flame-made alumina supported Pd-Pt nanoparticles: structural properties and catalytic behavior in methane combustion

Abstract: Bimetallic palladium–platinum nanoparticles supported on alumina were prepared by flame spray pyrolysis. The as-prepared materials were characterized by scanning transmission electron microscopy (STEM), CO chemisorption, nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), thermogravimetric analysis (TGA) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The materials were tested for the catalytic combustion of methane with a focus on the thermal stability of the noble metal particles. After flame synthesis the noble metal components of the materials were predominantly in oxidized state and finely dispersed on the alumina matrix. Reduction afforded small bimetallic Pd–Pt alloy particles (< 5 nm) supported on Al2O3 ceramic nanoparticles. The addition of small amounts of platinum made the palladium particles more resistant against sintering at high temperatures and further lowered the deactivation observed during methane combustion.

Competitive CO and CO2 methanation over supported noble metal catalysts in high throughput scanning mass spectrometer

Abstract: High-throughput synthesis and screening methods have been developed for the heterogeneously catalyzed gas phase hydrogenation of CO and CO 2 (‘methanation’) over zirconia and ceria supported noble and base metal catalysts at 300–400 °C and ambient pressure. The discovery libraries, for primary screening, consisted of 11 × 11 arrays of 111 catalysts on 3 in. quartz wafers, and 16 × 16 arrays of 256 catalysts on 4 in. quartz wafers. Catalysts were prepared by liquid dispensing techniques and screened for catalytic activity in scanning mass spectrometers (SMS). This primary screening tool uses quadrupole mass spectrometry for rapid serial detection. More than 500 potential catalysts could be screened in a single day. A kinetic model based on fast equilibration by the reverse water–gas-shift reaction in parallel with about an order of magnitude slower CO hydrogenation is in good agreement with CO x conversion data. CO 2 is mainly reverse shifted to CO. Ru, Rh, and Ni were found to promote methanation whereas Pt tends to catalyze the reverse WGS reaction. Methanation activity can be enhanced by some acidic and redox dopants or suppressed by basic dopants. High conversions were achieved in SMS demonstrating the minimal scalability risk for short contact time reactions.

Selective formation of metal layers in an integrated circuit

Abstract: A method for enhancing the reliability of copper interconnects and/or contacts, such as the bottom of vias exposing top surfaces of buried copper, or at the top of copper lines just after CMP. The method comprises contacting the exposed copper surface with a vapor phase compound of a noble metal and selectively forming a layer of the noble metal on the exposed copper surface, either by a copper replacement reaction or selective deposition (e.g., ALD or CVD) of the noble metal.

Affinity order among noble metals and CeO2

Abstract: The affinity order among four kinds of major noble metals (Ru, Rh, Pd, and Pt) and CeO2 was investigated by XRD and XAFS techniques. The states of noble metals supported on CeO2 and the sintering behavior of CeO2 on calcination differed depending upon the kinds of noble metals. Rh and Pd retained their Rh O Ce and Pd O Ce bonds even after calcination at 800 °C, and Pt kept its Pt O Ce bond at 500 °C, while Ru on CeO2 formed bulk RuO2 on calcination at temperatures as low as 500 °C. Rh effectively prevented sintering of CeO2. The order of the affinity of the noble metals toward CeO2 was assumed to be Rh > Pd > Pt > Ru.

High density remote plasma enhanced atomic layer deposition of ruthenium thin films

Abstract: As the silicon process migrates to small device geometries, new deposition process technology will be required to solve the problems obtained with high trench capacitor and ultra thin gate oxide. [1,2] Attempts to deposit material with atomic level control, atomic layer deposition as a technique which can be available good film uniformity and excellent step coverage have been reported. [1] Ruthenium is a noble metal has low electrical resistivity, high chemical inertness, thermal stability and hardness. Especially, ALD of ruthenium thin films using the metal electrode for trench DRAM ( Dynamic Random Access Memory) capacitor have been researched because the films have not only excellent uniformity also high work function energy of deposited films. [3 6] In this study, ruthenium thin films prepared by high density plasma ALD with Ru(EtCp) 2 as ruthenium precursor and NH 3 gas as plasma ion source from room temperature to 400°C. ECR remote plasma as high density plasma ion source has over 10 12 /cm 3 ion density, therefore, ligands of precursor gas can be removed easily. [7] Ruthenium thin films was grown to self-limited reaction process to be ranged from 250°C to 290°C deposited by conventional ALD on 44 nm thick TiN/ 4 nm thick Ti / 100 nm thick SiO 2 / p-type (100) Si.wafer. This result reveals that ALD process depends on chemical limited reaction with surface atom and precursor gas, furthermore, this temperature region possess sufficient energy to adsorb ruthenium precursor gas on substrate. Conventional ALD of ruthenium thin film deposited with oxygen and argon mixed gas as a reaction gas is formed ruthenium and ruthenium oxide, however, high-density remote plasma enhanced ALD of ruthenium deposited using NH 3 as a plasma gas is formed only ruthenium films by low angle HRXRD (High Resolution X-ray Diffraction) peaks. Ruthenium thin films deposited by HDPALD have surface morphology of 10 ∼ 12 □, and electrical resistivity of 11 ∼ 15 μΩ.cm.

Hydrodesulfurization of sulfur-containing polyaromatic compounds in light gas oil using noble metal catalysts

Abstract: We systematically monitored the hydrodesulfurization (HDS) activity of dibenzothiophene (DBT) and groups of substituted DBTs present in a SR-LGO over various noble metal catalysts (Ru, Rh, Ru-Rh, Pt, Pd and Pt-Pd) supported on alumina. The catalytic performances were compared to those obtained over a conventional CoMo catalyst. The Pd-based catalysts exhibited excellent HDS performances, especially for desulfurizing the refractory compounds. In particular, the 4,6-dimethyldibenzothiophene (4,6-DMDBT) HDS activity over the Pd or the Pd-Pt catalyst was equivalent to that over the CoMo catalyst. This was attributed to the exceptional hydrogenation (HYD) properties of the Pd-based catalysts, which enable desulfurization of the refractory compounds by considerably minimizing the effect of the steric hindrance due to their substituents. The synergetic effect observed on the bimetallic Pt-Pd system at low temperature for all the sulfur compounds was largely attenuated in the high temperature range, probably due to shifting of the HYD/dehydrogenation equilibrium to dehydrogenation. Despite a much lower metal loading (0.25 wt.%), the performances of the Rh catalyst were superior to those of the optimized 16 wt.% Ru catalyst. A synergetic effect was observed on the Ru-Rh catalyst, on which the DBT HDS activity was significantly enhanced compared to the activity of each corresponding monometallic catalyst. Further, we recently showed that the use of SiO2 or SiO2-Al2O3 as a support allows higher optimal Rh contents, making the Rh-based HDS catalysts even more promising. In brief, we obtained encouraging results, which showed for instance that the excellent properties of the noble metal catalysts in model HDS reactions are still observed during the HDS treatment of real feeds. # 2005 Elsevier B.V. All rights reserved.

Electrochemical synthesis of gold nanocrystals and their 1D and 2D organization.

Abstract: Size-controlled gold nanocrystals were conveniently synthesized through direct electroreduction of bulk AuCl4- ions in the presence of poly(N-vinylpyrrolidone) (PVP). PVP greatly enhanced the gold particle formation process and also significantly retarded the gold electrodeposition process, allowing the electrochemical synthesis of gold nanocrystals to be carried out in the form of simple electroreduction. This novel electrochemical method may be extended to synthesis of other noble metal nanoparticles with controllable size on a large scale. The PVPK90-protected gold nanocrystals spontaneously self-assembled into nearly ordered 2D close-packed arrays and interesting 1D nanostructures. The aggregation of unstable PVPK17-protected gold nanocrystals resulted in the formation of ultrathin single-crystalline films. PVP plays multifunctional roles in controlling the size and shape of gold nanocrystals and in inducing individual gold nanocrystals to construct 1D nanostructures. The nanoparticle self-assembling ...

Evaluation of a spinel based pigment system as a CO oxidation catalyst

Abstract: Commercially available black pigment consisting of mixed manganese, copper and iron oxides was tested as a catalyst as well as an oxidant for CO oxidation. The crystalline structures of the catalyst were determined by XRD as Cu 1.5 Mn 1.5 O 4 mixed with Fe 2 O 3 and Mn 3 O 4 oxides. The fresh catalyst with 30–300 nm size and a BET surface area of 18.5 m 2 g −1 was able to completely convert CO at 525 °C even at a significantly high CO O 2 He gas flow rate of 1000 ml min −1 (corresponding space velocity being ∼310,000 h −1 ). While the reaction rate was independent of oxygen concentration in the range tested (0.8–9.9 vol.% of O 2 at a constant CO concentration of 0.85%), it depended on CO concentration. The reaction order over fresh catalyst was measured to be 0.85 with respect to CO with an activation energy value of 47.9 kJ mol −1 . Application of a reduction followed by oxidation type of heat treatment on fresh catalyst induced the formation of fine clusters or domains of ∼5 nm on the surface of the catalyst particles. This refined morphology with high density of defects led to a great improvement in catalytic activity. Complete CO conversion was achieved at 180 °C over a heat treated catalyst. This change in morphology also led to higher reducibility of mixed oxide system after heat treatment as indicated by TPR results. The mixed oxides of transition metals can be a viable alternative to precious metal and noble metal containing catalysts for oxidation of CO.

Plasma‐Assisted Atomic Layer Deposition of Palladium

Abstract: A method is presented for the atomic layer deposition (ALD) of palladium using remote hydrogen plasma as the reducing source and agent. Palladium was deposited on iridium, tungsten and silicon at 80 °C using a remote inductively coupled hydrogen plasma with palladium(II) hexafluoroacetylacetonate as the precursor. In the case of the Pd film grown on Ir, the carbon and fluorine content were significantly reduced compared to previous thermal ALD results. Use of remote plasma eliminated the noble metal substrate requirement needed for thermal ALD, enabling films to be grown on W and Si. Ultra-thin Pd films grown on W and Si possessed a nearly random texture from reflection high-energy electron diffraction (RHEED) measurements. Atomic force microscopy (AFM) images showed very different surface morphologies for the different substrates suggesting very different substrate film interactions. X-ray photoelectron spectroscopy (XPS) measurements indicate high quality Pd films for all substrates, suggesting the substrate temperature was low enough to prevent dissociation of the hfac ligand and adequate C and F scavenging by the atomic hydrogen. The remote hydrogen plasma source results in the loss of selectivity but growth is evident on every surface used including surfaces that do not react strongly with the Pd precursor and are not catalytic towards the dissociation of molecular hydrogen.