Showing papers on "Noble metal published in 2002"

Complete oxidation of methane at low temperature over noble metal based catalysts: a review

Abstract: This review examines recent developments in the complete oxidation of methane at low temperature over noble metal based catalysts in patents and open literature. The abatement of natural gas vehicle (NGV) methane emissions is taken as one example among possible applications. The review develops current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane under oxidising conditions, focusing on low-temperature reaction conditions. The influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of metal particle size and reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds are examined. The latest studies concerned with improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature in the presence of water and sulphur containing compounds are reported. Possible routes for preparing catalysts able to meet future regulations concerning methane emissions from lean-burn NGV vehicles are discussed.

Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts

Abstract: Temperature programmed reduction (TPR) and hydrogen chemisorption combined with reoxidation measurements were used to define the reducibility of supported cobalt catalysts. Different supports (e.g. Al2O3, TiO2, SiO2, and ZrO2 modified SiO2 or Al2O3) and a variety of promoters, including noble metals and metal cations, were examined. Significant support interactions on the reduction of cobalt oxide species were observed in the order Al2O3>TiO2>SiO2. Addition of Ru and Pt exhibited a similar catalytic effect by decreasing the reduction temperature of cobalt oxide species, and for Co species where a significant surface interaction with the support was present, while Re impacted mainly the reduction of Co species interacting with the support. For catalysts reduced at the same temperature, a slight decrease in cluster size was observed in H2 chemisorption/pulse reoxidation with noble metal promotion, indicating that the promoter aided in reducing smaller Co species that interacted with the support. On the other hand, addition of non-reducible metal oxides such as B, La, Zr, and K was found to cause the reduction temperature of Co species to shift to higher temperatures, resulting in a decrease in the percentage reduction. For both Al2O3 and SiO2, modifying the support with Zr was found to enhance the dispersion. Increasing the cobalt loading, and therefore the average Co cluster size, resulted in improvements to the percentage reduction. Finally, a slurry phase impregnation method led to improvements in the reduction profile of Co/Al2O3.

Stabilization of inorganic nanocrystals by organic dendrons.

Abstract: A series of hydrophilic organic dendron ligands was designed and synthesized for stabilizing high-quality semiconductor and noble metal nanocrystals. The focal point of the dendron ligands is chosen to be a thiol group which is a universal coordinating site for compound semiconductor and noble metal nanocrystals. The methods for binding these dendron ligands onto the surface of the nanocrystals are simple and straightforward. The thin, about 1-2 nm, but closely packed and tangled ligand shell provides sufficient stability for the "dendron-protected nanocrystals" to withstand the rigors of the coupling chemistry and the standard separation/purification techniques. The chemistry presented can be immediately applied for the development of a new generation of biomedical labeling reagents based on high-quality semiconductor nanocrystals. It also provides an alternative path to apply noble metal nanocrystals for developing sensitive detection schemes for chemical and biochemical purposes. The concept may further provide an optimal solution for many other problems encountered in nanocrystal-related research and development, for which the stability of the nanocrystals is a critical issue. Furthermore, the experimental results confirmed that the photochemical stability of colloidal semiconductor and noble metal nanocrystals is the key for developing reliable and reproducible processing chemistry for these nanocrystals.

Copper-catalyzed Suzuki cross-coupling using mixed nanocluster catalysts.

Abstract: A small library of copper and noble metal nanoclusters is designed and synthesized These clusters are tested as catalysts in the Suzuki cross-coupling of various aryl halides with phenylboronic acid It is found that copper and copper/noble metal combination nanoclusters are active catalysts for this reaction, the most active being the combined copper/palladium clusters Iodo-, bromo-, and chloroarenes can be used In the case of p-nitrobromobenzene, a one-pot cross-coupling and selective hydrogenation is achieved

Fischer-tropsch synthesis: deactivation of noble metal promoted co/al2o3 catalysts

Abstract: Fresh and used, unpromoted and noble metal-promoted 15% Co/Al 2 O 3 catalysts were analyzed by XANES and EXAFS to provide insight into catalyst deactivation. XANES analysis of the catalysts gave evidence of oxidation of a fraction of the cobalt clusters by water produced during the reaction. Comparison of XANES derivative spectra to those of reference materials, as well as linear combination fitting with the reference data, suggest that some form of cobalt aluminate species was formed. Because bulk oxidation of cobalt by water is not permitted thermodynamically under normal Fischer–Tropsch synthesis (FTS) conditions, it is concluded that the smaller clusters interacting with the support deviate from bulk-like cobalt metal behavior and these may undergo oxidation in the presence of water. However, in addition to the evidence for reoxidation, EXAFS indicated that significant cobalt cluster growth took place during the initial deactivation period. Promotion with Ru or Pt allowed for the reduction of cobalt species interacting with the support, yielding a greater number of active sites and, therefore, a higher initial catalyst activity on a per gram catalyst basis. However, these additional smaller cobalt clusters that were reduced in the presence of the noble metal promoter, deviated more from bulk-like cobalt, and were therefore, more unstable and susceptible to both sintering and reoxidation processes. The latter process was likely in part due to the higher water partial pressures produced from the enhanced activity. The rate of deactivation was therefore faster for these promoted catalysts.

Controlled Electroless Deposition of Noble Metal Nanoparticle Films on Germanium Surfaces

Abstract: Thin noble metal films have been prepared as a result of the immersion of germanium substrates into dilute, aqueous solutions of AuCl4-, PdCl42-, or PtCl42-, respectively. Deposition proceeds via galvanic displacement in the absence of fluoride, pH adjusters, complexing agents, or external reducing agents. This manner of metal deposition serves as a cost-effective, high-throughput methodology with control over surface morphology and deposition rate by modulation of plating parameters such as concentration, temperature, and immersion time.

H2S sensing properties of noble metal doped WO3 thin film sensor fabricated by micromachining

Abstract: The purpose of this research is to develop a semiconductor-type H2S gas sensor with silicon-based microfabrication and micromachining technology. This successful approach allows for the production of small, geometrically well-defined sensors that are reliable and mechanically robust, and is compatible with VLSI processes. Individual sensor cost is also greatly reduced because the sensors are batch fabricated. The main sensing region is covered with a 300 μm ×300 μm WO3 thin film, which is deposited by RF sputtering on silicon wafer substrate. Platinum (Pt), gold (Au) or Au-Pt noble metals was then deposited onto WO3 thin film as activator layer by sputtering. Under 1 ppm H2S and at an operating temperature of 220 °C, the individual sensitivities of the Pt and the Au-Pt doped WO3 gas sensors are 23 and 5.5, respectively. The sensor response times of Pt, Au-Pt and Au doped WO3 thin films are 30, 2 and 8 s, respectively, and the recovery times are about 30, 30 and 160 s, respectively. The results show that the Pt doped WO3 gas sensor exhibits acceptable response time, recovery time and as well as a high sensitivity.

Oxygen reduction on silver catalysts in solutions containing various concentrations of sodium hydroxide – comparison with platinum

Abstract: In air cathodes for chlorine–sodium hydroxide production, silver is a suitable catalyst for the oxygen reduction reaction (ORR), as is platinum. The ORR mechanism, studied with both rotating disc and ring-disc electrodes and impedance spectroscopy, is first order towards O2. However, the reaction can involve a direct four-electron or two-electron pathway. Although the latter involves different chemistry, including decomposition of hydrogen peroxide, the two pathways are difficult to distinguish, probably because they have the same rate-determining step. Considering kinetics/solubility ratios, temperature increase favours the ORR kinetics on both metals, whereas an increase in sodium hydroxide concentration is only positive for silver: for high sodium hydroxide concentration, platinum properties are hindered by greater adsorbed oxygenated species coverage. Thus, silver becomes competitive to platinum in high concentration alkaline media.

In Situ Synthesis of Noble Metal Nanoparticles in Ultrathin TiO2−Gel Films by a Combination of Ion-Exchange and Reduction Processes

Abstract: We have explored in situ synthesis of noble metal nanoparticles in ultrathin TiO2−gel films, in view of the increasing importance of such nanoparticle-containing thin films. The two-step procedure consists of ion exchange in the ultrathin matrix and subsequent reduction of metal ions to nanoparticles by low-temperature H2 plasma, by aqueous NaBH4, and by UV irradiation. Smooth and transparent ultrathin TiO2 films containing noble metal (Ag, Au, Pd, Pt) particles of 2−10 nm were readily obtained by this approach. The size and distribution of metal nanoparticles could be controlled by changing reaction conditions. The film thickness was precisely controlled by the cycle of film assembly. The low-temperature H2 plasma treatment, first used for preparation of noble metal nanoparticles, showed interesting advantages over chemical and photoinduced reductions in fabrication of nanoparticle-containing inorganic thin films.

Surface properties and catalytic performance in CO oxidation of cerium substituted lanthanum–manganese oxides

Abstract: Perovskite type oxides may potentially replace noble metal catalysts as full oxidation catalysts of hydrocarbons, as present in combustion exhaust streams, due to their high activity and fair hydrothermal stability. Presently we have investigated the catalytic activities of a series of Ce substituted La–Mn perovskites, prepared by coprecipitation, in the oxidation of carbon monoxide. The composition, bulk structure and the surface properties were established using elemental analysis, XRD and XPS. The properties tested include the reducibility and reoxidation behavior, the thermal stability of these perovskites and the oxidation activity. It was observed that Ce is not totally incorporated in the perovskite lattice. For a high degree of substitution, an excess Ce forms a separate CeO 2 phase. Simultaneously, an increase in the atomic Mn 4+ /Mn 3+ ratio and a decreasing surface oxygen concentration are observed. This suggests that cation vacancies are created at A (La) sites, resulting in the formation of unsaturated Mn (B) site ions on the surface. The catalytic activity of the La–Mn perovskites systematically changes as a function of the degree of Ce substitution. A plausible explanation for this behavior is the nonstoichiometry induced by Ce substitution, which results in the formation of cation/anion vacancies near the surface. The specific surface area and the atomic surface composition play a less pronounced role in the catalytic activity.

The role of bismuth as promoter in Pd-Bi catalysts for the selective oxidation of glucose to gluconate

Abstract: Bismuth is a well-established promoter of noble metal-based catalysts for the selective liquid phase oxidation of alcohols, aldehydes and carbohydrates with molecular oxygen. Experiments were carried out to improve the understanding of the promoting role of bismuth in bimetallic Pd-Bi catalysts used for the selective oxidation of glucose to gluconate. In relationship with the fact that these catalysts undergo substantial bismuth teaching under the reaction conditions, particular attention was paid to the possible role played by the soluble fraction of bismuth in. the oxidative process. Carbon-supported Pd-Bi/C catalysts characterized by various Bi-Pd compositions (0.33 less than or equal to Bi/Pd less than or equal to 3.0, 10wt.%Pd + Bi) were prepared from acetate-type precursors, tested under various experimental conditions and analyzed by X-ray diffractometry and X-ray photoelectron spectroscopy (XPS). Whatever the initial catalyst composition, the surface intensity ratio measured by XPS in used catalysts lies in the range 0.4-0.6, suggesting that the dynamic state of the catalyst involves the association of one Bi and two to three I'd atoms. The leaching process and the promoting effect itself are discussed in line with the formation of Bi-glucose and Bi-gluconate complexes present in solution but also as adsorbed species at the catalyst surface. The performances of a monometallic Pd/C catalyst are significantly improved in the presence of adequate amounts of soluble Bi. The involvement of the soluble fraction of bismuth in the overall mechanistic scheme of glucose oxidative dehydrogenation is suggested. The detrimental effect of large amounts of soluble bismuth is attributed to a too extensive adsorption of Bi-glucose complexes on the surface I'd atoms. (C) 2002 Elsevier Science B.V. All rights reserved.

Noble metal-containing supported catalyst and a process for its preparation

Abstract: The invention provides a noble metal-containing supported catalyst which contains one of the noble metals from the group Au, Ag, Pt, Pd, Rh, Ru, Ir, Os or alloys of one or more of these noble metals in the form of noble metal particles on a powdered support material. The particles deposited on the support material have a degree of crystallinity, determined by X-ray diffraction, of more than 2 and an average particle size between 2 and 10 nm. The high crystallinity and the small particle size of the noble metal particles lead to high catalytic activity for the catalyst. It is particularly suitable for use in fuel cells and for the treatment of exhaust gases from internal combustion engines.

Effective catalysts for decomposition of aqueous ozone

Abstract: Activities of a series of oxide supports and dozens of supported metal catalysts toward the decomposition of aqueous ozone have been investigated. Under reaction conditions, active carbon (AC) showed comparatively high activity, while zeolite (HY and mordenite), Al 2 O 3 , SiO 2 , SiO 2 ·Al 2 O 3 and TiO 2 showed no or negligible activity. Noble metals showed high activity. Among four kinds (Al 2 O 3 , SiO 2 , SiO 2 ·Al 2 O 3 and TiO 2 ) of supported noble metal catalysts, SiO 2 supported noble metal catalysts showed the highest activity. A 3% Pd/SiO 2 was found to be the most effective catalyst among the tested catalysts. The order of the reaction on 3% Pd/SiO 2 was 1 with respect to O 3 concentration. Apparent activation energy of the reaction on 3% Pd/SiO 2 was about 3 kcal mol −1 . The reaction on Pd/SiO 2 was suggested to be diffusion controlled. The effect of a catalyst on this reaction was discussed.

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.

Volatile noble metal organometallic complexes

Abstract: A series of noble metal organometallic complexes of the general formula (I): MLaXb(FBC)c, wherein M is a noble metal such as iridium, ruthenium or osmium, and L is a neutral ligand such as carbonyl, alkene or diene; X is an anionic ligand such as chloride, bromide, iodide and trifluoroacetate group; and FBC is a fluorinated bidentate chelate ligand such as beta diketonate, beta-ketoiminate, amino-alcoholate and amino-alcoholate ligand, wherein a is an integer of from zero (0) to three (3), b is an integer of from zero (0) to one (1) and c is an 10 integer of from one (1) to three (3). The resulting noble metal complexes possess enhanced volatility and thermal stability characteristics, and are suitable for chemical vapor deposition(CVD) applications. The corresponding noble metal complex is formed by treatment of the FBC ligand with a less volatile metal halide. Also disclosed are CVD methods for using the noble metal complexes as source reagents for deposition of noble metal-containing films such as Ir, Ru and Os, or even metal oxide film materials IrO2, OsO2 and RuO2.

Noble Metal (Pt, Rh, Pd) Promoted Fe-ZSM-5 for Selective Catalytic Oxidation of Ammonia to N2 at Low Temperatures

Abstract: We have reported previously the excellent performance of Fe-exchanged ZSM-5 for selective catalytic oxidation (SCO) of ammonia to nitrogen at high temperatures (e.g., 400-500 °C). The present work indicates that the reaction temperature can be decreased to 250-350 °C when a small amount of noble metal (Pt, Rh or Pd) is added (by both doping and ion exchange) to the Fe-ZSM-5. The SCO activity follows the order: Pt/Fe-ZSM-5 > Rh/Fe-ZSM-5 > Pd/Fe-ZSM-5. The noble metal promoted Fe-ZSM-5 catalysts also show higher activity for NH3 oxidation than Ce-exchanged Fe-ZSM-5 at low temperatures. On the Pt promoted Fe-ZSM-5, near 100% of NH3 conversion is obtained at 250 °C at a high space velocity (GHSV = 2.3 × 105 h-1) and nitrogen is the main product. The presence of H2O and SO2 decreases the SCO performance only slightly. This catalyst is a good candidate for solving the ammonia slip problem that plagues the selective catalytic reduction (SCR) of NO with ammonia in power plants.

Supported noble metal nanometer catalyst particles containing controlled (111) crystal face exposure

Abstract: A noble metal nanometer-sized catalyst composition is described along with the method for preparation of the composition. The crystal face of the catalyst contains a preponderance of (111) type crystal phase exposure. The crystal phase exposure is controlled by sequestering the noble metal cation before deposition on a catalyst support. Controlled catalyst face exposition combined with the nanometer scale of the catalyst increases the catalyst selectivity and activity, particularly for hydrogenation and dehydrogenation reactions.

Metal alloy fine particles and method for producing thereof

Abstract: A novel method for preparing fine particles comprising a transition metal and a noble metal which are monodisperse and have almost no particle diameter distribution, and are transferable to a CuAu-I type L10 ordered phase, with safety and at a low cost, wherein a salt or a complex of at least one transition metal selected from Fe and Co and a salt or a complex of at least one transition metal selected from Pt and Pd (exclusive of the combination of Co - Pd) is dissolved in an organic solvent miscible with water or an alcohol in the presence of an organic protecting agent, and the resultant solution is heated under reflux in the presence of an alcohol in an inert atmosphere, to thereby prepare a binary alloy comprising a transition metal and a noble metal, or, a salt or a complex of at least one element selected from the group consisting of Cu, Bi, Sb, Sn, Pb and Ag is further dissolved in the above solvent and the resultant solution is heated under reflux in the presence of an alcohol in an inert atmosphere, to thereby prepare a ternary alloy comprising a transition metal, a noble metal and an additional element.

Brilliant Optical Properties of Nanometric Noble Metal Spheres, Rods, and Aperture Arrays

Abstract: T he efŽ ciency of optical processes can be greatly ampliŽ ed in the presence of nanoscale noble metal structures. Enhancement has been observed for a multitude of optical phenomena, including molecular luminescence,1–4 photochemistry,5–8 molecular absorbance,9 –12 optical transmission,13 ,14 and nonlinear processes15,16 such as second harmonic generation17,18 and hyper-Rayleigh scattering.19 ,20 Recently, the unique properties of noble metal structures have been utilized for imaging w ith sub-wavelength resolution,21–25 chemical and biological sensors,26–32 and novel photonic devices.33–41 The optical enhancements generated by noble metal structures are