Showing papers on "Noble metal published in 2008"

Ag@AgCl: A Highly Efficient and Stable Photocatalyst Active under Visible Light

Abstract: Nanoparticles (NPs) of noble metals can strongly absorb visible light because of their plasmon resonance, which is greatly influenced by their morphology and size. The phenomenon of plasmon resonance gives rise to important applications such as colorimetric sensors, photovoltaic devices, photochromic devices, and photocatalysts. Noble metal NPs exhibit characteristic optical and physical properties that are substantially different from those of the corresponding bulk materials. In particular, silver NPs show efficient plasmon resonance in the visible region, which Awazu et al. recently utilized to develop a plasmonic photocatalyst. In their study, TiO2 was deposited on NPs consisting of a silver core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. Under UV illumination, this plasmonic photocatalyst exhibits enhanced catalytic activity, which increases with decreasing thickness of the SiO2 shell. To enhance the activity of a plasmonic photocatalyst, it is desirable to deposit silver NPs directly onto the surface of an active dielectric substrate without a protective shell, because the near-field effect of the NPs will be more strongly felt by the substrate. Herein we show that such a photocatalyst can be obtained from silver chloride by exploiting its photosensitivity, and the resulting plasmonic photocatalyst is highly efficient and stable under visible-light illumination. Silver halides are photosensitive materials extensively used as source materials in photographic films. On absorbing a photon, a silver halide particle generates an electron and a hole, and subsequently the photogenerated electron combines with an Ag ion to form an Ag atom. Ultimately, a cluster of silver atoms is formed within a silver halide particle upon repeated absorption of photons. Due to this instability under sunlight, which provides the very basis for chemical photography, silver halides are seldom used as photocatalysts. Nevertheless, there have been reports that under UV/Vis illumination AgCl deposited on a conducting support photocatalyzes O2 production from water in the presence of a small excess of silver ions in solution, and that under UV illumination AgBr dispersed on a silica support photocatalyzes H2 production from CH3OH/H2O solution. [21] In their study on the AgBr/SiO2 photocatalyst, Kakuta et al. [21] observed that Ag species are formed on AgBr in the early stage of the reaction, and AgBr is not destroyed under successive UV illumination. As suggested by Kakuta et al., electron–hole separation may occur smoothly in the presence of Ag species, and the latter may catalyze H2 production from alcohol radicals formed by photo-induced holes. If so, silver NPs formed on silver halide particles might be expected to be a stable photocatalyst under visible-light illumination due to their plasmon resonance. This expectation led us to prepare a new photocatalyst active and stable under visible light, namely, AgCl particles with silver NPs formed on their surface, by first treating Ag2MoO4 with HCl to form AgCl powder and then reducing some Ag ions in the surface region of the AgCl particles to Ag species (for details, see the Experimental Section). For convenience, these are referred to as Ag@AgCl particles. The X-ray diffraction (XRD) pattern of the Ag@AgCl product clearly shows that the cubic phase of Ag with lattice constant a= 4.0861 A (JCPDS file: 65-2871) coexists with the cubic phase of AgCl with lattice constant a= 5.5491 A (JCPDS file: 31-1238; see Figure 1). Scanning electron microscopy (SEM) images of the Ag@AgCl product (Figure 2) reveal that silver NPs with diameters in the range of 20–150 nm are deposited on the surface of AgCl particles with diameters in the range of 0.2–1.3 mm. The UV/Vis diffuse-reflectance spectra of Ag@AgCl, AgCl, and N-doped TiO2 (used as reference photocatalyst) are compared in Figure 3. In contrast to AgCl and N-doped TiO2, Ag@AgCl has a strong adsorption in the visible region which is almost as strong as that in the UV region. This is attributed to the plasmon resonance of silver NPs deposited on AgCl particles. To evaluate the photooxidation capability of Ag@AgCl, we examined the decomposition of methylic orange (MO) dye in solution over the Ag@AgCl sample under visible-light irradiation as a function of time (Figure 4). For comparison, we also carried out decomposition of the MO dye in solution over the N-doped TiO2 reference photocatalyst under visible[*] P. Wang, Prof. Dr. B. Huang, X. Qin, Prof. X. Zhang, Dr. J. Wei State Key Lab of Crystal Materials Shandong University, Jinan 250100 (China) E-mail: bbhuang@sdu.edu.cn Homepage: http://www.icm.sdu.edu.cn/index.php

Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts

Abstract: In recent decades, fuel cell technology has been undergoing revolutionary developments, with fundamental progress being the replacement of electrolyte solutions with polymer electrolytes, making the device more compact in size and higher in power density. Nowadays, acidic polymer electrolytes, typically Nafion, are widely used. Despite great success, fuel cells based on acidic polyelectrolyte still depend heavily on noble metal catalysts, predominantly platinum (Pt), thus increasing the cost and hampering the widespread application of fuel cells. Here, we report a type of polymer electrolyte fuel cells (PEFC) employing a hydroxide ion-conductive polymer, quaternary ammonium polysulphone, as alkaline electrolyte and nonprecious metals, chromium-decorated nickel and silver, as the catalyst for the negative and positive electrodes, respectively. In addition to the development of a high-performance alkaline polymer electrolyte particularly suitable for fuel cells, key progress has been achieved in catalyst tailoring: The surface electronic structure of nickel has been tuned to suppress selectively the surface oxidative passivation with retained activity toward hydrogen oxidation. This report of a H2–O2 PEFC completely free from noble metal catalysts in both the positive and negative electrodes represents an important advancement in the research and development of fuel cells.

Alkaline Polymer Electrolyte Fuel Cells Completely Free From Noble Metal Catalysts

Abstract: In recent decades, fuel cell technology has been undergoing revolutionary developments, with fundamental progress being the replacement of electrolyte solutions with polymer electrolytes, making the device more compact in size and higher in power density. Nowadays, acidic polymer electrolytes, typically Nafion, are widely used. Despite great success, fuel cells based on acidic polyelectrolyte still depend heavily on noble metal catalysts, predominantly platinum (Pt), thus increasing the cost and hampering the widespread application of fuel cells. Here, we report a type of polymer electrolyte fuel cells (PEFC) employing a hydroxide ion-conductive polymer, quaternary ammonium polysulphone, as alkaline electrolyte and nonprecious metals, chromium-decorated nickel and silver, as the catalyst for the negative and positive electrodes, respectively. In addition to the development of a high-performance alkaline polymer electrolyte particularly suitable for fuel cells, key progress has been achieved in catalyst tailoring: The surface electronic structure of nickel has been tuned to suppress selectively the surface oxidative passivation with retained activity toward hydrogen oxidation. This report of a H2–O2 PEFC completely free from noble metal catalysts in both the positive and negative electrodes represents an important advancement in the research and development of fuel cells.

Selective degradation of wood lignin over noble-metal catalysts in a two-step process.

Abstract: Direct conversion of lignin into alkanes and methanol was carried out in a two-step process (hydrogenolysis and hydrogenation) involving initial treatment of white birch wood sawdust with H2 in dioxane/water/phosphoric acid using carbon supported Ru, Pd, Rh, or Pt as catalysts. The resulting monomers and dimers obtained by selective C-O hydrogenolysis were then hydrogenated in near-crit. water empolying Pd/C as the catalyst. The study is of interest with respect to prodn. of biofuel from lignin.

On the polyol synthesis of silver nanostructures: glycolaldehyde as a reducing agent.

Abstract: The polyol synthesis is a popular method of preparing metal nanostructures, yet the mechanism by which metal ions are reduced is poorly understood. Using a spectrophotometric method, we show, for the first time, that heating ethylene glycol (EG) in air results in its oxidation to glycolaldehyde (GA), a reductant capable of reducing most noble metal ions. The dependence of reducing power on temperature for EG can be explained by this temperature-dependent oxidation, and the factors influencing GA production can have a profound impact on the nucleation and growth kinetics. These new findings provide critical insight into how the polyol synthesis can be used to generate metal nanostructures with well-controlled shapes. For example, with the primary reductant identified, it becomes possible to evaluate and understand its explicit role in generating nanostructures of a specific shape to the exclusion of others.

Niobium oxide-supported platinum ultra-low amount electrocatalysts for oxygen reduction.

Abstract: We demonstrate a new approach to synthesizing high-activity electrocatalysts for the O2reduction reaction with ultra low Pt content. The synthesis involves placing a small amount of Pt, the equivalent of a monolayer, on carbon-supported niobium oxide nanoparticles (NbO2 or Nb2O5). Rotating disk electrode measurements show that the Pt/NbO2/C electrocatalyst has three times higher Pt mass activity for the O2reduction reaction than a commercial Pt/C electrocatalyst. The observed high activity of the Pt deposit is attributed to the reduced OH adsorption caused by lateral repulsion between PtOH and oxide surface species. The new electrocatalyst also exhibits improved stability against Pt dissolution under a potential cycling regime (30 000 cycles from 0.6 V to 1.1 V). These findings demonstrate that niobium-oxide (NbO2) nanoparticles can be adequate supports for Pt and facilitate further reducing the noble metal content in electrocatalysts for the oxygen reduction reaction.

Selective methanation of CO over supported noble metal catalysts: Effects of the nature of the metallic phase on catalytic performance

Abstract: The catalytic performance of Al2O3-supported noble metal catalysts for the methanation of CO, CO2 and their mixture has been investigated with respect to the nature of the dispersed metallic phase (Ru, Rh, Pt, Pd). Experiments have been conducted using feed compositions relevant to those of reformate gas streams, both in the absence and in the presence of water. It has been found that, for all experimental conditions investigated, Ru and Rh are significantly more active than Pt and Pd. Selectivity toward hydrogenation products depends strongly on the noble metal catalyst employed, as well as on whether solo- or co-methanation of CO/CO2 is occuring. For hydrogenation of CO alone, selectivity toward CH4 increases with increasing temperature at the expense of higher hydrocarbons. In presence of water in the feed, catalytic activity of Ru is not affected, while that of Rh is reduced. On the other hand, the performance of Pt and Pd is poor since they promote the undesired water-gas shift (WGS) reaction. In hydrogenation of CO2 alone, selectivity toward higher hydrocarbons is negligible for all catalysts investigated but the reaction is accompanied by production of CO via the reverse WGS reaction. In all cases, addition of water in the feed results in decrease of catalytic activity. In combined hydrogenation of CO/CO2 mixtures, conversion of CO2 is completely suppressed until conversion of CO reaches its maximum value. Selectivity toward methane, which is typically higher than 70%, increases with increasing temperature and approaches 100% when CO2 conversion is initiated. Addition of 30% water vapor in the feed does not affect CO hydrogenation over Ru and Rh catalysts but retards CO2 hydrogenation, thereby expanding the temperature window of operation.

Preferential Oxidation of Carbon Monoxide in the Presence of Hydrogen (PROX) over Noble Metals and Transition Metal Oxides: Advantages and Drawbacks

Abstract: The steam reforming reaction of hydrocarbons and organic fuels, in general, is followed by a two-stage reaction of water gas shift, which allows increasing the hydrogen yield and a final purification step for CO removal to use hydrogen in an ammonia plant or a PEM fuel cell. This paper is focused on the CO Preferential Oxidation, CO PROX (or CO selective oxidation in excess hydrogen) reaction, considered as the simplest and cost effective process to achieve the less than 10 ppm CO. The objective of this paper is to review the performances of noble metals (Pt, Ru, Rh, Pd), gold and transition metal oxides catalysts in this reaction. Although the results reported are largely influenced by the experimental conditions (reactant flow composition, mass of catalyst, duration of experiment …) a comparison of advantages and drawbacks for each type of catalysts is proposed in terms of activity and selectivity as well as of CO2 and H2O influences. A special attention will be paid to copper-doped ceria catalysts which appear to be very active and selective in a range of temperatures appropriate for fuel cell application. The performances, the stability and the low cost of these formulations compared to noble metal-based catalysts make them very attractive for an industrial application.

Hydrogen production via steam reforming of bio-oil components over calcium aluminate supported nickel and noble metal catalysts

Abstract: Steam reforming of two representative bio-oil components, acetic acid and acetone, was investigated thermally and catalytically over nickel (5 wt%) and noble metal (0.5 wt% Rh or Ir) catalysts supported on calcium aluminates (CaO·2Al2O3 and 12CaO·7Al2O3). The thermal reactions (with or without water) were studied in the presence of inert quartz particles and showed that at 750 °C, acetone and to a lesser extent acetic acid undergo a series of homogeneous reactions forming CO, CO2, CH4 and H2 in concentrations which depend on the organic, and the presence or absence of water. Characteristic of acetone thermal steam reforming is that high amounts of acetic acid are produced. The catalysts prepared were tested at three reaction temperatures 550–650–750 °C using steam/carbon = 3 and space velocities around 30 000 h−1. The results showed that acetic acid is easily reformed over the catalysts to hydrogen rich gas with yield approaching that of equilibrium. Hydrogen yields depend on the metal type and loading and the ratio of CaO to Al2O3 of the support. The best performance in terms of highest hydrogen yield is achieved with the 5 wt% Ni/CaO·2Al2O3 catalyst, while the 0.5 wt% Rh/CaO·2Al2O3 catalyst presents the highest resistant to coking. These catalysts were tested in acetone reforming showing also very high activity, low coking deposition rate and slight superiority of the Rh catalyst in terms of hydrogen yield.

Synthesis and Characterization of Noble Metal (Pd, Pt, Au, Ag) Nanostructured Materials Confined in the Channels of Mesoporous SBA-15

Abstract: Highly dispersed metal (Pd, Pt) nanoparticles and uniformly distributed metal (Au, Ag) nanowires have been synthesized in ordered mesoporous silica SBA-15 via conventional incipient wetness impregnation followed by novel glow discharge plasma reduction. N2 adsorption−desorption isotherms and the low-angle X-ray diffraction (XRD) patterns indicate that the parent ordered mesoporous structure was well-maintained during the synthesis process. The wide-angle XRD patterns and transmission electron microscope images demonstrate that spherical Pd and Pt nanoparticles as well as rodlike Au and Ag nanowires were fabricated within the channels of SBA-15. The diameters of the metal nanoparticles and the metal nanowires were effectively controlled by the mesopores of the SBA-15 host. The population of the metal nanoparticles and the length of the metal nanowires can be tuned by the metal loading amount. In particular, the novel plasma reduction at ambient temperature is green, economical, and non-time-consuming, show...

Controllable Pt/ZnO Porous Nanocages with Improved Photocatalytic Activity

Abstract: Composite Pt/ZnO porous nanocages with ultrathin porous ZnO shell layers and ultrafine embedded Pt nanoparticles were facilely fabricated by ultrasonic irradiation-assisted two-step etching of Zn/ZnO core/shell nanoparticle colloids. The Pt cluster size can be well adjusted by the applied ultrasonic power. These Pt/ZnO nanocages exhibit excellent photocatalytic performance and can be further improved by the control of the embedded noble metal nanoparticles, which can be attributed to the abundant nanoscale Schottky contacts in the Pt−ZnO metal−semiconductor interfaces as well as to the large specific surface area due to the unique porous structure. The selective etching route used here could be of considerable universality for fabrication of a series of noble metal/oxide porous nanostructures as photocatalysts, such as the (Au, Ag, Pt, Pd)/(ZnO, TiO2) system.

Synthesis of Noble Metal Nanoparticles Embedded in the Shell Layer of Core−Shell Poly(styrene-co-4-vinylpyridine) Micospheres and Their Application in Catalysis

Abstract: The noble metal nanoparticles of Pd, Au, and Ag embedded in the shell layer of core−shell poly(styrene-co-4-vinylpyridine) micospheres were synthesized, and the catalytic activity of the shell-embedded Pd nanoparticles was investigated. To increase the accessible active site and therefore increase the catalytic activity of noble metal nanoparticles, the in situ synthesized noble metal nanoparticles are selectively immobilized in the outer shell layer of the core−shell poly(styrene-co-4-vinylpyridine) microspheres, which are synthesized by one-stage soap-free emulsion polymerization in water and contain a core of polystyrene and a coordinative shell of poly(4-vinylpyridine). It is found the Pd nanoparticles embedded in the shell layer of the core−shell micospheres are an efficient and easily reusable catalyst for Suzuki reactions performed in water.

Polyelectrolyte functionalized carbon nanotubes as a support for noble metal electrocatalysts and their activity for methanol oxidation

Abstract: A highly effective polyelectrolyte functionalization of multi-walled carbon nanotubes (MWCNTs) by poly(diallyldimethylammonium chloride) (PDDA-MWCNTs) was employed for low temperature fuel cell applications. PDDA-MWCNTs were employed as support materials for the in situ deposition and formation of platinum nanoparticles, via the self-assembly between the negative Pt precursor and positively charged functional groups of PDDA-functionalized MWCNTs. The effect of the functionalization on the deposition and distribution of Pt nanoparticles was investigated in detail. Compared with MWCNTs functionalized by conventional acid-oxidation treatment (AO-MWCNTs), the PDDA-functionalized MWCNTs cause no structural damage on MWCNTs and provide high density and homogeneous surface functional groups for the anchoring Pt nanoparticles. Pt nanoparticles with an average particle size of 1.8 ± 0.4 nm and loading as high as 60 wt% were realized on PDDA-MWCNTs supports. The Pt/PDDA-MWCNTs electrocatalysts show significantly higher electrochemically active surface area and higher electro-catalytic activity for methanol oxidation than that of Pt/AO-MWCNTs and E-TEK Pt/C electrocatalysts.

Multipolar second-harmonic generation in noble metal nanoparticles

Abstract: Second-harmonic generation from noble metal nanoparticles with a noncentrosymmetrical shape is theoretically investigated by using finite element method simulations. The relative weight of the dipolar and quadrupolar responses is investigated in terms of both light polarization and size dependence of the harmonic scattered intensity. It is shown that, even for small deformations as compared with purely spherical particles, the dipolar response dominates and scales as the nanoparticle surface area squared. The difference between gold and silver metal nanoparticles is also addressed.

Gold/platinum hybrid nanoparticles supported on multiwalled carbon nanotube/silica coaxial nanocables: Preparation and application as electrocatalysts for oxygen reduction

Abstract: A simple approach combining sonication and sol−gel chemistry was employed to synthesize silica coated carbon nanotube (CNTs) coaxial nanocables. It was found that a homogeneous silica layer can be coated on the surface of the CNTs. This method is simple, rapid, and reproducible. Furthermore, gold nanoparticle supported coaxial nanocables were facilely obtained using amino-functionalized silica as the interlinker. Furthermore, to reduce the cost of Pt in fuel cells, designing a Pt shell on the surface of a noble metal such as gold or silver is necessary. High-density gold/platinum hybrid nanoparticles were located on the surface of 1-D coaxial nanocables with high surface-to-volume ratios. It was found that this hybrid nanomaterial exhibits a high electrocatalytic activity for enhancing oxygen reduction (low overpotential associated with the oxygen reduction reaction and almost four-electron electroreduction of dioxygen to water).

Adsorption and Switching Properties of Azobenzene Derivatives on Different Noble Metal Surfaces: Au(111), Cu(111), and Au(100)

Abstract: The adsorption and switching behavior of 3,3′,5,5′-tetra-tert-butylazobenzene (meta-TBA) are investigated by low-temperature scanning tunneling microscopy on three different metal substrates: Au(111), Cu(111), and Au(100). The trans state is the most stable configuration after adsorption, displaying similar appearances in the STM images, independent of the substrate. However, the self-assembly and switching behavior is highly dependent on the chemistry and corrugation of the surface. On the Au(111) surface, the tip-induced isomerization is probed successfully and different driving mechanisms are characterized. The experimental images are in good agreement with calculated ones. However, the switching effect is completely suppressed on Cu(111) and Au(100).

Selective-Area Atomic Layer Deposition Using Poly(methyl methacrylate) Films as Mask Layers

Abstract: Selective-area atomic layer deposition (ALD) was achieved using poly(methyl methacrylate) (PMMA) films as growth inhibiting mask layers. The PMMA films were prepared from PMMA−toluene solution by spin-coating and patterned by UV lithography through a mechanical mask. The patterned PMMA films were tested in several ALD processes, both noble metals and oxides. The tested noble metal processes were iridium, platinum, and ruthenium, and the oxide processes were Al2O3 and TiO2. Al2O3 was deposited using AlCl3 and H2O and trimethylaluminum (TMA) and H2O. TiO2 was deposited using Ti(OMe)4 and H2O. The growth temperatures were 250−350 °C.

Noble Metal Catalysts for Mercury Oxidation in Utility Flue Gas: Gold, Palladium and Platinum Formulations

Abstract: The use of noble metals as catalysts for mercury oxidation in flue gas remains an area of active study. To date, field studies have focused on gold and palladium catalysts installed at pilot scale. In this article, we introduce bench-scale experimental results for gold, palladium and platinum catalysts tested in realistic simulated flue gas. Our initial results reveal some intriguing characteristics of catalytic mercury oxidation and provide insight for future research into this potentially important process.

First-principles study of water on copper and noble metal (110) surfaces

Abstract: Water structure and dissociation kinetics on a model open metal surface: Cu(110), has been investigated in detail based on first-principles electronic structure calculations. We revealed that in both monomer and overlayer forms, water adsorbs molecularly, with a high tendency for diffusion and/or desorption rather than dissociation on clean surfaces at low temperature. Studying water on other noble metal (110) surfaces confirms that Cu(110) is the borderline between intact and dissociative water adsorption, differing in energy by only 0.08 eV. This may lead to promising applications in hydrogen generation and fuel cells.