Search results for "noble metal"
showing 10 items of 44 documents
Ab Initio Simulation of Clusters: Relativistic Effects in Structure and Bonding of Noble Metal Nanoparticles
2005
Resolving the atomic and electronic structures of nanoclusters represents an important preliminary for their controlled use in future nanotechnologies. Here we show through the comparison of density-functional calculations with high-resolution photoelectron spectroscopy that 1.4 nm nanoparticles of silver (negatively charged clusters of 53 to 58 atoms) are icosahedral-based structures displaying a perfect icosahedral-induced electronic shell structure for Ag 55 − and slightly perturbed shell structures for the neighboring cluster sizes. At variance, 55-atom gold clusters exhibit several isomeric structures of low symmetry, with a largely diminished electronic shell structure. This surprisin…
Pd/Co3O4 catalyst for CH4 emissions abatement: study of SO2 poisoning effect
2007
A catalyst with 0.7 wt% Pd load supported over Co3O4 oxide was investigated in the methane oxidation by operating under CH4/O2 stoichiometric conditions. The effect of the noble metal addition on the activity of bare Co3O4 was evaluated. Samples were characterized by BET, XRD, TPR and XPS analyses. The SO2 poisoning of Pd catalyst and Co3O4 was studied by performing CH4 oxidation tests under stoichiometric conditions in SO2 (1 ppm or 10 ppm). Experiments evidenced that in our conditions the low amount of SO2 doesn’t influence the Pd behaviour, whereas in presence of 10 ppm of SO2 some deactivation occurs that becomes more evident above 450 °C at which the catalyst doesn’t reach 100% of meth…
Efficient Anodic and Direct Phenol-Arene C,C Cross-Coupling: The Benign Role of Water or Methanol
2012
C,C cross-coupling reactions for the synthesis of nonsymmetrical biaryls represent one of the most significant transformations in contemporary organic chemistry. A variety of useful synthetic methods have been developed in recent decades, since nonsymmetrical biaryls play an evident role in natural product synthesis, as ligand systems in homogeneous catalysis and materials science. Transformation of simple arenes by direct C,H activation belongs to the cutting-edge strategies for creating biaryls; in particular the 2-fold C,H activation is of significant interest. However, in most examples very costly noble metal catalysts, ligand systems, and significant amount of waste-producing oxidants …
Raman Spectroscopic Fingerprints of Atomically Precise Ligand Protected Noble Metal Clusters: Au 38 (PET) 24 and Au 38− x Ag x (PET) 24
2021
Distinct Raman spectroscopic signatures of the metal core of atomically precise, ligand-protected noble metal nanoclusters are reported using Au38 (PET)24 and Au38- x Agx (PET)24 (PET = 2-phenylethanethiolate, -SC2 H4 C6 H5 ) as model systems. The fingerprint Raman features (occurring <200 cm-1 ) of these clusters arise due to the vibrations involving metal atoms of their Au23 or Au23- x Agx cores. A distinct core breathing vibrational mode of the Au23 core has been observed at 90 cm-1 . Whereas the breathing mode shifts to higher frequencies with increasing Ag content of the cluster, the vibrational signatures due to the outer metal-ligand staple motifs (between 200 and 500 cm-1 ) do not s…
SnO2 : Sb - A new material for high-temperature MEMS heater applications: Performance and limitations
2007
MEMS micro heater devices capable of long-term operation at temperatures up to 1000 degrees C are presented. The enhanced long-term stability has been achieved by employing antimony-doped tin oxide (SnO2:Sb) as a substitute for the conventionally used noble metal heater resistors. A detailed investigation of its high-temperature stability reveals that degradation is caused by out-diffusion of Sb impurities from the SnO2 film. (c) 2007 Elsevier B.V. All rights reserved.
The co-reactant role during plasma enhanced atomic layer deposition of palladium
2020
Atomic layer deposition (ALD) of noble metals is an attractive technology potentially applied in nanoelectronics and catalysis. Unlike the combustion-like mechanism shown by other noble metal ALD processes, the main palladium (Pd) ALD process using palladium(ii)hexafluoroacetylacetonate [Pd(hfac)2] as precursor is based on true reducing surface chemistry. In this work, a thorough investigation of plasma-enhanced Pd ALD is carried out by employing this precursor with different plasmas (H2*, NH3*, O2*) and plasma sequences (H2* + O2*, O2* + H2*) as co-reactants at varying temperatures, providing insights in the co-reactant and temperature dependence of the Pd growth per cycle (GPC). At all te…
Characterisation and radiolysis of modified lithium orthosilicate pebbles with noble metal impurities
2017
Modified lithium orthosilicate (Li4SiO4) pebbles with additions of titanium dioxide (TiO2) are suggested as an alternative tritium breeding ceramic for the European solid breeder test blanket module. The noble metals – platinum (Pt), gold (Au) and rhodium (Rh), can be introduced into the modified Li4SiO4 pebbles during the melt-based process, due to the corrosion of Pt-Rh and Pt-Au alloy crucible components. In this study, the surface microstructure, chemical and phase composition of the modified Li4SiO4 pebbles with different contents of the noble metals was analysed. The influence of the noble metals on the radiolysis was evaluated after irradiation with accelerated electrons (E = 5 MeV),…
Maximum Noble-Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum
2014
International audience; Platinum is the most versatile element in catalysis, but it is rare and its high price limits large-scale applications, for example in fuel-cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst's surface. To maximize the noble-metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria ``nanopocket'', which binds Pt2+ so strongly…
Crosslinked Spherical Nanoparticles with Core-Shell Topology
2000
Core–shell microgels are crosslinked nanosized spherical particles with a chemical composition that is different on the surface compared to the core region. By employing a core with special optical properties, e.g., a core labeled either with organic dye molecules or noble metal clusters (see Figure), these particles are perfectly suited as optical tracers in diffusion measurements. Here, the shell may be important for several reasons: (i) as a protective coating to suppress any influence of the labels on particle mobility, (ii) to optically separate individual particles even at high concentrations, and (iii) to compatibilize the particles with e.g., polymeric chains. Recent developments in…
Oxide-based nanomaterials for fuel cell catalysis:the interplay between supported single Pt atoms and particles
2017
The concept of single atom catalysis offers maximum noble metal efficiency for the development of low-cost catalytic materials. Among possible applications are catalytic materials for proton exchange membrane fuel cells. In the present review, recent efforts towards the fabrication of single atom catalysts on nanostructured ceria and their reactivity are discussed in the prospect of their employment as anode catalysts. The remarkable performance and the durability of the ceria-based anode catalysts with ultra-low Pt loading result from the interplay between two states associated with supported atomically dispersed Pt and sub-nanometer Pt particles. The occurrence of these two states is a co…