Role Of Ag Research Articles

Hydrogen Production by Steam Reforming of Methanol over a Ag/ZnO One Dimensional Catalyst

One dimensional (1-D) and three dimensional (3-D) ZnO were growth by a hydrothermal method. ZnO 1-D was employed as a support for silver nanoparticles in order to design a new catalyst and used on the steam reforming of methanol (SRM) reaction for H2 production. The catalytic activity of the Ag/ZnO sample with low content of Ag showed better performance on the SRM reaction than on high silver loading catalyst. So, the sample with small Ag particle size showed best performance in methanol conversion than catalyst with big Ag particle size, this finding could be attributed to the high ZnO/Ag ratio. According to results of SEM and TEM techniques the catalytic activity: methanol conversion, H2 and low CO production observed on the Ag/ZnO 1-D catalyst occurs in the edge sites rather than the rim sites. The role of Ag is to accept the hydrogen to be released to the gas phase. In addition, the 1.5Ag/ZnO 1-D catalyst showed good stability during the reaction.

Silver pick up during tail formation in thermosonic wire bonding process

Cu ball bonding processes are significantly less robust than Au ball bonding processes. One reason for this are higher variations observed, e.g. in the free-air ball (FAB) formation process. There is a strong influence the tail bond process has on subsequent FAB formation. Tail tips and bond fractographs made with Cu and Au wires are investigated using scanning electron microscopy. Au and Cu wires pick up Ag material (Ag pick up) from the Ag metallization of Cu leadframe diepads during wire tail breaking. Ag pick up by Cu wire is more dominant than that by Au wire. Ultrasonic friction energy is necessary in order for Ag material pick up to occur. The impact force plays an important role for Ag pick up; less pick up is observed with a higher impact force. The hardness of the free-air ball (FAB) with Ag pick up is reduced by up to 4% compared to that of a FAB made from a tail broken at the neck of a ball bond and therefore having no Ag pick up. No significant change of FAB diameter is observed in these two cases.

Ag + sorption on natural and Na-exchanged clinoptilolite from Eastern Rhodopes, Bulgaria

The study was carried out to examine the sorption of silver by natural and preliminary sodium-exchanged clinoptilolite from Beli plast deposit, Bulgaria. The effect of varying the process parameters like pH, adsorption time, and initial metal concentration was tested. It was found that the optimum pH is 4, and that the process has a fast kinetic – reaching equilibrium in 45 min. Preliminary treatment of clinoptilolite by exchange in sodium chloride solution increased its sorption ability towards Ag ions compared with the natural material. The sorption equilibrium was most well described by the Freundlich adsorption model. It was found that the ion-exchange plays main role in Ag + immobilization on the basis of correlation observed between the Ag + uptake and the amount of released metal ions (Ca 2+, K +, Mg 2+ and Na +). The desorption of silver from clinoptilolite by use of 0.1 N HNO 3 was investigated.

Photochemical Synthesis of Water-Soluble Gold Nanorods: The Role of Silver in Assisting Anisotropic Growth

The role of Ag+ ions in the ultraviolet-driven photochemical synthesis of Au nanorods (NRs) in aqueous surfactant mixtures has been investigated in order to elucidate the mechanism that drives anisotropic nanoparticle growth. The samples, grown in the presence of varying amounts of Ag+ ions for scheduled irradiation times, have been characterized by ultraviolet−visible−near infrared (UV−vis−NIR) absorption spectroscopy, analytical transmission electron microscopy (ATEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and extended X-ray absorption fine structure (EXAFS) measurements. Moreover, the time evolution of size and shape distribution has been investigated by statistical analysis of the relevant TEM data. EXAFS measurements at the Ag K-edge have unambiguously disclosed the presence of Ag species in the final product, identifying their chemical state as well as the most probable lattice environment around them with a reasonably high level of confidence. The extensive sample knowledge gained by the combination of spectroscopic, structural, and morphological measurements has provided reliable information regarding the most relevant processes underlying the Ag+-assisted formation of Au NRs by the photochemical route. An induction period prior to occurrence of fast nanoparticle nucleation has been identified, which has been correlated to the slow accumulation of a critical concentration of Au(I)−surfactant species from reduction of their Au(III) parent precursors. The role played by Ag in directing Au growth toward the formation of NRs has been clarified through demonstration of preferential adsorption of zerovalent Ag species on {110} facets of the growing Au nanoparticles, which can be therefore responsible for restricting crystal development along the relevant crystallographic directions.

Improvement in the Thermoelectric Figure of Merit by La/Ag Cosubstitution in PbTe

The thermoelectric properties of La-doped and Ag/La codoped PbTe were investigated in the temperature range of 300 to ∼720 K. All samples crystallize in the NaCl-type structure without noticeable secondary phase and exhibit narrow bandgaps of 0.26−0.30 eV, typical of PbTe. La doping (≤5 at %) in PbTe significantly enhances the room temperature electrical conductivity to >5000 S/cm, indicating that La is an efficient electron donor. Hall coefficient data confirm the significant increase in electron carrier concentration from ∼2 × 1018 cm−3 at 300 K for PbTe to ∼5.3 × 1019 cm−3 for Pb0.99La0.01Te, and ∼1.7 × 1020 cm−3 for Pb0.95La0.05Te. Ag doping in Pb0.99La0.01Te has an opposite effect on the electron carrier concentration (∼2.0 × 1019 cm−3 at 300 K for Ag0.05Pb0.99La0.01Te and ∼1.5 × 1019 cm−3 for Ag0.1Pb0.99La0.01Te), consistent with the role of Ag as electron acceptor. The temperature dependence of mobility is discussed in detail using the physics of electron scattering and possible scattering mechanis...

Adding further complexity to the polybasite structure: The role of Ag in the B layer of the -M2a2b2c polytype

We report data on the composition and crystal structure of the most Ag-rich (15.63 apfu) natural polybasite yet discovered. It shows the - M 2 a 2 b 2 c polytype. The crystal studied was found in a sample (mineralogical collection of the Royal Ontario Museum) from Gowganda, Timiskaming District, Ontario, Canada. Electron microprobe analysis yields the formula [Ag6(Sb1.78As0.18)∑=1.96S7] [Ag9(Ag0.63Cu0.43)∑=1.06S4]. Lattice parameters are a = 26.2625(4), b = 15.1623(5), c = 24.1061(6) A, β = 90.045(5)°, V = 9599.0(4) A3. The structure was refined in the space group C 2/ c to R = 0.0581 using 7725 observed reflections [ I > 2σ( I )]. The refinement shows that one of the three structural positions of the B module layer usually occupied by Cu is dominated by Ag. Crystal-chemical characteristics are compared with published data on the other members of the pearceite-polybasite group. Some remarks concerning nomenclature are also given.

Synthesis of Thermally Stable and Highly Active Bimetallic Au−Ag Nanoparticles on Inert Supports

A general two-step approach has been developed for the synthesis of very small and sintering-resistant bimetallic gold−silver nanoparticles on inert supports including commercial silica and alumina. In this approach, gold particles were formed in the first step on amino-functionalized silica or alumina support. Our density functional theory (DFT) calculations on selected model clusters show that the surface atoms of the gold particles formed in the first step carry slightly negative charges, which facilitates the subsequent Ag+ adsorption on the gold particle surface. Upon Ag+ adsorption and reduction by NaBH4 in the second step, specific nanoparticles with gold−silver alloy core and a silver nanoshell have been formed, as confirmed by our ultraviolet−visible spectroscopy (UV−vis), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) characterizations. Such particles have been found to be highly thermally stable, and their sizes remain substantially unchanged (∼3 nm) even upon calcination in air at 500 °C. After the final reduction treatment in H2, a randomly distributed alloy composed of gold and silver is formed, and the resultant Au−Ag alloy particles are highly catalytically active for CO oxidation, even superior to Au/TiO2. The role of Ag in stabilizing the particles has been discussed.

Catalytic Conversion of NO and C3H6 in Exhaust Gases Over Silver Catalysts Under Stoichiometric or Excess Oxygen

The role of Ag in simultaneously catalyzing NO reduction and C3H6 oxidation was shown to be strongly dependent on the redox properties of its local environment. Under an atmosphere of 1,000 ppm NO, 3,000 ppm C3H6, and 1% O2 and a GHSV of 30,000 h−1, a perovskite La0.88Ag0.12FeO3 prepared by reactive grinding is active giving a complete NO conversion and 92% C3H6 conversion at 500 °C. These values are much higher than the NO conversion of 55% and C3H6 conversion of 45% obtained over a 3 wt.% Ag/Al2O3 catalyst under the same conditions. Under an excess of oxygen (10% O2) a good SCR performance with a plateau of N2 yield above 97% over a wide temperature window of 350–500 °C along with C3H6 conversion of 90% at 500 °C was observed over Ag/Al2O3, while minor N2 yields (∼10% at 250–350 °C) and high C3H6 conversions (reaching ∼100% at 450 °C) were obtained over La0.88Ag0.12FeO3. Abundant molecular oxygen is desorbed from Ag substituted perovskite after 10% O2 adsorption as verified by O2- temperature programmed desorption (TPD). This reflects the strongly oxidative properties of La0.88Ag0.12FeO3, which lead to a satisfactory NO reduction at 1% O2 due to the ease of nitrate formation but to a significant C3H6 combustion above that value. The formation of nitrate species over the less oxidizing Ag/Al2O3 was accelerated under an excess of oxygen resulting in an excellent lean NO reduction behavior. The redox properties of silver catalysts could be adjusted via mixing perovskite with alumina for an optimal elimination of both NO and C3H6 over the whole range of oxygen concentration between 0 to 10%.

Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films

The ordered cocrystallization of nanoparticles into binary superlattices enables close contact of nanocrystals with distinct physical properties, providing a route to 'metamaterials' design. Here we present the first electronic measurements of multicomponent nanocrystal solids composed of PbTe and Ag(2)Te, demonstrating synergistic effects leading to enhanced p-type conductivity. First, syntheses of size-tuneable PbTe and Ag(2)Te nanocrystals are presented, along with deposition as thin-film nanocrystal solids, whose electronic transport properties are characterized. Next, assembly of PbTe and Ag(2)Te nanocrystals into AB binary nanocrystal superlattices is demonstrated. Furthermore, binary composites of varying PbTe-Ag(2)Te stoichiometry (1:1 and 5:1) are prepared and electronically characterized. These composites show strongly enhanced (conductance approximately 100-fold increased in 1:1 composites over the sum of individual conductances of single-component PbTe and Ag(2)Te films) p-type electronic conductivity. This observation, consistent with the role of Ag(2)Te as a p-type dopant in bulk PbTe, demonstrates that nanocrystals can behave as dopants in nanostructured assemblies.

4d core hole formation in silver and its role in Ag − secondary ion formation/survival

The low energy mono-atomic secondary ion intensities of Ag − over the transient regions from Ag under 1–5 keV Cs + primary ion impact are compared. These energies are used since they appear to be above and below the collisional excitation threshold for 4d core hole formation. The lack of any work function dependence in both suggests that (a) Ag − forms via resonance charge transfer of electrons from the broad 5s–5p band to Ag atoms bearing 4d core holes (Auger is disregarded for energetic reasons), and that (b) the route via which 4d core holes are formed, whether it be via collective oscillations or collisional excitation, is of minimal importance. The similarity in X-ray induced plasmon loss features from clean and Cs implanted Ag confers with these results.

Study of sodium and silver phosphate glasses doped with some metal chlorides

Several samples of sodium and silver phosphate glasses doped with 1%, 5% and 10% iron, manganese and zinc chlorides and control samples of undoped sodium and silver phosphate glasses were prepared by melt quenching technique. Sodium phosphate glasses doped with 1%, 5% and 10% silver chloride were also prepared to see the role of Ag + ions in modifying the structure of sodium phosphate glass. These glasses were characterized by elemental analyses, X-ray diffraction and DSC studies. The infrared spectra of these glasses were studied in the region 4000–400 cm −1. Spectral analyses confirm the existence of characteristic P–O–P linkages of linear polyphosphate chains and presence of O–P–O units in the phosphate tetrahedra. The spectra also reveal the formation of P–O–M (where M = Fe, Mn, Zn) bonds in these glasses except in Na 2O–P 2O 5 glasses doped with AgCl. The formation of P–O–M bonds which replace P O⋯Na + or P O⋯Ag + is in accordance with the variation of glass transition temperature ( T g) values.

The role of Ag [sbnd]O [sbnd]Al species in silver–alumina catalysts for the selective catalytic reduction of NO x with methane

We examined the role of silver and alumina in Ag–alumina catalysts for the selective catalytic reduction (SCR) of NO x by methane in gas streams containing excess oxygen. A cogelation technique was used to prepare Ag–alumina materials with high dispersion of silver even at high metal loadings (>10 wt%) and after air calcination at 650 °C. Typically, a part of silver is present as fine nanoparticles on the alumina, whereas another part is ionic, bound with the alumina as [Ag O Al] species. Dilute nitric acid leaching was used to remove the silver particles and all weakly bound silver from the surface of these materials. Complementary structural characterization was performed by HRTEM, XPS, XRD, and UV–vis DRS. We found that the higher the initial silver content, the higher the amount of the residual [Ag O Al] species after leaching. NO–O 2-TPD tests identified that silver does not modify the surface properties of the alumina. The SCR reaction-relevant NO x adsorption takes place on alumina. Temperature-programmed surface reaction (TPSR) and kinetic measurements at steady state were used to check the reactivity of the adsorbed NO x species with methane and oxygen to form dinitrogen. Only the alumina-adsorbed nitrates react with CH 4 to produce N 2 in the presence of oxygen, beginning at ∼300 °C as found by TPSR. Moreover, the SCR reaction rates and apparent activation energies are the same for the leached and parent Ag–alumina catalysts. Thus, metallic silver nanoparticles are spectator species in CH 4-SCR of NO x . These catalyze the direct oxidation of methane at temperatures as low as 300 °C, which explains the lower methane selectivity for the SCR reaction measured over the parent samples.

Role of Ag in the formation of interfacial intermetallic phases in Sn-Zn soldering

This study explored the effect of Ag as the substrate or alloying element of solders on the interfacial reaction in Sn-Zn soldering. Results show that instead of Ag-Sn compounds, ζ-AgZn and γ-Ag5Zn8 form at the Sn-Zn/Ag interface. The addition of Ag in Sn-Zn solders leads to the precipitation of e-AgZn3 from the liquid solder on preformed interfacial intermetallics. The morphology of this additional AgZn3 is closely related to the solidification process of Ag-Zn intermetallics and the under intermetallic layer.

Role of Ag+ in the Band Structures and Photocatalytic Properties of AgMO3 (M: Ta and Nb) with the Perovskite Structure.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Role of Ag+ in the Band Structures and Photocatalytic Properties of AgMO3 (M: Ta and Nb) with the Perovskite Structure

Photophysical and photocatalytic properties of perovskite-type materials AgMO3 (M: Ta and Nb) consisting of Ag+ and d0 ions were investigated. The band gaps of AgTaO3 and AgNbO3 were 3.4 and 2.8 eV, respectively, being 0.6 eV smaller than the band gaps of NaTaO3 (4.0 eV) and NaNbO3, even if the crystal structures of AgMO3 were similar to those of NaMO3. It was found from the electronic band structure study, using the plane-wave-based density functional method, that a hybrid orbital of Ag 4d and O 2p formed a valence band at a more negative level than O 2p orbitals. AgTaO3 showed photocatalytic activity for pure water splitting into H2 and O2 under UV-light irradiation. AgNbO3 has arisen as a new visible-light-driven photocatalyst possessing the ability to evolve H2 or O2 from water in the presence of sacrificial reagents.

Expression and function of transgenic HLA-DQ molecules and lymphocyte development in mice lacking invariant chain.

Invariant chain (Ii) is a non-MHC-encoded molecule, which plays an accessory role in the proper assembly/expression of functional MHC class II molecules and there by plays an important role in Ag processing/presentation. The phenotype of mice lacking Ii depends on the allotype of the MHC class II molecule. In some mice strains, Ii deficiency results in reduction in expression of class II molecules accompanied by defective CD4(+) T cell development. Responses to conventional Ags/superantigens are also compromised. In this study, we describe for the first time the functionality of human class II molecules, HLA-DQ6 and HLA-DQ8, in transgenic mice lacking Ii. HLA transgenic Ii(-/-) mice expressed very low levels of surface DQ6 and DQ8 accompanied by severe reduction in CD4(+) T cells both in the thymus and periphery. In vitro proliferation and cytokine production to an exogenous superantigen, staphylococcal enterotoxin B (SEB) was diminished in HLA-transgenic Ii(-/-) mice. However, SEB-induced in vivo expansion of CD8(+) T cells expressing TCR Vbeta8 family in DQ8.Ii(-/-) mice was comparable with that of DQ8.Ii(+/+) mice. Systemic IFN-gamma production following in vivo challenge with SEB was reduced in DQ8.Ii(-/-) mice and were also protected from SEB-induced toxic shock. Although the T cell response to a known peptide Ag was diminished in DQ8.Ii(-/-) mice, DQ8.Ii(-/-) APCs were capable of presenting that peptide to primed T cells from wild-type DQ8 mice as well as to a specific T cell hybridoma. Differentiation of mature B cells was also affected to a certain extent in DQ8.Ii(-/-) mice.

Coincidence Doppler broadening study of the pre-precipitation stage of an Al-Cu-Mg-Ag alloy

The role of Ag in the pre-precipitation process of an Al-1.9 at.% Cu-0.3 at. % Mg-0.2 at.% Ag alloy has been investigated in view of the interaction between the quenched-in vacancies and Ag atoms. Positron annihilation methods, coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopies, and transmission electron microscopy have been employed. In an as-quenched state, quenched-in vacancies V in the All 9 at% Cu-0.3 at.% Mg-0.2 at.% Ag alloy are associated with Ag atoms as well as Mg atoms (V-Mg-Ag) while V-Mg complexes are formed in the Al-1.9at.% Cu-0.3 at.% Mg alloy. The recovery of the vacancies after 15s ageing at 180°C is suppressed by the existence of Ag atoms. These results indicate that the V-Mg complexes are stabilized by the associated Ag atoms, which promotes the co-clustering of Ag and Mg atoms by ageing rather than by the formation of dislocation loops.

Coincidence Doppler broadening study of the pre-precipitation stage of an Al[sbnd]Cu[sbnd]Mg[sbnd]Ag alloy

Abstract The role of Ag in the pre-precipitation process of an Al[sbnd]1.9 at.% Cu[sbnd]0.3at.% Mg[sbnd]0.2at.% Ag alloy has been investigated in view of the interaction between the quenched-in vacancies and Ag atoms. Positron annihilation methods, coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopies, and transmission electron microscopy have been employed. In an as-quenched state, quenched-in vacancies V in the Al[sbnd]1.9at.% Cu [sbnd]0.3 at. % Mg[sbnd]0.2 at. % Ag alloy are associated with Ag atoms as well as Mg atoms (V[sbnd]Mg[sbnd]Ag) while V[sbnd]Mg complexes are formed in the Al[sbnd]1.9 at. % Cu[sbnd]0.3 at. % Mg alloy. The recovery of the vacancies after 15s ageing at 180°C is suppressed by the existence of Ag atoms. These results indicate that the V[sbnd]Mg complexes are stabilized by the associated Ag atoms, which promotes the co-clustering of Ag and Mg atoms by ageing rather than by the formation of dislocation loops.

Antigen presentation by liver cells controls intrahepatic T cell trapping, whereas bone marrow-derived cells preferentially promote intrahepatic T cell apoptosis.

Systemic activation and proliferation of CD8(+) T cells result in T cell accumulation in the liver, associated with T cell apoptosis and liver injury. However, the role of Ag and APC in such accumulation is not clear. Bone marrow chimeras were constructed to allow Ag presentation in all tissues or alternatively to restrict presentation to either bone marrow-derived or non-bone marrow-derived cells. OVA-specific CD8(+) T cells were introduced by adoptive transfer and then activated using peptide, which resulted in clonal expansion followed by deletion. Ag presentation by liver non-bone marrow-derived cells was responsible for most of the accumulation of activated CD8(+) T cells. In contrast, Ag presentation by bone marrow-derived cells resulted in less accumulation of T cells in the liver, but a higher frequency of apoptotic cells within the intrahepatic T cell population. In unmodified TCR-transgenic mice, Ag-induced T cell deletion and intrahepatic accumulation of CD8(+) T cells result in hepatocyte damage, with the release of aminotransaminases. Our experiments show that such liver injury may occur in the absence of Ag presentation by the hepatocytes themselves, arguing for an indirect mechanism of liver damage.

Role of Ag and Mg on precipitation of T 1 phase in an Al-Cu-Li-Mg-Ag alloy

Role of Ag and Mg on precipitation of T 1 phase in an Al-Cu-Li-Mg-Ag alloy