Ag2O Oxidation Research Articles

Oxidation behavior and mechanism of Ag-9.8Sn-1.3Bi-0.4Cu alloy powders in different oxidant

The oxidation behavior of Ag-9.8Sn-1.3Bi-0.4Cu alloy powders is investigated used by XRD, SEM, EPMA and TEM. The results show that the oxidation behavior in air and air+Ag2O is different for Ag-9.8Sn-1.3Bi-0.4Cu microparticles, but that is similar for Ag-9.8Sn-1.3Bi-0.4Cu nanoparticles. The size of oxides formed in air is larger than that in air+Ag2O and the size of CuO is larger than that of SnO2 and Bi2Sn2O7. The oxides mainly distribute on powder surface in air oxidation, while those mainly distribute on grain boundaries in air+Ag2O oxidation. The nucleation priority order of oxides is SnO2 >Bi2O3 >Bi2Sn2O7 >CuO during oxidation both in air and air+Ag2O.

Wear mechanism of Ag as solid lubricant for wide range temperature application in micro-beam plasma cladded Ni60 coatings

The effect of Ag content on the microstructure, compositions, and wear resistance of micro-beam plasma cladded Ni60 coating was studied. The friction layer formed on the wear surface at different temperatures and its influence on tribological properties were discussed, and wear mechanism of Ag added Ni60 coating were clarified. The results show that adding Ag can effectively improve the wear resistance of cladding layer. The friction coefficient of the cladding layer decreases from 0.40, 0.10 and 0.22–0.20, 0.08 and 0.18 at 25 ℃, 300 ℃ and 600 ℃, respectively. At room temperature, the wear rate of nickel based coating is 14.13 * 10−7 mm/Nm, and reduces to 10.55 * 10−7 mm/Nm with 5%Ag addition. The excellent tribological performance at room temperatures is attributed to the existence of Ag layer formed in cladding process. The addition of silver significantly reduced the degree of abrasive wear. However, at higher temperature of 300 ℃, Ag precipitates from the coating and begins to generate Ag2O oxide, which avoids the direct contact of metal and effectively reduces the friction coefficient. The wear mechanism is oxidation wear and abrasive wear. At 600 ℃, high temperature softening results in obvious adhesive wear of the coating, and Ag particles accumulate on the surface of the cladding layer in the form of "sweating", and results in the formation of self-lubricants with NiO and AgCrO2. Therefore, oxide lubrication layer formation such as Ag2O, NiO and AgCrO2 at high temperatures is benefit for low friction coefficient at elevated temperature.

Recycling electrochemical processing of Ag(I) species dispersed in polypyrrole towards Ag0 nanoparticles

Silver(I) hexafluorophosphate was dispersed in electrosynthezed polypyrrole hexafluorophosphate by precipitation, using an aqueous Ag(I) nitrate solution. The electrochemistry of the resulting composite was studied in aqueous hexafluorophosphate. The examined potential window encompassed the reversible redox process of the polymer and the redox transformations of the inorganic components, i.e. silver species, taking place within this window. The electrocrystallization of silver nanoparticles was controlled by dc potential. The Ag nanoparticles entrapped in polypyrrole, generated at reduction potentials of the composite, were 101 nm in diameter. The slightly larger AgNP of a mixed chemical nature was formed by a spontaneous discharge of the oxidized polypyrrole - salt composite towards the open circuit potential. Electrochemical processing and characterization of the composite were done using the dc and ac methods. Several other methods of chemical and structural identification were employed. The influence of oxygen for the processing of the composite towards metallic nanoparticles was shown. Electrochemical chemically irreversible oxidation of Ag2O to AgO in the polypyrrole matrix was found.

Biomimetic Oxidation of Monolignol Acetate and p-Coumarate by Silver Oxide in 1,4-Dioxane.

Lignin acylated with acetate and/or p-coumarate is common in many herbaceous plants. Herein, the biomimetic oxidation of γ-acylated monolignols with Ag2O was studied to understand the effect of γ-acyl groups on monolignol polymerization. The oxidation of sinapyl acetate gave γ-acylated and α-acylated β-O-4 dimers in 71 and 9.5% yields, respectively. The oxidation of sinapyl p-coumarate produced γ-acylated β-O-4 and γ-acylated tetralin β-β dimers in 53 and 16% yields, respectively. Only the sinapyl alcohol moiety in sinapyl p-coumarate reacted, and the p-coumarate moiety remained unchanged, suggesting that p-coumaric acid is not incorporated into the lignin backbone in the acylated lignins. All of the γ-acylated monolignols used in this study produced the γ-acylated β-O-4 dimers, which suggests that the γ-acylated monolignols act as lignin monomers. The relatively high yields of the β-O-4 dimers indicate that Ag2O oxidation of the monolignols can be used as an easy method for synthesizing the β-O-4 dimer model compounds.

Investigation of the Na2O/Ag2O ratio on the synthesis conditions and properties of the 80TeO2–10ZnO–[(10−x)Na2O–xAg2O] glasses

Properties of the tellurite glasses 80TeO2–10ZnO–[(10−x)Na2O–xAg2O] are investigated as a function of the substitution ratio x between Na2O and Ag2O. One observe that the variation of glass transition temperature decreases monotonously with x and that surface crystallization mechanism is favored. The assignment of the Raman bands and their relation with the underlying glass structure is discussed. While both Na2O and Ag2O oxides act as glass network modifiers, their progressive equimolar substitution does not lead to a meaningful evolution in the structure of the TZ[Na10-xAgx] glass. The refractive index and the cut-off wavelength are found to increase with x. The compositional and wavelength dependence of refractive indices, cut-off wavelength and optical band gap energy is discussed in terms of changes brought about by Na2O/Ag2O substitution in the glass network. Finally, it is shown that interaction between the platinum crucible and the glass melt increases with the silver content in the glass.

Quasi-Instantaneous Bacterial Inactivation on Cu-Ag Nanoparticulate 3D Catheters in the Dark and Under Light: Mechanism and Dynamics.

The first evidence for Cu-Ag (50%/50%) nanoparticulate hybrid coatings is presented leading to a complete and almost instantaneous bacterial inactivation in the dark (≤5 min). Dark bacterial inactivation times on Cu-Ag (50%/50%) were observed to coincide with the times required by actinic light irradiation. This provides the evidence that the bimetal Cu-Ag driven inactivation predominates over a CuO/Cu2O and Ag2O oxides inducing a semiconductor driven behavior. Cu- or Ag-coated polyurethane (PU) catheters led to bacterial inactivation needing about ∼30 min. The accelerated bacterial inactivation by Cu-Ag coated on 3D catheters sputtered was investigated in a detailed way. The release of Cu/Ag ions during bacterial inactivation was followed by inductively coupled plasma mass-spectrometry (ICP-MS) and the amount of Cu and Ag-ions released were below the cytotoxicity levels permitted by the sanitary regulations. By stereomicroscopy the amount of live/dead cells were followed during the bacterial inactivation time. By Fourier transform infrared spectroscopy (FTIR), the systematic shift of the -(CH2) band stretching of the outer lipo-polysaccharide bilayer (LPS) was followed to monitor the changes leading to cell lysis. A hydrophobic to hydrophilic transformation of the Cu-Ag PU catheter surface under light was observed within 30 min followed concomitantly to a longer back transformation to the hydrophobic initial state in the dark. Physical insight is provided for the superior performance of Cu-Ag films compared to Cu or Ag films in view of the drastic acceleration of the bacterial inactivation observed on bimetal Cu-Ag films coating PU catheters. A mechanism of bacterial inactivation is suggested that is consistent with the findings reported in this study.

Biomimetic oxidative coupling of sinapyl acetate by silver oxide: preferential formation of β-O-4 type structures.

Biomimetic oxidations of sinapyl alcohol and sinapyl acetate were carried out with Ag2O to better understand the high frequency of β-O-4 structures in highly acylated natural lignins. The major products from the Ag2O oxidation of sinapyl alcohol were sinapyl aldehyde (14% yield), β-O-4-coupled dimer (32% yield), and β-β-coupled dimer (3% yield). In contrast, the Ag2O oxidation of sinapyl acetate produced β-O-4-coupled dimer in 66% yield. Oligomeric products with predominantly β-O-4 structures were also obtained in 18% yield. The yield of the β-O-4-coupled products from sinapyl acetate was much higher than that from sinapyl alcohol. Computational calculations based on density functional theory showed that the negative charge at Cβ was significantly reduced by the γ-acetyl group. The computational calculations suggest that the Coulombic repulsion between Cβ and O4 in sinapyl acetate radicals was significantly reduced by the γ-acetyl group, contributing to the preferential formation of β-O-4 structures from sinapyl acetate.

Preparation, characterization and photocatalytic activity of metal-loaded NaNbO 3

Fe-, Ni-, Co- and Ag- loaded NaNbO 3 catalysts were prepared and their activities have been investigated in the reaction of photocatalytic hydrogen generation. Me/NaNbO 3 were synthesized by impregnation of NaNbO 3 in an aqueous solution of metal nitrates and then by calcination at the temperature of 400 °C. The crystallographic phases and optical and vibronic properties were examined by X-ray diffraction (XRD) and diffuse reflectance (DR) UV–vis and resonance Raman spectroscopic methods, respectively. Morphology and chemical composition of the produced samples were studied using a high-resolution transmission electron microscope (HR-TEM) and an energy dispersive X-Ray spectrometer (EDX) as its mode. The detailed analysis has revealed that all the investigated catalysts exhibit high crystallinity and the presence of Fe 2O 3, NiO, Co 3O 4 and Ag 2O oxides on Me/NaNbO 3 was confirmed. Finally, the influence of different metal loadings (Fe, Ni, Co and Ag) on the photocatalytic activity of NaNbO 3 for photocatalytic hydrogen generation has been investigated. Here we report that among all the Me/NaNbO 3 photocatalysts Ag-loaded NaNbO 3 exhibited higher photocatalytic efficiency for photocatalytic hydrogen generation than NaNbO 3.

Effect of orientation of crystal face of silver and its alloying with gold on properties of thin anodic Ag(I) oxide films: II. Photopotential

The current efficiency of the formation of anodic oxide on polycrystalline silver is shown to decrease with an increase in the concentration of KOH solutions, while the rate-limiting stage remains the solid-phase mass transfer. Photopotential in nano-size Ag(I) oxide films anodically formed on polycrystalline silver is independent of the OH− ion concentration, which means that a photoresponse is generated in the bulk oxide. The n-type conductivity of oxide films on silver, Ag-Au alloys, and low-index silver crystal faces, which was determined previously when measuring photocurrent, is confirmed. Replacing polycrystalline silver with its monocrystals results in a substantial decrease in the photopotential amplitude due to the decrease in the deviation from a stoichiometric composition. The electron mobility and the partial electronic photoconductivity in the anodic Ag(I) oxide depend on the orientatinon of the crystal face in silver and the gold content. At E = 0.56 V, a series of changes in these characteristics correlates to the changes in other structure-dependent parameters of Ag2O oxide (the optical absorption coefficient α, the concentration of donor defects ND, the width of the spatial charge region W, and the Debye screening length LD).

Organic sulfur oxidation to sulfate in soil samples for total sulfur determination by turbidimetry

Sulfur in the soil occurs in two basic forms, organic and inorganic S. The organic form accounts for 95 % of S in most soils. The effectiveness of organic S to oxidate to sulfate was evaluated for total S determination in soil samples by wet (acid) and dry-ash (alkaline) oxidation methods. To evaluate the wet method and the possible use as a reference when evaluating the dry method proposed here, a reference standard from the US National Institute of Standards and Technology (NIST) was used (Montana Soil - NIST 2710). The dry-ash oxidation process with alkaline oxidizing agents is one of the simplest oxidation methods of organic S to the sulfate form and was compared with the wet process. The objective of the study was to develop a dry method that would be easy to apply and allow the complete conversion of organic S to sulfate in soil samples and later detection by turbidimetry. The effectiveness of organic S oxidation to sulfate was evaluated by means of three alkaline oxidation mixtures: NaHCO3 + Ag2O, Eschka mixture (17 % Na2CO3, 66 % MgO, and 17 % K2CO3), and NaHCO3 + CuO. The procedure to quantify the sulfate concentration was based on the reaction with barium chloride and turbidimetric detection. Sulfur quantification in the standard sample by the wet method proved adequate, precise and accurate. It should also be pointed out that no significant differences were found (95 % reliability) between the wet and dry processes (NaHCO3 and Ag2O oxidation mixture) in six different Brazilian soils. The proposed dry method can therefore be used in the preparation of soil samples for total S determination.

ANODIC formation of thin Cu(I) and Ag(I) oxide films on Cu‐Au and Ag—Au alloys

AbstractThe anodic dissolution of Cu, Ag and their alloys with gold under the conditions of the formation of copper or silver oxides was examined by the technique of cyclic voltammetry, chronoammetry and photopotential measurements. The anodic formation and cathodic reduction of Cu(I) oxide on copper and Cu‐Au alloys or Ag(I) oxide on silver and Ag‐Au alloys with XAu≤30 at.% are controlled by migration in the oxide film. The peculiarity of Ag‐Au system is connected with the duplex structure of the oxide film and the initial selective dissolution of silver from Ag15Au and Ag30Au alloys. Ag2O oxide is an n‐type semiconductor with an excess of silver atoms. Oxide layers formed on monocrystalline silver are more stoichiometric than the layer formed on polycrystalline Ag. Copyright © 2008 John Wiley & Sons, Ltd.

Kinetic peculiarities of anodic dissolution of silver and Ag–Au alloys under the conditions of oxide formation

The kinetics of anodic dissolution of silver and Ag–Au alloys ( X Au = 0.1–30 at.% Au) in aqueous alkaline solution under the conditions of the formation of silver oxides has been examined. The techniques of cyclic voltammetry, chronoammetry, and photopotential measurements have been used. It was established that the anodic formation and cathodic reduction of Ag 2O on silver and alloys are controlled by migration in the oxide layer. Ag 2O oxide is an n-type semiconductor with an excess of silver atoms. Oxide layers formed on monocrystalline Ag(1 1 1) and Ag(1 1 0) are more stoichiometric than the layer formed on polycrystalline Ag.

Band structure and chemical bonding in Cu2O and Ag2O oxides

The band structure, total and projected densities of states, and distributions of the valence and difference electron densities for copper and silver oxides are calculated in the framework of the density functional theory in the local approximation with ab initio norm-conserving pseudopotentials in the basis set of pseudoatomic orbitals. The results obtained are compared with the experimental data and calculations performed by other authors. The energy spectrum and spatial distribution of electrons in crystals are similar to each other. Metal ions are bonded to each other through charge density channels with a weakly pronounced maximum at the center of the empty tetrahedron.

Interaction of a polycrystalline silver powder with ozone

AbstractThe interaction of a polycrystalline silver powder with ozone (5 mol.% O3 in O2) was investigated over the temperature range 300–923 K. Silver powder samples were reacted with ozone for 30 min at specified temperatures in this range. All experiments were performed at atmospheric pressure. Following treatment, samples were cooled rapidly to room temperature under a nitrogen gas purge and then characterized by scanning electron microscopy, powder x‐ray diffraction, Raman spectroscopy and x‐ray photoelectron spectroscopy. The morphology, structure and chemical composition of the oxide products formed at different reaction temperatures were examined. The silver powder blackened immediately upon contact with the reaction gas mixture at 300 K, due to the formation of silver (I) oxide (Ag2O). After reaction with O3 for 30 min at 300 K, a mixed‐oxide phase comprised of Ag2O and monoclinic silver (I, III) oxide (AgO) was formed. The AgO was formed at the gas/oxide interface via the oxidation of Ag2O. A tetragonal AgO polymorph was formed when the silver powder was reacted with ozone at 473 K. The only product of silver oxidation by O3 at 673 K was Ag2O, which remained stable in the reaction gas to temperatures in excess of 773 K. The silver powder was not oxidized by ozone at 923 K, although the powder did sinter extensively during the treatment. The experimental data presented are in good accord with available thermodynamic data pertaining to the thermal stability of Ag2O and AgO in ozonized oxygen at atmospheric pressure. Copyright © 2002 John Wiley & Sons, Ltd.

Rubber Chemicals Derived from Conjugate Addition. II. Synthesis of a Novel Class of Benzoquinone Diimines

Abstract Various 2-alkylthio-5-amino- and 2,5-diamino-1,4-phenylenediamines were prepared by treatment of N-(1,3-dimethylbutyl)- N′-phenyl-1,4-quinonediimine with thiols and amines using air as an oxidizing agent. Key 2-alkylthio-1, 4-benzoquinone diimines were prepared via Ag2O oxidation of the corresponding 1,4-phenylenediamines.

Synthesis of redox derivatives of lysine and their use in solid-phase synthesis of a light-harvesting peptide

Redox-active amino acids were synthesized for incorporation into peptide assemblies to study photoinitiated electron or energy transfer. 4′-Methyl-2,2′-bipyridine-4-carboxylic acid was obtained in 72% yield by consecutive SeO 2 and Ag 2O oxidation without isolation of intermediates. The side chain ε-amino group of Boc- l-lysine methyl ester or γ-carboxyl group of Boc- l-glutamic acid α-methyl ester was coupled to a redox moiety (transition-metal chromophore, electron donor, electron acceptor, metal ligand, or triplet-energy transmitter) using 4-(dimethylamino)pyridine, (1-benzotriazoleoxy)tris(dimethylamino)phosphonium hexafluorophosphate, N-methylmorpholine, and 1-hydroxybenzotriazole. Use of one equivalent of 4-(dimethylamino)pyridine provided the amide coupling product in 80–97% isolated yield. Selective hydrolysis of the methyl esters with lithium hydroxide provided the redox Boc-amino acids in 70–98% yield. These redox modules are suitable for solid-phase assembly of light-harvesting peptides, as illustrated by the synthesis of the partially α-helical 11-residue redox triad that contains a phenothiazine electron donor, a ruthenium(II)tris(bipyridine) chromophore, and an anthraquinone electron acceptor. Upon laser excitation at 420 nm, the peptide triad underwent photoinduced electron transfer to create a charge-separated state with a lifetime of 53 ns and decayed with a first-order rate constant of 1.9 × 10 8 s −1.

Synthesis of redox derivatives of lysine and related peptides containing phenothiazine or tris(2,2′‐bipyridine)ruthenium(II)

Boc-L-Lysine derivatives and lysine-containing peptides bearing the electron donor 10H-phenothiazine (PTZ) or the redox chromophore tris(2,2'-bipyridine)ruthenium(II) dication ([Rub3]2+, where b is 2,2'-bipyridine) have been synthesized and characterized. SeO2 oxidation (53% yield) of 4,4'-dimethyl-2,2'-bipyridine, Ag2O oxidation (85% yield) of the monoaldehyde, complexation (96% yield) of 4'-methyl-2,2'-bipyridine-4-carboxylic acid (m-OH) with Rub2Cl2, activation (81% yield) of the acid [Rub2m-OH]2+ (PF6-)2, and condensation (83% yield) of the succinimido ester [Rub2m-OSu]2+ (PF6-)2 with Boc-Lys furnished the protected redox-chromophore module [Boc-Lys(Rub2m)-OH]2+ (PF6-)2 in 29% overall yield over five steps. The first two steps constitute the first practical synthesis of the monocarboxylic acid m-OH (45% overall yield). Also prepared were m-OSu, Boc-Lys(m)-OH, Boc-Lys(m)-OCH3, and [Rub2m-NHCH3]2+ (PF6-)2. Activation (91% yield) of 3-(10H-phenothiazine-10)propanoic acid (PTZpn-OH) and condensation (92% yield) of the succinimido ester PTZpn-OSu with Boc-Lys furnished the protected electron-donor module Boc-Lys(PTZpn)-OH (84% overall yield). The latter was used in solid-phase syntheses of two redox tripeptides, CH3CO-Ala-Lys(PTZpn)-Ala-OH and [Rub2m-Ala-Lys(PTZpn)-Ala-OH]2+ (PF6-)2. The electrochemical properties of these redox amino acids and peptides were similar to those of PTZpn-OH, [Rub2m-OH]2+ (PF6-)2, or [Rub2m-NHCH3]2+ (PF6-)2. Lys(PTZpn), [Lys(Rub2m)]2+ (PF6-)2, and other redox modules may be useful for engineering light-harvesting proteins, photovoltaic cells, and other molecular electronic devices.

Ag electrode reaction in NaOH solution studied by in-situ Raman spectroscopy

Ag electrode reaction in NaOH solution has been studied by in-situ measurement of surface enhanced Raman scattering. The Raman spectrum is measured while the electrode potential is swept. During and after the reduction process of oxide layers grown on the electrode surface, we observe Raman lines due to atomic oxygen and hydroxyl species adsorbed on the Ag surface. Atomically adsorbed oxygen which gives a vibrational frequency of 330 cm−1 exists in a limited potential range of the electrochemical reduction process. Two lines are observed at 470 and 860 cm−1 in a wider electrode potential range. These lines are due to the AgO stretching vibration (470 cm−1) and the AgOH bending vibration (860 cm−1) of the adsorbed hydroxyl species. The oxygen atoms migrate through the oxide and Ag film facing the electrolyte and then they become adsorbed on the rough Ag film which grows on the electrode surface during the reduction process. The oxygen atom in this adsorption state is rather unstable and it turns to hydroxyl species. These Raman lines are observed also in the anodic sweep, but the intensities are weaker than in the case of the cathodic sweep. Three additional lines appear at 500, 700 and 1615 cm−1 for an electrode potential more negative than −0.8 V (versus SCE). They are observed even in the potential range for hydrogen evolution. These Raman lines are assigned as vibrations of interfacial water molecules sandwiched between the electrode and Na+ ions located in the outer Helmholtz plane. Photoelectrochemical oxidation of Ag2O grown on the electrode surface to AgO occurs if the thickness of Ag2O exceeds a couple of monolayers.

Electrochemical formation and reduction of silver oxides in alkaline media

The anodic oxidation of silver electrodes in NaOH solution and the reduction of the silver oxides formed were studied by potential step chronoamperometry. Oxidation of Ag to Ag 2O is a diffusion-controlled reaction, the diffusion control being established in the solid phase. Oxidation of Ag 2O to AgO proceeds via a nucleation and growth-controlled process. The amount of AgO decreased with increasing step height. The current—time curves for this reaction have been analysed with the Kolmogoroff—Avrami equation. Reduction of AgO to Ag 2O occurs initially on the outside of the electrode, and the rate of the reaction is limited by diffusion of ions across the thickening layer of Ag 2O. Reduction of Ag 2O to Ag proceeds via a nucleation and growth reaction.

Un nouvel exemple de transposition: l'épimérisation en C(3) d'hydroxy‐3‐δ1‐pyrazolines dérivées de sucres

A Novel Example of Reversible Ring Opening: The Epimerization at C(3) of Sugar 3‐Hydroxy‐Δ1‐pyrazolinesReaction of 1 (either geometrical isomer) with hydrazine followed by in situ Ag2O oxidation led to two pairs of interconverting isomers 4 ⇄ 5 and 6 ⇄ 7. By the same treatment, (Z)‐10 and (or) (E)‐10 gave the pair 11 ⇄ 12. Acetylation of 4 ⇄ 5 led to a non interconverting mixture of 8 and 9. This fact, and the lack of incorporation of 18O when the epimerization took place in the presence of H218O indicated that the most probable mechanism consisted in a reversible ring opening (D ⇄ E ⇄ F). The kinetic parameters of these reactions are given and structural assignments proposed for the new compounds.