Kinetics of oxygen adsorption on silver films

Kinetics of oxygen adsorption on silver films prepared by metal vacuum evaporation and condensation technique has been studied. The adsorption proceeds in accordance with the kinetic law of Zeldovich-Roginskii. Oxygen dissolution in the silver subsurface layers result in a decrease of the initial adsorption rate and an enhanced exponential decrease of the adsorption rate as a function of surface coverage.     

Catalytic properties of silver nanoparticles supported on silica spheres

In this work, we investigate the catalytic properties of silver nanoparticles supported on silica spheres. The technique to support silver particles on silica spheres effectively avoids flocculation of nanosized colloidal metal particles during a catalytic process in the solution, which allows one to carry out the successful catalytic reduction of dyes. The effects of electrolytes and surfactants on the catalytic properties of silver particles on silica have been investigated. It is found that the presence of surfactants depresses the catalytic activity of the silver particles to some extent by inhibiting the adsorption of reactants onto the surface of the particles. Electrolytes either increase the migration rate of reactants in the solution resulting in an increase in the catalytic reaction rate or inhibit the adsorption of reactants onto the surface of the silver particles leading to a loss in the activity of the metal particles.     

混合式格点法研究氧在银或银合金表面的吸附动力学

本文改进了混合式格点法对第一时间步的计算方法, 在保持原有精度的基础上, 减少计算时间约三个数量级。用这一方法, 研究了氧在银及其合金表面的吸附动力学。计算表明: 氧分子在银表面有效吸附的反应阈值是6.29kJ/mol, 这和实验所得的活化能相同。当氧分子动量大于45a.u.或合金中金配比大于0.30时,氧分子均无法在银及其合金表面形成稳定吸附, 这此结果和实验一致。计算中没有发现分子氧直接解离成原子氧的现象。从计算结果中推测, 处在振动激发态的氧分子比处在振动基态的氧分子更容易吸附在银表面。     

银表面甲醇氧化反应机理的ab initio计算研究 1: 银表面静态吸附物种的几何构型和吸附性质

本文用原子簇模型(CM)的从头计算方法, 计算了银表面甲醇氧化反应中的静态吸附物种的优化几何构型及吸附性质。计算表明在清洁银表面甲醇、甲醛只存在物理吸附; 当表面存在吸附氧原子时, 甲醇可在银表面形成两种分子态吸附;甲醛与表面羟基OH(a)或氢原子H(a)共存时在银表面能够形成化学吸附, 且CH2O(a)极易与O(a)反应生成深度氧化中间体η^2-甲二氧基; 中间产物甲氧基在无氧的银表面能够形成稳定吸附, 在富氧银表面极易进一步氧化脱氢生成产物甲醛。通过计算与实验结果的对照, 我们对反应机理作了初步讨论。     

Adsorption of the silver ion on soda-glass-I

A study of the adsorption of silver ions on soda-glass, using a radiochemical technique, has been made, and the effects of time and temperature investigated. A simple kinetic treatment allows estimation of the rate constants for the adsorption and desorption processes.     

Fluorescence quenching of eosin upon adsorption by colloidal silver mediated in aqueous solution and reverse micelles

The optical effects of the adsorption of eosin on the colloidal silver particle have been investigated in aqueous solutions. It was found that upon adsorption the fluorescence of eosin was effectively quenched. This was explained as the photoinduced interfacial electron transfer from the excited singlet state of eosin to the silver particle. Decreasing pH of the solution favors the adsorption of eosin and so enhances the fluorescence quenching. For comparison, the fluorescence quenching in reverse micelles was also investigated. The quenching behavior was much different from that mediated in aqueous solution. This was attributed to the unique microenvironment of reverse micelles.     

Behaviour of silver—silver malonate and silver—silver succinate electrodes in aqueous media

The silver—silver oxalate electrode has been employed by many workers^1–3 in aqueous media as the second order reference electrode, but no work seems to have been done so far on the study of the behaviour of silver—silver malonate and silver—silver succinate electrodes. The present work deals with the study of these electrodes in ionic equilibria of malonate and succinate ions in aqueous media. These electrodes, in conjunction with a saturated calomel electrode, have been employed in the poten- tiometric determination of malonate and succinate ions in aqueous media. In additon, the effect of the added salts, such as, potassium nitrate and sucrose on the behaviour of these electrodes has also been examined in this media.     

Adsorption of urea on a polycrystalline silver electrode; comparison of electrochemical and radiometric methods

The adsorption of urea on a polycrystalline silver electrode was studied by radiometry and impedance spectroscopy. The differential capacity of the silver electrode in 0.01 M NaClO₄ solution containing urea in concentrations from 10^–6 to 5×10^–4 M has been determined. The isotherms of urea adsorption, found from the capacitance and radiometric measurements have been compared. The experimental data were described by the Langmuir isotherm, and the Gibbs energy of adsorption was calculated. The urea adsorption takes place in the entire range of the applied potential. The process is reversible with respect to the electrode potential and the bulk urea concentration.Dedicated to Professor Gyorgy Horanyi on the occasion of his 70th birthday     

Adsorption of small molecules on silver clusters

We report investigations of adsorption of N(2) and O(2) molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n = 6 to 7 and for cations, in agreement with experiments, from n = 4 to 5. With the refined structures, adsorption energies of N(2) and O(2) molecules have been calculated. We have identified characteristic drops in the adsorption energies of N(2) that further link our calculations and experiments, and confirm the reported 2D-3D transition for cations. We have found that perturbations caused by physisorbed molecules are small enough that the structures of most Ag clusters remain unchanged, even though physisorption stabilizes the 3D Ag(7)(+) structure slightly more than the 2D counterpart. Results for pure O(2) adsorption indicate that charge transfer from Ag(n)(+) to O(2) occurs when n > 3. Below that size oxygen essentially physisorbes such as nitrogen to the cluster. We interpret the experimentally observed mutually cooperative co-adsorption of oxygen and nitrogen using results from density functional theory with generalized gradient approximations. The key to the enhancement is N(2)-induced increase in charge transfer from Ag(n)(+) cations to O(2).     

Density functional study of the interaction of chlorine atom with small neutral and charged silver clusters

Chlorine adsorption on small neutral, anionic, and cationic silver clusters Ag(n) (n=2-7) has been studied using the PW91PW91 density functional method. It was found that the adsorption of chlorine on the lowest-energy bare clusters does not always produce the lowest-energy complexes. In addition, the binding of chlorine can greatly change the geometries of the silver clusters in some cases. Among various possible adsorption sites, bridge site is energetically preferred for the neutral Ag(n) while top site is energetically more preferred for the anionic Ag(n) with n< or =6. For cationic clusters, adsorptions on bridge and face sites have similar binding energies, which are much larger than those on top sites. Natural bond orbital analyses show that irrespective of charge state, electrons always transfer from silver atoms to adsorbate and silver acts like alkali metals in the interaction with chlorine atom. Significant odd-even alternation patterns in the properties of the complexes have been observed: Even-electron clusters often have higher ionization energies, lower electron affinities, and higher dissociation energies than their odd-electron neighbors. It was also found that chlorine atoms bind more strongly with odd-electron bare clusters than with even-electron bare clusters. These patterns reveal that even-electron clusters are more stable than odd-electron clusters.     

Preparation and characterization of Cr(CO)(4)dpp (chromium tetracarbonyl 2,3-bis(2'-pyridyl)pyrazine) adsorbed on silver nanoparticles

A transition metal carbonyl species, Cr(CO)(4)dpp, has been successfully attached to bare silver nanoparticles prepared by laser ablation of a metal foil in ethanol. Transmission electron microscopy (TEM) images have shown that at least a portion of the silver nanoparticles have been capped by the chromium species, and ligand shells corresponding to Cr(CO)(4)dpp multilayer adsorption onto the silver nanoparticles of 30-50 nm diameter have been observed. The detection of the strongest Raman-active nu(CO) band of Cr(CO)(4)dpp at 2004 cm(-1) revealed that the species has been adsorbed without decomposition. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) signals recorded of the chromium-capped silver nanoparticles were also consistent with the nondecomposition adsorption process. Density functional calculations have been used to reproduce the Raman spectrum using Ag(7)(+) as a model surface. A large binding energy of about 122 kJ/mol has also been computed between silver and nitrogen atoms thus lending support to Cr(CO)(4)dpp being chemisorbed onto the silver surface.     

Density functional study of the interaction of carbon monoxide with small neutral and charged silver clusters

CO adsorption on small neutral, anionic, and cationic silver clusters Ag(n) (n = 1-7) has been studied with use of the PW91PW91 density functional theory (DFT) method. The adsorption of CO on-top site, among various possible sites, is energetically preferred irrespective of the charge state of the silver cluster. The cationic silver clusters generally have a greater tendency to adsorb CO than the anionic and neutral silver ones, except for n = 3 and 4, and the binding energies reach a local minimum at n = 5. The binding energies on the neutral clusters, instead, reach a local maximum at n = 3, which is about 0.87 eV, probably large enough to be captured in the experiments. Binding of CO to the silver clusters is generally weaker than that to the copper and gold counterparts at the same size and charge state. This is due to the weaker orbital interaction between silver and CO, which is caused by the larger atomic radius of the silver atom. In contrast, Au atoms with a larger nuclear charge but a similar atomic radius to silver owing to the lanthanide contraction are able to have a stronger interaction with CO.     

Sulphate adsorption at chemically deposited silver thin film electrodes: time-dependent behaviour as studied by internal reflection step-scan infrared spectroscopy

Stable silver thin films were chemically deposited on a germanium substrate. Ex-situ STM images showed that these films are formed by grains with a diameter between 20 and 100 nm. The silver films have been used as the working electrode in infrared spectroelectrochemical experiments with an internal reflection (Kretschmann) configuration. The adsorption of sulphate anions in neutral or slightly acidic solutions has been studied. The high intensity of the S–O stretching bands in the absorption spectra allowed the monitoring of the time-dependent behaviour of the adsorption/desorption processes in time-domain step-scan experiments.     

Plasmonic photocatalytic system using silver chloride/silver nanostructures under visible light

Plasmonic photocatalytic nanostructured system was investigated on silver chloride/silver nanoparticles under visible light. Silver chloride/silver nanoparticles were readily prepared using dispersing agent and light irradiation. The d-spacing analysis, high resolution-transmission electron microscopy, X-ray diffraction analysis and diffuse-reflectance spectroscopy demonstrated that silver nanoparticles were introduced on the surface of silver chloride nanoparticles and then silver chloride/silver nanostructured photocatalytic materials were successfully synthesized. The as-synthesized plasmonic photocatalysts exhibited the enhanced photocatalytic performance over nitrogen-doped titania nanomaterials. The improved catalytic activity was originated from the enhanced adsorption for visible light, electron–hole separation, and the formation of chloride atoms in silver chloride/silver nanostructured materials.     

Olefin adsorption on silica-supported silver salts--a DFT study

Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag+ cations, their counteranions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin pi states, which is supported by analyses of electron density difference plots and density of states. The counteranions, such as NO3- were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag+ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.     

Adsorption of cations onto the surfaces of silver nanoparticles

The effects of cations on the absorption spectra of silver sols have been investigated by the UV-vis spectrometry and TEM. Experiments showed that injection of certain amounts of transition metal cations into silver sols resulted not only in the aggregation of silver nanoparticles but also in the appearance of a new band centered near 510 nm in the absorption spectra of silver sols. However, the new band was not observed in the presence of alkaline earth metal cations or the Mv2+ cations. The peak position of the new band depends on the nature as well as the concentration of metal cations used. Comparing the peak positions of the new bands, it was found that the new band induced by the injection of Cr3+ was red-shifted with respect to those induced by Cu2+, Zn2+, or the Cd2+ cations. It is reasonable that this band near 510 nm should be attributed to the coeffects of the adsorption of metal cations onto the surfaces of silver nanoparticles and the aggregation of silver nanoparticles.     

Flame atomic absorption spectrometric determination of silver after preconcentration on Amberlite XAD-16 resin from thiocyanate solution

A method of silver preconcentration by using a column containing Amberlite XAD-16 resin and this future determination by a flame AAS after elution is proposed. The effect of the factors such as pH, the nature of complexing agent, sample volume, flow rate, the type and concentration of elution solution on the preconcentration efficiency have been investigated. The influence of some matrix elements on the recovery of silver were also examined. It was found, that the quantitative recovery of thiocyanate complex of silver was obtained from nitric acid solution (pH 2) as 99.20+/-0.07% at the 95% confidence level. A preconcentration factor up to 75 could be obtained. The detection limit of silver was 0.047 mg l(-1). The adsorption of silver onto Amberlite XAD-16 can be formally described by a Langmuir equation with maximum adsorption capacity 4.66 mg g(-1) (0.043 mmol g(-1)). The proposed method was applied to determination of silver in standard alloy with relative error 6.25%.     

Chemical and biological treatment of waste water with a novel silver/ordered mesoporous alumina nanocomposite

One of the serious problems in the present century is chemical and biological pollution of the environment. Nanocomposites are multiphase solid materials that have been used as adsorbent of pollutants such as dyes, pesticides, anions and etc. in the last decades. In this study, a novel nanocomposite including silver nanoparticles and ordered mesoporous alumina (OMA) has been synthesized and used for the removal of dyes pollutants (methyl orange, bromothymol blue and reactive yellow) from aqueous solution. The characterization of synthesized nanocomposite has been performed by TEM, N₂ adsorption/desorption, XRD and energy dispersive X-ray spectroscopy analysis. The adsorption kinetic and equilibrium data have been obtained by UV–vis spectroscopy. The results show that the silver/OMA nanocomposite (Ag/OMA nanocomposite) is good adsorbent for the removal of anionic dyes from aqueous solution, and also this nanocomposite has a biocidal action against both Gram-negative and Gram-positive bacteria.     

Catalytic activity of silver colloids on polarographic reduction of oxygen

The catalytic electroreduction of O₂ at DME in the presence of colloidal silver has been studied. Two colloids, characterized by different catalytic activity, have been prepared and tested. The influence of the height of the mercury head, pH, and temperature on both the first and the second wave of the polarographic reduction of O₂ have been investigated. A mechanism, which takes into account the possible adsorption of OH^? ions on the silver catalyst, is proposed for the H₂O₂ decomposition: the experimental points fit the calculated curve of log k_s vs. pH well. Thermodynamic parameters are also derived.     

Preparation of calix[4]arene methylene crosslinked resins with 2-pyridyl pendant group and their adsorption behavior towards silver ion

Three different resins namely 1:5 MC-resin, 1:3 MC-resin and 1:1 MC-resin have been synthesized by methylene crosslinking of 2-pyridylcalix[4]arene. Adsorption behavior of these resins towards the metal ions existing in photographic waste was investigated. The resins show absolute efficiency for adsorption of silver ion with no affinity for other coexisting ions. First two resins form 1:1 complex whereas the third one forms 2:1 complex with silver ion. Maximum loading capacity of silver ion on the present resins was found to be 1.15, 1.29 and 0.69 mol kg^?1, respectively. Column chromatographic separation of silver ion in presence of excess of sodium ions was also carried out with 1:5 MC-resin. Selective adsorption of silver ions over excess of sodium ions was achieved.