Semiempirical calculation of silver clusters with chemisorbed oxygen

The results of a calculation of the electronic structure of clusters containing a small number of silver atoms with chemisorbed oxygen atoms by an expanded Hückel method are presented. It was shown that chemisorption of oxygen leads to a sharp increase in the stability of the clusters and an increase in their electron affinity, and the energy gain in this case is sufficient for dissociation of molecular oxygen with its addition to silver atoms in the cluster.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 2, pp. 133–137, March–April, 1993.     

Adsorption of oxygen on copper and silver

Oxygen adsorption on silver has been studied using field-emission microscopy. It has been shown that adsorption is an activated process which proceeds on rough planes through the stage of formation of adsorption islands, O_ads. Using a semi-empirical interacting bonds method, the heat of oxygen adsorption has been estimated in a dissociative from on the (111), (100) and (201) planes of silver and on the (111) and (100) planes of copper.

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. , _. (111), (100), (201) (111), (100) .

     

Interfacial study of benzenesulfinate chemisorbed on silver

The oxidative stability of self-assembled monolayer (SAM) of thiols on silver was examined with matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). A benzenesulfinate (BS) monolayer on silver was also prepared and investigated with MALDI-TOF-MS and surface-enhanced Raman scattering (SERS). The presence of sulfinate, sulfonate, and thiosulfonate fragment ions reveals that thiolate monolayers oxidize in the air. The relative abundance of C6H5SO2- and C6H5SO3- species in the mass spectra of the monolayer with different air-exposure times provides an estimation of the oxidation progress. In the vibrational spectrum, the large red shift of v(SOO-), combined with the lack of a shift of v(C-S) upon adsorption, indicates its bidentate O-coordination. The orientation of the confined molecules, based on the Raman surface selection rules, was derived from the preferential enhancement of the different functional groups. The benzene ring of BS was found almost normal to the metal surface as a result of the intermolecular forces; that geometry excludes the possibility of the pi-system of the benzene ring from participating as another binding site. The SER spectra of BS obtained via a selective etching process confirms these observations.     

Adsorption isotherms for benzene on diatomites from China

In this paper, benzene adsorption isotherm and their hysteresis on two important local diatomites were determined at 25℃, and their silicon hydroxyl group (SiOH) number was determined, their properties were reported, and the relationship between surface structure, surface SiOH number per nm2 and adsorption isotherm with hysteresis was discussed. The specific surface was also calculated from the isotherms, and pore-size distribution was determined.     

UPS和功函数研究氧在电解银上的吸附

本文报导了用紫外光电子能谱(UPS), 热脱附(TDS)和表面功函数连续测量等手段对氧与电解银表面相互作用过程中的电荷传递, 成键以及吸附动力学等研究的实验结果。     

Adsorption isotherms on nicotinamide-imprinted polymer stationary phase

The molecular-imprinted technique is applied for the preparation of a polymer selector by using methacrylic acid as functional monomer, ethyleneglycol dimethacrylate as the cross-linker, 2,2'-azobisisobutyronitrile as the initiator, and nicotinamide as the template. The adsorption isotherms of nicotinamide and nicotinic acid and the competitive adsorption isotherms of nicotinamide and nicotinic acid on the imprinted stationary phase are determined using rectangular pulse frontal analysis and static method. Aqueous solution is used as the mobile phase in frontal analysis. It is found that the adsorption data fit well to both Langmuir and Freundlich isotherm models.     

Molecular orientation of thiourea chemisorbed on copper and silver surfaces

Surface-enhanced Raman spectroscopy has been used to study chemisorbed thiourea complexes on Cu and Ag electrodes. It was found that the thiourea molecule was bonded via its S atom to the Cu and Ag ad-atoms on the substrates.     

Titration of oxygen chemisorbed on platinum: Experiment and simulation

The titration of chemisorbed oxygen by carbon monoxide to form carbon dioxide has been studied from 373 to 673 K over polycrystalline platinum. The pressure transients for CO and CO₂ have been measured and simulated numerically. A complex Langmuir–Hinshelwood mechanism is found which fits all the data, and it is not necessary to invoke Eley–Rideal kinetics. The results fall into two temperature regimes, above and below 473 K, which are characterized by different Arrhenius parameters. A change in activation energy with oxygen coverage is also found below 473 K.     

Adsorption isotherms of homologous alkyldimethylbenzylammonium bromides on sodium montmorillonite

The adsorption of homologous alkyldimethylbenzylammonium bromides, [C(6)H(5)CH(2)N(CH(3))(2)R]Br, on sodium montmorillonite from aqueous NaCl solutions at room temperature has been studied. R stands for the methyl-, butyl-, hexyl-, octyl-, decyl-, and dodecyl-group, and the corresponding ammonium cations will be denoted as C1+, C4+, C6+, C8+, C10+, and C12+, respectively. C1+, the reference cation, attains the plateau region of adsorption at a level close to the cation exchange capacity (CEC) of the clay. The chain-length dependence on adsorptivity of the homologous cations exhibits an unexpected peculiarity. In the case of short-chain homologues of C1+ their adsorption onto sodium montmorillonite decreases in the order C1+>C4+>C6+. This behavior is due, presumably, to the growing steric hindrances at the surface of clay, which occur because of the limited area available for the bulky organic cations at the exchange sites. These limitations appear to be out-balanced in the case of higher homologues for which the increasingly growing hydrophobic effects lead to the expected sequence of adsorptivity of the cations, i.e., C1+相似文献   

Adsorption of gas mixtures on heterogeneous solid surfaces II. Adsorption isotherms for gaseous mixtures whose pure-gas isotherms show the Freundlich,Tóth and Langmuir behaviours

Summary In this paper the problem of determining adsorption from binary gaseous mixtures on the basis of pure-gas adsorption parameters has been theoretically discussed. The author analysed adsorption systems whose adsorption from pure gases is in agreement with the equation ofTóth, Freundlich orLangmuir.

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Zusammenfassung In der vorliegenden Arbeit wurde das Problem zur Ermittlung der Adsorption aus binären Gasmischungen auf der Basis von Adsorptionsparametern der reinen Gase theoretisch diskutiert. Es wurden nur solche Adsorptionssysteme analysiert, bei denen die Adsorption der reinen Gase mit der Gleichung vonTóth, Freundlich oderLangmuir übereinstimmt.

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With 1 table     

Unenhanced Raman scattering from pyridine chemisorbed on a stepped silver surface: Implications for proposed sers mechanisms

In an effort to separate chemical from electromagnetic sources of enhancement in surface-enhanced Raman scattering, we have investigated the adsorption     

Adsorption isotherms for the system n-heptane/ethanol/HDPE

The equilibrium isotherms for the adsorption of n-heptane and ethanol on high density polyethylene (HDPE) are reported at 27–110°C and at 20–100 mmHg.

The experimental apparatus and procedures are described for three methods, namely flow, static and chromatographic.

The obtained results are discussed on the basis of the Henry law for adsorbates at low coverage on solids.     

Proteins at liquid interfaces: II. Adsorption isotherms

Adsorption isotherms for the three proteins β-casein, bovine serum albumin, and lysozyme at the air-water and oil-water interfaces have been determined independently using ellipsometry and surface radioactivity methods; the surface pressure and surface potential were also monitored. Saturated monolayer coverage occurs via irreversible adsorption of 2–3 mg M^?2 of protein; the resultant films generate surface pressures of about 20 mN m^?1 and are 50–60 Å thick. Molecules adsorbed in the first layer dominate the film pressures so that further adsorption causes no change in the pressure although the film thickness can increase to more than 100 Å. The molecules which give rise to this increase in film thickness are reversibly adsorbed with respect to aqueous substrate exchange. The experimental isotherm data and the Langmuir adsorption isotherm are in close agreement at low protein concentrations. However, comparison with the Gibbs adsorption equation is not valid, although reasonable agreement can be achieved if some account is taken of the fact that the protein molecules in the first layer are irreversibly adsorbed.     

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.     

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).     

Modeling of oxygen adsorption on silver

Different adsorption forms of oxygen on silver are discussed. Four main types of oxygen forming at different temperatures and oxygen pressures have been distinguished. A kinetic model describing the formation and transformations of the oxygen forms and taking into account the surface amorphization has been proposed. Numerical modeling of stationary concentrations using this model gives evidence for a temperature window ΔT=500–800 K, where a quasimolecular oxygen state (E=530.5 eV, T_des=800–900 K) can exist at high oxygen pressures.     

Evidence for molecularly chemisorbed oxygen on TiO2 supported gold nanoclusters and Au(111)

In this study, we present evidence for the existence of a molecularly chemisorbed oxygen species on a Au/TiO2 model catalyst and a Au(111) single crystal following exposure of these samples to an oxygen plasma-jet molecular beam. We present evidence for the molecularly chemisorbed oxygen species from thermal desorption, collision-induced desorption, and heat of adsorption/reaction-induced desorption measurements. Thermal desorption measurements reveal a peak desorption temperature at approximately 145 K which corresponds to an activation energy for desorption of approximately 0.35 eV.     

Reactivity of molecularly chemisorbed oxygen on a Au/TiO2 model catalyst

We present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalyst sample to a radio-frequency-generated plasma jet of oxygen results in co-population of both atomically and molecularly chemisorbed oxygen species on the sample. We tested the reactivity of the molecularly chemisorbed oxygen by comparing the CO2 produced from a sample populated with both species to the CO2 produced from a sample that has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. Samples that are populated with both species consistently result in greater CO2 produced than samples with only atomic oxygen. We interpret this result to indicate that molecularly chemisorbed oxygen on the sample can directly participate in the CO oxidation reaction. The reactivity of molecularly chemisorbed oxygen has been investigated for five different gold coverages (0.5, 0.75, 1, 1.25, and 2 ML), and we observe that there is a greater fractional difference in the CO2 produced (difference between sample populated with both molecularly and atomically adsorbed oxygen and sample populated solely with atomically adsorbed oxygen) for the 1 ML Au coverage than for the other coverages for equivalent oxygen plasma-jet exposures. However, it is not possible to unambiguously conclude that this observation is directly related to a particle size effect on the chemistry since the absolute O(2,a) and O(a) content on the various surfaces is different for all the coverages studied because of the plasma-jet technique that we employed for populating the surfaces with oxygen. Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gold coverage and hence particle size.