Study of hydrated Na-faujasite zeolites by thermally stimulated currents technique

The dielectric relaxation of the two faujasite zeolites NaX and NaY as a function of their hydration state, has been studied. The technique of thermally stimulated current (TSC) has been used. Two distinct processes of cationic relaxation have been identified. They correspond to local hops of cations of sites III' and II of the supercage. Analysis of the observed TSC peaks has been performed by the thermal windowing method (TSC/RMA). The activation energies of the two relaxations have been determined as a function of the adsorbed water molecules. The influence of the water molecules on the movement of the cations in the considered sites was explained. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

A129Xe NMR study of highly cadmium-exchanged zeolite Y

The adsorption isotherms of carbon monoxide and xenon as well as the¹²⁹Xe NMR chemical shifts of xenon in highly (68 and 87%) cadmium-exchanged zeolite NaY were measured. The complete set of experimental data can quantitatively be reproduced with a model that considers localized adsorption of both adsorbate molecules on cadmium and sodium cation sites in the supercages. The concentrations of the supercage cadmium cations as well as their characteristics like adsorption constants for Xe and CO and¹²⁹Xe NMR chemical shifts were determined.     

Isotope exchange study of the dissociation of metal —humic substance complexes

Isotope exchange was employed to study dissociation of metal cations from their complexes with humic substances (HS). Dissociation of cation from HS controls the rate of isotope exchange between two identical metal-HS solutions (but for the presence of a radiotracer) divided by a dialysis membrane. The rate of isotope exchange of Eu/¹⁵²Eu and Co/⁶⁰Co in the systems with various HS was monitored as a function of pH, ionic strength, and the degree of HS loading with metal. The apparent rate of Eu-HS dissociation was found to be enhanced by decreasing pH, increasing ionic strength, and increasing metal loading. Co-HS dissociation was too fast to be followed by the method. For interpretation of the experimental kinetic data, the multiple first order law has been applied. Based on the results, a concept of HS as a mixture of two types of binding sites is discussed.     

Gas diffusion in zeolite beds: PFG NMR evidence for different tortuosity factors in the Knudsen and bulk regimes

We present molecular dynamics simulations in the micro-canonical ensemble of a Lennard-Jones model of nitrogen confined in realistic models for saccharose-based carbons developed in our previous work. We calculate the velocity autocorrelation function and mean-squared displacement, and the self-diffusivities from the latter. We observe that the self-diffusivity increases with temperature and exhibits a maximum with loading or adsorbate density. To the best of our knowledge, a maximum in self-diffusivities has not been observed in molecular dynamics simulations of fluids confined in slit pores.Received: 1 January 2003, Published online: 30 October 2003PACS: 61.43.Gt Powders, porous materials - 68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation - 82.75.Fq Synthesis, structure determination, structure modeling     

Strong dependence of ferrimagnetic properties on Na concentration in Na-K alloy clusters incorporated in low-silica X zeolite

Na concentration (x) dependence of ferrimagnetic properties is investigated for Na-K alloy clusters incorporated in low-silica X (LSX) zeolite. In the LSX zeolite, β-cages of inner diameter ≈7 Å are arranged in a diamond structure, and supercages of inner diameter ≈13 Å are formed among them. The used LSX zeolite contains xNa⁺ and (12−x)K⁺ cations per β-cage or supercage. Guest nK atoms are loaded into the zeolite, namely the loading density is given by n per β-cage. The samples at x=4 have been reported to show Néel’s N-type ferrimagnetism in the specific region of n. This ferrimagnetism is explained by the model of antiferromagnetic coupling between two non-equivalent magnetic sublattices of clusters, the ones in β-cages and the others in supercages. In the present study, the value of x is changed from 4 to 0. Ferrimagnetic properties are found to show strong x-dependence. A systematic increase in loading densities of ferrimagnetic region is clearly observed with decreasing x. A remarkable change in temperature dependence of spontaneous magnetization is observed depending on x. Na⁺ cations are known to be mainly distributed in β-cages. Hence, the decrease in Na concentration is proposed to change the electronic potential depth for clusters in β-cage, which leads to important differences in the interaction between electrons localized in β-cages and those in supercages.     

Study of adsorption states and interactions of CO on evaporated noble metal surfaces by infrared absorption spectroscopy: I. Silver

We have used infrared absorption spectroscopy to study the adsorption of CO at low temperature on evaporated silver films as a function of the coverage of CO and the deposition temperature of the silver. We observe two adsorption regimes when a cold silver film is exposed to CO gas. If the silver deposition temperature, (or the highest temperature at which the silver has been annealed), is above the threshold temperature of 150 K, then only physisorbed CO is observed. For sample temperatures below 25 K, these physisorbed molecules are oriented perpendicular to the metal surface. Films deposited at temperatures below 150 K, however, contain ≈ 0.01 monolayer of chemically active sites at which CO chemisorbs. The infrared band due to chemisorbed CO shifts to lower frequency with increasing coverage. We have analyzed this shift and separated the static and dynamic contributions. The static, chemical shift is caused in part by the change in the work function induced by surrounding adsorbates. The dynamics shift is fully explained by a dipole-dipole interaction; we find no evidence for a vibrational coupling through the metal. We have analyzed the vibrational polarizability and infrared absorption strength of the absorbed CO, and find no evidence for the infrared enhancement suggested by some theories of surface enhanced Raman scattering.     

When scanning tunneling microscopy gets the wrong adsorption site: H on Rh(100)

At low tunneling resistance, scanning tunneling microscopy (STM) images of a Rh(100) surface with adsorbed hydrogen reproducibly show protrusions in all bridge sites of the surface, leading to a naive interpretation of all bridge sites being occupied with H atoms. Using quantitative low-energy electron diffraction and temperature programmed desorption we find a much lower H coverage, with most H atoms in fourfold hollow sites. Density functional theory calculations show that the STM result is due to the influence of the tip, attracting the mobile H atoms into bridge sites. This demonstrates that STM images of highly mobile adsorbates can be strongly misleading and underlines the importance of additional analysis techniques.     

Work function and LEED study of Na and O2 covered surfaces of W(112)

Coadsorption of Na and O on W (112) surfaces has been studied with LEED and work function measurement. The composition and sequence of deposition of the adsorbates have been varied systematically. The results suggest that Na and O form a W-O-Na double layer, regardless of the sequence of deposition. It is found that the underlying O layer causes increased interaction among the Na atoms which improves the long range older of the Na array. An expansion of the Na layer due to O adsorption has been observed.     

Interactions between coadsorbed species on the Rh(111) surface as a function of the separation distance between them

The interaction energies as a function of the separation distance between two molecules of CO and between CO and H or O coadsorbed on the Rh(111) surface have been calculated by the Extended Hückel Method. Weak interactions of the order of a few hundredths of eV occur between the adsorbates via the metal lattice. The interaction energy is found to change its signs as the distance between the adsorbates increases. For first neighbor occupation sites the interaction energies are positive. Second and third neighbor interaction energies between CO and CO are, however, negative, the latter being stronger than the former. The interaction energies between CO and H or O are positive for first and second order neighbors and, in general, negative for third order neighbors. The computed interaction energies allow one to explain the observed LEED patterns of adsorbed CO at low temperatures. The compression of the p(2 × 2) LEED structure of CO to the (√3 × √3)R30° structure, induced by the subsequent exposure to hydrogen or oxygen, is explained on the basis of the more favorable interaction energies when CO and H, or CO and 0 are segregated.     

On the contribution of charge transfer excitations to SERS

Resonant Raman scattering due to dynamical charge transfer from the silver metal to the lowest unoccupied orbital of pyridine and other adsorbates contributes 1 to 2 orders of magnitude to SERS. Several experimental observations indicate, that this mechanism is particularly effective at sites of atomic scale roughness.     

Influence of acceptor and donor adsorbates (CO,K, NH3) on Pt surface core-level shifts

4f core-level shifts have been measured for clean surfaces of Pt(111), Pt(331), and Pt(557). Surface peaks due to terrace sites are shifted toward lower binding energy (0.32 ± 0.05 eV) from the bulk peak, whereas peaks from step atoms are shifted by 0.58 ± 0.05 eV also to lower binding energy. The intensity ratios for the two sites differ considerably between the stepped Pt surfaces. Chemisorption of carbon monoxide on the Pt(331) surface is preferential to step sites, with a Pt 4f binding energy shift of ～ 1.29 eV toward higher binding energy. Chemisorption of potassium and ammonia also produces Pt 4f surface shifts which are at higher binding energy than the bulk peak. These experiments do not support the concept of electron donation by these adsorbates into metal d orbitals. The results are discussed in view of, and supported by, tight-binding LCAOMO calculations of potassium and ammonia interacting with a Pt(111) thin film.     

Coadsorption of electropositive and electronegative elements: I. Cs and H2 on W(100)

A systematic experimental study of Cs and CsH₂ adsorption on W (100) surfaces by LEED and work function measurements has been made. It was found that at high coverages the electropositive and electronegative adsorbates formed a double layer. The arrangement of the adsorbates was independent of the sequence of deposition. The two adsorbates affected the structures of each other: a Cs layer caused an increase in ordering and binding energy of H on W and the existence of H on W(100) caused an increase in the maximum density of Cs atoms that can be deposited on the surface. This increase in Cs density was accompanied by an increase in work function. The coverage of Cs at the work function minimum was also shifted to a smaller value with increasing amount of initially adsorbed H₂. The work function result was compared with a recent non-uniform electron gas theory of work function. The adsorption of H₂ on Cs-covered W was found to be an activated process and the sticking coefficient decreased exponentially with increasing Cs coverage. It appears that adsorption of Cs did not form a duolayer on W (100), as reported by MacRae et al., rather it formed a single layer on top of some patches of hydrogen impurity. The small amount of H impurity was not ordered unless there was a layer of Cs on top.     

Lateral interactions and nonequilibrium in adsorption and desorption. Part 1. Experimental results for (2 × 2)-(3O + NO)/Ru(0 0 1)

Extending earlier vibrational spectroscopy and thermal desorption measurements on this system, its geometrical structure and its adsorption/desorption kinetics have been investigated in detail. The adsorption and desorption of NO proceed, with little influence on the 3O template, on its (2 × 2) lattice of empty hcp sites. The desorption kinetics, with splitting of the TPD spectra into two peaks, are far from the expected behavior for independent sites. Also, the vibrational band structure shows dispersion beyond dipole-dipole interactions. So, despite the quite large NO-NO distance and their screening by the O atoms, there is clear evidence for static and dynamic lateral interactions which should be extractable from the data. A qualitative analysis suggests that these interactions are due to elastic coupling between the positions and vibrations, respectively, of the O and NO adsorbates. However, quantitative conclusions cannot be drawn directly as the kinetic data cannot be interpreted in a quasiequilibrium approach, as would be the normal procedure, due to the presence of strong nonequilibrium effects. The lack of internal equilibration is presumably caused by slow diffusion. The results are sufficiently complete and detailed to justify the effort of theoretical modeling with the aim to quantitatively describe both the lateral interactions and the nonequilibrium effects.     

The Effect of Cations on Uranyl Transport and Fluorescence in Mesoporous Silica Gel

The fluorescence intensity and transport kinetics of uranyl into mesoporous silica gel was measured in the presence of six naturally occurring cations. It was shown that the presence of the cations can reduce the fluorescence intensity of the uranyl through collision quenching and through competition for the silica gel surface sites. Stern-Volmer quenching coefficients were obtained by measuring the uranyl fluorescence as a function of cation concentration. The cations compete with uranyl to occupy silica gel surface sites and cause a decrease in uranyl fluorescence intensity and a reduction in the uranyl saturation time constant. Energy-dispersive x-ray spectroscopy (EDS) was used to measure the weight percentage of uranium and the cations in the silica gel samples and these results correlated well with the results of the saturation time constant measurements. The results of this study show that, at high concentrations, the presence of cations in water can influence the fluorescence intensity and transport kinetics of uranyl into mesoporous silica gel.     

Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.     

Chemical effects on vibrational properties of adsorbed molecules on metal surfaces

Vibrational properties of chemisorbed molecules on metal surfaces are studied with a focus on the coverage (θ) dependent chemical shift of the frequencies. The electronic properties of an incomplete monolayer of adsorbates are calculated by means of the coherent potential approximation, in which the electron hopping between the adsorbates (band formation effect) and the depolarization effect due to the proximity of ionized adsorbed molecules are taken into account consideration. It is found that in a weakly chemisorbed system, such as CO/Cu, the negative shift in chemical origin amounts to the positive dipole shift at low coverage. It is also shown that the variation of the back-donated charge with θ gives rise to the coverage dependent polarizability, which in turn influences the frequency shift estimated by the previous dipole coupling theory. The coverage dependent back-donation also plays a significant role in the work function change Δϕ of the substrate. The polarity of a weakly chemisorbed CO remains unchanged compared to a free CO (⁻CO⁺) so that Δϕ exhibits the initial lowering in the presence of the positive dipoles. The increase in the back-donated charge with θ causes the decrease in the effective dipole moment towards a compensation of the positive hole due to 5σ donation. A simple explanation is offered to clarify the characteristic difference of the work function change between the strongly and weakly chemisorbed CO molecules on metal surfaces. In particular, a possible origin of the work function minimum observed for CO/Cu systems is discussed in terms of the coverage dependent back-donation.     

Video STM studies of adsorbate diffusion at electrochemical interfaces

Direct in situ studies of the surface diffusion of isolated adsorbates at an electrochemical interface by high-speed scanning tunneling microscopy (video STM) are presented for sulfide adsorbates on Cu(100) in HCl solution. As revealed by a quantitative statistical analysis, the adsorbate motion can be described by thermally activated hopping between neighboring adsorption sites with an activation energy that increases linearly with electrode potential by 0.50 eV per V. This can be explained by changes in the adsorbate dipole moment during the hopping process and contributions from coadsorbates.     

Extended speech intelligibility index for the prediction of the speech reception threshold in fluctuating noise

The extension to the speech intelligibility index (SII; ANSI S3.5-1997 (1997)) proposed by Rhebergen and Versfeld [Rhebergen, K.S., and Versfeld, N.J. (2005). J. Acoust. Soc. Am. 117(4), 2181-2192] is able to predict for normal-hearing listeners the speech intelligibility in both stationary and fluctuating noise maskers with reasonable accuracy. The extended SII model was validated with speech reception threshold (SRT) data from the literature. However, further validation is required and the present paper describes SRT experiments with nonstationary noise conditions that are critical to the extended model. From these data, it can be concluded that the extended SII model is able to predict the SRTs for the majority of conditions, but that predictions are better when the extended SII model includes a function to account for forward masking.     

Active role of oxide support during CO oxidation at Au/MgO

The oxidation of CO at MgO supported gold aggregates is studied by means of density functional theory calculations. In addition to serving as a structural promoter holding the gold particles, the supporting oxide also takes an active role in the bonding and activation of adsorbates bound to the gold. The oxide stabilizes a peroxolike reaction intermediate, CO.O2, and causes steric repulsion to CO. The most reactive site at Au/MgO appears where the gold shelters the MgO thereby creating a cavity where several low-coordinated Au atoms and Mg2+ cations from the substrate can interact simultaneously with an adsorbate.     

FT-IR study of the surface properties of K2O-TiO2

K₂O-TiO₂ samples have been prepared by impregnation using KOH in water solution. The surface area decreases by increasing the loading, after calcination at 673 K. Samples containing about one K⁺ atom each 11 Ų reveal a complete modification of the titania surface, attributed to the formation of a “surface-K₂O-phase”. Such a surface presents, after activation, coordinatively unsaturated K⁺ cations that act as weak Lewis acid sites and are also able to interact with hydrocarbons such as propylene and basic sites where ammonia is hydrogen-bonded and water and formic acid are dissociated. Neither ammonia nor propylene dissociate to a detectable extent at room temperature on K₂O-TiO₂. Surface hydroxy groups are also observed, having a low acidic character.