Types Of Adsorption Sites Research Articles

Atomic structure and tip-induced reconstruction of bromide covered Cu(1 1 0) electrodes

The behavior of specifically adsorbed bromide on Cu(1 1 0) in 10 mM HBr has been studied using cyclic voltammetry in combination with high resolution in situ electrochemical scanning tunneling microscopy (EC-STM). At cathodic potentials near the onset of the hydrogen evolution reaction the structure of the bare copper surface was observed by EC-STM. A variation of the electrode potential in positive direction causes a specific anion adsorption leading to the formation of a highly ordered superstructure with a quasi-hexagonal symmetry. The two-dimensional lattice of this adlayer can be described by a c(3 × 2) unit cell. The bromide anions are arranged parallel to the close packed copper rows and are located in three different types of adsorption sites. These inequivalently bound bromide species are imaged with different brightness in the STM-pictures. As a result the bromide adlayer exhibits a long-ranged wavy superstructure superimposed on the atomic corrugation. Tip induced copper corrosion is used to obtain highly ordered nanostructures due to a locally confined electrochemical annealing process proceeding along the [0 0 1]-direction of the substrate. This annealing process results in the formation of Cu(1 0 0)-facets.

Copper Sorption Mechanisms on Smectites

AbstractDue to the importance of clay minerals in metal sorption, many studies have attempted to derive mechanistic models that describe adsorption processes. These models often include several different types of adsorption sites, including permanent charge sites and silanol and aluminol functional groups on the edges of clay minerals. To provide a basis for development of adsorption models it is critical that molecular-level studies be done to characterize sorption processes. In this study we conducted X-ray absorption fine structure (XAFS) and electron paramagnetic resonance (EPR) spectroscopic experiments on copper (II) sorbed on smectite clays using suspension pH and ionic strength as variables. At low ionic strength, results suggest that Cu is sorbing in the interlayers and maintains its hydration sphere. At high ionic strength, Cu atoms are excluded from the interlayer and sorb primarily on the silanol and aluminol functional groups of the montmorillonite or beidellite structures. Interpretation of the XAFS and EPR spectroscopy results provides evidence that multinuclear complexes are forming. Fitting of extended X-ray absorption fine structure spectra revealed that the Cu-Cu atoms in the multinuclear complexes are 2.65 Å apart, and have coordination numbers near one. This structural information suggests that small Cu dimers are sorbing on the surface. These complexes are consistent with observed sorption on mica and amorphous silicon dioxide, yet are inconsistent with previous spectroscopic results for Cu sorption on montmorillonite. The results reported in this paper provide mechanistic data that will be valuable for modeling surface interactions of Cu with clay minerals, and predicting the geochemical cycling of Cu in the environment.

Synthesis of a Novel Magnetic Resin and the Study of Equilibrium in Cation Exchange with Amino Acids

Magnetic cation-exchange resins usually consist of small polymeric particles with a magnetic material dispersed within their structure. Because the mass-transfer kinetics through these solid particles depends on their size, very small diameter particles are desirable. Nevertheless, faster kinetics can also be obtained by using particles consisting of spherical magnetic cores covered with a thin layer of polymeric adsorbent material. In the present work, we describe the synthesis of a novel type of ion-exchange resin, in which the magnetic core was a single spherical stainless steel particle and the covering layer was poly(styrene-co-divinylbenzene). To check their ion-exchange characteristics, the equilibrium adsorption of the amino acids methionine, phenylalanine, and tryptophan on magnetic and nonmagnetic resins was investigated. The experimental results are explained in terms of a model that considers that the surface of the ion exchangers is heterogeneous, with two different types of adsorption sites ...

Carbon Monoxide on γ-Alumina Single Crystal Surfaces with Gold Nanoparticles

The adsorption and oxidation reaction of CO on γ-Al2O3 single-crystal thin films with Au nanoparticles were studied as a function of the particle size using reflection−adsorption infrared spectroscopy, temperature-programmed desorption (TPD), and the molecular beam technique in a temperature range of 93 K < Ts < 213 K. It was found that CO undergoes a precursor adsorption on the Au particles ending up in an on-top position. The activation energy of this process is calculated from the molecular beam data to (Ed − Ea) = 3.53 kJ/mol. The amount of CO adsorbed increases with increasing particle size. Two types of adsorption sites could be deduced from the TPD data. The oxidation of CO, however, was never observed in the present experiments.

CO Adsorption on Pd Nanoparticles: Density Functional and Vibrational Spectroscopy Studies

Adsorption of CO on nanosize Pd particles was studied theoretically by density functional method and spectroscopically by means of infrared reflection absorption spectroscopy (IRAS) and sum frequency generation (SFG). A density functional approach was applied to three-dimensional crystallites of about 140 atoms. The model clusters were chosen as octahedral fragments of the face centered cubic (fcc) bulk, exhibiting (111) and (001) facets. Bare and adsorbate-decorated cluster models were calculated with Oh symmetry constraints. Various types of adsorption sites were inspected: 3-fold hollow, bridge, and on-top positions at (111) facets; 4-fold hollow and on-top sites at (001) facets; bridge positions at cluster edges; on-top positions at cluster corners; and on single Pd atoms deposited at regular (111) facets. Adsorption properties of the relatively small regular cluster facets (111) and (001) are calculated similar to those of corresponding ideal (infinite) Pd surfaces. However, the strongest CO bonding...

Study of Fourier transform infrared-temperature-programmed desorption of benzene, toluene and ethylbenzene from H-ZSM-5 and H-Beta zeolites

In the present study, an infrared (IR) high temperature cell was used, in combination with a Fourier transform infrared (FTIR) spectrometer for the development of an alternative temperature-programmed desorption (TPD) procedure. Three different adsorbates, i.e., benzene, toluene and ethylbenzene were non-isothermally desorbed from two zeolites H-ZSM-5 and H-Beta. The FTIR-TPD profiles were fitted with the help of the complementary error function. The fitting process was carried out with the help of a computer program which allows us to calculate two parameters, the temperature, T 0 (K) and the temperature range Δ T (K), which, in conjunction with the complementary error function, characterizes the FTIR-TPD profile. Was found that the parameter T 0 is linked with the adsorption energy of the adsorbate in the zeolite and the parameter Δ T was correlated with the transport process of the desorbed molecules inside the zeolites during the desorption process and with the presence of more than one type of adsorption sites. In conclusion, was confirmed that the FTIR-TPD methodology is appropriate for in situ observation of adsorbed molecules on zeolites, and that this technique makes available information concerning the adsorbed state of guest molecules in non-isothermal desorption.

Single-Molecule Imaging and Repositioning of 1,3-Butadiene Adsorbed on Pd(110) Surface

We report here single-molecule imaging and repositioning of 1,3-butadiene molecules adsorbed on a palladium (110) surface by scanning tunneling microscopy (STM) at cryogenic temperature of 4.7 K. The atomically resolved STM images reveal that the adsorbed 1,3-butadiene molecules have two types of adsorption sites, where the molecular axis is oriented along [110] (N type) or [001] (R type). Based on the STM images, we determine the adsorption sites of each type. In addition, repositioning of R-type 1,3-butadiene molecules is demonstrated by controlling tip-adsorbate distance and by moving the tip laterally. The target molecules have been laterally moved and simultaneously rotated to change their adsorption sites from R type to N type on the next row.

Reaction Kinetics on Heterogeneous Model Catalysts: The CO Oxidation on Alumina-Supported Pd Particles

We have employed multimolecular beam techniques to study the transient and steady-state kinetics of the CO oxidation on alumina-supported Pd model catalysts as a function of particle size and surface structure. The model systems were prepared under UHV conditions on a well-ordered alumina film on NiAl(110) and were previously characterized with respect to their geometric and electronic structure and their morphology. Crossing two molecular beams on the sample surface we have systematically probed the CO2 production rate over a wide range of reactant fluxes and at different sample temperatures. Characteristic differences as a function of particles size are observed in both the transient and steady-state regime. In order to relate these effects to the differences in structure and adsorption properties, we have performed microkinetic simulations of the entire series of transient experiments. Whereas it is found that the kinetics on large and ordered Pd particles can in general be described by a homogeneous surface model, significant deviations remain with respect to the kinetics on small and defect-rich particles. In order to semiquantitatively simulate these effects, we consider a heterogeneous surface model, which takes into account the simultaneous presence of different types of adsorption sites. Depending on their distribution, surface diffusion between these sites is included. It turns out that the differences observed for the small particles can be qualitatively understood by a simple model, where we add a small fraction of weakly CO binding sites to the regular adsorption properties. This type of modified adsorption behavior is in agreement with previous desorption studies.

Density functional theory studies of the adsorption of ethylene and oxygen on Pt(111) and Pt3Sn(111)

Density functional theory, employing periodic slab calculations, was used to investigate the interactions of ethylene and oxygen with Pt(111) and Pt3Sn(111). The predicted energetics and structures of adsorbed species on Pt(111) are in good agreement with experimental data. The binding energies of π-bonded ethylene, di-σ-bonded ethylene, and ethylidyne species are weaker on Pt3Sn(111) than on Pt(111) by 21, 31, and 50 kJ/mol, respectively. Hence, the electronic effect of Sn on the adsorption of ethylene depends on the type of adsorption site, with adsorption on three-fold site weakened more than adsorption on two-fold and one-fold sites. Oxygen atoms bond as strongly on Pt3Sn(111) as on Pt(111), and these atoms prefer to adsorb near Sn atoms on the surface. The addition of Sn to Pt(111) leads to a surface heterogeneity, wherein ethylidyne species prefer to adsorb away from Sn atoms and oxygen atoms prefer to adsorb near Sn atoms. Implications of this surface heterogeneity on hydrocarbon reaction selectivity on Pt-based catalysts are discussed.

Chromatographic behaviour of low molar-mass polyesters in normal-phase high-performance liquid chromatography

The normal-phase chromatographic retention behaviour of polyesters on bare silica and on a polymer-based polyamine (PA) column has been studied with a variely of binary mobile phases under isocratic conditions. The dependence of experimental retention data on the degree of polymerization (p) and on mobile phase composition (φ) was characterized by to an approach developed by Jandera et al. The bulky repeating unit and the relatively highly polar end groups of the polyesters both had a large influence on retention behaviour. The two effects in combination explain the molar-mass-independent retention observed experimentally at a particular mobile phase composition for all the mobile phase—stationary phase combinations investigated. These conditions were found to be independent of the type of end group. End group separation on a silica column improves when the polarity of the less polar solvent is increased. End group separation is better on the PA column because of a greater difference between the adsorption energy of the alcohol and acid end groups. Better prediction of retention data on the PA column was achieved by use of an approach which assumes two different types of adsorption site. Results enabled further understanding of retention behaviour in normalphase gradient polymer-elution chromatography (NPGPEC) and explained both the dependence of the order of elution onp and differences between the end-group selectivity of different systems.

Adsorption and reduction of NO over activated coke at low temperature

NO adsorption and reduction with NH 3 over activated coke at low temperatures were studied by temperature-programmed desoption (TPD) and step-response experiments. NO adsorption is inhibited by competitive adsorption of NH 3 in the absence of oxygen, and is significantly increased in the presence of oxygen. This phenomenon cannot be explained by either the oxidation of NO in the gas phase, nor the creation of oxidized surface on the activated coke. A hypothesis is proposed to interpret the phenomenon, which suggests that on the surface of the activated coke there are at least two types of adsorption sites, one of them adsorbs NO 2 and/or the oxidized NO species rather than NO. NO–NH 3–O 2 reaction proceeds via both the ED mechanism and the LH mechanism. NO conversion decreases with increasing temperature in the range of 30–250°C, which suggests that the adsorption of reactants, especially NH 3, is the rate-limiting step.

Toward a Better Understanding on the Adsorption Behavior of Aromatics in 12R Window Zeolites

Benzene adsorption behavior in a large family of 12R window zeolites (X, Y, EMT, Beta and LTL) has been examined by means of in-situ FTIR spectroscopy and correlated with the zeolite structure, the type and number of counter-ions, and the negative charge on framework oxygen atoms of zeolites. The effect of coadsorption of HCl, NH3 and CH3NH2 on the benzene location has also been studied. The present work illustrates that besides the benzene adsorption on counter ions of zeolites, the 12R windows could also be the adsorption sites for benzene. Upon adsorption of coadsorbates such as HCl, NH3 and CH3NH2, the migration of preadsorbed benzene molecules from one type of adsorption sites towards another, i.e. from 12R windows towards the cations for HCl and opposite direction for NH3 and CH3NH2, has been evidenced. The lack of adsorption of benzene on 12R windows of NaBeta even upon coadsorption of a series of basic molecules reveals that benzene adsorption on 12R windows is most likely governed by a molecular recognition effect where benzene molecule and 12R window should have the adapted chemical and structural properties like in enzyme-substrate system and zeolites can be referred to as solid enzymes or zeo-enzymes. This paper indicates also that the adsorption properties of zeolites can be modified and accommodated by introduction of a co-adsorbate.

The mechanism of cadmium surface complexation on iron oxyhydroxide minerals

Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite.Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ∼2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous.

Modeling of adsorption and bifunctional conversion of n-alkanes on Pt/H-ZSM-22 zeolite catalyst

The hydroconversion of C6–C9 n-alkanes on Pt/H-ZSM-22 was investigated by measuring and modeling of the adsorption and catalytic properties of this strongly shape-selective zeolite catalyst. Henry adsorption constants and adsorption isotherms of C6–C9 n- and iso-alkanes on H-ZSM-22 were determined with chromatographic techniques and fitted to different models. While n-alkanes have access to the internal pore structure of ZSM-22, only the linear part of the iso-alkanes adsorbs in the pore entrances. Besides pore and pore mouth adsorption, external surface adsorption plays an important role. In the hydroconversion of C6–C9 n-alkanes, nonselective adsorption of n- and iso-alkanes on the external surface of the ZSM-22 crystals determines the concentration of the reacting species and the reaction orders. The skeletal rearrangement reactions occur exclusively in the pore mouths at the external surface of the zeolite crystals. The hydrocracking reactions were modeled with a lumped reaction scheme, accounting for adsorption on the different types of adsorption sites. The best agreement between model and experiments was obtained with an external surface adsorption model.

Time-dependent Raman spectra from two types of adsorption sites at Ag electrodes

Time-dependent surface-enhanced Raman scattering (SERS) form thiourea (TU) and ClO 4 − coadsorbed at Ag electrodes was investigated using a potential step. The intensity–time profiles for the different vibrational modes can be classified into two categories according to the response rate of the band intensity to the potential step, i.e. a fast response within 5 s and a slow response up to 5 min. More interestingly, all band intensities show the fast response characteristics after the electrode was made to undertake a negative excursion to the potential of −2.0 V. This phenomenon is explained in terms of co-existence of two types of adsorption sites (SERS active sites), i.e., neutral Ag atom(s) and partial charged Ag atom(s). It is shown that time-dependent SERS combined the potential step method may be helpful not only for gaining a deeper insight into the SERS mechanism(s), but also for revealing the nature and structural dynamics of adsorption sites formed by strong chemical interaction.

Catalyst Design to Change Selectivity of Photocatalytic Oxidation

Different selectivities for photocatalytic oxidation (PCO) of ethanol were observed for two types of adsorption sites on TiO2, and this identification led to the design of a photocatalyst with enhanced selectivity to acetaldehyde, a partial oxidation product. Transient photocatalytic oxidation (PCO), steady-state reaction, temperature-programmed desorption (TPD), temperature-programmed oxidation, and isotope labeling were combined to determine the reactivity of the different adsorption sites. During PCO, weakly bound ethanol preferentially formed gas-phase acetaldehyde, whereas strongly bound ethanol (which decomposes during TPD) primarily produced CO2. Weakly bound ethanol appears to adsorb on sites that are not available for acetaldehyde adsorption. This information, combined with the fact that acetaldehyde decomposes to a strongly bound intermediate during TPD, led to the design of a catalyst that was modified with acetaldehyde TPD products, which were stable during PCO. The TPD products of acetaldehyde preferentially poison the sites where ethanol is strongly bound so that selectivity to acetaldehyde increased during ethanol PCO. Higher selectivity was seen during both transient and steady state experiments, and at 54–60% conversion the ratio of partial to complete oxidation on the poisoned catalyst was five times that on fresh TiO2.

Diffusion of adsorbates on random alloy surfaces

In this work the diffusion of non-interacting adsorbates on a random AB alloy surface is considered. For this purpose a simple cubic (sc), body-centered cubic (bcc) or face-centered cubic (fcc) auxiliary metal lattice is introduced. The auxiliary lattice is truncated parallel to its (100) plane in such a way that the fourfold hollow positions of the metal surface form a regular net of adsorption sites with square symmetry. The adsorption energy of each adsorption site is determined by its own environment, i.e. by the numbers of direct A or B neighbors. The Monte-Carlo method has been utilized to simulate surface diffusion of adsorbates on such energetically heterogeneous alloy surfaces and to calculate the tracer, jump and chemical diffusion coefficients. The chemical diffusion coefficient was calculated via two different approaches: the fluctuation and the Kubo-Green method. The influence of energetical heterogeneities on the surface diffusion is largely pronounced at low temperatures and low surface coverages, where most of the adatoms are trapped by deep adsorption sites. It was found that at low temperatures the sequential occupation of the different types of adsorption sites can be observed.

Controlled-Rate Thermal Desorptton

A new way of conducting thermal desorption experiments is described. The method consists in maintaining the desorption rate constant by a rigorous control of temperature. The equations for two models of constant rate desorption are described: from a surface with only one type of adsorption site, and with two different types. The technique is applied to two real systems and the kinetic parameters are determined. The results are compared with those obtained by using TPD. The main advantages are discussed and the methodology and modifications required for a TPD set-up to work at a constant desorption rate are described.

Biospecific interactions of vitamin K-dependent factors with phospholipid-like polystyrene derivatives: Part II: factor IX

We previously demonstrated that phosphorylated polystyrene derivatives exhibit phospholipid-like behaviour and therefore are able to interact with factor II, one of the vitamin K-dependent coagulation factors. Under the same conditions as for factor II, we examined the interactions of factor IX with phosphorylated resins of various compositions in phosphate groups: these studies were carried out with or without albumin precoating of the polymers and either in the presence or absence of calcium ions. Adsorption experiments show that, in the absence of calcium ions, only one class of adsorption sites of factor IX can be evidenced with the interactions taking place through the formation of binary complexes, whereas in the presence of calcium ions, the affinity of factor IX for phosphorylated resins becomes very high and two types of adsorption sites have been evidenced with biospecific ternary complexes being formed. The domains of predominance of these complexes were determined. Moreover, the only functional groups borne by the phosphorylated polystyrene resins involved in factor IX-polymer interactions are phosphodiester groups. Comparison between factor II and factor IX adsorption onto the same polymers leads to the conclusion that the observed differences probably reflect the differences in the Gla domains of the vitamin K-dependent factors. Finally, this study demonstrates that phosphorylated polystyrene derivatives can be used as stationary phases for purification of factor IX by highly specific liquid biochromatography.

METAL-INDUCED SULFATE ADSORPTION BY SOILS: III. APPLICATION OF LANGMUIR EQUATIONS1

The one- and two-surface Langmuir equations were evaluated in studies of the effect of metal type, valence, and concentration on SO42− adsorption by four diverse soils, two from Iowa (dominated by permanent charge) and one each from Chile and Costa Rica (two highly weathered soils with variable charge). The adsorption parameters, Xm and k, calculated using the "one-surface" Langmuir equation, showed that SO42− adsorption by the soils in the presence of trivalent metals was relatively higher than in the presence of mono- or divalent metals. When SO42− and metal were added at equivalent concentrations, the Xm values varied from 14.9 mmolckg−1 for Rathbun soil when Cs+ was the metal ion to 303 mmolckg−1 for Osorno soil when In3+ was the metal ion. The corresponding k values were 0.109 and 0.040, respectively. The Xm values for SO42− adsorption by the soils when the metals were added at a constant concentration (12.0 mmolcL−1) were, in general, less than those when SO42− and its metal ion were added at equivalent concentrations. At the constant initial metal concentration, the Xm values ranged from 10.0 mmolckg−1 for Rathbun soil when K+ or Cs+ was the metal ion to 93.7 mmolckg−1 for Osorno soil when In3+ was the metal ion. The corresponding k values were 0.163 (K+), 0.224 (Cs+), and 1.28 (In3+). The calculated parameters (Xml, k1, Xm2, and k2) of the "two-surface" Langmuir equation for the SO42− adsorption data of the two Iowa soils with predominant permanent charge showed that this equation gave one line instead of two when SO42− and metals were added at equivalent concentrations, suggesting that, in the range of SO42− concentration used, one type of adsorption site or mechanism was involved. The two-surface Langmuir equation adequately described SO42− adsorption data when the initial metal concentration was constant. The estimated adsorption maximum (Xm1 + Xm2) values by the "two-surface" Langmuir equation were always greater than those estimated by the "one-surface" Langmuir equation when the metal ions were added at a constant concentration. For all the SO42− adsorption data, two bonding constants were found, suggesting the presence of two different adsorption sites or different mechanisms, each with a different bonding energy. The results provide evidence that SO42− adsorption by soils is caused by more than one mechanism and that the associated metal ion significantly affects SO42− adsorption, regardless of the mechanism involved.