Model For Coadsorption Research Articles

Controlling the Adsorption Enthalpy of CO2 in Zeolites by Framework Topology and Composition

Zeolites are often investigated as potential adsorbents for CO(2) adsorption and separation. Depending on the zeolite topology and composition (Si/Al ratio and extra-framework cations), the CO(2) adsorption heats at low coverages vary from -20 to -60 kJ mol(-1), and with increasing surface coverage adsorption heats either stay approximately constant or they quickly drop down. Experimental adsorption heats obtained for purely siliceous porous solids and for ion-exchanged zeolites of the structural type MFI, FER, FAU, LTA, TUN, IMF, and -SVR are discussed in light of results of periodic density functional theory calculations corrected for the description of dispersion interactions. Key factors influencing the stability of CO(2) adsorption complexes are identified and discussed at the molecular level. A general model for CO(2) adsorption in zeolites and related materials is proposed and data reported in literature are evaluated with regard to the proposed model.

Coadsorption model for first-principle description of roles of donors in heterogeneous Ziegler–Natta propylene polymerization

Systematic periodic density functional calculations were conducted to clarify the mechanism for donors to exert steric and electronics influences on propylene polymerization using heterogeneous Ziegler–Natta catalysts. It was concluded that TiCl4 preferentially adsorbs as mononuclear species on the MgCl2 (110) surface, and the coadsorption of donors with it is energetically viable. The coadsorption of donors on the (110) surface reinforces the electron density of the Ti mononuclear species and sterically transfers the underlying C2 symmetry to convert the originally aspecific mononuclear species into isospecific one. The nearest coadsorption of ethylbenzoate (EB) not only sterically induces the isospecificity of the Ti mononuclear species but also electrostatically improves the regiospecificity in propylene insertion. In addition, EB prevents sterically demanding chain transfer to propylene, increasing the molecular weight of the produced polypropylene. Thus theoretically derived “coadsorption model” is highly consistent with a variety of experimentally known facts and believed to be useful for the ab initio prediction of new donor structures.

Adsorption of CO and O2 on W2C(0001)

CO, O(2), and H(2) adsorption on a clean W(2)C(0001)√13×√13 R ± 13.9° reconstructed surface at room temperature (RT) were investigated using high-resolution electron energy loss spectroscopy (HREELS). The W(2)C(0001) adsorbs CO molecularly and adsorbs O(2) dissociatively, but does not adsorb H(2) at RT. In the CO adsorption system, two C-O stretching (antisymmetric CCO stretching) modes were found at 242.3 meV (1954 cm(-1)) and at 253.0 meV (2041 cm(-1)). The low-frequency site is occupied at first with subsequent conversion to the high-frequency site with increasing coverage. Additionally, a small peak was apparent at 104.5 meV (843 cm(-1)), and a middle peak at 50-51 meV (400-410 cm(-1)), which are assignable to a symmetric stretching mode and a hindered translational mode, respectively, of a CCO (ketenylidene) species. These observations are consistent with the CO adsorption model on top of the surface carbon. For oxygen adsorption, two adsorption states were found at 65.2-68.1 meV (526-549 cm(-1)) and 73.6 meV (594 cm(-1)): typical frequencies to oxygen adsorption on metal surfaces. Results suggest that atomic oxygen adsorption occurred on a threefold hollow site of the second W layer.

CO2 and CH4 Separation by Adsorption Using Cu-BTC Metal−Organic Framework

Molecular simulations have shown that the metal−organic framework Cu-BTC (Cu3(BTC)2) is an interesting candidate for the separation of CO2 by adsorption. In this work, the first experimental binary and ternary adsorption data of CO2, CH4, and CO on the Cu-BTC are reported. These data are analyzed and compared with coadsorption models that are built from pure component isotherm data. Cu-BTC has a CO2/CH4 selectivity of ∼8 and a high delta loading (=difference between adsorption capacity under conditions of adsorption and desorption) and therefore appears to be a good compromise between zeolites, with high selectivity for CO2, but low delta loadings, and activated carbons, with high delta loadings, but low selectivity, for pressure swing adsorption applications.

Monte Carlo model of CO adsorption on supported Pt nanoparticle

For molecular simulations with thousands of atoms it is desirable to use a lattice gas model because it is fast and easy-to-use for computations. Unfortunately, simulation of adsorption on heterogeneous surfaces within this model is rather complicated due to a large variety of available adsorption site types. We propose the combined model with lattice representation of adsorbent atoms and arbitrary location of adsorbate atoms. Using this model simulation of CO adsorption on supported Pt nanoparticles has been performed. With the proposed approach the above-mentioned difficulties were successfully overcome.

Potential-assisted deposition of mixed alkanethiol self-assembled monolayers

Preparation of self-assembled monolayers (SAMs) usually involves passive incubation. The recently developed potential-assisted deposition is indeed more selective as well as 100-fold faster than passive adsorption, thereby enhancing the reproducibility of the monolayer deposition. This article aims to identify the electrodeposition conditions necessary to prepare mixed alkanethiol SAMs on gold surface. Parameters such as concentrations in solution, electrode polarization and deposition time were examined for two chain lengths, C 3 (mercaptopropionic acid, MPA) and C 18 (octadecanethiol, ODT). The kinetics and composition of the SAMs were systematically characterized by reductive stripping analysis and by surface plasmon enhanced ellipsometry. Control of the surface concentration of the stable binary SAM was achieved by carefully adjusting the MPA fraction at between 70% and 95% in the deposition solution. Three theoretical models were developed. For MPA or ODT SAMs, whatever the adsorbate and the time period, the second order model gives the best fit. For the binary monolayer (MPA–ODT), a co-adsorption model was developed to describe the kinetics before 10 s.

A surface adsorption model for electroless cobalt alloy thin films

Thin cobalt alloy films have been obtained using electroless deposition solution with two reducing agents: dimethylamine borane (DMAB) and sodium hypophosphite. This system allows spontaneous and self-activated deposition of barrier layers on Cu lines and via contacts for ultra large scale integration (ULSI) interconnects applications. This work presents a study of the solution composition effects on the material properties and composition of the films. First, we present the deposition rates, the electrical resistance, the various element profiles in the thin film, and the thin film roughness. Next, we discuss the film’s composition and its dependence on the ratio between the reducing agents composition in the solution. The experimental results suggest that the film phosphorous and boron composition is determined by the surface adsorption rates of the reducing agents. Therefore, a surface co-adsorption model of the two reducing agents is proposed, formulated, analyzed, and compared to the experimental results. Finally, we discuss the model and its significance to the formation of high-quality ultra-thin barrier layers.

Adsorption of organic molecules on electrodes: 2005 Frumkin Memorial Medal Award lecture

This Award lecture, delivered on the occasion of the presentation of the 2005 Frumkin Memorial Medal to the author, consists of two parts: a memorial section and a scientific section. In the scientific section the successive development of models within Frumkin theory of adsorption of organic molecules on electrodes are presented. In particular, (i) the versions of theory taking into account the dependence of attraction constant in adlayer on the electrode potential, (ii) generalization of Frumkin’s, Parsons’s and Hansen’s models, (iii) model for coadsorption of two compounds, and (iv) model taking into account the diffuse part of the double layer are reported.

Modeling spreading-pressure-dependent binary gas coadsorption equilibria using gravimetric data

The present work proposes an approach to building nonideal coadsorption models in a thermodynamically consistent fashion, including the effects of pressure and spreading pressure, from simple gravimetric measurements. This is an “inverse problem” of parameter determination from appropriate and limited experimental data.The approach relies on the nonideal adsorbed solution theory, which includes activity coefficients and their dependence on spreading pressure, and on an original form of the excess Gibbs energy of mixing. A fully analytical development leads to explicit relations between the infinite dilution activity coefficients and three sets of independent information: the parameters of this excess Gibbs function, the limiting slopes of measured binary gravimetric curves at two different total pressures, and the properties of the single-component isotherms. From there, the four parameters of the model may be determined quasi-analytically and uniquely. The method is exemplified with the coadsorption of CO 2 and CH 4 on activated carbon, and a heterogeneous set of data. On one hand, the total adsorbed mass of the two components is measured at 1 bar by “incremental gravimetry.” On the other hand, data obtained from independent batch-type equilibration measurements at 2 bar allow a comparison of calculated and measured data for the individual component concentrations. It is emphasized, however, that only total adsorbed mass data are needed for application of the method.

Adsorption Properties of Nitrogen, Carbon Dioxide and Nitrogen Dioxide onto Mesoporous Chromium Silicate

The adsorption properties of nitrogen, carbon dioxide and nitrogen dioxide onto mesoporous chromium silicates (Cr-FSM-16) were studied by spectroscopic analyses. The Cr-oxide moieties are highly dispersed in the framework of FSM-16. The UV-vis spectra of the Cr-FSM-16 exhibit three distinct absorption bands, which can be assigned to charge transfer from O2 − to Cr6 + of the tetrahedrally coordinated Cr-oxides moieties. The results of initial heat of adsorption of CO2 indicated the interaction between Cr6 + = O2 − in Cr-FSM-16 and CO2 was weak. The decomposition of NO2 by FSM-16 or Cr-FSM-16 was caused by Si = OH site or Cr6 + = O2 − site, respectively. The co-adsorption models for NO2 and CO2 onto Cr-FSM-16 or FSM-16 were proposed in the end.

Sorption of Cations and Anions from a Binary Electrolyte at Uncharged Electrode Surface: Comparing the Monolayer and Two-Layer Adsorption Models

Equations for calculating coverage and decrease in the surface tension Δσ during co-adsorption of cations and anions (CA) on an uncharged interface from a binary electrolyte with concentration c are derived. The equations are valid if electric centers of CA are localized in two parallel layers. The Δσ vs. lnc curves, computed within this model and a model for co-adsorption in a common layer are compared. The Δσ vs. lnc curves, calculated within either model to an accuracy substantially exceeding experimental accuracy, may be formally described with an equation of the Frumkin isotherm for neutral molecules and a corresponding equation for Δσ. The link of effective parameters of the Frumkin isotherm with the adsorption parameters of mixed adsorption isotherms of CA depends on the model.

Effects of competitive carbon monoxide adsorption on the hydrogen response of metal–insulator–semiconductor sensors: the role of metal film morphology

We have investigated the effects of carbon monoxide coadsorption on the performance of hydrogen-sensitive palladium metal–insulator–semiconductor (MIS) sensors, and have found that device performance is highly dependent on the morphology of the metal film. On smooth, continuous films, experiments utilizing H2/CO mixtures show that CO induces a time-delayed increase in response to H2. Based on a kinetic model of CO and hydrogen adsorption on the Pd film, this increase is interpreted as being due to trapping of H atoms at the Pd–SiO2 interface by CO adsorbed on the metal surface. However, MIS sensors fabricated so that a large concentration of voids exist in the metal film respond in a markedly different fashion: on these devices, the addition of CO causes a time-delayed decrease in the sensor response. Possible explanations for this attenuating CO effect are discussed. The experimental and modeling results presented here suggest that sensor behavior in the presence of competitively adsorbing mixtures can be quite complex, with film imperfections playing an important role in determining the response.

Electronic structure investigation of the room temperature coadsorption of oxygen and potassium on Ni(100): from oxygen submonolayer coverage to saturated NiO/Ni(100) via an Ni(100)-(3×3)-(K+O) structure.

We have investigated the room temperature potassium-promoted oxidation of Ni(100) by means of O 1s and K 2p X-ray photoemission spectroscopy (XPS), relative work function and O 1s X-ray absorption spectroscopy (XAS) measurements. Within the potassium coverage range we examine in this study ( θ K=0.10–0.25 ML), coadsorption of about 0.5 ML of oxygen results in the formation of a (3×3) pattern which is peculiar to the coadsorption of alkalis and oxygen on Ni(100). This structure is stable against further oxygen uptake or against adsorption of other species such as CO. However, by means of O 1s XPS and XAS observations we determine an overall potassium-enhanced oxidation rate of about two orders of magnitude. The O 1s XPS data show that oxygen coadsorbed with K on Ni(100) is still bonded primarily to Ni and thus rule out the formation of potassium oxide or other K x O y compounds such as peroxide or superoxide. The line-shape analysis of the K 2p XPS data show that upon oxygen coadsorption the alkali metal layer undergoes a metal–insulator type of transition. The O 1s XPS data rule out the possibility that this transition is due to a large charge transfer between coadsorbed species so other mechanisms are discussed. The work function changes induced by O on the K-precovered surface are consistent with the formation of a bilayered structure in which oxygen is adsorbed underneath the alkali metal layer at K coverages above the work function minimum. However, we find that the alkali-induced minimum of the work function is not related to the occurrence of structural properties, in particular the formation of the (3×3) structure, or electronic properties such as the metal–insulator transition of the K layer. Therefore a coplanar alkali–oxygen coadsorption model cannot be ruled out. The O 1s XAS data show that the presence of potassium on Ni(100) lowers the O coverage for the onset of the formation of cubic NiO.

The bonding of CO to metal surfaces

The atom and symmetry specific properties of x-ray emission spectroscopy have been applied to the investigation of CO adsorbed on Ni(100) and Cu(100) surfaces. In comparison to ab initio electronic structure calculations, obtained in density functional theory, we develop a consistent electronic structure model of CO adsorption on transition and noble metals and extend to a conceptual model of the surface chemical bond. A strong CO–substrate interaction is found, characterized by significant hybridization of the initial CO orbitals and the metal bands. In the π system an allylic configuration is found as the result of orbital mixing between the CO 1π, 2π* and the metal dπ-band which is manifested experimentally in the observation of an oxygen lone-pair state. In the σ system experimental evidence of equally strong orbital mixing has been found. Energetically, the adsorbate–substrate complex is stabilized by the π-interaction but is destabilized by the σ-interaction. Furthermore, the internal C–O bond carried by the π-interaction is weakened upon adsorption, which is opposite for the internal C–O σ bond that is strengthened. The equilibrium properties of CO adsorbed on these metals are found to be the direct result of the balance between the σ- and π-interactions; both in terms of the total energy and the local bond properties.

Static Adsorption, by Activated Carbons, of Vapour Mixtures Corresponding to Immiscible Liquids

It is shown that the static adsorption of benzene and water vapours by a typical industrial active carbon can be described by the model of independent co-adsorption proposed for compounds which are not miscible in the liquid state. The model assumes that the vapours are adsorbed according to their respective DA isotherms, each component using the micropore volume left unoccupied by the other. A good agreement is found between the calculated and the experimental adsorption, but in the case of water the model must take into account the decrease of the characteristic energy E(H2O) with increasing co-adsorption of benzene. This behaviour is confirmed by earlier studies of water adsorption and of immersion calorimetry into water, following variable pre-adsorption of n-nonane.

High-resolution core-level photoemission of the CO-induced Pd(110) surface reconstruction

The complex behaviour of the CO/Pd(110) adsorption system has been followed with high-resolution XPS of both the C 1s and the surface core-level shift (SCLS) of the Pd 3d features, from initial exposures to a monolayer coverage. For low exposures (<0.45 ML) the SCLS shifts from −0.5 to +0.43 eV. The C 1s emission first appears as a low-energy band at around 284.8 eV characteristic of CO in high coordination sites, indicating that reconstruction begins with initial exposure. On increasing exposure up to saturation at room temperature ( ϴ CO=0.75 ML) with its (4×2) CO LEED structure, a second higher binding-energy band assignable to two-fold bridged molecules appears and becomes dominant; concomitant with this is a further change in the SCLS to +0.63 eV. The features observed for ϴ CO=0.75 ML coverage are fully consistent with the model of CO adsorption on a (1×2) missing-row reconstructed surface where the C 1s peaks at 286.3 and 286.1 eV binding energy are assigned to two-fold bridge sites on the ridges and in the grooves, respectively. In the process of reconstruction, the latter appear before the former. At low temperatures the (2×1)p2mg “herring-bone” CO overlayer on the unreconstructed (110) surface is characterised by a large SCLS of +0.94 eV and a single C 1s feature at 286.3 eV, proving that this structure involves CO adsorbed in two-fold bridge sites.

Adsorbate–adsorbate repulsions—the coverage dependence of the adsorption structure of CO on Cu(110) as studied by electron-stimulated desorption ion angular distribution

The coverage dependent orientation of CO adsorbed on a Cu(110) surface was studied by the electron-stimulated desorption ion angular distribution (ESDIAD) technique. A neutral excited (CO*) species is imaged and in addition positive ions are measured. The adsorption temperature was varied between 32 K and 150 K. By applying the ESDIAD technique at a temperature below 80 K it was possible to decrease the beamwidths drastically, to determine the angular distributions better than ±0.5°, and to study the adsorption of CO chemisorbed and physisorbed on the surface. With increasing CO coverage we observe three distinct ESDIAD patterns. Starting from a normal beam pattern with an elliptical cross section with the major axis oriented in the 〈11̄0〉 direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into a pattern of two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the 〈11̄0〉 directions by ∼9°. With further increasing CO coverage an additional central peak develops with an elliptical broadening now in the 〈001〉 direction. The changes of the pattern are reversible as shown by decreasing the coverage by thermal desorption. Based on these ESDIAD and digital low energy electron diffraction results, a linear-chain model for CO adsorption is proposed. Temperature programmed desorption measurements also indicate the presence of repulsive CO–CO interactions in the adlayer.

Dynamics and structure of chemisorbed CO on Cu(110): An electron-stimulated desorption ion angular distribution study

The adsorption of CO on a Cu(110) surface has been studied in the temperature range between 32 K and 200 K by the ESDIAD (electron-stimulated desorption ion angular distribution) technique. Here a desorbing neutral excited CO* species is spatially imaged. At coverages up to θ=0.2, a pronounced anisotropy in the librational motion of CO adsorbed in the atop configuration has been observed. The higher amplitude of libration is along the copper atom rows ([11̄0]-direction). Large changes in the observed CO* pattern in the low temperature range between 32 K and 70 K give an indication of a shallow potential well and the thermal population of the higher librational levels. With increasing CO coverage three distinct ESDIAD patterns are observed. Starting from a single normal beam with an elliptical cross-section having the major axis in the [11̄0]-direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the [11̄0]-directions. With further CO coverage increase, an additional central beam grows with a slight elliptical broadening in the [001]-direction. Based on ESDIAD and digital low-energy electron diffraction measurements, a linear-chain model for CO adsorption on atop sites of Cu(110) is proposed.

Tight-binding model of CO adsorption at [formula omitted] surface

Chemisorption of CO on Pt overlayer deposited on Ni(111) is studied within a semi-empirical self-consistent tight-binding method. It is argued that the chemisorption energy is lower than at Pt(111) mainly due to the possible overlayer compression. The result has bearing also on the (111) surface of PtNi alloys. Surface core-level shifts are evaluated within the initial-state picture. The previously found correlation between core-level shifts and the chemisorption-energy variation is again confirmed in situations when the changes are significant. Trends in additional core-level shifts induced by the admolecule are mentioned.

The synergistic effect of halide ions and some selected thiols as a combined corrosion inhibitor for pickling of mild steel in sulphuric acid solution

The effect of halide ions on the electrochemical and corrosion behaviour of mild steel in sulphuric acid solution with and without some selected thiols, viz. 2-mercaptobenzoxazole, 2-mercaptobenzothiazole and 2-mercaptobenzimidazole, has been studied. Each of the halide ions and the investigated thiols were found to inhibit the corrosion process of mild steel when added separately to the corrosive medium, while the inhibition efficiency of thiols is higher than that of halides. It was found that the presence of halide ions together with the different thiols in the corrosive medium decreases the inhibition effect, specially at low concentrations. The surface coverage values which are obtained both from weight loss and polarization studies are approximately the same. The decrease in corrosion rate associated with an increase in the cathodic overvoltage and a positive shift in the corrosion potential, in the presence of the investigated thiols, denotes inhibition of a mixed type (predominantly anodic). Synergism of the investigated thiols and halides is discussed from the viewpoint of a model of co-adsorption of halide ions and thiols. Adsorption of the inhibitor was found to follow the Langmuir isotherm.