Adsorption Charge Transfer Research Articles

Non-dissociative adsorption of diatomic molecules on nanoclusters at room temperature

Experiments using vibrationally resolved ultraviolet photoelectron spectroscopy (UPS) provide evidence that dissociative chemisorption of N 2 becomes less stable than the molecular chemisorption on small clusters consisting of less than 10 metal atoms, which can efficiently dissociate diatomic molecules in the bulk form. This result is different from the generally accepted view that undercoordinated atoms are chemically more reactive. Our observation can be rationalized with a less efficient screening of the positive holes in smaller clusters created by a metal to adsorbate charge transfer, caused by the lower number of the directly neighbouring atoms.

Adsorption of CO and CN − on transition metal surfaces: a comparative study of the bonding mechanism

A comparative study of the chemisorption of the isoelectronic species CO and CN − on Rh, Ni, Pd and Pt(1 1 1) surfaces has been performed using the cluster model approach and the density functional theory. The CO bond is weakened whereas the CN − one is strengthened upon chemisorption in agreement with experimental evidence based on shifts to lower or higher frequencies, respectively, with respect the vibrational frequency of the free adsorbate. The details of the chemisorption bond have been studied using different techniques including a topological analysis of the electron localization function and the projection orbitals method. Finally, a correlation between the substrate–adsorbate charge transfer and the frequency shift has been found.

Complete analysis of the surface-enhanced Raman scattering of pyrazine on the silver electrode on the basis of a resonant charge transfer mechanism involving three states

A new general procedure to interpret surface-enhanced Raman scattering (SERS) spectra has been developed in order to clarify the controversy concerning the relevant enhancement mechanism of this type of spectra. The analysis consists of detecting the presence of the charge transfer (CT) enhancement mechanism by correlating the most enhanced SERS bands with the ab initio calculated geometries (ΔQ) and vibrational frequencies (Δv) of the isolated molecule and its radical anion. This CT mechanism is assumed to be identical to that of resonance Raman between the electronic ground state of the metal–adsorbate complex and charge transfer excited states. We consider that these excited states arise when one electron is transferred from the metal to pyrazine. For this reason, they have been labeled from the point of view of pyrazine on the basis on the symmetry of the doublet states of its radical anion. The SERS spectra of pyrazine recorded on silver surface at several electrode potentials have been analyzed on the basis of the Franck–Condon and Herzberg–Teller contributions related to B3u2–1Ag and Au2–1Ag transitions. A great deal of experimental facts related to the relative enhancement of in-plane Ag, B3g, and B1u modes as well as out-of-plane B3u, B2g, and Au vibrations has been explained. Likewise, it is possible to account for the observation of Raman inactive fundamentals as well as the influence of the forbidden Au2–1Ag transition on the SERS spectra if the symmetry of the metal–adsorbate complex and the nonplanarity of the Au2 state are taken into account. All the results point out that the CT mechanism is mainly responsible for the SERS features of this molecule studied here.

A SERS spectroelectrochemical investigation of the interaction of 2-mercaptobenzothiazole with copper, silver and gold surfaces

The interaction of the sulfide mineral flotation collector, 2-mercaptobenzothiazole, with silver, copper and gold surfaces has been investigated by surface enhanced Raman scattering (SERS) spectroscopy. 2-mercaptobenzothiazole, the copper, silver and gold compounds of this species, and the dithiolate, 2,2′-dithiobis(benzothiazole) were characterised by 13C NMR and Raman spectroscopy to provide a basis for identifying surface species. SERS investigations showed that, at pH 4.6 where the solution species is in the protonated form, and at 9.2, where it is present as the ion, adsorption on each metal occurs over a wide potential range. Attachment of the organic compound occurs through bonding between the exocyclic sulfur atom and metal atoms in the surface. X-ray photoelectron spectroscopy confirmed that the adsorbed layer was of monolayer thickness. Adsorption of the protonated 2-mercaptobenzothiazole occurs on copper at pH 4.6 at potentials below that at which charge transfer adsorption commences.

Sub-picosecond Injection of Electrons from Excited [Ru(2,2′-bipy-4,4′-dicarboxy)2(SCN)2] into TiO2 Using Transient Mid-Infrared Spectroscopy*

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge transfer adsorption peaks and probing the absorption of electrons injected into the TiO2 conduction band at 1.52 {micro}m and in the range of 4.1 to 7.0 {micro}m, we have directly observed the arrival of the injected electrons. Our measurements indicate an instrument-limited {approx}50-fs upper limit on the electron injection time under ambient conditions in air. We have compared the infrared transient absorption for noninjecting (blank) systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2, and found no indication of electron injection at probe wavelengths in the mid-IR (4.1 to 7.0 {micro}m). At 1.52 {micro}m interferences exist in the observed transient adsorption signal for the blanks.

Femtosecond dynamics of adsorbate charge transfer processes

In this paper, experimental results on the (sub)-femtosecond dynamics of adsorbate-substrate charge transfer processes are reported. A new method to study such phenomena by a combination of high-resolution excitation and decay spectroscopy of adsorbate core resonances is presented. The results were obtained by using the enhanced capabilities regarding high-resolution soft X-ray spectroscopy offered by third generation synchrotron sources. Timescales for charge transfer processes for physisorbed and chemisorbed adsorbates are derived and compared to expectations from resonance lineprofiles.

Early Events in Intercalation Reactions: The Preintercalation State

Ultrahigh vacuum scanning tunneling spectroscopy (UHV-STS) is used to make the first in situ measurements of reactant charge transfer with the host basal planes during the early events in lamellar intercalation processes to clarify the role and importance of charge transfer in intercalation reactivity. The level of basal-plane charge transfer is determined from the Fermi level shift relative to the host (TiS2) conduction-band edge. Prior to intercalation, the model reaction of ammonia with lamellar TiS2 exhibits low-level, pressure-independent adsorbate (ammonia) charge transfer to the basal planes, identifying a preintercalation state in which adsorbate charge transfer plays a key role in intercalation reactivity. Such a state can explain the anomalously-high reactivity exhibited by other molecular intercalation systems, such as hydrazine/TiSe2, where available evidence suggests strong preintercalation adsorbate−host charge transfer. Once intercalation occurs, ammonium forms and cointercalates with ammon...

Calculation of NEXAFS spectra for surface–adsorbates: Hydroxyl on Cu(111)

Using OH adsorbed on the copper(111) surface as model system we demonstrate the feasibility of calculations of NEXAFS spectra of chemisorbed species. We employ a static exchange method that is implemented for large systems through direct atomic orbital techniques. The general cluster convergence of the spectra ranging up to 49 cluster atoms is investigated and found to be satisfactory. The special character of the strong metal to adsorbate charge transfer involving occupied OH orbitals and of the OH–metal ionic bond are shown to have particular consequences for both the analysis and the computation of the NEXAFS spectra. For the larger clusters the discrete spectra are dominated by adsorbate to cluster transitions, which can be understood from the similarity of electronic structure of the adsorbed OH moiety with that of free OH− for which all transitions end up in the continuum.

Comparative study of the surface properties and the reactivity of coal pyrite and mineral pyrite

Cyclic voltammetry (CV), flotation response, evolved-gas analysis (EGA), and X-ray photoelectron spectroscopy (XPS) were employed to evaluate the major differences in the surface property and surface reactivity of a selected coal pyrite and a mineral pyrite. Surface properties of pyrite were evaluated in terms of flotation response in the presence of both an anionic fluorosurfactant and sodium hydrosulfide. The charge transfer adsorption and oxidation potential of sodium hydrosulfide were evaluated using cyclic voltammetry. Drastic differences in the flotation response have been observed between the coal pyrite and the mineral pyrite. Significant differences in the cyclic voltammograms at pyrite surfaces of these two sources were also observed. Measurements by XPS and EGA were used to determine the oxidation products and the rate of oxidation of the coal-pyrite and the mineral-pyrite surfaces.

Dynamics of desorption induced by electronic transitions: Desorption of neutrals from physisorbed layers

The dynamical theory of desorption from physisorbed layers irradiated by electrons or photons is presented. The basic steps of the microscopic mechanism are: ionization to a more strongly bound state, acceleration towards the surface, reneutralization, and finally, desorption. Desorption yields and the kinetic energy distributions of desorbing neutrals obtained in the classical and in the fully quantum-mechanical approach are numerically compared. Comparison with the existing experimental data is also made. Next, the classical trajectory approximation is applied to study the metal — adsorbate charge transfer process. The neutralization rate obtained is compared with the one following from the conventional perturbation theory.

Charge transfer adsorption in silicon vapor-phase epitaxial growth: A Ishitani et al, J appl Phys, 63, 1988, 390–394

Charge transfer adsorption in silicon vapor-phase epitaxial growth: A Ishitani et al, J appl Phys, 63, 1988, 390–394

Charge transfer adsorption in silicon vapor-phase epitaxial growth

Silicon vapor-phase epitaxial growth with SiH2Cl2 is theoretically studied. The optimized geometries and total energies of the species, generated from SiH2Cl2, are calculated by using an ab initio molecular orbital method. The charge transfer of the interaction between a silicon surface and SiCl2 is considered. Based on the computational result that SiCl−2 has the lower total energy that SiCl2, a new adsorption mechanism, named charge transfer adsorption, is proposed. By using this charge transfer adsorption followed by the surface reaction at the hollow bridge site, the epitaxial growths on the silicon (001), (111), and (110) surfaces are discussed. The epitaxial growths take place in different ways for these three surfaces because of the specific locations of the hollow bridge sites.

Charge Transfer and Surface Reaction in Silicon Vapor Phase Epitaxial Growth

ABSTRACTSilicon vapor phase epitaxial growth with SiH2Cl2 is theoretically studied. The optimized geometries and total energigs of the species, generated from SiH2Cl2, are calculated by using ab initio molecular orbital method. As the intgraction between silicon surface and SiCl2 the charge transfer is considered. Based on the computational result that SiCl2 - has the lower total energy than SiCl2, a new adsorption mechanism, named, charge transfer adsorption, is p~oposed. By using this charge transfer adsorption followed by the surface reaction at the hollow bridge site, the epitaxial growths on the silicon (001), (111), and (110) surfaces are discussed.

Theory of competitive adsorption and its application to the anodic dissolution of nickel and other iron-group metals—II. The steady state in the prepassive, passive and transpassive potential ranges

The theory developed in Part I has been extended to include the action of chemisorbed hydroxide. Equilibrium charge transfer adsorption is able to predict the negative reaction orders with respect to hydroxide ions which have been reported for solutions high in chloride and hydrogen ions, as well as the occurrence of a maximum and its dependence on the composition of the solution. It has been concluded that it is likely that passivation is in the first instance due to the chemisorption of hydroxide. If, on the other hand, hydroxide ions are assumed to have a catalytic action in metal dissolution, albeit with a very low anodic desorption rate, then the relevant equations are in addition able to predict passive and transpassive behaviour. Thus the steady state anodic current—potential relationship in the potential range between hydrogen evolution and oxygen evolution can in principle be expressed in one equation. The remarkable agreement between predicted and observed anodic behaviour makes it likely that competitive adsorption plays an important role in the passive range, even in the presence of an oxide film. Finally it has been investigated to what degree the theory can contribute to a better understanding of the behaviour of passive metals.

Flüssigchromatographische stoffklassentrennung höhersiedender aromaten mit hilfe von charge-transfer-komplex bildenden kompaktphasen

The outstanding suitability of the charge-transfer adsorption in the separation of compound classes by the formation of π—π complexes with higher boiling aromatic compounds of complicated petroleum hydrocarbons could be demonstrated. 2,4,6-Trinitrophenylamino groups bonded by propyl spacer bridges to spheric silica gel served as an preferred acceptor. On the basis of the previously introduced term for the compound class or group selectivity log σ G, the conditions for the separation of the aromatic compounds into subclasses as well as the influence of the eluent on the selectivity and on the number of theoretical plates are discussed. The behaviour of hetero compounds and the nature of relevant adsorptive interactions are dealt with. Various examples demonstrate that the chromatographic analysis of compound classes of higher hydrocarbons can supply important information about the behaviour of the mixtures under technological conditions.

Adsorption and absorption of diatomic gases by zirconium: Studies of the dissociation and diffusion of CO, NO, N 2, O 2 and D 2

The adsorption of D 2, CO, N 2, NO and O 2 on polycrystalline Zr has been studied at 300 K. The behaviour of D 2 differs in every respect from that of the other gases: continuous absorption occurs at 300 K apparently with some charge transfer to the metal from the deuterium. Work function, electron impact, Auger, and diffusion data indicate that adsorption of CO, N 2, NO and O 2 is almost entirely dissociative at 300 K; heating of these adsorbed phases leads to rapid surface → bulk diffusion and very little desorption. Coadsorption experiments tend to support the view that these adsorbates do not exist as simple overlayers at 300 K, but form instead a rather complex selvedge at the solid-vacuum interface in which adatoms penetrate and become intimately mixed with the metal lattice. Nitrogen seems to form an underlayer. Diffusion coefficients for surface → bulk transport of C, N and O are obtained over a range of temperatures, with the following results: D( C) = 3.6 × 10 −16 exp( −64000 RT ) m 2 s -1 ; D( N) = 2.2 × 10 −16 exp( 57000 RT ) m 2 s −1 ; D( O) = 7.4 × 10 −16 exp( −50400 RT ) m 2 s −1 with the activation energies quoted in joules. Very marked relative attenuation effects are observed in the valence band Zr Auger spectra upon adsorption of CO, N 2, NO and O 2. These effects cannot be accounted for in terms of conventional escape depth arguments: they are ascribed to extensive metal → adsorbate charge transfer which has its origins in the very electropositive nature of Zr.

Charge-transfer adsorption in aqueous media

Charge-transfer adsorption in aqueous media

Charge-transfer and water-mediated chromatography : II. Adsorption of nucleotides and related compounds on acriflavin-sephadex

Low-molecular-weight compounds are linearly adsorbed on acriflavin-Sephadex, and desorption by variation of pH, salt or temperature is therefore not necessary. The V E / V T values of the tested substances under the experimental conditions varied in the range 1–15, thus allowing good separations on moderate-sized columns. The adsorption capacity is surprisingly high, indicating the great potentiality for large-scale fractionation of oligonucleotides. Secondary cooperative adsorption effects can be controlled by temperature, pH, ionic strength and/or polarity of the eluent (which should increase the versatility of charge-transfer adsorption in nucleotide chemistry).

Explorations into the field of charge-transfer adsorption

Theoretical conditions for charge-transfer adsorption are discussed briefly. The adsorbents are synthesized by covalently attaching electron donors or acceptors to suitable matrices such as agarose and Sephadex. Charge-transfer chromatography can be used for fractionation of aromatic substances or heterocyclic compounds such as nucleotides. Metal chelate gels are also described. These are selective adsorbents for peptides and proteins.

Charge-transfer adsorption chromatography

Charge-transfer adsorption chromatography