Structural, electronic, and vibrational properties of acetylene on Pd(100) doped with Sn or Pb: A DFT cluster model study

Some structural, electronic, and vibrational properties of the acetylene (C₂H₂) molecule adsorbed at various sites on the Pd(100) surface doped with Sn or Pb are determined theoretically. The calculations were performed using the B3LYP hybrid density functional, and the Sn- or Pb-doped Pd(100) surfaces were represented by a cluster model approach. It is found that the geometry of the C₂H₂ molecule adsorbed in di- σ configurations is highly perturbed with respect to the structure of acetylene in the gas phase. By contrast, the geometry of acetylene adsorbed in π configurations on the doped surfaces shows a much smaller distortion. Apart from calculating the properties of the adsorbed C₂H₂ molecule, the effect of the dopants, i.e. Sn and Pb atoms, on these properties is established by comparing the properties of acetylene adsorbed on the Sn- or Pb-doped Pd(100) surfaces with its properties on the monometallic Pd(100) surface. The results indicate that the geometry of the adsorbed C₂H₂ molecule is similar on the doped and monometallic Pd(100) surfaces.     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.     

Extensive oxygen exchange between carbon dioxide and magnesium oxide surface

Temperature-programmed desorption study of C¹⁸O₂ adsorbed on MgO has revealed that the desorbed carbon dioxide is composed exclusively of C¹⁶O₂, indicating that the adsorbed CO₂ species roll over the MgO surface.     

Electronic, Energetic, and Geometric Properties of Methylene-Functionalized C60

Chemical functionalization of C₆₀ fullerene with one to six carbene (CH₂) molecule(s) has been investigated using density functional theory. We have found that the reaction is regioselective so that a CH₂ molecule prefers to be adsorbed atop a C–C bond which is shared between two hexagonal rings of the C₆₀, releasing energy of ?3.95 eV. Singly occupied molecular orbital (SOMO) of the CH₂ interacts with LUMO of the C₆₀ via a [2 + 1] cycloaddition reaction. Energy of the reaction and work function of the system are decreased by increasing the number of adsorbed CH₂ molecules. The HOMO/LUMO energy gap of C₆₀ is slightly changed and the electron emission from its surface is facilitated upon the functionalization.     

Site- and surface species-dependent propylene oxidation with molecular oxygen on gold surface

A complete fundamental understanding of propylene oxidation with molecular O₂ on Au surface is achieved, in which site-and surface species-dependent reaction behaviors are revealed.     

Photosensitization of Nanocrystalline TiO2 Electrode Modified with C60 Carboxylic Acid Derivatives

C₆₀ carboxylic acid derivatives can be readily adsorbed on the surface of nanocrystalline TiO₂ film. The C₆₀ carboxylic acids adsorbed on nanocrystalline TiO₂ films act as charge‐transfer sensitizer. The electron transport from TiO₂ to the C₆₀ derivatives results in the generation of the cathodic photocurrent. The short‐circuit photocurrent of a C₆₀ tetracarboxylic acid is 0.45 μA/cm² under 464 nm light illumination. The photoelectric behaviour of ITO electrodes modified by the same C₆₀ carboxylic acids is different from that of the modified TiO₂ electrodes, and shows anodic photocurrent.     

Effect of pH and Ionic Strength on the Mechanism of Association of Rodenticides with Natural Organic Components of Soil

The mechanisms of association of hydrophobic organic contaminants with natural organic matter (for example humic acid, HA) in soil and sediment are a major objective of environmental and geochemical research. This paper discusses a general model for studying the process of association between a series of rodenticides and humic acid, by use of a C₁₈ stationary phase. An approach based on extended Langmuir distribution isotherms was used to study the effect of bulk solvent pH and ionic strength (adjusted by addition of sodium cation) on the mechanism of HA–rodenticide binding. The results demonstrated that: (i) HA can be adsorbed on the surface of the C₁₈ phase; (ii) the rodenticides can be associated with HA adsorbed on the C₁₈ surface; and (iii) ionic strength and bulk solvent pH both modify the conformation of HA and thus its mechanism of association with the rodenticide molecules.     

Etudes par spectroscopie dans l'ultra-violet et le visible, du fullerène C60 en solution dans le n-hexane ou adsorbé sur des solides diélectriques

Ultraviolet and visible spectroscopy studies of the fullerene C₆₀ melted in n-hexane or adsorbed on dielectric solids. Experiments of spectroscopy have been carried out, between the far ultraviolet and the near infrared, with pure C₆₀ or containing endohedral complexes rare gas atom-C₆₀, melted in n-hexane, and also pure C₆₀ adsorbed on surfaces of solids.Several phenomena have been observed: 1. a charge transfer due to the influence of interactions between the molecules of C₆₀ and surfaces of solids, yielding new transitions observed in the far ultraviolet ; and interpreted as super excited states of Rydberg type; 1.1. red and blue shifts for some transitions of adsorbed C₆₀, interpreted on the basis of attractive, or turned repulsive, dispersion forces; 1.1.1. emission processes for several transitions of adsorbed C₆₀, explained on the basis of a resonant coupling between these transitions and modes of the surface plasmon induced by the collective excitations of the C₆₀ molecule; this kind of process might be drawn to the Faraday instability in acoustics.     

The influence of preadsorbed hydrogen and CO on adsorption of ethylene on the Pd(111) surface

The coadsorption of C₂H₄ with H₂ and CO on Pd(111) has been investigated at 300 and 330 K At 300 K two forms of adsorbed ethylene coexist on the surface in the presence of ethylene gas: a molecular form desorbing as C₂H₄ at 330 K and a dissociatively adsorbed form (giving only hydrogen in desorption spectra) which is stable both in vacuum and in hydrogen at 10^?8 Torr. The molecular form seems to be a precursor state for hydrogenation and for dissociative adsorption. Both processes are controlled by the amount of coadsorbed hydrogen which in turn is controlled by CO coverage.     

Surface Adsorbed Hydroxyl: A Double-Edged Sword in Electrochemical CO2 Reduction over Oxide-Derived Copper

Oxide-derived Cu (OD−Cu) featured with surface located sub-20 nm nanoparticles (NPs) created via surface structure reconstruction was developed for electrochemical CO₂ reduction (ECO₂RR). With surface adsorbed hydroxyls (OH_ad) identified during ECO₂RR, it is realized that OH_ad, sterically confined and adsorbed at OD−Cu by surface located sub-20 nm NPs, should be determinative to the multi-carbon (C₂) product selectivity. In situ spectral investigations and theoretical calculations reveal that OH_ad favors the adsorption of low-frequency *CO with weak C≡O bonds and strengthens the *CO binding at OD−Cu surface, promoting *CO dimerization and then selective C₂ production. However, excessive OH_ad would inhibit selective C₂ production by occupying active sites and facilitating competitive H₂ evolution. In a flow cell, stable C₂ production with high selectivity of ∼60 % at −200 mA cm⁻² could be achieved over OD−Cu, with adsorption of OH_ad well steered in the fast flowing electrolyte.     

Molecular orientation of phthalimide adsorbed on colloidal silver

Surface Enhanced Raman Scattering (SERS) obtained for phthalimide (PIMH) and potassium phthalimide (PIMK) are identified with the phthalimide anion adsorbed on the surface of colloidal silver. The molecular orientation on the surface is discussed in terms of the relative intensities of vibrational modes assigned to irreducible representations of the _C2ν point group.     

The role played by an iodine-containing promoter in the formation of active polycrystalline silver catalyst surface

The programmed temperature desorption method was used to study the interaction of oxygen with the surface of a polycrystalline silver catalyst promoted with iodine. Ethyl iodide almost did not interact with the unoxidized surface of silver. The adsorption of C₂H₅I on the oxidized catalyst surface resulted in the formation of two adsorbed iodine forms, silver iodide and iodine deeply dissolved in subsurface silver crystal lattice layers. The character of oxygen adsorption from the iodine-containing surface of the catalyst was determined by the amount and form of adsorbed iodine. In the presence of a iodine-containing promoter, the concentration of oxide-like oxygen sharply decreased, and the amount of strongly bound atomically adsorbed oxygen responsible for the selective transformation of ethylene glycol into glyoxal increased.     

Explaining Fullerene Dispersion by using Micellar Solutions

An effective computational strategy to describe the dispersion of C₆₀ by surfactants is presented. The influence of parameters such as surfactant concentration and molecular length on the final morphology of the system is explored to explain the experimental results and to understand the incorporation of C₆₀ inside micelles. Both neutral and charged amphiphilic molecules are simulated. The long‐discussed problem of the location of fullerenes in micelles is addressed and C₆₀ is found in the hydrocarbon‐chain region of the micelles. If the available hydrophobic space increases, C₆₀ is localized in the inner part of the micellar core. Short, charged amphiphilic stabilizers are more efficient at dispersing fullerenes monomolecularly. Two different phases of C₆₀ are observed as the C₆₀/surfactant ratio varies. In the first, aggregates of C₆₀ are entrapped inside the micelles, whereas, in the second, colloidal nanoC₆₀ is formed with surfactants adsorbed on the surface.     

Molecular rearrangement induced by hydrogen co-adsorption: C2H4 on Pd(110)

The symmetry of low-coverage (0.1 ML) ethylene adsorbed on Pd(110) and the co-adsorption effect of hydrogen have been investigated by high-resolution electron energy-loss spectroscopy. Ethylene adsorbs with π-bond character intact on both clean and H-covered Pd(110) with characteristic C–C stretching vibrational energies at 178 and 186 meV, respectively. The symmetry of the adsorbed ethylene, however, drastically changes upon the co-adsorption of hydrogen: the C–C axis which is tilted to the clean Pd(110) surface (C₁ symmetry) is rearranged such that it becomes parallel to the H-covered Pd(110) surface (C₂ symmetry).     

Adsorption of triallylamine on Si(111) and its coadsorption with triethylgallium – A combined HREELS and XPS study

The adsorption of triallylamine [(C₃H₅)₃N; TAA] on Si(111)-(7 × 7) under UHV conditions was studied by means of surface sensitive electron spectroscopy. The High-Resolution Electron Energy Loss Spectroscopy (HREELS) yields the spectrum of vibration modes of the adsorbed species. X-ray Photoelectron Spectroscopy (XPS) gives insight into the chemical environment and the relative concentrations in the near surface region. The tertiary amine TAA physisorbes at room temperature without dissociation. Successive annealing steps induce the dissociation of the physisorbed phase at temperatures above 400°C. Further annealing leads to partial desorption of the allyl groups from the surface. At temperatures above 600°C the remaining allyl groups are fully dissociated. Hydrogen leaves the surface and nitrogen and carbon start to diffuse into the substrate. The surface chemistry of triallylamine adsorbed on a heated substrate behaves in a very similar way. The coadsorption of TAA with triethylgallium [(C₂H₅)₃Ga; TEG] in the temperature range between 500 and 800°C induces no significant change of the surface reactions. Only a small amount of gallium could be detected at the surface. The nucleation of GaN has not been observed, neither on Si(111) nor on Al₂O₃(0001) substrates.     

Competitive hydrogenation in binary diene systems on a palladium catalyst

Summary Hydrogenations of individual C₅ dienic substrates (1,3-cyclopentadiene, isoprene, 1,4-pentadiene, cis-1,3-pentadiene, trans-1,3-pentadiene) and their binary mixtures on a heterogeneous palladium catalyst were studied in cyclohexane at 25°C and 2 MPa. Selectivities of the competitive hydrogenations were determined and the substrates relative adsorption coefficients calculated. Effects of the diene structure on their reactivity and the stability of the surface complex are discussed. It was found that differences in selectivity of the competitive hydrogenations of C₅ dienes are caused by the difference both in adsorptivity values and in reactivity of adsorbed molecules. The presence of a five-membered ring in the C₅ dienes leads, , in significant reduction of surface complex stability as compared with acyclic structures of C₅ dienes. On the other hand, it has a very positive effect on the rate of surface reaction.     

Static secondary ionization mass spectrometry analysis of tributyl phosphate on mineral surfaces: Effect of Fe(II)

The static secondary ionization mass spectrometry (SIMS) spectrum of tri-n-butyl phosphate (TBP) on a variety of basalt and quartz samples is affected by the chemical composition of the mineral surface. When TBP is adsorbed on Fe(II)-bearing surfaces, the compound undergoes concomitant H^? abstraction and reduction, followed by the elimination of two C₄H₈ molecules to form an ion at m/z 137⁺. When TBP is adsorbed to quartz or other nonreducing surfaces, it merely undergoes protonation and elimination of three C₄H₈ molecules to form H₄PO ₄ ⁺ . When TBP is adsorbed to Fe(III)-bearing surfaces, it undergoes H^? abstraction and elimination of two C₄H₈ molecules, to form an ion at m/z 153⁺. These conclusions are supported by model studies that employed FeO, Fe₂0₃, TBP, and tributyl phosphite. The results show that the SIMS spectrum is very sensitive to the mode of TBP adsorption on the mineral surface.     

The adsorption and decomposition of ethylene on Ni(100)

The adsorption behavior of ethylene on Ni(100) at a variety of temperatures has been studied using temperature programmed desorption, and X-ray and UV photoemission. The adsorption of ethylene at 98 K results in molecular adsorption with a saturation C/Ni ratio of 0.76. Heating this surface to any temperature between 213 and 683 K reduces the C/Ni ratio to 0.5. Exposure to ethylene at 300 K leads to decomposition producing surface carbide, adsorbed hydrogen atoms and an adsorbed C_xH_2x species. A comparison with other work on Ni(111) indicates that ethylene adsorption processes are structure sensitive.     

The surface adsorption and micelle formation of the mixed aqueous solutions of fluorocarbon and hydrocarbon surfactants: II. Sodium perfluorooctanoate-sodium decylsulfate system

The adsorption and micellar behavior of the mixed solutions of sodium perfluorooctanoate (7CFNa) and sodium decylsulfate (C₁₀SNa) have been studied at constant ion strength of 0.1m. The adsorption was calculated from the surface (and interfacial) tension-concentration curves by applying Gibbs equation. It was found that the cmc's of 7CFNa and C₁₀SNa are of nearly the same value (1.66 × 10⁻²m and 1.45 × 10⁻²m, respectively), but γ_cmc of 7CFNa solution is ∼23 mNm⁻¹, which is much lower than that of C₁₀SNa solution. This implies that 7CFNa would have a much higher surface activity than C₁₀SNa and be adsorbed preferentially; for instance, 7CFNa has a surface mole fraction of about 0.8 in the saturated adsorption layer of the 1:1 mixed solution. The mole fractions of 7CFNa at the surface are always greater than those in the bulk solutions. The adsorption at the n- heptane-aqueous solution interface is quite different from that at the air-solution surface. Here C₁₀SNa is preferentially adsorbed owing to “Mutual phobicity” between the HC-chain of n-heptane and the FC-chain of 7CFNa at the interface. All the cmc's obtained from the γ-log m relations of 7CFNa (or C₁₀SNa) in the mixed solutions have nearly the same value and the cmc-x curves show a positive deviation from the ideal case. This further indicates that in the mixed solutions of fluorocarbon and hydrocarbon surfactants no completely miscible micelle but essentially the individual micelle of each surfactant exist due to the “Mutual phobicity” between FC- and HC-chain in the micellization process.     

Analysis of dynamic surface tension of tetraethyleneglycol monooctyl ether at air/water interface

The dynamic surface tension of the aqueous solutions of tetraethyleneglycol monooctyl ether (C₈E₄), a nonionic surfactant, was measured at different concentrations and temperatures. Present data at 298.15 K clearly indicate that the mechanism of adsorption is purely diffusion controlled at low concentrations (0.1~0.4 mmol/kg), and there is a switchover in adsorption mechanism to the mixed diffusion-kinetic control at higher concentrations. The calculated activation energies increase with concentration, and thus, with surface density, but decrease with temperature. The magnitude of activation energy and its increase with surface density suggest that the barrier is due to the free surface site formation by overcoming mainly the attractive van der Waals forces between the chain of adsorbed C₈E₄ molecules.