Showing papers on "Chemisorption published in 1999"

Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes

Abstract: Hydrogen adsorption on crystalline ropes of carbon single-walled nanotubes (SWNT) was found to exceed 8 wt.%, which is the highest capacity of any carbon material. Hydrogen is first adsorbed on the outer surfaces of the crystalline ropes. At pressures higher than about 40 bar at 80 K, however, a phase transition occurs where there is a separation of the individual SWNTs, and hydrogen is physisorbed on their exposed surfaces. The pressure of this phase transition provides a tube-tube cohesive energy for much of the material of 5 meV/C atom. This small cohesive energy is affected strongly by the quality of crystalline order in the ropes.

Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures

Abstract: UV−visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO3, and dispersed WOx species on ZrO2 surfaces. UV−visible absorption edge analysis, CO2 chemisorption, and Raman spectroscopic results show that three distinct regions of WOx coverage on ZrO2 supports appear with increasing WOx surface density: submonolayer region (0−4 W nm-2), polytungstate growth region (4−8 W nm-2), and polytungstate/crystalline WO3 coexistence region (>8 W nm-2). The structure and catalytic activity of WOx species on ZrO2 is controlled only by WOx surface density (W nm-2), irrespective of the WOx concentration, oxidation temperature, and ZrO2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WOx species that are well dispersed on the ZrO2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few a...

Zeolite structure and reactivity by combined quantum-chemical- classical calculations

Abstract: Proton-energy differences, ammonia adsorption, and D/H-exchange barriers for methane at selected isolated Bronsted sites in zeolites FAU, MFI, BEA, ERI, and CHA are studied by combined quantum-chemical−classical (QM/MM) calculations in an attempt to understand the factors that determine the reactivity at these Bronsted sites. The barrier of the D/H-exchange reaction for methane was found to correlate well with the calculated ammonia chemisorption energy, but even better with the O−Al−O angle of the free zeolite Bronsted site the reaction is taking place on, provided the Si−O−Al−O−Si moiety over which the reaction takes place is more or less collinear. The barrier is considerably higher if this collinearity is weaker, which may be explained by the necessity of costly backbone distortions to accommodate the geometrical requirements of the transition state. This is confirmed by similarly strong correlations with the O−Al−O angle change going from the free acid site to zeolite−ammonium ion bidentate structure...

Preparation and Optical Properties of Colloidal Gold Monolayers

Abstract: Colloidal gold adsorbed onto positively charged substrates forms a two-dimensional layer with a maximum coverage of 1 colloid/1000 nm2 as shown by optical absorption spectroscopy, small-angle X-ray scattering, and atomic force microscopy. Positively charged substrates are obtained by either chemisorption of end-group-functionalized silanes or by polycation adsorption. Bulk diffusion controls the gold adsorption; thus lower coverages can also be achieved. The optical spectra not only contain information on single colloids but also on their mutual interactions, as well as on the polymeric and solvent environments. Absorption spectra based on Mie and Maxwell−Garnett theory are calculated. The films remain stable on exchange of the external solvent. The local dielectric environment is shown to not only influence the position of the absorption band but also, more drastically, the intensity (by a factor of 2.5), in agreement with theoretical predictions (±15%). Particle aggregation induced by branched polycatio...

Development of a Structure‐Activity Relationship for Oil Field Corrosion Inhibitors

Abstract: Corrosion rate data for mild steel corrosion inhibitors in carbonated brine media were fitted to the Temkin adsorption isotherm. The fundamental constants of the Temkin adsorption isotherm, i.e., molecular interaction and adsorption equilibrium constants, were used to glean important information about the behavior of adsorbed corrosion inhibitors. Adsorption data were also used to calculate , , and , and the results demonstrated that some compounds are chemisorbed onto the electrode. Significantly, it has been found that chemisorption of corrosion inhibitors can yield very good film persistency (i.e.. corrosion protection even when the inhibitor is no longer present). Molecular modeling using the quantum mechanical program PCSpartan plus, along with a comparative analysis of adsorption data for a broad range of inhibitor molecules, has been used to derive a tentative structure/activity relationship for some oil field corrosion inhibitors. © 1999 The Electrochemical Society. All rights reserved.

Gold/Titania Interfaces and Their Role in Carbon Monoxide Oxidation

Abstract: The chemisorption properties of differently prepared model gold/titania interfaces have been compared with the aim of gaining a better understanding of the synergistic interplay between the constituents in gold/titania catalysts used in low-temperature CO oxidation. The structurally different gold/titania interfaces were prepared using various techniques, including wet chemical deposition (dip coating) and physical vapor deposition of TiO2 on flat and highly oriented Au(111)/mica films and immobilization of gold colloids on TiO2/Au(111)/mica films as well as on TiO2 powders. The low-temperature activity of small gold colloids anchored on films was corroborated by DRIFTS measurements. CO, CO2, and O2 adsorption/desorption studies were performed on the flat model catalysts with TDS, XPS, and ISS. All flat model systems did not show any significant CO adsorption. Oxygen desorption was evidenced by TDS. The adsorptive properties of powder model catalysts were investigated with DRIFTS, pulse thermal analysis, ...

Electron-Hole Pair Creation at Ag and Cu Surfaces by Adsorption of Atomic Hydrogen and Deuterium

Abstract: Hot electrons and holes created at Ag and Cu surfaces by adsorption of thermal hydrogen and deuterium atoms have been measured directly with ultrathin metal film Schottky diode detectors on Si(111). When the metal surface is exposed to these atoms, charge carriers are excited at the surface, travel ballistically toward the interface, and have been detected as a chemicurrent in the diode. The current decreases with increasing exposure and eventually reaches a constant value at the steady-state coverage. This is the first direct evidence of nonadiabatic energy dissipation during adsorption at transition metal surfaces.

Method for forming dielectric film of capacitor having different thicknesses partly

Abstract: The present invention discloses a method for forming a dielectric film having improved leakage current characteristics in a capacitor. A lower electrode having a surface and a rounded protruding portion is formed on a semiconductor substrate. The surface and the protruding portion define at least one concave area. A chemisorption layer is then formed on the surface and the rounded protruding portion by supplying a first reactant. Also, a physisorption layer is formed on the chemisorption layer from the first reactant. Next, a portion of the physisorption layer is removed and a portion of the physisorption layer is left on the concave area. Subsequently, the chemisorption layer and the portion of the physisorption layer on the concave area react with a second reactant to form a dielectric film on the surface of the lower electrode. The thickness of said dielectric film is greater on the concave area than on the protruding portion, thereby reducing leakage current.

Bond Activation at Monatomic Steps: NO Dissociation at Corrugated Ru(0001)

Abstract: The NO bond activation at a corrugated Ru(0001) surface is investigated using density functional theory. Monatomic steps in the Ru surface are found to offer completely new reaction pathways with highly reduced energy barriers compared to reaction at a flat surface. The calculated energy barriers are found to be dominated by final state effects. The favorable barriers at the step edges result from the attractive chemisorption potential energies of the noninteracting reaction products, atomic N and O, and from a minimal degree of intramolecular repulsion mediated through the substrate.

Organosulfur Monolayers at Gold Surfaces: Reexamination of the Case for Sulfide Adsorption and Implications to the Formation of Monolayers from Thiols and Disulfides

Abstract: This paper presents the results from a reexamination of the adsorption of organosulfides on gold films. Our original goal was to explore the adsorption of different organosulfur precursors (e.g., sulfides) as a basis for addressing details related to gold−sulfur interactions in thiol-derived monolayers. The results of our previous work (J. Am. Chem. Soc. 1994, 116, 11616) were, however, complicated by the purity of most organosulfides as well as by the competitive adsorption of thiol and disulfide impurities. By using preparative gas chromatographic purification, we were able to reduce significantly the levels of both types of impurities from the organosulfide precursors, as confirmed by mass spectrometric analyses. Spectroscopic (infrared and X-ray photoelectron) and electrochemical characterizations of adlayers formed on gold from these highly purified organosulfides have led us to reach new conclusions regarding our earlier work. First, the amount of adsorbed sulfide corresponds to only a small fractio...

CO2 reforming of methane over Ni–Ru and Ni–Pd bimetallic catalysts

Abstract: This paper reports the effect of noble metal addition (Ru, Pd) to supported Ni catalysts towards the reaction of CO2 reforming of methane. On the basis of FT-IR spectra of adsorbed CO, H2 chemisorption and TPR measurements it has been proposed that the different behaviour of the Ni-based bimetallic catalysts can be related to different metal–metal interactions occurring on the catalysts. In particular, the strong improvement in the activity and stability observed in the case of ex-nitrate Ni–Ru catalysts has been attributed to an enrichment of the catalyst surface in nickel due to the formation of Ni–Ru clusters with the surface mainly covered by Ni. This leads to an increase in the metallic dispersion of Ni and favours the formation of more reactive intermediate carbonaceous species.

The role of K2O in the selective reduction of NO with NH3 over a V2O5(WO3)/TiO2 commercial selective catalytic reduction catalyst

Abstract: To elucidate the nature of the acid sites of the V2O5(WO3)/TiO2 catalyst upon K2O addition and its relation to the selective reduction of NO with NH3, measurements were made by means of infrared and Raman spectroscopy, NH3 chemisorption, and NO reduction measurements as a function of the K2O loading. The catalytic activity was found to decrease rapidly with the K2O loading, irrespective of the similar textural properties of all samples. Addition of K2O modified the vanadium species on the catalyst surface. For large additions of K2O, the potassium partially reacted with V2O5 to form KVO3. The amount of NH3 chemisorbed on the catalyst was observed to decrease with both the loading of K2O and the temperature. The adsorption of NH3 on both Bronsted and Lewis acid sites was confirmed. The strength and the number of Bronsted acid sites decrease largely with the loading of K2O in parallel with the decrease of the SCR activity, suggesting that the SCR reaction involves NH3 adsorption on the Bronsted acid sires. At low surface coverage of NH3, the isosteric heat of NH3 chemisorption was determined to be 370 kJ/mol for 0 wt.% K2O addition. With increasing K2O amount, the heat of adsorption decreased and was 150 kJ/mol for the catalyst with higher amounts of K2O addition. The results obtained imply that potassium disturbs the formation of the active ammonia intermediates, NH4+, resulting in deactivation of the catalyst. (C) 1999 Elsevier Science B.V. All rights reserved.

The adiabatic molecule–metal surface interaction: Theoretical approaches

Abstract: Ab initio methods for calculating the adiabatic electronic properties of a single isolated molecule interacting with a metal surface are reviewed. First the fundamental approaches of Anderson, Grimley, and Newns for chemisorption, as well as of Zaremba and Kohn, for physisorption, are outlined. Then the density-functional theory and its approximations are considered. The different models for the adsorbate system are described. They comprise those in which the system has a finite volume---i.e., the cluster, the slab, and the supercell models---and those which take into account the semi-infinite nature of the substrate---i.e., the embedding approach based either on the Dyson equation or on Green's-function matching. Those definitions are also introduced that we deem important for the understanding of the physical properties of systems to be presented in this article. The lack of full screening in a localized region around the adsorbate, and hence the existence of long-range Friedel's oscillations induced by the adsorbate in the metal, are discussed. The way in which the lack of full screening influences the calculated adsorption energies is estimated by the grand-canonical functional. Recent ab initio results on physisorption of a noble-gas atom on metals deal mainly with the limits of validity of the simpler effective-medium theory and with the anticorrugating effect of He. Atomic chemisorption is considered in order to deal with the concept of bonding at a metal. Dissociative chemisorption calculations mainly treat the ${\mathrm{H}}_{2}$ metal system. Here both the adiabatic electronic properties and the sticking probabilities recently obtained using the ab initio potential-energy surfaces are analyzed. Carbon monoxide chemisorption, lateral interactions between adsorbates, adatom diffusion, and chemisorption on stepped surfaces are presented as prototypes of the large variety of ab initio results currently available. Finally, the conclusions are devoted to the respective merits of the different theoretical approaches and to some future directions.

Activation energy for oxygen chemisorption on carbon at low temperatures

Abstract: The kinetics of oxygen chemisorption on resin chars were investigated in this study. The Elovich equation was employed to facilitate the interpretation of the chemisorption process. It was found that the activation energy for chemisorption is not constant and varies with surface coverage. This observation can be explained by a distributed kinetic parameter model, here implemented as a distributed activation energy model. The energy distribution model reveals that the amount of mass uptake obtained in a typical chemisorption study does not involve full coverage of all of the active sites on carbon. It is concluded that the information on active sites obtained from low-temperature chemisorption cannot be directly applied to gasification at higher temperatures.

Analysis of the Reaction of Carbon with NO/N2O Using Ab Initio Molecular Orbital Theory

Abstract: An attempt was made to analyze the reaction of carbon with NO or N2O by using an ab initio molecular orbital theory. This method allows the simulation of the chemisorption process of these gas molecules on zigzag and armchair edge sites of carbon. It was found that NO adsorption with the N atom down is more thermally favorable than the adsorption with the O atom down, while the O down mode is more favorable than the N down one for N2O adsorption because the former process releases a stable N2 molecule to form a surface oxygen complex. The chemisorption of NO or N2O with its bond axis parallel to the edge line gave the most stable chemisorbed species. The presence of surface oxygen complexes (quinone-type carbonyl group) on the edge decreases the strength of some bonds in NO- and N2O-chemisorbed species and consequently lowers their thermal stability. Furthermore, the N2 formation process in the C−NO/N2O reaction was analyzed and possible N2 formation routes were proposed. The routes predicted by the molec...

Molecularly chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beam and electron energy-loss spectroscopy study

Abstract: High translational energy adsorption of oxygen on the (111) surface of platinum was examined with electron energy loss spectroscopy (EELS) and molecular beam techniques. EEL spectra indicate that over an incident energy range of 0.2–1.37 eV and on a Pt(111) surface held at 77 K, oxygen adsorbs in an associative chemisorbed state—yielding to the dissociated state only after sufficient substrate heating. Simple direct dissociation appears negligible for all incident kinetic energies studied. At near-zero surface coverages, exclusive population of the peroxolike molecular precursor is observed for adsorption at these high translational energies, while both superoxolike and peroxolike forms are detected for low energy adsorption (0.055 eV). This peculiarity represents evidence that translational energy is effective in differentially populating reaction intermediates and provides better quantification of potential energy barriers to dissociation. We estimate the activation barrier for dissociation from the peroxolike precursor to be approximately 0.29 eV. Initial adsorption probability measurements over a wide range of surface temperatures and high incident kinetic energies corroborate a molecular chemisorption mediated mechanism.

Carbon-supported ruthenium catalyst for the synthesis of ammonia. The effect of the carbon support and barium promoter on the performance

Abstract: The dispersion and activity of carbon-based ruthenium catalysts for ammonia synthesis were examined. Barium was used as a promoter. Parallel XRD and extensive chemisorption (H 2 , O 2 and additionally CO) studies of a series of unpromoted ruthenium/carbon materials showed that a more developed texture (porosity and surface area) of the carbon support results in a significant increase in the ruthenium dispersion. It was found that the presence of ultra small mesopores (diameters p =6.0 MPa, T =673 K, x NH 3 =8%, H 2 :N 2 =3:1) the reaction rate referred to the number of Ru surface atoms (TOF) was by an order of magnitude higher than that of a commercial fused iron catalyst. The TOF values are about constant for large Ru crystallites (diameters>3 nm) but they decrease, very likely, for smaller ones.