Showing papers on "Chemisorption published in 2000"

Reaction of NO2 with Zn and ZnO: Photoemission, XANES, and Density Functional Studies on the Formation of NO3

Abstract: Synchrotron-based high-resolution photoemission and X-ray absorption near-edge spectroscopy (XANES) have been used to study the interaction of NO2 with polycrystalline surfaces of metallic zinc and zinc oxide. NO2 exhibits a complex chemistry on metallic zinc. After adsorbing nitrogen dioxide, N, O, NO, NO2, and NO3 are present on the surface of the metal. At room temperature the NO2 molecule mainly dissociates into O adatoms and gaseous NO, whereas at low temperatures (<250 K) chemisorbed NO2 and NO3 dominate on the surface. NO2 is a very good oxidizing agent for preparing ZnO from metallic zinc. Zn reacts more vigorously with NO2 than metals, such as Rh, Pd, or Pt which are typical catalysts for the removal of NOx molecules (DeNOx process). At 300 K, the main product of the reaction of NO2 with polycrystalline ZnO is adsorbed NO3 with little NO2 or NO present on the surface of the oxide. No evidence was found for the full decomposition of the NO2 molecule (i.e., no NO2 → N + 2O). The results of density ...

Density-functional study of bulk and surface properties of titanium nitride using different exchange-correlation functionals

Abstract: The adsorption and diffusion of hydrogen on the (100) surface of titanium nitride was studied using density-functional theory (DFT) and the generalized gradient approximation (GGA) for the exchange and correlation energy. The adsorption site was found to be on top of the titanium atom with the chemisorption energy of -2.88 eV. The diffusion barrier was determined as 0.73 eV along the path connecting the neighboring titanium atoms. The surface energies and surface relaxations of the three most important surfaces of TiN were studied. The surface energies have the following order: ${S}_{100}l{S}_{110}l{S}_{111}.$ Three different GGA functionals, the Perdew-Wang 1991 (PW91), the Perdew-Burke-Ernzerhof (PBE), and the revised PBE (RPBE) functionals, were tested on crystals, small molecules and TiN surfaces. The RPBE functional when applied to the surface studies of TiN was found to produce slightly lower values of surface energies and of hydrogen adsorption energies than the PW91 functional.

MnOx-CeO2 binary oxides for catalytic NOx sorption at low temperatures. Sorptive removal of NOx

Abstract: Interactions of nitrogen oxides (NOx) with (n)MnOx−(1 − n)CeO2 binary oxides were studied to use them for sorptive NOx removal at low temperatures (≃150 °C) The formation of MnOx−CeO2 solid solutions with the fluorite-type structure at n ≃ 05 was found to be quite effective in accelerating NOx sorption from flowing mixtures of 008% NO, 2% O2, and He balance (W/F = 024 g·s/cm3) The cumulative NOx uptake was increased by decreasing the reaction temperature and/or by increasing O2 concentration, indicative of chemisorption via oxidation of NO/NO2 In situ FT-IR diffuse reflectance spectrometry demonstrated that adsorption of NOx as bidentate, monodentate, and ionic nitrates is responsible for the large uptake These adsorbates are produced by oxidative adsorption, which is caused by NO oxidation to NO2 by lattice oxygens bound to Mn and subsequent coordination to Ce in adjacent surface sites XPS and O2-TPD studies suggested that the active site for NO oxidation should be related to the redox of Mn and

Real-time observation of adsorbate atom motion above a metal surface

Abstract: The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10–15 seconds) time resolution. Unusual changes in the surface electronic structure within 160 femtoseconds after excitation, observed by time-resolved two-photon photoemission spectroscopy, are attributed to atomic motion in a copper–cesium bond-breaking process. Describing the change in energy of the cesium antibonding state with a simple classical model provides information on the mechanical forces acting on cesium atoms that are “turned on” by photoexcitation. Within 160 femtoseconds, the copper–cesium bond extends by 0.35 angstrom from its equilibrium value.

A Comparative Theoretical Study of the Hydrogen, Methyl, and Ethyl Chemisorption on the Pt(111) Surface

Abstract: Chemisorbed hydrogen and various intermediate hydrocarbon fragments play an important role in the important reaction of ethylene hydrogenation to ethane, which is catalyzed by Pt(111). As a first step toward building a theoretical mechanism of the ethylene hydrogenation process, binding site preferences and geometries of chemisorbed hydrogen, methyl, and ethyl on the Pt(111) surface are presented and rationalized. State-of-the-art Pseudopotential Planewave Density Functional Theory is employed for calculating accurate binding energies and geometries for the adsorbates. A comprehensive theory of hydrogen and methyl chemisorption on Pt(111) is developed with the help of Crystal Orbital Hamilton Population formalism within the extended Huckel molecular orbital theory. The symmetry properties of the surface Pt orbitals as well as the mixing of Pt s, p, and d orbitals in pure Pt is shown to be crucial in determining the strength of subsequent interaction with an adsorbate. It is suggested that hydrogen moves f...

A First Principles Analysis of C−H Bond Formation in Ethylene Hydrogenation

Abstract: The Horiuti−Polanyi mechanism for ethylene hydrogenation over Pd(111) is examined using first-principle density functional quantum chemical calculations. Cluster and periodic slab DFT-GGA calculations were carried out to determine the modes and energies of chemisorption for a sequence of proposed intermediates, along with overall reaction energies and activation barriers for each of the speculated elementary steps. The DFT-calculated binding energies for ethylene (π), ethylene (di-σ), ethyl, vinyl, ethylidyne, atomic oxygen, and atomic carbon on the Pd19 cluster (and the Pd(111) slab) were found to be −30 (−27), −60 (−62), −130 (−140), −237 (−254), −620 (−636), −375 (−400), and −610 (−635) kJ/mol. The slab results were found to be within 20 kJ/mol of the cluster results. Frequency calculations along with predicted chemisorption energies indicate that ethylene adsorbs in both π- and di-σ-configurations. At moderate temperatures, the binding energies for π- and di-σ-bound ethylene are comparable. At low sur...

Electrochemical studies of the adsorption behavior of serum proteins on titanium

Abstract: Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used to examine the adsorption behavior of bovine serum albumin (BSA) and bovine fibrinogen on titanium in phosphate buffer pH 7.4, over the temperature range 295−343 K. It was shown that the surface charge density is directly proportional to the amount of the adsorbed protein (surface concentration), thus indicating that the adsorption is accompanied by the transfer of charge, i.e. chemisorption. On the other hand, the resulting adsorption pseudocapacitance obtained under the potentiostatic conditions not only depends on the protein surface concentration but also is a very complex function of parameters that are, in turn, dependent on structural, physical, and chemical properties of the proteins. Both techniques were shown to be very sensitive to the conformational behavior of the proteins. The adsorption of BSA onto a Ti surface resulted in a bimodal isotherm at all the temperatures studied, while the adsorption of fibrinogen resul...

Field-dependent chemisorption of carbon monoxide and nitric oxide on platinum-group (111) surfaces: Quantum chemical calculations compared with infrared spectroscopy at electrochemical and vacuum-based interfaces

Abstract: Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru exhibit uniformly atop, and Pd hollow-site binding, throughout the experimentally accessible interfacial fields. These trends are analyzed with reference to metal d-band parameters by decomposing the field-dependent DFT binding energies into steric (electrostatic plus Pauli) repulsion, and donation and back-donation orbital components. The increasing tendency towards multifold CO coordination seen at more negative fields is due primarily to enhanced back-donation. The decreasing propensity for atop vs multifold CO binding seen in moving from the lower-left to the upper-right Periodic corner of the Pt-group elements is due to the combined effects of weaker donation, stronger back-donation, and weaker steric repulsion. The uniformly hollow-site binding seen for NO arises from markedly stronger back-donation and weaker donation than for CO. The metal-dependent zero-field DFT vibrational frequencies are in uniformly good agreement with experiment; a semiquantitative concordance is found between the DFT and experimental frequency-field (“Stark-tuning”) slopes. Decomposition of the DFT bond frequencies shows that the redshifts observed upon chemisorption are due to donation as well as back-donation interactions; the metal-dependent trends, however, are due to a combination of several factors. While the observed positive Stark-tuning slopes are due predominantly to field-dependent back-donation, their observed sensitivity to the binding site and metal again reflect the interplay of several interaction components.

QUANTUM CHEMISTRY OF OXIDE SURFACES: FROM CO CHEMISORPTION TO THE IDENTIFICATION OF THE STRUCTURE AND NATURE OF POINT DEFECTS ON MgO

Abstract: The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.

Structure-reactivity correlations for oxide-supported metal catalysts: new perspectives from STM

Abstract: Deposition of metals onto planar oxide supports provides a convenient methodology for modeling important aspects of supported metal catalysts. In this work, scanning tunneling microscopy (STM), in conjunction with traditional surface-science techniques, is used to monitor the morphological changes of oxide-supported metal clusters upon exposure to reactants at elevated pressures. Of special concern is the relationship between catalytic activity/selectivity and surface structure, e.g., metal–support interaction and intrinsic cluster size effects. Au and Ag clusters were vapor-deposited onto TiO2(110) under ultrahigh vacuum (UHV) conditions. Characterization of cluster size and density as a function of metal coverage is correlated with catalytic reactivity. Oxygen-induced cluster ripening occurs upon exposure of Au/TiO2(110) and Ag/TiO2(110) to 10.00 Torr O2. The morphology of the metal clustering induced by O2 exposure implies the chemisorption of O2 onto the metal clusters and the TiO2 substrate at room temperature. Ag and Au clusters exhibited a bimodal size distribution following O2 exposure due to Ostwald ripening, i.e., some clusters increased in size while other clusters shrank. A volatile oxide species is proposed to form at high oxygen pressures, accelerating intercluster atom transport. The oxide substrate was found to play a role in the kinetics of cluster ripening. STM shows that oxide-supported metal clusters are very reactive to O2 and that nanoclusters are particularly susceptible to adsorbate-induced restructuring.

In Situ FTIR Spectroscopic Studies of Adsorption of CO, SCN-, and Poly(o-phenylenediamine) on Electrodes of Nanometer Thin Films of Pt, Pd, and Rh: Abnormal Infrared Effects (AIREs)

Abstract: Pt, Pd, and Rh films of a few nanometers in thickness supported on glassy carbon (GC) and other substrates were prepared by electrochemical voltammetry. STM patterns illustrated that the prepared thin films are composed of crystallites of layer structure and exhibit a low surface roughness. Studies of in situ FTIR spectroscopy on chemisorption of CO and SCN- and formation of a polymer of o-phenylenediamine (POPD) on electrodes of nanometer thin films have been conducted to explore the abnormal infrared effects (AIREs), which consist of two main characteristics: (1) inversion of IR bands; (2) the enhancement of IR absorption of adsorbates. The results demonstrated that the AIREs depend mainly on the structure and the chemical nature of nanometer thin films. In all cases of chemisorption on thin films of platinum-group metals supported on GC or supported on polymer-covered GC, the direction of IR bands of adsorbates is inverted in comparison with the direction of IR bands of the same adsorbates on correspo...

Adsorption of NO on the TiO2(110) Surface: An Experimental and Theoretical Study

Abstract: The chemisorption properties of NO on the oxidized TiO2(110) surface have been investigated using both experimental and theoretical methods. The results of temperature-programmed desorption measurements indicate that for NO exposures less than 1.1 × 1014 molecules/cm2 NO adsorbs weakly and desorbs at ∼127 K. The thermal desorption kinetics are almost independent of the coverage of adsorbed NO molecules. The experimental activation energy for NO desorption from the nondefective TiO2(110) surface is 8.4 kcal/mol in the limit of zero coverage. Above a critical NO surface exposure of 5.5 × 1014 molecules/cm2, partial conversion of NO to N2O is observed yielding N2O desorption processes at ∼169 and ∼250 K. The weak interaction between the NO molecule and the TiO2(110) surface has been also revealed from first-principles calculations based on density functional theory and the pseudopotential method in which NO molecules are adsorbed at the in-plane Ti cation sites. These calculations employ slab geometry and pe...

Modification of Cu/ZnO/SiO2 catalysts by high temperature reduction

Abstract: In this overview results obtained in three laboratories on Cu/ZnO/SiO 2 catalysts prepared by homogeneous deposition–precipitation are summarised. It is concluded that the system adopts a layered ‘zincsilite’ structure. In this copper–zinc hydrosilicate the Cu and/or Zn cations are located in octahedral sites and in the interlayer. When copper or zinc is present in excess of the zincsilite ratio (Cu y + Zn 1− y : Si = 0.75) or at relatively high copper content ( y ≥ 0.40), copper–zinc and copper hydroxocompounds are formed, the latter yielding CuO after calcination. Cu/ZnO/SiO 2 shows increased activity in ester hydrogenolysis and methanol synthesis reactions as a function of reduction temperature, in contrast to the unpromoted catalyst. Inert treatment at 700 K significantly decreases activity, whereas subsequent reductive treatment restores the high activity mode. The copper metal surface area of the promoted catalyst increases modestly with reduction temperature when the catalyst is carefully evacuated preceding a N 2 O chemisorption measurement. Significantly lower surface areas result after prolonged evacuation at higher temperatures. It was shown using infrared spectroscopy that upon high temperature (700 K) reduction cationic copper sites are created. Investigation of 63 Cu/ 68 ZnO/SiO 2 by low energy ion scattering demonstrates that the surface of the catalyst reduced at 700 K is enriched in zinc. In the 600 K reduced catalyst the zinc enrichment was less prominent. It is proposed that the Cu–ZnO interface plays a crucial role in catalysing the reactions studied. It is not yet possible to decide between two models arriving at a situation where this interface is enlarged: (1) Reversible formation of flat epitaxial particles upon high temperature reduction from the fraction of the copper dissolved in the mixed oxide catalyst precursor phase. (2) Migration of partly reduced ZnO on top of copper.

Hydrolysis of the amorphous silica surface. II. Calculation of activation barriers and mechanisms

Abstract: Using a previously derived model of the dry, amorphous, hydrophilic SiO2 surface, the reactivity of generic defect sites on the surface with respect to water, and the local network rearrangement that accompanies hydrolysis at these sites, is investigated using cluster models. Ab initio methods are used to calculate reaction barriers and reaction pathways. Consequences of the various types of hydrolysis product found are discussed with reference to potential sites for polymer chemisorption on the hydrolyzed, amorphous SiO2 surface.