Showing papers on "Chemisorption published in 1990"

Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films

Abstract: An empirical many-body potential-energy expression is developed for hydrocarbons that can model intramolecular chemical bonding in a variety of small hydrocarbon molecules as well as graphite and diamond lattices. The potential function is based on Tersoff's covalent-bonding formalism with additional terms that correct for an inherent overbinding of radicals and that include nonlocal effects. Atomization energies for a wide range of hydrocarbon molecules predicted by the potential compare well to experimental values. The potential correctly predicts that the \ensuremath{\pi}-bonded chain reconstruction is the most stable reconstruction on the diamond {111} surface, and that hydrogen adsorption on a bulk-terminated surface is more stable than the reconstruction. Predicted energetics for the dimer reconstructed diamond {100} surface as well as hydrogen abstraction and chemisorption of small molecules on the diamond {111} surface are also given. The potential function is short ranged and quickly evaluated so it should be very useful for large-scale molecular-dynamics simulations of reacting hydrocarbon molecules.

Novel mechanism for the formation of chemisorption phases: the (2×1)O-Cu(110) added-row reconstruction

Abstract: Scanning tunneling microscopy investigations on nucleation and growth of the (2\ifmmode\times\else\texttimes\fi{}1)O-Cu(110) structure revealed that the new phase is formed by condensation of mobile chemisorbed O atoms with Cu adatoms evaporating from steps and diffusing across the terraces of the substrate surface. This process can be regarded as two-dimensional precipitation from a dilute mixed fluid into a solid phase rather than a solid-solid transformation. The resulting structure is more appropriately described to be of ``added-row'' rather than of ``missing-row'' type.

Chemisorption on metal surfaces

Abstract: Presents a coherent summary of the current understanding of chemisorption phenomena, stressing theoretical concepts and developments rather than experimental techniques. The rapid experimental development of surface science requires a more systematic approach to the explosion of chemisorption data. To this end, the common characteristics of chemisorption systems are first reviewed before the differences are discussed and trends identified, in a search for physical concepts that can systematise the observational data and perhaps make it possible to predict the behaviour of systems that have not yet been studied. The major topics discussed are the adiabatic potential energy surface, the electronic structure problem, the Newns-Anderson model, atomic and molecular chemisorption, and reactions and heterogeneous catalysis. A comprehensive review of experimental results is not attempted within the concept-oriented approach of this study. It is shown that simple models are able to describe semi-quantitatively the chemisorption bond for simple gas atoms, and that there is some understanding of the surface and adsorbate parameters that determine important experimental observables such as the chemisorption energy, bond lengths, and vibrational frequencies. For molecular chemisorption and the dissociation of molecules on metal surfaces the understanding is less well developed, but there is some qualitative understanding of a number of trends.

Surface reconstruction of Cu(110) induced by oxygen chemisorption.

Abstract: The dynamics of the reconstruction of the Cu(110) surface induced by oxygen chemisorption has been studied by scanning tunneling microscopy. The nucleation and growth of the CU(110)-(2\ifmmode\times\else\texttimes\fi{}1)O reconstructed phase shows up as ``added rows'' of Cu-O atoms which grow preferentially in the [001] direction. The Cu atoms are supplied by diffusion from terrace edges.

Metal crystallinity effects in electrocatalysis as probed by real-time FTIR spectroscopy: electrooxidation of formic acid, methanol, and ethanol on ordered low-index platinum surfaces

Abstract: The electrooxidation kinetics of 0.05-0.25 M formic acid, methanol, and ethanol in 0.1 M HClO{sub 4} on ordered Pt(111), Pt(100), and Pt(110) surfaces were examined by means of FTIR spectra obtained during slow (2-10 mV s{sup {minus}1}) thin-layer potential sweeps. This procedure enables the potential-dependent CO coverage, {theta}{sub CO}, formed by dissociative reactant chemisorption to be followed in real time in relation to the appearance of CO{sub 2} and partial electrooxidation products as assayed quantitatively from their characteristic infrared bands. Terminally bonded (i.e., on-top) CO is the major form of this adsorbate detected under most conditions from the characteristic C-O stretching frequencies at ca. 2,030-2,060 cm{sup {minus}1}, although bridging CO was also observed in some cases. For formic acid electrooxidation, high CO coverages ({theta}{sub CO} {approx lt} 0.7) are formed on Pt(100) and Pt(110) that inhibit severely the reaction at low overpotentials during the positive-going sweep.

Adsorption and Desorption

Abstract: In this chapter models of adsorption processes are reviewed. Both adsorption from the gas phase and from the liquid phase onto a solid are discussed, but adsorption from the gas phase is emphasized. Much of the material in earlier chapters is obviously pertinent background information for the topic of adsorption processes. In Section 5.2.4 the various degrees of interaction of gases at a solid surface, physical adsorption (physisorption), chemisorption, and finally the development of a new phase were described. In the remainder of Chapter 5, the forms of bonding of an adsorbate to the surface were reviewed. This chapter will build on these concepts, moving on to discussions of amount adsorbed, the kinetics of adsorption/desorption, and electron transfer during adsorption (ionosorption).

Recoil effects in surface dissociation

Abstract: Using real‐time wave packet propagation we consider the effects of lattice recoil, inelasticity and surface temperature in strongly activated dissociation reactions of diatomic molecules at surfaces. The energy diagram governing the dissociation, modeled as suggested by electronic structure calculations for H2 dissociation at Cu surfaces, consisted of an entrance channel barrier separated from the chemisorption region by a ridge, where dissociation takes place. Lattice recoil is simulated by coupling this ‘‘stiff‐barrier’’ PES to a harmonic oscillator. Calculations were carried out for masses and potential parameters appropriate to H2/D2 dissociation on Cu and N2 dissociation on Fe. Barrier recoil was found to suppress the dissociation probability as compared with its stiff‐barrier value. The effect, marginal for H2 and D2 but pronounced in the case of N2, can be understood in terms of dynamical increases in the barrier width and height. Simulations where the N2–Fe barrier was excited in the initial state...

The Bond-Order Conservation Approach to Chemisorption and Heterogeneous Catalysis: Applications and Implications

Abstract: Publisher Summary For chemisorption phenomena on transition metal surfaces, including surface reactivity, the bond-order conservation-Morse potential (BOC-MP) model appears to provide such a framework in which the intricacies of real phenomena are coherently interrelated. The BOC-MP model is a simple, truly “back-of-the-envelope’’ model that can be directly used by practitioners in the field. It efficiently describes and interrelates a wide variety of chemisorption phenomena. Most important, the model maps out metal surface reactions providing insight into both regularities and details. In principle, any metal surface reaction can be treated this way. The only requirement is to retain the rigor and simplicity of the model projections. This analytic model proved to be efficient for treating energetics of atomic and diatomic adsorbates on transition metal surfaces, particularly, the heat of chemisorption and activation barriers for dissociation and recombination. Recently, the BOC-MP model has been extended to treat energetics of polyatomic adsorbates as well, which made it possible to analyze mechanisms of catalytic heterogeneous reactions of practical importance. The chapter surveys major applications and implications of BOC-MP modeling to chemisorption and heterogeneous catalysis.

Theoretical Aspects of Heterogeneous Catalysis

Abstract: 1 A Theoretical View and Approach to the Physics and Chemistry of Zeolites and Molecular Sieves.- Quantum Mechanical Description of Al-Site-Related Properties in Zeolites.- Statistical Mechanics and Molecular Dynamics Simulations Applied to a Water-Ferrierite System.- 2 Conceptual Background for the Conversion of Hydrocarbons on Heterogeneous Acid Catalysts.- Nomenclature and Representation of Alkylcarbenium and Alkylcarbonium Ions Relevant to Alkane Conversion.- Relative Stabilities of Alkylcarbenium and Alkylcarbonium Ions.- Rearrangements of Acyclic Alkylcarbenium Ions in Superacids.- Alkylcarbenium Ions on the Surface of Heterogeneous Catalysts.- Carbocation Chemistry and Bifunctional Conversion of Short-Chain Alkanes.- Carbocation Chemistry and Bifunctional Conversion of Long-Chain Alkanes.- General Conclusions.- 3 The Role of Next Nearest Neighbors in Zeolite Acidity and Activity.- The Aluminosilicate Active Site.- Buffered Behavior and Zeolite Structure.- Stability Criteria and the Next-Nearest-Neighbor Model.- Paraffin Cracking Activity.- Cumene Cracking Activity.- Summary and Conclusions.- 4 Electronegativity Equalization, Solid-State Chemistry, and Molecular Interactions.- Electronegativity of an Atom in a Molecule.- Explicit Expression for the Effective Electronegativity.- The Electronegativity Equalization Method (EEM).- The Solid State.- Molecular Interactions.- Framework: Intrinsic Properties.- Framework: Molecular Interactions.- Bronsted Acidity: Intrinsic Properties.- Bronsted Acidity: Molecular Interactions.- The Active Site.- 5 Quantum-Chemical Studies of Zeolites.- Zeolite Models and Quantum-Chemical Methods.- Zeolite Properties.- Interaction of Zeolite Sites with Molecules.- Conclusions.- 6 Theoretical Studies of Transition Metal Sulfide Hydrodesulfurization Catalysts Suzanne Harris.- Periodic Trends in the HDS Activity of Simple TMS.- Promoted TMS Catalysts.- Electronic Structure of the Simple and Promoted TMS.- Structural Effects in Layered TMS.- Active Site Theory.- Conclusions.- 7 Factors Affecting the Reactivity of Organic Model Compounds in Hydrotreating Reactions.- General Features.- Reactivity of Aromatic Molecules.- Reactivity of Saturated Molecules.- Theoretical Modeling of the Reactivity.- Conclusions.- 8 Theoretical Investigation of Metal-Support Interactions and Their Influence on Chemisorption.- Experimental Investigations of Geometric Structures and Electronic Properties of Metal-Support Systems: Support Influence on Adsorbates.- The Support Influence on Chemisorption.- Surface Modeling and Computational Methods.- Metal-Support Interfaces and Support Influence on Chemisorption: Recent Theoretical Results.- Conclusions.- Note Added in Proof.- 9 Mechanisms and Intermediates of Metal Surface Reactions: Bond-Order Conservation Viewpoint.- The BOC-MP Model of Chemisorption.- Mapping of Surface Reactions.- Concluding Remarks.- 10 Structure and Electronic Factors in Heterogeneous Catalysis: C?C, C?O, and C-H Activation Processes on Metals and Oxides.- Atom Superposition and Electron Delocalization Molecular Orbital (ASED-MO) Approach.- Studies in Bond Activation.- Acetylene Bonding to Transition Metals: Structure and Electronic Effects.- CO Binding to Transition Metal Surfaces: A New Interaction.- CO Binding to ZnO and Surface Ion Relaxations.- CH Activation in Alkanes and Alkenes.- Conclusions.- 11 Application of Band-Structure Calculations to Chemisorption.- Method of Calculation.- Atomic Adsorbates.- Molecular Stereochemistry.- CO Adsorption.- Saturated Hydrocarbons.- Ethylene Adsorption.- Chemisorbed Butadiene.- Concluding Remarks.- 12 Quantum-Chemical Studies of the Acidity and Basicity of Alumina.- Acid-Base Properties of ?-Alumina.- Structure of Acidic and Basic Sites.- Models for Molecular Orbital Calculations of Alumina.- Types of Surface Hydroxyls.- Bronsted Acidity.- Basicity.- Lewis Acidity.- Conclusions.

Molecular beam studies of H2 and D2 dissociative chemisorption on Pt(111)

Abstract: Molecular beam techniques have been utilized to measure the dissociative chemisorption probability at zero surface coverage S0 for D2(H2) on Pt(111) as a function of initial energy Ei, angle of incidence θi, surface temperature Ts, isotopic mass and nozzle temperature Tn. S0 shows a large increase with translational energy, but no threshold in Ei, a peaking at θi=0°, and an independence with Ts, isotope and Tn. These results are interpreted in terms of direct dissociative chemisorption on the Pt(111) terraces. The dynamical picture that emerges is that although there is no significant barrier to dissociation along the minimum energy path, barriers do exist along nonoptimal reactive trajectories. Thus, the ‘‘translational activation’’ and other dynamical observations are intimately related to the multidimensional aspects of the dissociative potential energy surface. Some aspects of the dissociative chemisorption, however, still seem somewhat surprising within this general description.

Modern concepts in surface science and heterogeneous catalysis

Abstract: Surface science studies using small-area single crystals and a combination of electron, ion, photon, and molecular beam scattering techniques have been exploring surface properties on the molecular level. Many new phenomena were discovered that could be used to recast the models or concepts we employ to describe surfaces. The surface structure exhibits relaxation, reconstruction, and the presence of steps and kinks on the atomic scale. Chemisorption causes adsorbate-induced restructuring of surfaces, and the substrate has a significant influence on the growth mode of the deposited material (epitaxy). The surface chemical bond is cluster like, thermal activation is needed for chemical bond breaking, and rough, more open surfaces are markedly more reactive than flat surfaces with close atomic packing. The adsorbate-adsorbate interaction that may be repulsive or attractive induces weakening of the adsorbate-substrate bond and ordering in the surface monolayer. Surface dynamics studies reveal low potential energy barriers for the diffusion of molecules along the surface (two-dimensional phase approximation) and rapid energy transfer between incident gas and surface atoms. Catalyzed surface reactions may be surface structure sensitive or structure insensitive and coadsorbed promoter atoms act by altering the structure and/or the bonding of adsorbed molecules.

Chemisorption of carbon monoxide, hydrogen, and oxygen on ordered tin/platinum(111) surface alloys

Abstract: The chemisorption of CO, H{sub 2}, and O{sub 2} on ordered Pt-Sn surface alloys has been investigated by using thermal desorption mass spectroscopy (TDMS), high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), Auger electron (AES), X-ray photoelectron (XPS) and ultraviolet photoelectron spectroscopy (UPS), and low-energy ion scattering spectroscopy (LEISS). Alloying with Sn causes only slight decreases in the CO desorption peak temperature compared to Pt(111). A (2{radical}3{times}2{radical}3)R30{degree} LEED pattern is observed for the saturation CO coverage on the p(2{times}2) ordered alloy surface. No ordered CO overlays were observed by LEED from the ({radical}3{times}{radical}3)R30{degree} alloy surface. HREELS indicates the atop-bonded CO is the most strongly adsorbed species on both ordered alloy surfaces, as on Pt(111), and that both atop and bridge sites are populated for saturation CO coverage on both ordered alloy surfaces.

Characterization of acidity in ZSM-5 zeolites: an x-ray photoelectron and IR spectroscopy study

Abstract: An x-ray photoelectron spectroscopic (XPS) method is proposed for the identification and quantitation of Broensted and Lewis acid sites in ZSM-5 zeolites. The method consists of deconvoluting the N{sub 1s}XPS level of chemisorbed pyridine and measuring the relative intensities of the peak components. It was found that pyridine is chemisorbed in three different states on ZSM-5 zeolites corresponding to N{sub 1s} binding energy of 398.7, 400.0, and 401.8 eV, respectively. The first peak at 398.7 eV was assigned to N{sub 1s} level of pyridine adsorbed on Lewis sites, while the second and third were assigned to N{sub 1s} levels of pyridine adsorbed on relatively weak and strong Broensted acid sites, respectively. Comparison of the concentrations of the various acid sites as determined from the relative intensities of the N{sub 1s} components with IR spectroscopic data showed that XPS has potential applications in the identification and the quantitative determination of Broensted and Lewis acid sites in zeolites.

On the nature of dense CO adlayers

Abstract: We have performed Monte Carlo simulations in order to study the ordered structures formed by CO on Pt{111} at high coverage The results are compared with LEED and infrared (IR) spectra The calculations are based on a recently constructed potential energy surface for CO on Pt{111} and a CO–CO interaction potential deduced from the variation of the CO binding energy with coverage Ordered adsorbate structures are obtained at θ=05, 06, 067, and 071 in the simulations The so‐called compression structures (θ>05) are stabilized by the energy lowering which results when CO molecules at the high density domain walls move away from the on‐top sites because of the unbalanced repulsive CO–CO interactions If this relaxation channel is blocked, disordered adsorbate structures occur We present the resulting (θ, T) phase diagram and discuss its qualitative properties The LEED data show ordered structures at θ=05, 06, and 071, but, in contrast to previous results, or perhaps to the interpretation thereof, not at θ=067 The IR data show that the compression structures still consist of CO molecules adsorbed on distinct surface sites Finally, we discuss the changes in adsorbate structures which would result from variations in the CO–substrate potential energy surface and, in the light of these results, briefly look at the Cu{111}–CO, Ni{111}–CO, and Pd{111}–CO chemisorption systems

Influence of adsorbed carbon monoxide on the electrocatalytic oxidation of simple organic molecules at platinum and palladium electrodes in acidic solution : a survey using real-time FTIR spectroscopy

Abstract: The influence of adsorbed CO formed by dissociative chemisorption on the electrooxidation pathways and kinetics of 22 alcohols and aldehydes on polycrystalline platinum and palladium electrodes in relation to the adsorbed CO formed by dissociative chemisorption has been examined systematically by means of real-time sequences of FTIR spectra obtained during slow (2 mV s{sup {minus}1}) voltammetric sweeps in 0.1 M HClO{sub 4}. The overall objective is to explore the reactant structure-dependent role that adsorbed CO (and other CO{sub 2}-producing species) plays in the organic electrooxidation. The reactants examined include short-chain primary and secondary alcohols, aldehydes, and small bifunctional molecules (e.g., ethylene glycol) containing alcohol, aldehyde, and/or carboxylic acid units.

Decomposition mechanisms of SiHx species on Si(100)‐(2×1) for x=2, 3, and 4

Abstract: Silane adsorption at a surface temperature of 150 K and the surface decomposition of SiH3 and SiH2 have been investigated on the Si(100)‐(2×1) surface using static secondary ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). Silane dissociatively chemisorbs at 150 K to form SiH3 and H. At saturation, the combined coverage of these two is approximately 0.4 groups/1st layer Si atom (0.2 SiH4 adsorbed/1st layer Si atom). Using SiH4, the surface coverage of SiH3 species is varied, and the coverage‐dependant kinetics of SiH3 decomposition are examined using temperature programmed SSIMS. Changes in SiH4 exposure and source of SiH3 (di‐ vs monosilane) cause changes in surface SiH3 stability. The stability changes are interpreted as due to blocking of empty sites (dangling bonds, db) required for SiH3 decomposition to SiH2 and H. It is shown here that the decomposition temperature of SiH3 can vary from 200 to 600 K, depending on the dangling bond coverage (θdb). Subsequently, evidence for ...

Theory of the oxygen-induced restructuring of Cu(110) and Cu(100) surfaces

Abstract: A model calculation based on the effective-medium theory of the oxygen-induced reconstruction of the (110) and (100) surfaces of Cu is presented. Equilibrium structures are calculated from a minimization of the total energy of the system. Missing-row-type reconstructions are found to be most stable in both cases, and an analysis is presented, showing what the driving force is behind these reconstructions.

Chemisorption of high coverages of atomic oxygen on the Pt(111), Pd(111), and Au(111) surfaces

Abstract: Studies of the interaction of oxygen with some transition metal surfaces are limited by activation barriers to adsorption and reaction of O2. These barriers can be surmounted by using oxidants more powerful than O2 to chemisorb oxygen on these surfaces under ultrahigh vacuum (UHV) conditions, but at high ‘‘effective’’ pressures of O2. We report here on the use of very reactive molecules, such as nitrogen dioxide (NO2) and ozone (O3) to study the interaction of high concentrations of oxygen atoms on the Pt(111), Pd(111), and Au(111) single crystal surfaces. Chemisorbed oxygen adatom coverages of Θ0≤0.75 monolayer (ML) on Pt(111), Θ0≤1.4 ML on Pd(111), and Θ0≤0.80 ML on Au(111) surfaces have been characterized. On the Pd(111) surface, higher concentrations (Θ0≤3.1 ML) can be formed and the initial stages of oxidation, including migration of oxygen into the subsurface region and formation of Pd oxide can be studied. This method of cleanly forming high coverages of oxygen on metal surfaces allows for new insi...

Dynamics of the chemisorption of N2 on W(100): Precursor‐mediated and activated dissociation

Abstract: The dissociative chemisorption probability of N2 on W(100) is found to proceed by way of two dynamically distinct channels. At low kinetic energies Ei, dissociation proceeds primarily by way of a precursor‐mediated process, where the dissociation probability is found to fall with increasing Ei, reflecting the energy dependence of the trapping probability into this state. Dissociation at low energies is also strongly dependent on surface temperature Ts which effects the fraction of trapped species that desorb. For energies above about 0.45 eV, the dissociation probability is found to rise from a minimum of about 0.14 at Ts=800 K to over 0.45 at Ei=5 eV. Over this range we believe that kinetic energy enables the incident molecules to directly overcome a barrier in the reaction coordinate. Throughout the entire range of energies we observe only slight variations of the dissociation probability with the angle of incidence, with no discernible sensitivity for energies below ∼0.5 eV. For energies between 1 and ...

Decomposition of silane on Si(111)‐(7×7) and Si(100)‐(2×1) surfaces below 500 °C

Abstract: Using static secondary ion mass spectrometry (SIMS) to observe the silicon hydride species formed by silane adsorption on atomically clean single crystal silicon surfaces, two distinct adsorption mechanisms are identified. Dissociation to SiH3 plus H occurs on the Si(100)‐(2×1) surface, which contains pairs of dangling bonds located on Si dimers (with Si–Si distance ≊2.4 A). In contrast, SiH2 formation in the adsorption step is indicated on the Si(111)‐(7×7) surface, where adjacent dangling bonds are separated by more than 7 A. Lower limits on the silane reactive sticking coefficient (SR) are evaluated using hydrogen coverage (ΘH) measurements after calibrated SiH4 exposures, and this limit is ≊10−5 for 25 °C gas and 100–500 °C surface temperatures. Within experimental error, SR is the same for both mechanisms on the two clean surfaces (ΘH near zero). Dependence of SR on ΘH is reported at 400 °C for both surfaces, and differences appear as ΘH exceeds 0.1 H/Si. Silane adsorption is weakly activated on Si(1...

Infrared study of carbon monoxide hydrogenation over rhodium/ceria and rhodium/silica catalysts

Abstract: Carbon monoxide hydrogenation over ceria-promoted Rh or Rh/SiO{sub 2} catalysts was investigated by means of Fourier transform infrared spectroscopy. In the chemisorption of CO at room temperature, a new absorption band was observed near 1,725 or 1,696 cm{sup {minus}1} on a reduced 5% Rh-5% CeO{sub 2}/SiO{sub 2} or 5% Rh/CeO{sub 2} catalyst, respectively. This new absorption band was attributed to a C- and O-bonded species, located at the metal/support interface. The appearance of this species was related to the shift to lower temperatures of the peak ascribed to CO dissociation in the temperature-programmed desorption spectra after CO adsorption. A flow high-pressure IR cell allowed to study the surface species in situ during CO + H{sub 2} reaction and to follow the reactivity of these species toward pure H{sub 2}. The results suggest that the drop in methane formation may be related to a lower reactivity of surface carbon in the presence of the promoter.