Showing papers on "Chemisorption published in 2002"

Catalytic Role of Gold in Gold-Based Catalysts: A Density Functional Theory Study on the CO Oxidation on Gold

Abstract: Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO(2) has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O(2) dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O(2) dissociation on Au (O(2)-->2O(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O(2) on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O(2) can adsorb on the interface of Au/TiO(2) with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O(2) dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O(2)-->CO(2)+O, and CO+O-->CO(2); and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.

Surface Science: Foundations of Catalysis and Nanoscience

Abstract: Acknowledgements. Introduction. 1. Bulk and Surface Structure. 1.1. Clean Surface Structure. 1.2. Reconstruction and Adsorbate Structure. 1.3. Band Structure of Solids. 1.4. the Vibratins of Solids. 1.5. Summary of Important Concepts. 1.6. Frontiers and Challenges. 1.7. Further Reading. 1.8. Exercises. References. 2. Experimental Probes and Techniques. 2.1. Ultrahigh Vacuum. 2.2. Light and Electron Sources. 2.3. Molecular Beams. 2.4. Scanning Probe Techniques. 2.5. Low Energy Electron Diffraction (LEED). 2.6. Electron Spectroscopy. 2.7. Vibration Spectroscopy. 2.8. Other Surface Analytical Techniques. 2.9. Summary of Important Concepts. 2.10. Frontiers and Challenges. 2.11. Further Reading. 2.12. Exercises. References. 3. Chemisorption, Physisorption and Dynamics. 3.1. Types of Interactions. 3.2. Binding Sites and Diffusion. 3.3. physisorption. 3.4. Nondissociative Chemisorption. 3.5. Dissociative Chemisorption: H2 on a Simple Metal. 3.6. What Determines the Reactivity of Metals? 3.7. Atoms and Molecules Incident on a Surface. 3.8. Microscopic Reversibility in Ad/desorption Phenomena. 3.9. The Influence of Individual Degrees of Freedom on Adsorption and Desorption. 3.10. Translations, Corrugation, Surface Atom Motions. 3.11. Rotatins and Adsorption. 3.12. Vibrations and Adsorption. 3.13. Competitive Adsorptin and Collision Induced Processes. 3.14. Classificatin of Reactin Mechanisms. 3.15. Measurement of Sticking Coefficeints. 3.16. Summary of Important Concepts. 3.17. Frontiers and Challenges. 3.18. Further Reading. 3.19. Exercises. 4. Thermodynamics and Kinetics of Adsorption and Desporption. 4.1. Thermodynamics of Ad/desorption. 4.2. Adsorption Isotherms from Thermodynamics. 4.3. Lateral Interactions. 4.4. Rate of Desorption. 4.5. Kinetics of Adsorption. 4.6. Adsorption Isotherms from Kinetics. 4.7. Temperature Programmed Desorption (TPD). 4.8. Summary of Important Concepts. 4.9. Frontiers and Challenges. 4.10. Further Reading. 4.11. Exercises. References. 5. Liquid Interfaces. 5.1. Structure of the Liquid/Said Interface. 5.2. Surface Energy and Surface tension. 5.3. Liquid Films. 5.4. Langmuir Films. 5.5. Langmuir-Blodgett Films. 5.6. Self-assembled Monolayers (SAMs). 5.7. thermodynamics of Liquid Interfaces. 5.8. Electrified and Charged Interfaces. 5.9. Summary of Important Concepts. 5.10. Frontiers and Challenges. 5.11. Further Reading. 5.12. Exercises. References. 6. Heterogeneous Catalysis. 6.1. The Prominence of Heterogeneous Reactions. 6.2. Measurement of Surface Kinetics and reaction Mechanisms. 6.3. Haber-Bosch Process. 6.4. From Microscopic Kinetics to Catalysis. 6.5. Fischer-Tropsch Synthesis and Related Chemistry. 6.6. The Three-Way Automotive Catalyst. 6.7. Promoters. 6.8. Poisons. 6.9. Bimetallic and Bifuncionall Catalysts. 6.10. Rate Oscillations and Spatiotemporal Pattern Formation. 6.11. Sabatier Analysis and Optimal Catalysts Selection. 6.12. Summary of Important Concepts. 6.13. Frontiers and Challenges. 6.14. Further Reading. 6.15. Exercises. References. 7. Growth and Epitaxy. 7.1. Stress and Strain. 7.2. Types of Interfaces. 7.3. Surface Energy, Surface tension and Strain Energy. 7.4. Growth Modes. 7.5. Nucleation Theory. 7.6. Growth Away from equilibrium. 7.7. Techniques for Growing Layers. 7.8. catalytic Growth of Nanotubes and Nonwhites. 7.9. Etching. 7.10. Summary of Important Concepts. 7.11. Frontiers and Challenges. 7.12. Further Reading. 7.13. Exercises. References. 8. Laser and Nonthermal Chemistry: Photon and Electron Stimulated chemistry and Atom Manipulation. 8.1. Photon Excitation of Surfaces. 8.2. Mechanisms of electron and Photon Stimulated Processes. 8.3. Photon and electron Induced Chemistry at Surfaces. 8.4. Charge Transfer and Electrochemistry. 8.5. Tip Induced Process: Mechanisms of Atom Manipulation. 8.6. Summary of Important Concepts. 8.7. Frontiers and Challenges. 8.8. Further Reading. 8.9. Exercises. References. Appendix: Fundamental Constants and Conversion Factors. Appendix II: Abbreviations. Appendix III: Symbols. Appendix IV: Useful Mathematical Expressions. Index.

Catalytic oxidation of methane over CeO2-ZrO2 mixed oxide solid solution catalysts prepared via urea hydrolysis

Abstract: In this study, CeO 2 -ZrO 2 mixed oxide catalysts were prepared via urea hydrolysis and tested for methane oxidation. Highly uniform solid solution particles of ceria-zirconia were obtained under the conditions of this study. The incorporation of Zr into the CeO 2 lattice was found to promote the redox properties. The methane oxidation activity of the mixed oxides was found to be dependent on the Ce:Zr ratio, which relates to the degree of reducibility. It was postulated that the cubic phase, fluorite structure, which is mainly found in Ce 1 − x Zr x O 2 (where x 1 − x Zr x O 2 (where x >0.5). The catalytic activity decreased with an increasing Zr content. The mixed oxide catalyst, Ce 0.75 Zr 0.25 O 2 solid solution, was reported to exhibit the highest activity for methane oxidation. Kinetic studies of methane oxidation over such a mixed oxide catalyst (Ce 0.75 Zr 0.25 O 2 ) showed that the methane oxidation rates strongly depend on methane concentration, but only slightly on the oxygen concentrations. The Langmuir–Hinshelwood mechanism (oxygen dissociative chemisorption on the active sites and non-dissociative chemisorption of methane) can satisfactorily fit the experimental results obtained from the kinetic studies for this catalyst. The activation energy of methane oxidation is calculated based on this surface reaction mechanism as being 100.8 kJ/mol.

Structure and energetics of alkanethiol adsorption on the Au(111) surface

Abstract: The interaction of thiol molecules with the Au(111) surface was investigated with state-of-the-art first-principles methods. We report theoretical evidence for the existence of a physisorption precursor to chemisorption, in agreement with experiment. The origins of inconsistency in recent studies regarding the adsorption site, geometry, and energetics of CH3S on the Au(111) surface were also investigated. We show that the chemisorption site is between the hollow and bridge sites, with a large molecular tilting angle relative to the surface normal. The molecular structure of the overlayer is coverage dependent, with the molecular tilting angle increasing with decreasing coverage. Increasing chain length up to three carbon atoms affects both the chemisorption energetics and the tilt angle. The inconsistency of tilting angles, reported for the fcc site is found to be a consequence of multiple local minima. The ordered structure of thiol molecules at different coverages was also investigated, confirming the r...

Surface Modification of Colloidal Gold by Chemisorption of Alkanethiols in the Presence of a Nonionic Surfactant

Abstract: Surface modification of colloidal gold with 11-mercaptoundecanoic acid or 16-mercaptohexadecanoic acid was performed in the absence or in the presence of the nonionic surfactant polyoxyethylene (20) sorbitan monolaurate (Tween 20). The stability of the colloidal systems was assessed with optical absorption spectroscopy. The surface-modified nanoparticles were stable only within a narrow range of intermediate pH values when chemisorption of alkanethiols was performed in the absence of Tween 20. This was explained in terms of partial ionization of the surface carboxylic groups and charge neutralization at high pH values by counterions present in the buffer solutions. Formation of a physisorbed monolayer of Tween 20 onto the nanoparticles prior to chemisorption of alkanethiols resulted in surface-modified colloidal gold that was stable over a broader range of pH values. Parallel experiments demonstrated that self-assembled monolayers could form on flat substrates in the presence of Tween 20. Therefore, possi...

Atomic and molecular adsorption on Rh(111)

Abstract: A systematic study of the chemisorption of both atomic (H, O, N, S, C), molecular (N2, CO, NO), and radical (CH3, OH) species on Rh(111) has been performed Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using both PW91 and RPBE functionals, have been employed to determine preferred binding sites, detailed chemisorption structures, binding energies, and the effects of surface relaxation for each one of the considered species at a surface coverage of 025 ML The thermochemical results indicate the following order in the binding energies from the least to the most strongly bound: N2

Interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes

Abstract: Density functional theory has been used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the H2 molecule outside the nanotube are in the range 0.04–0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the...

Isopropanol oxidation by pure metal oxide catalysts: number of active surface sites and turnover frequencies

Abstract: The objective of the present study was to determine the number of active surface sites and their nature, redox or acidic, for bulk metal oxide catalysts using isopropanol as a chemical probe molecule. Isopropanol oxidation activity on the following metal oxides was investigated: MgO, CaO, SrO, BaO, Y 2 O 3 , La 2 O 3 , CeO 2 , TiO 2 , ZrO 2 , HfO 2 , V 2 O 5 , Nb 2 O 5 , Ta 2 O 5 , Cr 2 O 3 , MoO 3 , WO 3 , Mn 2 O 3 , Fe 2 O 3 , Co 3 O 4 , Rh 2 O 3 , NiO, PdO, PtO, CuO, Ag 2 O, Au 2 O 3 , ZnO, Al 2 O 3 , Ga 2 O 3 , In 2 O 3 , SiO 2 , SnO 2 and Bi 2 O 3 . On average, the number of active surface sites for isopropanol dissociative adsorption on these catalysts was ∼2–4 μmol/m 2 . The number of active surface sites enabled quantification of the turnover frequency (TOF) for these catalysts. The TOF values for the various pure metal oxides were normalized at 200 °C. The TOFs of catalysts showing redox activity vary by six-orders of magnitude (10 2 to 10 −4 s −1 ). For catalyst showing acidic activity, the TOFs varied by over eight-orders of magnitude (10 1 to 10 −7 s −1 ). The reaction products from isopropanol oxidation at low conversions reflected the nature of the active surface sites, redox or acidic, on these catalysts. Redox surface sites yield acetone and acidic surface sites yield propylene. Small amounts of isopropyl ether formation are sometimes also observed via bimolecular recombination of surface isopropoxide species on acidic surface sites. All catalysts with the exception of Fe 2 O 3 and TiO 2 , exhibited extremely high selectivity to either redox or acidic products. Except for the sharp decrease in TOFs towards redox products with increasing bulk M–O heats of formation at low −Δ H f , no correlations were found between the TOFs and bulk metal oxide properties (TPR–H 2 and −Δ H f ). However, an inverse relation was found between the TOFs (redox) and the surface isopropoxide intermediate decomposition temperature at low decomposition temperatures. At moderate and high decomposition temperatures, the TOFs (redox) were almost independent of the surface isopropoxide decomposition temperature. The selectivity of the metal oxide catalysts was found to be independent of the TOFs.

High-resolution XPS study of decanethiol on Au(111): Single sulfur-gold bonding interaction

Abstract: The chemisorption of 1-decanethiol on the Au(111) single crystal has been studied with synchrotron-based, high-resolution photoemission spectroscopy with molecular film prepared from both gas-phase dosing and solution immersion to vary surface coverage over a wider range. The structure of the molecular film, determined separately via low-energy electron diffraction, includes a c(23 × √3) stripe phase and c(3 × 2√3) saturated phases. Careful curve fitting of the S 2p2/3 core level reveals that there is only one sulfur species at a binding energy of 162.1 eV in the film and the spectrum of the S 2p core level does not vary with the surface coverage and existence temperature of the decanethiolate. This finding is inconsistent with the sulfur-pairing model proposed based on X-ray scattering and standing wave studies. Up to two C 1s core levels at 284.0 and 285.0 eV can be observed, depending on the surface coverage. Angle-resolved X-ray photoelectron spectroscopy measurements are utilized to provide a direct ...

Study of Surface Reactivity of Cobalt Oxides: Interaction with Methanol

Abstract: In this paper, the interaction between cobalt oxides (Co3O4 and CoO) and methanol is studied. CoO was obtained by heating under high-vacuum (HV) conditions and characterized by X-ray photoelectron spectroscopy (XPS). The Co3O4 powder sample was characterized by means of XPS, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray diffraction (XRD), and thermal analysis. The interaction between Co3O4 and methanol was studied both at atmospheric pressure (by means of DRIFT spectroscopy) and under HV conditions (by means of XPS and quadrupolar mass spectroscopy, QMS), whereas the chemisorption of methanol on the CoO surface was studied only under HV conditions. Methanol chemisorbs mainly molecularly on the cobalt oxide surfaces, the alcohol dissociation being more evident at higher temperatures. In the case of Co3O4, the formation of formate and polymers of formaldehyde is evident around 473−523 K, whereas under HV conditions, formaldehyde and several decomposition and fragmentation produc...

Hydrogen adsorption on sp 2 -bonded carbon: Influence of the local curvature

Abstract: The interaction of atomic hydrogen and low-energy hydrogen ions with ${\mathrm{sp}}^{2}$-bonded carbon is investigated on the surfaces of ${\mathrm{C}}_{60}$ multilayer films, single-walled carbon nanotubes, and graphite (0001). These three materials have been chosen to represent ${\mathrm{sp}}^{2}$-bonded carbon networks with different local curvatures and closed surfaces (i.e. no dangling bonds). Chemisorption of hydrogen on these surfaces reduces emission from photoemission features associated with the \ensuremath{\pi} electrons and leads to a lowering of the work function up to 1.3 eV. It is found that the energy barrier for hydrogen adsorption decreases with increasing local curvature of the carbon surface. Whereas in the case of ${\mathrm{C}}_{60}$ and single-walled carbon nanotubes, hydrogen adsorption can be achieved by exposure to atomic hydrogen, the hydrogen adsorption on graphite (0001) requires ${H}^{+}$ ions of low kinetic energy (\ensuremath{\sim}1 eV). On all three materials, the adsorption energy barrier is found to increase with coverage. Accordingly, hydrogen chemisorption saturates at coverages that depend on the local curvature of the sample and the form of hydrogen (i.e., atomic or ionic) used for the treatment.

Proton transfer reactions on semiconductor surfaces.

Abstract: The concept of proton affinity on semiconductor surfaces has been explored through an investigation of the chemistry of amines on the Ge(100)-2 × 1, Si(100)-2 × 1, and C(100)-2 × 1 surfaces. Multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, temperature programmed desorption (TPD), and density functional theory (DFT) calculations were used in the studies. We find that methylamine, dimethylamine, and trimethylamine undergo molecular chemisorption on the Ge(100)-2 × 1 surface through the formation of Ge−N dative bonds. In contrast, primary and secondary amines react on the Si(100)-2 × 1 surface via N−H dissociation. Since N−H dissociation of amines at semiconductor surfaces mimics a proton-transfer reaction, the difference in chemical reactivities of the Ge(100)-2 × 1 and Si(100)-2 × 1 surfaces toward N−H dissociation can be interpreted as a decrease of proton affinity down a group in the periodic table. The trend in proton affinities of the two surfaces is explained in terms o...

Studies into the Storage of Hydrogen in Carbon Nanofibers: Proposal of a Possible Reaction Mechanism

Abstract: Substantial levels of hydrogen, up to 6.5 wt %, have been stored in carbon nanofibers (CNFs) under conditions of 12 MPa pressure and ambient temperature. The magnitude of this result cannot be interpreted in terms of physisorption on the external surface alone. Kinetic studies indicate that a slow chemisorption process is involved. The rate of uptake corresponds to that of hydrogen dissociation on graphite edge sites. Such a finding proposes a novel mechanism, offering a plausible explanation for these unusually high experimental observations. This involves the initial dissociation of hydrogen, believed to be catalyzed by carbon edge sites, which constitute the majority of the nanofiber surface, a property which is probably an important contributory factor toward their high hydrogen storage capacities.

Metal nanoparticles as models of single crystal surfaces and supported catalysts: Density functional study of size effects for CO/Pd(111)

Abstract: Large octahedral and cuboctahedral palladium clusters, ranging from Pd55 to Pd146, have been investigated by means of all-electron relativistic density functional calculations. Adsorption of CO molecules on the (111) facets of these clusters was also studied. In particular, we focused on the interaction of CO (a single molecule per facet) with threefold hollow sites to inspect the variation of the calculated adsorption parameters with cluster size. We considered how observables calculated for that adsorption position on cluster facets relate to adsorption properties of the corresponding site at the single crystal surface Pd(111). We demonstrated for the first time that, with three-dimensional cluster models proposed here, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. We also addressed size effects on interatomic distances and the cohesive energy of bare Pd nanoclusters whose structure was fully optimized under the imposed Oh symmetry constraint. These quantities were found to correlate linearly with the average coordination number and the inverse of the cluster radius, respectively, allowing a rather accurate extrapolation to the corresponding values of Pd bulk. Finally, we considered the size convergence of adsorption properties of the optimized Pd clusters, as probed by CO adsorption. We also outlined implications of using these symmetric clusters for investigating adsorption and reactions on oxide-supported nanoparticles of model Pd catalysts.

Characterization of Titania Loaded V-, Fe-, and Cr-Incorporated MCM-41 by XRD, TPR, UV−vis, Raman, and XPS Techniques

Abstract: Transition metal (Me = V, Fe, and Cr) incorporated into MCM-41 mesoporous molecular sieves (Si/Me = 80) have been synthesized by hydrothermal methods and were loaded with TiO2 utilizing a sol−gel technique. These materials were found (refs 22, 23) to be photoactive for the destruction of organics with visible light. A combination of various physicochemical techniques such as N2 physisorption, O2 chemisorption, diffuse reflectance UV−vis (DR−UV−vis), X-ray diffraction (XRD), Raman, temperature program reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the chemical environment of these transition metals in the prepared photocatalysts. The dispersion of transition metals as determined by O2 chemisorption suggests that they are well dispersed inside the MCM-41 framework, but the dispersion values decreased with the loading of TiO2. This indicates that the loaded titania promotes the transformation of incorporated metal ions into different phases. DR−UV−vis spectra of the Me-...

A Model for the Chemical Vapor Deposition of Poly(para-xylylene) (Parylene) Thin Films

Abstract: A kinetic model is developed for the chemical vapor deposition of poly(para-xylylene), or parylene, thin polymer films. The growth process is modeled as a multistep process that includes physisorption of monomer on the surface and subsequent chemisorption. The chemisorption step is equivalent to a propagation reaction between the monomer and a radical chain end, and each chemisorption produces a new chemisorption site. The sticking coefficient of the monomer as a function of substrate temperature is extracted from the measured data using the model and is determined to be 2.0 × 10-5 at 60 °C, increasing to 1.4 × 10-3 at −60 °C. The heat of physisorption for the monomer is also extracted from the experiment, and the value found (75 kJ/mol) is reasonable when compared to those of other similar molecules. The model fits experimental kinetic data well for a large range of pressures and temperatures, and it should be appropriate for use with all parylene-family polymers.

Polymers near metal surfaces: selective adsorption and global conformations.

Abstract: We study the properties of a polycarbonate melt near a nickel surface as a model system for the interaction of polymers with metal surfaces by employing a multiscale modeling approach. For bulk properties, a suitably coarse-grained bead-spring model is simulated by molecular dynamics methods with model parameters directly derived from quantum chemical calculations. The surface interactions are parametrized and incorporated by extensive quantum mechanical density functional calculations using the Car-Parrinello method. We find strong chemisorption of chain ends, resulting in significant modifications of the melt composition when compared to an inert wall.

Oxygen incorporation into Fe-doped SrTiO3: Mechanistic interpretation of the surface reaction

Abstract: The surface part of the oxygen incorporation reaction into the model material Fe-doped SrTiO3 is investigated by in situ optical spectroscopy. Experiments close to and far from equilibrium (small and large pO2 changes) and the effect of UV irradiation on the reaction rate allow us to draw the following conclusions with respect to the kinetic processes at the surface: (i) molecular oxygen species participate in or before the rate determining step (RDS), and (ii) a single conduction band electron participates in or before the RDS of the incorporation reaction even though the bulk is p-type conducting. Probable reaction mechanisms comprising chemisorption, charge transfer, dissociation and oxygen ion transfer steps are discussed.