Showing papers on "Chemisorption published in 2003"

Concepts of modern catalysis and kinetics

Abstract: Preface. 1 INTRODUCTION TO CATALYSIS. 1.2 What is Catalysis. 1.2 Catalysts Can Be Atoms, Molecules, Enzymes and Solid Surfaces. 1.3 Why is Catalysis Important? 1.4 Catalysis as a Multidisciplinary Science. 1.5 The Scope of This Book. 1.6 Catalysis in Journals. 1.7 General References to Textbooks in Catalysis. 2 KINETICS. 2.1 Introduction. 2.2 The Rate Equation and Power Rate Laws. 2.3 Reactions and Thermodynamic Equilibrium. 2.4 Temperature Dependence of the Rate. 2.5 Integrated Rate Equations: Time Dependence of Concentrations in Reactions of Different Orders. 2.6 Coupled Reactions in Flow Reactors: The Steady-state Approximation. 2.7 Coupled Reactions in Batch Reactors. 2.8 Catalytic Reactions. 2.9 Langmuir Adsorption Isotherms. 2.10 Reaction Mechanisms. 2.11 Entropy, Entropy Production, Auto Catalysis and Oscillating Reactions. 2.12 Kinetics of Enzyme-catalyzed Reactions. 3 REACTION RATE THEORY. 3.1 Introduction. 3.2 The Boltzmann Distribution and the Partition Function. 3.3 Partition Functions of Atoms and Molecules. 3.4 Molecules in Equilibrium. 3.5 Collision Theory. 3.6 Activation of Reacting Molecules by Collisions: The Lindermann Theory. 3.7 Transition State Theory. 3. 8 Transition State Theory of Surface Reactions. 3.9 Summary. 4 CATALYST CHARACTERIZATION. 4.1 Introduction. 4.2 X-Ray Diffraction (XRD). 4.3 X-Ray Photoelectron Spectroscopy (XPS). 4.4 Extended X-Ray Absorption Fine Structure (EXAFS). 4.5 Electron Microscopy. 4.6 Mossbauer Spectroscopy. 4.7 Ion Spectroscopy: SIMS, LEIS, RBS. 4.8 Temperature-programmed Reduction, Oxidation and Sulfidation. 4.9 Infrared Spectroscopy. 4.10 Surface Science Techniques. 4.11 Concluding Remarks 5 SOLID CATALYSTS. 5.1 Requirements of a Successful Catalyst. 5.2 Structure of Metals, Oxides and Sulfides and their Surfaces. 5.3 Characteristics of Small Particles and Porous Material. 5.4 Catalyst Supports. 5.5 Preparation of Supported Catalysts. 5.6 Unsupported Catalysts. 5.7 Zeolites. 5.8 Catalyst Testing. 6 SURFACE REACTIVITY. 6.1 Introduction. 6.2 Physisorption. 6.3 Chemical Bonding. 6.4 Chemisorption. 6.5 Important Trends in Surface Reactivity. 7 KINETICS OF REACTIONS ON SURFACES. 7.1 Elementary Surface Reactions. 7.2 Kinetic Parameters from Fitting Langmuir-Hinshelwood Models. 7.3 Micro-kinetic Modeling. 8 HETEROGENEOUS CATALYSIS IN PRACTICE: HYDROGEN. 8.1 Introduction. 8.2 Steam Reforming Process. 8.3 Reactions of Synthesis Gas. 8.4 Water Gas Shift Reaction. 8.5 Synthesis of Ammonia. 8.6 Promoters and Inhibitors. 8.7 The Hydrogen Society. 9 OIL REFINING AND PRETROCHEMISTRY. 9.1 Crude Oil. 9.2 Hydrotreating. 9.3 Gasoline Production. 9.4 Petrochemistry: Reactions of Small Olefins. 10 ENVIRONMENTAL CATALYSIS. 10.1 Introduction. 10.2 Automotive Exhaust Catalysis. 10.3 Air Pollution by Large Stationary Sources. QUESTIONS AND EXERCISES. Appendix A. Appendix B. Index.

Insights into Photoexcited Electron Scavenging Processes on TiO2 Obtained from Studies of the Reaction of O2 with OH Groups Adsorbed at Electronic Defects on TiO2 (110)

Abstract: In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO2(110). The electron...

Theoretical evaluation of hydrogen storage capacity in pure carbon nanostructures

Abstract: Carbon nanotubes have been proposed as promising hydrogen storage materials for the automotive industry. By theoretical analyses and total-energy density functional theory calculations, we show that contribution from physisorption in nanotubes, though significant at liquid nitrogen temperature, should be negligible at room temperature; contribution from chemisorption has a theoretical upper limit of 7.7 wt %, but could be difficult to utilize in practice due to slow kinetics. The metallicity of carbon nanotube is lost at full hydrogen coverage, and we find strong covalent C–H bonding that would slow down the H2 recombination kinetics during desorption. When compared to other pure carbon nanostructures, we find no rational reason yet why carbon nanotubes should be superior in either binding energies or adsorption/desorption kinetics.

Photoluminescence and electronic interaction of anthracene derivatives adsorbed on sidewalls of single-walled carbon nanotubes

Abstract: Dye molecules (anthracene derivatives) are observed to strongly adsorb to single-walled carbon nanotubes (SWNTs). The adsorption coverage of anthracene molecules on SWNTs varied with the aromatic ring substituents. The observed red shifts of emission peaks of the absorptive adduct appear to depend on the energy level of the lowest unoccupied molecular orbital (LUMO) of the adsorbate, consistent with adsorption by a charge-transfer interaction, in which the SWNT is electron donor and anthracene is acceptor. The anthracene absorptive adducts can be displaced by adsorption of pyrene.

Cluster expansion method for adsorption: Application to hydrogen chemisorption on graphene

Abstract: A systematic method is presented for determining the ground state of absorbed species on a substrate based on a cluster expansion of the configurational energy. It is shown that the method can determine the ground state of a strongly relaxing system using a few first-principles total energy calculations of small cells only. The method is applied to a particularly challenging case, the two-sided hydrogen chemisorption on a free standing graphene sheet, where, as a function of hydrogen coverage, the carbon hybridization goes from ${\mathrm{sp}}^{2}$ to ${\mathrm{sp}}^{3}.$ The method should require still fewer calculations and yield still more accurate results in the case of physisorption where longer-ranged strain effects are less important.

FTIR study of low-temperature water-gas shift reaction on gold/ceria catalyst

Abstract: Chemisorption and reactivity of the molecules involved in the water-gas shift (WGS) reaction on gold/ceria catalyst have been studied at 90 and 300 K by FTIR spectroscopy. Forward and reverse WGS reaction at 300 K and up to 573 K have been investigated, too. The FTIR results show that gold causes a strong modification of the surface properties of the support. The nanosized metallic gold particles in close contact with defective ceria play an essential role for the genesis of high catalytic activity in WGS reaction at low temperature and appear to be of crucial importance in explaining the remarkably high stability of this catalytic system. An electronic interaction between small gold metallic nanoparticles and ceria has been evidenced.

Different surface chemistries of water on Ru[0001]: from monomer adsorption to partially dissociated bilayers.

Abstract: Density functional theory has been used to perform a comparative theoretical study of the adsorption and dissociation of H2O monomers and icelike bilayers on Ru{0001}. H2O monomers bind preferentially at atop sites with an adsorption energy of ∼0.4 eV/H2O. The main bonding interaction is through the H2O 1b1 molecular orbital which mixes with Ru dz2 states. The lower-lying set of H2O molecules in an intact H2O bilayer bond in a similar fashion; the high-lying H2O molecules, however, do not bond directly with the surface, rather they are held in place through H bonding. The H2O adsorption energy in intact bilayers is ∼0.6 eV/H2O and we estimate that H bonding accounts for ∼70% of this. In agreement with Feibelman (Science 2002, 295, 99) we find that a partially dissociated OH + H2O overlayer is energetically favored over pure intact H2O bilayers on the surface. The barrier for the dissociation of a chemisorbed H2O monomer is 0.8 eV, whereas the barrier to dissociate a H2O incorporated in a bilayer is just 0...

DFT Study of Cysteine Adsorption on Au(111)

Abstract: The adsorption of the cysteine amino acid (H−SCβH2−CαH−NH2−COOH) on the (111) surface of gold is studied by means of periodic density functional calculations. Results for different adsorption sites and molecular configurations show that chemisorption involving S(thiolate)−Au bonds on Au(111) is favored by starting with either cysteine or cystine gas-phase molecular precursors. In the most stable adsorption configuration, the sulfur headgroup sits at the bridge site between two surface Au atoms, and the S−Cβ bond is tilted by 57° with respect to the surface normal, whereas the in-plane orientation of the molecular backbone plays a secondary role. The analysis of the electronic properties shows that the hybridization of the p-like S states with the d-like Au states produces both bonding and antibonding occupied orbitals, and the process is well described by a model for the interaction of localized orbitals with narrow-band dispersive electron states. The bonding orbitals well below the Fermi level contribut...

Supported bimetallic cluster catalysts

Abstract: We review synthesis, structural characterization, and catalysis by supported bimetallic clusters. Initial steps in the preparations include (a) adsorption of molecular metal clusters with preformed metal−metal bonds; (b) synthesis in zeolite cages; and (c) adsorption of a metal complex on a supported complex or cluster of another metal. Chemisorption capacities of supported oxophilic metal−noble metal clusters for hydrogen and CO are typically less than those of supported clusters of just the noble metal. Catalytic properties distinguishing supported bimetallic clusters from monometallics include high selectivities for oxygenates in CO hydrogenation and different selectivities for CO vs CC bond hydrogenation. Advantages of precise synthesis with organometallic precursors (smallness and uniformity of the supported clusters with bonds between the two metals) are offset by the expensive preparations and difficulty of catalyst regeneration. Applications of supported bimetallic clusters are most likely in spec...

Growth Per Cycle in Atomic Layer Deposition: A Theoretical Model

Abstract: Atomic layer deposition (ALD) is used in advanced applications where thin layers of materials with precise thickness down to the nanometer scale are needed. Growth of materials by ALD takes place through repeating the separate, saturating reactions of at least two gaseous reactants with a solid substrate. When surface saturation is systematically utilized, the growth obtained per ALD reaction cycle is a well-defined quantity that depends on i) the reactants used, ii) the ALD processing temperature, and iii) sometimes the substrate material. A model is derived to describe the growth per cycle in ALD as a function of the chemistry of the growth when compounds are used as reactants. Two main types of chemisorption may occur:i) ligand exchange reaction of the ML n reactant with surface a groups, where ligands are removed from the surface as gaseous aL, and ii) dissociation or association, where all parts of the ML n reactant are attached to the surface. A simple mathematical model based on the mass balance of chemisorption relates the growth per cycle to the size of the ML n reactant and the chemisorption mechanisms involved. Steric hindrance of the ligands causes saturation of chemisorption if a limited number of bonding sites does not cause it. Because of the steric hindrance, the growth per cycle remains less than a monolayer. The applicability of the model is illustrated through several theoretical examples.

Activity of SiO2 supported gold-palladium catalysts in CO oxidation

Abstract: Bimetallic Au-Pd catalysts supported on silica with different Au/Pd atomic ratios were prepared by simultaneous reduction of palladium and gold precursors by ethanol in the presence of the polymer, polyvinylpyrrolidone (PVP). Formation of alloyed particles was detected by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and CO chemisorption measurements. The catalysts were tested in the catalytic oxidation of CO using a plug–flow reactor. The CO conversion was determined as a function of temperature. The monometallic palladium and the palladium-rich catalysts behaved quite similarly and were the most active catalysts. A drastic reduction of the CO oxidation activity was observed for the 50/50 Au/Pd catalyst and for samples with increasing amount of gold.

Scanning tunneling microscopy and spectroscopy of gallium oxide deposition and oxidation on GaAs(001)-c(2×8)/(2×4)

Abstract: The surface structures formed upon deposition of O2 and Ga2O onto the technologically important arsenic-rich GaAs(001)-c(2×8)/(2×4) surface have been studied using scanning tunneling microscopy and spectroscopy, and the results are compared to density functional theory calculations. O2 chemisorbs by displacing first layer arsenic atoms bonded to second layer gallium atoms. Oxygen chemisorption pins the Fermi level at less than 5% monolayer coverage by creating a donor and acceptor site within the band gap originating from the gallium atom bonded between the two O atoms. In contrast, Ga2O chemisorbs by inserting into arsenic dimer pairs at elevated surface temperatures. A monolayer of Ga2O forms a (2×2) surface structure with a crystalline interface that is electronically unpinned: there are no states within the band gap. The unpinned interface results from Ga2O restoring the surface arsenic and gallium atoms to near-bulk charge.

Chemical treatment of γ-Al2O3 and its influence on the properties of Co-based catalysts for Fischer–Tropsch synthesis

Abstract: Alumina has been pretreated in the presence of medium and then used to prepare the supported Co catalysts. These modified supports and the respective catalysts have been thoroughly characterized by means of methods such as BET, pore size distribution, X-ray diffraction (XRD), ammonia temperature-programmed desorption (NH 3 -TPD), pyridine infrared spectroscopy (Py-FTIR), KBr-IR spectroscopy (FTIR), temperature-programmed reduction (TPR), oxygen titration, hydrogen chemisorption, in situ diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) and CO hydrogenation to understand how chemically-treated alumina influences the properties of Co catalysts. These characterizations clearly show the changes of morphology (surface area, pore volume, pore size distribution and crystallite phase) as well as chemical properties (e.g. acidity) of the supports. Although the cobalt oxide crystallite sizes of the oxidic catalyst precursor are almost unaffected by different support pre-treatment, the reducibility of these catalysts vary greatly. And the support pre-treatment remarkably influences the adsorption and catalytic properties of these Co catalysts. The acetic acid-treated support has a negative effect on the catalytic properties of Co catalyst, whereas the ammonia and ammonium nitrate-treated samples show pronounced effects on the catalytic behaviors of Co catalysts.

Synthesis, Spectroscopy, and Catalytic Properties of Cationic Organozirconium Adsorbates on “Super Acidic” Sulfated Alumina. “Single-Site” Heterogeneous Catalysts with Virtually 100 Active Sites

Abstract: Sulfated alumina (AlS), a highly Bronsted acidic sulfated metal oxide, is prepared by the impregnation of γ-alumina with 1.6 M H2SO4, followed by calcination at 550 °C for 3 h. 13C CPMAS NMR spectroscopy of the chemisorbed 13Cα-enriched organozirconium hydrocarbyl Cp‘2Zr(13CH3)2 (2*)/AlS (Cp‘ = η5-(CH3)5C5) reveals that the chemisorption process involves M−C σ-bond protonolysis at the strong surface Bronsted acid surface sites to yield a “cation-like” highly reactive zirconocenium electrophile, Cp‘2Zr13CH3+. In contrast, chemisorption of 2* on dehydroxylated alumina (DA) yields a similar cation via methide transfer to surface Lewis acid sites, while chemisorption onto dehydroxylated silica yields a μ-oxo Cp‘2Zr(13CH3)−OSi⋮ species. Two complementary active site kinetic assays for benzene hydrogenation show that, unlike typical heterogeneous and supported organometallic catalysts, 97 ± 2% of all Cp‘ZrMe3 (3)/AlS sites are catalytically significant, demonstrating that the species identified by 13C CPMAS NMR...

Oxidation of carbon nanotubes by singlet O2.

Abstract: Chemisorption of singlet 1 � g O2 on single-walled carbon nanotubes is reexamined by first principles calculations, and the reaction barrier is substantially lower than previously reported when the spin on O2 is correctly treated. The process is initiated by the cycloaddition of a singlet O2 on top of a C-C bond and ended with an epoxy structure with each of the two oxygen atoms occupying a bridge position. The overall process is exothermic, with an activation barrier as low as 0.61 eV for the (8, 0) tube. Our results raise the possibility that carbon nanotubes with small diameters could be degraded after exposure to air and sunlight, similar to the degradation of natural rubber and synthetic plastics.

A Periodic DFT Study of Isobutene Chemisorption in Proton-Exchanged Zeolites: Dependence of Reactivity on the Zeolite Framework Structure

Abstract: Isobutene chemisorption within proton-exchanged zeolites is investigated using periodic density functional theory method. This allows us to consider the effect of the zeolite micropore dimension to reactivity. The isobutene reaction pathways that proceed through primary and tertiary carbocation-like transition states have been investigated. The results agree with predicted reactivity trends. Activation energies of isobutene chemisorption are estimated to be around 100 and 25 kJ/mol for primary and tertiary transition states, respectively. Destabilization of transition state complexes and products are as observed before. Interestingly, because of the steric constraints, the chemisorbed alkoxy species appeared to become as unstable as protonated hydrocarbons. The more significant result is the correlation of the zeolite micropore dimension with activation energies. Fluctuations of the activation energies are observed as a function of the match of the transition state structures with the zeolite cavities. We...

Chemisorption of Acetone on Carbon Nanotubes

Abstract: Temperature-programmed desorption experiments show that acetone chemisorbs on nanotubes while physisorption occurs on graphite. Computed high binding energies for chemisorption using hybrid quantum mechanical and semiempirical calculations are in good agreement with the experimental thermal desorption data. The strong chemical interactions between acetone and the nanotube surface are established as being due to the effects of curvature and topological defects.

Resolution of an ancient surface science anomaly: Work function change induced by N adsorption on W{100}

Abstract: For many decades it has been assumed that an adsorbate centered above a metal surface and with a net negative charge should increase the work function of the surface. However, despite their electronegativity, N adatoms on Wf100g cause a significant work function decrease. Here we present a resolution of this anomaly. Using density functional theory, we demonstrate that while the N atom carries a negative charge, of overriding importance is a reduction in the surface overspill electron density into the vacuum, when that charge is engaged in bonding to the adatom. This novel interpretation is fundamentally important in the general understanding of work function changes induced by atomic adsorbates.