Showing papers on "Chemisorption published in 2005"

Activated Carbon Adsorption

Abstract: Preface Introduction Activated Carbon and Its Surface Structure Crystalline Structure of Activated Carbons Porous Structure of Activated Carbons Chemical Structure of the Carbon Surface Carbon-oxygen Surface Groups and Their Characterization Influence of Carbon-Oxygen Surface Groups on Surface Characteristics of Carbons Active Sites on Carbon Surfaces Modification of Activated Carbon Surface Adsorption Energetics, Models, and Isotherm Equations Adsorption on a Solid Surface Adsorption Equilibrium Energetics of Adsorption Classification of Adsorption Isotherms Adsorption Isotherm Equations and their Applicability to Adsorption Data Adsorption Hysteresis Activated Carbon Adsorption from Solutions Types of Adsorption from Solution Phase Types of Adsorption Isotherms Factors Influencing Adsorption from Binary Solutions Determination of Individual Adsorption Isotherms from Composite Isotherms Thickness of the Adsorbed Layer Chemisorption from Binary Solutions Traube's Rule Carbon Molecular Sieves Preparation of Carbon Molecular Sieves Characterization of Carbon Molecular Sieves Adsorption by Carbon Molecular Sieves Activated Carbon Adsorption Applications Liquid Phase Applications Gas Phase Applications Activated Carbon Adsorption in Nuclear Technology Activated Carbon Adsorption in Vacuum Technology Medicinal Applications of Activated Carbon Adsorption Activated Carbon Adsorption of Gas Storage Activated Carbon Adsorption and the Environment: Removal of Inorganics from Wastewater Activated Carbon Adsorption of Inorganics from Aqueous Phase (General) Activated Carbon Adsorption of Copper Activated Carbon Adsorption of Chromium Activated Carbon Adsorption of Mercury Activated Carbon Adsorption of Cadmium from Aqueous Solutions Activated Carbon Adsorption of Cobalt Activated Carbon Adsorption of Nickel Removal of Lead from Water Adsorptive Removal of Zinc Activated Carbon Adsorption of Arsenic Adsorptive Separation of Cations in Trace Amounts from Aqueous Solutions Mechanism of Metal Ion Adsorption by Activated Carbons Activated Carbon Adsorption and the Environment: Adsorptive Removal of Organic Compounds Activated Carbon Adsorption of Halogenated Organic Compounds Activated Carbon Adsorption of Natural Organic Matter (NOM) Activated Carbon Adsorption of Phenolic Compounds Adsorption of Nitro and Amino Compounds Adsorption of Pesticides Adsorption of Dyes Activated Carbon Adsorption of Drugs and Toxins Adsorption of Miscellaneous Organic Compounds Mechanism of Adsorption of Organics by Activated Carbons Activated Carbon Adsorption and Environment: Removal of Hazardous Gases and Vapors Removal of Volatile Organic Compounds (VOC) at Low Concentrations Removal of Oxides of Nitrogen from Flue Gases Removal of Sulphur Dioxide from Flue Gases Evaporated Loss Control Devices Protection of upper Respiratory Tract in Hazardous Environment Activated Carbon Adsorption of Mercury vapors Removal of Organic Sulphur Compounds Adsorptive Removal of Miscellaneous Vapors and Gases Author Index Subject Index

Adsorption and desorption of oxygen probed from ZnO nanowire films by photocurrent measurements

Abstract: Sensing mechanisms of ZnO nanowire films are investigated by performing transient photocurrent measurements. The current jumps upon ultraviolet or green laser illumination. The amplitude in the jump is strongly dependent on the ambient atmosphere. The decay rate is much more sensitive to the ambient atmosphere than the rise rate. The fast rise is due to rapid photogenerated carriers and the slow decay is controlled by slow surface process. Our experimental results indicate that the oxygen physisorption rate is slower than the chemisorption rate.

Kinetics of Catalytic Reactions

Abstract: Foreword Preface List of Symbols 1. Regular Symbols 2. Greek Symbols 3. Subscripts 1: Introduction 2: Definitions and Concepts 2.1 Stoichiometric Coefficients 2.2 Extent of Reaction 2.3 Rate of Reaction 2.4 Turnover Frequency or Specific Activity 2.5 Selectivity 2.6 Structure-Sensitive and Structure4nsensitive Reactions 2.7 Elementary Step and Rate Determining Step (RDS) 2.8 Reaction Pathway or Catalytic Cycle 2.9 Most Abundant Reaction Intermediate (MARI) 2.10 Chain Reactions 2.11 Reaction Rates in Reactors 2.12 Metal Dispersion (Fraction Exposed) 2.13 Meta1Support Interactions (MSI) References 3: Catalyst Characterization 3.1 Total (BET) Surface Area 3.2 Pore Volume and Pore Size Distribution 3.2.1 Hg Porosimetry Method 3.2.2 N2 Desorption Method 3.2.3 Overall Pore Size Distribution 3.3 Metal Surface Area, Crystallite Size, and Dispersion 3.3.1 Transmission Electron Microscopy (TEM) 3.3.2 X-Ray Techniques 3.3.2.1 Line Broadening of X-Ray Diffraction (XRD) Peaks 3.3.2.2 Extended X-Ray Absorption Fine Structure (EXAFS) 3.3.3 Magnetic Measurements 3.3.4 Chemisorption Methods 3.3.4.1 H2 Chemisorption 3.3.4.2 CO Chemisorption 3.3.4.3 02 Chemisorption 3.3.4.4 H2-02 Titration Techniques 3 3.5 Relationships Between Metal Dispersion, Surface Area, and Crystallite Size References Problems 4: Acquisition and Evaluation of Reaction Rate Data 4.1 Types of Reactors 4.1.1 Batch Reactor 4.1.2 Semi-Batch Reactor 4.1.3 Plug-Flow Reactor (PFR) 4.1.4 Continuous Flow Stirred-Tank Reactor (CSTR) 4.2 Heat and Mass Transfer Effects 4.2.1 Interphase (External) Gradients (Damkohler Number) 4.2.1.1 Isothermal Conditions 4.2.1.2 Nonisothermal Conditions 4.2.2 Intraphase (Internal) Gradients (Thiele Modulus) 4.2.1.1 Isothermal Conditions 4.2.2.2Nonisothermal Conditions 4.2.2.3 Determining an Intraphase (Internal) Effectiveness Factor from a Thiele Modulus 4.2.3 Intraphase Gradients (Weisz-Prater Criterion) 4.2.3.1 Gas-Phase or Vapor-Phase Reactions 4.2.3.2 Liquid-Phase Reactions 4.2.4 Other Criteria to Verify the Absence of Mass and Heat Transfer Limitations (The Madon-Boudart Method) 4.2.5 Summary of Tests for Mass and Heat Transfer Effects References Problems 5: Adsorption and Desorption Processes 5.1 Adsorption Rate 5.2 Desorption Rate 5.3 Adsorption Equilibrium on Uniform (Ideal) Surfaces-Langmuir Isotherms 5.3.1 Single-Site (Nondissociative) Adsorption 5.3.2 Dual-Site (Dissociative) Adsorption 5.3.3 Derivation of the Langmuir Isotherm by Other Approaches 5.3.4 Competitive Adsorption 5.4 Adsorption Equilibrium on Nonuniform (Nonideal) Surfaces 5.4.1 The Freundlich Isotherm 5.4.2 The Temkin Isotherm 5.5 Activated Adsorption References Problems 6: Kinetic Data Analysis and Evaluation of Model Parameters for Uniform (Ideal) Surfaces 6.1 Transition-State Theory (TST) or Absolute Rate Theory 6.2 The Steady-State Approximation (SSA) 6.3 Heats of Adsorption and Activation Barriers on Metal Surfaces: BOC-MP/UBI-QEP Method 6.3.1 Basic BOC-MP/UBI-QEP Assumptions 6.3.2 Heats of Atomic Chemisorption 6.3.3 Heats of Molecular Chemisorption 6.3.4 Activation Barriers for Dissociation and Recombination on Metal Surfaces 6.4 Use of a Rate Determining Step (RDS) and/or a Most Abundant Reaction Intermediate (MARl) 6.5 Evaluation of Parameter Consistency in Rate Expressions for Ideal Surfaces References Problems 7: Modeling Reactions on Uniform (Ideal) Surfaces 7.1 Reaction Models with a RDS Unimolecular Surface Reactions 7.2 Reaction Models with a RDS Bimolecular Surface Reactions 7.3 Reaction Models with a RDS Reactions between an Adsorbed Species and a Gas

The Electronic Structure Effect in Heterogeneous Catalysis

Abstract: Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture is given of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal surfaces as catalysts for chemical reactions can be related to certain characteristics of the surface electronic structure.

Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes

Abstract: Interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes SWNT is investigated using first-principles plane wave method. It is found that the binding energy of the H2 physisorbed on the outer surface of the bare SWNT is very weak, and cannot be enhanced significantly either by increasing the curvature of the surface through radial deformation, or by the coadsorption of a Li atom that makes the semiconducting tube metallic. Although the bonding is strengthened upon adsorption directly to the Li atom, its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of a Pt atom. A single H2 is chemisorbed to the Pt atom on the SWNT either dissociatively or molecularly. The dissociative adsorption is favorable energetically and is followed by the weakening of the Pt-SWNT bond. Out of two adsorbed H2, the first one can be adsorbed dissociatively and the second one is chemisorbed molecularly. The nature of bonding is a very weak physisorption for the third adsorbed H2. Palladium also promotes the chemisorption of H2 with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and these results advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.

Hydrogen chemisorption on Al2O3-supported gold catalysts.

Abstract: Hydrogen is dissociatively adsorbed on the gold particles in Au/Al2O3 catalysts, as demonstrated by a combination of in-situ X-ray absorption spectroscopy, chemisorption, and H/D exchange experimen...

Characterizations and activities of the nano-sized Ni/Al2O3 and Ni/La-Al2O3 catalysts for NH3 decomposition

Abstract: A series of nano-sized Ni/Al2O3 and Ni/La-Al2O3 catalysts that possess high activities for NH3 decomposition have been successfully synthesized by a coprecipitation method. The catalytic performance was investigated under the atmospheric conditions and a significant enhancement in the activity after the introduction of La was observed. Aiming to study the influence of La promoter on the physicochemical properties, we characterized the catalysts by N-2 adsorption/desorption, XRD, H-2-TPR, chemisorption and TEM techniques. Physisorption results suggested a high specific surface area and XRD spectra showed that nickel particles are in a highly dispersed state. A combination of XRD, TEM and chemisorption showed that Ni-0 particles with the average size lower, than 5.0 nm are always obtained even though the Ni loading ranged widely from 4 to 63 %. Compared with the Ni/Al2O3 catalysts, the Ni/La-Al2O3 ones with an appropriate amount of promoter enjoy a more open mesoporous structure and higher dispersion of Ni. Reduction kinetic studies of prepared catalysts were investigated by temperature-programmed reduction (TPR) method and the fact that La additive partially destroyed the metastable Ni-Al mixed oxide phase was detailed. (c) 2005 Elsevier B.V. All rights reserved.

Molecular Distortions and Chemical Bonding of a Large π-Conjugated Molecule on a Metal Surface

Abstract: Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are $0.18\ifmmode\pm\else\textpm\fi{}0.03\text{ }\AA{}$ closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled---through charge transfer into the former lowest unoccupied molecular orbital---to the primary, extended chemisorption bond via the perylene skeleton.

Metal-Catalysed Reactions of Hydrocarbons

Abstract: Metals and Alloys.- Small Metal Particles and Supported Metal Catalysts.- Chemisorption and Reactions of Hydrogen.- The Chemisorption of Hydrocarbons.- Introduction to the Catalysis of Hydrocarbon Reactions.- Exchange of Alkanes with Deuterium.- Hydrogenation of Alkenes and Related Processes.- Hydrogenation of Alkadienes and Polyenes.- Hydrogenation of Alkynes.- Hydrogenation of the Aromatic Ring.- Hydrogenation of Small Alicyclic Rings.- Dehydrogenation of Alkanes.- Reactions of the Lower Alkanes with Hydrogen.- Reactions of Higher Alkanes with Hydrogen.- Index.

In situ spectroscopic study of the oxidation and reduction of Pd(111).

Abstract: Using a photoemission spectroscometer that operates close to ambient conditions of pressure and temperature we have determined the Pd-O phase diagram and the kinetic parameters of phase transformations. We found that on the (111) surface oxidation proceeds by formation of stable and metastable structures. As the chemical potential of O2 increases chemisorbed oxygen forms followed by a thin surface oxide. Bulk oxidation is a two-step process that starts with the metastable growth of the surface oxide into the bulk, followed by a first-order transformation to PdO.

Room-Temperature Hydrogen Uptake by TiO2 Nanotubes

Abstract: TiO(2) nanotubes can reproducibly store up to approximately 2 wt % H(2) at room temperature and 6 MPa. However, only about 75% of this stored hydrogen can be released when the hydrogen pressure is lowered to ambient conditions, suggesting that both physisorption and chemisorption are responsible for the hydrogen uptake. FTIR spectroscopy, temperature-programmed desorption (TPD), and pressure-composition (P-C) isotherms suggest that 75% of the H(2) is physisorbed and can be reversibly released upon pressure reduction. Approximately 13% is weakly chemisorbed and can be released at 70 degrees C as H(2), and approximately 12% is bonded to oxide ions and released only at temperatures above 120 degrees C as H(2)O.

Chemisorption of CO2 on Nickel Surfaces

Abstract: CO2 chemisorption on the Ni(111), Ni(100), and Ni(110) surfaces was investigated at the level of density functional theory. It was found that the ability of CO2 chemisorption is in the order of Ni(110) > Ni(100) > Ni(111). CO2 has exothermic chemisorption on Ni(110) and endothermic chemisorption on Ni(111), while it is thermally neutral on Ni(100). It is also found that there is no significant lateral interaction between the adsorbed CO2 at 1/4 monolayer (ML) coverage, while there is stronger repulsive interaction at 1/2 ML. On all surfaces, the chemisorbed CO2 is partially negatively charged, indicating the enhanced electron transfer, and the stronger the electron transfer, the stronger the CO bond elongation. The bonding nature of the adsorbed CO2 on nickel surfaces has been analyzed. The thermodynamics of CO2 dissociative chemisorption, compared with CO and O adsorption, has been discussed, and the thermodynamic preference is in the sequence Ni(100) > Ni(111) > Ni(110).

Molecular Dynamics Simulation of the Structure and Hydroxylation of Silica Glass Surfaces

Abstract: The surface structure of silica glasses has been simulated using molecular dynamics. The surface hydroxyl concentration was estimated to be 4.5/nm 2 , based on surface defect statistics. Hydroxyl-silica potentials were developed and used to study the hydroxylation of silica surface. It is found that the energy of chemisorption of water declines in the sequence: three coordinated silicon (Si 3 ) and non-bridging oxygen (NBO) on separate sites, Si 3 and NBO on combined sites, two- and three-membered rings. Partial hydroxylation of the most reactive sites, which leads to an OH coverage of 2.5/nm 2 , was studied. Structural relaxation after hydroxylation was observed.

First-principles analysis of the effects of alloying Pd with Ag for the catalytic hydrogenation of acetylene-ethylene mixtures.

Abstract: The effect of alloying Pd with Ag on the hydrogenation of acetylene is examined by analyzing the chemisorption of all potential C1 (atomic carbon, CH, methylene, and methyl) and C2 (acetylene, vinyl, ethylene, ethyl, ethane, ethylidene, ethylidyne, and vinylidene) surface intermediates and atomic hydrogen along with the reaction energies for the elementary steps that produce these intermediates over Pd(111), Pd75%Ag25%/Pd(111), Pd50%Ag50%/Pd(111), and Ag(111) surfaces by using first-principle density functional theoretical (DFT) calculations. All of the calculations reported herein were performed at 25% surface coverage. The adsorption energies for all of the C1 and C2 intermediates decreased upon increasing the composition of Ag in the surface. Both geometric as well as electronic factors are responsible for the decreased adsorption strength. The modes of adsorption as well as the strengths of adsorption over the alloy surfaces in a number of cases were characteristically different than those found over ...

The structure, energetics, and nature of the chemical bonding of phenylthiol adsorbed on the Au(111) surface: implications for density-functional calculations of molecular-electronic conduction.

Abstract: The adsorption of phenylthiol on the Au(111) surface is modeled using Perdew and Wang density-functional calculations. Both direct molecular physisorption and dissociative chemisorption via S–H bond cleavage are considered as well as dimerization to form disulfides. For the major observed product, the chemisorbed thiol, an extensive potential-energy surface is produced as a function of both the azimuthal orientation of the adsorbate and the linear translation of the adsorbate through the key fcc, hcp, bridge, and top binding sites. Key structures are characterized, the lowest-energy one being a broad minimum of tilted orientation ranging from the bridge structure halfway towards the fcc one. The vertically oriented threefold binding sites, often assumed to dominate molecular electronics measurements, are identified as transition states at low coverage but become favored in dense monolayers. A similar surface is also produced for chemisorption of phenylthiol on Ag(111); this displays significant qualitativ...

Correlation of Water Activation, Surface Properties, and Oxygen Reduction Reactivity of Supported Pt–M/C Bimetallic Electrocatalysts Using XAS

Abstract: An analysis of X-ray absorption spectroscopy XAS data X-ray absorption near-edge structure XANES and extended X-ray absorption fine structure EXAFS at the Pt L3 edge for Pt‐M bimetallic materials M = Co, Cr, Ni, Fe and at the Co K edge for Pt‐Co is reported for Pt‐M/C electrodes in HClO4 at different potentials. The XANES data are analyzed using the method, which utilizes the spectrum at some potential V minus that at 0.54 V reversible hydrogen electrode RHE representing a reference spectrum. These data provide direct spectroscopic evidence for the inhibition of OH chemisorption on the cluster surface in the Pt‐M. This OH chemisorption, decreasing in the direction Pt Pt‐Ni Pt‐Co Pt‐Fe Pt‐Cr, is directly correlated with the previously reported fuel cell performance electrocatalytic activities of these bimetallics, confirming the role of OH poisoning of Pt sites in fuel cells. EXAFS analysis shows that the prepared clusters studied have different morphologies, the Pt‐Ni and Pt‐Co clusters were more homogeneous with M atoms at the surface, while the Pt‐Fe and Pt‐Cr clusters had a “Pt skin.” The cluster morphology determines which previously proposed OH inhibition mechanism dominates, the electronic mechanism in the pres

Three-site model for hydrogen adsorption on supported platinum particles: influence of support ionicity and particle size on the hydrogen coverage.

Abstract: Pt L3 X-ray absorption edge data on small supported Pt particles (N < 6.5) reveals that at very low H2 pressure or high temperature the strongest bonded H is chemisorbed in an atop position. With decreasing temperature or at higher H2 pressure only n-fold (n = 2 or 3) sites are occupied. At high H2 pressure or low temperature, the weakest bonded H is positioned in an “ontop” site, with the chemisorbing Pt already having a stronger bond to a H atom in an n-fold site. DFT calculations show that the adsorption energy of hydrogen increases for Pt particles on ionic (basic) supports. The combination of the DFT calculations with hydrogen chemisorption data and the analysis of the Pt L3 X-ray absorption spectra implies that both the H coverage and/or the type of active Pt surface sites, which are present at high temperature catalytic reaction conditions, strongly depend on the ionicity of the support. The consequences for Pt catalyzed hydrogenolysis and hydrogenation reactions will be discussed.

Evolution of small copper clusters and dissociative chemisorption of hydrogen.

Abstract: The structural evolution of small copper clusters of up to 15 atoms and the dissociative chemisorption of H2 on the minimum energy clusters are studied systematically using density functional theory. The preferred copper sites for chemisorption are identified and the transition state structures and activation barriers for clusters four to nine atoms are determined and found to be inconsistent with the empirical Bronsted-Evans-Polanyi relationship. The physicochemical properties of the clusters are computed and compared with the bulk and surface values. The results indicate that a phase transition must occur in the going from cluster to bulk. Recent advances in nanocatalysis have underscored the fundamental importance of transition metal (TM) clusters [1‐3]. The physicochemical properties of the clusters are structure and size sensitive and can sometimes change dramatically with the addition or removal of one atom from the clusters. With a large variation in size, TM clusters can bridge the homogeneous and the heterogeneous catalysis and offer useful physical insight into the evolution of structure and properties from atoms or molecules to bulk. Over the last few decades, much research activity has been made to address the physical properties of clusters, but very little has been done on the chemical reactivity of TM clusters, which constitute the majority of nanocatalysts widely used in the contemporary homogeneous and heterogeneous catalytic systems [4 ‐7].

First-principles study of Ti-catalyzed hydrogen chemisorption on an Al surface: a critical first step for reversible hydrogen storage in NaAlH4.

Abstract: Complex metal hydrides are perhaps the most promising hydrogen storage materials for a gradual transformation to a hydrogen-based economy. We have used a computational approach to aid the ongoing experimental effort to understand the reversible hydrogen storage in Ti-doped NaAlH(4) and propose a plausible first step in the rehydrogenation mechanism. The study provides insight into the catalytic role played by the Ti atoms on an Al surface in the chemisorption of molecular hydrogen and identifies the local arrangement of the Ti atoms responsible for the process. Our results can potentially lead to ways of making other similar metal hydrides reversible.

One-dimensional PtO2 at Pt steps: Formation and reaction with CO

Abstract: Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) oxide structure forms at the steps of the Pt(332) surface after exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.