Showing papers on "Chemisorption published in 2009"

Ozone Adsorption on Graphene: Ab Initio Study and Experimental Validation

Abstract: We have investigated ozone adsorption on graphene using the ab initio density functional theory method. Ozone molecules adsorb on the graphene basal plane with a binding energy of 0.25 eV, and the physisorbed molecule can chemically react with graphene to form an epoxide group and an oxygen molecule. The activation energy barrier from physisorption to chemisorption is 0.72 eV, and the chemisorbed state has the binding energy of 0.33 eV. These binding energies and energy barrier indicate that the ozone adsorption on graphene is gentle and reversible. An atomic layer deposition experiment on ozone treated graphite has confirmed the presence of uniform hydrophilic groups on the graphene basal plane. This finding can be applied to diverse chemical functionalization of graphene basal planes.

Chemical Reactivity of Reduced TiO2(110): The Dominant Role of Surface Defects in Oxygen Chemisorption

Abstract: O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature-programmed desorption, and electron-stimulated desorption. On the annealed surface, two oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e., 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ∼20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of 2 when the OHb concentration is enhanced—without changing the concentration of subsurface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species ...

Reactions at Solid Surfaces

Abstract: Preface. 1. Basic principles. 1.1. Introduction: The surface science approach. 1.2. Energetics of chemisorption. 1.3. Kinetics of chemisorption. 1.4. Surface diffusion. References. 2. Surface structure and reactivity. 2.1. Influence of the surface structure on reactivity. 2.2. Growth of two-dimensional phases. 2.3. Electrochemical modification of surface structure. 2.4. Surface reconstruction and transformation. 2.5. Subsurface species and compound formation. 2.6. Epitaxy. References. 3. Dynamics of molecule/surface interactions. 3.1. Introduction. 3.2. Scattering at surfaces. 3.3. Dissociative adsorption. 3.4. Collision-induced surface reactions. 3.5. Hot adparticles. 3.6. Particles coming off the surface. 3.7. Energy exchange between adsorbate and surface. References. 4. Electronic excitations and surface chemistry. 4.1. Introduction. 4.2. Exoelectron emission. 4.3. Internal electron excitation: chemicurrents . 4.4. Electron-stimulated desorption. 4.5. Surface photochemistry. References. 5. Principles of heterogeneous catalysis. 5.1. Introduction. 5.2. Active sites. 5.3. Langmuir Hinshelwood versus Eley Rideal mechanism. 5.4. Coadsorption. 5.5. Kinetics of catalytic reactions. 5.6. Selectivity. References. 6. Mechanisms of heterogeneous catalysis. 6.1. Synthesis of ammonia on iron. 6.2. Synthesis of ammonia on ruthenium. 6.3. Oxidation of carbon monoxide. 6.4. Oxidation of hydrogen on platinum. References. 7. Oscillatory kinetics and nonlinear dynamics. 7.1. Introduction. 7.2. Oscillatory kinetics in the catalytic CO oxidation on Pt(110). 7.3. Forced oscillations in CO oxidation on Pt(110). References. 8. Spatiotemporal self-organization in surface reactions. 8.1. Introduction. 8.2. Turing patterns and electrochemical systems. 8.3. Isothermal wave patterns. 8.4. Modification and control of spatiotemporal patterns. 8.5. Thermokinetic effects. 8.6. Pattern formation on microscopic scale. References. Index.

Pd-sensitized single vanadium oxide nanowires: Highly responsive hydrogen sensing based on the metal−insulator transition.

Abstract: Exceptionally sensitive hydrogen sensors were produced using Pd-nanoparticle-decorated, single vanadium dioxide nanowires. The high-sensitivity arises from the large downward shift in the insulator to metal transition temperature following the adsorption on and incorporation of atomic hydrogen, produced by dissociative chemisorption on Pd, in the VO2, producing ∼1000-fold current increases. During a rapid initial process, the insulator to metal transition temperature is decreased by >10 °C even when exposed to trace amounts of hydrogen gas. Subsequently, hydrogen continues to diffuse into the VO2 for several hours before saturation is achieved with only a modest change in the insulator to metal transition temperature but with a significant increase in the conductivity. The two time scales over which H-related processes occur in VO2 likely signal the involvement of two distinct mechanisms influencing the electronic structure of the material one of which involves electron−phonon coupling pursuant to the mod...

Role of B5-Type Sites in Ru Catalysts used for the NH3 Decomposition Reaction

Abstract: A series of activated carbon supported Ru catalysts have been reduced at different temperatures under hydrogen flow, and in some cases under ammonia flow, in order to modify the morphology and the particle size of the metallic active sites. CO chemisorption and transmission electron microscopy have been applied to follow the variations of these particles. The samples have been tested in the ammonia decomposition reaction, where systematic differences in catalytic activities as consequence of the support modification as well as due to the changes in the Ru particle sizes have been detected. Furthermore when potassium is added as catalyst promoter the sintering of Ru particles is significantly diminished and thus the changes in catalytic activities are inhibited. The electronic states of the Ru particles have been evaluated by determination of the chemisorption heats of the CO probe molecule. A part of other promoter or support effects it seems to exit a critical mean size for Ru particles on where maximum of catalytic activity is achieved. This behavior can be rationalized by the presence of surface highly active B5 sites, which consist of an arrangement of three Ru atoms in one layer and two further Ru atoms in an internal layer. These especial surface sites are expected to be in a higher proportion over Ru crystallites of those critical sizes, namely for Ru diameter sizes in the rage of 3–5 nm.

Surface reactions during atomic layer deposition of Pt derived from gas phase infrared spectroscopy

Abstract: Infrared spectroscopy was used to obtain absolute number information on the reaction products during atomic layer deposition of Pt from (methylcyclopentadienyl)trimethylplatinum [(MeCp)PtMe3] and O2. From the detection of CO2 and H2O it was established that the precursor ligands are oxidatively decomposed during the O2 pulse mainly. Oxygen atoms chemisorbed at the Pt lead to likewise ligand oxidation during the (MeCp)PtMe3 pulse however the detection of a virtually equivalent density of CO2 and CH4 also reveals a concurrent ligand liberation reaction. The surface coverage of chemisorbed oxygen atoms found is consistent with the saturation coverage reported in surface science studies.

Study of Pd-Au bimetallic catalysts for CO oxidation reaction by DFT calculations.

Abstract: First-principles calculations were carried out to examine the catalytic activity of Pd and Pd–Au alloy surfaces for CO oxidation. The influences of surface-ligand effect and lattice strain effect on activity were demonstrated. The catalytic efficiency of Pd–Au bimetallic systems depends largely on the surface composition of Pd and Au. The addition of Au significantly improves the activity of a Pd–Au bimetallic slab with an Au-rich surface due to the dominant Au-induced ligand effect on both O and CO chemisorption. Among the various cases considered, the system with Au on the surface of the Pd16Au4 slab exhibits the lowest energy barrier of 0.21 eV, which is decreased by 0.66 eV compared to that of the pure Pd(111) surface. It is predicted that the [Pd](Au) core/shell nanoparticle catalyst should have a higher activity for CO oxidation as it combines the advantages of the ligand effect of Au and the strain effect of Pd.

Determination of the Particle Size, Available Surface Area, and Nature of Exposed Sites for Silica-Alumina-Supported Pd Nanoparticles: A Multitechnical Approach

Abstract: In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR spectroscopy) to understand the effects of the activation temperature and activation atmosphere (air or H2) on the particle size distribution, the fraction, and the type of exposed surface sites of Pd nanoparticles supported on a high surface area SiO2−Al2O3 (SA) support. Pd particle distribution has been carefully determined by a high statistic TEM study, from which the cuboctahedral-like shape of the metal particles is demonstrated. Assuming a model of perfect cuboctahedral particles, from the TEM particle size distribution we estimated the expected average Pd first shell coordination number. This value is slightly larger than that directly found by EXAFS owing to the fraction of very small Pd particles (d < 6−8 A) that basically escape TEM detection. The same geometrical model allows prediction, from TEM particle size distribution, of the metal dispersion observed by CO chemisorption (S/VChemi). The S/VChe...

Adsorption and reactivity of CO(2) on defective graphene sheets.

Abstract: Density-functional calculations have been performed to investigate the adsorption of CO2 on defected graphite (0001) represented by a single graphene sheet. The interaction with a vacancy defect gives a computed molecular binding energy of ∼136 meV in a strong physisorbed state. Subsequently, chemisorption by lactone group formation will occur after overcoming a barrier of ∼1 eV relative to the gas phase, with an exothermicity of about 1.4 eV. Further reaction paths from this chemisorbed state lead to dissociation of the CO2 through the formation of epoxy groups followed by oxygen recombination and desorption of O2, after overcoming successive energy barriers of ∼0.9 and ∼1.0 eV. The global minimum (“O2 desorbed + graphene sheet”) entails an energy release of about 3.4 eV with respect to the initial state.

Infrared characterization of biotinylated silicon oxide surfaces, surface stability, and specific attachment of streptavidin.

Abstract: Biotinylation of silicon oxide surfaces, surface stability, and evolution of these functionalized surfaces under biospecific attachment of streptavidin were studied using Fourier transform infrared spectroscopy. Adsorption and stability of species or changes in the resulting surfaces were monitored after each step of the attachment processes. The silicon oxide surface was initially derivatized by 3-aminopropyltriethoxysilane, and the quality of the 3-aminopropylsiloxane (APS) surface was monitored using the Si−O−Si and Si−O−C region of its vibrational spectrum. A strong correlation between surface quality and presilanization atmospheric moisture content was established. The vibrational fingerprint of biotinylation was determined, both for physisorption and chemisorption to the surface. A new band (i.e., not previously associated with biotin) at ∼1250 cm−1 was identified as a vibrational mode of the biotin ureido group, making it possible to track changes in the biotinylated surface in the presence of stre...

Dealloyed Core-Shell Fuel Cell Electrocatalysts

Abstract: We review our recent work on dealloyed nanoparticle electrocatalysts and address their synthesis, structural characterization and surface catalytic performance in low-temperature Polymer Electrolyte Membrane fuel cells (PEMFCs). The active form of the catalyst is obtained by voltammetric dealloying of non-noble metal rich Pt alloy precursors. In the dealloying process, the less noble precursor component, here Cu, is selectively removed from the surface of the precursor alloy particles and hence a Pt enriched particle shell is formed. Single fuel cell tests showed that, when used on the cathode of PEMFCs, dealloyed Pt catalysts show reactivities for the oxygen reduction reaction (ORR) which are up to 6 times higher than those of conventional pure Pt fuel cell catalysts. Similarly, the stability of dealloyed nanoparticle catalysts is superior to that of pure Pt particles. X-ray based structural and compositional studies suggested a core—shell particle structure as the active form of the catalyst consisting of a Pt enriched particle shell surrounding a Pt alloy core. At the present time, this catalyst system constitutes one of the most active fuel cell catalyst system reported in the literature. I. THE CONCEPT OF DEALLOYED NANOPARTICLE CATALYSTS Despite much recent focus on the development of advanced Li ion batteries for use as power source for short-range inner city transportation 255 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Vol. 25. No. 4. 2009 Dealloyed Core-Shell Fuel Cell Eleclrocatalysts applications, low temperature fuel cells continue to be the solution of choice for medium and long range transportation technologies based on their gravimetric power density as well as the gravimetric energy density of commonly used fuels". Wider use of low-temperature fuel cell technology is hampered by performance, cost, and durability issues associated with materials and components of a single fuel cell. Figure 1 displays a cross section of the layered structure of a low temperature PEMFC showing the anode (left) and cathode (right) gas diffusion layers (GDLs), which sandwich the anode and cathode catalyst layers and the proton exchange membrane. Figure 1 also schematically shows the molecular as well as electrical pathways of hydrogen fuel molecules, oxygen molecules, protons as well as of electrons. The overall performance of a PEMFC in terms of its practical cell voltage is limited by kinetic, ohmic, and mass transport processes for low, medium and high current densities, respectively. Of these, the kinetic surface catalytic reactions cause the most severe fuel cell voltage losses. * Anode \ fl( / Cathode ·>' '..·· Electrical | O2+*H* + 4e -> 2 H2O Energy Fig. 1: Cross sectional SEM micrograph through a membrane electrode assembly sandwiched between gas diffusion layers. Reaction processes at anode and cathode, mass and charge flows are indicated (from ref) In particular, the electrocatalytic Oxygen Reduction Reaction (ORR) at the cathode according to O2 + 4H + +4e->2H 2 O E°=+1.23 V/RHE (1) represents the key challenge to improved PEMFCs". In acidic media, Pt catalysts offer the highest catalytic activities which made first unsupported, later high surface area carbon-supported Pt particles the ORR catalyst of 256 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Peter Strasser Reviews in Chemical Engineering choice· . Figure 2 illustrates the associative (via O2H) as well as the dissociative (via O) pathways of reaction (1) from molecular oxygen to water. Key for the catalysis is the chemisorption energy of the adsorbed oxygenated intermediates, such as Pt-O2H, Pt-O, and Pt-OH. On pure Pt, atomic oxygen is bonded too strongly and requires considerable overpotentials to react to Pt-OH. As a result of this, Pt is covered by (hydr)oxide adsorbates near the equilibrium potential of reaction (1). There is a consensus within the fuel cell catalysis community that a moderate reduction of the Pt-O chemisorption energy would result in significant ORR activity increases. H2O2 Hydrogen peroxide 0=0 Oxygen adsorption Oxygenated OH intemediates

Physisorption and chemisorption of alkanes and alkenes in H-FAU: a combined ab initio–statistical thermodynamics study

Abstract: The sorption in H-FAU zeolite of C4-C12 n-alkanes, and linear and branched C2-C8 alkenes has been quantified up to 800 K by combining QM-Pot(MP2//B3LYP) with statistical thermodynamics calculations. The physisorption strength increases linearly with increasing carbon number by 8.5 kJ mol(-1) and does not depend on the detailed alkane or alkene structure. Van der Waals interactions are dominant in physisorption, but alkenes are additionally stabilized by 20 kJ mol(-1) by formation of a pi-complex. Protonation of an alkene leads to the formation of alkoxides, which are more stable than the physisorbed species. As for physisorption a linear relation between the chemisorption energy and the carbon number is obtained. Protonation energies are independent of the carbon number but depend on the type of CC double bond being protonated. The relative stability difference between the secondary and tertiary alkoxides is 15 kJ mol(-1) in favor of the former. Both physisorption and chemisorption are accompanied with entropy losses which increase linearly with the carbon number. A typical compensation effect is obtained: the stronger the stabilization of the sorbed species the more pronounced the entropy loss. For temperatures ranging from 0 to 800 K, all of the derived linear relations expressing the physisorption and/or chemisorption enthalpy and entropy of the alkanes and the alkanes as function of the carbon number are independent of temperature. A good agreement between calculated and experimental values for alkanes is obtained at 500 K.

From electronic structure to catalytic activity: a single descriptor for adsorption and reactivity on transition-metal carbides.

Abstract: Adsorption and catalytic properties of the polar (111) surface of transition-metal carbides (TMC's) are investigated by density-functional theory. Atomic and molecular adsorption are rationalized with the concerted-coupling model, in which two types of TMC surface resonances (SR's) play key roles. The transition-metal derived SR is found to be a single measurable descriptor for the adsorption processes, implying that the Bronsted-Evans-Polanyi relation and scaling relations apply. This gives a picture with implications for ligand and vacancy effects and which has a potential for a broad screening procedure for heterogeneous catalysts.