Home About us Contact
|
Home About us Contact | ||
|
|
||
Adsorption Sites (adsorption + site)
Selected AbstractsCharged soc metal-organic framework for high-efficacy H2 adsorption and syngas purification: Atomistic simulation studyAICHE JOURNAL, Issue 9 2009Jianwen Jiang Abstract H2 adsorption and syngas purification in charged soc metal-organic framework are investigated using atomistic simulations. As experimentally observed, the extraframework NO3, ions are entrapped in carcerand-like capsule with negligible mobility. At low pressure, H2 adsorption occurs concurrently at multiple sites near the exposed indium atoms and organic components. The capsule is accessible at high pressure through the surrounding channels by restricted windows. Adsorption sites identified are remarkably consistent with inelastic neutron scattering measurements. The isotherm and isosteric heat of H2 adsorption predicted match well with experimental data. As loading rises, the isosteric heat remains nearly constant, revealing the homogeneity of adsorption sites. CO2/H2 selectivity in syngas adsorption is up to 600 and substantially higher than other nanoporous materials. With a trace of H2O, the selectivity increases slightly at low pressure due to promoted adsorption of CO2 by H2O bound proximally to the exposed indium atoms, but decreases at high pressure as a consequence of competitive adsorption of H2O over CO2. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Energy density analysis of cluster size dependence of surface,molecule interactions (II): Formate adsorption onto a Cu(111) surfaceJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2006Hiromi Nakai Abstract Adsorption of formate (HCOO) onto a Cu(111) surface has been treated theoretically using 18 kinds of Cun (6 , n , 56) clusters. The energy density analysis (EDA) proposed by Nakai has been adopted to examine surface,molecule interactions for different cluster sizes. EDA results for the largest model cluster Cu56 have shown that the adsorption-induced energy density variation in Cu atoms decays with distance from the adsorption site. Analysis of this decay, which can be carried out using the EDA technique, is important because it enables verification of the reliability of the model cluster used. In the case of formate adsorption onto the Cu(111) surface, it is found that at least a four-layer model cluster is necessary to treat the surface,molecule interaction with chemical accuracy. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 917,925, 2006 [source] The transient response study of CO, CO2, and O2 adsorption and CO oxidation over La0.4Sr0.6Co0.4Mn0.6O3THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2008Rong Li Abstract The transient behaviour caused by the change of the component concentration for CO oxidation on the perovskite-type catalyst La0.4Sr0.6Co0.4Mn0.6O3 was investigated. Results showed that CO was not adsorbed on the catalyst surface and CO oxidation was carried out between the surface oxygen species and gas phase CO. On the other hand, CO2 can be adsorbed on the catalyst surface but its adsorption site was different from the forming site. On a étudié le comportement transitoire causé parle changement dans la concentration des composantes pour l'oxydation du CO sur un catalyseur de type perovskite La0.4Sr0.6Co0.4Mn0.6O3. Les résultats montrent que le CO n'est pas adsorbé á la surface du catalyseur et que l'oxydation du CO s'est produite entre les espèces d'oxygène de surface et la phase gazeuse du CO. En outre, le CO2 peut être adsorbé sur la surface du catalyseur mais son site d'adsorption est différent du site de formation. [source] Unexpected Deformations Induced by Surface Interaction and Chiral Self-Assembly of CoII -Tetraphenylporphyrin (Co-TPP) Adsorbed on Cu(110): A Combined STM and Periodic DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2010Philip Donovan Abstract In a combined scanning tunnelling microscopy (STM) and periodic density functional theory (DFT) study, we present the first comprehensive picture of the energy costs and gains that drive the adsorption and chiral self-assembly of highly distorted CoII -tetraphenylporphyrin (Co-TPP) conformers on the Cu(110) surface. Periodic, semi-local DFT calculations reveal a strong energetic preference for Co-TPP molecules to adsorb at the short-bridge site when organised within a domain. At this adsorption site, a substantial chemical interaction between the molecular core and the surface causes the porphyrin macrocycle to accommodate close to the surface and in a flat geometry, which induces considerable tilting distortions in the phenyl groups. Experimental STM images can be explained in terms of these conformational changes and adsorption-induced electronic effects. For the ordered structure we unambiguously show that the substantial energy gain from the molecule,surface interaction recuperates the high cost of the induced molecular and surface deformations as compared with gas phase molecules. Conversely, singly adsorbed molecules prefer a long-bridge adsorption site and adopt a non-planar, saddle-shape conformation. By using a van der Waals density functional correction scheme, we found that the intermolecular ,,, interactions make the distorted conformer more stable than the saddle conformer within the organic assembly. These interactions drive supramolecular assembly and also generate chiral expression in the system, pinning individual molecules in a propeller-like conformation and directing their assembly along non-symmetric directions that lead to the coexistence of mirror-image chiral domains. Our observations reveal that a strong macrocycle,surface interaction can trigger and stabilise highly unexpected deformations of the molecular structure and thus substantially extend the range of chemistries possible within these systems. [source] Effect of deposition temperature on the properties of thin polymer films deposited by plasma-enhanced CVD using cyclo-hexa-hydrocarbons as monomersELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 4 2010Kazunori Moriki Abstract Plasma CVD is a candidate technology for the fabrication of optical polymer waveguides. It can deposit a film on any surface geometry and any substrate material at a temperature under 200 °C in a vacuum process. It also provides good thickness controllability and uniformity of the deposition film. In the present study, the effects of the deposition temperature on film properties, specifically the refractive index, deposition rate, and molecular structure, are discussed. The refractive index decreases as the deposition temperature rises. The logarithm of the deposition rate increases with the reciprocal of the temperature and the gradient of the deposition rate depends on the relative abundance of double bonds in the monomer source. The gradient does not change when CF4 is used instead of Ar as the gas mixed into the plasma, although the deposition rate increases by a factor of about five. We speculate that the deposition rate increases due to the increase in the abundance precursors produced by the presence of CF4 in the plasma and due to an increased abundance of dangling bonds on the surface of the deposition film caused by F radicals. We further speculate that the precursors incorporated into the polymer are selected on the substrate by the density of adsorption sites and the adsorption energy. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(4): 27,35, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10171 [source] The impact of metabolic state on Cd adsorption onto bacterial cellsGEOBIOLOGY, Issue 3 2007K. J. JOHNSON ABSTRACT This study examines the effect of bacterial metabolism on the adsorption of Cd onto Gram-positive and Gram-negative bacterial cells. Metabolically active Gram-positive cells adsorbed significantly less Cd than non-metabolizing cells. Gram-negative cells, however, showed no systematic difference in Cd adsorption between metabolizing and non-metabolizing cells. The effect of metabolism on Cd adsorption to Gram-positive cells was likely due to an influx of protons in and around the cell wall from the metabolic proton motive force, promoting competition between Cd and protons for adsorption sites on the cell wall. The relative lack of a metabolic effect on Cd adsorption onto Gram-negative compared to Gram-positive cells suggests that Cd binding in Gram-negative cells is focused in a region of the cell wall that is not reached, or is unaffected by this proton flux. Thermodynamic modeling was used to estimate that proton pumping causes the pH in the cell wall of metabolizing Gram-positive bacteria to decrease from the bulk solution value of 7.0 to approximately 5.7. [source] Models for the adsorption and self-assembly of ethanol and 1-decanethiol on Au(111) surfaces.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2010A comparative study by computer simulation Abstract Results from computer simulations, based on different models to study theadsorption and self-assembly of the ethanol and 1-decanethiol on gold surfaces, Au(111), are presented. Canonical ensemble Monte Carlo simulations were performed at 298 K using two different force fields. One from DFT calculations, where the gold electrode has an explicit structure (corrugated electrode), and the other representing an electrode, in which the structure is taken into account on an average way (flat electrode). The behavior of the ethanol adsorption on gold surfaces, with and without the 1-decanethiol presence, is analyzed. The introduction of molecular flexibility is also discussed. The relative surface density for the ethanol oxygen, adsorbed on gold, and the density profiles, in different conditions, show that the structure of the surface has a fundamental role on the way the adsorption takes place, not only on the preferential adsorption sites of the surface but also on the ethanol distribution over the electrodes. Potentials of mean force have also been calculated for the two surface models, giving the free energy barriers to the 1-decanethiol crossing of the solvent adsorption layers. The average tilt angle, obtained with a single thiol molecule in the simulation box, presents the values: ,26° for the rigid molecule model and 74° ± 18° for the flexible one. These differences are analyzed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Hydrodenitrification with PdCu Catalysts: Catalyst Optimization by Experimental and Quantum Chemical ApproachesISRAEL JOURNAL OF CHEMISTRY, Issue 1 2006Irena Efremenko A continuous process for nitrate and nitrite abatement from drinking water by catalytic hydrogenation has been developed in our lab. We describe the experimental process development procedure, and support it with semiempirical quantum chemical methods. Comparisons of activated carbon (ACC) and silica glass fiber (GFC) cloths as supports for mono- and bimetallic Pd-Cu catalysts show the former to be 45-fold and 15-fold more active for nitrite and nitrate hydrogenation, respectively, than the latter. Catalysts prepared by selective deposition of Cu on Pd/ACC led to better activity for nitrate hydrogenation than catalysts prepared by co-impregnation or ion exchange methods. The optimal Cu:Pd atomic ratio was found to be 1:2. The computational results show the following: (i) The dispersion of Pd catalysts supported on ACC is much higher than that on GFC due to the larger surface area and higher density of adsorption sites, and that accounts for the higher activity of PdCu/ACC; (ii) Nanosized Pd particles supported on ACC have a semispherical shape and possess preferentially close-packed triangular surfaces, while Pd/GFC particles are extended in the direction parallel to the support surface and show both fcc (100) and (111) planes; (iii) The interaction of Cu atoms with both supports is stronger than that of Pd; adsorbed Cu atoms show a greater ability to form monometallic than bimetallic bonds and that should result in poor mixing of the metal upon co-impregnation, as was observed experimentally; (iv) Cu atoms in bimetallic PdCu particles admit a significant positive charge; the experimentally measured solubility of metal atoms correlates with their calculated charges. The best catalyst (2 wt%Pd-0.6 wt%Cu/ACC) was employed in a novel continuous flow reactor for nitrate hydrogenation in distilled and tap water. The advantages of the reactor investigated over a conventional packed bed reactor are discussed, suggesting a potential for further process intensification. [source] Charged soc metal-organic framework for high-efficacy H2 adsorption and syngas purification: Atomistic simulation studyAICHE JOURNAL, Issue 9 2009Jianwen Jiang Abstract H2 adsorption and syngas purification in charged soc metal-organic framework are investigated using atomistic simulations. As experimentally observed, the extraframework NO3, ions are entrapped in carcerand-like capsule with negligible mobility. At low pressure, H2 adsorption occurs concurrently at multiple sites near the exposed indium atoms and organic components. The capsule is accessible at high pressure through the surrounding channels by restricted windows. Adsorption sites identified are remarkably consistent with inelastic neutron scattering measurements. The isotherm and isosteric heat of H2 adsorption predicted match well with experimental data. As loading rises, the isosteric heat remains nearly constant, revealing the homogeneity of adsorption sites. CO2/H2 selectivity in syngas adsorption is up to 600 and substantially higher than other nanoporous materials. With a trace of H2O, the selectivity increases slightly at low pressure due to promoted adsorption of CO2 by H2O bound proximally to the exposed indium atoms, but decreases at high pressure as a consequence of competitive adsorption of H2O over CO2. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Material design using molecular modeling for hydrogen storageAICHE JOURNAL, Issue 2 2009F. Darkrim Lamari Abstract Using grand canonical Monte-Carlo simulations, the adsorption capacities and isosteric heats of hydrogen on activated carbons, graphite nanofibers, and bundles of carbon nanotubes are estimated for identical thermodynamic states. These computations allow a systematic, meaningful, and unbiased comparison of the adsorption properties of hydrogen in such porous materials. The comparison shows that the hydrogen storage capacity can be optimized, but only to a limited extent, in adjusting the material pore sizes and functionalizing a part of the adsorption sites. Therefore, at room temperature and up to 70.0 MPa, for the three models of carbonaceous adsorbents, the hydrogen maximal excess adsorption is of the order of 2% of the adsorbent mass. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Surface characterization of salmeterol xinafoate powders by inverse gas chromatography at finite coverageJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 3 2005Henry H.Y. Tong Abstract In our previous studies, surface analysis by inverse gas chromatography (IGC) at infinite dilution (zero coverage) was performed on four salmeterol xinafoate (SX) powdered samples, viz, two supercritical CO2 -processed Form I (SX-I) and Form II (SX-II) polymorphs, a commercial granulated SX (GSX) raw material and its micronized product (MSX). Both GSX and MSX are also of the same Form I polymorph. To further probe the differences in surface properties between the samples, the present study has extended the IGC analysis to the finite concentration range of selected energy probes. The adsorption isotherms of the SX samples were constructed using (nonpolar) octane, (polar acidic) chloroform, and (polar basic) tetrahydrofuran as liquid probes. Type II adsorption isotherms with weak knees were observed with each probe for all SX Form I samples. The extents of probe adsorption by the samples at various relative pressures follow the rank order: SX-II,>,GSX,,,MSX,>,SX-I, indicating that the SX-I has fewer high-energy adsorption sites than GSX and MSX. Type III isotherms were observed for SX-II with the two polar probes, indicative of weak adsorbate,adsorbent interactions. The additional information generated shows that IGC analysis at finite coverage is a valuable complementary tool to that at infinite dilution. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:695,700, 2005 [source] Adsorption of Magnesium by Bottom Soils in Inland Brackish Water Shrimp Ponds in AlabamaJOURNAL OF THE WORLD AQUACULTURE SOCIETY, Issue 4 2010Harvey J. Pine Low-salinity (2.0,9.0 g/L) well waters used for inland culture of marine shrimp in Alabama are imbalanced with respect to ionic composition. Inputs of potassium (muriate of potash) and potassium-magnesium sulfate (Kmag®) fertilizers are used to correct these imbalances. Potassium is lost in overflow and intentional discharge, seepage, and through adsorption by bottom soils by exchangeable and non-exchangeable processes. This study was initiated to determine if bottom soils removed magnesium in the same manner as potassium. Laboratory soil,water mesocosms revealed that soils strongly adsorbed magnesium. The rate of adsorption tended to decline over time, indicating establishment of the equilibrium. Magnesium losses for the three soils ranged from 1405 to 1713 mg/tank (average = 1568 mg/tank). The cation exchange capacity (CEC) of the soils varied from 10.4 to 44.0 cmolc/kg (average = 24.6cmolc/kg). The decline in magnesium increased with higher soil CEC. In another trial, repeated exposures of soils to solutions of 40 mg Mg2+/L failed to saturate exchange sites, but rather maintained equilibrium with other base cations on soil adsorption sites. Dissolved sulfate resulting from additions of magnesium with magnesium sulfate heptahydrate (MgSO4·7H2O) was also monitored. Although difficulties of analysis occurred, sulfate was not adsorbed appreciably by the soils. [source] Effect of the dimer structure on indium adsorption and diffusion on a GaSb surfacePHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7-8 2009Min Xiong Abstract The adsorption properties and surface diffusion of an indium adatom on a GaSb(001)-(2 × 3) structure have been studied using first-principles calculations. We find that the indium adatoms are preferentially adsorbed in the low energy trenches along the [110] direction at bridge positions of surface dimers. Besides, the adsorption sites connecting these trenches along [110] present distinct properties for different surface dimer structures. For the structure with Sb,Sb homodimers, the dimer arrangement reduces significantly the adatom diffusion barrier along the [110] direction and the calculations on diffusion coefficients demonstrate that [110] is the fast diffusion direction. While on the structure with Ga,Sb heterodimers, the adsorption sites are separated by the heterodimers causing a considerable diffusion barrier and [10] becomes the fast diffusion direction accordingly. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] IR spectroscopy of adsorbates on ultrathin metal filmsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2005Annemarie Pucci Abstract Metal films with thickness in the nanometer range are optically transparent. With IR transmittance spectroscopy the in-plane film conductivity with its correlation to the film-growth process can be studied without electrical contacts and, on metal-island films, adsorbate vibrations can be observed because of surface enhanced IR absorption (SEIRA). Their analysis enables insight into the adsorbate-metal bonding and therefore gives information on the available adsorption sites and the crystalline facets correlated to. As in IR reflection absorption spectra dipole,dipole interaction of molecules on different sites modifies the vibration lines according to the degree of disorder ("atomic roughness"). Depending on that roughness IR spectra of adsorbate vibrations may be further modified because of their interaction with electronic excitations of the film. So, the limited facet size on cold-condensed metal films leads to additional IR activity: Raman lines of certain centrosymmetric adsorbate molecules (C2H4) are observed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Enhancement of cellulose saccharification kinetics using an ionic liquid pretreatment stepBIOTECHNOLOGY & BIOENGINEERING, Issue 5 2006Anantharam P. Dadi Abstract Hydrolysis of cellulose to glucose in aqueous media catalyzed by the cellulase enzyme system suffers from slow reaction rates due in large part to the highly crystalline structure of cellulose and inaccessibility of enzyme adsorption sites. In this study, an attempt was made to disrupt the cellulose structure using the ionic liquid (IL), 1- n -butyl-3-methylimidazolium chloride, in a cellulose regeneration strategy which accelerated the subsequent hydrolysis reaction. ILs are a new class of non-volatile solvents that exhibit unique solvating properties. They can be tuned to dissolve a wide variety of compounds including cellulose. Because of their extremely low volatility, ILs are expected to have minimal environmental impact on air quality compared to most other volatile solvent systems. The initial enzymatic hydrolysis rates were approximately 50-fold higher for regenerated cellulose as compared to untreated cellulose (Avicel PH-101) as measured by a soluble reducing sugar assay. © 2006 Wiley Periodicals, Inc. [source] Understanding Adsorption and Interactions of Alkane Isomer Mixtures in Isoreticular Metal,Organic FrameworksCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2007Li Zhang Dr. Abstract Novel metal,organic frameworks (MOFs) may lead to advances in adsorption and catalysis owing to their superior properties compared to traditional nanoporous materials. A combination of the grand canonical Monte Carlo method and configurational-bias Monte Carlo simulation was used to evaluate the adsorption isotherms of C4,C6 alkane isomer mixtures in IRMOF-1 and IRMOF-6. The amounts of adsorbed linear and branched alkanes increase with increasing pressure, and the amount of branched alkanes is larger than that of the linear ones. The locations of the alkane isomer reveal that the Zn4O clusters of the IRMOFs are the preferential adsorption sites for the adsorbate molecules. The interaction energy between the Zn4O cluster and the adsorbate is larger than that between the organic linker and the adsorbate. It was further confirmed that the Zn4O cluster plays a much more important role in adsorption by pushing a probe molecule into the pore at positions closer to the Zn4O cluster. It is difficult for branched alkane molecules to approach the Zn4O cluster of IRMOF-6 closely owing to strong spatial hindrance. In addition, the adsorption selectivity is discussed from the viewpoints of thermodynamics and kinetics, and the diffusion behavior of n -butane and 2-methylpropane were investigated to illustrate the relationship between diffusion and adsorption. [source] Entropy Effects in Atom Distribution and Electrochemical Properties of AuxPt1,x/Pt(111) Surface AlloysCHEMPHYSCHEM, Issue 7 2010Andreas Bergbreiter Abstract We report on the structural and electrochemical properties of AuxPt1,xsurface alloys prepared by Au vapour deposition onto Pt(111) followed by annealing to 1000 K. Driven by configurational entropy, Pt and Au atoms are distributed homogeneously over the surface. On the nm scale, however, atomically resolved scanning tunnelling microscopy images with chemical contrast reveal the formation of nm-sized Pt-rich and Au-rich aggregates, similar to the behaviour recently reported for PdxRu1,x/Ru(0001) [H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H. E. Hoster, R. J. Behm, Surf. Sci. 2009, 603, 1439]. Based on the STM data, we determine the abundance of specific adsorption sites for different Au contents, and we derive effective pair interaction parameters that allow reproducing the lateral distribution in Monte Carlo simulations. Cyclic voltammograms of the surface alloys have many similarities with Pt(111). Had and OHad related features both decrease with increasing amount of Au. Both seem to adsorb only on Pt sites, but Had requires smaller ensembles of Pt atoms than OHad. The onset potential for Had -formation decreases with increasing Au content. This is can be explained by an effect of the Au atoms on the entropy of adsorption. [source] Enhancement of Methanol Tolerance in DMFC Cathode: Addition of Chloride IonsCHEMPHYSCHEM, Issue 10 2008Sunghyun Uhm Dr. Abstract In the operation of a direct methanol fuel cell, the modification by chloride ions on the surface of a Pt cathode can facilitate the extraordinary increase of power performance and long-term stability. Analyzing the results of cyclic voltammograms and electrochemical impedance spectroscopy, the positive shift of Pt oxidation onset potential and the depression of oxidation current are observed, which results from the role of chloride as surface inhibitor. In addition, O2 temperature-programmed desorption and X-ray photoelectron spectroscopy also reveal that the suppression of Pt surface oxide can be best understood in terms of lower binding of oxygen species by the alteration of electronic state of Pt atoms. Such a reduced surface oxide formation not only provides more efficient proton adsorption sites with high selectivity but also decreases the mixed potential by crossover methanol, resulting in higher performance and stability even under high voltage long-term operation. [source] Microporous Niobia,Silica Membrane with Very Low CO2 PermeabilityCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 5 2008Vittorio Boffa Dr. Abstract A sol,gel-derived microporous ceramic membrane with an exceptionally low permeability for CO2 from gaseous streams was developed and characterized. The sols were prepared from a mixture of niobium and silicon alkoxide precursors by acid-catalyzed synthesis. Microporous films were formed by coating asymmetric ,-alumina disks with the polymeric sol (Si/Nb=3:1), followed by calcination at 500,°C. The membrane consists of a 150-nm-thick layer with a Si/Nb atomic ratio of about 1.5. The single-gas permeance of small gas molecules such as H2, CH4, N2, and SF6 decreases steadily with kinetic diameter. Hydrogen, helium, and carbon dioxide follow an activated transport mechanism through the membrane. The permeance of CO2 in this membrane is much lower than that in pure silica, and its behavior deviates strongly from the general trend observed with the other gases. This is attributed to a relatively strong interaction between CO2 and adsorption sites in the niobia,silica membrane. [source] | ||||||||||||||||||||||||||||