Showing papers on "Chemisorption published in 2001"
TL;DR: In this article, a series of activated carbon fibers (ACFs) were produced by treatment with ammonia to yield a basic surface and the micropore sizes of these chemically modified fibers were determined with nitrogen adsorption experiments and they were shown to increase with increasing activation time and temperature.
452 citations
TL;DR: This work observed reaction-induced electron excitations during gas interactions with polycrystalline silver for a variety of species with adsorption energies between 0.2 and 3.5 electron volts, and the measured time dependence of the electron current can be understood in terms of the strength and mechanism of Adsorption.
Abstract: The energy released in low-energy chemisorption or physisorption of molecules on metal surfaces is usually expected to be dissipated by surface vibrations (phonons). Theoretical descriptions of competing electronic excitations are incomplete, and experimental observation of excited charge carriers has been difficult except at energies high enough to eject electrons from the surface. We observed reaction-induced electron excitations during gas interactions with polycrystalline silver for a variety of species with adsorption energies between 0.2 and 3.5 electron volts. The probability of exciting a detectable electron increases with increasing adsorption energy, and the measured time dependence of the electron current can be understood in terms of the strength and mechanism of adsorption.
293 citations
TL;DR: In this article, a novel chemisorption method was employed for the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on one-component metal oxide catalysts.
Abstract: A novel chemisorption method was employed for the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on one-component metal oxide catalysts (MgO, CaO, SrO, BaO, Y2O3, La2O3, CeO2, TiO2, ZrO2, HfO2, V2O5, Nb2O5, Ta2O5, Cr2O3, MoO3, WO3, Mn2O3, Fe2O3, Co3O4, Rh2O3, NiO, PdO, PtO, CuO, Ag2O, Au2O3, ZnO, Al2O3, Ga2O3, In2O3, SiO2, GeO2, SnO2, P2O5, Sb2O3, Bi2O3, SeO2 and TeO2). The number of surface active sites for methanol dissociative adsorption corresponds to ∼3 μmol/m2 on average for many of the metal oxide catalysts. Furthermore, the methanol oxidation product distribution at low conversions reflects the nature of the surface active sites on metal oxides since redox sites yield H2CO, acidic sites yield CH3OCH3 and basic sites yield CO2. The distribution of the different types of surface active sites was found to vary widely for the different metal oxide catalysts. In addition, the commonality of the surface methoxy intermediate during dissociative chemisorption of methanol and methanol oxidation on oxide catalysts also allows for the quantitative determination of the turnover frequency (TOF) values. The TOF values for the various metal oxide catalysts were found to vary over seven orders of magnitude (10−3 to 104 s−1). An inverse relationship (for metal oxide catalysts displaying high (>85%) selectivity to either redox or acidic products) was found between the methanol oxidation TOF values and the decomposition temperatures of the surface M–OCH3 intermediates reflecting that the decomposition of the surface M–OCH3 species is the rate-determining step during methanol oxidation over the metal oxide catalysts.
251 citations
TL;DR: In this article, a brief resume of the relevant electrochemical and heterogeneous catalysis literature is expounded together with an analysis of the problem of chirality in two dimensions in order to show that, by utilizing an electrochemical surface science ap...
Abstract: The combination of electrochemical methods with the use of well-defined kinked metal surfaces allows the experimentalist to examine many fundamental aspects of asymmetric reactions at solid surfaces in a new and detailed manner. For example, by systematically changing both the stereogenic center on a particular chemisorbing molecule and also the molecular architecture of the metal surface, relationships between adsorption geometry and reactive sites on the metal surface may be deduced. In addition, by measuring the differential rate of chemisorption of chiral auxiliaries on R and S metal surfaces insights in to the mechanism of the so-called “Orito” reaction (whereby enantioselective hydrogenation of α-ketoesters is observed) may be deduced. In the present article, a brief resume of the relevant electrochemical and heterogeneous catalysis literature is expounded together with an analysis of the problem of chirality in two dimensions in order to show that, by utilizing an electrochemical surface science ap...
214 citations
TL;DR: The mesoporous silica SBA-15, functionalised with propylthiol groups during synthesis and rendered porous (mean pore diameter 51 A) by extraction of surfactant template molecules, shows strong and size selective adsorption of proteins, selectively excluding those with molecular weights of ca. 40000 u and above.
Abstract: The mesoporous silica SBA-15, functionalised with propylthiol groups during synthesis and rendered porous (mean pore diameter 51 A) by extraction of surfactant template molecules, shows strong and size selective adsorption of proteins, selectively excluding those with molecular weights of ca. 40000 u and above. A model for the adsorption process is proposed, in which reversible physisorption is followed by irreversible chemisorption. Adsorption of proteins on an unfunctionalised SBA-15 from which the template has been removed by calcination, (mean pore diameter 56 A) shows shape selective and reversible adsorption of proteins with molecular weights of ca. 43000 u and below.
212 citations
TL;DR: In this paper, the surface structure of the NOx decomposition is reconstructed during the catalytic reaction, and a general consensus emerges on (i) the chemisorption of the reactant(s) which causes adsorbate-induced restructuring of the surfaces, (ii) the substrate, which is influenced by the redox properties of the support and by the particle sizes of the metallic aggregates, (iii) the fact that low-coordinated surface atoms are the more active in NO bond breaking, and (iv) the general pathways of
Abstract: The mechanism of NOx decomposition is reviewed. This reaction is first studied on metal or oxide single crystals, in order to get solid basis from catalysis on well-defined surface structures. If we understand few things about this reaction, at least the catalytic surface is known. This statement is not 100% correct, as the surface is reconstructed during the catalytic reaction; and this point is considered in the manuscript. Then we analyse this reaction on metallic catalysts supported on metal oxides. The crystallites are more or less dispersed, and the support can play the role of a modifier or of a promoter. A general consensus emerges on (i) the chemisorption of the reactant(s) which causes adsorbate-induced restructuring of the surfaces, (ii) the substrate, which is influenced by the redox properties of the support and by the particle sizes of the metallic aggregates, (iii) the fact that low-coordinated surface atoms are the more active in NO bond breaking, and (iv) the general pathways of the reaction, where redox mechanisms involving nitrosyl species, as intermediates, moving to nitrite-nitrosyl or dinitrosyl species are proposed. All these points are discussed.
208 citations
TL;DR: In this paper, the authors proposed a model aiming to generalize the important factors that affect dissociation reactions, such as the dissociation barrier with the association barrier, the chemisorption energies of A and B at the final state and the bonding energy of AB in the gas phase.
Abstract: Dissociative adsorption is one of the most important reactions in catalysis. In this communication we propose a model aiming to generalize the important factors that affect dissociation reactions. Specifically, for a dissociation reaction, say AB→A+B, the model connects the dissociation barrier with the association barrier, the chemisorption energies of A and B at the final state and the bonding energy of AB in the gas phase. To apply this model, we have calculated CO dissociation on Ru(0001), Rh(111), Pd(111) (4d transition metals), Os(0001), Ir(111), and Pt(111) (5d transition metals) using density function theory (DFT). All the barriers are determined. We find that the DFT results can be rationalized within the model. The model can also be used to explain many experimental observations.
204 citations
TL;DR: In this paper, the gradient corrected density function theory calculations on the adsorption and thermally activated diffusion pathways of hydrogen on the {111} surfaces of Ni, Pd, and Pt were presented.
Abstract: We present gradient corrected density function theory calculations on the adsorption and thermally activated diffusion pathways of hydrogen on the {111} surfaces of Ni, Pd, and Pt. We find that the variation of the adsorption energy as a function of adsorption site shows considerable differences between the three metals. For Ni and Pd, the adsorption energies vary as a function of hydrogen coordination with the 3-fold hollow sites the most stable and the 1-fold atop site considerably less stable. On Pt the adsorption energies for all the sites are similar indicating that diffusion across the surface will be faster on Pt than on Ni or Pd. The activation energies for diffusion have been calculated with that for Pt (2−3 kJ mol-1) considerably smaller than for Ni or Pd (13 kJ mol-1). The calculated adsorption and activation energies are in good agreement with experimental investigations of the structure, energetics, and diffusion properties.
180 citations
TL;DR: In this paper, the authors used density functional theory to study the adsorption of hydroxyl at low and high coverages and investigate the nature of the intermediate in the H2O formation reaction on Pt(111).
Abstract: Density functional theory has been used to study the adsorption of hydroxyl at low and high coverages and also to investigate the nature of the intermediate in the H2O formation reaction on Pt(111). At low coverages [1/9 of a monolayer (ML) to 1/3 ML] OH binds preferentially at bridge and top sites with a chemisorption energy of ∼2.25 eV. At high coverages (1/2 ML to 1 ML) H bonding between adjacent hydroxyls causes: (i) an enhancement in OH chemisorption energy by about 15%; (ii) a strong preference for OH adsorption at top sites; and (iii) the formation of OH networks. The activation energy for the diffusion of isolated OH groups along close packed rows of Pt atoms is 0.1 eV. This low barrier coupled with H bonding between neighboring OH groups indicates that hydroxyls are susceptible to island formation at low coverages. Pure OH as well as coadsorbed OH and H can be ruled out as the observed low temperature intermediate in the water formation reaction. Instead we suggest that the intermediate consists ...
171 citations
TL;DR: In this paper, the adsorption and dissociation of dioxygen on the surface of the Cu(1.1) surface have been studied using periodic self-consistent density functional calculations.
170 citations
TL;DR: In this article, the authors examined magnetic measurements (MMs), transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) for the estimation of cobalt dispersion in a series of Co/SiO 2 catalysts.
TL;DR: In this article, the number of surface sites active in methanol oxidation has been determined over a wide range of supported metal oxide catalysts using quantitative methanoline chemisorption and in-situ infrared titration techniques performed at an experimentally optimized temperature of 110 °C.
Abstract: Methanol oxidation over metal oxide catalysts is industrially important for the production of formaldehyde, but knowledge about the intrinsic catalysis taking place is often obscured by a lack of knowledge as to the number of active sites present on the catalyst surface. In the present study, the number of surface sites active in methanol oxidation has been determined over a wide range of supported metal oxide catalysts using quantitative methanol chemisorption and in-situ infrared titration techniques performed at an experimentally optimized temperature of 110 °C. It was found that a steric limitation of about 0.3 methoxylated surface species (e.g., strongly Lewis-bound CH3OHads and dissociatively adsorbed −OCH3,ads, which are the reactive surface intermediates in methanol oxidation) exists per active deposited metal oxide metal atom across all supported metal oxides. Hence, the use of methanol chemisorption for counting active surface sites is more realistic than other site-counting methods for the kine...
TL;DR: In this paper, the authors investigated several catalytic reactions on some transition metal surfaces, using density functional theory, and determined all the reaction barriers related to the potential energy surface of reactants on the surface, the total chemisorption energy of reactant, and the metal d orbital occupancy and the reactant valency.
Abstract: A catalyst preparation by design is one of the ultimate goals in chemistry. The first step towards this goal is to understand the origin of reaction barriers. In this study, we have investigated several catalytic reactions on some transition metal surfaces, using density functional theory. All the reaction barriers have been determined. By detailed analyses we obtain some insight into the reaction barrier. Each barrier is related to (i) the potential energy surface of reactants on the surface, (ii) the total chemisorption energy of reactants, and (iii) the metal d orbital occupancy and the reactant valency.
TL;DR: In this article, the role of pore size, pore volume, and pore surface chemistry on adsorption of sulfur dioxide and its catalytic conversion to sulfuric acid was investigated.
TL;DR: The previously unexplained experimental observations of the direct hydrogenation of fullerenes under high pressure lend further support for a mechanism for the dissociative chemisorption of H2 on carbon nanotubes.
Abstract: Based on first principles calculations, we propose a mechanism for the dissociative chemisorption of H2 on carbon nanotubes. The breaking of the H—H bond is concerted with the formation of two C—H bonds on two adjacent carbon nanotubes in solid phase, facilitated by the application of high pressure which shortens the interstitial distance between nanotubes. The process is reversible upon the release of external pressure and could make an important contribution to the observed hydrogen storage capacity of carbon nanotubes. The previously unexplained experimental observations of the direct hydrogenation of fullerenes under high pressure lend further support for such a mechanism.
TL;DR: In this article, the adsorption energies and structures of methanethiolate, SCH3, on the (111) surfaces of Au, Ag, and Cu have been studied using a density functional theory.
Abstract: The adsorption energies and structures of methanethiolate, SCH3, on the (111) surfaces of Au, Ag, and Cu have been studied using a density functional theory. The results obtained for the Au surface are in good agreement with experiments and previous calculations. The strength of the adsorption energies is found to be Cu>Ag>Au, and the nature of the chemisorption bond is discussed. The strong interaction between the SCH3 and Cu surface can be explained in a similar way to that as for the binding energy of SCH3 with metal atoms. Scalar-relativistic effects in the adsorption energies and adsorption structures, which dominate the differences observed between the Ag and Au surfaces, are studied using quasirelativistic and nonrelativistic pseudopotentials. The relativistic effects decrease the adsorption energy of SCH3 on the Au(111) surface, although the binding energy of the AuSCH3 complex is increased by relativity. The unexpected relativistic effects are also discussed.
TL;DR: In this paper, the adsorption of benzene on all three low-index surfaces of nickel has been studied using gradient-corrected density-functional calculations, based on ultrasoft pseudopotentials, residuum minimization techniques for the calculation of the electronic ground-state and of the Hellmann-Feynman forces and stresses, and on a conjugate-gradient technique for the optimization of the atomic structure.
Patent•
24 Oct 2001
TL;DR: In this paper, an electrostatic chuck (ESC) assembly is used to create a DC bias on the substrate to attract charged gas ions in the chamber to the substrate, resulting in improved chemisorption results.
Abstract: A process chamber for conducting an ALD process to deposit layers on a substrate includes an electrostatic chuck (ESC) to retain the substrate. Electrodes in the chuck assembly are biased so as to create a DC bias on the substrate to attract charged gas ions in the chamber to the substrate. Improved chemisorption results.
TL;DR: In this article, density functional calculations for the interaction of CO on different Cu-Pd(1.1) bulk and surface alloys are presented, in terms of changes in the adsorption sites and the change of the electronic structure occurring upon alloying.
TL;DR: In this article, the authors investigated the second rate-limiting factor in the case of these Al-based materials that was deduced to he related to Al-rich surface oxides.
Abstract: Oxygen reduction reaction (ORR) kinetics were investigated on hulk synthesized analogues of Al-Cu, Al-Cu-Mg, and Al-Cu-Fe-Mn intermetallic phases with and without chromate conversion coating (CCC) in 0.1 M Na 2 SO 4 + 0.005 M NaCl (pH 6) with minimal levels of total dissolved chromate. The results were compared to AA 2024-T3 and high purity Al. Cu, Cr, and Au. Net cathodic ORR mass transport-limiting current densities of Al-based materials, lacking large quantities of Fe, Mn, or Cu were lower than the theoretically predicted rates in the mass transport controlled regime of ideal electronic conductors. This suggests a second rate-limiting factor in the case of these Al-based materials that was deduced to he related to Al-rich surface oxides. A second rate-limiting effect was also seen for pure Cr, implying that Cr 2 O 3 inhibits ORR kinetics, CCC inhibited open circuit corrosion, via reduced ORR kinetics. It also serves as a diffusive barrier to O 2 transport. The role(s) of CCC as an enhanced electronic barrier to electron transfer or barrier to O 2 chemisorption remains unclear.
TL;DR: A real-time in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) technique was developed to study the adsorption of linear phosphates with different number of P atoms on titania surface from aqueous solutions as mentioned in this paper.
TL;DR: The results show that interactions with gold can be blocked by a silane layer so as not to significantly influence physisorption of molecules at the C-18/solution interface.
Abstract: Immobilized gold colloid particles coated with a C-18 alkylsilane layer have been characterized as a substrate for surface-enhanced Raman scattering (SERS) studies of adsorption onto hydrophobic surfaces Atomic force microscopy images, optical extinction spectra, and SERS measurements are reported as a function of accumulation of gold colloid on glass As the metal particles become increasingly aggregated on the surface, the SERS enhancement increases until the plasmon resonance shifts to wavelengths longer than the excitation laser The gold colloid substrates are stable and exhibit reproducible SERS enhancement When octadecyltrimethoxysilane is self-assembled over the gold, the metal surface is protected from exposure to solution-phase species, as evidenced by the inhibition of chemisorption of a disulfide reagent to the overcoated gold surface The results show that interactions with gold can be blocked by a silane layer so as not to significantly influence physisorption of molecules at the C-18/solu
TL;DR: The interaction of different bitartrate isomers on the Cu(110) surface has been investigated systematically by using the Vienna Ab-initio Simulation Package (VASP), which performs periodical density functional theory (DFT) calculations.
Abstract: In the present work the interaction of different bitartrate isomers on the Cu(110) surface has been investigated systematically by using the Vienna Ab-initio Simulation Package (VASP), which performs periodical density functional theory (DFT) calculations. Among all bitartrate isomers the R,R-configuration is the most stable under the (3 1, 1 2) domain on the Cu surface. Its optical isomer, the S,S-bitartrate, is 10 kJ mol(-)(1) less stable in the same domain. This energy difference is sufficient to produce the distinct chiral assemblies observed after the adsorption of each optical isomer on the Cu surface. The calculations also showed that these domains are not formed due to intermolecular H-bonds, in contrast with the previous proposal by Raval et al.(Nature 2000, 23, 376). In fact, there is a formation of optimal intramolecular H-bonds in the chemisorption structures. A favorable packing orientation is also needed for the respective chiral domains. For instance, the S,S-configuration suffers from a destabilizing packing energy of 21 kJ mol(-)(1) under the same domain, due to a short contact between the H atoms of the hydroxy groups. These intramolecular H-bonds cause also some distortions on the bitartrate molecule, which appear to be dependent on the relative position of the alpha-hydroxy groups. The stability of the extended asymmetric domains, when the surface is modified by a chiral additive, might have important consequences for understanding and optimizing the properties of enantioselective heterogeneous catalysts.
TL;DR: In this article, the authors investigated the mechanistic origin of the ligand effect in the selective oxidation of methanol to formaldehyde using a novel in situ IR cell designed to operate as a fixed-bed catalytic reactor with coupling to an online gas chromatograph for analysis of gas phase product distributions.
TL;DR: Abstractive chemisorption in the initial oxidation of Al(111) has been experimentally verified using variable incident energy O2 and the observations that both abstractive and dissociative chem isorption are activated processes are in contrast to current adiabatic models of the absorption process.
Abstract: ive chemisorption in the initial oxidation of Al(111) has been experimentally verified using variable incident energy ${\mathrm{O}}_{2}$. Scanning tunneling microscopy images show a transition between single O-adatom reaction products to more pairs of O-adatom reaction products as the ${\mathrm{O}}_{2}$ incident energy is raised from 0.025 to 0.8 eV. The ejected O atoms have been detected in the gas phase with resonant enhanced multiphoton ionization. The observations that both abstractive and dissociative chemisorption are activated processes are in contrast to current adiabatic models of the absorption process.
TL;DR: In this paper, X-ray photoelectron spectroscopy (XPS) technique was employed to characterize Al2O3-TiO2 support and MoO3/Al 2O3 -TiO 2 catalyst calcined at different temperatures from 773 to 1073
Abstract: X-ray photoelectron spectroscopy (XPS) technique was employed to characterize Al2O3-TiO2 support and MoO3/Al2O3-TiO2 catalyst calcined at different temperatures from 773 to 1073 K. The Al2O3-TiO2 (1:1.3 mole ratio) binary oxide support was obtained by a coprecipitation procedure with in situ generated ammonium hydroxide. A nominal 12 wt.% MoO3 was impregnated over the calcined (773 K) support by a wet impregnation method. The initial characterization by X-ray powder diffraction, Fourier transform-infrared (FT-IR), and O2 chemisorption techniques revealed that the impregnated MoO3 is in a highly-dispersed state on the surface of the support. XPS electron binding energy (Eb) values indicate that the MoO3/Al2O3-TiO2 catalyst contains the mixed-oxide elements in the highest oxidation states, Ti(IV), Al(III), and Mo(VI), respectively. However, the core level Eb of Al 2p slightly increased with increase of calcination temperature, and this effect was more prominent in the case of molybdena-doped samples. A better resolved Mo 3d doublet was observed at all calcination temperatures. This was explained as due the coverage of alumina surface by titania, thereby lowering the interaction between molybdena and alumina. The XPS atomic ratios indicate that the Ti/Al ratio is sensitive to the calcination temperature. The Mo/Al ratio was found to be more than that of Mo/Ti ratio and decreased with increasing calcination temperature. A clear difference between the Al2O3 and the TiO2 surfaces, in terms of surface free energy, isoelectric point, and surface hydroxyl distribution was considered to be responsible for different distributions of molybdena over these supports.
TL;DR: In this paper, it was shown that if the surface species formed by oxygen adsorption are restricted to be charge neutral, then oxygen cannot be exothermically adsorbed from the gas phase on the stoichiometric surface.
TL;DR: In this article, fast-scan cyclic voltammetry at high repetition rates was used to characterize adsorptive properties of dopamine (DA) at native and modified carbon-fiber microelectrode surfaces.
Abstract: Fast-scan cyclic voltammetry at high repetition rates was used to characterize adsorptive properties of dopamine (DA) at native and modified carbon-fiber microelectrode surfaces. Disk electrodes were fabricated from Thornel P55 fibers, and cylindrical electrodes, from Thornel T650 fibers. Their surfaces were modified by physisorption of 2,6-anthraquinone disulfonic acid (2,6-AQDS) or chemisorption of 4-carboxyphenyl or catechols. Chemisorption was accomplished via electrochemical reduction of diazonium salts. The degree of DA adsorption and its oxidation kinetics were found to vary for the two types of native carbon fiber electrodes and with the different chemical overlayers on the carbon surfaces. 2,6-AQDS measurably increased DA adsorption and desorption kinetics at P55 disks without a significant change in the measurement sensitivity, the response exhibiting temporal characteristics similar to that for nonadsorbing species. 4-Carboxyphenyl modification accelerated the DA adsorption rate and sensitivity...
TL;DR: A series of titania supported molybdenum catalysts were prepared by incipient wetness impregnation method and characterized by BET surface area, XRD, TPR, FTIR, ESCA, and low temperature oxygen chemisorption.
Abstract: A series of titania supported molybdenum catalysts were prepared by incipient wetness impregnation method and characterized by BET surface area, XRD, TPR, FTIR, ESCA, and low temperature oxygen chemisorption. Thiophene, cyclohexene and tetrahydrofuran were taken as model compounds for evaluating catalytic activities for hydrodesulfurization (HDS), hydrogenation (HYD), and hydrodeoxygenation (HDO) reactions, respectively. XRD results indicate that molybdenum oxide species are dispersed as a monolayer on the support up to 8 wt.% Mo and the formation of crystalline MoO 3 is observed above this loading. FTIR and TPR results showed that molybdenum oxide species were present predominantly in tetrahedral form at lower loading and polymeric octahedral forms are dominant at higher loading. Both oxygen chemisorption and rates of reaction were found to increase with increasing Mo loading up to 8 wt.% and then decrease with further increase in loading. HDS and HYD activities are more or less same but HDO activity is two times higher than HDS and HYD activities. The results are also interpreted with the help of other parameters, like dispersion, equivalent molybdenum surface area, surface coverage, crystalline size, quasi-turnover frequencies and intrinsic activities. ESCA results suggest that electron transfer is taking place from support to metal.