Showing papers on "Chemisorption published in 1991"

Molecular adsorption on oxide surfaces: Electronic structure and orientation of NO on NiO(100)/Ni(100) and on NiO(100) as determined from electron spectroscopies and ab initio cluster calculations

Abstract: We have investigated the adsorption of NO on a thin NiO(100) film of several layers thickness grown on top of a Ni(100) surface in comparison with data of an in vacuo cleaved NiO(100) single crystal. The layer exhibits a high defect density. We demonstrate via application of several surface-sensitive electron-spectroscopic techniques [i.e., x-ray photoelectron spectroscopy (XPS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), near-edge x-ray-absorption fine structure (NEXAFS), and high-resolution electron-energy-loss spectroscopy (HREELS)] that this layer has similar occupied (ARUPS) and unoccupied (NEXAFS) states as a bulk NiO(100) sample. In spite of its limited thickness, the band structure of the film exhibits dispersions perpendicular to the surface compatible with bulk NiO(100). It is shown that the electronic structure of the oxygen sublattice can be described in a band-structure picture while for the Ni sublattice electron localization effects lead to a breakdown of the band-structure picture. NO on NiO desorbs at 220 K. This indicates weak chemisorption. The NO coverage is close to 0.2 relative to the number of Ni surface atoms as determined by XPS.HREELS reveals that there is only one species on the surface documented by the observation of only one bond-stretching frequency. NEXAFS data on the system and a comparison with previous data on the system NO/Ni(100) indicate that the molecular axis of adsorbed NO is tilted by an angle of approximately 45\ifmmode^\circ\else\textdegree\fi{} relative to the surface normal. The N 1s XP spectra of the weakly chemisorbed species show giant satellites similar to the previously observed cases for weak chemisorption on metal surfaces. This is the first observation of an intense satellite structure for an adsorbate on an insulator surface, which shows that there must be sufficient screening channels even on an insulating surface. A theoretical assignment of the peaks is discussed. We compare the spectroscopic properties of the NO species on the thin-film oxide surface, which is likely to contain a certain number of defects, with NO adsorbed on a basically defect-free bulk oxide surface by thermal-desorption (TDS) and XP spectra. TDS and XP spectra of the bulk system are basically identical as compared with the oxide film, indicating that the majority of species adsorbed on the film is not adsorbed on defects but rather on regular NiO sites. Results of ab initio oxide cluster calculations are used to explain the bonding geometry of NO on regular NiO sites.

Surface science studies of the electronic and chemical properties of bimetallic systems

Abstract: Recent studies dealing with the electronic, chemical, and catalytic properties of well-defined bimetallic surfaces (prepared by vapor-depositing one metal onto a crystal face of a second metal) are discussed. The result show that a metal atom supported on a dissimilar metal can be electronically perturbed and this perturbation can dramatically alter the chemical and catalytic properties of both constituents of the bimetallic system. In many cases, the metal adatoms exhibit properties toward the chemisorption of H{sub 2}, O{sub 2}, CO, and CO{sub 2} and reactivities toward small hydrocarbons that are significantly different from those seen for the pure metal. For supported monolayers of Ni, Cu, and Pd a correlation is observed between shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. The shifts in core-level binding energies and CO desorption temperatures are a consequence of (1) electronic interactions between the metal overlayer and metal substrate and (2) variations that occur in the admetal-admetal interactions when the admetal adopts the lattice parameters of the substrate. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behavior of high surface area supported bimetallic catalysts. The coupling of an apparatus formore » the measurement of reaction kinetics at elevated pressures with an ultrahigh-vacuum system for surface analysis allows detailed studies of structure sensitivity, the effects of surface composition on catalytic activity, and, in certain cases, identification of reaction intermediates by postreaction analysis. The roles of ensemble' and ligand' effects in mixed-metal catalysts are discussed in the light of data obtained by using well-defined bimetallic surfaces.« less

Surface-chemistry of colloidal silver in aqueous-solution - observations on chemisorption and reactivity

Abstract: Colloidal silver particles (3-nm diameter) are oxidized in the absence of air by organic and inorganic electron acceptors (such as nitrobenzene, methylviologen, nitropyridine oxide, and hexacyanoferrateIII) when nucleophilic reagents (such as CN- and SH-) are present in the solution. The mechanism of catalyzed metal oxidation proposed previously,2 according to which the interaction of surface atoms with the nucleophiles leads to an excess negative charge in the metal interior which can be picked up by the electron acceptors, explains the observed phenomena. This mechanism contains reversible steps. An experiment is reported in which the reverse sequence of events is observed: chemisorption of Ag(CN)2- on silver particles, producing an excess positive charge inside the particles, followed by complete reduction of the Ag+ ions of the adsorbed complexes by excess electrons that are deposited by a reducing reagent (i.e., organic radicals generated radiolytically). Ag(CN)2- ions in solution are not reduced by radicals. © 1991 American Chemical Society.

Dynamics of the chemisorption of O2 on Pt(111): Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy

Abstract: We have used the thermal desorption spectroscopy of the O/O2+CO→CO2 system to probe the chemical nature of oxygen that remains on a Pt(111) surface following exposure to a supersonic O2 beam under various conditions. We find that for a surface temperature of 90 K, the resulting CO2 formation thermal desorption spectrum is the same for all beam kinetic energies employed up to 1.1 eV at normal incidence, in all cases resembling that assigned to the O2+CO co‐adsorbate system. This spectrum is clearly distinct from the O+CO case, where atomically chemisorbed oxygen is obtained either by thermal dissociation of O2 on the surface or by exposing the 90 K surface to a beam containing O atoms. These results imply that the dissociative chemisorption of O2 on Pt(111) proceeds by way of a molecular precursor even at relatively high incidence kinetic energies, at least as high as 1.1 eV. This interpretation readily accounts for the strong surface temperature dependence associated with dissociation under these conditio...

Chemisorption of -cysteine and 3-mercaptopropionic acid on gold and copper surfaces: An infrared reflection-absorption study

Abstract: Infrared reflection-absorption spectroscopy (IRAS) has been used to study the chemisorption of l -cysteine ( l -Cys) and 3-mercaptopropionic acid (MPA) on evaporated gold and copper surfaces. The infrared data show that monomolecular films are formed for both l -Cys and MPA on gold, and that the bonding to the surface occurs primarily via the SH group. More complex reactions occur on the copper surface. MPA forms a multilayer structure on copper when adsorbed from solution at pH 3.5. In the very first layer the SH group is coordinated to the copper surface and the CO2− group to dissolved copper ions. The subsequent layers consist of MPA molecules in the dimeric form. Dissolution of copper is not very pronounced at high pH values as evidenced by the formation of a sodium salt instead of a cuprous (cupric) complex when MPA is adsorbed from a NaOH solution at pH 11.5. l -Cys also forms a complex ionic structure on copper where the SH, NH2 and CO2− groups appear to be involved in the bonding to the copper surface and/or dissolved copper ions.

Adsorbed state of benzene on the Si(100) surface: Thermal desorption and electron energy loss spectroscopy studies

Abstract: The adsorbed state of benzene on the Si(100) surface at 90 and 300 K has been investigated by the use of thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS). Benzene is chemisorbed nondissociatively on Si(100) at 300 K, and the fractional saturation coverage corresponds to 0.27 benzene molecule per surface Si atom. It is proposed that chemisorbed benzene is di‐σ bonded to two adjacent Si atoms saturating the dangling bonds on Si(100). At 90 K, physisorbed multilayers of benzene molecules are formed in addition to the chemisorbed layer. The multilayers consist of the metastable transition layer (α2) and ‘‘bulk’’ multilayers (α3). These results are markedly different from those of benzene on the Si(111)(7×7) surface, and the origin of the crystal‐face specificity is discussed.

Simulations of buckminsterfullerene (C60) collisions with a hydrogen-terminated diamond {111} surface

Abstract: The stability of C{sub 60} during collisions with a hydrogen-terminated diamond (111) surface was studied by using molecular dynamics simulations. At a collision energy of 150 eV only nonreactive collisions occur. At higher energies nonreactive scattering and two types of reactive collisions occur: (1) exchange of one or more atoms between the molecule and the surface; (2) chemisorption of the C{sub 60} molecule. No dissociation of the rebounding molecules was observed on the time scale of the simulations. However, the scattered molecules contain a large amount of internal energy, which suggests that dissociation may occur at longer times.

Real-Time Measurements of the Gas-Phase Adsorption of n-Alkylthiol Mono- and Multilayers on Gold

Abstract: Results of surface acoustic wave, ellipsometric, and electrochemical experiments indicating that stable mono- or multilayer films result from the gas-phase chemisorption of n-alkylthiol molecules onto Au substrates are reported. Measurement of the mass and thickness of CH3(CHS Nuzzo et al.3e used ultra- high vacuum techniques to study the adsorption of CH3- SH and (CH3S)z to Au and found that both materials formed strongly bound films. The alkylthiol interaction did not involve cleavage of the S-H bond, but the disul- fide interacted with the Au substrate by a thiolate interaction. Solution-deposited alkylthiol films are also thought to involve a thiolate-Au interaction.2e We have used SAW devices to study the real-time adsorption characteristics of n-alkylthiol molecules at atmospheric pressure. SAW devices are extremely sen- sitive to the presence of surface-confined adsorbates as a result of their high operating frequencies and confinement of the acoustic energy to within one wavelength of the ~urface.~ The velocity of the SAW, which is perturbed in direct proportion to the mass loading of the device, is measured as a change in frequency when the device is used as the feedback element of an oscillator circuit. Changes in the attenuation of the acoustic wave can be related to the viscoelastic properties of the surface-confined adsorbate as we11.5

Detection of chemisorbed methyl and methylene groups : surface chemistry of methyl iodide on Pt(111)

Abstract: The thermal chemistry of methyl iodide on Pt(111) surfaces was studied by using thermal programmed desorption (TPD), X-ray photoelectron (XPS), and reflection-absorption (RAIRS) spectroscopies. Spectra obtained at low temperatures for both normal and fully deuterated methyl iodide indicate that chemisorption is molecular and that the adsorption geometry changes with coverage, starting with a tilted configuration after small doses and switching to a structure where the C-I bond is perpendicular to the surface above half-saturation. The only reaction that can be thermally activated at low coverages is a C-I bond breaking step, which is then followed by total dehydrogenation and by hydrogen desorption. At higher coverages, however, an appreciable amount of methane also desorbs, and both methyl and methylene groups form on the surface.

Precursor state in the chemisorption of CO on supported palladium clusters

Abstract: It is demonstrated that physisorption of CO molecules on a MgO (100) surface, supporting epitaxially grown Pd particles, acts as a precursor state for CO chemisorption on the particles. The adsorption rate of CO on the Pd particles is measured, at zero coverage by a molecular beam technique, as a function of the substrate temperature and for different particle sizes. A kinetic model describing the adsorption, desorption, diffusion, and capture by the clusters of CO molecules is given. Comparison with the experimental data gives the adsorption probability: 0.5±0.05 and the saddle energy for surface diffusion: 0.25±0.05 eV of CO molecules on MgO.

Hydrogen adsorption on supported cobalt, iron, and nickel

Abstract: This paper emphasizes concepts and fundamentals relating to the kinetics, energetics, and stoichiometries of adsorption of hydrogen on supported cobalt, iron and nickel, with emphasis on nickel. Relationships between catalyst and adsorption properties and the application of hydrogen chemisorption to the measurement of metal surface areas are discussed. Evidence is presented for nonstoichiometric adsorption of hydrogen on supported metals and the results are interpreted in terms (i) reversibility of adsorption and (ii) interactions of hydrogen with metal oxides present on or near metal crystallites. Contamination of the metal surface by support moieties can cause (i) the appearance of new adsorption states of hydrogen at higher binding energies and (ii) an increase in the adsorption activation energy for hydrogen which can lead to severe kinetic limitations in the adsorption process. Precalcination treatments and promoters such as potassium also cause the appearance of new high temperature adsorption states and significantly increase the adsorption activation energy for hydrogen.

Unusual chemisorption geometry of Na on Al(111)

Abstract: The adsorption of Na on Al(111) at room temperature has been studied by surface extended x-ray-absorption fine-structure (SEXAFS) experiments as well as by parameter-free calculations. For coverages of ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{Na}}$=0.16--0.33, the SEXAFS analysis shows that Na atoms occupy an unusual six-fold-coordinated substitutional site. The Na-Al bond length is determined as 3.31 \AA{}, consistent with metallic bonding. Ab initio density-functional-theory calculations for several adsorbate geometries show that the substitutional site has the lowest total energy.

Ammonia chemisorption studies on silicon cluster ions

Abstract: Silicon clusters in the size range from 5 to 66 atoms were generated by laser vaporization in a supersonic nozzle and injected into the ion trap of a specially‐designed Fourier transform ion cyclotron resonance apparatus. On the positively charged clusters ammonia chemisorption reaction rates were found to vary by over three orders of magnitude as a function of cluster size, with clusters of 21, 25, 33, 39, and 45 atoms being particularly unreactive, and cluster 43 being the most reactive. For the negative cluster ions, 43 was the only cluster found to react substantially. Although the reaction behavior of many clusters clearly indicated that several structural isomers were present with different reaction rates, the strikingly low net reactivity of such clusters as 39 and 45 provides evidence that they have effectively crystallized into a single specially stable form.

Chemisorption of Carbon Monoxide on Three Metal Surfaces: Ni(111), Pd(111) and Pt(111. A Comparative Study

Abstract: : At one-third coverage of carbon monoxide, a (3 x 3)R30 deg low energy electron diffraction (LEED) pattern has been observed on Ni(111), Pd(111) and Pt(111). Interestingly, the C-O stretching frequency indicates three completely different adsorption sites; a 2-fold bridge site for Ni(111), a 3-fold site for Pd(111), and an on-top site for Pt(111). As the coverage increases to one-half, a c(4x2) Leed pattern appears. For Ni(111) and pd (111), the adsorbate seems to prefer the 2-fold site. On Pt(111), half of the chemisorbed CO sits on the on- top site while the other half is located at the 2-fold site. Further increase in coverage results in titled CO molecules on Pt(111) and adsorption of CO at the on-top site of Ni(111) and Pd(111). To rationalize these observations, we carried out extended Huckel calculations within the tight-binding formalism. Our discussion is augmented by conceptual tools such as density of states and crystal orbital overlap population. We also examine the question of the adequacy of the Blyholder model in describing the adsorbate-surface interaction. (js)

Infrared spectroscopic studies of the reactions of alcohols over group IVB metal oxide catalysts. Part 1.—Propan-2-ol over TiO2, ZrO2 and HfO2

Abstract: Infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 °C from the dehydrogenation/dehydration reactions of propan-2-ol over a series of differently calcined catalysts of TiO2, ZrO2 and HfO2. The ZrO2 and HfO2 results were independent of the calcination pretreatment, and the surfaces of these oxides, like that from a TiO2 sample calcined at 800 °C, were dehydroxylated. Different results were obtained from a TiO2 sample calcined at 300 °C which had a hydroxylated surface. The acidic sites and reactivities of the surfaces of TiO2(300 °C) and TiO2(800 °C) were explored by pyridine adsorption and infrared spectroscopy. Only Lewis-acid sites were detected by pyridine.On raising the reaction temperature, in all cases the dehydrogenation reaction to give acetone occurred either before or simultaneously to the onset of the dehydration reaction to give propene. Acetone production was most pronounced over ZrO2 and HfO2 but also occurred more with TiO2(800 °C) than with TiO2(300 °C). The dehydrogenation reaction was largely quenched by pre-adsorbed pyridine on both TiO2 samples. The TiO2(300 °C) catalyst showed the presence of adsorbed propan-2-ol and 2-propoxide groups at room temperature. The dehydroxylated ZrO2, HfO2 and TiO2(800 °C) samples only showed appreciable amounts of 2-propoxide groups. In each case the 2-propoxide ions occurred in two different forms, presumably formed by adsorption on different types of sites.Both the acetone and propene products appeared as absorptions from 2-propoxide surface species decreased in intensity, so the latter are clearly reactive species. Gas-phase acetone production was followed by the chemisorption of acetone at a higher temperature. This subsequently decomposed to give surface acetate species, and finally at 400 °C to give CO2 and methane in the gas phase. Propene did not give rise to adsorbed species, or to further products in the gas phase.At the higher temperatures, above 300 °C, the reaction was always selective in favour of the dehydration reaction. However, each of the dehydroxylated catalysts showed some selectivity in favour of the dehydrogenation reaction over the earliest temperature range for alcohol decomposition, between 200 and 250 °C.A discussion is given of possible mechanistic pathways for the production of surface 2-propoxide species and the two types of products, based on the infrared-supported assumption that the different adsorbed forms of 2-propoxide [and possibly adsorbed propan-2-ol on TiO2(300 °C)] are reactive intermediates.

A reexamination of the chemisorption of trimethylaluminum on silica

Abstract: Fourier transform infrared spectroscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy have been used to study the chemisorption and decomposition of trimethylaluminum (TMA) on silica under high vacuum. By annealing a series of silica substrates from 425 to 1,573 K prior to TMA exposures at 300 K, the authors have examined the distributions of chemisorption products as a function of the relative concentrations of isolated hydroxyls (OH{sub i}), hydrogen-bonded hydroxyls (OH{sub H}), and siloxane bridges. The observed variation in the Si-methyl to Al-methyl population ratios supports a new chemisorption model in which a monomethylaluminum surface complex and methyl groups bonded to silicon are proposed as the majority species on the surface at 300 K. Although the initial reactive sticking probability for TMA on the silica substrates is < 0.01 at 300 K, TMA chemisorption affects OH{sub i}, OH{sub H}, and siloxane bridges on the surface with equivalent probability. The common reaction probability (equivalent rate constants) implies that similar requirements may be involved in the reactive adsorption that consumes each site. Decomposition of the monomethylaluminum adsorbate begins above 373 K and increases the population of methyl groups bonded to silicon on the surface. The methyl groups react to form methane{sub g} andmore » ethane{sub g} and adsorbed hydrocarbon fragments. In addition, the methyl groups also react further with the surface to form tetramethylsilane{sub g}. At coverages of one-third saturation and greater, the monomethylaluminum surface complex can also react with methyl groups to yield TMA{sub g} above 500 K.« less

Selective functionalization of gold microstructures with ferrocenyl derivatives via reaction with thiols or disulfides: characterization by electrochemistry and Auger electron spectroscopy

Abstract: Auger electron spectroscopy and electrochemistry show that ferrocenyl thiols or disulfides can be used to modify selectively the Au microelectrodes (∼2 μm×90 μm×0.1 μm) on a Si 3 N 4 substrate. The Auger technique shows the selectivity of chemisorption of thiol (−SH) or disulfide (―SS―) groups on Au relative to their physisorption on Si 3 N 4 to be at least 100:1. Immersion of Au electrodes into solutions containing (1-mercapto-3,6-dithiaheptanyl)octamethylferrocene (I), 11-mercaptoundecanoylferrocene (II), or bis[10-(ferrocenylcarbonyl)decyl] disulfide (III) yields modified Au electrodes showing about one monolayer of a reversibly redox active ferrocene reagent