Ion Scattering Spectroscopy Data Research Articles

Effects of Metal Coating on Self-Assembled Monolayers on Gold. 2. Copper on an Oligo(Phenylene-Ethynylene) Monolayer

We report studies on the modifications induced by the evaporation of copper overlayers on a self-assembled monolayer (SAM) of the oligo(phenylene-ethynyl) dithiol, 1-thio-4-[4'-[(4'-thio)phenylethynyl]-1'-ethynyl]-benzene (TTPEB). These SAMs were characterized after deposition from a tetrahydrofuran solution on polycrystalline gold substrates and after copper evaporation and its subsequent removal by nitric acid. Monolayers were studied via cyclic voltammetry (CV), UV-vis multiwavelength ellipsometry, external reflectance infrared (IR) spectroscopy, and ion scattering spectroscopy (ISS). The results obtained indicate that TTPEB SAMs display the same packing characteristics before and after copper evaporation and removal. However, as shown by IR spectroscopy, the monolayers undergo a reorganization process that involves an increase in tilt angle accompanied by rotation of aromatic rings that results in a decrease in the average molecular twist angle. ISS data suggest that copper diffuses through the monolayer after copper evaporation, a result that is significant for applications of this molecule in molecular electronic devices.

Determination of target mass from ISS data in the presence of inelastic losses

Ion scattering spectroscopy (ISS) is often used to identify the masses of surface atoms through measurement of scattered-ion energies. Mass analysis is straightforward when the scattering events can be described by binary elastic collision kinematics. However, inelastic and multiple scattering effects can alter the final energy of the detected particles, complicating the analysis. Here we present a method that uses ISS data collected at two or more observation angles to identify both the mass of the scattering center and any inelastic energy loss. Use of this technique can also allow detection and quantification of certain systematic errors, such as uncertainties in the initial beam energy, detected ion energy and scattering angle. While ion scattering is stressed here, the presented technique works equally well for the analysis of direct recoil ion energies. An example using He+ ISS data recorded on an Al–Ag alloy illustrates the abilities of this method.

Deposition dynamics and chemical properties of size-selected Ir clusters on TiO 2

We report a study of Ir n /TiO 2 samples prepared by size and energy-selected deposition of Ir n + ( n=1, 2, 5, 10, 15) on rutile TiO 2(1 1 0) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO 2 surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir n . This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential C 16O and C 18O doses, facile C 16O → C 18O exchange is observed for Ir 5 and larger clusters, but not for Ir 2. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiO x layer.

In situ laser annealing system for real-time surface kinetic analysis

For real-time analysis during thermal annealing, a continuous wave CO2 infrared laser was coupled to a surface analysis system equipped for x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The laser beam was directed into the vacuum chamber through a ZnSe window to the back side of the sample. With 10 W laser output, the sample temperature reached 563 K. The chamber remained below 10−8 Torr during annealing and allowed XPS and ISS data to be gathered as a function of time at selected temperatures. As a test example, real time Cu2O reduction at 563 K was investigated.

Formation of c(2×2) Mn–Ag superficial bilayer alloys on Ag(001): role of thermally activated surface atomic exchange and ordering

Ultra-thin Mn films have been evaporated in ultra-high vacuum on a Ag(001) substrate kept at temperatures between 210 and 340 K. A c(2×2) LEED (low-energy electron diffraction) diagram is obtained due to the formation of a two-layer-thick superficial Mn–Ag alloy. The intensity and width of the half integer spots are investigated as a function of coverage, deposition temperature, and time elapsed after Mn deposition. The brightest and sharpest diagram is obtained for ∼1.5 monolayers deposited at room temperature, which corresponds to a surface plane made of c(2×2) domains formed by Mn and Ag atoms in a checkerboard arrangement, while the subsurface plane is saturated in Mn. Depending on the preparation conditions, various c(2×2) surface alloys may be obtained that mainly differ in the Mn x Ag 1− x composition of the second atomic plane. A comparison of the time evolution of the LEED spot intensity with ion scattering spectroscopy data reveals that two different thermally activated phenomena determine the formation and nature of the c(2×2) superstructure: on the one hand, part of the deposited Mn atoms must exchange with Ag atoms from the substrate first plane, and on the other hand, the Mn and Ag atoms of the film surface plane diffuse laterally so that nucleation and growth of the c(2×2) domains take place.

Catalytic Activity and Surface Characterization Study of Pd Supported on Nanocrystalline and Polycrystalline CeO2

ABSTRACTThe catalytic activity of polycrystalline and nanocrystalline CeO2-supported Pd (Pd/pCeO2 and Pd/nCeO2) has been determined as a function of temperature and Pd loading. While the untreated nCeO2 support gives 50% methane conversion at 420°C, the untreated pCeO2 support exhibits little activity under the conditions examined due to its low surface area. A Pd loading of 5 wt% increases the activity of pCeO2 to 50% conversion at 260°C, while a 40 wt% Pd loading on nCeO2 exhibits a relatively smaller activity increase, yielding 50% conversion at 240°C. On a mass basis the 40 wt% Pd/nCeO2 catalyst is the most active tested in this study, but it is less active than the 5 wt% Pd/pCeO2 catalyst on a surface-area basis. Furthermore, the activity of the 40 wt% Pd/nCeO2 catalyst does not decrease during 100 hrs of exposure to CH4 and O2 at 250°C.X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) have been used to characterize the surfaces of both bare supports and Pd-containing catalysts before and after exposure to reactor conditions. The XPS results reveal that the Pd surface concentration is more than an order of magnitude higher for 5 wt% Pd/pCeO2 than for 5 wt% Pd/nCeO2 due to the larger surface area of nCeO2 and that the 40 wt% Pd/nCeO2 catalyst has a lower Pd loading on a surface-area basis than the 5 wt% Pd/pCeO2 catalyst. Most of the supported Pd is in the form of PdO, but higher PdO2/PdO ratios are observed for both CeO2 supports compared to Pd supported on ZrO2 or CO3O4. Furthermore, a significant amount of metallic Pd forms on Pd/nCeO2 but not on Pd/pCeO2 during reaction. The nanocrystalline and polycrystalline CeO2 behave differently chemically which is consistent with the fact that the nanocrystalline catalysts are less active on a surface-area basis. Accumulation of H20 on the Pd/pCeO2 surface during reaction is significant but not on the Pd/nCeO2 surface. This suggests that the rate limiting step may be H2O desorption on Pd/pCeO2 while for Pd on nCeO2 adsorption of methane appears to be the slow step. The ISS data indicate that the outermost atomic layer of Pd/nCeO2 consists mostly of O and C, which is not the case for Pd/pCeO2. Site blockage by these species may also contribute to the lower activity on a surface-area basis of Pd/nCeO2 compared to Pd/pCeO2.

Surface characterization study of the chemical alteration of an air-exposed polycrystalline tin foil during H-atom exposures

An air-exposed polycrystalline Sn foil surface has been examined before and after exposure to H atoms using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The foil initially contains large amounts of oxygen and carbon at the surface. C is present mostly as hydrocarbon contamination, and O is present as Sn hydroxide and surface water in addition to SnO, SnO 2 and transitional oxide (possibly Sn 3O 4), which has an O content between SnO and SnO 2. The C and O contents of the near-surface region are significantly reduced by exposure to the H-atom flux. The large hydroxyl group concentration is reduced through formation and desorption of water. A short 1-min exposure results in reduction of some of the SnO 2 or transitional oxide to SnO. Longer reduction periods result in the formation of a large amount of transitional oxide and some metallic Sn. ISS data indicate that the O and C contents of the outermost atomic layer initially increase upon exposure to H atoms due to a chemically induced driving force and then decrease. Migration of subsurface C and O becomes the rate-limiting step with regard to further removal of these species at room temperature.

Chemical alteration of the native oxide layer on LiGaO2(001) by exposure to hyperthermal atomic hydrogen

X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) have been used to examine the near-surface region and outermost atomic layer of air-exposed, solvent-cleaned, LiGaO2(001) substrates, respectively, before and after room-temperature exposures to the flux produced by a novel electron stimulated desorption hyperthermal H-atom source. The native oxide layer on the solvent-cleaned LiGaO2(001) substrate is nonhomogeneous and contains primarily LiO, Ga, and small amounts of C. Li is initially present in the near-surface region as Li2O, LiGaO2, and a small amount of LiOH. Several forms of O are present including adsorbed water, LiGaO2, Li, and Ga-oxides, and hydroxyl groups with Ga2O3 as the predominant species. Upon exposure to the hyperthermal H-atom flux, low-temperature removal of oxygen and carbon contaminants occurs, and the near-surface region approaches the stoichiometry of a clean LiGaO2(001) surface except for an increased O concentration. The usually difficult to observe Li 1s photoelectron peak increases in intensity significantly with H-atom exposure. XPS binding energies for LiGaO2(001) have been assigned for Li 1s, O 1s, Ga 3d, and 2p peaks at 56.0, 530.2, 19.4, and 1116.9 eV, respectively. According to the He+ ISS data, the H-atom flux is very effective in removing surface contamination and enriches the outermost atomic layer in O through diffusion of subsurface O to the surface under a chemically induced driving potential. This O enrichment implies that the diffusion of O to the surface is more rapid than removal of surface O by impinging H atoms.

Chemical alteration of the native oxide layer on InP(111) by exposure to hyperthermal atomic hydrogen

A surface characterization study using x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) has been performed on polished, solvent-cleaned, undoped InP(111) substrates before and after room-temperature exposure to the flux produced by a novel atomic hydrogen source based on electron-stimulated desorption of hyperthermal (1–3 eV) hydrogen atoms from a Cu-alloy membrane. The native oxide layer on the solvent-cleaned InP(111) substrate is nonhomogeneous and contains primarily C, O, and In, and very little P. Indium is present in the near-surface region as InPO4, In(PO3)3, InPO3, InP, and a relatively small amount of In2O3 in the subsurface region. Phosphorus is present as InPO4, In(PO3)3, InPO3, InP, P2O5, and elemental P. For In and P, InP is the predominant form. Before H-atom exposure the C is present as hydrocarbons, alcohols, and carbide with hydrocarbons as the predominant chemical state. During room-temperature exposure to the hyperthermal H-atom flux for 90 min, removal of oxygen and carbon contamination occurs with the O content decreasing by about 28% and the C content decreasing by 93% according to the XPS data. The In:P ratio is initially 2.28 indicative of an In-rich near-surface region. After a 15 min and then 90 min H-atom exposure, the In:P ratio decreases to 1.30 and 1.11, respectively. Complex chemical changes occur during the H-atom exposures. The P2O5 is eliminated, the In phosphates are converted to biphosphates and hydroxide and all forms of C are removed. Some of the phosphates and In2O3 are decomposed by the H-atom flux, but these chemical reactions occur slowly at room temperature. According to ISS data, the H-atom flux is very effective in removing contamination at the outermost atomic layer which is crucial for epitaxial growth of device-quality thin films.

Reaction and Surface Characterization Study of Higher Alcohol Synthesis Catalysts: VII. Cs- and Pd-Promoted 1:1 Zn/Cr Spinel

A 1:1 Zn/Cr spinel catalyst promoted with 5.9 wt% Pd and 1, 3, or 5 wt% Cs has been tested for methanol and isobutanol synthesis using a 1:1 H2:CO syngas feedstream. The reactor was operated at two different temperatures (400 and 440°C) and two different pressures (1000 and 1500 psig). The higher temperature and pressure settings result in the highest isobutanol production rates over most of the catalysts examined. As the amount of Cs promotor is increased at 440°C and 1500 psig, the total alcohol production rate increases and the methanol-to-isobutanol ratio decreases. A maximum isobutanol production rate of 142 g/kg-h and a methanol-to-isobutanol mole ratio of 1.0 are attained using the 5.0 wt% Cs/ 5.9 wt% Pd-promoted Zn/Cr catalyst. Furthermore, the addition of 3 and 5 wt% Cs results in an increase in the selectivity to total alcohols. The addition of Pd to these catalysts enhances both the total alcohol production rate and isobutanol production rate compared to Cs-promoted Zn/Cr catalysts which do not contain Pd. Surface characterization studies using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) were performed on the 5 wt% Cs/5.9 wt% Pd-promoted catalyst. The XPS and ISS data reveal that the fresh catalyst surface consists primarily of ZnO, ZnCr2O4, Cs2O and a small amount of PdO2. Since the catalysts are typically reduced before testing in the reactor, the catalyst sample was again characterized after a reductive pretreatment in 1×10−7Torr of H2at 300°C for 4 h. This pretreatment results in removal of the carbon contamination, thereby exposing underlying ZnO, Cs, and Pd. Characterization studies were also performed on a 5 wt% Cs/5.9 wt% Pd-promoted catalyst which had been removed from the reactor after 5 days of testing. The XPS data indicate that the near-surface region remains enriched in Cs and Pd and that the PdO2present on the pretreated catalyst is converted to PdO during reaction. Apparently, the reaction process results in agglomeration of the ZnO film present on the reduced catalyst, which is the active catalytic phase in higher alcohol synthesis (HAS).

Room-temperature oxidation of a GaAs(001) surface induced by the interaction of hyperthermal atomic oxygen and studied by x-ray photoelectron spectroscopy and ion scattering spectroscopy

In this study a hyperthermal oxygen atom source has been used to form an oxide layer on an Ar+-sputtered GaAs(001) surface at room temperature, and this layer has been examined using x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). XPS data indicate that the Ga in the near-surface region is oxidized predominantly to Ga2O3 with a significant contribution from GaAsO4 while the As is oxidized predominantly to an AsOx species with significant contributions from As2O3 and GaAsO4 and/or As2O5. The oxide layer thickness is estimated to be about 25 Å, and the XPS Ga:As atom ratio increases from 1.1 to 1.6 during the oxidation. The ISS data indicate that the resulting oxide layer formed is more electrically insulating than a native oxide layer on this surface.

Surface Characterization Study of the Thermal Decomposition of Ag2CO3

The changes in chemical states and composition of the surface region of a Ag2CO3 powder at various stages during thermal decomposition have been examined using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The near-surface region of the as-received powder consists primarily of Ag2CO3 although some hydrocarbon and alcohol contaminants also are present. A 155 °C anneal results in partial decomposition of Ag2CO3 to Ag metal and oxides and reduces the amounts of the C contaminants. An anneal at 170 °C causes further decomposition of the Ag2CO3 to Ag metal, Ag2O, and AgO. ISS data indicate that at 205 °C oxygen migrates more rapidly to the outermost atomic layer than it desorbs, resulting in an increased oxygen concentration. The AgO species undergoes further decomposition to Ag2O during a 340 °C anneal treatment. Between 340 and 430 °C the Ag2O decomposes, leaving only Ag metal and subsurface oxygen in the near-surface region of the sample. These results are consistent with temperature-programmed reaction (TPR) data which exhibit a CO2 peak at 260 °C and an O2 peak at 420 °C. Both the XPS and TPR data indicate that the thermal decomposition of these species are activated since their decompositions occur at temperatures much higher than predicted by equilibrium thermodynamic calculations.

Relative stability of an on-top and an inverted Mn monolayer on Ag(1 0 0): Experiment and theory

Mn films with a thickness of one monolayer (ML) have been deposited in ultra-high vacuum, at room temperature (RT), on a Ag(1 0 0) single crystal. Photoemission and ion scattering spectroscopy data indicate that after Mn evaporation a substantial fraction of the Mn atoms is located within the second topmost atomic layer. If the film is left at RT or mildly (60 °C) annealed, the Mn atoms tend to exchange further with Ag atoms, increasing the Mn concentration in the second atomic plane. Eventually the second atomic plane of the sample is constituted mainly by Mn atoms, whereas the first atomic layer is almost a pure Ag plane, i.e. an inverted atomic Mn ML is formed. To directly compare experiment and theory, we have performed ab initio electronic band structure calculations on the energetic stability of 1 ML of Mn on top of Ag(1 0 0) versus 1 ML of Mn covered by one Ag atomic plane. The case of a 2 ML-thick MnAg alloy is also investigated. The relation between magnetism and stability of Mn films is discussed.

Oxidation Study of a Polycrystalline Ni/Cr Alloy II

The oxygen uptake of a sputtered and annealed polycrystalline Ni/Cr alloy surface has been examined using Auger electron spectroscopy (AES). The O2 exposures were carried out at 10-7 Torr and room temperature. Then the compositional and chemical-state changes induced at a 100-langmuir O2-exposed surface with increasing annealing times and temperatures were examined using AES, ion-scattering spectroscopy (ISS), and X-ray photoelectron spectroscopy (XPS). These three techniques probe different depths beneath the surface, so they provide depth-sensitive information about the complex chemical processes and compositional changes that occur during annealing. With annealing to 500 °C the near-surface region becomes enriched in Cr according to the ISS and AES data. XPS data indicate that Ni is present as Ni metal, NiO, and Ni(OH)2 and that Ni metal is the predominant chemical state of Ni in the region probed by this technique. Cr is present as metallic Cr, CrO2, CrO3, Cr2O3, and Cr(OH)x with CrO2 being the predominant chemical state below 500 °C. Both XPS and AES data indicate that NiO is reduced to Ni metal as Cr metal is oxidized to CrOx. Oxygen dissolution into the bulk of the sample occurs after annealing the sample at 500 °C or above. After a 650 °C anneal for 2 h, very little of the oxides remain in the near-surface region, but NiO and CrOx are still present in the subsurface region probed by XPS. The annealing treatments result in reversion of the O2-exposed surface nearly to the initially cleaned Ni/Cr surface.

Oxidation study of a polycrystalline Ni/Cr alloy I: room-temperature exposure to O 2

The changes induced at a cleaned polycrystalline Ni/Cr alloy surface upon increasing oxygen exposure (0 to 100 L) at room temperature were studied by Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and X-ray photoelectron spectroscopy (XPS). Cr oxides, NiO and hydroxides form near the surface during the initial 5 L exposure. More of the oxygen present is bonded as NiO than Cr oxide, but a larger fraction of the near-surface Cr is oxidized. The Cr oxide formed consists of a mixture of CrO 2, Cr 2O 3 and CrO 3 with CrO 2 as the predominant form. With increasing exposure to 100 L, both metallic Cr and Ni become increasingly oxidized, but most of the Ni within 3.5 nm of the surface remains in metallic form after a 100 L exposure. ISS data indicate that the outermost atomic layer becomes enriched in Cr with oxygen exposure, while AES data indicate a small enrichment of Cr in the near-surface region sampled by this technique. AES data also indicate that the oxygen uptake is rapid to 20 L and then slow above that exposure.

Study of Cs-Promoted, α-Alumina-Supported Silver, Ethylene-Epoxidation Catalysts

In an attempt to gain a better understanding of the structure of ethylene epoxidation catalysts and the role of the Cs promotor, a model study using Al and Ag foils was performed. Air-exposed Al and Ag foils were dipped into a 400 ppm CsOH solution, rinsed with distilled H2O, and characterized using X-ray photoelectron spectroscopy (XPS). The data indicate that Cs readily adsorbs at the Al2O3surface on the Al foil. Very little Cs adsorbs to the Ag foil which contains a thin oxide film at the surface. A sputter-cleaned Ag foil (oxide free), however, adsorbs significant quantities of CsOH indicating that the Cs bonds to a Ag metal surface but not to Ag oxide. Characterization studies were also performed on two Al foils: one which was dipped into a heated, 85% lactic acid solution containing dissolved Ag2O and the other which was dipped into an identical solution but containing CsOH as well. XPS and ion scattering spectroscopy data show that the Al foil does not adsorb Ag from the solution unless the CsOH is present. Furthermore, the XPS data indicate that there is a chemical interaction (electronic effect) between the Al, Cs, and Ag. These results support the conclusion of the previous part of this study that one of the roles of the Cs is to act as a binder between the Ag and alumina support in ethylene epoxidation catalysts.

The use of low-energy ion scattering spectroscopy for the quantitative determination of adsorption sites in surface chemistry studies

Ion scattering spectroscopy (ISS) was used in conjunction with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) for the study of the adsorption of hydrogen, oxygen, ethyl iodide, and 2-propyl iodide on Ni(100). In all those systems the decrease in Ni ISS signal intensity seen during the uptake could be explained by shielding of the Ni surface atoms by the adsorbate. In addition, since the ISS data could be correlated nicely with results from other techniques, the conclusion was reached that matrix effects and changes in neutralization probabilities (due to variations in work function) were negligible for the systems studied here. Additional ISS studies for the adsorption of 2-propyl iodide on oxygen-precovered Ni(100) indicated that those molecules bond preferentially to Ni (not O) sites. This confirms previous results which showed that the products of that system associated with chemistry on the Ni sites — propane, propene, and hydrogen — dominated the TPD spectra for low coverages of either 2-propyl iodide or oxygen. In a broader sense, this work also shows that ISS can be used to determine adsorption sites in complex systems where other techniques are not applicable.

Study of a polycrystalline Ni/Cr alloy V. Hydrogen-atom exposure

An air-exposed polycrystalline nichrome (Ni/Cr) alloy was exposed to a hydrogen-atom flux at room temperature and then sputtered with Ar + for 15 min. X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) were used to examine the changes, which occur at the surface of the sample during these treatments. The near-surface region of the alloy initially consists primarily of nickel hydroxide and carbon contamination. Small amounts of Na and Cl contaminants are also present on the surface, according to the ISS data. The amount of carbon contamination and oxygen in the near-surface region is decreased by the H-atom exposure, and the Ni and Cr concentrations are increased. The hydroxyl groups are rapidly removed during the exposure to H atoms presumably through formation of water molecules, which desorb. Ni(OH) 2, NiO, Ni 0, CrO 2 and Cr 0 are present in the near-surface region after the H-atom exposure. A subsequent 15-min, 1-keV Ar + sputter removes the remaining carbon, nearly all of the oxygen, as well as the Na and Cl contaminants. The Ni is mostly metallic, and Cr is present as Cr 0 and CrO 2. In previous studies, multiple ion-beam sputtering and annealing cycles were necessary to obtain this level of cleanliness. The process using a H-atom beam flux provides an alternative and less time-consuming method for cleaning Ni/Cr alloy surfaces.

Thermal Conversion of 2-Propyl Iodide on O/Ni(100): Changes in Product Distribution with Varying Oxygen Coverages

The oxidation of 2-propyl iodide on oxygen-covered Ni(100) surfaces has been studied with temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and ion-scattering spectroscopy (ISS). It was found that the product distribution is strongly dependent on oxygen preexposure, with partial oxidation being favored at low oxygen coverages and total oxidation dominating on thin oxide films. XP I 3dcore-level spectra indicate that the adsorption of 2-propyl iodide below 100 K is molecular, and ISS data strongly suggest preferential bonding to Ni sites, Annealing the 2-propyl iodide adsorbed on O/Ni(100) surfaces between 120 to 200 K generates 2-propyl groups on the nickel sites via dissociation of the C–I bond, the same as on the clean surface. For submonolayer oxygen coverages the 2-propyl groups then follow one of two reaction pathways: they either undergo hydrogenation–dehydrogenation on the nickel sites to form propane, propene, and hydrogen, or, in the case of those formed near the oxygen sites, migrate and react to form 2-propoxide groups. The 2-propoxide moieties are stable on the surface up to ∼325 K, at which point some undergo β-hydride elimination to yield acetone. The fact that the rate of desorption of acetone from the reaction of 2-propyl iodide with oxygen is reaction limited is supported by the observation that the desorption of molecular acetone from Ni(100) occurs below 300 K. Also, other TPD experiments indicate that propene does not react to yield acetone on O/Ni(100) surfaces, and that the peak shapes and temperatures for acetone desorption from the reaction of 2-propyl iodide and 2-propanol on O/Ni(100) are nearly identical, suggesting that they form from a common intermediate, 2-propoxide.

Study of Cs-Promoted, α-Alumina-Supported Silver, Ethylene Epoxidation Catalysts: III. Characterization of Cs-Promoted and Nonpromoted Catalysts

A nonpromoted Ag/α-Al2O3catalyst and a Ag/α-Al2O3catalyst promoted with 420 ppm Cs used for ethylene epoxidation were studied using scanning electron microscopy (SEM), ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). SEM, ISS, XPS, and AES data indicate that the Cs-promoted catalyst consists of a thin film of Ag covering most of the support surface. Both a thin film and small Ag clusters exist on the nonpromoted catalyst, but the alumina is only partially covered. ISS and SIMS depth profiling data taken from the promoted catalyst indicate that most of the Cs lies beneath the surface although a small amount is present at the surface. The surface characterization data suggest that the Cs coats the support material during the preparation and acts as a binder between the Ag and the support resulting in a larger Ag coverage of the α-alumina. Total oxidation of ethylene and ethylene oxide occur primarily on the alumina surface. The enhancement in selectivity toward ethylene epoxidation may result from the fact that Cs addition results in the thin Ag film covering the alumina.