Surface Science Experiments Research Articles

Microkinetic analysis of transient ethylene oxidation experiments on silver

A microkinetic model which successfully explains the steady-state kinetics of ethylene oxidation on silver is used to simulate transient surface science experiments. The model explains a number of important TPR experiments in the literature. It is demonstrated that a model with a common intermediate for epoxidation and combustion may explain these experiments, lending further support for the crucial role of oxametallacycles in ethylene epoxidation. It is suggested that the role of subsurface oxygen is to serve as an oxygen reservoir in transient experiments without oxygen atmospheres, but are otherwise unimportant.

Selectivity driven design of bimetallic ethylene epoxidation catalysts from first principles

The sequence of key elementary steps for ethylene epoxidation on silver was developed from surface science experiments and computational chemistry. This molecular-level mechanistic information was used as an input in computational screening of potential bimetallic alloy catalysts. The aim of the computational screening was to identify a catalyst that would offer higher selectivity to ethylene oxide (EO) than the traditional monometallic silver catalyst. Computational screening led to formulation of a new Cu/Ag alloy that is more selective than Ag to EO. These predictions were verified by steady-state experiments with monolith-supported catalysts.

New Perspectives on Direct Heterogeneous Olefin Epoxidation

AbstractFor Abstract see ChemInform Abstract in Full Text.

Gallium nitride surface preparation optimised using in situ scanning tunnelling microscopy

Effective preparation of clean GaN surfaces is vital for surface studies, and also has a key role to play in the development of nitride-based electronic devices, due to the importance of forming good metal–semiconductor contacts. We identify the effect of two common surface cleaning treatments (annealing in ammonia, and sputtering in nitrogen followed by in vacuo annealing) on surface reconstruction, morphology and stoichiometry, by employing a combination of in situ RHEED and scanning tunnelling microscopy (STM), and ex situ X-ray photoelectron spectroscopy (XPS). We show how the defect densities in as-treated GaN surfaces may be reduced by growing thin layers of GaN by low-pressure MBE, a technique compatible with most experimental surface science systems. By using a combination of in situ etching and low-pressure growth, we show that it is possible to obtain very similar surface structures to those obtained by full-scale MBE.

Control of ethylene epoxidation selectivity by surface oxametallacycles.

Surface science experiments, DFT calculations, and kinetic isotope effect data are utilized to understand the elementary steps that govern the selectivity of silver catalysts for the partial oxidation of ethylene to produce ethylene oxide. It is proposed that selective and unselective pathways proceed via a common intermediate, the surface oxametallacycle. The structures of the transition states leading from this intermediate to selective and unselective products are calculated. From the calculated Gibbs free energies of activation for competing pathways, it is possible to predict selectivity to ethylene oxide as well as the magnitude of the kinetic isotope effect. The proposed mechanism is qualitatively and quantitatively in accord with experimental results.

Construction of a reaction coordinate and a microkinetic model for ethylene epoxidation on silver from DFT calculations and surface science experiments

Density functional theory (DFT) calculations have been used to investigate a possible reaction coordinate for the epoxidation of ethylene on silver. These studies have been motivated by recent advances in the understanding of the mechanisms of olefin epoxidation. We find various surface intermediates and transition states along the reaction coordinate with structures, binding energies, and vibrational frequencies that are in good agreement with experimental results. We also discuss the uniqueness of silver in providing an optimal environment for highly selective ethylene epoxidation. It was concluded that ethylene reacts with adsorbed oxygen to form a surface intermediate, identified as a surface oxametallacycle. This intermediate reacts through a transition state to form gas-phase ethylene oxide. The kinetic and thermodynamic parameters obtained in these studies are used to formulate a simple microkinetic model. The microkinetic model is used to estimate parameters needed for the formulation of a rate law. The resulting rate law agrees very well with observations from macroscopic measurements of ethylene epoxidation rates under steady-state conditions.

New Perspectives on Direct Heterogeneous Olefin Epoxidation

Important commercial direct oxidation processes include the epoxidation of ethylene to ethylene oxide and the newer epoxidation of butadiene to epoxybutene (EpB), both carried out with silver catalysts. However, detailed reaction mechanisms for these processes are still matters of debate. The guiding hypothesis of our research is that surface oxametallacycles are key intermediates in selective olefin epoxidation. By a combination of surface science experiments and density functional theory (DFT) calculations, we have synthesized the first stable surface oxametallacycles and have verified their identities and structures. In the case of EpB chemistry, we have been able to demonstrate direct connections between surface oxametallacycles and epoxide products. The EpB ring opens with an activation energy of 8.4 kcal/mol on Ag(110) to form a stable surface oxametallacycle. Spectroscopic results for this species are in excellent agreement with DFT calculations for an oxametallacycle bound to three silver atoms of a seven-atom cluster. This oxametallacycle undergoes 1,2 and 1,4 ring-closure reactions during temperature programmed desorption to form EpB and 2,5-dihydrofuran, respectively. This reaction represents the first demonstration of surface oxametallacycle ring closure to form an epoxide, and we suggest that surface oxametallacycles are of general importance in silver-catalyzed olefin epoxidation.

Full potential calculations on the electron bandstructures of Sphalerite, Pyrite and Chalcopyrite

The bulk electronic structures of Sphalerite, Pyrite and Chalcopyrite have been calculated within an ab initio, full potential, density functional approach. The exchange term was approximated with the Dirac exchange functional, the Vosko–Wilk–Nusair parameterization of the Cepler–Alder free electron gas was used for correlation and linear combinations of Gaussian type orbitals were used as basis functions. The Sphalerite (zinc blende) band gap was calculated to be direct with a width of 2.23 eV. The Sphalerite valence band was 5.2 eV wide and composed of a mixture of sulfur and zinc orbitals. The band below the valence band located around −6.2 eV was mainly composed of Zn 3d orbitals. The S 3s orbitals gave rise to a band located around −12.3 eV. Pyrite was calculated to be a semiconductor with an indirect band gap of 0.51 eV, and a direct gap of 0.55 eV. The valence band was 1.25 eV wide and mainly composed of non-bonding Fe 3d orbitals. The band below the valence band was 4.9 eV wide and composed of a mixture of sulfur and iron orbitals. Due to the short inter-atomic distance between the sulfur dumbbells, the S 3s orbitals in Pyrite were split into a bonding and an anti-bonding range. Chalcopyrite was predicted to be a conductor, with no band-crossings at the Fermi level. The bands at −13.2 eV originate from the sulfur 3s orbitals and were quite similar to the sulfur 3s bands in Sphalerite, though somewhat shifted to lower energy. The top of the valence band consisted of a mixture of orbitals from all the atoms. The lower part of the same band showed metal character. Computational modeling as a tool for illuminating the flotation and leaching processes of Pyrite and Chalcopyrite, in connection with surface science experiments, is discussed.

Applications of Storage Ring Infrared Spectromicroscopy and Reflection-Absorption Spectroscopy to Geochemistry and Environmental Science

Infrared radiation extracted from a storage ring affords new opportunities for scientific exploration in the areas of geochemistry, geomicrobiology and environmental science. In this review paper, the fundamental interactions between infrared light and matter are discussed, followed by an introduction to the source properties of infrared radiation emitted from relativistically accelerated electrons in a storage ring. The most important of these properties is the brightness of the source. A bright source can deliver a higher density of photons onto a small (less than 20 μm × 20 μm) sample at normal incidence than a lab based globar source, which is necessary to produce diffraction limited spatially resolved infrared images. Alternatively, this source can couple well to grazing incidence geometry for surface science experiments affording the opportunity to examine low frequency adsorbate-substrate vibrational bands. Several examples of infrared spectromicroscopy applications and surface science applications are reviewed.

Atomic description of elementary surface processes: diffusion and dynamics

A large fraction of processes which are at the foundation of our technological society involve physical and chemical properties of surfaces. Catalytic reactions and semiconductor devices production are two of the most important ones.This paper describes a sample of some of the most relevant surface science experiments which have been recently performed, in order to understand elementary surface processes of model catalytic reactions and in semiconductor technology at the atomic level. The focus is on experiments performed with scanning tunneling microscopy and atomic force microscopy which have represented, in some cases, real breakthroughs in our understanding of these phenomena.We then present an overview of possible experimental technique developments that can be foreseen for the future and that may give us a more in-depth understanding of the elementary processes which form the basis of important complex surface phenomena. Finally, some of the challenging tasks that lie ahead for surface scientists and the collateral opportunities are discussed.

Tunable pulsed vacuum ultraviolet light source for surface science and materials spectroscopy based on high order harmonic generation

We report here the development of a tunable vacuum ultraviolet light source, providing subpicosecond pulses over the wavelength range from 114 (∼11 eV) to 32 nm (∼39 eV), designed for surface science experiments. The source is based on high order harmonic generation. The experimental setup is described in detail and the harmonic yield as a function of the gas type (xenon or argon) is compared with theoretical descriptions. We address in particular the tunability of the source, desirable for surface science applications. Absolute characterization of the harmonic yield has also been performed and validated with time-resolved fluorescence measurements. The results are extremely promising, with intensities of ∼1010 photons/s, with regard to the use of this coherent source for surface science and spectroscopy.

GROWTH, MORPHOLOGY, INTERFACIAL EFFECTS AND CATALYTIC PROPERTIES OF Au ON TiO2

We survey recent studies and present new data on the growth, interactions, structure and chemistry of gold deposited on TiO 2(110) surfaces. The noble metal Au on TiO 2(110) is a model system for weak interaction of a metal with an oxide substrate; it is also of interest because Au on TiO 2 has unusually high activity as a catalyst for CO oxidation at room temperature. In this review, we present results on the growth of ultrathin films of Au on TiO 2(110), as well as the morphology, interface formation, epitaxy, structure and electronic properties of Au on TiO 2 single crystal and planar films. The results are compared to studies of Au on high-surface-area TiO 2 catalysts in an attempt to bridge the gap between surface science experiments and the high pressure conditions of catalyst operation.

Dissociative Adsorption of Dinitrogen on a Multipromoted Iron-Based Ammonia Synthesis Catalyst: Linking Properties of Catalysts and Single-Crystal Surfaces

Isothermal N2 adsorption experiments were conducted with a multipromoted iron-based ammonia synthesis catalyst. From these measurements a simple model describing the adsorption process over the entire range of N coverages was developed. The model is consistent with both surface science experiments conducted with single-crystal iron surfaces and earlier experiments on promoted iron-based catalysts. The results from these experiments were well described, if it was assumed that N2 dissociation occurred at only a few active sites, and that the rest of the adsorption sites were filled via diffusion from these sites. The number of active sites amounted to 4 μmol/g, which is approximately 5% of the total number of sites on the catalyst capable of binding nitrogen. The initial sticking coefficient, s0, of dinitrogen on the active sites was found to be s0=10−5 exp(−(4 kJ/mol)/RT).

ARTOSS – A new surface science experiment to study the hydrogen inventory in multi-component materials

The design of a new instrument is described along with results demonstrating the capabilities of in situ preparation and analysis. The purpose of the ARTOSS experiment is the preparation and analysis of multi-component surface layers on materials – in particular those which are well-suited for the first wall of present and future fusion devices – and their behaviour with respect to the hydrogen inventory in such materials. Relevant preparation and analysis techniques are combined to achieve this aim. For preparation, surfaces can be cleaned in situ and layer preparation includes vapour deposition, ion beam bombardment at energies between 100 eV and 20 keV as well as admission of gases. Photoelectron spectroscopies analyse the chemical state of elements within the surface layers, ion beam techniques enable quantitative measurements, including hydrogen isotope analysis. For ion beam analysis, low-energy (up to 5 keV) as well as high-energy ions (3 MV tandem accelerator) are available in situ. Thermal desorption and re-emission measurements complete the capabilities for hydrogen detection in multi- component materials. Sample transfer into the ultra-high vacuum system is possible via a sample load-lock. Results are presented for carbon layers on beryllium and surface modifications of beryllium by C+ and CO+ ion bombardment to demonstrate the capabilities of the new experiment.

A UHV compatible source for a highly polarized thermal atomic beam of radioactive 8Li

A beam of the radioactive isotope 8Li is prepared at thermal velocities. The nuclei are highly spin polarized by transverse optical pumping of the thermal beam. The installation is ultra-high vacuum (UHV) compatible in a non-UHV accelerator environment. Since the atomic beam is used in a surface science experiment, where contamination must be avoided, special emphasis is given to the vacuum coupling of the accelerator/ 8Li production/surface experimental areas. The atomic beam is produced by stopping the nuclear reaction products and evaporating them again from high-temperature graphite. To enhance the atomic beam, a novel tubular thermalizer is applied. The thermal polarized atomic beam intensity is approximately 5×10 8 atoms/s sr.

Effect of temperature and illumination on pyrite oxidation between pH 2 and 6

The effect of heat and illumination with visible light on the oxidation of pyrite with dissolved molecular oxygen in solutions between pH 2 and 6 has been investigated using a combination of surface science experiments and batch oxidation experiments. The rate of the oxidation of pyrite is strongly dependent on temperature. It is, however, not possible to cast the temperature dependence in a simple Arrhenius equation because the magnitude of the activation energy depends on the progress variable chosen. Activation energies based on proton release rate, sulfate release rate, and total iron release rate vary by as much as 40 kJ mol-1, suggesting that the oxidation mechanism of the sulfur and iron component of pyrite are largely independent of each other. This difference in mechanism can also explain why the reaction rates on the basis of these three different progress variables do not show the same pH dependence. Exposed to visible light, the rate of pyrite oxidation is under most conditions accelerated by less than a factor of two. Some of this acceleration may be accounted for by a light-induced heating of the pyrite surface. Surface science experiments employing photoelectron spectroscopy show no evidence for significant changes in the chemical composition of the surface as a function of exposure to visible light. The batch sorption experiments show, however, that the reaction stoichiometry changes somewhat, which indicates that there might be a change in reaction mechanism as a result of exposure to visible light.

A Growth Technique to Make Extensive Atomically Flat Silicon Surfaces

AbstractWe have carried out a series of experiments aimed at producing arrays of mesas on both Si(001) and Si(111) which are free from atomic steps. These are of interest in CMOS technology and for quantum well structures. They also provide interesting substrates for fundamental surface science experiments. In previous work we have created atomically flat regions surrounded by ridges through an evaporation method. The present work ‘inverts’ the previous process by using a pattern of trenches to define the mesas and then depositing Si to grow the atomic steps off the edges. The mesas are created on Si wafers, which are ~1° from the (111) and (001) plane by lithography and reactive ion etching. Step-free mesas were formed on Si(111) but not yet on Si(001). Both the evaporation and this new growth technique rely on step flow to move the steps to the edges of the flat areas. Although the evaporation method is simpler, an advantage of the growth technique is that it can be carried out at lower temperature. The maximum size of mesa that can be made free of atomic steps depends on the combination of temperature and deposition rate. On very large step-free terraces nucleation of islands and concentric arrays of mono-atomic steps are observed; these correspond to the vacancy pits observed with the evaporation method.

Heterogeneous chemistry in the troposphere: A modern surface chemistry approach to the study of fundamental processes

There is an increasing amount of evidence that points to the importance of halogen chemistry in the troposphere in addition to the well known important chemistry in the stratosphere. Likely sources of halogens in the troposphere include reactions of gas phase pollutants with particles containing alkali halide salts such as sea salt particles. In this article we describe how modern ultra-high vacuum surface science experiments can provide significant new insight into the detailed mechanisms of reactions that are important to the chemistry of particles in the atmosphere. An introduction to what is known about the surface chemistry of NaCl is provided followed by a description of the experimental techniques. Studies from our laboratory that are described here have shown that the reaction of dry nitric acid with NaCl is self-limiting leading to a chemically inert surface that is covered with a layer of sodium nitrate. Our experiments also show that very low vapour pressures of water provide enhanced ionic mobility on the surface leading to a recrystallization of the nitrate layer and phase separation from the NaCl. This results in open areas of clean NaCl that are then available for further reaction. The reaction of water with NaCl surfaces which have been previously 'corroded' by reaction with nitric acid and water leads to the dissociative adsorption of water to form surface OH+/- species. This is in stark contrast to the completely reversible adsorption of water on defect free NaCl surfaces. A model is also presented that is consistent with the information provided by the detailed ultra-high vacuum surface science experiments and explains the dependence of the reaction of nitric acid with NaCl on the nitric acid pressure.

Chemical bond manipulation for nanostructure integration on wafer scale

In this paper, we have briefly summarized our activity in the area of chemical bond manipulation for the integration of nanostructures on a full wafer scale. Chemical bond manipulation involves a judicious combination of surface phenomena: reactions or diffusion, and growth process such as molecular beam epitaxy (MBE). Here, we present our results on oxidation, metallization and nitridation and their role in the formation of nanostructures. We find that oxygen changes the bonding partner from Ge to Si and this phenomenon can be controlled by controlling the annealing temperature. We have employed this phenomenon for the fabrication of novel, multiperiod Si/SiO2/Ge layered structure which exhibits interesting light emitting properties. Further, by making use of selective diffusion of cobalt atoms through Ge layers it is possible to incorporate metallic features into Ge quantum dots. Moreover, it is possible to fabricate Si nanopillars through high temperature reaction of nitric oxide. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrence of these two simultaneous processes result in the formation of nanometre-sized Si pillars capped by silicon nitride. All these results emphasize the fact that we can extend information obtained through traditional surface science experiments for the fabrication of novel structures on a full wafer scale.

Molecular Studies of Catalytic Reactions on Crystal Surfaces at High Pressures and High Temperatures by Infrared−Visible Sum Frequency Generation (SFG) Surface Vibrational Spectroscopy

Infrared−visible sum frequency generation (SFG) is a surface-specific vibrational spectroscopy that can operate in a pressure range from ultrahigh vacuum (uhv) to atmospheric pressures. SFG is therefore one of the few surface science techniques that permits atomic scale monitoring of surface species during catalytic reactions at high pressures (around 1 atm) and high temperatures. Using single-crystal surfaces of transition metals, platinum and rhodium, reaction rates can be simultaneously determined by gas chromatography, and correlations between the concentration of adsorbates under reaction conditions and the observed turnover numbers can help to elucidate the reaction mechanism. To bridge the gap to traditional surface science experiments, SFG is also employed under uhv or low pressures. The technique has been successfully applied to the adsorption and oxidation of CO, hydrocarbon conversion such as ethylene hydrogenation and cyclohexene hydrogenation and dehydrogenation on Pt(111). The experiments de...