Combination Of Surface Science Techniques Research Articles

Adsorption, thermal conversion, and catalytic hydrogenation of acrolein on Cu surfaces

The adsorption, thermal chemistry, and catalytic hydrogenation of acrolein on copper model surfaces was characterized by a combination of surface-science techniques, namely, reflection–absorption infrared spectroscopy (RAIRS) and temperature program desorption (TPD), quantum mechanics (DFT) calculations, and catalytic kinetic measurements using a so-called “high-pressure cell”. Adsorption of acrolein on the Cu surface was found to involve both the carbonyl oxygen atom and the CC bond. Thermal activation leads to early dissociation and dehydrogenation to produce acetylene and carbon monoxide below 160 K, and later to the production of ketene (220 K) and propene (260 K). Under vacuum hydrogenation with H2 is not possible, but upon predosing of the surface with atomic H all the possible hydrogenation products, namely, propanal, 1-propanol, and allyl alcohol, were detected in TPD experiments, at 162, 190, and 210 K, respectively. Finally, catalytic hydrogenation under atmospheric pressures is slow and only produces propanal, at turnover frequencies below 0.2 s−1.

An In Situ and Real-Time Plasmonic Approach of Seed/Adhesion Layers: Chromium Buffer Effect at the Zinc/Alumina Interface

The effect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated films by a combination of surface science techniques, among which a very flexible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al2O3(0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn films during their growth are analyzed from sub-monolayer to multilayer thicknesses. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. With regard to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the sub-monolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coefficient from 0 to nearly 1 due to a diffusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr film percolates, it forms an adhesion layer on which Zn grows in a nearly two-dimensional mode. In all cases, Cr films are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces, which, in line with atomistic simulations, is assigned to the formation of more favorable Cr–Al2O3 and Cr–Zn than Zn–Al2O3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of films is of great practical interest. It shows that noncontinuous and partially oxidized films of additives, closer to realistic cases of application, can strongly enhance the sticking of films. Also, anchoring a functional film by discrete predeposited seeds can keep its properties intact.

Ni-modified Fe3O4(001) surface as a simple model system for understanding the oxygen evolution reaction

Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.

Adsorption and reaction pathways of 7-octenoic acid on copper.

The surface structure and reaction pathways of 7-octenoic acid are studied on a clean copper substrate in ultrahigh vacuum using a combination of reflection-absorption infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning-tunneling microscopy, supplemented by first-principles density functional theory calculations. 7-Octenoic acid adsorbs molecularly on copper below ∼260 K in a flat-lying configuration at low coverages, becoming more upright as the coverage increases. It deprotonates following adsorption at ∼300 K to form an η2-7-octenoate species. This also lies flat at low coverages, but forms a more vertical self-assembled monolayer as the coverage increases. Heating causes the 7-octenoate species to start to tilt, which produces a small amount of carbon dioxide at ∼550 K and some hydrogen in a peak at ∼615 K ascribed to the reaction of these tilted species. The majority of the decarbonylation occurs at ∼650 K when CO2 and hydrogen evolve simultaneously. Approximately half of the carbon is deposited on the surface as oligomeric species that undergo further dehydrogenation to evolve more hydrogen at ∼740 K. This leaves a carbonaceous layer on the surface, which contains hexagonal motifs connoting the onset of graphitization of the surface.

Decoupling of graphene from Ni(111) via formation of an interfacial NiO layer

A combination of surface science techniques (STM, XPS, ARPES) and density-functional theory calculations was used to study the decoupling of graphene from Ni(111) by oxygen intercalation. The formation of an antiferromagnetic (AFM) NiO layer at the interface between graphene and the ferromagnetic (FM) Ni is found, where graphene protects the underlying AFM/FM sandwich system. It is found that graphene is fully decoupled in this system and strongly p-doped via charge transfer with a position of the Dirac point of (0.69±0.02) eV above the Fermi level. Our theoretical analysis confirms all experimental findings, addressing also the interface properties between graphene and AFM NiO.

Thermally Selective Formation of Subsurface Oxygen in Ag(111) and Consequent Surface Structure

A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance. This is due to the coexistence of several different surface reconstructions on oxidized silver surfaces. A further complication is subsurface oxygen (Osub). Osub are O atoms absorbed into the near surface region of a metal, and are expected to alter the surface in terms of chemistry and structure; however, these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how Osub is formed and how its presence alters the surface structure. Using a combination of surface science techniques to quantify Osub formation and the resultant surface structure, we observed that once 0.1 ML of Osub formed, the surface was dramatically, and uniformly, reconstructed to striped structures at the expense of all other surface structures. Furthermore, Osub formation was hin...

Formation of surface oxides and Ag2O thin films with atomic oxygen on Ag(111)

The nature of the oxygen species adsorbed to silver surfaces is a key component of the heterogeneously catalyzed epoxidation of ethylene and partial oxidation of methanol over silver catalysts. We report the formation of two different silver–oxygen species depending on the flux and energy of incident gas-phase oxygen atoms on an Ag(111) surface. A combination of surface science techniques was used to characterize the oxidized surfaces. Atomic oxygen was generated with an Ir filament; lower temperatures created surface oxides previously reported. When O was deposited with a higher filament temperature, the surface became highly corrugated, little subsurface oxygen was observed, and thin layers of Ag2O were likely formed. These results show that the energy and flux of oxygen are important parameters in the chemical identity and abundance of oxygen on silver surfaces and suggest that formation of the Ag2O thin film hinders formation of subsurface oxygen.

Morphology and thermal stability of AlF3 thin films grown on Cu(100)

The growth of ultrathin epitaxial layers of aluminum fluoride on Cu(100) has been studied by a combination of surface science techniques. Deposition at room temperature results in step decoration followed by the formation of dendritic two-dimensional islands that coalesce to form porous films. Ultrathin layers (up to 2 monolayers in thickness) are morphologically unstable upon annealing; de-wetting takes place around 430K with the formation of three-dimensional islands and leaving a large fraction of the Cu surface uncovered. Films several nanometers thick, on the contrary, are stable up to ca. 730K where desorption in molecular form sets on. The effect of electron irradiation on the AlF3 has also been characterized by different spectroscopic techniques; we find that even small quantities of stray electrons from rear electron beam heating can provoke significant decomposition of the aluminum fluoride, resulting in the release of molecular fluorine and the formation of deposits of metallic aluminum. These features make AlF3 an interesting material for spintronic applications.

Growth, Structure and Morphology of Supported Metal Clusters Studied by Surface Science Techniques

This paper reviews on the basic theoretical background of the nucleation, growth and morphology of supported clusters, from thermodynamics and kinetics approaches. The application of surface science techniques to study, in situ, the formation and the structure of supported metal clusters is discussed from several examples. The combination of surface science techniques with microscopy techniques (ex situ TEM or in situ STM and AFM) is very powerful to study supported metal clusters. The influence of the substrate on the structure of metal clusters is also discussed from recent molecular dynamics calculations and experimental observations.

Investigations of Active Sites for Methane Activation in the Oxidative Coupling Reaction over Pure and Li-Promoted MgO Catalysts

The oxidative coupling of methane to ethane over model MgO catalysts prepared under well-controlled, ultrahigh vacuum (UHV) conditions has been studied using a combination of surface science techniques and elevated-pressure kinetic measurements. The utilization of electron energy loss spectroscopy has allowed us to characterize a variety of defects and to identify the active sites which are responsible for the methane coupling reaction. The results indicate that [Li+O−] centers are not likely directly involved in the methane activation step, but rather promote the production of F-type centers in the near-surface region which are responsible for this key step in the methane coupling reaction. The origin of [Li+O−] and F-type defects and their densities and stabilities are discussed.

Role of F centers in the oxidative coupling of methane to ethane over lithium-promoted magnesium oxide catalysts

We report a study of the partial oxidation of methane to ethane over a model MgO catalyst prepared under well-controlled, ultrahigh vacuum (UHV) conditions using a combination of surface science techniques and elevated pressure kinetics measurements.