L. Savio

Carbon Dioxide Hydrogenation on Ni(110)

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

Low-energy acoustic plasmons at metal surfaces

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons—higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

Tuning surface reactivity by in situ surface nanostructuring

As recently demonstrated, the morphology of a surface can be modified on the mesoscopic scale by ion sputtering. Here we show by microscopy and spectroscopy that the chemical properties of the surface are strongly affected by nanostructuring and that surface reactivity can be tuned by changing surface morphology. For the otherwise inert Ag(001) surface significant O2 dissociation takes place on the nanostructured surface, thus allowing us to control the relative coverage of admolecules and adatoms. The dissociation probability is determined by the experimentally tunable density of kinks.

Oxygen interaction with disordered and nanostructured Ag(001) surfaces

We investigated O2 adsorption on Ag(001) in the presence of defects induced by Ne+ sputtering at different crystal temperatures, corresponding to different surface morphologies recently identified by scanning tunneling microscopy. The gas-phase molecules were dosed with a supersonic molecular beam. The total sticking coefficient and the total uptake were measured with the retarded reflector method, while the adsorption products were characterized by high resolution electron energy loss spectroscopy. We find that, for the sputtered surfaces, both sticking probability and total O2 uptake decrease. Molecular adsorption takes place also for heavily damaged surfaces but, contrary to the flat surface case, dissociation occurs already at a crystal temperature, T, of 105 K. The internal vibrational frequency of the O2 admolecules indicates that two out of the three O2− moieties present on the flat Ag(001) surface are destabilized by the presence of defects. The dissociation probability depends on surface morpholo...

Self-assembly of (S)-glutamic acid on Ag(100): a combined LT-STM and ab initio investigation.

Self-assembly of organic molecules at metal surfaces is of greatest importance in nanoscience; in fact, it opens new perspectives in the field of molecular electronics and in the study of biocompatible materials. Combining an experimental low-temperature scanning tunneling microscopy investigation with ab initio calculations, we succeeded to describe in detail (S)-glutamic acid adsorption on Ag(100) at T = 350 K. We find that (S)-glutamic acid organizes in a squared structure and, at variance with the majority of cases reported in literature, it adsorbs in the neutral form, 4.6 A above the surface plane. The interaction with the poorly reactive Ag substrate is only due to weak van der Waals forces, while H-bonds between carboxyl groups and the formation of a OCOH-OCOH-OCOH-OCOH cycle at the vertex of the squares are the main responsible for the self-assembly.

Formation of channels for oxygen migration towards subsurface sites by CO oxidation and growth of the surface oxide phase on Ag(0 0 1)

The mechanism of oxygen incorporation in Ag is still poorly known. As recently demonstrated [Phys. Rev. B 63 (2001) R1404], oxygen adatoms removal by CO oxidation leaves the Ag(0 0 1) surface in a modified state in which oxygen segregation from the bulk and the formation of a surface oxide phase can be induced by further CO exposure. Here we show that the same channels, forming during CO oxidation and linking surface and subsurface sites, allow also for the migration of oxygen adatoms into the subsurface region. When dosing O2 on an Ag(1 0 0) surface, on which oxygen had been previously removed by CO oxidation, we observe indeed the formation of the same surface oxide phase produced by oxygen segregation. A characterisation of this phase by X-rays photoemission spectroscopy and high resolution electron energy loss spectroscopy is given, from which we deduce that it extends several layers deep into the volume. 2002 Elsevier Science B.V. All rights reserved.

Selective Production of Reactive and Nonreactive Oxygen Atoms on Pd(001) by Rotationally Aligned Oxygen Molecules

Sticking together: The occupation of different sites by oxygen atoms that are produced by the dissociation of O(2) on Pd(100) is determined by the initial rotational alignment of the parent molecules. The atom locations are characterized by different chemical reactivities in the reaction with CO to form CO(2) (see picture), which are followed by synchrotron radiation (SR) experiments with a supersonic molecular beam (SMB).

Dynamics of the gas-surface interaction in presence of well defined defects

The active sites for catalytic reactions in heterogeneous catalysis are often minority sites related to defects at the surface. However, in most studies performed so far under controlled conditions, nearly perfect low Miller index surfaces were used, giving rise to the so-called structure gap in the surface science approach to catalysis. In order to overcome these limits and understand how defects affect reactions, surfaces damaged on purpose by ion bombardment or surfaces aligned along high Miller index planes are studied. In this paper we shall discuss the cases of C2H4 and O2 interacting with Ag(4 1 0). We shall demonstrate that the open steps are in both cases active sites for adsorption and in the case of O2 also for dissociation, while flat (1 0 0) planes are much less reactive. Using a supersonic molecular beam, which allows to define exactly the angle of incidence and the impact energy of the gas-phase molecules, we show that the distribution of the activation barriers to adsorption at defects can be measured. For O2/Ag(4 1 0) we find that its height is definitely smaller at open steps than at (1 0 0) terraces. The O2 sticking probability at terraces is moreover strongly reduced, implying that the reactivity of the Ag atoms is influenced by the terrace width. 2002 Elsevier Science B.V. All rights reserved.

(S)-Glutamic Acid on Ag(100): Self-Assembly in the Nonzwitterionic Form

The fundamental understanding of adsorption and self-organization of biological molecules at surfaces is of greatest importance for a huge variety of possible applications, ranging from molecular electronics to the study of biocompatible materials, hygiene, and biofouling. In spite of that, the characterization of the interactions of organic molecules of biological interest with surfaces is far from being complete. In the present paper we report on a combined microscopic (scanning tunneling microscopy (STM)) and spectroscopic (X-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy) study of glutamic acid (Glu) adsorption and self-assembly on Ag(100) at different temperature. STM allows one to determine the structures of the Glu layers, for which empirical models are proposed, while photoemission spectra exclude adsorption in the zwitterionic form, which is the most common especially for weakly interacting substrates.

Enhanced Chemical Reactivity of Pristine Graphene Interacting Strongly with a Substrate: Chemisorbed Carbon Monoxide on Graphene/Nickel(1 1 1)

Graphene is usually considered a chemically inert material. Theoretical studies of CO adsorption on free‐standing graphene predict quite low adsorption energies (<0.1 eV). However, we show here by vibrational spectroscopy and scanning tunneling microscopy that the nondissociative chemisorption of CO occurs at cold, pristine graphene grown on Ni(1 1 1). The CO adlayer remains stable up to 125 K, although some coverage survives flashes to 225 K. This unexpected result is explained qualitatively by the modification of the density of states close to the Fermi energy induced by the relatively strong graphene–substrate interaction. The value of the adsorption energy allows us to estimate an equilibrium coverage of the order of 0.1 monolayers at 10 mbar pressure, which thus paves the way for the use of graphene as a catalytically active support under realistic conditions.