Yuichi Fujimori

Oxidation of Au by Surface OH: Nucleation and Electronic Structure of Gold on Hydroxylated MgO(001)

The nucleation and electronic structure of vapor-deposited Au on hydroxylated MgO(001) surfaces has been investigated under ultrahigh vacuum conditions. Hydroxylated MgO(001) surfaces with two different hydroxyl coverages, 0.4 and 1 monolayer, respectively, were prepared by exposure to water (D(2)O) at room temperature. Scanning tunneling microscopy experiments show significantly higher gold particle densities and smaller particle sizes on the hydroxylated MgO surface as compared to gold deposited on clean MgO(001). Infrared spectroscopy and X-ray photoelectron spectroscopy experiments were performed to reveal details about the initial nucleation of gold. Gold atoms are found to chemically interact with a specific type of hydroxyl groups on the MgO surface, leading to the formation of oxidized gold particles. The enhanced adhesion of Au particles, which is due to the formation of strong Au-O interfacial bonds, is responsible for the observed higher stability of small Au clusters toward thermal sintering on hydroxylated MgO surfaces. The results are compared to similar studies on Au/TiO(2)(110) model systems and powder samples prepared by the deposition-precipitation route.

Carbon Dioxide Activation and Reaction Induced by Electron Transfer at an Oxide-Metal Interface

A model system has been created to shuttle electrons through a metal-insulator-metal (MIM) structure to induce the formation of a CO2 anion radical from adsorbed gas-phase carbon dioxide that subsequently reacts to form an oxalate species. The process is completely reversible, and thus allows the elementary steps involved to be studied at the atomic level. The oxalate species at the MIM interface have been identified locally by scanning tunneling microscopy, chemically by IR spectroscopy, and their formation verified by density functional calculations.

CO+NO versus CO+O2 Reaction on Monolayer FeO(111) Films on Pt(111)

Thin oxide films grown on metal single crystals are used in many “surface science” research groups in attempts to understand the surface chemistry of metal oxides. In addition, these films are employed as suitable supports for modeling highly dispersed metal catalysts (for reviews, see Refs. [1]–[4]). However, in the case of ultrathin films that are only a few angstroms in thickness, the metal substrate underneath the film often affects the properties of metal clusters by charge transfer through the film. These observations can, in principle, be traced back to the so-called “electronic theory of catalysis” developed in the 1950s and 1960s, and are primarily based on a Schottky barrier model, which predicts, in particular, that by varying the thickness of oxide films, the reactivity of heterogeneous catalysts can be controlled. However, these ideas faded away, primarily because of a lack of successful examples of the promotional effects of thin oxide films on catalytic activity and/or selectivity. Recently, it has been demonstrated that a thin oxide film grown on a metal may exhibit higher catalytic activity than the metal substrate under the same reaction conditions. Indeed, a thin FeO(111) film grown on Pt(111) is active for CO oxidation at 450 K, a temperature far below that at which Pt(111) itself is active. Furthermore, the rate enhancement was observed on Fe3O4-supported Pt nanoparticles, [13] which underwent encapsulation by an FeO(111) film as a result of the strong metal– support interaction. It has been suggested that, in the millibar pressure range (1 mbar=100 Pa) of O2, the bilayer Fe–O film on Pt(111) transforms into a trilayer O–Fe–O film that catalyzes CO oxidation according to a Mars–van Krevelen-type mechanism. A density functional theory (DFT) study corroborated this scenario. The DFT results showed that, by overcoming a small energy barrier of about 0.3 eV, O2 is chemisorbed on the Fe atom, which is pulled out of the pristine FeO film. In the chemisorbed state, electrons are transferred from the oxide/metal interface to oxygen, resulting in a O2 2 species, which then dissociates, thus forming a local O–Fe–O trilayer structure. Further DFT studies revealed that the reaction is site-specific within the large Moir unit cell formed due to an approximately 10% mismatch between the FeO(111) and Pt(111) lattices. This mismatch explains scanning tunneling microscopy (STM) results that showed the formation of close-packed O–Fe–O islands rather than a continuous FeO2 film. [15]

Formation of Water Chains on CaO(001): What Drives the 1D Growth?

Formation of partly dissociated water chains is observed on CaO(001) films upon water exposure at 300 K. While morphology and orientation of the 1D assemblies are revealed from scanning tunneling microscopy, their atomic structure is identified with infrared absorption spectroscopy combined with density functional theory calculations. The latter exploit an ab initio genetic algorithm linked to atomistic thermodynamics to determine low-energy H2O configurations on the oxide surface. The development of 1D structures on the C4v symmetric CaO(001) is triggered by symmetry-broken water tetramers and a favorable balance between adsorbate-adsorbate versus adsorbate-surface interactions at the constraint of the CaO lattice parameter.

Vibrational spectroscopic observation of ice dewetting on MgO(001)

The properties of the interfacial water monolayer on MgO(001) during growth of multilayer ice and, in particular, the dewetting of crystalline ice on MgO(001) are revealed by vibrational sum frequency generation and infrared reflection absorption spectroscopy.