High Activity of Au/K/TiO₂(110) for CO Oxidation: Alkali-Metal-Enhanced Dispersion of Au and Bonding of CO

Images from scanning tunneling microscopy show high mobility for potassium (K) on an oxidized TiO₂(110) surface. At low coverages, the alkali metal occupies mainly terrace sites of the o-TiO₂(110) system. The results of X-ray photoelectron spectroscopy indicate that K is fully ionized. The electron transferred from K to the titania affects the reactivity of this oxide, favoring the dispersion of Au particles on the terraces of the o-TiO₂(110) surface. When small coverages of K and Au are present on the o-TiO₂(110) system, only a few K–Au pairs are formed and the alkali metal affects Au chemisorption mainly through the oxide interactions. Addition of K to Au/o-TiO₂(110) enhances the reactivity of the system, opening new reaction paths for the adsorption and oxidation of carbon monoxide. CO can undergo disproportionation (2CO → C_ads + CO_2,ads) on K/o-TiO₂(110) and Au/K/o-TiO₂(110) surfaces. The Au–KO_<i>x</i> interface binds CO much better than plain Au–TiO₂, increasing the surface coverage of CO and facilitating its oxidation. Kinetic tests show that K promotes CO oxidation on Au/TiO₂. Turnover frequencies of 2.1 and 10.8 molecules (Au site)⁻¹ s^–1 were calculated for oxidation of CO on Au/o-TiO₂(110) and Au/K/o-TiO₂(110) catalysts, respectively

Spectromicroscopy of a Model Water–Gas Shift Catalyst: Gold Nanoparticles Supported on Ceria

Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water–gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1–2 nm thick) CeO_2–<i>x</i>(111) crystallites on a Rh(111) substrate. Low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influence the Au 4f binding energy; more heavily reduced ceria promoted a larger shift to higher binding energy. Understanding the redox behavior of the gold/ceria system is an important step in elucidating the mechanisms behind its catalytic activity

Water Dissociates at the Aqueous Interface with Reduced Anatase TiO₂ (101)

Elucidating the structure of the interface between natural (reduced) anatase TiO₂ (101) and water is an essential step toward understanding the associated photoassisted water splitting mechanism. Here we present surface X-ray diffraction results for the room temperature interface with ultrathin and bulk water, which we explain by reference to density functional theory calculations. We find that both interfaces contain a 25:75 mixture of molecular H₂O and terminal OH bound to titanium atoms along with bridging OH species in the contact layer. This is in complete contrast to the inert character of room temperature anatase TiO₂ (101) in ultrahigh vacuum. A key difference between the ultrathin and bulk water interfaces is that in the latter water in the second layer is also ordered. These molecules are hydrogen bonded to the contact layer, modifying the bond angles