Akira Nambu

High catalytic activity of Au/CeOx/TiO2(110) controlled by the nature of the mixed-metal oxide at the nanometer level

Mixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry. Recently, there has been a strong interest in understanding phenomena associated with the deposition of oxide nanoparticles on the surface of a second (host) oxide. Here, scanning tunneling microscopy, photoemission, and density-functional calculations are used to study the behavior of ceria nanoparticles deposited on a TiO2(110) surface. The titania substrate imposes nontypical coordination modes on the ceria nanoparticles. In the CeOx/TiO2(110) systems, the Ce cations adopt an structural geometry and an oxidation state (+3) that are quite different from those seen in bulk ceria or for ceria nanoparticles deposited on metal substrates. The increase in the stability of the Ce3+ oxidation state leads to an enhancement in the chemical and catalytic activity of the ceria nanoparticles. The codeposition of ceria and gold nanoparticles on a TiO2(110) substrate generates catalysts with an extremely high activity for the production of hydrogen through the water–gas shift reaction (H2O + CO → H2 + CO2) or for the oxidation of carbon monoxide (2CO + O2 → 2CO2). The enhanced stability of the Ce3+ state is an example of structural promotion in catalysis described here on the atomic level. The exploration of mixed-metal oxides at the nanometer level may open avenues for optimizing catalysts through stabilization of unconventional surface structures with special chemical activity.

Au ↔ N Synergy and N-Doping of Metal Oxide-Based Photocatalysts

N-doping of titania makes photocatalytic activity possible for the splitting of water, and other reactions, under visible light. Here, we show from both theory and experiment that Au preadsorption on TiO2 surfaces significantly increases the reachable amount of N implanted in the oxide. The stabilization of the embedded N is due to an electron transfer from the Au 6s levels toward the N 2p levels, which also increases the Au-surface adhesion energy. Theoretical calculations predict that Au can also stabilize embedded N in other metal oxides with photocatalytic activity, such as SrTiO3 and ZnO, producing new states above the valence band or below the conduction band of the oxide. In experiments, the Au/TiN(x)O(2-y) system was found to be more active for the dissociation of water than TiO2, Au/TiO2, or TiO(2-y). Furthermore, the Au/TiN(x)O(2-y) surfaces were able to catalyze the production of hydrogen through the water-gas shift reaction (WGS) at elevated temperatures (575-625 K), displaying a catalytic activity superior to that of pure copper (the most active metal catalysts for the WGS) or Cu nanoparticles supported on ZnO.

Adsorbate-Driven Morphological Changes of a Gold Surface at Low Temperatures

Using STM, infrared absorption reflection spectroscopy experiments and density functional calculations we show that low temperature adsorption of CO on gold surfaces modified by vacancy islands leads to morphological changes and the formation of nanosized Au particles. These results demonstrate a dynamic response of a surface during adsorption with consequences for the surface reactivity.

Recent progress in coincidence studies on ion desorption induced by core excitation

This paper reports on recent studies of photostimulated ion desorption (PSID) using electron ion coincidence (EICO) spectroscopy combined with synchrotron radiation. H+ desorption from H2O dissociatively adsorbed on Si(111) and SiO2/Si(111) surfaces (H2O/Si(111) and H2O/SiO2/Si(111)) was studied for Si L-edge excitation. The Si 2p–H+ photoelectron photoion coincidence (PEPICO) and Si 2p photoelectron spectra of H2O/Si(111) and H2O/SiO2/Si(111) show that H+ desorption probability increases as the number of positive charges at the Si site increases. The H+ desorption probability per Si 2p ionization for the Si4+ site was estimated and found to be 5–7 × 10−5. We proposed a mechanism that H+ desorption is induced by Si 2p photoionization accompanied by a Si LVVV double-Auger transition. This article also reviews recent EICO work on site-specific ion desorption of 1,1,1-trifluoro-2-propanol-d1 (CF3CD(OH)CH3) adsorbed on Si(100) surfaces, and on the mechanisms of PSID of poly(tetrafluoroethylene) (PTFE) and TiO2(110). Clear site-specific ion desorption was observed for the C 1s core excitation of a CF3CD(OH)CH3 sub-monolayer on Si(100). A spectator-Auger-stimulated ion-desorption mechanism was proposed for F+ desorption induced by a transition from F 1s to σ(C–F)* of PTFE. O+ desorption induced by O 1s excitation of TiO2(110) was attributed mainly to three-hole final states resulting from multi-electron excitation/decay. For O+ desorption induced by Ti core excitation of TiO2(110), on the other hand, charge transfer from an O 2p orbital to a Ti 3d orbital, instead of the interatomic Auger transition, was proposed to be responsible for the desorption. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a useful tool for studying PSID.

Development of a compact electron ion coincidence analyzer using a coaxially symmetric mirror electron energy analyzer and a miniature polar-angle-resolved time-of-flight ion mass spectrometer with four concentric anodes.

A compact electron ion coincidence (EICO) analyzer that uses a coaxially symmetric mirror electron energy analyzer and a miniature polar-angle-resolved time-of-flight ion mass spectrometer with four concentric anodes was developed for surface science and surface analysis. The apparatus is especially useful in the study of ion desorption stimulated by an Auger process because information on the mass, yield, desorption polar angle, and kinetic energy of ions can be obtained for the selected core-ionization-final-states or the selected Auger-final-states. The analyzer can be used also for analysis of the configuration of specific surface molecules because the desorption polar angles reflect the direction of surface bonds. The EICO analyzer was evaluated by measuring polar-angle-resolved-ion yield spectra and coincidence spectra of Auger-electron and polar-angle-resolved H(+) from condensed water.