Yoshihide Watanabe

Size-dependent catalytic activity and geometries of size-selected Pt clusters on TiO2(110) surfaces

Scanning tunneling microscope observations show a geometrical transition from a planar structure to a 3D structure at n = 8. This geometrical transition resulted in a significant decrease in the activation energy of the CO oxidation reaction. The upper-layer Pt atoms of the 3D cluster structure that starts to form at n = 8 are low-coordinated Pt atoms, and they may play an important role in the CO oxidation reaction.

A new experimental setup for high-pressure catalytic activity measurements on surface deposited mass-selected Pt clusters

A new experimental setup to study catalytic and electronic properties of size-selected clusters on metal oxide substrates from the viewpoint of cluster-support interaction and to formulate a method for the development of heterogeneous catalysts such as automotive exhaust catalysts has been developed. The apparatus consists of a size-selected cluster source, a photoemission spectrometer, a scanning tunneling microscope (STM), and a high-pressure reaction cell. The high-pressure reaction cell measurements provided information on catalytic properties in conditions close to practical use. The authors investigated size-selected platinum clusters deposited on a TiO2(110) surface using a reaction cell and STM. Catalytic activity measurements showed that the catalytic activities have a cluster-size dependency.

Atomic-resolution imaging of size-selected platinum clusters on TiO2(110) surfaces

Size-selected Pt(n) (n=4,7-10,15) clusters were deposited on TiO(2)(110)-(1x1) surfaces and imaged at atomic resolution using an ultrahigh-vacuum scanning tunneling microscope with a carbon nanotube tip. Clusters smaller than Pt(7) lay flat on the surface with a planar structure and a planar-to-three-dimensional transition occurred at n=8 for Pt(n) clusters on TiO(2). However, both Pt(8) and Pt(9) had two types of geometric structures. The geometric structures depend strongly on the number of atoms in the deposited cluster possibly because of the differences in binding energies in different-sized clusters and different degrees of interaction with the surface. We obtained atomic-resolution images of size-selected clusters on surfaces for the first time, enabling the identification of atomic alignments in the clusters on the surface.

Epitaxial growth of CeO2(111) film on Ru(0001): scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study.

We formed an epitaxial film of CeO2(111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce(4+)/Ce(3+) ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2(111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2(111) with wide terrace width and sharp step edges.

Calcined sepiolite-supported Pt/Fe catalyst

Calcined sepiolite-supported Pt (Pt/CSp) catalysts indicate the high activity for complete oxidation of hydrocarbons (HC). The simultaneous oxidation of HC and SO 2 on the Fe-impregnated calcined sepiolite-supported Pt (Pt/Fe-CSp) catalysts was studied in comparison with that on Pt/CSp catalysts. There was a negligible difference in HC activity. Nevertheless, SO 2 oxidation activity on the Pt/Fe-CSp catalyst was suppressed more than that on the Pt/CSp catalyst. The calcination temperature of sepiolite affects the quantity of Fe additive. XRD and NMR show that the quantity of Fe additive depends on the distortion of the local structure of the calcined sepiolite. At the same time, the quantity of Fe additive affects the SO 2 oxidation activity. Fresh Pt/Fe-CSp catalyst absorbs SO 2 . The absorbed SO 2 reacted with Mg of CSp to form MgSO 4 on the catalyst. After this absorption was saturated, the selectivity of the oxidation was maintained. XPS and Mossbauer spectroscopy show that most of Fe on the Pt/Fe-CSp catalyst is Fe 3+ Pulsed chemisorption analysis of the redox cycle shows that Fe on the calcined sepiolite has a high oxygen storage capacity (OSC). This redox reaction suggests the mechanism of this selective oxidation reaction. The selective oxidation property is expected to be useful for a catalyst of purification for exhaust gas from a boiler or a combustion engine using a fuel which contains sulfur.

Atomically precise cluster catalysis towards quantum controlled catalysts

Abstract Catalysis of atomically precise clusters supported on a substrate is reviewed in relation to the type of reactions. The catalytic activity of supported clusters has generally been discussed in terms of electronic structure. Several lines of evidence have indicated that the electronic structure of clusters and the geometry of clusters on a support, including the accompanying cluster-support interaction, are strongly correlated with catalytic activity. The electronic states of small clusters would be easily affected by cluster–support interactions. Several studies have suggested that it is possible to tune the electronic structure through atomic control of the cluster size. It is promising to tune not only the number of cluster atoms, but also the hybridization between the electronic states of the adsorbed reactant molecules and clusters in order to realize a quantum-controlled catalyst.

Catalytic properties of thin films by simultaneous oblique sputter deposition of two materials from different directions

Abstract Catalytic properties of simultaneously obliquely sputter-deposited platinum-metal oxide thin films were studied. We attempted to study the effect of combination of the metal oxide with platinum on the NO-CO reaction by minimizing differences in the particle size and the dispersion of platinum in the samples. Simultaneously obliquely sputter-deposited Pt-metal oxide thin film systems gave comparable particle sizes and dispersion of platinum in different combinations of several metal oxides with platinum. The catalytic property of platinum was changed by the choice of metal oxide. We found this simultaneous oblique sputter deposition to be an excellent technique for obtaining a model catalyst suitable for exploring the synergistic catalytic reaction and the role of promoters at the interfaces of different materials, which facilitates the survey of industrial catalysts.

Cluster size dependence of Pt core-level shifts for mass-selected Pt clusters on TiO2(110) surfaces

In order to examine cluster size dependence, mass-selected platinum clusters, Ptn (n=2–5, 7, 8, 10, 15), were deposited on TiO2(110) surfaces at room temperature under soft landing conditions, and their core levels were investigated by x-ray photoelectron spectroscopy. Pt core-level shifts with cluster size are observed. The binding energies of Pt 4f7/2 decrease steeply with increasing cluster size up to n=7 for Ptn and decrease gradually for n≥8. This inflection point (n=8) agrees well with the cluster size at a geometric transition (planar-to-three-dimensional) seen with scanning tunneling microscopy (STM) [N. Isomura, X. Wu, and Y. Watanabe, J. Chem. Phys. 131, 164707 (2009)]. It was found that the core-level shifts of mass-selected Pt clusters deposited on TiO2 are closely correlated with cluster geometries determined directly by atomic-resolution STM imaging.

Alumina-supported sub-nanometer Pt10 clusters: amorphization and role of the support material in a highly active CO oxidation catalyst

Catalytic CO oxidation is unveiled on size-selected Pt_(10) clusters deposited on two very different ultrathin (≈0.5–0.7 nm thick) alumina films: (i) a highly ordered alumina obtained under ultra-high vacuum (UHV) by oxidation of the NiAl(110) surface and (ii) amorphous alumina obtained by atomic layer deposition (ALD) on a silicon chip that is a close model of real-world supports. Notably, when exposed to realistic reaction conditions, the Pt_(10)/UHV-alumina system undergoes a morphological transition in both the clusters and the substrate, and becomes closely akin to Pt_(10)/ALD-alumina, thus reconciling UHV-type surface-science and real-world experiments. The Pt_(10) clusters, thoroughly characterized via combined experimental techniques and theoretical analysis, exhibit among the highest CO oxidation activity per Pt atom reported for CO oxidation catalysts, due to the interplay of ultra-small size and support effects. A coherent interdisciplinary picture then emerges for this catalytic system.

A new optimizing technique of a diesel engine aftertreatment system using HC DeNox catalyst

Abstract A new method that optimizes a control map of hydrocarbon (HC) addition to diesel exhaust gas for HC type Selective Catalytic Reduction DeNO x catalyst system has been developed. This method comprises a numerical model and a new optimization technique. The numerical model is capable of predicting performance of HC DeNO x with diesel fuel as a supplemental reductant. The control map of HC addition was optimized with the evolutionary programming based on the evolution of living things. As a result, NO x conversion with the optimized control map was found to be greater by 21% than that with the conventional control on Japanese 10–15 mode.