Yasuhiro Iwasawa

Adsorption of Na atoms and oxygen-containing molecules on MgO(100) and (111) surfaces

The adsorption of Na atoms and oxygen-containing molecules such as H 2 O, CH 3 OH, CO 2 , HCOOH and HCOOCH 3 has been studied on the annealed MgO(100) and (111) surfaces by means of XPS, UPS and LEED. Annealing a (111) surface at 1000 K yielded microfacets ~ 20 A across. This faceted (111) surface showed a high adsorption activity compared with the flat (100) surface. Hence it is suggested that the coordinatively unsaturated edge-sites or the multicentered valley-sites on the microfaceted surface play important roles in the chemisorption.

Regulation of reaction intermediate by reactant in the water-gas shift reaction on CeO2, in relation to reactant-promoted mechanism

Reaction regulation by intermediate-reactant interaction in the water-gas shift reaction (H2O + CO → H2 + CO2; WGSR) on CeO2 was investigated in relation to the reactant-promoted mechanism on MgO and ZnO by FT-IR. Terminal OH groups on partially reduced CeO2 reacted with CO to produce bridge formates. Bridge formates were converted to bidentate formates above 443 K, and this transformation occurred at room temperature when water coexisted. The decomposition of the surface formates was affected by the water molecule; 70% of the bidentate formates decomposed to OH + CO (backward decomposition) and 30% of them decomposed to H2 + CO2 (forward decompose to H2 and unidentate carbonates. The decomposition of the unidentate carbonates to CO2 was promoted by coadsorbed water. The results are also discussed in comparison with the formates increased from 94 kJ mol−1 without water to 193 kJ mol−1 with water. Isotope effects were observed on the hydrogen atoms of both formate and hydroxyl, giving a proposed transition state in the reaction mechanism. The bidentate formates reacted with the adjacent hollow-site OH to decompose to H2 and unidentate carbonates. The decomposition of the unidentate carbonates to CO2 was promoted by coadsorbed water. The results are also discussed in comparison with the results for WGSR on MgO and ZnO.

Reconstruction of TiO2(110) surface: STM study with atomic-scale resolution

Abstract We succeeded in STM imaging of TiO 2 (110) with atomic-scale resolution. Sputtering and subsequent annealing under UHV at 600–900 K resulted in the (110)−(1 × 1) terraces bounded by steps parallel to the [111] or [001] direction. Five-fold coordinated Ti 4+ cations ordered in (1 × 1) periodicity were resolved on the terraces. Further annealing at 900–1100 K formed strings composed of double ridges along the [001] direction on the (110)−(1 × 1) terraces. The strings covered the surface to give a (1 × 2) superstructure when annealed at 1150 K. LEED patterns consistent with the transformations were observed.

A new approach to active supported Au catalysts

In this article we attempted to review the key issues relevant to tremendous oxidation activity of supported Au catalysts. We described in detail newly developed Au catalysts, remarkably active for low-temperature CO oxidation, using Au phosphine complexes and clusters as precursors for gold metal particles and as-precipitated wet metal hydroxides as precursors for oxide supports. Particular attention was placed on chemical interaction of the Au complexes and clusters with the oxide surfaces and its critical role in preparation of tremendously active supported Au. Simultaneous transformations of both the gold and support precursors during temperature-programmed calcination provoked the formation of Au catalysts highly active for the low-temperature CO oxidation. A new aspect of CO oxidation mechanisms was also discussed.

STM-imaging of formate intermediates adsorbed on a TiO2(110) surface

We have succeeded in STM-imaging of formates adsorbed on TiO2(110) with atomic-scale resolution, formates being regarded as intermediates for catalytic decomposition reaction of formic acid on TiO2(110). Dispersive and (2×1)-ordered overlayers of formates were resolved with positive sample bias voltages. The formates were adsorbed on the rows of fivefold coordinated Ti4+ cations along the [001] axis. Their individual images were elongated in the [110] direction. The anisotropic feature reflects the distribution of the tunneling orbital, the LUMO, of the formates.

Adsorption of CH3OH, HCOOH and SO2 on TiO2(110) and stepped TiO2(441) surfaces

Abstract A TiO2(441) surface was prepared whose electronic states and chemisorption properties for CH3OH, HCOOH and SO2 were compared with those of a TiO2(110) surface by means of XPS, UPS and LEED at 298 K. The (441) surface had a regular step structure which is indexed as [3(110) × (111)]. Its work function was smaller by 0.7 eV than that of the (110) surface due to the pinned Fermi level originated from a small amount of Ti3+ species. CH3OH and HCOOH were molecularly adsorbed on both the surfaces, giving a p(2×1) structure for HCOOH adsorbed on (110). It was suggested from the change of the work function that they were absorbed with their dipole axes normal to the local crystal plane on the step sites, thus the adsorbed species at step sites being more inclined as compared with those on the (110) terrace. SO2 was absorbed on (441) forming SO2-3 and also reacted with Ti3+ probably at the step to form S2-, while only SO2-3 was detected on the (110) surface.

Modification of surface electronic structure on TiO2(110) and TiO2(441) by Na deposition

Abstract The geometric and electronic properties of Na overlayers deposited on TiO 2 (rutile) (110) and stepped (441) surfaces have been examined by means of XPS, UPS, XAES (X-ray excited Auger electron spectroscopy), EELS and LEED. Na atoms form a smooth atomic layer interacting with surface oxygen atoms on TiO 2 (110). A surface model consisting of ordered Na 2 O dimers is proposed for a c(4×2) structure which appears at 0.5 ML Na coverage. At the first monolayer, the Na 2 O units are considered to complete an array along the [001] direction in (1×1) periodicity. Bonding of Na with oxygen causes charge transfer to the substrate, resulting in a downward band bending toward the surface. With the increase of the band bending, the Fermi level crosses an unfilled surface state localized in five-fold coordinated Ti 4+ ions, which leads to the reduction of these Ti 4+ to Ti 3+ . The strong interaction of the TiO 2 (110) surface toward the Na overlayer is ascribed to surface states just above the valence band maximum localized on protruded ridge oxygen atoms, from comparison with previous results for a flat MgO(100) surface. Almost the same picture is drawn for a stepped TiO 2 (441) surface.

Silica-supported aminopyridinium halides for catalytic transformations of epoxides to cyclic carbonates under atmospheric pressure of carbon dioxide

Silica-supported 4-pyrrolidinopyridinium iodide was prepared by quaternization of 4-pyrrolidinopyridine with silica-supported alkyl iodide. The pyrrolidinopyridinium structure on the silica surface was confirmed by solid-state 13C CP MAS NMR. The silica-supported 4-pyrrolidinopyridinium iodide showed excellent catalytic performances for transformations of various epoxides to cyclic carbonates under atmospheric pressure of carbon dioxide (CO2). The reactions took place without any solvents or additives other than the catalyst. The catalyst was reusable with retention of activity and selectivity. 1-n-Hexyl-4-pyrrolidinopyridinium as a homogeneous catalyst showed a lower catalytic performance than the supported catalyst. Bifunctional catalysis involving acidic surface silanol and the basic 4-pyrrolidinopyridinium iodide was proposed.

Reactant-promoted reaction mechanism for catalytic water-gas shift reaction on MgO

The behavior of reaction intermediates in the catalytic water-gas shift reaction (WGSR) on the MgO surface was studied by means of FT-IR spectroscopy. The hydroxyl groups on top of coordinatively unsaturated Mg atoms reacted with CO to produce three kinds of surface formates of unidentate, bidentate, and bridge types in the order bridge > unidentate > bidentate. Unidentate-type formate was produced at room temperature and decomposed at ca. 450 K. The formation and decomposition of bridge- and bidentate-type formates proceeded at higher temperatures (ca. 400 and 550 K, respectively). In the presence of adsorbed water, unidentate-type formate changed into bridge-type formate and hence most formates are bridge type under reaction conditions. Contrary to previous reports, formate intermediates were never converted to H{sub 2} and CO{sub 2} in the absence of H{sub 2}O. While all the formates decompose to CO and surface OH by themselves, adsorbed water promoted the decomposition of formates to the WGSR products H{sub 2} and CO{sub 2} through electronic interaction between the adsorbed water and the formate. The reaction rate of WGSR at steady state agreed with the decomposition rate of the bridge-type formate intermediate to H{sub 2} and CO{sub 2} in the presence of water.

Chemical Design Surfaces for Active Solid Catalysts

Publisher Summary This chapter focuses on the chemical design surfaces for active solid catalysts that can be regarded as a promising and scientific way to prepare well-defined active surfaces with excellent catalytic properties as well as to elucidate reaction mechanisms, including dynamic changes of active sites. These are new generation of catalysts resulting from scientific development at the boundary between homogeneous and heterogeneous chemistry. The use of such chemically and structurally controllable surface systems, rather than the more conventional supported catalysts with heterogeneous metal centers, can lead to novel information on the origins of catalysis. Once the essential processes of activity, selectivity, and specificity have been identified, it should be possible to incorporate this knowledge into the rational synthesis of practical catalysts that are deliberately tailored for specific reaction chemistries. Chemical surface design has strategic advantages in the synthesis of catalysts with novel and desired surface structures and compositions, and in the elucidation of heterogeneous catalysis on a molecular level.