Akira Sasahara

Reconstructive activation of bimetallic surfaces: Catalytic reduction of NO with H2 on Pt(100), Pt(110), Rh(100), Rh(110) and bimetallic single crystal surfaces of Rh/Pt(100), Rh/Pt(110), Pt/Rh(100), and Pt/Rh(110)

Abstract When Pt–Rh alloy or Pt/Rh bimetallic surfaces were exposed to O 2 or NO, Rh atoms underwent segregation at temperatures higher than ca. 400 K, and was followed by the characteristic restructuring. That is, Pt–Rh(100) alloy, Rh/Pt(100) and Pt/Rh(100) bimetallic surfaces commonly form a p(3×1) structure by heating in O 2 , and Pt/Rh(110) and Rh/Pt(110) surfaces heated in O 2 give c(2×4) and c(2×2) LEED pattern, respectively. Interestingly, the catalytic activity of the bimetallic surfaces for NO+H 2 reaction was markedly enhanced by the reconstruction and became structure insensitive. An STM image in atomic resolution revealed an ordered Rh and Pt composite structure for the p(3×1) Pt–Rh(100)/O alloy surface. These results suggest that the reconstructed bimetallic surfaces may be composed of the active sites, having a common local structure.

Individual Na Adatoms on TiO2(110)-(1×1) Surface Observed Using Kelvin Probe Force Microscope

The work function of Na-deposited TiO2(110) surfaces was mapped using a Kelvin probe force microscope (KPFM). Individual Na adatoms were identified as topographic protrusions on atomically flat terraces of the TiO2 surface. The work function locally decreased by 0.1–0.5 eV on each adatom. The observed modulations of work function were interpreted with the electric permanent dipole moment caused by the electron transfer from the adatom to the substrate.

Cyclic voltammogram of Rh(100), Pt-deposited Rh(100) and chemically reconstructed PtRh(100) surfaces

Abstract A Pt Rh (100) bimetallic surface was prepared by electrochemical deposition of Pt ions on Rh(100). Annealing of the Pt Rh (100) in O2 led to chemical restructuring, and the LEED pattern changed from p(1 × 1) to a very sharp p(3 × 1) pattern. The cyclic voltammogram was measured for a clean Rh(100), a Pt-deposited Rh(100) and the reconstructed p(3 × 1) surface. The voltammogram was very sensitive to the topmost layer of the electrode, and the reconstructed p(3 × 1) surface showed a very similar voltammogram to Rh(100) surface. This proves the formation of a hybrid surface of Rh-O/Pt/Rh(100) which we proposed previously.

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H2 was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H2 on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.

Image topography of alkyl-substituted carboxylates observed by noncontact atomic force microscopy

Abstract A series of carboxylates (HCOO − , CH 3 COO − and (CH 3 ) 3 CCOO − ) adsorbed on TiO 2 (1 1 0) were observed by noncontact atomic force microscopy. Different carboxylates were identified molecule-by-molecule in monolayers of mixed composition. The molecule-dependent image topography was quantitatively related to the atom geometry of alkyl group terminating the molecules. The image topography exhibited a good correlation with the physical topography (altitude of topmost hydrogen atoms) of the alkyl groups, though the former was not a one-to-one projection of the latter. The deviation from the one-to-one relation indicates a long-range nature of tip–molecule force.

Optically excited near-surface phonons of TiO2 (110) observed by fourth-order coherent Raman spectroscopy

We observed the fourth-order and third-order optical responses in the time domain on a TiO(2) (110) surface covered with trimethyl acetates. Coherent vibrations assignable to near-surface phonon modes were present at 179, 191, 359, 440, 507, 609, and 823 cm(-1) in the fourth-order responses. The amplitude and phase of each mode were determined with different azimuths and polarizations of pump and probe light pulses. Vibrational assignments and possible mechanisms to excite the vibrations were discussed.

Topography of anatase TiO2 film synthesized on LaAlO3(001)

The surface of an anatase titanium dioxide (TiO2) film grown on LaAlO3(001) was observed using noncontact atomic force microscopy (NC-AFM). After cleaning with cycles of argon ion sputtering and annealing in vacuum, (1 × 4)- and (1 × 5)-reconstructed terraces appeared. In addition to the terraces, the sputter-annealed surface included many agglomerations. The presence of Tin+ () shown by x-ray photoelectron spectroscopy indicates that the agglomerations are due to TiOx () species. Limited replenishment of oxygen by the substrate and the low diffusivity of the Tin+ to the substrate are possible causes for the generation of the TiOx.

Activation of the Pt deposited Rh(100) bimetallic surface by chemical ordering

Abstract When the Pt deposited Rh(100) surface, Pt/Rh(100), was exposed to 10 −7 Torr of O 2 at 780 K, Rh atoms are segregated and the LEED pattern changes from a c(2 × 2) to a p(3 × 1) + p4g p(2 × 2) pattern. This p(3 × 1) + p4g LEED pattern changed to a p(1 × 1) surface at room temperature by exposing it to H 2 and was recovered at room temperature by exposure to O 2 . The p(3 × 1) + p4g surface is stable at 780 K in vacuum but changes to a p(1 × 1) surface at 1000 K. If the p(1 × 1) surface obtained by annealing at 1000 K is heated in 10 −7 Torr of O 2 at 780 K, the p(3 × 1) + p4g surface is again recovered. The p(1 × 1) surface annealed at 1000 K is composed of the Pt-enriched topmost layer on a Rh rich 2nd layer but the p(3 × 1) + p4g surface prepared by treating with O 2 at 780 K is composed of the RhO surface compound on the Pt enriched 2nd layer. The catalytic reaction of NO + H 2 was compared on the two surfaces, the p(3 × 1) + p4g Rh-rich surface and the p(1 × 1) Pt-rich surface. The surface composed of RhO on the Pt layer was very active for the reaction but the Pt enriched p(1 × 1) surface was inactive for the reaction. From these results, we concluded that the RhO surface compound ordered on the Pt layer is an active structure for the catalysis.

Scanning tunnelling microscopy study of ammonia adsorption on TiO2(110)

We have used scanning tunnelling microscopy to study the adsorption of ammonia on TiO2(110). Our results confirm that NH3 adsorbs on 5-fold coordinated Ti sites, in line with previous studies. At higher coverages, our images show that the NH3 has a preferred periodicity in the [001] direction of twice the primitive unit cell distance.

Scanning Tunneling Microscopy Study of Surface Reconstructions of Rutile TiO2(111)

Structure of the rutile TiO2(111) surface prepared in ultra-high vacuum (UHV) was investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) for the first time. It is expected from the crystal structure that the stoichiometric TiO2(111) surface has more separated Ti metal sites compared to the well-studied rutile TiO2(110) surface. When the surface was annealed at 1024 K in UHV without Ar+ ion sputtering, a phase indexed as ( 1 &-1 1 &1 ) wasobservedwhereatomic-scaleprotrusions (probablyTiatoms) wereorderedrectangularly. AfterseveralcyclesofAr^+ ionsputteringandannealingupto 976 K, (1×1) anddouble-domain (1×2) smallpatchesappearedonthesurface. Thesethreephaseswereunstableagainsthigh-temperatureannealinginvacuum, probablybecausetheyaresensitivetosurfacestoichiometry.