星 永宏

The adsorption of hydroxide (OHad) on Pt has been studied on the low index planes of Pt using infrared reflection absorption spectroscopy (IRAS) in electrochemical environments. We discuss the correlation between the integrated band intensity of the bending mode of OH of Pt-OH (delta(PtOH)) and the charge density of the oxide formation of Pt. The band of dPtOH is observed around 1100 cm(-1), and the onset potential depends on the surface structure. The onset potential of dPtOH on Pt(110) and Pt(100) overlaps with the hydrogen adsorption/desorption potential region. The order of the integrated band intensity of delta(PtOH) at 0.9 V vs RHE is opposite to the order of the oxygen reduction reaction (ORR) activity. This finding supports that the OHad is one of the species deactivating the ORR.

燃料電池の酸素還元活性は表面構造によって大きく変化する。この講演では，白金単結晶電極を用いて酸素還元を活性化する表面構造を明らかにしたのち，白金の構造規整ナノ微粒子に類似の構造を構築し，単結晶電極との相関関係を論じる。あわせて，白金ナノ微粒子の耐久性に関しても言及する。

Correlation between Pt oxide and the activity for the oxygen reduction reaction (ORR) has been investigated on the low index planes of Pt (Pt(111), Pt(100), and Pt(110)) using voltammogram and rotating disk electrode (RDE). Pt oxide is formed by holding the potential at 1.0 V vs. RHE. The ORR activity decreases with the increase of the time of Pt oxide formation. The order of the ORR activity is Pt(100) < Pt(111) < Pt(110) in 0.1 M HClO4 after the formation of Pt oxide. This order is identical with that without Pt oxides. Formation of hardly reducible Pt oxide deactivates the ORR activity on Pt(111) remarkably. The amount of Pt oxides (PtOH, PtO and hardly reducible Pt oxide) increases as Pt(100) < Pt(111) < Pt(110) at 1.0 V.

Transitional structures of Cs+ at the outer Helmholtz plane (OHP) have been determined using time-resolved X-ray diffraction during the double-layer charging/discharging on the Ag(100) electrode in CsBr solution. At the double-layer potential region at which c(2 X 2)-Br is formed on Ag(100), the transient current comprises two exponential terms with different time scales: a rapid and a slow one are due to the dielectric polarization of water molecules and the transfer of Cs+, respectively. The slow term is composed of different dynamic processes of Cs+ during charging and discharging. When the potential is stepped in the positive direction, the coverage of Cs+ at the OHP decreases. In this step, the transient X-ray intensity at the (0 0 1) reflection, which is sensitive to the OHP structure, shows that Cs+ is released from the OHP according to exponential function of time. The decay of transient intensity of X-ray has a time scale similar to that of the current transient measurement. On the other hand, the accumulation process of Cs+ from the diffuse double layer to the OHP comprises two different kinetic processes after a potential step in the negative direction: a rapid one is the accumulation of Cs+ near the outer layer, and a slow one is the structural stabilization of the Cs+ layer.

Real surface structures of Pd(110) = 2(111)-(111) and Pd(311) = 2(100)-(111) have been determined using surface X-ray scattering (SXS) at 0.5V (RHE) in 0.1M HClO4 saturated with Ar and O-2. Both surfaces have unreconstructed (1 x 1) structures. These results differ from those of Pt(110) and Pt(311) of which surfaces are reconstructed to (1 x 2) in 0.1 M HClO4. Interlayer spacing between the first and the second layer d(12) is expanded on Pd(110) in O-2 saturated solution, whereas the spacing d(12) on Pd(311) is larger than that of the bulk in both Ar and O-2 saturated solutions. (c) The Electrochemical Society of Japan, All rights reserved.

Active sites for the oxygen reduction reaction (ORR) have been studied on n(111)-(111) and n(111)-(100) surfaces of Pt3Ni in 0.1 M HClO4 using rotating disk electrode (RDE). The activity for the ORR is decreased with the increase of the step atom density on n(111)-(111) surfaces. This fact is completely opposite to that of the same series of Pt on which the activity is enhanced at higher step atom density. The ORR gives the highest activity on n(111)-(100) surfaces of Pt3Ni with terrace atomic rows n = 3 and n = 5. These results show that the activity for the ORR of (100) step is superior to that of (111) step on Pt3Ni electrodes. (C) 2013 Elsevier B.V. All rights reserved.

The oxygen reduction reaction (ORR) has been studied on the n(111)-(111) and n(111)-(100) series of Pt3Co using a hanging meniscus rotating disk electrode (HMRDE) in 0.1 M HClO4 (n is the number of terrace atomic rows). The activity for the ORR on the n(111)-(111) series, which is estimated by the specific activity at 0.90 V (RHE) j(k), increases linearly with an increase in the step atom density d(S). On the other hand, the activity for the ORR on the n(111)-(100) series increases linearly with an increase of d(S) on the surfaces with n >= 9. On the surfaces with n < 9, however, the activity for the ORR decreases with an increase of d(S). We find a correlation between the ORR activity and theta(OH) at the step on the assumption that all the oxide species are Pt-OH.

Active sites for the oxygen reduction reaction (ORR) have been studied on four series of the high index planes of Pt in O. 1 M HCIO 4 using rotating disk electrode. The ORR activity linearly increases with the increase of the step atom density on n(111)一(111)andn(ll1)一(100)seriesfrom n=∞to n = 5 where n is the number of terrace atomic rows. The ORR activities on the surfaces with (100) terrace are lower than those with (111) terrace. giving no orientation dependence. The ORR activity increases as n(100)一(111)くn(100)一(110)くn(111)ー(100)くn(l11)一(111). Pt (331) = 3 (111)一(111) has the highest activity for the ORR. Active sites for the ORR are assigned to(l11)terrace edge or terrace atomic row neighboring to the edge.Key Words: Oxygen reduction reaction，H igh index planes，S tep，T

Structural effects on the oxygen reduction reaction (ORR) have been studied on the high index planes of Pt (n(1 1 1)-(1 II), n(1 1 1)-(1 00), n(1 0 0)-(1 1 1) and n(1 0 0)-(1 1 0)) using rotating disk electrode (RDE) in 0.1 M HClO4. The activity for the ORR increases with the increase of the step atom density on the surfaces with (1 1 1) terrace from n= infinity to n=5 (n is the number of terrace atomic rows), whereas the surfaces with (1 0 0) terrace do not show structural effects on the ORR activity. The activity of the surfaces with (1 1 1) terrace is higher than that on the surfaces with (1 0 0) terrace. The active sites for the ORR are located between the (1 1 1) terrace edge and the (1 1 1) terrace atomic row neighboring to the edge on Pt electrodes. (C) 2013 Elsevier Ltd. All rights reserved.

Surface and subsurface structures of PtSn surface alloy on Pt(111) were determined using in situ scanning tunneling microscopy (STM) and X-ray diffraction. Different ordered structures of the PtSn alloy layer were observed by STM in HClO4 at coverage of theta(Sn) <= 0.23. Superstructure of (root 3 X root 7)R19.1 degrees with small domain size was formed at theta(Sn) = 0.23. This structure promoted the catalytic activity for the ethanol oxidation reaction with high durability. X-ray structural analysis showed that the ratio of Sn in the subsurface was below 3(2)%, The PtSn alloy layer was mainly formed at the surface of the Pt(111) electrode. The Sn atoms protruded by 0.02 nm from the Pt layer, which was similar to the surface structure of Pt3Sn(111). One Pt atom in the (root 3 x root 7)R19.1 degrees structure contacts to one or two surrounding Sn atoms, which lead to the highest activity for the EOR.

The oxidation processes of a Pt(111) electrode in alkaline electrolytes depend on non-specifically adsorbed ions according to in situ X-ray diffraction and infrared spectroscopic measurements. In an aqueous solution of LiOH, an OHad adlayer is formed in the first oxidation step of the Pt(111) electrode as a result of the strong interaction between Li+ and OHad, whereas Pt oxidation proceeds without OHad formation in CsOH solution. Structural analysis by X-ray diffraction indicates that Li+ is strongly protective against surface roughening caused by subsurface oxidation. Although Cs+ is situated near the Pt surface, the weak protective effect of Cs+ results in irreversible surface roughening due to subsurface oxidation.

Lattice strain of Pt-based catalysts reflecting d-band status is the decisive factor of their catalytic activity toward oxygen reduction reaction (ORR). For the newly arisen monolayer Pt system, however, no general strategy to isolate the lattice strain has been achieved due to the short-range ordering structure of monolayer Pt shells on different facets of core nanoparticles. Herein, based on the extended X-ray absorption fine structure of monolayer Pt atoms on various single crystal facets, we propose an effective methodology for evaluating the lattice strain of monolayer Pt shells on core nanoparticles. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity.

The oxygen reduction reaction (ORR) have been studied on n(1 1 1)-(1 0 0) series of Pt using hanging meniscus rotating disk electrode (HMRDE) in 0.1 M HClO4 (n is the number of terrace atomic rows). The activity for the ORR, which is estimated by the reduction current density at 0.90 V (RHE)j(ORR,0.90V), increases linearly with the increase of the step atom density d(S) on the surfaces with 5 <= n. On the surfaces with n < 5, however, the activity for the ORR decreases with the increase of d(S). The deactivation of the ORR on the surfaces with n < 5 correlates with the formation of PtO-like species at the step. (C) 2012 Elsevier Ltd. All rights reserved.

Interfacial structures of cobalt(II) porphine (CoP) and 12,3,7,8,12,13,17,18-octaethyl-21H,23-H-porphine]cobalt(II) (CoOEP) have been studied on Au(111) electrode using electrochemical scanning tunneling microscopy (EC-STM), in-situ X-ray diffraction, and density functional theory (DFT) calculations. The adsorption of porphyrins affects the reconstruction of Au(111) surface. The adsorption of CoP causes a lifting of the reconstruction to a complete 1 x 1 structure of Au(111). On CoOEP modified Au(111), the unit cell periodicity of the reconstructed substrate structure expands compared with the root 3 x 23 structure of bare Au(111). The same expanded substrate structure was observed on Au(111) modified with OEP without the coordinated Co ion; the coordinated metal ion of the adsorbed porphyrin molecule does not affect the substrate structure. This result indicates that the interaction of conjugated pi electrons of porphyrin with the substrate is stronger than that of the coordinated Co ion. In-situ X-ray diffraction and DFT calculation support non-covalent interaction of porphyrins with the Au(111) surface. (C) 2012 Elsevier B.V. All rights reserved.

The dissolution of cubic and tetrahedral Pt nanoparticles has been studied on a Pt plate in 0.1 M NaCIO4 using conventional in situ atomic force microscopy (AFM). The Pt nanoparticles were dissolved during the potential cycle between 0.0 and 1.1 V (Ag/AgCl). The sides of the nanoparticles are dissolved faster than the upper parts. The height of cubic Pt nanoparticles does not change up to 200 cycles, shrinking to 80% of the initial height after 600 cycles. Tetrahedral Pt nanoparticles are dissolved from the top, forming a terrace at the upper part after 300 cycles. The durability of cubic Pt nanoparticles is higher than that of tetrahedral Pt nanoparticles.

Dissolution mechanism of cubic Pt nanoparticles has been studied with the use of atomic force microscopy on a carbon substrate in 0.1 M NaClO4. The shape of the nanoparticles does not change in the double layer region. The height of the nanoparticles increases slightly at the onset potential of the oxide film formation. The nanoparticles are dissolved from the sides in the oxide film formation region, whereas the change of the height is negligible. These results differ from those of the cubic Pt nanoparticles on a Pt substrate, on which the dissolution proceeds at the upper terrace. (C) 2011 Elsevier B.V. All rights reserved.

Ptは燃料電池の電極触媒として利用されている貴金属であるが、原子レベルでは表面に酸化皮膜を形成し、電極触媒活性を低下させることが報告されている。触媒活性向上のためには、この酸化皮膜の実態解明が不可欠である。そこで本研究では、Pt酸化物のうち、Pt-OHを赤外反射吸収分光法（IRAS）により観測した。また、Pt単結晶電極を用いることにより、Pt-OH形成と表面構造との相関関係を明らかにした。

酸性下において電位サイクルによる立方体，立方八面体および正四面体型Ptナノ微粒子の溶解を高速AFM観測した。電位走査範囲の上限を1.6 V_RHEに固定し，下限を0.05～0.7 V_RHEの間で変化させ溶解速度の違いを調べた。溶解が見られる下限電位は立方八面体＜正四面体≒立方体であった。溶解時の溶解速度は正四面体＜立方八面体＜立方体であった。この結果は微粒子の稜および面の安定性により説明できる。

We revealed the promotive effect of alkali metal cations on Br adlayer formation on an Ag(100) electrode by in-situ measurement of the x-ray specular rod. Alkali metal cations in the electrical double layer affect the onset potential of Br adsorption and the order-disorder transition. We determined the Cs structure in the electrical double layer during Br adlayer formation and found that (1) the Cs structure depends on the coverage of adsorbed Br, and (2) the amount of Cs increases at the initial stage of Br adlayer formation. Structural analysis suggests that hydrated Cs is localized in the area around adsorbed Br via noncovalent interactions. Formation of a Cs-Br complex promotes Br adsorption. © 2011 American Physical Society.

We revealed the promotive effect of alkali metal cations on Br adlayer formation on an Ag(100) electrode by in-situ measurement of the x-ray specular rod. Alkali metal cations in the electrical double layer affect the onset potential of Br adsorption and the order-disorder transition. We determined the Cs structure in the electrical double layer during Br adlayer formation and found that (1) the Cs structure depends on the coverage of adsorbed Br, and (2) the amount of Cs increases at the initial stage of Br adlayer formation. Structural analysis suggests that hydrated Cs is localized in the area around adsorbed Br via noncovalent interactions. Formation of a Cs-Br complex promotes Br adsorption.

Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H2SO4 using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands are due to the intermediate species for the HER. The band of the atop H appears on stepped surfaces with (110) step, whereas the asymmetric bridge H is observed on Pt(211) = 3(111)-(100) and Pt(311)= 2(111)(100) that have (100) step. The absence of the atop H on Pt( 100). Pt(211), and Pt(311) can be attributed to the relative stability of the bridge site. (C) 2011 Elsevier B.V. All rights reserved.

The oxygen reduction reaction (ORR) have been studied on n(1 1 1)-(1 0 0) series of Pd using hanging meniscus rotating disk electrode (HMRDE) in 0.1 m HClO(4) saturated with O(2). Activity for the ORR, which is estimated by the reduction current density at 0.90 V (RHE) j(ORR,0.90V). is enhanced linearly with the increase of the terrace atom density d(T), showing that (1 1 1) terrace is the active site for the ORR on n(1 1 1)-(1 0 0) series of Pd. Coverage of the oxide film theta(OX) of Pd increases with the increase of d(T) at 0.90 V (RHE). The correlation of the ORR activity and theta(OX) contradicts to that on Pt electrodes on which the ORR is deactivated with the increase of theta(OX) The oxide film is not relevant to the ORR on n(1 1 1)-(1 0 0) series of Pd at 0.90 V (RHE). (C) 2011 Elsevier B.V. All rights reserved.

Structural effects on the hydrogen oxidation reactions (HOR) and the hydrogen evolution reactions (HER) have been studied on (n-1)(1 1 1)-(1 1 0), n(1 0 0)-(1 1 1), n(1 0 0)-(1 1 0) and n(1 1 1)-(1 0 0) series of Pt using rotating disk electrode (RDE) in 0.1 M HClO(4) saturated with H(2). The value of the exchange current density J(0) increases linearly with the increase of the step atom density d(S) from n = infinity to n = 9, where n denotes the number of terrace atomic rows, on (n-1)(1 1 1)-(1 1 0), n(1 0 0)-(1 1 1), and n(1 0 0)-(1 1 0) series. This indicates that the activity for the HOR/HER of the step is higher than that of the terrace. The values of j(0) of (n-1)(1 1 1)-(1 1 0), n(1 0 0)-(1 1 1) and n(1 0 0)-(1 1 0) series become constant on the surfaces with n <= 9, whereas those of n(1 1 1)-(1 0 0) series increase with the increase of d(S). The activity for the HOR/HER per step atom increases in the sequence: n(1 1 1)-(1 0 0) < n(1 0 0)-(1 1 1) < n(1 0 0)-(1 1 0) < (n-1)(1 1 1)-(1 1 0). This result supports that the most active site for the HOR/HER is the linear (1 1 0) step. Decrease of the NOR current is found around 0.2 V(RHE) on the surfaces with (1 0 0) terrace, whereas the surfaces with (1 1 1) terrace give no decrease of the HOR current. (C) 2011 Elsevier B.V. All rights reserved.

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO4 at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110)= 2(111)4111) and Pt(311) = 2(100)-(111) = 2(111)(100), are reconstructed to a (1 x 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 x 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.

Surface structures of Pd(111) and Pd(100) electrodes have been determined at 0.50 V (RHE) in 0.1 M HClO4 saturated with Ar or O-2 using surface X-ray scattering (SXS). Both surfaces have unreconstructed (1 x 1) in-plane structure. The surface layers on Pd(111) do not relax in Ar saturated solution: the interlayer spacing between the first and the second layers d(12) agrees with that of the bulk. In O-2 saturated solution, the value of d(12) is expanded by 1.8% on Pd(111), whereas the value of d(23) (interlayer spacing between the second and third layers) is the same as that of the bulk. No relaxation is also observed on Pd(100) in Ar saturated solution. In O-2 saturated solution, however, d(23) as well as d(12) is expanded by 5.7% on Pd(100).

Structural effects on the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) have been studied on the high index planes of Pt and Pd in 0.1 M HClO₄. The exchange current density j_o of the HOR increases with the increase of the step atom density d_S from n = ∞ to 9 on(n−1)(111)-(110), n(111)-(100), n(100)-(111) and n(100)-(110) surfaces of Pt, where n shows the number of the terrace atomic rows. On the surfaces with n ≤ 9, however, the values of j₀ do not depend on d_S. These facts support that the step is the active site for the HOR on Pt electrodes, however, only part of the step atoms contributes to the HOR. Pt electrodes of (n−1)(111)-(110) series have the highest activity for the HOR. The activity for the ORR increases with the increase of the terrace atom density d_T on Pd electrodes. The step atoms do not affect the ORR. Pd(100) has the highest activity for the ORR. The specific activity of Pd(100) is 4.2 times as high as that of the standard Pt catalyst (TEC10E50E). The mass activity of Pd surfaces covered with monolayer of Pt (Pt/Pd(hkl)) is 7 times as high as that of TEC10E50E.

Real structure of cubic Pt nanoparticle has been studied at various potentials in 0.1 M NaClO4 with the use of atomic force microscopy (AFM). Cubic Pt nanoparticles in 10 nm height are clearly imaged from 0.10 V to 1.10 V (Ag/AgCl). The height of the nanoparticle increases 1.2 +/- 0.7 nm (10.4 +/- 6.8%) around the onset potential of oxygen evolution (1.20 V (Ag/AgCl)). The height increase is attributed to the formation of the oxide species at the inner layers of the nanoparticle. Dissolution of the nanoparticle starts from the upper terrace, not from the edge above 1.40 V (AgiAgCl). (C) 2010 Elsevier B.V. All rights reserved.

The surface structure of underpotentially deposited Bi has been determined on Au(111) in perchloric acid solution using surface X-ray diffraction (SXD), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. SXD analysis and STM images reveal that the catalytically active structure for the hydrogen peroxide reduction reaction (HPRR) is the (2 x 2)-Bi honeycomb Structure with theta(Bi) = 0.50. The stability is supported by DFT calculations. The hydrated perchlorate anion is located in the center of the honeycomb structure without hydrogen peroxide. DFT calculations predict that the Bi honeycomb structure promotes the dissociation of the O-O bond of hydrogen peroxide. Hydrogen peroxide expels the hydrated perchlorate anion, and then HPRR takes place at the honeycomb center.

Oxygen reduction reaction (ORR) has been studied on the low index planes of Pd modified with a monolayer of Pt (Pt/Pd(hkl)) in 0.1 M HClO(4) with the use of hanging meniscus rotating disk electrode. The activity for ORR on bare Pd(hkl) electrode depends on the surface structure strongly, however, voltammograms of ORR on Pt/Pd(hkl) electrodes do not depend on the crystal orientation. The specific activities of Pt/Pd(hkl) electrodes at 0.90 V (RHE) are higher than that on Pt(110) which has the highest activity for ORR in the low index planes of Pt. The mass activity on Pt/Pd(hkl) electrode is 7 times as high as a commercial Pt/C catalyst. (C) 2009 Elsevier B.V. All rights reserved.

Structural effects on hydrogen oxidation and evolution reactions (HOR/HER) have been studied on n(111)-(111) surfaces of Pt using a rotating disk electrode in 0.1 M HClO(4) saturated with H, at temperatures between 268 and 298 K. The exchange current density of HOR increases linearly with the increase of step atom density up to the surface with the terrace atomic rows n = 9; however, it becomes constant on the surfaces with n <= 9. The activation energy and pre-exponential factor of HOR/HER decrease with the increase of the step atom density, but they also become constant on the surfaces with n <= 9. These results support that the step atom is the active site of HOR/HER; however. a part of the step atoms contributes to HOR/HER on the surfaces with n <= 9.

Surface structures of shape-controlled Pt nanoparticles have been estimated using cyclic voltammetry (CV) and infrared reflection absorption spectroscopy (IRAS). Cubic and cubocatahedral Pt nanoparticles are prepared using a capping polymer. These nanoparticles give CVs similar to those of single crystal electrodes of Pt in sulfuric acid solution. The CV of cubic nanoparticles is similar to that of the Pt(510) [=5(100)-(110)] electrode, while the CV of cuboctahedral nanoparticles is reproduced well with the convolution of Pt(766) [=13(111)-(100)] and Pt(17 1 1) [=9(100)-(111)] electrodes. These results suggest that the planes of the cubic and cuboctahedral nanoparticles are composed of step-terrace and atomically flat terraces, respectively. Adsorbed carbonmonoxide (CO) on the shape-controlled nanoparticles gives the IR bands that are assigned to on-top and bridged CO. The band of on-top CO is deconvoluted to two bands: the higher and the lower frequency bands are assigned to the CO on the plane and the edges of the nanoparticles, respectively. on top CO adsorbed on the edges is oxidized at more negative potential then that on the planes. Edge sites of the nanoparticles promote CO oxidation.

Active sites for the oxygen reduction reaction (ORR) have been studied on the low index planes, n(100)(111) and n(100)-(110) series of Pd with the use of hanging meniscus rotating disk electrode (RDE) in 0.1 M HClO(4) saturated with O(2). The Kotekey-Levich plots give a linear line with a slope of about 4, showing four electron reduction for the ORR on Pd electrodes, as is the case of Pt. Activity of ORR, which is estimated by the reduction current density at 0.90 V (RHE) j(ORR.0.90V), gives the following order on the low index planes: Pd(110) - Pd(111) < Pd(100). This order is completely opposite to that of Pt in 0.1 M HClO(4): Pt(100) < Pt(111) < Pt(110). Specific activity of Pd(100) is nearly three times as high as that of Pt(110) at 0.90 V (RHE). The values of j(ORR,) (0.90) (V) increase linearly with the increase of the terrace atom density on n(100)-(111) and n(100)-(110) series of Pd. This fact supports that (100) structure is the active site for ORR on Pd. ORR activity does not depend on the step structure of n(100)-(111) and n(100)-(110) series.

We determined the structure of water at step edges on the stepped Pt(211) = 3(111)-(100) surface using surface X-ray diffraction. Adsorbed water molecules form a chain with a zigzag configuration along the step line through hydrogen bonds. The chain comprises two types of water molecules. One is adsorbed on the top of the step atom and has a shorter Pt-O distance than that adsorbed on terrace sites, which indicates stronger adsorption at step edges. The other is displaced from the precise on-top sites and has an elongated Pt-O distance. The hydrogen-bonding configuration of the water chain is different from the tetrahedral arrangement of ice. The interaction of the water molecules with the surface distorts the hydrogen bonds in the water chain.

Until the mid 1980's, there had been only few in situ methods available for structural determination of an electrode surface in solution at atomic and monolayer levels. Nowadays, many powerful in situ techniques, such as electrochemical scanning tunneling microscopy (EC-STM), infrared reflection absorption spectroscopy (IRAS), surface-enhanced Raman scattering (SERS), and surface-enhanced infrared reflection absorption spectroscopy (SEIRAS), second harmonic generation (SHG), sum frequency generation (SFG), and surface X-ray scattering (SXS) have been widely employed to characterize the electrode surfaces under potential control with atomic and/or molecular resolution. The object of this review is to highlight some of the progress on in situ methods at solid-liquid interface with atomic and molecular levels. Several selected topics are focused on, specifically adsorbed anions on metal surface, electrocatalysis of the carbon oxide oxidation and dioxygen reduction, and direct observation of single crystal electrode surfaces.

Voltammograms of the low index planes of Pd and Pd(S)-[n(100) x (111)] electrodes have been studied in 0.1 M NaOH. The voltammograms are more complicated than those in acid solutions. An anodic peak due to the desorption of absorbed hydrogen is found on Pd(111) and Pd(100). No peak due to the hydrogen desorption is observed on Pd(110). The onset potentials of the oxide film formation are more negative than those in acid solutions. Coverages of the oxide film on Pd(111) and Pd(100) in 0.1 M NaOH are lower than those in 0.1 M HClO(4). The coverage on Pd(110), however, has the same value as the one in 0.1 M HClO(4). On Pd(S)-[n(100) x (111)] electrodes, oxide film formation at the step can be distinguished from the one at the terrace in 0.1 M NaOH. This result differs from that in H(2)SO(4) in which the oxide film formation gives no peak at the step. (C) 2008 Elsevier B.V. All rights reserved.

The structural and electrochemical properties of Ag cubic-particles supported on highly oriented pyrolytic graphite (HOPG) have been studied using atomic force microscopy (AFM)) and voltammetry. Ag cubic-particles aggregate in an array on HOPG in solution. The AFM image of the Ag particles is similar to that obtained using transmission electron microscopy. Voltammogram of the Ag particles on HOPG electrode gives a pair of peaks at around -0.7 V (Ag/AgCl) in NaBr solution, which corresponds to the order-disorder structural transition of Br layer adsorbed on Ag(100) electrode. The surface structure and electrochemical properties of Ag cubic-particles are similar to those of Ag(100).

Surface structure of Pt(310) = 3(100)-(110), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO4. Clean Pt(310) surface has pseudo (1 x 1) structure on which lateral displacements of 2-9% and 0.3-1% are found along a and b directions, respectively, whereas the surfaces of Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) are reconstructed to (1 x 2) according to previous reports. Interlayer spacing between the first and the second layers d(12) is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(310). This result differs from that on Pt(111), where d(12) is expanded after CO adsorption. (C) 2008 Elsevier Ltd. All rights reserved.

Spectroscopic characterization of adsorbed water has been carried out on (111) surfaces of Pt, Pd, Rh, Au, and Cu in sulfuric acid solution using infrared reflection-absorption spectroscopy (IRAS). The absorption band of the HOH bending mode (delta(HOH)) is measured below the onset potential of (bi)sulfate adsorption. The band frequency of delta(HOH) of adsorbed water is lower than that of bulk water. The delta(HOH) band shifts to higher frequencies with the increase of the positive potentials on Pt, Pd, and Rh, whereas the band intensity is unchanged between the potentials of hydrogen evolution and (bi)sulfate adsorption. This suggests that water is adsorbed through an oxygen lone pair, with its molecular plane slightly tilted from the surface. On Au(111), the delta(HOH) band intensity decreases with the increase of the electrode potential. This result indicates that the orientation of water depends on the applied potential. No absorption band of water is found on Cu(111), which indicates different behavior of adsorbed water compared with the other metal electrode surfaces.

Surface structure of a stepped surface of Pt, Pt(31 1) (=2(100) - (111)), has been determined under potential control in 0.1 M HClO4 with the use of in situ surface X-ray scattering (SXS). The crystal truncation rods (CTRs) are reproduced well with the (1 x 2) missing-row model. Relaxation of surface layers, which is observed on the low-index planes of Pt, is not found on Pt(31 1) in the "adsorbed hydrogen region". CTRs at 0.10 (RHE) have the same feature as those at 0.50 V, showing that the surface layers of Pt(311) have no potential dependence. Scanning tunneling microscopy (STM) also supports the (1 x 2) structure of Pt(311) in 0.1 M HClO4.

Chlorobenzene was electrolyzed on poly cry stalline platinum electrode in acetonitrile with various water concentrations at -3.02 V(vs. Fc/Fc(+)). Formation rate of the reduction products depend on water concentration remarkably. Benzene is a main product at [H2O] <= 0.3 mol dm(-3). At [H2O] >= 0.35 mol dm(-3), toluene is obtained as well as benzene. Partial current density of toluene gives maximum at [H2O] = 0.44 mol dm(-3). Formation of toluene closely relates with production of methane resulting from the decomposition of acetonitrile.

Structural effects on intermediate species of methanol oxidation ore studied on low-index planes of platinum using in-situ infrared (IR) spectroscopy. A flow cell is designed for rapid migration of reactant and product species on the electrode surface. IR spectra show adsorption of formate and the formation of carbonate species on the Pt(111) surface at potentials higher than that of CO oxidation. The band assignments for carbonate and formate are confirmed by vibrational isotope shifts. On Pt(100), the absorption band of adsorbed formate is much smaller than that on Pt(117). On the other hand, there is no adsorbed formate on Pt(110) in the potential region examined. The band intensity of formote follows the order: Pt(117) > Pt(100) > Pt(110). This order is opposite to that of the current density in the regions of higher potential. Adsorbed formate on PtO 10 behaves like a catalyst-poisoning intermediate, like adsorbed CO.