星 永宏

© 2020 Elsevier Ltd The oxidation of dopamine (DA) on a boron-doped diamond (BDD) electrode was investigated using in situ attenuated total reflection infrared spectroscopy (ATR-IR) and infrared reflection absorption spectroscopy (IRAS). Voltammogram showed multiple anodic/cathodic peaks for the oxidation/reduction of DA and its oxidized derivatives on the BDD electrode. The oxidation/reduction species on the surface and in solution were assigned using ATR-IR and IRAS, respectively. The anodic oxidation of DA promoted polymerization, and the polymerized DA (PDA) was continuously deposited on BDD without being reduced by potential cycles, which resulted in the irreversible behavior of the voltammogram. A decrease in the oxidation current of DA by potential cycles was due to the deposition of PDA. Some intermediate quinone species, such as dopaminequinone and dopaminechrome, were reversibly reduced to hydroquinone.

We have studied Pt surfaces for which activity towards the oxygen reduction reaction (ORR) is enhanced by adsorbed amines: octylamine (OA) and an alkyl amine containing a pyrene ring (PA). Adsorbed OA/PA (molar ratio 9/1) enhances activity for ORR on n(111)–(111) surfaces of Pt for terrace atomic rows n greater than 7. The activity on Pt(111), the surface of which is a flat terrace, is particularly enhanced. On Pt(100), which also has a flat terrace, however, the ORR activity is greatly reduced. It is shown that adsorbed OA/PA enhances the ORR on surfaces with a wide (111) terrace, but deactivates the ORR on the (100) terrace.

In situ vibrational spectra of Pt oxides that cannot be measured with IR spectroscopy have been studied on the low index planes of Pt using surface enhanced Raman spectroscopy with bare Au nanoparticles (NPSERS). Two bands appear around 570 and 340 cm(-1) at higher potentials in 0.1 M HClO4 saturated with Ar, which are assigned to the stretching vibration of Pt-O(H) and the libration vibration of Pt-O, respectively. NPSERS spectra are measured in 0 2 saturated solution for the first time. The band intensities of Pt-O(H) and Pt-O in O-2 saturated solution are enhanced significantly compared with those in Ar saturated solution. The onset potentials of Pt-O and Pt-O(H) formation are 1.15 V(RHE) on Pt(100) and 1.2 V(RHE) on Pt (111) and Pt(110). The onset potential of Pt-O and Pt-O(H) and band shape differ from the results obtained using shell isolated surface enhanced Raman spectroscopy (SHINERS). The Pt-O and Pt-O(H) band intensities are normalized using COad as an internal standard. The Pt-O(H) band intensity depends on surface structures as Pt(110) < Pt(111) << Pt(100), whereas the Pt-O band gives a different intensity order for Pt(lll) and Pt(110) as Pt(111) < Pt(110) << Pt(100) in O-2 saturated solution.

Methanol oxidation reaction (MOR) has been studied on the low index planes of Pd (Pd(111), Pd(100) and Pd(110)) in 0.1 M HClO4 using voltammogram. Onset potential of methanol oxidation increases as Pd(110)similar to Pd(100) < Pd(111). The activity for the MOR is estimated using anodic peak current density of the voltammograms. The order of the activity is Pd(111)similar to Pd(110) << Pd(100): (100) surface enhances the activity for the MOR on Pd electrodes. (C) The Electrochemical Society of Japan, All rights reserved.

Infrared spectroscopy of adsorbed hydroxide (OHad) has been performed on the high index planes of Pt: n(111)-(100) and n(111)-(111) series. The bending mode of OHad (delta(ptOH)) appears on these surfaces around 1080 cm(-1), which is similar to the result on the low index planes of Pt. The band intensity of delta(ptOH) depends on surface structure and electrode potential. At 0.9 V vs RHE, the band intensity of delta(ptOH) increases with the decrease of the step atom density: the intensity of delta(ptOH) is higher on the surfaces with lower activity of the oxygen reduction reaction (ORR). These results suggest that the adsorption of OHad is site specific and the deactivating species of the ORR. (C) 2016 Elsevier B.V. All rights reserved.

The interfacial structure and activity for the oxygen reduction reaction (ORR) were investigated on a PtNi surface alloy on a Pt(111) electrode (PtNi/Pt(111)). The PtNi surface alloy was prepared by thermal annealing of Ni2+ modified on Pt(111) at 573-803 K. After optimizing the alloying temperature and the amount of added Ni, the ORR current density of PtNi/Pt(111) at 0.9 V (reversible hydrogen electrode) is enhanced 9.5 times compared with that of Pt(111), and the activity is decreased by 24% after 1000 potential cycles. The atomic composition and subsurface structure of PtNi/Pt(111) were determined by in situ infrared reflection-absorption spectroscopy and X-ray diffraction. The surface contains a (111)-oriented Pt-skin and the subsurface of the 2nd-5th layers of the PtNi alloy contains less than 11% Ni atoms. The layer spacings of the surface alloy layers are slightly expanded compared with those of bare Pt(111). Homogeneous alloying with a small amount of Ni in the subsurface layers achieves the high ORR activity and durability.

Understanding the electrocrystallization mechanisms of metal cations is of importance for many industrial and scientific fields. We have determined the transitional structures during underpotential deposition (upd) of various metal cations on Au(111) electrode using time-resolved surface X-ray diffraction and step-scan IR spectroscopy. At the initial stage of upd, a characteristic intensity transient appears in the time-resolved crystal truncation rod depending on metal cations. Metal cations with relatively high coordination energies of hydration water are deposited in two steps: first, the hydrated metal cations approached the surface and are metastably located at the outer Helmholtz plane, then they are deposited via the destruction of the hydration shell. However, Tl+ and Ag+, which have low hydration energy, are rapidly adsorbed on Au(111) electrode without any metastable states of dehydration. Therefore, the deposition rate is strongly related to the coordination energy of the hydration water. Metal cations strongly interacting with the counter coadsorbed anions such as Cu2+ in sulfuric acid causes the deposition rate to be slower because of the formation of complexes.

The oxygen reduction reaction (ORR) has been studied on kinked stepped surfaces of Pt inside the stereographic triangle with the use of rotating disk electrode (RDE) in 0.1 M HClO4. The kinked stepped surfaces are composed of (331) = 3(111)-(111) structure of which step line contains (100) kink: 3(111) - [m(111) + (100)], where m denotes the number of straight step atoms. The activity for the ORR decreases with increasing the kink atom density d (k) of the surface with m = 1, 3, 6, and 21, whereas the surface with m = 11 has specifically high activity for the ORR. The activity on Pt(16 15 5) = 3(111) - [11(111)-(100)] exceeds that of Pt(331) that gives the highest activity in stepped surfaces. The electricity of Pt oxide formation on Pt(16 15 5) m = 11 is the lowest in the kinked stepped surfaces examined.

Real time dissolution process of shape-controlled Pt nanoparticles (cubic, cuboctahedral, tetrahedral) has been studied using high-speed atomic force microscopy (AFM) during potential cycles in electrochemical environments. The nanoparticles are dissolved above 1.5 V(RHE) when the lower limit of the potential cycle is fixed at 0.05 V(RHE). Edges of cubic nanoparticles are dissolved at first, whereas the tops of cuboctahedral and tetrahedral nanoparticles are dissolved forming pseudo-flat structure at the upper terraces. The order of the durability is cuboctahedral < cubic < tetrahedral. When the upper limit of the potential cycle is fixed at 1.6 V(RHE), the order of the durability depends on the lower limit of the potential cycle: cubic cuboctahedral < tetrahedral between 0.05 and 02 V(RHE), cubic < tetrahedral cuboctahedral between 03 and 0.6 V(RHE). (C) 2016 Elsevier B.V. All rights reserved.

Activities for the oxygen reduction reaction (ORR) were investigated on Ni low index surfaces modified by electrochemical deposition of Pt (Pt/Ni(hkl)) and then thermal annealing. The ORR current density at 0.9 V (RHE) of the Pt/Ni(111) without annealing is three times higher than that of Pt(111). However, the Pt/Ni(111) without annealing has a low durability, with the activity reduction of more than 50% after 1000 potential cycles between 0.6 V and 1.0 V. The catalytic activity and durability are improved by annealing of Pt/Ni(111). The ORR activity of the Pt/Ni(111) with annealing at 650 K is 1.7 times higher than that without annealing and the decrease of the activity is less than 10% after 1000 potential cycles. The annealing process stabilizes the Pt-rich shell on Ni(111). (C) 2016 Elsevier B.V. All rights reserved.

The effects of alkali meal cation on the surface oxidation and alcohol oxidation reactions on Au(111) have been investigated using surface X-ray diffraction and infrared spectroscopy. It is known that alkali metal cations strongly affect the alcohol oxidation reactions on Pt(111); however, the oxidation reactions on Au(111) do not depend on alkali metal cations. Infrared spectroscopy reveals the formation of adsorbed OH in LiOH and CsOH solutions below the second anodic peak at 1.3 V. This result indicates that surface oxidation processes do not depend on alkali metal cations below 1.3 V. The interfacial structure, including the outer layer, of an Au(111) electrode has been determined using X-ray diffraction in LiOH and CsOH. During the surface oxidation, the Au(111) surface in CsOH gets roughened more remarkably than that in LiOH above 1.3 V. Li+ has a protective effect against surface roughening. Thus, the cationic effect is weaker in the potential region lower than the second anodic peak, which does not affect the lifting of the surface reconstruction and the alcohol oxidation reactions.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Adsorption of sulfuric acid anions (HSO<Sub>4</Sub><Sup>&minus;</Sup> or SO<Sub>4</Sub><Sup>2&minus;</Sup>) on single crystal electrodes of platinum and palladium in 0.05 M H<Sub>2</Sub>SO<Sub>4</Sub> has been studied using infrared reflection absorption spectroscopy (IRAS). Both Pt(111) and Pd(111) electrodes provide a single IRAS band around 1200 cm<Sup>&minus;1</Sup> in the region above 1000 cm<Sup>&minus;1</Sup>. Two bands appear around 1200 and 1100 cm<Sup>&minus;1</Sup> with Pt(100), Pt(110) and Pt(S)-[n(111)&times;(111)] electrodes. The intensity of the lower frequency band is enhanced with the increase of the step atom density. The IRAS spectra of Pd(100), Pd(110) and stepped surface of Pd (Pd(311)) differ completely from those of Pt electrodes: a single band is observed around 1200 cm<Sup>&minus;1</Sup>. Adsorbed geometry and adsorption site of the anion are discussed. The reduction of CO<Sub>2</Sub> is deactivated remarkably by the adsorption of sulfuric acid anion on the Pt(S)-[n(111)&times;(111)] electrodes. Active site for the CO<Sub>2</Sub> reduction is also discussed.

Electrochemical reduction of carbon dioxide was studied with various series of copper single crystal electrodes in 0.1 M KHCO3 aqueous solution at constant current density 5 mA cm(-2); the electrodes employed are Cu(S)-[n(1 0 0) x (1 1 1)], Cu(S)-[n(1 0 0) x (1 1 0)], Cu(S)-[n(1 1 1) x (1 0 0)], Cu(S)-[n(1 1 1) x (1 1 1)] and Cu(S)-[n(1 1 0) x (1 0 0)]. The electrodes based on (1 0 0) terrace surface give ethylene as the major product. The ethylene formation is further promoted by the introduction of (1 1 1) or (1 1 0) steps to the (1 0 0) basal plane. The highest C2H4 to CH4 formation ratio amounts to 10 in terms of the current efficiency for the (7 1 1) (=4(1 0 0) - (1 1 1)) surface as compared with the value 0.2 for the (I 11) electrode. The n (1 1 1) - (1 1 1) surfaces favor the formations of acetic acid, acetaldehyde and ethyl alcohol with the increase of the (1 1 1) step atom density. CH4 formation at the n (1 1 1) - (1 1 1) electrodes decreases with the increase of the (111) step atom density. The n (1 1 1) - (1 0 0) surfaces give higher gaseous products; the major product is CH4 with lower fraction of C-2+ compounds. (C) 2003 Elsevier Science B.V. All rights reserved.

The sulfuric acid anion (HSO4- or SO42-) adsorbed on Pd single-crystal electrodes was studied using infrared reflection absorption spectroscopy (IRAS) in 0.05 M H2SO4 solution. Surfaces examined are flat ones (Pd-(111) and Pd(100)) and stepped ones (Pd(110) (= 2(111 )-(111)) and Pd(311) (= 2( 111)-(100))). All of the surfaces give a single IRAS band around 1200 cm-1 that is assigned to S-O stretching vibration. The integrated band intensity depends on the crystal orientation significantly, i.e., Pd(311) ∼ Pd(110) ≪ Pd(100) &lt Pd-(111), showing that the flat surfaces adsorb more sulfuric acid anion than the stepped ones. Magnitude of the band shift (dv/dE) on the flat surfaces is nearly twice as high as that on the stepped ones.

Adsorption of sulfuric acid anion (HSO4- or SO42-) was studied using IRAS (infrared reflection absorption spectroscopy) on Pt(S)-[n(111) × (111)] and Pt(S)-[n(100) × (111)] electrodes in 0.05 M H2SO4. Pt(S)[n(111) × (111)] electrodes give two IRAS bands. One is from a sulfuric acid anion adsorbed on the terrace with 3-fold symmetry (∼1200 cm-1), and another is from that on the step with 2-fold symmetry (∼1200 cm-1 and ∼1100 cm-1). Relative band intensity from the 2-fold sulfuric acid anion (∼1100 cm-1) gets higher with the increase of step atom density. Pt(S)-[n(100) × (111)] electrodes also show two IRAS bands around 1200 and 1100 cm-1 which originate from the sulfuric acid anion adsorbed on the terrace and the step with 2-fold symmetry. The relative intensity of the lower frequency band increased with the increase of the step atom density.

Electrochemical reduction of CO2 was studied using single-crystal electrodes, Cu(111), Cu(100), Cu(S)-[n(100) × (111)], and Cu(S)-[n(100) × (110)] at a constant current density 5 mA cm-2 in0.1 M KHCO3 aqueous solution. Copper single crystals were prepared from 99.999% copper metal in graphite crucibles by the Bridgeman method. The crystal orientation was determined by the X-ray back reflection method. The Cu(111) electrode yields mainly CH4 from CO2, and the Cu(100) favorably gives C2H4. Introduction of (111) steps to Cu(100) basal plane, leading to Cu(S)-[n(100) × (111)] orientations, significantly promoted C2H4 formation and suppressed CH4 formation. The selectivity ratio C2H4/CH4 on Cu(711) (n = 4) amounted to 14, 2 orders of magnitude higher than that on Cu(111).

The effects of underpotential-deposited lead on the adsorption of CO on Pt(111) surfaces have been investigated in 0.1 M HClO4 by in situ infrared reflection absorption spectroscopy (IRAS). Lead coverages of about 0.4 on Pt(111) electrodes polarized at 0.1 V vs RHE prevent CO from adsorbing on multiply bonded sites, reducing the overall coverage to about a third of that observed in the absence of coadsorbed Pb under otherwise identical conditions, yielding a single, sharply-defined feature in the infrared reflection absorption spectra characteristic of atop-bonded CO. However, the integrated absorption band of atop CO at 0.1 V in the presence of coadsorbed Pb was found to be about 3 times higher than that predicted from the corresponding spectral features observed for Pb-free surfaces at that specific coverage corrected for dipole-dipole coupling interactions. Although much of this effect may be due to a Pb-induced decrease in the dielectric screening within the CO adlayer, the gains in intensity are too large to be explained solely on this basis, pointing to chemical factors, such as Pt-Pb charge polarization, as an important factor.

Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single cryst;al electrodes examined were stepped surfaces (Pt(S)-[n(111) x (111)]), Pt(S)-[n(lll)x (100)], Pt(S)-[n(100)x (111)]), and kinked step surfaces (Pt(S)-[n(110)x (100)] and Pt(S)[n(100) x (110)]). Atomically flat surfaces, Pt(lll) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111) &lt; Pt(100) &lt; Pt(S)-[n(lll) x (100)] &lt; Pt(S)-[n(lll) x (100)] &lt; Pt(S)-[n(lll) x (111)] &lt; Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111) x (111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2. (C) 2000 Elsevier Science Ltd. All rights reserved.

Structural effects on the rates of CO2 reduction were studied on Ag(111), Ag(100) and Ag(110) electrodes in 0.1 M KHCO3 using macroelectrolysis. The partial current density of each reduction product (CO, HCOO-, H-2) was measured at various potentials. All the Ag single crystal electrodes mainly gave CO at any potential. The partial current density of CO on the atomically stepped Ag(110) was remarkably higher than those on the flat Ag(111) and Ag(100), as is the case with Pt group metals. The partial current density of HCOO- gave a smaller orientation dependence. (C) 1997 Elsevier Science S.A.

The structural effect on the rate of CO2 reduction was studied with voltammetry on single crystals of Pd (Pd(111), Pd(100), Pd(110)) in 0.1 M HClO4 saturated with CO2. CO2 was reduced to an adsorbed product at potentials more negative than -0.1 V vs. RHE. The rate of CO2 reduction depends remarkably on the crystal orientation: Pd(100) &lt; Pd(lll) &lt; Pd(110). The Pd(111) surface shows uniquely high activity, whereas (111) is the surface of lowest activity in CO2 reduction for other Pt group metals (Pt, Ph, Ir). The rate of CO2 reduction at -0.5 V vs. RHE on Pd(110) is two orders of magnitude higher than that of Pt(110).

Electrochemical measurements were conducted for investigation of the dynamical process of adsorbed CO formation from CO2 and adsorbed hydrogen on Pt single crystal surfaces in 0.5 M H2SO4. The adopted monocrystal surfaces were Pt(111) and Pt(110) [= 2(111)-(111)], whose atomic orientations are composed of (111) type arrangement on the surface. The rate of the CO formation on Pt(110) was more than 10 times as high as those on Pt(111). Oxidation charge of CO at the final stage of the reaction was almost constant between 0.05 and 0.20 V (vs. rhe) on Pt(110), whereas the amount of adsorbed hydrogen, source material of the reaction, greatly varied in this potential range. This fact suggests that hydrogen atoms are regenerated on Pt(110). Such regeneration was not observed on Pt(111). The difference of the reactivity between Pt(111) and Pt(110) may be attributed to the four-fold hollow site on Pt(110).