Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACS Applied Materials and Interfaces  

In order to obtain a suitable design policy for the development of a next-generation polymer electrolyte fuel cell, we performed a visualization analysis of Pt and Co species following aging and degradation processes in membrane-electrode assembly (MEA), using a same-view. Nano-X-ray absorption fine structure (XAFS)/Scanning transmission electron microscope (STEM)-energy dispersive X-ray spectroscopy (EDS) technique that we developed to elucidate durability factors and degradation mechanisms of a MEA Pt3Co/C cathode electrocatalyst with higher activity and durability than a MEA Pt/C. In the MEA Pt3Co/C, after 5000 ADT-rec (rectangle accelerated durability test) cycles, unlike the MEA Pt/C, there was no oxidation of Pt. In contrast, Co oxidized and dissolved over a wide range of the cathode layer (∼70% of the initial Co amount). The larger the size of the cracks and pores in the MEA Pt/C and the smaller the ratio of Pt/ionomer of cracks and pores, the faster the rate of catalyst degradation. In contrast, there was no correlation between the size or Co/ionomer ratio of the cracks and pores and the Co dissolution of the MEA Pt3Co/C. It was shown that Co dissolved in the electrolyte region had an octahedral Co2+-O6 structure, based on a 150 nm × 150 nm nano-XAFS analysis. It was also shown that its existence suppressed the oxidation and dissolution of Pt. The MEA Pt3Co/C after 10,000 ADT-rec cycles had many cracks and pores in the cathode electrocatalyst layer, and about 90% of Co had been dissolved and removed from the cathode layer. We discovered a metallic Pt-Co alloy band in the electrolyte region of 300-400 nm from the cathode edge and square planar Pt2+-O4 species and octahedral Co2+-O6 species in the area between the cathode edge and the Pt-Co band. The transition of Pt and Co chemical species in the Pt3Co/C cathode electrocatalyst in the MEA during the degradation process, as well as a fuel cell deterioration suppression process by Co were visualized for the first time at the nano scale using the same-view nano-XAFS/STEM-EDS combination technique that can measure the MEA under a humid N2 atmosphere while maintaining the working environment for a fuel cell.

DOI： 10.1021/acsami.9b16393

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DOI： 10.1021/acsami.8b04407

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Chemistry Chemical Physics  

Photoelectron spectroscopy has the advantage of providing electric potentials by non-contact measurements based on the kinetic energy shift in component potential. We performed operando hard X-ray photoelectron spectroscopy (HAXPES) measurements with an 8 keV excitation source to measure the shift in electron kinetic energies as a function of the voltages of all the components at the anode and cathode electrodes of a polymer electrolyte fuel cell (PEFC). At the cathode electrode, when we increase the voltage between the cathode and anode from 0.2 to 1.2 V, the O 1s and F 1s peaks shift to a lower binding energy and the magnitude of the energy shift is equal to the voltage. The Pt 3d and C 1s peaks do not shift with the voltage since platinum nanoparticles and carbon supports at the cathode electrode have ground contact. In contrast to the cathode electrode, the peak shifts of all the components at the anode electrode show the same amount of shift as the voltages. It is clear that the change in the potential difference occurs only in an electrical double layer at the interface between the cathode electrode (Pt/C) and the electrolyte (Nafion and water), and that the anode electrode is in equilibrium as a pseudo-hydrogen electrode. Moreover, the electric potential variation of the cathode electrode in a PEFC under a power generation condition was also directly detected by operando HAXPES.

DOI： 10.1039/c7cp05436j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Chemistry Chemical Physics  

We performed in situ hard X-ray photoelectron spectroscopy (HAXPES) measurements of the electronic states of platinum nanoparticles on the cathode electrocatalyst of a polymer electrolyte fuel cell (PEFC) using a near ambient pressure (NAP) HAXPES instrument having an 8 keV excitation source. We successfully observed in situ NAP-HAXPES spectra of the Pt/C cathode catalysts of PEFCs under working conditions involving water, not only for the Pt 3d states with large photoionization cross-sections in the hard X-ray regime but also for the Pt 4f states and the valence band with small photoionization cross-sections. Thus, this setup allowed in situ observation of a variety of hard PEFC systems under operating conditions. The Pt 4f spectra of the Pt/C electrocatalysts in PEFCs clearly showed peaks originating from oxidized Pt(II) at 1.4 V, which unambiguously shows that Pt(IV) species do not exist on the Pt nanoparticles even at such large positive voltages. The water oxidation reaction might take place at that potential (the standard potential of 1.23 V versus a standard hydrogen electrode) but such a reaction should not lead to a buildup of detectable Pt(IV) species. The voltage-dependent NAP-HAXPES Pt 3d spectra revealed different behaviors with increasing voltage (0.6 → 1.0 V) compared with decreasing voltage (1.0 → 0.6 V), showing a clear hysteresis. Moreover, quantitative peak-fitting analysis showed that the fraction of non-metallic Pt species matched the ratio of the surface to total Pt atoms in the nanoparticles, which suggests that Pt oxidation only takes place at the surface of the Pt nanoparticles on the PEFC cathode, and the inner Pt atoms do not participate in the reaction. In the valence band spectra, the density of electronic states near the Fermi edge reduces with decreasing particle size, indicating an increase in the electrocatalytic activity. Additionally, a change in the valence band structure due to the oxidation of platinum atoms was also observed at large positive voltages. The developed apparatus is a valuable in situ tool for the investigation of the electronic states of PEFC electrocatalysts under working conditions.

DOI： 10.1039/c6cp06634h

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DOI： 10.1021/jacs.5b04256

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DOI： 10.1021/acs.jpclett.5b00750

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DOI： 10.1002/anie.201408845

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DOI： 10.1039/c3cp54457e

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DOI： 10.1039/c3cp52323c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Angewandte Chemie - International Edition  

A bridge too far: Time-resolved surface-enhanced infrared spectroscopy (SEIRAS) coupled with chronoamperometry shows that formic acid is oxidized via bridge-bonded adsorbed formate, and that formate decomposition is the rate-determining step in the electrooxidation of formic acid to CO2 on Pt. The contribution of direct oxidation of formic acid via weakly adsorbed HCOOH is negligible at best. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/anie.201004782

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry C  

Electro-oxidation of CO adsorbed on a polycrystalline Pt electrode in the potential region of hydrogen absorption is examined by fast time-resolved surface-enhanced infrared absorption spectroscopy coupled to voltammetry or chronoamperometry. Oxidation dynamics at a weak preoxidation peak around 0.5 V (vs RHE) and the main oxidation peak around 0.7 V observed in stripping voltammetry are focused. IR spectra show that the shift of bridge-bonded CO to atop sites triggers the partial oxidation of CO adsorbed at atop sites to yield the preoxidation peak. It is also shown that CO on terraces becomes very mobile in the main oxidation region after some amount of CO being oxidized via a nucleation-and-growth mechanism and that terrace CO is oxidized faster than CO adsorbed at step edges. The result is interpreted in terms of a Langmuir-Hinshelwood type mechanism involving adsorbed CO and an oxygen-containing species (most likely OH) adsorbed at steps. The oxidation mechanism is essentially identical to that proposed in some earlier studies, but more convincing spectroscopic evidence of the mechanism is presented. © 2009 American Chemical Society.

DOI： 10.1021/jp900582c

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Language：Japanese   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Surface Science Society of Japan  

Molecules adsorbed on rough metallic surfaces give very strong infrared absorption. The infrared spectroscopy using this effect, surface-enhanced infrared absorption spectroscopy (SEIRAS), has successfully been applied to <I>in situ</I>, time-resolved monitoring of reactions at the electrochemical interface and provided several new insights into electrochemical surface science. Mechanistic studies on the fundamental electrocatalytic reactions relevant to fuel cells are focused in this review. The reactions treated in this review are hydrogen evolution reactions on Pt and Ag, and electrochemical oxidation of methanol, formaldehyde, and formic acid on Pt, through which it is highlighted that time-resolved analysis of reactions is quite essential for understanding reaction mechanisms at the molecular scale.

DOI： 10.1380/jsssj.30.68

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry C  

Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode is used to examine the structure of water on a polycrystalline Pt electrode in H2SO4 and HClO4 as a function of applied potential. The electrode surface covered with CO is used as the reference in recording spectra, which enables us to obtain the absolute infrared spectrum of the interfacial water layer (monolayer or bilayer) in contact with the surface with negligible interference from the bulk water. The spectrum of the interfacial water is largely different from that of bulk water and changes around the potential of zero charge of the electrode. The spectral changes are ascribed to the potential-dependent reorientation of water molecules from a weakly hydrogen-bonded oxygen-up orientation at the negatively charged surface to a strongly hydrogen-bonded nearly flat orientation at the positively charged surface in agreement with theoretical simulations reported in the literature. Clear experimental evidence of the formation of a stable ice-like structured water on the positively charged surface is reported. © 2008 American Chemical Society.

DOI： 10.1021/jp710386g

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Chemistry Chemical Physics  

Pt nanoparticles having the same size (∼10 nm) but different shapes (cubic or octahedral/tetrahedral), as determined by transmission electron microscopy, were synthesized via a polyol-based synthetic procedure. Their respective electrocatalytic activities for methanol oxidation were characterized by cyclic voltammetry and chronoamperometry in both sulfuric and perchloric acid electrolytes, which showed clear shape (surface orientation) dependences. Furthermore, the octahedral/tetrahedral Pt nanoparticles displayed an unexpectedly large enhancement in methanol electro-oxidation activity; about 3-fold increase in transient intrinsic activity and 10-fold increase in CO tolerance steady-state activity when compared to commercial Pt black. Gaseous and methanolic CO adsorption on the synthesized nanoparticles were also investigated by surface-enhanced IR absorption spectroscopy in perchloric acid electrolyte, which suggested that the different trends observed might be related to the electronic effects specific to a given ensemble of the nanofacets. © the Owner Societies.

DOI： 10.1039/b802708k

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Chemistry Chemical Physics  

A mechanistic study of electrocatalytic oxidation of formic acid on Pd in sulfuric and perchloric acids is reported. Surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) shows the adsorption of CO, bridge-bonded formate, bicarbonate, and supporting anions on the electrode surface. Poisoning of the Pd surface by CO, formed by dehydration of formic acid, is very slow and scarcely affects formic acid oxidation. The anions are adsorbed more strongly in the order of (bi)sulfate > bicarbonate > perchlorate, among which the most strongly adsorbed (bi)sulfate considerably suppresses formic acid oxidation in the double layer region. The oxidation is suppressed also at higher potentials in both acids by the oxidation of the Pd surface. Adsorbed formate is detected only when formic acid oxidation is suppressed. The results show that formate is a short-lived reactive intermediate in formic acid oxidation and is hence detected when its decomposition yielding CO2 is suppressed. The high electrocatalytic activity of Pd can be ascribed to the high tolerance to CO contamination and also high catalytic activity toward formate decomposition. © 2008 the Owner Societies.

DOI： 10.1039/b805955a

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry C  

When formaldehyde is oxidized on a Pt electrode under galvanostatic (constant current) conditions, the electrode potential oscillates spontaneously. The oscillatory behavior has been examined by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS) for understanding the mechanism and kinetics of the reaction at the molecular scale. SEIRAS reveals that CO and formate are adsorbed on the electrode surface and that their band intensities (coverages) oscillate synchronously with the oscillation of potential. SEIRAS coupled to cyclic voltammetry or linear sweep voltammetry suggests that formaldehyde is oxidized to CO2 via two parallel processes: the direct path via adsorbed formate and the indirect path via adsorbed CO. The two processes are kinetically coupled and autocatalytically activate and deactivate formaldehyde oxidation to yield the potential oscillations. The oxidation of the electrode surface also contributes to the oscillations for large applied currents. © 2007 American Chemical Society.

DOI： 10.1021/jp0743020

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Electrochimica Acta  

Hydrogen evolution reaction (HER) on a polycrystalline Pt electrode has been investigated in Ar-purged acids by surface-enhanced infrared absorption spectroscopy and electrochemical kinetic analysis (Tafel plot). A vibrational mode characteristic to H atom adsorbed at atop sites (terminal H) was observed at 2080-2095 cm-1. This band appears at 0.1 V (RHE) and grows at more negative potentials in parallel to the increase in hydrogen evolution current. Good signal-to-noise ratio of the spectra enabled us to establish the quantitative relation between the band intensity (equivalently, coverage) of terminal H and the kinetics of HER, from which we conclude that terminal H atom is the reaction intermediate in HER and the recombination of two terminal H atoms is the rate-determining step. The quantitative analysis of the infrared data also revealed that the adsorption of terminal H follows the Frumkin isotherm with repulsive interaction. © 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.electacta.2006.12.007

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry B  

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H12COOH and H 13COOH) reveals that formic acid is oxidized to CO2 via adsorbed formate and the decomposition (oxidation) of formate to CO2 is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account. © 2006 American Chemical Society.

DOI： 10.1021/jp0618911

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry B  

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H12COOH and H13COOH) reveals that formic acid is oxidized to CO2 via adsorbed formate and the decomposition (oxidation) of formate to CO2 is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account. © 2006 American Chemical Society.

DOI： 10.1021/jp061891l

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry B  

We have modeled temporal potential oscillations during the electrooxidation of formic acid on platinum on the basis of the experimental results obtained by time-resolved surface-enhanced infrared absorption spectroscopy (J. Phys. Chem. B 2005, 109, 23509). The model was constructed within the framework of the so-called dual-path mechanism; a direct path via a reactive intermediate and an indirect path via strongly bonded CO formed by dehydration of formic acid. The model differs from earlier ones in the intermediate in the direct path. The reactive intermediate in this model is formate, and the oxidation of formate to CO2 is rate-detemining. The reaction rate of the latter process is represented by a second-order rate equation. Simulations using this model well reproduce the experimentally observed oscillation patterns and the temporal changes in the coverages of the adsorbed formate and CO. Most properties of the voltammetric behavior of formic acid, including the potential dependence of adsorbate coverages and a negative differential resistance, are also reproduced. © 2006 American Chemical Society.

DOI： 10.1021/jp061129j

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physical Chemistry B  

The mechanism of temporal potential oscillations that occur during galvanostatic formic acid oxidation on a Pt electrode has been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS). Carbon monoxide (CO) and formate were found to adsorb on the surface and change their coverages synchronously with the temporal potential oscillations. Isotopic solution exchange (from H13COOH to H12COOH) and potential step experiments revealed that the oxidation of formic acid proceeds dominantly through adsorbed formate and the decomposition of formate to CO 2 is the rate-determining step of the reaction. Adsorbed CO blocks the adsorption of formate and also suppresses the decomposition of formate to CO2, which raises the potential to maintain the applied current. The oxidative removal of CO at a high limiting potential increases the coverage of formate and accelerates the decomposition of formate, resulting in a potential drop and leading to the formation of CO. This cycle repeats itself to give the sustained temporal potential oscillations. The oscillatory dynamics can be explained by using a nonlinear rate equation originally proposed to explain the decomposition of formate and acetate on transition metal surfaces in UHV. © 2005 American Chemical Society.

DOI： 10.1021/jp055220j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Angewandte Chemie - International Edition  

(Graph Presented) Revised model: Current oscillations in the electrooxidation of formic acid at a fixed potential can be observed by time-resolved surface-enhanced infrared absorption spectroscopy (see picture). Real-time probing of the reaction dynamics on the electrode surface provides new insight into the oscillations that are not accessible by conventional techniques. The reaction mechanism involves adsorbed formate as a reactive intermediate. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/anie.200501009

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Electrochimica Acta  

The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5 × 10-5 M Sn2+ in 0.5 M H 2SO4 has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of νmax = 0.35 (Sn to surface Pt atoms). Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm-1 band associated with bridge-bonded CO and the 2070 cm-1 band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm-1 band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2 × 2)-3CO saturation structure on Pt(1 1 1). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV. © 2003 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.electacta.2003.07.009

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Solid State Electrochemistry  

The co-catalytic effect of W on the oxidation of CO and methanol is investigated by using differential electrochemical mass spectrometry (DEMS). DEMS reveals that CO oxidation starts at 120 mV, overlapping with W oxidation. The action of W consists in shifting the pre-peak from 450 mV (as observed on pure Pt) to 200 mV. In this shifted pre-peak only 2% of the total adsorbed CO is oxidized independently from the W coverage, as compared to 10% on pure Pt. A correlation between the surface coverage of W as determined by XPS with the W oxidation peak charge in cyclic voltammetry suggests that the oxidation is a six-electron process.

DOI： 10.1007/s10008-003-0440-6

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Electrochimica Acta  

The co-catalytic effect of Mo on the oxidation of adsorbed CO is examined on different types of electrodes using differential electrochemical mass spectrometry (DEMS). Mo surface composition is determined by XPS. Mo was either deposited electrochemically onto Pt(111), Pt(332), smooth and porous polycrystalline Pt electrodes, or prepared by co-sputter deposition. From a comparison of the surface coverage of Mo on Pt(111) and the corresponding oxidation charge, a number of six electrons per adsorbed Mo is estimated, meaning that below 0.2 V adsorbed Mo has a formal oxidation number of zero. Since there is a linear relationship between the oxidation charge of Mo and the suppression of CO adsorption when varying the Mo coverage, we conclude that CO does not adsorb on Mo. From the oxidation charge and the number of CO adsorption sites suppressed, we obtain a number of three electrons per site; therefore, one Mo atom blocks approximately two Pt sites. DEMS reveals, that CO2 formation starts around 0.15 V, overlapping with the onset of Mo oxidation. The action of Mo is to shift the usual prepeak (weakly adsorbed state) from 0.5 to 0.3 V, whereas the main oxidation peak (strongly adsorbed state) is hardly affected. This resembles the effect of Sn; however, Sn forces up to 50% of the adsorbed CO from the strongly adsorbed state into the weakly adsorbed state, independent of the Sn coverage, whereas in the case of Mo, only around one tenth of the adsorbed CO is oxidized in the prepeak, independent of Mo coverage. Therefore, we explain the effect of Mo by an oxygen spillover effect, which is only working for CO in the weakly adsorbed state, and not by an electronic, repulsive interaction as in the case of Sn. With sequential step decoration on Pt(332) by Ru and Mo, it is shown that a synergetic effect of both co-catalysts is obtained. The effect of Mo on the oxidation of COad formed by adsorbing methanol and on the oxidation of methanol is very small. © 2002 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0013-4686(02)00338-9

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Langmuir  

The influence of monatomic steps on the oxidation of adsorbed CO is examined using potentiodynamic and galvanostatic experiments. Galvanostatic experiments allow a better determination of the lowest possible oxidation potential than cyclic voltammetry. By comparing results on Pt(111), Pt(665), Pt(332), and Pt(755), we found that steps with a local (110) geometry are more active than those with a (100) geometry. Steps can be decorated by Ru, thus leading to surfaces with a known atomic arrangement of the components. During galvanostatic oxidation of adsorbed CO on such surfaces a second potential plateau is observed, the value of which depends on the density of steps. These transients suggest the existence of an electronic effect in addition to the widely accepted bifunctional mechanism: Due to the electronic effect, only CO in the neighborhood of Ru is destabilized, whereas CO adsorbed at more distant sites is not influenced and has to diffuse "uphill" to the reactive sites, leading to an apparently slow diffusion. In addition, the spillover effect according to the bifunctional mechanism lowers the height of the activation barrier for CO oxidation and therefore has an influence on all adsorbed CO.

DOI： 10.1021/la011308m

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Electrochimica Acta  

Model bimetallic catalyst surfaces were generated by depositing small submonolayer amounts of Ru, Sn and Mo on Pt(111) and Pt(332) single crystal electrodes. In particular, in the case of Pt(332), a step decoration was achieved and thus a catalyst surface with a known atomic arrangement of the constituents. Ternary catalysts were modelled by sequential deposition of Sn and Ru. To study the influence on the rate of oxidation of adsorbed CO in cyclic voltammetry, CO2 formation rates were monitored by differential electrochemical mass spectrometry (DEMS), which allows a separation from pseudocapacitive effects, e.g. oxygen adsorption. Contrary to PtRu alloy electrodes, adsorbed CO on the Ru modified single crystal electrodes is oxidized in two oxidation peaks. In accordance with Monte Carlo simulations [1], this is due to slow diffusion of adsorbed CO to Ru sites. The CO adsorption state corresponding to the 2nd peak is also not oxidized during an extended potential stop at the onset of the 1st peak. Ternary model catalysts were used to test whether a synergetic effect of Ru and Sn, which influence the CO oxidation quite differently, is possible. The ternary model catalyst behaved like a superposition of the corresponding binary catalysts, probably because separate 2D (on Pt(111) or 1D (on Pt(332)) islands of Ru or Sn were formed instead of an atomically mixed overlayer. Mo shifts the onset potential for oxidation of adsorbed CO to even lower potentials (0.15 to 0.2 V) than Sn. However, at such a low potential only about 10% of the adsorbate is oxidized, the main oxidation peak is hardly influenced. © 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/s0013-4686(00)00654-x

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