Ru Clusters Synthesized Chemically from Dissolved Carbonyl: In Situ

Sebastian Fiechter, Iris Dorbandt, Peter Bogdanoff, Gerald Zehl, Hendrik Schulenburg, and Helmut Tributsch , Michael Bron , Jörg .... Nicol√°s Alo...
0 downloads 0 Views 114KB Size
Download PDF
5238

J. Phys. Chem. B 2001, 105, 5238-5243

Ru Clusters Synthesized Chemically from Dissolved Carbonyl: In Situ Study of a Novel Electrocatalyst in the Gas Phase and in Electrochemical Environment W. Vogel,† V. Le Rhun,‡ E. Garnier,‡ and N. Alonso-Vante*,‡ Laboratory of Electrocatalysis, UMR CNRS 6503, UniVersite´ de Poitiers, 40 AVenue du Recteur Pineau, F-86022 Poitiers Cedex, France and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: December 31, 2000; In Final Form: March 14, 2001

Highly dispersed nanocrystalline Ru particles were prepared under mild conditions in organic solvents such as xylene (Xyl) or dichlorobenzene (Dcb) from the Ru carbonyl precursor. The route of this chemical synthesis determines the physicochemical properties of these materials. This phenomenon is clearly shown in the voltammetric current-potential characteristics of Rux(Xyl) and Rux(Dcb). Although both materials have a similar particle size, in the reduced state, Rux(Xyl) shows a higher electrochemical pseudocapacitance than Rux(Dcb) by a factor of 10. However, in ambient conditions, the particles slowly oxidize to an amorphous RuxOy species. Their interfacial behavior is similar. Both species and their differences have been characterized by in situ X-ray diffraction. Samples prepared in xylene, RuxOy(Xyl), can be partly reduced in 1 bar of hydrogen at room temperature to ∼2 nm size hcp ruthenium and grown to ∼8 nm particles at 200 °C. In contrast, samples prepared in dichlorobenzene, RuxOy(Dcb), under the same conditions can only be reduced at 140 °C, to form perfect 2 nm hcp ruthenium clusters of narrow size distribution. This sample is reversibly reoxidized to the amorphous RuxOy species at 200 °C in oxygen. The large 8 nm Ru particles based on the xylene preparation are, however, stable under these conditions. The driving force for such a difference in the electrochemical behavior lies on the disorder precursor state of nano-ruthenium.

1. Introduction In electrochemical energy converting systems, a key factor in determining efficiency is the existence of an interaction between the reactants and the surface atoms of the electrode.1-6 For this purpose, the surface atoms must possess the necessary ingredients such as composition and7-10 crystallographic orientation,1-12 as well as electronic properties13-17 to enhance the reactivity (electrocatalysis) and selectivity in multielectron charge-transfer reactions. In heterogeneous catalysis, ruthenium carbonyl complexes have been used to prepare chlorine-free, highly dispersed Ru catalysts on different supports,18 which are very active for ammonia synthesis as well as for NO reduction.19,20 Recently, an alternative route to design novel nanoscale materials for electrocatalysis, in mild conditions in organic solutions, has been to use the reactivity of transition metal carbonyl complexes with dissolved chalcogenes. Cluster-like compounds, in the form of powder or colloids, based on MoxRuySez, RuxSey are the products.21-23 These compounds have proved to be effective electrode materials for the molecular oxygen reduction in acid medium and have the advantage of being selective in the presence of methanol.24 Taking advantage of the material preparation in mild conditions via a transition-metal carbonyl complex, ruthenium particles (Rux) were synthesized.25-27 The solvents used were xylene (Xyl) and 1,2-dichlorobenzene (Dcb). In this work, to explore the different electrochemical activities of Rux resulting from its chemical synthesis, we will characterize both of these two compounds in the gas-phase aiming at * To whom correspondence should be addressed. [email protected]. † Fritz-Haber-Institut der Max-Planck-Gesellschaft. ‡ Universite ´ de Poitiers.

E-mail:

correlating the surface states with the reactivity toward oxygen, either to form an oxide or molecular oxygen on the nanostructured ruthenium material. 2. Experimental Section 2.1. Sample Preparation. Synthesis in Xylene and 1,2Dichlorobenzene. Nanodivided ruthenium was synthesized, as previously described,23,26 in two solvents: xylene (Xyl; Merck Nr. 8687.1000) and 1,2-dichlorobenzene (Dcb) (Merck Nr. 6.03298.1000) using as precursor Ru3(CO)12 (Alfa Nr. 617002). The synthesis temperature was determined by the boiling point of the solvent, i.e., 140 (Xyl) and 180 °C (Dcb). The electrocatalysts in powder form were recovered after 20 h of reaction. Care was taken to wash the synthesized powder with ethyl ether in order to eliminate traces of unreacted chemical precursor. As recently reported,25 the Rux samples react slowly with the oxygen from air to an amorphous RuxOy. For X-ray experiments (section 3), the state “as received” corresponds to t > 400 h after synthesis. However, for electrochemistry, we measured the state “as prepared”, i.e., t < 20 h after synthesis, thus, Rux (section 4). Electrode Preparation and Electrochemical Measurements. Electrodes were prepared from catalysts in powder form according to the procedure reported by Schmidt et al.27 In short, this procedure consists of mixing a determined amount of the catalyst with a volume of distilled water (1 mg mL-1). An aliquot of this suspension was deposited onto mirror polished glassy carbon (GC) disk electrodes (diameter 3 mm) and dried under a nitrogen atmosphere. Thereafter, the GC electrode surface was covered by 4 µL of a water diluted Nafion solution (50:50 %volume) to attach the catalyst particles after evaporating the solvent. The catalyst loading corresponds to 57 µg cm-2.

10.1021/jp0100654 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/15/2001

Ru Clusters Synthesized Chemically

Figure 1. WAXS patterns of RuxOy(Xyl) after different times of storage in air. The dashed line is the calculated scattering of a small rutiletype RuO2 cluster.

The electrochemical characteristics were measured in 0.5 M H2SO4. The measurements were done potentiostatically (Autolab Pgstat 30) in a standard one-compartment electrochemical cell provided with a double jacket for temperature regulation. A GC rod and a homemade reference hydrogen electrode RHE (0.05V/ SHE) were used as counter and reference electrodes, respectively. Prior to measurements, the electrolyte was purged with nitrogen and the electrode was cycled during 10 min at 50 mVs-1 between 0.1 and 0.7 V/RHE. During measurements, a gentle flow of nitrogen was kept above the electrolyte surface. 2.1. X-ray Diffraction. X-ray in Situ Measurements. The diffraction experiments were performed with a Guinier diffractometer (HUBER, Cu KR1, λ ) 0.154 06 nm, -45° transmission geometry). The two samples were pressed into translucent pellets 15 mm × 4.5 mm × 0.1 mm in size, sandwiched between two 0.1 mm beryllium platelets, and inserted in the sample holder of a specially designed in situ cell described elsewhere.28,29 To avoid long-term instability, the two samples were exchanged at every step of the angular scan by a small shift of the whole cell parallel to the plane of the samples. Spectra of both samples were corrected for the background scattering and for the usual angular factors (absorption, polarization, and geometry factor). At certain angles, narrow sections of the scan are cut out because of the intense Be peaks superimposed to the diffraction patterns. DFA Simulation. Numerical simulation with the help of Debye functions (Debye function analysis, DFA) yields information on the intrinsic structure of the colloids and on the size distribution of the assembly of coherently scattering particles within the colloid. In brief, the DFA method is based on the Debye functions of a sequence of model clusters with increasing size. These are added up to compare them with the experimental intensity. Such a discrete distribution of particles with specific nuclearities is, in general, unreal. However, a continuous distribution of particle sizes can be deduced from the discrete DFA sizes, which fit equally well to the experiment. A set of free parameters is used for the number of fractions of the individual clusters. Additional parameters are (i) the exponent of the Debye-Waller factor, (ii) the average spacing within the individual clusters, and (iii) an additive constant correcting for errors in the background subtraction. Details of the method are given in refs 30 and 31. 3. Gas-Phase Studies by in Situ XRD 3.1. Oxidation in Ambient Conditions. Figure 1 shows the wide-angle X-ray scattering patterns (WAXS) of Rux(Xyl) after storage in air. In ambient conditions, the samples are not stable

J. Phys. Chem. B, Vol. 105, No. 22, 2001 5239

Figure 2. Direct in situ comparison of the gas-phase reduction of RuxOy(Dcb) and RuxOy(Xyl). The diffracted intensity around the oxide peak (lower curves) and of the main Ru metal peak (upper curves) is measured versus time and temperature (a) at RT and (b) with increasing temperature.

but partially oxidize and are fully oxidized after prolonged storage. After storage for 17 days (thick line), the Rumet peak at b ) 0.48 Å-1 (b ) 2 sin θ/λ; θ: Bragg angle, λ: wavelength) is still visible, but this peak nearly disappears after 8 months of storage in air at room temperature (RT; thin line). We have calculated the expected diffraction curve of a very small rutile-type RuO2 cluster. The dashed line in Figure 1 is simply calculated from the JCPDS data by broadening all lines according to a spherical size of 7 Å. This is a crude approximation, but the diffuse peaks roughly agree with those observed, except for the intrinsic structure of the ruthenium oxide. 3.2. Reduction of RuxOy(Dcb) and RuxOy(Xyl). We have used a special in situ X-ray diffraction technique that allows for the direct comparison of RuxOy(Dcb) and RuxOy(Xyl) at identical reducing conditions. Both samples are placed as thin pellets next to one another in the sample holder. The counter is set to 2θ ) 32°, the diffraction angle of the ruthenium oxide peak. After a 10 s counting time for both samples with intermediate sample exchange, the diffraction angle is changed to 2θ ) 44°, the ruthenium metal peak position. This procedure is cycled continuously while the treatment conditions are changed. Figure 2a shows the thus-obtained time-resolved XRD signals (RuxOy(Xyl), full circles; RuxOy(Dcb), open circles) in 1 bar of hydrogen at RT. For an ongoing reduction, the oxide signal (lower intensity lines) should decrease, whereas the metal signal (higher intensity lines) is expected to increase. This behavior is in fact observed for the RuxOy(Xyl) sample. The primary reduction is already accomplished within a few minutes. However, RuxOy(Dcb) remains nearly unaffected at RT. In a second run, the temperature was raised to 161 °C (the dashed line is the temperature curve). Reduction of RuxOy(Dcb) only takes place at about 140 °C. Moreover, the process of reduction proceeds much slower as compared to the RuxOy(Xyl) sample (note the logarithmic time scale in Figure 2). At this temperature, the reduced RuxOy(Xyl) particles already begin to grow via coalescence and the ruthenium metal peaks narrow (compare Figure 4). Accordingly, the intensity at the metal peak drops, although there is no further reduction. The reason for the decrease of the total intensity, scattered into the receiving window of the counter, is related to the narrowing of the metal peaks. In the diffraction pattern, the three first ruthenium reflections are already well resolved and the crystallite size is

5240 J. Phys. Chem. B, Vol. 105, No. 22, 2001

Vogel et al.

Figure 3. DFA simulation (solid line) of the diffraction pattern of sample Rux(Dcb) after reduction in hydrogen at 161 °C. The insert to the figure shows the mass fractions of the hcp model clusters Rux versus their diameters used for this simulation. The smallest clusters with x ) 13, 57, and 153 are closed shell models, whereas the larger clusters are irregularly shaped hcp model particles.

of the order of 8 nm (compare Figure 4b). However, the particle size of RuxOy(Dcb) is still small, with an average size of about 2 nm. This is demonstrated in Figure 3, which shows a DFA simulation of the corresponding diffraction pattern using a sequence of small hcp model cluster with increasing size. The weight fractions of the clusters are used as free parameters for this fit. They are shown in the insert of Figure 3. The described experiment clearly shows the higher stability of RuxOy(Dcb) against hydrogen reduction, as well as a higher thermal stability as compared to those Rux particles prepared in xylene. The two following figures show the differences between both preparations represented by their diffraction patterns (i) in the “as received” state (solid line), (ii) after reduction in hydrogen at medium temperature (full circles), and (iii) after reduction at high temperature (open circles). The low reducibility of RuxOy(Dcb) ends with small (∼2 nm) Rux metal clusters (Figure 4a), whereas the unstable RuxOy(Xyl) sample ends with Rux particles of ∼8 nm (Figure 4b). The gray bar in Figure 4a indicates the range for the oxide signal measurement shown in Figure 2. 3.3. Reoxidation of the Reduced RuxOy(Dcb). As long as the reduced ruthenium cluster remains small enough, the metal phase can be reoxidized in pure oxygen at sufficiently high temperatures. Figure 5 shows the diffraction pattern of RuxOy(Dcb) (corresponding to Figure 3 in the reduced state) after exposure to 1 bar of pure oxygen at (a) 200 and (b) 250 °C. The main metal peak at b ) 0.48 Å-1 nearly disappeared after treatment in oxygen, only a small rather narrow peak is left (arrow in Figure 5). At sufficiently mild oxidation conditions, the metal clusters are nearly fully oxidized and the remainder is an amorphous RuxOy phase as seen from the diffuse diffraction pattern. At 250 °C, however, the RuxOy phase transforms to larger crystallites that evidently have the RuO2 rutile structure. The structure factors of this compound are plotted in Figure 5 as vertical bars. The average size of these crystallites can be estimated as 4-5 nm from the line width of the (110) and (101)-RuO2 peaks with a rather broad size distribution. At the same conditions, the previously reduced RuxOy(Xyl) sample does not oxidize at all, because the Rux metal particles are already grown to a critical size above which no bulk oxide is formed. This is in line with the fact that the residual Ru-

Figure 4. Diffraction patterns at different degrees of reduction in hydrogen (“as received”, medium temperature, and high temperature), (a) RuxOy(Dcb), and (b) RuxOy(Xyl). The grayish bar in a indicates the intensity range around the oxide peak as displayed in Figure 2.

Figure 5. Diffraction patterns of the fully reduced RuxOy(Dcb) sample (compare Figure 4a) after two sequential states of reoxidation in oxygen. Vertical bars and Miller indices represent the structure factors for the RuO2 phase. The arrow indicates a peak related to a remainder of the Ru metal phase.

metal peak (arrow in Figure 5) is much narrower than that in Figure 3. Only the largest particles in the size distribution resist oxidation. 4. Electrochemical Studies 4.1. Electrochemical Behavior. Figure 6 contrasts typical voltammograms at 50 mV/s in 0.5 M H2SO4 of the same amount of catalysts (57 µg cm-2) of Rux(Xyl) (dashed line) and

Ru Clusters Synthesized Chemically

J. Phys. Chem. B, Vol. 105, No. 22, 2001 5241

Figure 8. Cathodic (full symbols) and anodic (empty symbols) specific capacitance of ruthenium particles issued from xylene (squares) and dichlorobenzene (triangles) solvents. The evolutions (decrease and increase) of this capacitance is due to the oxide-like species formation onto the ruthenium particles. The line binding the points serve as an optical guide.

Figure 6. Voltammograms in nitrogen outgassed 0.5 M H2SO4 electrolyte at 50 mV/s of Rux(Xyl) (dashed line) and Rux(Dcb) (full line). The samples were measured respectively 3 and 24 h after synthesis, see text.

Figure 7. Voltammograms in nitrogen outgassed 0.5 M H2SO4 at 50 mV/s of RuxOy(Xyl) (full line) and RuxOy(Dcb) (dashed line). The samples (unexposed to H2) were measured after a prolonged exposure to air at RT, t ) 6744 and 3528 h, respectively.

Rux(Dcb) (full line). Rux(Xyl) and Rux(Dcb) were measured 3 and 24 h after synthesis, respectively. Corresponding curves of the reduced states (both samples measured electrochemically 1.5 h after H2 treatment) are not depicted because they are not different from those shown here previous to treatment. Furthermore, Figure 7 depicts the typical characteristics of samples, unexposed to H2, after a prolonged exposure to oxygen from air at RT (t > 1000 h). In the whole potential interval explored

(0.1 V to 0.7 V/RHE), a large pseudocapacitance (due to a significant amount of charge stored in the elecytrochemical double layer via adsorption process) is mainly observed for the Rux(Xyl) sample. A surface redox reaction centered at 0.35 V is also apparent. This characteristic is also present, although to a lesser extent, in the Rux(Dcb) sample. In the oxidized state, i.e., after exposure to oxygen from air (t > 1000 h), both types of samples apparently tend to attain a similar kind of interfacial behavior, as is testified by the voltammograms in Figure 7. This similarity can also be appreciated by the shape of the anodic current at ca. 0.36 V/RHE, the point where the curve changes slope, for RuxOy(Xyl) and 0.1 V more positive for RuxOy(Dcb). This result gives evidence of water interaction either onto the oxide-like species or onto remaining metallic particles. This latter seems not to be the case, as is deduced from results depicted in Figure 6, at least for the precursor synthesized in dichlorobenzene. Therefore, the striking difference observed in the voltammetric curves between the “reduced” as well as the “oxidized” state of Rux can be because of the adsorption effect of the electrolyte leading to the so-called pseudocapacitance.32 This phenomenon is dependent on the oxide-like coverage, induced by oxygen from water or air. Inferred from Figure 7, the oxygen, from air, must be strongly coordinated either to Rux(Xyl) or Rux(Dcb). The surface modification interplay induced by oxygen from air and/or water onto Rux(Xyl)Rux(Dcb) particles was monitored with cyclic voltammetry. The integrated anodic or cathodic charges divided by the potential interval (∆E ) 0.6V; cf. Figures 6 and 7) deliver the specific capacitance (F g-1). This specific capacitance as a function of time after synthesis of samples exposed to air (that is, from Rux to RuxOy) is contrasted in Figure 8. It looks clear that the Rux(Xyl) has a specific capacitance ca. 10 times that of Rux(Dcb) at least to a time span t < 1000 h. At longer exposure time, the specific capacitance of Rux(Dcb) almost attains the same level to that of RuxOy(Xyl), because of the formation of RuxOy(Dcb). We believe that the decrease of the specific capacitance of Rux(Xyl) is due to its high reactivity with oxygen. This latter may act as metal center blocking sites for water interaction. RuO2 is known to be a good electrocatalyst for the oxygen evolution reaction.33 Therefore, the formation of a ruthenium oxide-like species (RuxOy), which is strongly dependent on the nature of the surface, was tested for the oxygen evolution reaction. This phenomenon is depicted in Figure 9, which shows the current-potential characteristics (Tafel plots) for water

5242 J. Phys. Chem. B, Vol. 105, No. 22, 2001

Vogel et al.

Figure 9. Tafel plots of oxygen evolution reaction on Rux(Xyl) (curves 1 and 3) and Rux(Dcb) (curves 2 and 4), for the “as prepared” samples and/or thermally treated samples under hydrogen at 90 °C (full lines) and for the samples after prolonged exposure to air (dashed lines). The Tafel slopes are indicated on each curve by dotted lines. Inset: massif Ru-metal electrode. Scan rate: 5 mV s-1.

TABLE 1: Tafel Slopes for the Oxygen Evolution on Ruthenium Cluster Based Catalysts catalyst Ru (massif) Ruy(Xyl) Rux(Dcb) a

b/mV dec-1

catalyst

b/mV dec-1

55 87 153

a

58 141 157

RuO2 RuxOy(Xyl) RuxOy(Dcb)

Reference 37.

oxidation obtained at a scan rate of 5 mV s-1. Once again we found some differences for both samples, in the reduced states (curves 1 and 2) as well as in the oxidized one (curves 3 and 4). For the sake of comparison, a massif Ru-metal electrode was also measured, as is depicted in the inset of the figure. One can clearly observe a common characteristic between the nanostructured materials and the ruthenium massif metal: the oxidation of the metallic surfaces between 0.7 V and 1.35 V/RHE before evolution of molecular oxygen. For the same amount of catalysts, these oxide species take place more cathodically onto samples issued from xylene either on Rux or RuxOy (cf. curves 1 and 3, Figure 9). Here, again the material Rux(Dcb) and/or RuxOy(Dcb) is more stable toward the oxidation. The Tafel slope is represented by the short linear portion of the semilogarithm plot (see dotted straight lines). These values are summarized in Table 1. 5. Discussion and Summary 5.1. Comparison of Rux(Xyl) and Rux(Dcb). In the synthesis route employed, the nanostructured precursors, i.e., Rux, depends on the nature of the solvents. These latter apparently influence the way ruthenium atoms are being rearranged in order to form clusters. The recovering of the initial precursor state (metallic) does not take place at the same temperature. In an initial study,23 via XRD measurements, we observed a shift to lower 2θ values of the main peak (101) of metallic ruthenium attributed to a loss of neighboring order. For Rux(Xyl), this main peak was

further shifted to lower values. These preliminary conclusions are strongly supported by the present in situ XRD results. The suppression of the intermediate hexagonal (102) peak on both Rux precursors testifies a very high degree of stacking faults. Indeed, the DFA simulations (cf. Figure 3) revealed a bimodal size distribution represented by a small fraction of unfaulted closed shell hcp model clusters (x ) 13, 57, and 153), whereas the larger ones (e.g., 20 Å < d < 30 Å) are strongly faulted by stacking defects. The respective Debye parameters are B ) 3 Å2 and 0.15 Å2. The mean Ru-Ru distances were found to be appreciably contracted against bulk ruthenium by 3.8% and 0.7% for Rux(Xyl) and Rux(Dcb). It is evident that the observed properties of these novel compounds have implications in the way of interacting with water molecules or with molecular oxygen. For this reason, we can consider them as models, though they pave the way for the understanding of the complex interplay of nanocrystallinity (in which some disorder is present) versus the amorphous state (totally disordered structure).23 Moreover, the effect of oxygen as a ligand to form RuxOy is a clear indication of the instability of these nanostructured materials. In this respect, we recall that the role of the chalcogen, as ligand, is to stabilize the metallic nanostructure, as is demonstrated in various papers.22,34 Nevertheless, in RuxSey compounds, the channels to coordinate molecular oxygen remain open. This explains why the reduced state, Rux, behaves in a similar way with or without selenium.25,26 5.2. Order/Disorder Implications of Nanostructured Precursors in the Electroactivity. The easiness to release oxygen in the RuxOy form (enhanced in the material issued from xylene) must be related to the surface state generated in producing such nanoparticles. This is a particular aspect which emerges from these model compounds. The electrochemical monitoring of this phenomenon (see Figure 8) leads to the so-called pseudocapacitance. As deduced from Figure 6 (cf. also Figures 7 and 8), this pseudocapacitance must be related to absorbed species from the electrolyte onto the nanoparticle surface (redox surface reaction). This statement is supported by the fact that on the homologous material, Rux(Dcb), the surface reaction takes place to a lesser extent. On the other hand, it has been reported in the literature that on amorphous RuO2 the charge is stored in the bulk,35,36 in contrast to the crystalline ruthenium dioxide; this phenomenon is 2 orders of magnitude lower. These data are in line with our findings and can explain the increase of the specific capacitance of RuxOy(Dcb), which is also an amorphous oxide. However, the decrease of the specific capacitance on RuxOy(Xyl) can be dominated by surface reaction, which is the essence of electrocatalysis. Although the goal of our work was not to produce “super” capacitors, it is, however, interesting to write down some figures, considering only the Rux(Xyl) which behaves voltametrically (cf. Figure 6) similar to RuxSey(Xyl).25,26 The initial average value of the specific capacitance on Rux(Xyl) is ca. 207 F g-1 (see Figure 8). The average surface per unit mass of our nanostructured materials is 60 m2 g-1.26 This gives an estimate of 345 µF cm-2 which, compared to the double layer capacity on metals (ca. 20 µF cm-2), represents an increment of the interfacial capacity by a factor of 17. Certainly, this property may influence the electrocatalytic activity in the reduction of the molecular oxygen23 or in its evolution, see Figure 9. This is in fact reflected in the mass current density potential curves. For a mass current, less energy is required to preoxidize the nanoprecursor, compare curves 1 and 2 or 3 and 4 of Figure 9, at i ) 10-1 mA g-1. From Table

Ru Clusters Synthesized Chemically 1, the magnitude of the Tafel slopes suggests that the mechanism of oxygen evolution is different, as compared to the values obtained for Ru massif and RuO2 thin layer37 electrodes. In summary, nanostructured ruthenium materials prepared in organic solvents from carbonyl compounds present striking differences in the initial stages which influence the oxidation process leading to amorphous species. The recovery of the metallic nanostructured precursor also depends on temperature. These in situ observations via XRD measurements also encompasse the electrochemical behavior of these materials. This is important information in tailoring nanostructured materials for catalysis or electrocatalysis. Acknowledgment. This work was in part supported by a Grant Joule N˚JOE3-CT97-0063. References and Notes (1) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967. (2) Herrero, E.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (3) Chrzanowski, W.; Wieckowski, A. J. Am. Chem. Soc. 1997, 119, 194. (4) Markovic, N. M.; Ross, P. N. Electrochim. Acta 2000, 45, 4101. (5) Hoshi, N.; Hori, Y. Electrochim. Acta 2000, 45, 4263. (6) Conway, B. E.; Liu, T.-C. Langmuir 1990, 6, 268. (7) Nishimura, K.; Kunimatsu, K.; Enyo, M. J. Electroanal. Chem. 1989, 260, 167. (8) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. Catal. Lett. 1996, 36, 1. (9) Lucas, C. A.; Markovic, N. M.; Grgur, B. N.; Ross, P. N. Surf. Sci. 2000, 448, 65. (10) Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E.; Sung, Y.-E.; Kim, S.; Thomas, S.; Wieckowski, A. J. Phys. Chem. B 2000, 104, 3518. (11) Triaca, W. E.; Arvia, A. J. J. Appl. Electrochem. 1990, 20, 347. (12) Trudeau, M. L.; Ying, J. Y. Nanostruct. Mater. 1996, 7, 245.

J. Phys. Chem. B, Vol. 105, No. 22, 2001 5243 (13) Howells, A.; Harris, T.; Scherson, D. A. Solid State Ionics 1997, 94, 115. (14) Mukerjee, S.; Srinivasan, S.; McBreen, J. J. Electrochem. Soc. 1995, 142, 1409. (15) Jaegermann, W.; Pettenkofer, Ch.; Alonso-Vante, N.; Schwarzlose, Th.; Tributsch, H. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 513. (16) Tong, Y.; Rice, C.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2000, 122, 1123. (17) Metikos, M.; Jukic, A. Electrochim. Acta 2000, 45, 4159. (18) Aika, K. I.; Takano, T.; Murata, S. J. Catal. 1992, 136, 126. (19) Rosowski, F.; Hornung, A.; Hinrichsen, O.; Herein, D.; Muhler, M.; Ertl, G. Appl. Catal. A 1997, 151, 443. (20) Hornung, A.; Muhler, M.; Ertl, G. Catal. Lett. 1998, 53, 77. (21) Alonso-Vante, N.; Giersig, M.; Tributsch, H. J. Electrochem. Soc. 1991, 138, 639. (22) Solorza-Feria, O.; Ellmer, K.; Giersig, M.; Alonso-Vante, N. Electrochim. Acta 1995, 40, 567. (23) Le Rhun, V.; Garnier, E.; Pronier, S.; Alonso-Vante, N. Electrochem. Commun. 2000, 2, 475. (24) Alonso-Vante, N. J. Phys. Chim. Phys. 1996, 93, 702. (25) Le Rhun, V.; Garnier, E.; Vogel, W.; Alonso-Vante, N. 51st Meeting of ISE, 3-5 Sept. 2000, Warsaw, Poland. (26) Le Rhun, V.; Alonso-Vante, N. J. New Mater. Electrochem. Syst. 2000, 3, 331. (27) Schmidt, T. J.; Paulus, A. U.; Gasteiger, H. A.; Alonso-Vante, N.; Behm, R. J. J. Electrochem. Soc. 2000, 147, 2620. (28) Vogel, W. Cryst. Res. Technol. 1998, 33, 1141. (29) Hartmann, N.; Imbihl, R.; Vogel, W. Catal. Lett. 1994, 28, 373. (30) Gnutzmann, V.; Vogel, W. J. Phys. Chem. 1990, 94, 499. (31) Vogel, W.; Rosner, B.; Tesche, B. J. Phys. Chem. 1993, 97, 11611. (32) Higgins, R. A. Solid State Ionics 2000, 134, 179. (33) Trasatti, S. In Electrochemistry of NoVel Materials; Lipkowski, J., Ross, Ph. N., Eds.; VCH Publishers: New York, 1994; p 207. (34) Alonso-Vante, N.; Cattarin, S.; Musiani, M. J. Electroanal. Chem. 2000, 481, 200. (35) Jow, T. R.; Zheng, J. P. J. Electrochem. Soc. 1998, 145, 49. (36) Zheng, J. P. Electrochem. Solid State Lett. 1999, 2, 359. (37) Alonso-Vante, N.; Colell, H.; Stimming, U.; Tributsch, H. J. Phys. Chem. 1993, 97, 7381.