Ti Electrodes by EXAFS

Chem. B , 1998, 102 (19), pp 3736–3741. DOI: 10.1021/jp9728114. Publication Date (Web): April 22, 1998. Copyright © 1998 American Chemical Society ...
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J. Phys. Chem. B 1998, 102, 3736-3741

Characterization of the Structure of RuO2-IrO2/Ti Electrodes by EXAFS Toshihide Arikawa,† Yoshio Takasu,*,† Yasushi Murakami,† Kiyotaka Asakura,‡ and Yasuhiro Iwasawa§ Department of Fine Materials Engineering, Faculty of Textile Science and Technology, Shinshu UniVersity, 3-15-1 Tokida, Ueda 386-8567, Japan; Research Center for Spectrochemistry, Faculty of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan; and Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: August 28, 1997; In Final Form: December 31, 1997

The structure of oxide layers of the RuO2-IrO2/Ti electrode system and the thermal decomposition processes of RuCl3 and IrCl3 to form their respective oxides have been mainly analyzed by EXAFS (extended X-ray absorption fine structure) and XRD (X-ray diffraction). Upon heating of the respective chlorides in air, both chlorides convert into their respective oxides. The coordination numbers of the oxide ions around both the ruthenium and iridium ions increased with an increase in the calcination temperature and attained 6, which is the theoretical value of the standard samples of rutile RuO2 and IrO2. The changes in the coordination number with respect to the calcination temperature were accompanied by changes in the lattice constants of these oxides. This suggests that the deviation of these parameters from the standard sample is caused by the lattice defects of the oxide ions. A dependence of the radial distribution functions of EXAFS on the composition of the RuO2-IrO2/Ti electrode system showed that RuO2 forms a solid solution with IrO2 for the binary oxide electrode system.

Introduction RuO2-based oxide-coated titanium electrodes are important anodes in the chlor-alkali industry.1 Oxides such as TiO2 and IrO2 are used as additives to increase the activity, selectivity, and stability of these electrodes toward chlorine evolution.2-6 These oxide coatings are usually prepared by thermal decomposition of metal chlorides and/or metal alkoxides dip-coated on titanium substrates. Because these anodes are significantly anticorrosive in comparison with the graphite anodes, they are usually called a DSA (dimensionally stable anode). The preparation conditions, especially the calcination temperature, of these oxide-coated electrodes affect their oxide structure and thereby the electrochemical properties.7-10 The calcination temperature of these types of oxide electrodes is usually 300550 °C, which is between the thermal decomposition temperature of RuCl3 and IrCl3 and the temperature where considerable oxidation of the titanium substrate begins. Since the temperature is not high, not only may a small amount of residual chloride ion still exist in the oxide lattice but also many oxide ion defects may be formed. This makes it more difficult to understand the oxide structure and physical properties.8,11-13 The structure of these types of electrodes has been examined using XRD (Xray diffraction), EPMA (electron probe X-ray microanalysis), or XPS (X-ray photoelectron spectroscopy), etc.; however, it is known that the oxide layers are not uniform in their structure and composition. For instance, the XRD diffraction peaks are broad, so that the detailed analysis of the oxide structure is practically impossible. On the other hand, the extended X-ray absorption fine structure (EXAFS) can give direct information on the local environment of the metal ions. * To whom all correspondence should be addressed. † Shinshu University. ‡ Faculty of Science, The University of Tokyo. § Graduate School of Science, The University of Tokyo.

Figure 1. Rutile structure of RuO2 and IrO2.

TABLE 1: Coordination Number (CN) and Bond Length of Each Bonding for RuO2 and IrO2a bond length (Å) shell

bond

CN

RuO2

IrO2

1 2 3 4

Me-O Me-O Me-Me Me-Me

2 4 2 8

1.94 1.98 3.11 3.54

1.94 2.00 3.15 3.55

a

The bond lengths were calculated using the JCPDS cards.

In the present paper, we report both the characterization of the structure of the oxide layers of the RuO2-IrO2/Ti electrode system and the thermal decomposition processes of RuCl3 and IrCl3 to form the respective oxides mainly using EXAFS. Experimental Section Preparation of the RuO2-IrO2/Ti Electrodes. Substrates of the oxide-coated electrodes used in this study were 99.5% pure titanium foil (5 µm in thickness) or titanium plate (1 mm in thickness). They were chemically polished in 30% HF + 60% H2O2 + 10% H2O solution at 50 °C for 10 s, washed

S1089-5647(97)02811-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/22/1998

Structure of RuO2-IrO2/Ti Electrodes

J. Phys. Chem. B, Vol. 102, No. 19, 1998 3737

(a)

(b)

Figure 2. High-resolution scanning electron micrographs of the ultrafine RuO2 particles which were finally calcined at (a) 300 and (b) 450 °C.

ultrasonically with distilled water, and then dried. The oxide electrodes were prepared by 10 repetitions of the following coating process: dipping of the Ti substrate into a 1-butanol solution of the RuCl3 (Permelec Electrode, Ltd., Japan) and/or IrCl3 (Permelec Electrode, Ltd., Japan), drying at 60 °C for 10 min, and thermal decomposition of the coated metal salts at various temperatures for 10 min. After a series of the above processes, the oxide-coated electrodes were annealed at the same temperature for 1 h. To obtain suitable absorbance during EXAFS measurements, 4-10 sheets of the cut samples (∼4 × 8 mm in size) were layed on top of one another. The oxidecoated layer formed on the titanium substrate by calcination at a temperature lower than 300 °C tended to peel off from the titanium substrate; therefore, the peeled-off layer was kneaded with acetone, and then the paste was held between polyethylene films for the EXAFS measurement. Measurements. A high-resolution scanning electron microscope (HR-SEM; JEOL JSM-6000F) was used to determine the diameter of the oxide particles of the calcined samples of RuO2 and IrO2 prepared from RuCl3 and IrCl3. XRD patterns of the samples were obtained using a Cu KR radiation system (30 kV, 60 mA, Rigaku CN-2028). Highly pure silicon powder (60 mesh in diameter, 99.999% in purity, Wako Pure Chemical Industries, Ltd.) was used as the internal standard. The EXAFS (extended X-ray absorption fine structure) spectra at both the Ru K absorption edge (22 119 eV) and that of the Ir LIII-edge (11 212 eV) were measured at Beam Line 10B (BL10B) in the Photon Factory of the National Laboratory for High Energy Physics (KEK-PF) (Proposal No. 95G011). The electron energy and currents for the storage ring were 2.5 GeV and 250-300 mA, respectively. High-intensity white X-rays generated by the synchrotron were monochromatized using a Si(311) channel cut monochromator. X-ray absorption measurements were carried out in the range 21 673.6-23 502 eV (for Ru K-edge) and 10 867-12 579 eV (for Ir LIII-edge) in a transmission mode. The energy resolution of Ru K-edge was 6 eV, and that of the Ir LIII-edge was 1.4 eV. Two 34 cm long ionization chambers were used for detecting the incident (I0) and transmitted (I) X-ray intensities. The I0 and I chambers were filled with Ar for the Ru K-edge, N2 for the Ir LIII-edge, Kr for the Ru K-edge, and Ar(50) + N2(50) for the Ir LIIIedge. The reason we did not use Kr + Ar but Ar for the I chamber is as follows: (1) The use of the mixture of Kr and Ar may decrease the accuracy due to the probable change of beam intensity caused by the fluctuation of the gas flux. (2) The absorption of the beam by Kr is µt ) 5 behind the I

chamber; however, there would be no problem, because the total current was only 10-9-10-10 A and little difference was found when we used two series of I chamber filled with Ar. The treatment of the harmonics rejection of Ir LIII-edge was as follows: (1) Since an Si(311) channel cut monochromator was used, the harmonics energy was ca. 33 keV where the beam intensity decreases. (2) The absorption of the high-energy beam was suppressed by the use of Ar + N2 for the I chamber. (3) The beam intensity must be still sufficiently strong since the absorption of the energy by the sample was restricted less than µt ) 3. Anhydrous ruthenium(IV) oxide (Wako Pure Chemical Industries, Ltd.), iridium(IV) oxide (Wako Pure Chemical Ind., Ltd.), anhydrous ruthenium(III) chloride (Permelec electrode, Ltd.) and anhydrous iridium(III) chloride (Permelec electrode, Ltd.) were used as reference samples. The measurements were performed at 20 °C. The analysis of the EXAFS data was performed using a software of Rigaku EXAFS. The EXAFS oscillations were extracted from the observed data by subtracting the smoothly varying part, µs(E), which was estimated by the cubic spline method. The oscillation was then normalized with the edge height, µ0(E), estimated from Victreen’s equation,

χ(k) ) (µ(E) - µs(E))/µ0(E)

(1)

where k is the photoelectron energy related to the photon energy E by

k ) (2m/h2(E - E0))1/2

(2)

E0 is the threshold energy, and it was taken to be equal to the energy of the inflection point for convenience. We performed the Fourier transformation of k3-weighted χ(k) over the region 30-160 nm-1. During the Fourier transformation, the Hanning window function was used to minimize the termination errors. The Fourier transform peaks were filtered, and data were then analyzed by means of a least-squares curve-fitting method using the theoretical EXAFS equation

k3χ(k) )

∑Sjk2NjFj(k) exp(-2σj2k2) sin(2krj + φj(k))/rj2

(3)

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Figure 4. Fourier transforms of k3χ(k) of Ru K-edge of the standard samples of rutile-type RuO2 (a) and RuCl3 (e) and the heat-treated samples of RuCl3 which were finally calcined at temperatures of (b) 450, (c) 300, and (d) 250 °C. The range of Fourier transform was 3.016.0 Å.

Figure 3. (A, top) The XRD profiles of the standard samples of RuO2 (a) and RuCl3 (e) and the heat-treated samples of RuCl3 which were finally calcined at various temperatures: (b) 450, (c) 300, and (d) 250 °C. (B, bottom) XRD profiles around rutile (110) plane of the standard sample of RuO2 (a) and the heat-treated samples of RuCl3 which were finally calcined at various temperatures: (b) 450 and (c) 300 °C. A highly pure Si powder was used as the internal standard. The cell constants were calculated with the peak angle.

where Nj, rj, and σj represent the coordination number, bond length, and Debye-Waller factor of the jth shell, respectively. Sj is an amplitude reduction factor obtained from the reference sample. Nj, rj, σj, and E0 altogether are the fitting parameters. φj and Fj are the phase shift and amplitude functions of the jth shell atom, respectively. The empirical phase shift and amplitude functions were used for the calculation. Both Hanning windows of dk1 and dk2 were 1.0. Structures of RuO2 and IrO2. Figure 1 and Table 1 show the rutile structures of RuO2 and IrO2 and their bond lengths, respectively. The bond lengths were calculated using the JCPDS cards. Since the bond length of the first coordination oxygen is almost same as that of the second coordination oxygen and they could not be differentiated by the EXAFS analysis, they were treated as one bonding of Me-O. That is, the coordination

Figure 5. Curve fitting for the Ru-O bond in the heat-treated sample of RuCl3 which was finally calcined at 450 °C: solid line, observed data, dotted line; calculated data determined by using parameters of the standard RuO2 sample.

number and bond length of the first peak of Ru-O are 6 and 1.97 Å, respectively. In analogy with RuO2, the coordination number of Ir-O for the first peak of IrO2 is 6 and the bond length is 1.98 Å. Since data for the bond length of RuCl3 and that of IrCl3 are not published, those were estimated from the curve fitting with theoretical data. For the first coordination number and the expected bond length of RuCl3 are 6 and 2.37 Å, and those of IrCl3 are 6 and 2.35 Å, respectively. We have checked the validity of these parameters by comparing the analyzed EXAFS data for rutile with the crystallographic data. Results and Discussion RuO2/Ti Electrodes. Figure 2 shows the high-resolution scanning electron micrographs of the ultrafine particles of RuO2

Structure of RuO2-IrO2/Ti Electrodes

J. Phys. Chem. B, Vol. 102, No. 19, 1998 3739

TABLE 2: Results of Curve-Fitting Analysis for the First Peak of the Ru K-Edge EXAFS Dataa RuCl3b 250 °C 300 °C 350 °C 400 °C 450 °C RuO2b

CN

(elem)

r/Å

∆σ2/Å2

∆E0/eV

∆r/Å

6.0 5.6 5.2 5.4 5.5 6.0 6.0

(Cl) (Cl) (O) (O) (O) (O) (O)

2.37 2.37 1.97 1.97 1.97 1.97 1.97

0.0002 0.0017 0.0010 0.0010 0.0013

1.94 1.47 -3.36 -3.01 1.39

0.8-2.5 0.8-2.0 0.8-2.0 0.8-2.0 0.8-2.0

a CN, coordination number; (elem), element; r, bond length; ∆σ2, Debye-Waller factor; ∆E0, deviation from edge energy of standard sample; ∆r, range of inverse Fourier transformation. b Standard sample.

which were finally calcined at (a) 300 and (b) 450 °C. The particle size of the former is 5-20 nm in diameter, and that of the latter is 30-80 nm. Figure 3a shows the XRD profiles of the standard samples of RuO2 and RuCl3 and three heat-treated samples of RuCl3 which were finally calcined at various temperatures: (b) 450, (c) 300, and (d) 250 °C. The XRD peaks of the calcined samples treated at 300 and 450 °C gave the profiles of the rutiletype RuO2, but they are broad. This suggests that the oxide particles are fine and/or amorphous-like. Between 250 and 300 °C, RuCl3 changed into rutile RuO2. In Figure 3b, the XRD profiles around the rutile (110) peak for the RuO2 and two calcined samples of RuCl3 heat-treated at 300 and 450 °C are shown. The peak positions of these calcined samples appear at a lower angle than the standard RuO2 sample. An analysis of these peaks shows that the length of the a-axis decreases while that of the c-axis increases with an increase in the heattreatment temperature of RuCl3. Due to the small differences in the cell constants between the test sample and the reference sample, little change was detected in the EXAFS analysis. Figure 4 shows the Fourier transforms of k3χ(k) of the Ru K-edge of the standard samples of rutile-type RuO2 (a) and RuCl3, (e) and the calcined specimens of RuCl3 treated at various temperatures: (b) 450, (c) 300, and (d) 250 °C. The range used for Fourier transforming was 3.0-16.0 Å. From the comparison of parts d and e of Figure 4, the first peak in Figure 4 (d) is due to the Ru-Cl bond, though it could not be detected by the XRD measurement. The radial distribution function of the sample calcined at 200 °C gives the structure of RuCl3, whereas those samples calcined at higher than 300 °C show almost the same structure as RuO2. Concerning the radial distribution function of (a)-(c) in Figure 4, the first large peak is composed of the peaks due to the sum of the Ru-O bond (no. 1 in Figure 1) and the Ru-O bond (no. 2 in Figure 1), the second peak is due to the Ru-Ru bond (no. 3 in Figure 1), and the third peak is a Ru-Ru bond (no. 4 in Figure 1). A successful curve fitting for the Ru-O bond in a calcined sample of RuCl3 treated at 450 °C (RuO2) is shown in Figure 5, where the solid line shows the observed data and the dotted line shows the calculated data determined using parameters of the standard RuO2 sample. The results of the curve-fitting analysis for the Ru K-edge EXAFS data of the RuCl3 and calcined samples of it are presented in Table 2. The range of fitting analyses was 0.82.5 or 0.8-2.0 Å. Generally, the parameters determined by the curve-fitting method have errors with respect to the bond distance and the coordination number, i.e., 0.03 Å and 10%, respectively. The coordination number around ruthenium ion for RuCl3 calcined at 250 °C was less than 6. This can be interpreted by the simultaneous coordinations of Ru-O and

Figure 6. (A, top) XRD profiles of the standard samples of IrO2 (a) and IrCl3 (e) and the heat-treated samples of IrCl3 which were finally calcined at temperatures of (b) 450, (c) 400, and (d) 350 °C. (B, bottom) The XRD profiles around rutile (110) plane of the standard sample of IrO2 (a) and the heat-treated samples of IrCl3 which were finally calcined at temperatures of (b) 450 and (c) 400 °C. A highly pure Si powder was used as the internal standard. The cell constants were calculated with the peak angle.

Ru-Cl or disorder of the Ru-Cl bonding. When we carried out two shell fitting of Ru-Cl and Ru-O assuming the total coordination number to be 6, we could not obtain any physically meaningful fitting results, indicating that the contribution of Ru-O bonding was small at 250 °C. We analyzed the bonding using cumulant expansion and found that the coordination number was still 6 with the larger Debye-Waller and C4 factor. Thus Ru-Cl bonding in the sample was rather disordered, and hence the apparent coordination number seemed to be smaller than 6. The other samples showed more good fitting for 1 shell fitting than 2 shell fitting. For the samples calcined at a temperature higher than 300 °C, the Ru-O bond distances show no change within the calculation errors, whereas the coordination numbers increased with an increase in the calcination temper-

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Arikawa et al.

Figure 7. Fourier transforms of k3χ(k) of Ir K-edge of the standard samples of rutile-type IrO2 (a) and IrCl3 (f) and the heat-treated samples of IrCl3 which were finally calcined at temperatures of (b) 450, (c) 400, (d) 350, and 300 °C. The range of Fourier transform was 3.016.0 Å.

ature. The XRD patterns and the radial distribution functions determined from the EXAFS spectrum represent the specimens calcined at a temperature higher than 300 °C to form the rutiletype RuO2. Hence, the dependence of the coordination number on the calcination temperature means that the oxygen defect in the RuO2 lattice depends on the calcination temperature. It may be due to the chloride ion remaining in the oxide layer even after the calcination. In early works,14,15 a measured content of chloride ion in the oxide layer was ca. 5 wt %. However, it seems not to form the Ru-Cl bonding. IrO2/Ti Electrodes. Figure 6a shows the XRD profiles of the standard samples of IrO2 and IrCl3 and calcined samples of IrCl3 which were heat-treated at various calcination temperatures. Between 350 and 400 °C, IrCl3 changed into rutile IrO2. The samples calcined at temperatures higher than 400 °C give similar XRD patterns of IrO2. The XRD profiles around the rutile (110) peak for the IrO2 and two calcined samples of IrCl3 heat-treated at 300 and 450 °C are shown in Figure 6b. The peak positions of these calcined samples appear at a lower angle than the standard IrO2 sample. As in the case of the RuO2 system, an analysis of these peaks shows that the length of the a-axis decreases while that of the c-axis increases with an increase in the calcination temperature of IrCl3. Figure 7 shows Fourier transforms of k3χ(k) of the Ir LIIIedge of the standard samples of rutile-type IrO2 (a) and IrCl3

(f) and the calcined samples of IrCl3 treated at various temperatures: (b) 450, (c) 400, (d) 350, and (e) 300 °C. The range of the Fourier transform was 3.0-16.0 Å. The peak due to an Ir-Cl bond is clearly detected, while it could not be detected by the XRD measurement. The radial distribution function of the calcined sample of IrCl3 treated at 300 °C (e) shows the same structure as that of IrCl3, whereas those of the calcined samples treated at a temperature higher than 400 °C give almost the same structure as IrO2. For the specimen calcined at 350 °C (d), both peaks of an Ir-Cl bond and an Ir-O bond are detected in the radial distribution function. The decomposition temperature of IrCl3 is higher than that of RuCl3. Concerning the radial distribution functions of (a)-(c) in Figure 7, the first large peak is composed of the peaks due to an Ir-O bond (no. 1 in Figure 1) and Ir-O bond (no. 2 in Figure 1), the second peak is due to the Ir-Ir bond (no. 3 in Figure 1), and the third peak is an Ir-Ir bond (no. 4 in Figure 1). Although the rutile structure of IrO2 is almost same as that of RuO2, the radial distribution function of IrO2 differs from that of RuO2. This is due to the difference in the phase shift of the photoelectrons. Table 3 presents the results of the curve-fitting analyses for the Ir LIII-edge EXAFS data. The ranges of fitting analyses were 1-2.0 or 1-2.5 Å. These samples were fitted for oneshell and two-shell fitting; the former showed a more suitable value than the latter except for the sample calcined at 350 °C. However, the sample calcined at 300 °C may have the coordination number of ca. 0.7 for oxide ion. For the calcined samples treated at a temperature higher than 350 °C, the distances of the Ir-O bonds gave no change, whereas the coordination numbers increased with an increase in the calcination temperature. Figure 8 shows curve fittings for the first peak of the calcined sample treated at 450 °C; the solid line shows observed data, and the dotted line shows the calculated data determined with the parameters of the standard IrO2 sample. RuO2-IrO2/Ti Electrodes. Since the phase shift of Ir-Ir is different from the Ir-Ru, the second and third Fourier transformed peaks behave in a complicated way if the RuO2 and IrO2 form solid solutions. When Ir content is large, the second and third peaks are similar to the corresponding peaks in IrO2. As Ru content increases, the Ir-Ir bonding is replaced by Ir-Ru, and its peak decreases rapidly because the phase shifts of Ir-Ru and Ir-Ir are different and smear out with each other. When Ru content is further increased and the Ir is mostly surrounded by Ru atoms or Ir is regarded as an impurity of RuO2 lattice, the Fourier transform of Ir shows a structure similar to that found in the Fourier transform of RuO2. On the other hand, if RuO2 and IrO2 form a separate phase, Ir gives the unchanged Fourier peak irrespective of composition. The observed results are the followings.

TABLE 3: Results of Curve-Fitting Analysis for the First or the First and the Second Peak of the Ir LIII-Edge EXAFS Data first peak a

IrCl3 300 °C 350 °C 350 °Cb 400 °C 450 °C IrO2a

CN (elem)

r/Å

∆σ2/Å2

6.0 (Cl) 4.8 (Cl) 3.5 (O) 3.8 ( 1.4 (O) 5.5 (O) 5.9 (O) 6.0 (O)

2.35 2.36 2.01 2.00 ( 0.00 1.98 1.99 1.99

0.0004 0.0023 0.0065 ( 0.0027 0.0023 0.0013

second peak ∆E0/eV 1.7 4.1 -1.0 ( 2.0 1.8 0.3

CN (elem)

r/Å

∆σ2/Å2

∆E0/eV

3.2 (Cl) 2.2 ( 1.4 (Cl)

2.34 2.36 ( 0.01

0.0045 0.0012 ( 0.0024

-0.6 3.5 ( 0.5

∆r/Å 1.0-2.5 1.0-2.5 1.0-2.5 1.0-2.0 1.0-2.0

a Standard sample. b The sum of the coordination numbers for the oxide ion and the chloride ion was constrained to be equal to 6. CN, coordination number; (elem), element; r, bond length; ∆σ2, Debye-Waller factor; ∆E0, deviation from edge energy of standard sample; ∆r, range of inverse Fourier transformation.

Structure of RuO2-IrO2/Ti Electrodes

J. Phys. Chem. B, Vol. 102, No. 19, 1998 3741 Thus, we conclude that the RuO2 and IrO2 form a solid solution. Conclusion

Figure 8. Curve fitting for the Ir-O bond in the heat-treated sample of IrCl3 which was finally calcined at 450 °C: solid line, observed data; dotted line, calculated data determined by using parameters of the standard IrO2 sample.

The precise structure of the RuO2-IrO2/Ti electrode system has been examined by using EXAFS, XRD, and HR-SEM. These oxide electrodes were prepared by the thermal decomposition of RuCl3 and/or IrCl3 which were dip-coated with 1-butanol on Ti substrates. The XRD analyses suggested that the a-axis of the respective oxides increased in comparison with the standard oxides, RuO2 and IrO2, while the c-axis decreased. Although the XRD measurements could give no information on the metal-Cl bond for this oxide system, EXAFS measurements clarified the metal-Cl bonds in the radial distribution functions. Therefore, the thermal decomposition processes of these oxides were successively followed by the EXAFS measurement. The first peak of the radial distribution functions presenting metal-O bonds showed that the coordination number increased with an increase in the calcination temperature, while little change was observed for the bond length. The change in the plane distance observed in the XRD measurements must be due to the change in the coordination number around the respective noble metal ions. With the difference in the radial distribution function which was caused by the phase shift of the photoelectron, it was demonstrated that the binary oxide system of RuO2-IrO2 forms a solid solution. Acknowledgment. X-ray absorption measurements were carried out under the approval of the Photon Factory Advisory Committee (Proposal No. 95G011). The authors thank Dr. M. Nomura and the staff of the Photon Factory for their help in the EXAFS measurements. The present work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas “Catalysis Chemistry of Unique Reaction Fields, Extreme Environment Catalysis” from the Ministry of Education, Science and Culture, Japan. References and Notes

Figure 9. Dependence of the radial distribution functions calculated from the EXAFS of Ir LIII-edge on the composition of RuO2-IrO2/Ti electrode system. The standard samples of rutile-type IrO2 (a) and the heat-treated samples of RuCl3-IrCl3: (b) IrO2/Ti, (c) RuO2 (20%)IrO2 (80%)/Ti, (d) RuO2 (40%)-IrO2 (60%)/Ti, (e) RuO2 (50%)-IrO2 (50%)/Ti, (f) RuO2 (60%)-IrO2 (40%)/Ti, and (g) RuO2 (80%)-IrO2 (20%)/Ti.

The dependence of the radial distribution function determined from the EXAFS of the Ir LIII-edge on the composition of RuO2-IrO2/Ti electrodes is shown in Figure 9. The observed radial distribution functions continuously change with the change in the oxide composition. That is, the second and third peaks in the Fourier transforms decrease and change their shape when Ru content increases. In the previous studies, it was revealed that the RuO2-IrO2 electrode surfaces were enriched with Ir16,17 while the in-depth composition linearly changed with the nominal composition of the dipping solution used in the sample preparation process.18 The results shown in Figure 9 are in good agreement with the result of the in-depth composition.

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