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Infrared Reflection Absorption Spectroscopy of the Sulfuric Acid Anion Adsorbed on Pd(S)-[n(111) × (111)] Electrodes Nagahiro Hoshi,* Makiko Kuroda, Takehiko Ogawa, Osamu Koga, and Yoshio Hori Department of Applied Chemistry, Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Received November 15, 2003. In Final Form: February 15, 2004 Adsorption of the sulfuric acid anion (HSO4- or SO42-) has been studied on Pd(S)-[n(111) × (111)] electrodes (n ) 2, 3, 5, 9, 20, infinity) using in situ infrared reflection absorption spectroscopy (IRAS). A single band is observed around 1200 cm-1 on all the electrodes. The band is assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with three- or onefold geometry. This result differs from the case of Pt-stepped surfaces on which two IRAS bands are observed around 1200 and 1100 cm-1. The maximum coverage of the sulfuric acid anion is enhanced with the increase of the terrace width. The surfaces with n more than 3 have similar IRAS band shifts (dv/dE). Pd-stepped surfaces, for which the terrace is wide enough for the anion adsorption, adsorb the anion on the terrace rather than the step.
1. Introduction Studies on adsorbed anions are important for revealing the double layer structure of an electrode and solution interface. Pt group metals (Pt, Ir, Rh, Pd) are known to adsorb sulfuric acid anion (HSO4- or SO42-) strongly. The adsorbed sulfuric acid anion lowers the activity of electrochemical reactions,1-4 because it blocks the active sites for the reactions. Revealing the adsorption sites of the anion will provide important information on the structure of active sites for electrochemical reactions. The geometry and superstructure of the adsorbed sulfuric acid anion have been studied using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). STM studies show that the sulfuric acid anion has the identical x3 × x7 superstructure on (111) planes of Pt,5 Rh,6 Ir,7,8 and Pd.9 Kunimatsu et al. first reported IRAS spectra of the adsorbed sulfuric acid anion on the Pt polycrystal electrode.10,11 The study was extended to the well-defined surfaces, such as the low index planes of Pt,12-21 Au,22-24 Rh,25 Ir,8 Pd,26 and stepped surfaces of Pt.27 In the spectral range above 1000 cm-1, IRAS spectra give a band around * To whom correspondence should be addressed. E-mail: hoshi@ faculty.chiba-u.jp. Phone & fax: +81-43-290-3384. (1) Markovic´, N. M.; Ross, P. N. J. Electroanal. Chem. 1992, 330, 499. (2) Kita, H.; Gao, Y.; Nakato, T.; Hattori, H. J. Electroanal. Chem. 1994, 373, 177. (3) Kita, H.; Gao, Y.; Ohnishi, K. Chem. Lett. 1994, 73. (4) Hoshi, N.; Suzuki, T.; Hori, Y. J. Electroanal. Chem. 1996, 416, 61. (5) Funtikov, A. M.; Stimming, U.; Vogel, R. J. Electroanal. Chem. 1997, 428, 147. (6) Wan, L.-J.; Yau, S.-L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (7) Wan, L.-J.; Hara, M.; Inukai, J.; Itaya, K. J. Phys. Chem. B 1999, 103, 6978. (8) Senna, T.; Ikemiya, N.; Ito, M. J. Electroanal. Chem. 2001, 511, 115. (9) Wan, L.-J.; Suzuki, T.; Sashikata, K.; Okada, J.; Inukai, J.; Itaya, K. J. Electroanal. Chem. 2000, 484, 189. (10) Kunimatsu, K.; Samant, M. G.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1988, 243, 203. (11) Kunimatsu, K.; Samant, M. G.; Seki, H. J. Electroanal. Chem. 1989, 258, 163. (12) Faguy, P. W.; Markovic´, N. M.; Azˇic´, R. R.; Fierro, C. A.; Yeager, E. B. J. Electroanal. Chem. 1990, 289, 245.
1200 cm-1 on all the (111) surfaces in H2SO4 solutions. The band is assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with threefold geometry. There is, however, no consensus on whether HSO4- or SO42- is adsorbed on the (111) surfaces: some papers insist HSO4- adsorption,12,14,16 and the others propose the coadsorption of SO42- and H3O+.17-19 Identification of the adsorbed anion is out of the scope of this paper; we address the adsorbed anion as the sulfuric acid anion (HSO4- or SO42-). On (100) and (110) surfaces, the geometry of the adsorbed sulfuric acid anion depends on metals. Two IRAS bands are observed around 1100 and 1200 cm-1 on Pt(100) and Pt(110) electrodes. The bands are assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with twofold geometry.20,21 The lower and the higher frequency bands arise from the coordinated and the uncoordinated SO stretching vibrations, respectively. However, a single IRAS band appears around 1200 cm-1 on the (100) and (110) surfaces of Au,24 Rh,25 and Pd.26 (13) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J. L., Ross, P. N., Jr., Eds.; VCH: New York, 1991; Chapter 7. (14) Sawatari, Y.; Inukai, J.; Ito, M. J. Electron Spectrosc. 1993, 64/65, 515. (15) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 961. (16) Shingaya, Y.; Ito, M. Chem. Phys. Lett. 1996, 256, 438. (17) Thomas, S.; Sung, Y.-E.; Kim, H. S.; Wiekowski, A. J. Phys. Chem. 1996, 100, 11726. (18) Faguy, P. W.; Marinkovic´, N. S.; Azˇic´, R. R. Langmuir 1996, 12, 243. (19) Faguy, P. W.; Marinkovic´, N. S.; Azˇic´, R. R. J. Electroanal. Chem. 1996, 407, 209. (20) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 2093. (21) Iwasita, T.; Nart, F. C.; Rodes, A.; Pastor, E.; Weber, M. Electrochim. Acta 1995, 40, 53. (22) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (23) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (24) Moraes, I. R.; Nart, F. C. J. Electroanal. Chem. 1999, 461, 110. (25) Moraes, I. R.; Nart, F. C. J. Braz. Chem. Soc. 2001, 12, 138. (26) Hoshi, N.; Kuroda, M.; Koga, O.; Hori, Y. J. Phys. Chem. B 2002, 106, 9107. (27) Hoshi, N.; Sakurada, A.; Nakamura, S.; Teruya, S.; Koga, O.; Hori, Y. J. Phys. Chem. B 2002, 106, 1985.
10.1021/la036149g CCC: $27.50 © 2004 American Chemical Society Published on Web 05/04/2004
IRAS of the Sulfuric Acid Anion
Langmuir, Vol. 20, No. 12, 2004 5067 IRAS spectra were measured using a JIR-6000M spectrometer (JEOL) with p-polarized light with resolution 4 cm-1. The window of the IRAS cell was a CaF2 plate with a thickness of 2 mm. Reflected IR light was detected with a mercury cadmium telluride detector cooled by liquid nitrogen. The spectra were averaged over 1000 scans using subtractively normalized interfacial Fourier transform infrared spectroscopy. The reference potential was 0.20 V (reversible hydrogen electrode, RHE), whereas the sample potential was changed between 0.25 and 0.80 V (RHE). All the potentials are shown with respect to a RHE.
3. Results and Discussion
Figure 1. Voltammograms of the Pd(S)-[n(111) × (111)] series in 0.05 M H2SO4 saturated with Ar. The value of n shows the number of terrace atomic rows.
The band is assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with onefold geometry. On Pt(S)-[n(111) × (111)] electrodes, two IRAS bands are observed around 1100 and 1200 cm-1.27 The ratio of the intensity of the lower frequency band to that of the higher one (IL/IH) is enhanced with the increase of the step atom density. We proposed the model that the terrace and the step adsorb the sulfuric acid anion with threefold and twofold geometry, respectively. We extend the study to Pd(S)-[n(111) × (111)] surfaces in this paper and compare the results with those of Pt(S)-[n(111) × (111)]. 2. Experimental Section We prepared single crystals of Pd in a teardrop shape with cross sections between 0.30 and 0.35 cm2 according to Clavilier et al.28 The detailed procedure was shown in our previous report.26 The crystal was oriented using the reflection beam of the He/Ne laser from (111) and (100) facets29,30 and polished mechanically using a diamond slurry. The single-crystal electrode was annealed in a H2/O2 flame at about 1300 °C, cooled in a stream of Ar, and transferred to an IRAS cell with a droplet of ultrapure water on the surface. We prepared Pd(S)-[n(111) × (111)] electrodes with terrace atomic rows n ) 2, 3, 5, 9, 20, and ∞. The electrolytic solution was prepared from ultrapure water treated with Milli-Q plus low total organic carbon (Millipore) and suprapure grade chemicals (Merck). The purity of Ar is higher than 99.9999%.
3.1. Voltammograms. Pd single crystals provide voltammograms characteristic of their orientation in H2SO4 solutions. Figure 1 shows the representative voltammograms of Pd(S)-[n(111) × (111)] electrodes. Previous studies show that the sharp redox peaks at 0.25 V are related with the adsorption and desorption of sulfuric acid anion at the terrace, because the peaks were not observed in 0.1 M HClO4.26,31 Anodic peaks at 1.1 and 0.9 V arise from the oxide film formation at the (111) terrace and step, respectively.31-33 Voltammograms in Figure 1 are identical with those reported previously; thus, we judged that the crystals are correctly oriented. 3.2. IRAS Spectra. Figure 2 shows the IRAS spectra of Pd(S)-[n(111) × (111)] electrodes in 0.05 M H2SO4. We cannot measure the spectra below 1000 cm-1 because of the cutoff of the IR light due to the CaF2 window. A positivegoing band is obtained around 1200 cm-1 above 0.30 V. The band frequencies are almost identical with those on Pt(111) in H2SO4 solutions.12-19 General consensus has been reached that the sulfuric acid anion is adsorbed on Pt(111) with three fold geometry. There have been, however, several arguments on the band assignments. Faguy et al. assigned the band to the asymmetric stretching vibration of the S-O bond of HSO4- (∼1200 cm-1) that is forbidden on the surface. They insist that surface selection rule is broken on the electrode surface.12 Ito et al. assigned the band to the symmetric stretching vibration of the S-O bond of HSO4- according to the surface selection rule.14 They showed that the band, which is located around 1050 cm-1 in solution phase, shifts significantly to a higher wavenumber on a positively charged surface on the basis of molecular orbital calculation. They later attributed the significant band shift to
Figure 2. IRAS spectra of Pd(S)-[n(111) × (111)] electrodes in 0.05 M H2SO4. The sample potentials (ES) are shown in the figures. Reference spectra were collected at 0.20 V.
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Table 1. IR Bands of the Cobalt Complex with Sulfate with a C3v Point Group {[Co(SO4)(NH3)5]Cl}35 wavenumber, irreducible
cm-1
vs(SO)a
vas(SO)a
vs(SO*)a
1044 A1
1135 E
974 A1
a
vs, symmetric stretching vibration; vas, asymmetric stretching vibration; SO, S-O bond uncoordinated to metal; SO*, S-O bond coordinated to metal.
the back-donation of electrons from the antibonding orbital of HSO4-.34 Nart el al. explained that the asymmetric band shape around 1200 cm-1 arises from the overlap of the symmetric stretching vibrations of uncoordinated S-O [vs(SO)] and coordinated S-O [vs(SO*)] of the sulfate anion.15 They suggest that coordination through three oxygen atoms produces a strong mechanical hindrance that causes a band shift to a higher wavenumber. Taking the former arguments into account, we discuss the IRAS spectra on Pd single-crystal electrodes according to the following assumptions:26 1. The surface selection rule is valid for the IRAS of the sulfuric acid anion. 2. The IR bands of the adsorbed sulfuric acid anion locate at a higher wavenumber than those of the metal complexes with sulfate. 3. When the sulfuric acid anion is adsorbed with C3v symmetry, vs(SO) locates near vs(SO*). 4. The adsorbed sulfuric acid anion has the same symmetry as adsorbed SO42-. We assign the band around 1200 cm-1 on Pd(S)-[n(111) × (111)] electrodes to the symmetric SO stretching vibration vs(SO) of the sulfuric acid anion adsorbed with three- or onefold geometry according to the IR data of cobalt complexes with sulfate,35 as is the case of Pd(111).26 The wavenumber of the observed vs(SO) is higher than that of the cobalt complexes with C3v symmetry (Table 1).35 The band shift to a higher wavenumber may arise from positively charged surface,14 back-donation,34 or strong mechanical hindrance.20 We cannot distinguish the threefold anion from the onefold one using IRAS; both of them have C3v symmetry, and the band shift (dv/dE) of the onefold anion is almost identical with that of the threefold one on the (111) terrace.26 The threefold anion, however, will be adsorbed on the (111) terrace at low coverage, because the symmetry and the atomic distance of the Pd(111) surface are almost identical with those of the bottom plane of the tetrahedral sulfuric acid anion. A single IRAS band supports that the anion is adsorbed nearly upright on the surfaces. If the anion is tilted significantly, the IRAS spectra will give the additional band of the forbidden vibration vas(SO) at the wavenumber higher than vs(SO).26,35 We previously reported that the IRAS spectra on Pt(S)-[n(111) × (111)] gave two bands around 1200 and 1100 cm-1, which indicate that the twofold anion is coadsorbed with the threefold one.27 According to the IR data of the cobalt complex with sulfate (Table 2)35 and the assumptions mentioned above, the lower and higher (28) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (29) Motoo, S.; Furuya, N. J. Electroanal. Chem. 1984, 172, 339. (30) Furuya, N.; Koide, S. Surf. Sci. 1989, 220, 18. (31) Hoshi, N.; Kagaya, K.; Hori, Y. J. Electroanal. Chem. 2000, 485, 55. (32) Solomun, T. J. Electroanal. Chem. 1987, 217, 435. (33) Sashikata, K.; Matsui, Y.; Itaya, K.; Soriaga, M. P. J. Phys. Chem. 1996, 100, 20027. (34) Shingaya, Y.; Ito, M. Chem. Phys. Lett. 1996, 256, 438. (35) Tanaka, N.; Sugi, H.; Fujita, J. Bull. Chem. Soc. Jpn. 1964, 37, 640.
Table 2. IR Bands of the Cobalt Complex with Sulfate with C2v Symmetry35 {[(NH3)4Co(NH2)(SO4)Co(NH3)4](NO3)3} wavenumber, cm-1 irreducible
vs(SO)
vas(SO*)
vas(SO)
vs(SO*)
1109 A1
1064 B1
1171 B2
997 A1
a v , symmetric stretching vibration; v , asymmetric stretching s as vibration; SO, S-O bond uncoordinated to metal; SO*, S-O bond coordinated to metal.
Figure 3. Potential dependence of the integrated band intensity of the positive-going bands around 1200 cm-1.
frequency bands were assigned to vs(SO*) and vs(SO) of the sulfuric acid anion adsorbed with twofold geometry, respectively. The integrated intensity of the lower frequency band is enhanced with the increase of the step atom density. This result supports that Pt(S)-[n(111) × (111)] surfaces adsorb the twofold anion at the step. Pd(S)-[n(111) × (111)] electrodes provide, however, no band around 1100 cm-1 (Figure 2). This fact indicates that no twofold anion is adsorbed on Pd(S)-[n(111) × (111)]. Broad negative-going bands appear above 0.4 V on each electrode (Figure 2). This band may be assigned to the SO stretching vibration of the HSO4- (∼1200 cm-1) and SO42anion (∼1100 cm-1) in the solution phase.36,37 When an electrode adsorbs the sulfuric acid anion, the concentration of HSO4- and SO42- decreases in the thin layer so that negative-going bands appear in the IRAS spectra. The wavenumber of the negative-going band is lower than that of the free HSO4- anion (∼1200 cm-1). That is probably because the positive-going band hides the real peak of the negative band. 3.3. Band Intensity of the IRAS Spectra. The bipolar bands in Figure 2 are fitted with two Gaussian curves. The integrated intensity and the peak position of the band are evaluated after the peak separation. The integrated intensity of the positive-going band is enlarged with the increase of the sample potentials as shown in Figure 3. The positive-going band definitely originates from the adsorbate, because the band position depends on the potential. The intensity of the negative-going band is also enhanced as the potential increases, showing the increase of the consumption of sulfuric acid anion in the solution phase due to the adsorption. A previous paper reports that the static electric field also affects the intensity of the IRAS band.38,39 The increase of the applied potential, however, decreases the integrated intensity of the IRAS band of the adsorbed sulfuric acid anion38,39 and CO.40 (36) Clarke, J. H. R.; Woodward, L. A. Trans. Faraday Soc. 1961, 57, 1286. (37) Dawson, B. S.; Irish, D. E.; Toogood, G. E. J. Phys. Chem. 1986, 90, 334. (38) Nart, F. C.; Iwasita, T. Electrochim. Acta 1992, 37, 2179. (39) Nart, F. C.; Iwasita, T. Electrochim. Acta 1996, 41, 631. (40) Lambert, D. K. J. Chem. Phys. 1991, 94, 6237.
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Figure 4. Peak position of the IRAS band plotted against the potential. Table 3. Band Shift (dv/dE) of the Sulfuric Acid Anion Adsorbed on Pd(S)-[n(111) × (111)] Electrodes electrode
dv/dE, cm-1 V-1
Pd(111) Pd(10 10 9) (n ) 20) Pd(997) (n ) 9) Pd(553) (n ) 5) Pd(331) (n ) 3) Pd(110) (n ) 2)
65 70 65 80 61 38
These results suggest that the coverage of the sulfuric acid anion increases at positive potentials, as is the case of Pt single crystals27 and low-index planes of Pd.26 The maximum coverage of the anion is enhanced as the terrace width increases. This fact indicates that the (111) terrace is more likely to adsorb the anion than the step. The onset potential of the adsorption of the sulfuric acid anion agrees with the positive end of the sharp redox peaks of the voltammograms in Figure 1. Considering the fact that these redox peaks were not observed in HClO4 solution, the redox peaks may arise from the adsorption and desorption of hydrogen which interacts with sulfuric acid anion strongly.26,31 Although the coverage of sulfuric acid anion increases monotonically at positive potentials, no peak due to the anion adsorption is observed between 0.4 and 0.8 V in the voltammograms in Figure 1. That is because the electricity due to the anion adsorption is as small as the double layer charge, as is the case of the iodine adsorption on Au(111).41 3.4. Band Shift of the IRAS Spectra. The peak of the IRAS band is plotted against the potentials in Figure 4.
The plots give good linear lines on all the electrodes. The band shift (dv/dE) is summarized in Table 3. A band shift is generally caused by the donation and back-donation of electrons between the adsorbate and the electrode surface,12,42 the Stark effect,43 and dipole-dipole interaction among adsorbates.44 The intensity of the dipole-dipole interaction depends on the coverage of the adsorbate, whereas the other factors are affected by the applied potential. A previous report tried to separate the dipoledipole coupling from the applied potential effects, suggesting that the band center frequency is proportional to the square root of the coverage of the sulfate ion adsorbed on the Pt polycrystal electrode.38 However, the band center frequency of Pd(553) (n ) 5) is almost identical with that of Pd(111) at 0.8 V (Figure 4), even though the estimated coverage of the sulfuric acid anion differs remarkably, coveragePd(553)/coveragePd(111) ) 0.57 (Figure 3). This fact supports that the dipole-dipole interaction hardly affects the band shift of the sulfuric acid anion on Pd(S)-[n(111) × (111)] electrodes. Almost the same band shift is obtained on the surfaces with n more than 3, suggesting that the sulfuric acid anion is adsorbed on the common sites of these surfaces. The band shift of Pd(110) (n ) 2) is rather smaller than the other surfaces with n more than 3. The first layer of Pd(110) (1 × 1) consists of only one step. The sulfuric acid anion will be adsorbed on the step of Pd(110) with onefold geometry because of the steric hindrance with the step atoms. The onefold anion adsorbed at the step may provide the small band shift. The surfaces with n more than 3 may adsorb no sulfuric acid anion with onefold geometry on the step, because the band shift shows little orientation dependence. If the onefold sulfuric acid anion is adsorbed at the step sites, the apparent band shift will be smaller with the increase of the step atom density. Former study using STM reported that Pd(110) has a (1 × 2) structure in an ultrahigh vacuum.45 Pd(110) (1 × 2) will adsorb the sulfuric acid anion on the terrace, because the terrace of Pd(110) (1 × 2) is as wide as Pd(331) (n ) 3). If Pd(110) is reconstructed to (1 × 2), the band shift will be the same as Pd(331) (n ) 3). The small band shift may indicate that Pd(110) has an unreconstructed (1 × 1) structure in the sulfuric acid solution. 3.5. Model of the Adsorbed Sulfuric Acid Anion. The possible adsorption model of the sulfuric acid anion
Figure 5. Model of the adsorption structure of the sulfuric acid anion.
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is shown in Figure 5. STM study shows that the x3 × x7 superstructure is completed on Pd(111) at 0.4 V (RHE).9 The STM image shows no potential dependence between 0.4 and 0.8 V. Integrated intensity of the IRAS band, however, shows that the coverage of sulfuric acid anion is enhanced with the increase of the potential on Pd(111). We explained this inconsistency on the assumption that the mobile onefold anion is adsorbed on the (111) terrace above 0.4 V, because STM cannot image mobile species. There is no room for the threefold anion adsorption in the unit cell of the x3 × x7 superstructure; thus, the mobile anion may have a onefold geometry.26 We assume that the IRAS band at 0.4 V on Pd(111) originates from the threefold sulfuric acid anion providing the x3 × x7 superstructure. The integrated band intensity at 0.4 V on Pd(111) (I0.4V,(111)) can be the measure of the maximum coverage of the threefold sulfuric acid anion on n(111)-(111) electrodes, because the existence of the step reduces the adsorption site for the threefold anion. The integrated intensity of the IRAS band on the surfaces with n more than 5 exceeds I0.4V,(111). Thus, the surfaces (41) Lei, H-.W.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 1996, 413, 131. (42) Holloway, S.; Korsknov, J. K. J. Electroanal. Chem. 1984, 161, 193. (43) Lambert, D. K. Phys. Rev. Lett. 1983, 50, 2106. (44) Person, B. N. J.; Ryberg, R. Phys. Rev. B 1981, 24, 6954. (45) Yoshinobu, J.; Tanaka, H.; Kawai, M. Phys. Rev. B 1995, 51, 4529.
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with n more than 5 may adsorb the mobile onefold anion as well as the threefold one. On the other hand, the maximum integrated intensity of Pd(331) (n ) 3) is lower than I0.4V,(111). This fact suggests that Pd(331) adsorbs only the threefold anion. Conclusion 1. Pd(S)-[n(111) × (111)] electrodes give a single IRAS band around 1200 cm-1, which is assigned to the SO stretching vibration of the sulfuric acid anion adsorbed with threefold or onefold geometry. 2. The maximum integrated intensity of the IRAS band is enhanced with the increase of the number of terrace atomic rows n, suggesting that the (111) terrace site is more likely to adsorb the sulfuric acid anion than the step. 3. The integrated intensity of the IRAS band increases at positive potentials. 4. The shift of the IRAS band (dv/dE) does not depend on the crystal orientation at the surfaces with the terrace atomic rows more than 3. Acknowledgment. This study was supported by a grant-in-aid of scientific research from Ministry of Education and Science of Japan 14550785 and Asahi Glass Foundation. LA036149G
