Mechanism of Selective Oxygen Reduction on Platinum by 2, 2

Mechanism of selective oxygen reduction on platinum by 2,2'-bipyridine in the presence of methanol has been investigated by in situ surface-enhanced i...
0 downloads 0 Views 408KB Size
Download PDF
Langmuir 2005, 21, 3037-3043

3037

Mechanism of Selective Oxygen Reduction on Platinum by 2,2′-Bipyridine in the Presence of Methanol Hidenobu Shiroishi, Yusuke Ayato, and Tatsuhiro Okada* National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8565, Japan

Keiji Kunimatsu Clean Energy Research Center, University of Yamanashi, Kofu 400-8510, Japan Received November 24, 2004. In Final Form: January 12, 2005 Mechanism of selective oxygen reduction on platinum by 2,2′-bipyridine in the presence of methanol has been investigated by in situ surface-enhanced infrared absorption spectroscopy. The addition of 2,2′bipyridine caused the decrease of adsorbed water molecules and those existing near the surface of platinum. The formation of both CO and formate, the latter being the intermediate in the non-CO path for methanol oxidation, depressed in the presence of 2,2′-bipyridine, suggests that 2,2′-bipyridine hinders methanol oxidation via both non-CO and CO paths on platinum. The geometrical effect of 2,2′-bipyridine adsorbed onto platinum was also investigated by multisite Monte Carlo simulation. It is indicated that selective oxygen reduction is caused by the difference in the number of required adsorption sites between methanol and dioxygen molecules. The suppression of Pt oxide species by 2,2′-bipyridine is found to be another factor that enhances the oxygen reduction.

1. Introduction Direct fuel cells are promising power sources for portable devices because of their system simplicity. Although various organic compounds were employed as fuels,1-6 methanol has various advantages of having a high energy density per unit volume, and being liquid at room temperature makes it easy to handle. However, direct methanol fuel cells (DMFCs) presently suffer from high overvoltage of the methanol oxidation reaction and methanol crossover through an electrolyte membrane. The latter causes not only the negative shift of the cathode potential but also useless consumption of methanol.7-10 Although polyperfluorosulfonic acid membranes are normally employed as the polymer electrolyte in DMFCs because of their thermal and chemical stabilities and excellent ionic conductivity, these membranes also have high methanol permeability. The methanol crossover occurs by methanol diffusion resulting from the concentration gradient through the polymer electrolyte and the electro-osmotic drag. To improve the energy * To whom correspondence may be addressed: telephone, + 81 29 861 4464; fax, +81 29 861 4678; e-mail, [email protected]. (1) Vigier, F.; Coutanceau, C.; Perrard, A.; Belgsir, E. M.; Lamy, C. J. Appl. Electrochem. 2004, 34, 439. (2) Kobayashi, T.; Otomo, J.; Wen, C.; Takahashi, H. J. Power Sources 2003, 124, 34. (3) Fujiwara, N.; Yasuda, K.; Ioroi, T.; Siroma, Z.; Miyazaki, Y.; Kobayashi, T. Electrochem. Solid-State Lett. 2003, 6, A257. (4) Tayhas, G.; Palmore, R. Trends Biotechnol. 2004, 22, 99. (5) Kariya, N.; Fukuoka, A.; Ichikawa, M. Chem. Commun. 2003, 690. (6) Yamada, K.; Asazawa, K.; Yasuda, K.; Ioroi, T.; Tanaka, H.; Miyazaki, Y.; Kobayashi, T. J. Power Sources 2003, 115, 236. (7) Ku¨ver, A.; Vogel, I.; Vielstich, W. J. Power Sources 1994, 52, 77. (8) Ravikumar, M. K.; Shukla, A. K. J. Electrochem. Soc. 1996, 143, 2601. (9) Ren, X.; Springer, T. E.; Gottesfeld, S. J. Electrochem. Soc. 2000, 147, 92. (10) Gattrell, M.; MacDougall, B. In Handbook of Fuel CellsFundamentals, Technology and Applications; Vielstich, W., Gasteiger, H. A., Lamm, A., Eds.; Wiley: New York, 2003; Vol. 2.

density per unit volume of DMFC devices, the feeding methanol concentration should be increased, but this causes performance degradation of the cathode. Many studies have been performed to overcome the issue in two ways. Developing new polymer electrolytes is one of the most promising ways;11,12 however it is still difficult to obtain a high proton-conductive polymer membrane with low methanol permeability and high mechanical, chemical, and thermal stabilities because many physical properties of methanol are similar to water. The other is to develop methanol-tolerant oxygen reduction catalysts such as platinum nickel alloy,13 transition metal chalcogenide,14,15 and heat-treated porphyrins.16,17 Recently, we reported the selective oxygen reduction on platinum in the presence of methanol using some additives that have a pyridyl structure.18 Fortunately the migration of these additives from the cathode to the anode was negligibly small so that no interference to the anode catalyst layer was anticipated when they were used at the cathode side of fuel cells. This approach might be one of the effective solutions to reduce the negative potential shift of the cathode due to the methanol crossover effect of DMFC. In this paper, mechanism of selective oxygen reduction on platinum by 2,2′-bipyridine has been investigated by a spectroscopic analysis of the electrode surface using in (11) Fedkin, M. V.; Zhou, X.; Hoffman, M. A.; Chalkova, E.; Weston, J. A.; Allcock, H. R.; Lvov, S. N. Mater. Lett. 2002, 52, 192. (12) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. B 2000, 38, 3277. (13) Drillet, J.-F.; Ee, A.; Friedemann, J.; Ko¨tz, R.; Schnyder, B.; Schmidt, V. M. Electrochim. Acta 2002, 47, 1983. (14) Alonso-Vante, N.; Bogdanoff, P.; Tributsch, H. J. Catal. 2000, 190, 240. (15) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Alonso-Vante, N.; Behm, R. J. J. Electrochem. Soc. 2000, 147, 2620. (16) Jiang, R.; Chu, D. J. Electrochem. Soc. 2000, 147, 4605. (17) Gupta, S.; Tryk, D.; Zecevi, K.; Aldred, W.; Guo, D.; Savinell, R. F. J. Appl. Electrochem. 1998, 28, 673. (18) Shiroishi, H.; Ayato, Y.; Kunimatsu, K.; Okada, T. Chem. Lett. 2004, 33, 792.

10.1021/la0471184 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005

3038

Langmuir, Vol. 21, No. 7, 2005

Shiroishi et al.

situ surface-enhanced infrared absorption spectroscopy (SEIRA). Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is superior to conventional infrared reflection absorption spectroscopy (IRRAS) for the observation of bands derived from adsorbed species because IR beam does not pass the solution phase.19,20 The geometrical effect of 2,2′-bipyridine adsorbed on platinum surface is also discussed by multisite Monte Carlo simulation. 2. Experimental Section 2.1. In Situ Surface-Enhanced Infrared Absorption Spectroscopy. Details of ATR-SEIRA spectroscopy are described in the literature.19-23 A thin Pt film was fabricated on the base plane of a hemicylindrical silicon prism (Nippon Pastec Co., Ltd., radius 12.5 mm, length 25 mm) as follows. Palladium was first deposited on the base plane with 0.5% HF-1 mM PdCl2 for 5 min at room temperature. After the surface was rinsed with water, platinum was deposited on the base plane by contacting with the Pt plating solution at 333 K for a few minutes. The silicon prism was mounted in a spectroelectrochemical cell with a reversible hydrogen reference electrode and a spiral platinum wire counter electrode. The cell was then placed in a homemade reflection optics at an angle of incidence of 65° as shown in the literature.23 In situ SEIRA measurements were performed with a Fourier transform infrared spectrometer equipped with a MCT detector (Digilab FTS7000). The chemically deposited Pt surface was cleaned by cycling potential between 0.05 and 1.5 V before the measurements. Acquisition time in the SEIRA measurement was about 30 s per spectrum. After spectra were measured at each potential in 0.1 M HClO4, 2,2′-bipyridine (abbreviated to 2,2′bpy) was added to the spectroelectrochemical cell. The potential of the working electrode was maintained at 0.1 V for 30 min, and then the FT-IR measurements were conducted. Finally, methanol was added to the cell, and the cell was permitted to stand at 0.1 V for 30 min before the FT-IR measurements. All potentials shown in this paper are referred to the reversible hydrogen electrode (RHE).

3. Results and Discussion In situ spectroelectrochemical measurements Figure 1 shows the polarization curves of oxygen reduction in the absence and presence of methanol in 0.1 M HClO4 using a rotating ring disk electrode (RRDE) measurement. Also compared are the polarization curves in the same condition but with a 2,2′-bpy additive. In the absence of methanol, the onset potential where oxygen reduction current rose was improved by 0.026 V compared to that without bpy, as was observed in sulfuric acid.18 This is because 2,2′-bpy molecules adsorbed on platinum interfere with the formation of platinum oxide species. Without 2,2′-bpy, the overpotential of oxygen reduction increased ca. 0.2 V in the presence of 0.1 M methanol, but with 2,2′-bpy this was improved to 0.06 V. Figure 2 shows a series of ATR-SEIRA spectra which is referenced by those measured at the same applied potential without methanol and additives in 0.1 M HClO4. In the absence of methanol, the spectra of the Pt surface in the water band region obtained by SEIRAS were similar to that of Au surface by SEIRAS21 rather than that by IRRAS,24-26 except for the coverage tendency of water (19) Osawa, M.; Ataka, K.; Yoshii, K.; Yotsuyanagi, T. J. Electron Spectrosc. Relat. Phenom. 1993, 64/65, 371. (20) Osawa, M.; Yoshii, K.; Ataka, K.; Yotsuyanagi, T. Langmuir 1994, 10, 640. (21) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (22) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680. (23) Ayato, Y.; Kunimatsu, K.; Osawa, M.; Okada, T. Phys. Chem. Chem. Phys., submitted.

Figure 1. Polarization curves for oxygen reduction in 0.1 M CH3OH-0.1 M HClO4. Scan rate is 5 mV s-1. Rotating speed of the disk electrode is 300 rpm. (a) 0 M MeOH-0.1 mM bpy; (b) 0.1 M MeOH-0.1 mM bpy; (c) 1 M MeOH-0.1 mM bpy; (d) 0 M MeOH-0 M bpy; (e) 0.1 M MeOH-0 M bpy; (f) 1 M MeOH-0 M bpy.

molecules.24 Hence we assigned the OH stretching bands mainly based on the result of the Au surface by SEIRAS.21 The addition of 2,2′-bpy caused decreasing intensities of OH stretching oscillators around 3520 cm-1 which consist of OH stretching modes for hydrogen-bonded water and oxonium ion.21,24 Also the decrease of HOH bending oscillators around 1630 cm-1 occurred in the region of 0.1-1.1 V, suggesting that adsorbed water molecules and those existing near the surface of platinum were released from the surface. The intensity of the band around 1120 cm-1 attributed to a Cl-O stretching mode of perchlorate accumulated on the Pt surface with 2,2′-bpy was larger than that without 2,2′-bpy except around 0.4 V where a potential of zero charge on Pt(111) exists.25 Although it might seem strange that the band intensity increased with decreasing applied potential below 0.4 V in the presence of 2,2′-bpy, this occurs because perchlorate ion can easily be close to the surface due to the elimination of water molecules near the surface which were accumulated with decreasing potential without 2,2′-bpy.24 After addition of methanol, the intensity of OH stretching oscillators increased a little in the whole potential range in contrast to that of perchlorate ion, whereas the band around 3670 cm-1 observed in the absence of 2,2′bpy, whose frequency was similar to that of monomeric H2O and non-hydrogen bonded OH, did not appear in the presence of 2,2′-bpy. The intensity of the band had a linear relationship with those of linear CO stretching and formate stretching in the absence of 2,2′-bpy. The band assigned to linear CO stretching consisted of two components, and its intensity decreased to 1.5% by 2,2′-bpy as compared to that without 2,2′-bpy at 0.5 V. A high-frequency component of the band disappeared above 0.7 V, whereas a low-frequency component was left up to 1.0 V, which would be derived from the CO molecules close to 2,2′-bpy. The peak position of bridged CO stretching oscillators also shifted to lower wavenumber. A band around 1320 cm-1 assigned to the symmetric OCO stretching mode of the formate species, which was recently reported by Osawa and co-workers as the intermediate of non-CO path for methanol oxidation,22 was not observed in the presence of 2,2′-bpy; therefore, it is concluded that 2,2′-bpy interferes (24) Hirota, K.; Song, M.-B.; Ito, M. Chem. Phys. Lett. 1996, 250, 335. (25) Iwashita, T.; Xia, X. J. Electroanal. Chem. 1996, 411, 95. (26) Habib, M. A., Bockris, J. O’M. Langmuir 1986, 2, 388.

Selective Oxygen Reduction on Pt by Bipyridine

Langmuir, Vol. 21, No. 7, 2005 3039

Figure 2. Series of SEIRA spectra of the Pt plating electrode in 0.1 M HClO4 at various potentials under N2 (‚ ‚ ‚) in 0.1 M CH3OH-0 mM bpy, (- - -) in 0 M CH3OH-0.1 mM bpy, and (s) in 0.1 M CH3OH-0.1 mM bpy. (A) OH stretching band. Baselines were corrected from 4000 to 2400 cm-1 linearly. (B) CO stretching band of linear and bridged CO on Pt. Baselines were corrected at 2400 cm-1 as zero at each potential. Figure 2. (C) HOH bending band, formate and perchlorate bands. Baselines were corrected at 2400 cm-1 as zero at each potential.

Figure 3. Dependence of integrated band intensity at symmetric ring stretch of 2,2′-bpy around 1440 cm-1: O, 0 M CH3OH; b, 0.1 M CH3OH.

Figure 4. Dependence of the fraction of platinum sites on which 2,2′-bpy molecules were adsorbed on the concentration of 2,2′-bpy in 0.1 M perchloric acid.

with the formation of intermediates and eventually suppresses methanol oxidation on platinum. The bands derived from 2,2′-bpy molecules were observed around 3100 cm-1 (C-H stretching), 1440 cm-1 (symmetric ring stretching), and 1535 cm-1 (asymmetric ring stretching).27 Figure 3 shows the dependence of the band intensity around 1440 cm-1 on applied potential, which had the strongest intensity among the three bands. Since the band intensities were almost unchanged in this potential region both in 0 M and in 0.1 M methanol, we could confirm that bpy molecules adsorbed onto platinum were neither removed nor exchanged for methanol in the potential region. Thus we could estimate the fraction of platinum sites occupied by bpy using hydrogen adsorption/ desorption region in cyclic voltammograms.

Figure 4 shows the dependence of the fraction of platinum sites occupied by 2,2′-bpy, xbpy, on the concentration of 2,2′-bpy in 0.1 M HClO4. The xbpy increased with increasing concentration, and reached plateau at 1 mM, where 36% of the platinum sites can adsorb hydrogen atom. We call a “Free site” a Pt site that is not occupied by 2,2′-bpy and define xfree as the fraction of “Free site” through discussion

(27) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acta 1983, 39A, 97.

xfree ) 1 - xbpy

(1)

We normalized methanol oxidation currents that were measured in the absence of oxygen gas, by xfree, to estimate the methanol oxidation activity per a “Free site”. Figure 5 depicts the plots for dependence of normalized methanol oxidation current on the fraction of 2,2′-bpy sites. If methanol oxidation activity per one Pt site does not change, current normalized by “Free sites” would be independent

3040

Langmuir, Vol. 21, No. 7, 2005

Shiroishi et al. k1

k2

CH3OHsol 98 CH3OHads 98 *CtO| + 4H+ + 4e- (2)

Figure 5. Relationship between methanol oxidation current normalized by “Free area” and the fraction of platinum sites on which 2,2′-bpy molecules were adsorbed at various applied potentials: O, 0.60 V; b, 0.65 V; 0, 0.70 V; 9, 0.75 V; 4, 0.80V.

of fraction of 2,2′-bpy sites because methanol oxidation is not a diffusion-controlled reaction on platinum. However, the normalized current decreased with increasing fraction of 2,2′-bpy sites indicating that methanol oxidation activity per a “Free site” decreased with increasing fraction of 2,2′-bpy sites. There are two possible reasons why 2,2′-bpy reduces methanol oxidation on platinum rather than oxygen reduction. One is the geometrical effect of 2,2′-bpy adsorbed on platinum, since a methanol molecule is larger than a dioxygen molecule, and the other reason is an electronic effect by 2,2′-bpy to platinum orbital. We performed a simple Monte Carlo simulation to study the geometric effect of 2,2′-bpy adsorbed on Pt surface. Multisite Monte Carlo Simulation. Monte Carlo simulation has been applied to many studies on adsorption in various systems.28 Dynamic Monte Carlo simulations have been used to describe kinetics of CO electrooxidation.29-31 In these simulations, it was assumed that one adsorbed species occupied only one adsorption site. Monte Carlo studies in which several adsorption sites were occupied by one molecule have been reported in recent years.32-34 Christov and Sundmacher adopted a threesite methanol model both in square and in hexagonal lattice to simulate methanol adsorption on Pt/Ru.35 Methanol oxidation was extensively investigated in the last 4 decades.36-40 The first process of methanol oxidation is the adsorption from the bulk solution onto platinum, which is followed by successive dehydrogenation of methanol to form linear and bridged CO:41 (28) Steele, W. Appl. Surf. Sci. 2002, 196, 3. (29) Petukhov, A. V.; Akemann, W.; Friedrich, K. A., Stimming, U. Surf. Sci. 1998, 402-404, 182. (30) Koper, M. T. M.; Jansen, A. P. J.; Santen, R. A.; Lukkien, J. J.; Hilbers, A. J. J. Chem. Phys., 1998, 109, 6051. (31) Saravanan, C.; Markovic´, N. M.; Head-Gordon, M.; Ross, P. N. J. Chem. Phys. 2001, 114, 6404. (32) Rosenstock, Z.; Riess, I. J. Catal. 2001, 201, 286. (33) Budinski-Petkovic´, Lj.; Kozmidis-Luburic´, U. Physica A 2001, 301, 174. (34) Barone, G.; Duca, D. J. Catal. 2002, 211, 296. (35) Christov, M.; Sundmacher, K. Surf. Sci. 2003, 547, 1. (36) Bagotzky, V. S.; Vassiliev, Yu. B. Electrochim. Acta 1967, 12, 1323. (37) Iwashita, T. Electrochim. Acta 2002, 47, 3663. (38) Wasmus, S.; Ku¨ver, A. J. Electroanal. Chem. 1999, 461, 14. (39) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (40) Kunimatsu, K.; Kita, H. J. Electroanal. Chem. 1987, 218, 155. (41) Gasteiger, H. A.; Markovic´, N. M.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem., 1993, 97, 12020.

where *CtO| expresses methanol dehydrogenation fragments bonded to the Pt surface. It is widely accepted that methanol adsorption requires more than two platinum sites. Several researchers postulated that three platinum sites were needed to adsorb methanol,35,36,41 whereas Parsons and VanderNoot suggested that four platinum sites are required to adsorb methanol.39 Gasteiger et al. described that they adopted a three-site methanol model in their statistical calculation by which they estimated the optimum ratio between platinum and ruthenium for methanol oxidation because there were found no significant differences among the number of methanol adsorption sites in their caluculation.41 The number of platinum sites required for methanol adsorption is still open for discussion, thus we performed Monte Carlo simulation of methanol adsorption using a two-dimensional square lattice model for Pt(100) and a hexagonal lattice model for Pt(111) assuming that one methanol occupied two to four sites (Figure 6). Hubbard and co-workers studied the conformation of bipyridyls adsorbed on platinum at -0.2 V vs Ag|AgCl (1 M KCl) by electron energy-loss spectroscopy (EELS) and Auger spectroscopy (AES).42 They revealed that the maximum amount of 2,2′-bpy adsorbed on platinum was half as large as that of 4,4′-bpy. On the basis of EELS, they concluded that 2,2′-bpy takes a fixed noncoplanar conformation, and one of the pyridine rings was vertically oriented (Figure 7, left). On the basis of their results we adopted the multisite adsorption models of 2,2′-bpy shown in Figure 7 and their rotated patterns into the simulation, where two kinds of adsorption sites exist. One is the site completely covered with 2,2′-bpy (gray sites in Figure 7), and the other is the site partially covered with 2,2′-bpy (hatching sites) and is able to accept a hydrogen atom. Since the object of this simulation is to estimate the geometrical influence of 2,2′-bpy on methanol adsorption, we made the following simple assumptions: 1. Adsorbed species neither change nor diffuse on the surface of platinum, which is close to the assumption called “molecular adsorption frozen in disorder” as was adopted in the simulation of methanol adsorption on Pt/Ru alloys.35 2. 2,2′-bpy molecules on the surface are randomly dispersed and there are no interaction between them because the interaction between 2,2′-bpy molecules on platinum is weak due to its conformation. 3. Methanol molecules are not adsorbed on the sites where a 2,2′-bpy molecule is already adsorbed, and no interaction between methanol and 2,2′-bpy is considered. Procedures used for the simulation of the adsorption process are as follows, both in the square lattice and the hexagonal lattice whose sizes are 1000 × 1000 with periodic boundary condition: 1. Trials of a specific number of 2,2′-bpy additions are performed in the lattice. A specific angle is chosen randomly from the all-available angles to put a 2,2′-bpy molecule at a randomly selected site. If no available angle exists at the site, nothing occurs. 2. Twofold methanol adsorption was performed until the lattice was saturated with methanol. After available angles are sought at a randomly selected site, a methanol molecule is put at a randomly chosen angle in the similar manner as above. If there is no available angle to put, nothing occurs. (42) Chaffins, S. A.; Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T. Langmuir 1990, 6, 957.

Selective Oxygen Reduction on Pt by Bipyridine

Langmuir, Vol. 21, No. 7, 2005 3041

Figure 6. Methanol adsorption models used in the simulation for (A) two-dimensional hexagonal lattice and (B) twodimensional square lattice site models.

Figure 8. Relationship between the fraction of platinum sites adsorbing methanol molecules (xMeOH) and that adsorbing 2,2′bipyridine (xbpy) with (A) two-dimensional hexagonal lattice and (B) two-dimensional square lattice site models: O, two site model; b, three site model; ], four site model; 9, xMeOH experimentally obtained by the hydrogen adsorption/desorption region in cyclic voltammograms of a polycrystalline platinum electrode in 0.1 M HClO4.

Figure 7. Adsorption models of 2,2′-bpy used in the simulation for (A) two-dimensional hexagonal lattice and (B) twodimensional square lattice site models. Gray sites are completely covered with 2,2′-bpy. A hatching site is partially occupied by 2,2′-bpy and can accept a hydrogen atom.

3. Similarly, 3-fold and 4-fold methanol adsorptions were performed respectively after methanol molecules were removed from the lattice. 4. Methanol molecules were erased from the lattice, and the above steps 1-3 were repeated until the lattice was filled with 2,2′-bpy molecules. Figure 8 depicts the dependence of the fraction of platinum sites adsorbing methanol (xMeOH) on that occupied by 2,2′-bpy at various methanol models using the simulation. The simulation with a four-site methanol model reproduced the relationship actually obtained by the hydrogen adsorption/desorption region in cyclic voltammograms of a polycrystalline platinum electrode in 0.1 M HClO4 both in hexagonal and in square lattice site models, as also shown in Figure 8. Figure 9 shows the dependence of the fraction of methanol adsorption sites normalized by the fraction of “Free sites” on the fraction of the 2,2′-bpy adsorption sites to estimate the number of methanol molecules per a Pt free site. The values are comparable with the normalized

current at any potentials shown in Figure 5, although in strict sense kinetic Monte Carlo simulations are required to compare the values with the normalized current. Simulation with the four-site methanol model nicely reproduces the tendency that the normalized current approaches 0 along with the fraction of 2,2′-bpy sites both in square lattice and in hexagonal lattice models. Extensive studies on oxygen reduction have also been performed during four decades and well reviewed in the literature.43,44 The mechanism of oxygen reduction on platinum is being revealed by DFT studies in recent years.45 According to their studies, oxygen reduction occurs on platinum through two overall processes. One is the two-electron reduction to peroxide, which takes place at a single platinum site with an end-on oxygen molecule. The other is four-electron reduction to water at a dual site with a di-σ bonded oxygen.46 It can be assumed that oxygen molecules are promptly reduced on platinum not to interfere with the adsorption of “next” oxygen molecules below 0.9 V, thus four-electron oxygen reduction can occur with at least two free nearest neighbor sites unlike methanol oxidation. In Figure 10, the selectivity between (43) Ross, P. N., Jr. In Handbook of Fuel Cells-Fundamentals, Technology and Applications; Vielstich, W., Gasteiger, H. A., Lamm, A., Eds.; Wiley: New York, 2003; Vol. 2. (44) Appleby, A. J. J. Electroanal. Chem. 1993, 357, 117. (45) Sidik, R. A.; Anderson, A. B., J. Electroanal. Chem. 2002, 528, 69. (46) Adzic, R. R.; Wang, J. X. J. Phys. B 1998, 102, 8988.

3042

Langmuir, Vol. 21, No. 7, 2005

Figure 9. Dependence of the fraction of methanol adsorption sites normalized by the fraction of “Free sites” (left axis) on the fraction of the platinum sites on which bpy molecules were adsorbed with (A) two-dimensional hexagonal lattice and (B) two-dimensional square lattice site models: O, two site model; b, three site model; ], four site model. Relationships between experimental methanol oxidation current normalized by “Free sites” and the fraction of platinum sites on which 2,2′-bpy molecules were adsorbed at 0.75 V are also superposed (right axis).

Shiroishi et al.

Figure 11. Snapshots of adsorption simulation displayed only by 50 × 50 area for a two-dimensional hexagonal lattice model: (a) 2,2′-bpy model and (b) single site adsorbate model.

Figure 12. Relationship between oxygen reduction current normalized by “Free area” and the fraction of platinum sites on which 2,2′-bpy molecules were adsorbed at various applied potentials: O, 0.95 V; b, 1.00 V; 0, 1.05 V.

site adsorption model as an additive is also shown in Figure 10, which is expressed by the following equation

xO2/xMeOH ) x(2 sites)/x(4 sites) ) Figure 10. Dependence of xO2/xMeOH on the fraction of the platinum sites occupied by 2,2′-bpy with a four site methanol model: O, two-dimensional hexagonal lattice model; b, two-dimensional square lattice model. The line is calculated by eq 3.

oxygen reduction and methanol oxidation by geometrical effect is represented as the ratio between the simulated fraction of oxygen sites and that of the methanol sites as a function of xbpy. The selectivity toward oxygen reduction increased drastically above xbpy ) 0.5, suggesting that one of the reasons selective oxygen reduction occurs by the addition of 2,2′-bpy is due to the difference in the number of required adsorption sites between methanol and dioxygen molecules. The simple statistical relation of a single

(1 - xbpy)2/(1 - xbpy)4 ) (1 - xbpy)-2 (3) It is revealed that the geometrical selectivity by a single site model is higher than that by the multisite 2,2′-bpy model, since occupied sites are concentrated in the multisite model (Figure 11). However, the ratio increases with the increase in xbpy, and it reaches ca. 2 at xbpy ) 0.6, but this is not enough to explain the observed selectivity toward oxygen reduction against methanol oxidation as seen in Figure 1. This is because the turnover number per one site of oxygen reduction is higher than that of methanol oxidation, and the concentration of oxygen is much lower than that of methanol. Figure 12 depicts the plots for the dependence of oxygen reduction current normalized by the “Free site” on the

Selective Oxygen Reduction on Pt by Bipyridine

fraction of 2,2′-bpy sites in the absence of methanol, which is experimentally obtained by RRDE measurements in 0.1 M HClO4. The absolute value of the normalized current increases with the fraction of 2,2′-bpy sites, reaches a maximum around xbpy ) 0.6, and decreases sharply at xbpy > 0.6. The initial increase is due to the suppression of the formation of Pt oxide species by 2,2′-bpy.18 Overall, reaction selectivity toward oxygen reduction as compared to methanol oxidation with 2,2′-bpy reached about six times as large as that without 2,2′-bpy. 4. Conclusion The addition of 2,2′-bpy caused the decrease of the intensities of OH stretching and HOH bending oscillators and the increase in perchlorate ion adsorption near the interface, suggesting that adsorbed water molecules at the interface were expelled from the surface. After addition of methanol, perchlorate ion near the interface was replaced by the water molecules. The

Langmuir, Vol. 21, No. 7, 2005 3043

formation of intermediates for methanol oxidation was suppressed by 2,2′-bpy adsorbed on platinum. According to the multisite Monte Carlo simulation, the four-site methanol model nicely reproduced the relationship between the normalized current and the fraction of 2,2′-bpy sites both in square lattice and in hexagonal lattice models. It is concluded that the observed selectivity toward oxygen reduction against methanol oxidation is derived from both the geometrical effect and the suppression of platinum oxide species by 2,2′-bpy. Acknowledgment. The authors gratefully acknowledge Professor M. Osawa and Mr. A. Miki at Hokkaido University for kindly providing the ATR-SEIRAS developed by them. H. Shiroishi wishes to acknowledge the NEDO fellowship program for its financial support. LA0471184