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Langmuir 2008, 24, 3590-3601
Adsorption/Oxidation of CO on Highly Dispersed Pt Catalyst Studied by Combined Electrochemical and ATR-FTIRAS Methods: Oxidation of CO Adsorbed on Carbon-Supported Pt Catalyst and Unsupported Pt Black Keiji Kunimatsu,† Takako Sato,† Hiroyuki Uchida,‡ and Masahiro Watanabe*,† Clean Energy Research Center and Interdisciplinary Graduate School of Medicine and Engineering, UniVersity of Yamanashi, 4 Takeda, Kofu 400-8510, Japan ReceiVed June 13, 2007. In Final Form: December 15, 2007 ATR-FTIRAS measurements combined with linear potential sweep voltammetry were conducted to investigate oxidation of CO adsorbed on a highly dispersed Pt catalyst supported on carbon black, Pt/C, and carbon-unsupported Pt black catalyst, Pt-B. Bands ν(CO) of atop- and bridge-bonded COs were resolved into those of COs adsorbed at terrace and step edge sites by curve-fitting analysis. At the high coverage near the saturation, a band around 1950-1960 cm-1 assigned to asymmetric bridge-bonded CO, CO(B)asym, was observed to develop on both Pt/C and Pt-B, which was the predominant type on the latter. Preferential oxidation of atop-CO adsorbed at the step edge site was commonly observed on both Pt/C and Pt-B during the potential sweep from 0.05 to 1.2 V. However, it has been found that CO(B)asym is the most reactive species. The high reactivity of the CO(B)asym on Pt/C and Pt-B is demonstrated for the first time in the present report. Adsorption of CO on the Pt/C and Pt-B resulted in growth of a sharp ν(OH) band around 3642-3645 cm-1 which is assigned to non-hydrogen-bonded water molecules coadsorbed with CO. The ν(OH) band frequency exhibits a linear increase with potential with a Stark tuning rate of ca. 20 cm-1/V. Analysis of the potential dependence of this band in the CO oxidation potential region led us to conclude that this is the oxygen-containing species to oxidize adsorbed CO. Stark tuning rates of ν(CO) bands for the COs at the terrace and step edge sites on both Pt/C and Pt-B are almost independent of the adsorption sites for both atop- and bridge-bonded COs. However, CO(B)asym exhibits tuning rates of 41 cm-1/V and 37 cm-1/ V on Pt/C and Pt-B, respectively, which is in between the rates of atop and symmetric bridge-bonded COs.
1. Introduction Adsorption and oxidation of CO on Pt electrodes has been extensively studied due to its fundamental importance to the electrochemical surface science and the development of COtolerant anode catalysts for polymer electrolyte fuel cells.1-5 The studies have been accelerated by the use of single-crystal electrodes coupled with new methods for in situ surface analysis such as IRAS,6,7 SFG,8,9 ECSTM,10,11 and SXS.12 In particular, studies by combined ECSTM and IRAS studies10,13-15 have * To whom correspondence should be addressed. E-mail: m-watanabe@ yamanashi.ac.jp. † Clean Energy Research Center. ‡ Interdisciplinary Graduate School of Medicine and Engineering. (1) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G., II; Philpott, M. R. Surf. Sci. 1985, 158, 596. (2) Beden, B.; Lamy, C.; De Tacconi, N. R.; Arvia, A. J. Electrochim. Acta 1990, 35, 691. (3) Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (4) Markovic, N. M.; Ross, P. N., Jr. Surf. Sci. Rep. 2002, 45, 117. (5) (a) Cuesta, A.; Couto, A.; Rinco´n, A.; Pe´rez, M. C.; Lo´pez-Cudero, A.; Gutie´rrez, A. C. J. Electroanal. Chem. 2006, 586, 184. (b) Lopez-Cudero, A.; Cuesta, A.; Gutierrez, C. J. Electroanal. Chem. 2006, 586, 204. (c) Lopez-Cudero, A.; Cuesta, A.; Gutierrez, C. J. Electroanal. Chem. 2005, 579, 1. (6) Nichols, R. J. Adsorption of Molecules at Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; Chapter 7. (7) Furuya, N.; Motoo, S.; Kunimatsu, K. J. Electroanal. Chem. 1988, 239, 347. (8) Tadjeddine, A.; Peremans, A.; Guyot-Sionnest, P. Surf. Sci. 1995, 335, 210. (9) Lu, G.-Q.; White, J. O.; Wieckowski, A. Surf. Sci. 2004, 564, 131. (10) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (11) Wakisaka, M.; Ohkanda, T.; Yoneyama, T.; Uchida, H.; Watanabe, M. Chem. Commun. 2005, 21, 2710. (12) Lucas, C. A.; Markovic´, N. M.; Ross, P. N. Surf. Sci. 1999, 425, L381. (13) Severson, M. W.; Stuhlmann, C.; Villegas, I.; Weaver, M. J. J. Chem. Phys. 1995, 103, 9832. (14) Yoshimi, K.; Song, M. B.; Ito, M. Surf. Sci. 1996, 368, 389.
correlated the observed ν(CO) spectra of adsorbed COs to the adlayer structure derived from the STM images. Recently, the studies have been extended to the single-crystal Pt16-28 surfaces with a controlled density of step edges and terrace width. Preferential initial adsorption of CO at step edge sites,17,18,21-23 higher reactivity for CO oxidation on surfaces with higher step density,24-27 and the role of the step sites to (15) Batista, E. A.; Iwasita, T.; Vielstich, W. J. Phys. Chem. B 2004, 108, 14216. (16) Rodes, A.; Gomez, R.; Feliu, J. M.; Weaver, M. J. Langmuir 2000, 16, 811. (17) Greenler, R. G.; Burch, K. D.; Kretzschmar, K.; Klauser, R.; Bradshaw, A. M.; Hayden, B. E. Surf. Sci. 1985, 152/153, 338. (18) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf. Sci. 1985, 149, 394. (19) Motoo, S.; Furuya, N. Ber. Bunsen-Ges. 1987, 91, 457. (20) (a) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (b) Orts, J. M.; Feliu, J. M.; Aldaz, A.; Clavilier, J.; Rodes, A. J. Electroanal. Chem. 1990, 281, 199. (c) Clavilier, J.; El Achi, K.; Rodes, A. Chem. Phys. 1990, 141, 1. (21) Watanabe, S.; Inukai, J.; Ito, M. Surf. Sci. 1993, 293, 1. (22) (a) Yates, J. T., Jr. J. Vac. Sci. Technol., A: 1995, 13, 1359. (b) Xu, J.; Yates, J. T., Jr. Surf. Sci. 1995, 327, 193. (c) Xu, J.; Henriksen, P.; Yates, J. T., Jr. J. Chem. Phys. 1992, 97, 5250. (d) Xu, J.; Yates, J. T., Jr. J. Chem. Phys. 1993, 99, 725. (23) (a) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484. (b) Kim, C. S.; Korzeniewski, C.; Tornquist, W. J. J. Chem. Phys. 1994, 100, 628. (c) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Chem. Phys. 1994, 101, 9113. (d) Kim, C. S.; Korzeniewski, C. Anal. Chem. 1997, 69, 2349. (e) Korzeniewski, C.; Severson, M. W. Spectrochim. Acta, Part A 1995, 51A, 499. (24) Akemann, W.; Friedrich, K. A.; Linke, U.; Stimming, U. Surf. Sci. 1998, 402-404, 571. (25) Lebedeva, N. P.; Koper, M. T. M.; Herrerro, E.; Feliu, J. M.; Van Santen, R. A. J. Electroanal. Chem. 2000, 487, 37. (26) Lebedeva, N. P.; Rodes, A.; Feliu, J. M.; Koper, M. T. M.; Van Santen, R. A. J. Phys. Chem. B 2002, 106, 9863. (27) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; Van Santen, R. A. J. Phys. Chem. B 2002, 106, 12938. (28) Hoshi, N.; Sakurada, A.; Nakamura, S.; Teruya, S.; Koga, O.; Hori, Y. J. Phys. Chem. B 2002, 106, 1985.
10.1021/la702441x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/21/2008
Adsorption/Oxidation of CO on Pt Catalyst
adsorb an oxygen-containing species to oxidize CO from terraces22,25-27 have been generally recognized in these studies. Understanding of the role of these defect sites in CO adsorption and its oxidation has been developed on the basis of the studies on the model platinum surfaces. Highly dispersed Pt nanoparticles supported on carbon used as a practical fuel cell catalyst consist of integrated nanosized terraces and steps. Extending the concepts and understandings developed through the studies on the model platinum surfaces to practical fuel cell catalysts is an important step to the next stage. In the past several years, in situ infrared spectroscopic characterization of CO on highly dispersed platinum nanocatalysts has been developed along this line.29-38 In such studies, the Pt nanoparticles are supported on a gold or graphite33,35 reflecting substrate to conduct IRAS measurements. However, the technique has suffered from a well-known problem of band anomalies, which makes it difficult to conduct quantitative analysis of band intensities as well as band positions of CO adsorbed on the catalysts. Such anomalies have been minimized and controlled by keeping the thickness of the catalyst layer as thin as possible.29-32,34 In our previous report,39 we conducted in situ ATR-FTIRAS characterization of COs adsorbed on a highly dispersed Pt/C as well as carbon-unsupported Pt black catalysts. Spectra of adsorbed COs on these Pt/C and Pt black catalysts free from the band anomalies were successfully observed by the ATR method. Simultaneous adsorption of COs at terrace and step-edge sites and development of a new band around 1950 cm-1 assigned to asymmetric bridge-bonded CO, CO(B)asym, adsorbed preferentially on (100) terraces were the new findings on these catalysts. Intensity of the CO(B)asym is significantly higher on a carbonunsupported Pt-B catalyst than Pt/C. Growth of a sharp ν(OH) band of non-hydrogen-bonded water molecules around 3650 cm-1 and concomitant development of the band around 3500 cm-1 was clearly observed during CO adsorption on the Pt/C and Pt-B at the saturation coverage. The band around 3650 cm-1 assigned to non-hydrogen-bonded water coadsorbed with CO has been reported in several recent reports on chemically deposited Pt;40,41 electrodeposited Pt, Pd, Ru, and Rh;42 Pd-modified Au;43 (29) Park, S.; Tong, Yu Ye; Wieckowski, A.; Weaver, M. J. Electrochem. Commun. 2001, 3, 509. (30) (a) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 9719. (b) Park, S.; Weaver, M. J. J. Phys. Chem. B 2002, 106, 8667. (c) Park, S.; Wasileski, S. A.; Weaver, M. J. Electrochim. Acta 2002, 47, 3611. (31) (a) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Colloid Surf. 1998, 134, 193. (b) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Electrochim. Acta 2001, 47, 689. (c) Friedrich, K. A.; Geyzers, K. P.; Dickinson, A. J.; Stimming, U. J. Electroanal. Chem. 2002, 524-525, 261. (d) Maillard, F.; Savinova, E. R.; Simonov, P. A.; Zaikovskii, V. I.; Stimming, U. J. Phys. Chem. B 2004, 108, 17893. (32) (a) Arenz, M.; Stamenkovic, V.; Blizanac, B. B.; Mayrhofer, K. J.; Markovic, N. M.; Ross, P. N. J. Catal. 2005, 232, 402. (b) Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tada, T.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2005, 127, 6819. (c) Mayrhofer, K. J. J.; Arenz, M.; Blizanac, B. B.; Stamenkovic, V.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2005, 50, 5144. (d) Stamenkovic, V.; Arenz, M.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2004, 108, 17915. (33) (a) Lu, G.-Q.; Sun, S.-C.; Chen, S.-P.; Cai, L.-R. J. Electroanal. Chem. 1997, 421, 19. (b) Lu, G.-Q.; Sun, S.-G.; Cai, L.-R.; Chen, S.-P.; Tian, Z.-W.; Shiu, K.-K. Langmuir 2000, 16, 778. (34) Rice, C.; Tong, Y. Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger, J-M.; Lamy, C. J. Phys. Chem. B 2000, 104, 5803. (35) (a) Christensen, P. A.; Hamnett, A.; Munk, J.; Troughton, J. L. J. Electroanal. Chem. 1994, 370, 251. (b) Munk, J.; Christensen, P. A.; Hamnett, A.; Skou, E. J. Electroanal. Chem. 1996, 401, 215. (36) Zhu, Y.; Uchida, H.; Watanabe, M. Langmuir 1999, 15, 8757. (37) Ortiz, R.; Cuesta, A.; Ma´rquez, O. P.; Ma´rquez, J.; Me´ndez, J. A.; Gutie´rrez, C. J. Electroanal. Chem. 1999, 465, 234. (38) Bo, A.; Sanicharane, S.; Sompalli, B.; Fau, Q.; Gurau, B.; Liu, R.; Smotkin, E. S. J. Phys. Chem. B 2000, 104, 7377. (39) Sato, T.; Kunimatsu, K.; Uchida, H.; Watanabe, M. Electrochim. Acta 2007, 53, 1265. (40) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500.
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and sputtered PtRu44,45 by ATR-FTIRAS. Its adsorption state on such electrodes and the role in CO oxidation has been discussed. In particular, the active role of the H2O species found on PtRu44,45 was interpreted in terms of the bifunctional mechanism46 for methanol oxidation on such a Pt-based bimetallic electrocatalyst. This is directly related to the issue concerning the nature of the oxygen-containing species, often described as adsorbed OH, postulated in the reactant pair or Langmuir-Hinshelwood-type reaction mechanism.2,22,46,47 However, the natures of the oxygencontaining species to oxidize CO and of a reaction intermediate other than adsorbed COs involved in the overall CO oxidation have been still elusive. The final goal of our series of works is to elucidate the relationship between CO tolerance and activity for electrochemical CO oxidation at Pt-based alloy catalysts. An aim of the present work is to conduct combined electrochemical and spectroscopic studies of the CO oxidation mechanism at Pt/C and Pt black catalysts. The central issues are elucidations of the role of terrace/ edge sites, reactivity of asymmetric bridge COs, and role of the water molecules with different degrees of hydrogen bonding with respect to CO oxidation. 2. Experimental Section We employed ATR-FT-IRAS for the in situ monitoring of the processes associated with CO adsorption on carbon-supported Pt fuel cell catalysts in 0.1 M HClO4. The experimental details have been described elsewhere39 and are not repeated in detail here. Carbonsupported or unsupported Pt electrocatalysts were dispersed on a gold film (ca. 100 nm thick) deposited chemically on the reflecting plane of a Si ATR prism of 20 × 25 mm in a hemicylindrical shape. The procedure of the chemical deposition of the Au film has been described elsewhere.39,48 50 wt % Pt /C catalyst with average Pt particle size of 2.6 nm (TEK10E50E, purchased from Tanaka Kikinzoku Kogyou) was loaded on the Au surface to have 10 µg Pt/cm2 by dropping an ethanol suspension containing the Pt/C catalyst. The catalyst layer was dried in air at room temperature under the presence of ethanol vapor, after which Nafion solution was dropped on the catalyst surface to make a 0.05 µm thick Nafion film. After drying in air, the film was subjected to heat treatment at 130 °C for half an hour. The high-resolution TEM image and particle size distribution of the Pt/C catalyst were reported in our previous work.39 ATR-FT-IRAS measurements were conducted also on a carbon unsupported Pt black catalyst, purchased from Japan Engelhard, to examine the CO oxidation mechanism on the catalyst which accommodates the predominant adsorbed CO species, asymmetric bridge-bonded CO on (100) terraces.39 The procedure to have catalyst loading of 10 µg Pt/cm2 on Au was similar to that on Pt/C. Measurements on the Pt black were conducted with a cast Nafion film. The average particle size of Pt-B estimated by XRD was ca. 10-17 nm. A crystal rubber O-ring, Valflon, made of perfluoro elastmers, of 14.8 mm inner diameter was used to fit the prism to the spectroelectrochemical cell with a flange at the cell front. The geometrical surface area defined by the O-ring is 1.72 cm2. The electrode surface was cleaned by repeated cycles of the electrode potential between 0.05 and 1.0 V vs RHE in 0.1 M HClO4. The reference electrode was a sealed-type reversible hydrogen electrode in 0.1 M HClO4 under 1atm H2 atmosphere, RHE, to which all the potentials are (41) Shiroishi, H.; Ayato, Y.; Kunimatsu, K.; Okada, T. J. Electroanal. Chem. 2005, 581, 132. (42) Yan, Y.-G.; Li, Q.-X.; Huo, S.-J.; Ma, M.; Cai, W.-B.; Osawa, M. J. Phys. Chem. B 2005, 109, 7900. (43) Pronkin, S.; Wandlowski, Th. Surf. Sci. 2004, 573, 109. (44) Yajima, T.; Wakabayashi, N.; Uchida, H.; Watanabe, M. Chem. Commun. 2003, 828. (45) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 2654. (46) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (47) Gilman, S. J. Phys. Chem. 1964, 68, 70. (48) Miyake, H.; Ye, Shen; Osawa, M. Electrochem. Commun. 2002, 4, 973.
3592 Langmuir, Vol. 24, No. 7, 2008 referred in this report. The 1 M HClO4 solution was made up by diluting Merck suprapure HClO4 by Milli-Q water with 18.2 MΩ cm and TOC level below 5 ppb. Pre-electrolysis was conducted for further purification of the 1 M HClO4 in a glass cell using two Pt gauze electrodes with Pt/Pt at 10 mA for 2 days. It was diluted with the Milli-Q water to 0.1 M and then deaerated in a glass flask by bubbling UHP grade N2 before it was used in the spectroelectrochemical cell. Adsorption of CO was conducted by bubbling UHP-grade 1% (He balance) or 100% CO into 0.1 M HClO4 for 1 h under potential control at 0.05 V. Then, the electrolyte solution was purged by UHP grade N2 for an hour to make CO-free atmosphere, under which ATR-FTIRAS measurement coupled with linear potential sweep voltammetry was conducted at 5 mV/s. The time resolution was set at 1 s for the scan rate, and the observed spectra are an average of 8 interferometer scans over the potential range of 5 mV. The interferometer scan rate was set at 40 kHz. The ATR-FTIRAS measurements were conducted by using Digilab FTS6000 and FTS7000 spectrometers equipped with an MCT detector using nonpolarized infrared radiation with a spectral resolution of 4 cm-1. The angle of incidence was set at 70°, and all the measurements were conducted at room temperature. The spectral results are shown in the absorbance units defined as -log(I/I0), where I and I0 represent the spectral intensities at the sample and reference states, respectively. The reference spectrum was collected in N2 atmosphere at the adsorption potential, 0.05 V. In most cases, observed spectra have uncompensated bands of water vapor with a rotational fine structure in the O-H stretching and bending regions. Spectral subtraction was performed in such cases for the uncompensated bands by using a water vapor spectrum observed at a constant potential.
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Figure 1. (A) Potential dependence of ATR-IR spectra observed on Pt/C catalyst (10 µg Pt/cm2) simultaneously with the CO stripping CV shown in Figure 3A. (B) Detailed potential dependence of the ν(CO) band of the linearly bonded CO between 0.1 and 0.8 V.
3. Results and Discussion 3.1. CVs of Pt/C and Pt Black Catalysts Supported on Au. The cyclic voltammograms (CV) of Pt/C and Pt black catalysts (10 µg Pt/cm2) supported on the chemically deposited Au film on a Si ATR prism were reported in our previous study. Surface areas of the Pt/C and Pt black determined from the hydrogen desorption charge assuming 210 µC/cm2 are 17.5 cm2 and 4.47 cm2, respectively. The smaller surface area of the Pt black is due to its average particle size, ca. 10-17 nm as estimated from its XRD pattern, which is several times larger than that of the Pt/C, 2.6 nm.39 The apparent roughness factors of the Pt black and Pt/C electrodes were 2.62 and 10.1, respectively. 3.2. Potential Dependence of the ATR-FTIRAS Spectra of CO Adsorbed on Pt/C and Pt Black. 3.2.1. Oxidation of Adsorbed COs on Pt/C. Adsorption of CO was conducted by bubbling UHP grade 1% and 100% CO into 0.1 M HClO4 for 1 h at controlled rates of 3 mL/min and 20 mL/min, respectively, under potential control at 0.05 V. Then, the electrolyte solution was purged by N2 for an hour to make the solution and atmosphere free of CO. Figure 1A,B shows the ATR-FTIRAS spectra, respectively, observed simultaneously at 5 mV/s between 0.05 and 1.2 V for CO adsorbed at 0.05 V in 1% CO. The CO stripping CV observed simultaneously is shown in Figure 3A. The CO coverage, θCO, calculated from the charge ratio, QCO/2SQH, were 0.69 and 0.99 in 1% and 100% CO atmospheres, respectively. The QCO,the CO oxidation charge, was determined from Figure 3A for 1% CO and Figure 5A for 100% CO, while SQH, the hydrogen desorption charge, was determined by CV measurements at 50 mV/s in 0.1 M HClO4 before CO bubbling. We can note that the peak potentials in the CO stripping CV in Figure 3A and Figure 5A are 0.737 and 0.762 V, respectively, and stripping is completed at potentials as high as 1.2 V. Such CO stripping characteristics are very different from conventional polycrystalline Pt electrodes on which the main current peak is located usually between 0.6 and 0.7 V at 5 mV/s for high CO coverages. We checked to determine whether Nafion coating
Figure 2. Curve-fitting analyses of the ν(CO) bands of CO adsorbed on 10 µgPt/cm2 Pt/C electrode at 0.05 V in 1% CO (He balance), A, and 100% CO, B, followed by 60 min N2 purging.
applied on the catalyst is a cause for such a high CO stripping overpotential. This was done on the Pt-B catalyst with and without Nafion coating, but essentially no effect was recognized (data not shown). On the other hand, a high CO stripping overpotential on Pt nanoparticles fixed on a gold substrate has been reported in several papers. The overpotential depends markedly on Pt particle size, i.e., higher overpotential on smaller Pt particles.30a,31,32b,49 Our data shown in Figure 3A and Figure 5A are consistent with the general trend reported. The high overpotential was interpreted in terms of stronger CO-Pt binding and concomitant decrease in CO diffusion on small Pt particles with a high density of defect sites,31d,49 whereas a different ability to dissociate water to form OHad on defect sites was concluded on the basis of a “CO-annealing” effect on CO stripping CV.32b (49) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Electrochim. Acta 2000, 45, 3283.
Adsorption/Oxidation of CO on Pt Catalyst
Figure 3. (A) CO stripping CV of Pt/C catalyst (10 µg Pt/cm2) supported on Au film surface in 0.1 M HClO4 observed at 5 mV/s. CO adsorbed at 0.05 V in 1% CO (He balance) followed by N2 purging. (B) Potential dependence of the integrated band intensities of CO(L)ter, CO(L)edge, and CO(B)sym. observed simultaneously with the CV.
Figure 4. Potential dependence of ATR-IR spectra observed simultaneously with the CO stripping CV shown in Figure 5A.
Figure 5. (A) CO stripping CV of Pt/C catalyst (10 µg Pt/cm2) supported on Au film surface in 0.1 M HClO4 observed at 5 mV/s. CO adsorbed at 0.05 V in 100% CO followed by 60 min N2 purging. (B) Potential dependence of the band intensities of CO(L)terrace, CO(L)edge, CO(B)sym., and CO(B)asym. adsorbed on 10 µgPt/cm2 Pt/C electrode at 0.05 V in 100% CO. The intensities were derived from spectra in Figure 4 by curve-fitting analysis.
For CO adsorbed in 1% CO, two ν(CO) bands without the optical anomalies located in the 2000-2050 cm-1 and 18001900 cm-1 regions, respectively, are assigned to the linearly,
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i.e., atop-, and bridge-bonded COs, respectively, from their ν(CO) frequencies. The two well-defined peaks at 2036 cm-1 and 2016 cm-1 seen in the atop CO band at 0.1 V were assigned to CO at terrace and step edge sites, respectively.39 All the CO bands shift to higher wavenumbers with increasing potential up to ca. 0.8 V. This is followed by a sharp intensity decrease due to CO oxidation at the higher potentials. The detailed potential dependence of the atop CO band between 0.1 and 0.8 V is shown in Figure 1B. The spectrum acquired at the end of the positive going sweep, 1.2 V, was used as the reference. We can note a few points from these spectra. First, for atop CO, the 2016 cm-1 peak, or shoulder, at 0.1 V assigned to CO at the step edge sites loses its intensity continuously between 0.1 and 0.8 V, while the intensity of the terrace CO band at 2036 cm-1 remains almost unchanged as shown in Figure 1B. The atop CO band shows a characteristic downshift of its peak position with a concomitant band intensity decrease in the CO oxidation region as shown in Figure 1A. Second, we see no bands below 1500 cm-1 which could be assigned to intermediates of CO oxidation such as carboxyl, COOH, and formate, HCOO (see Figure 1A). The former is usually assumed to be the reaction product between adsorbed CO and OH,27,50 and is expected to give a vibration of a carbonyl bond, CdO, around 1700 cm-1. The latter is known as an intermediate of methanol oxidation to exhibit a band due to symmetric vibration of OCO around 1320 cm-14.1,51 However, we see a band around 1100 cm-1 which develops during the CO stripping process. Its frequency is red-shifted by ca. 18 cm-1 compared to free ClO4- ions, and we assign the band to the asymmetric ClsO stretch of perchrolate ions which adsorb on the vacant sites of Pt made available upon CO stripping.41 Arenz et al.32b interpreted an abnormal increase of the Stark tuning rate of atop CO during CO oxidation on carbon-supported Pt catalysts in terms of compression of the CO layer by adsorbed anions, ClO4- or impurity Cl-. The compression would lead to a high local CO density on Pt, which causes a blue shift of the CsO stretching frequency due to increased dipole-dipole coupling in the CO layer. The same analysis was made by Stamenkovic et al. on Pt(111).52 The data presented in Figure 1A directly proves such an anion adsorption after CO stripping. While the Stark shift of the CsO stretch frequency was analyzed in terms of anion adsorption, its effect on the band intensity has not been discussed yet. In principle, the band intensity is expected to increase due to increased dipole-dipole coupling. However, we could not observe such an effect during CO oxidation as shown below. We conducted curve fitting of the spectra observed during the CV measurements by GRAMS to determine the potential dependence of CO(L) and CO(B) adsorbed on terrace and edge sites, respectively, as well as of the asymmetric bridge CO, CO(B)asym. Such a curve fitting analysis was done in our previous report39 and also in a SFG study of CO on a Pt(100) electrode to deconvolute an atop CO band into two components at higher and lower wavenumbers ascribed to terrace and edge sites, respectively.53 Examples of such curve fitting are shown in Figure 2A,B for COs adsorbed at 0.05 V on Pt/C in 1% CO and 100% CO, respectively. All the bands, assigned in the same way as our previous work,39 are summarized in Table 1. It should be noted that the CO(B)edge bands in 1% CO and 100% CO and the CO(50) Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem. 2007, 599, 221. (51) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680. (52) Stamenkovic, V.; Chou, K. C.; Somorjai, G. A.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2005, 109, 678. (53) Vidal, F.; Busson, B.; Six, C.; Tadjeddine, A.; Dreesen, L.; Humbert, C.; Peremans, A.; Thiry, P. J. Electroanal. Chem. 2004, 563, 9.
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Table 1. Assignments of the Bands on Pt/C at 0.05 V Separated by Curve Fitting CO(L)ter.
CO(L)edge
1% CO 2036 cm-1 2014, 1993 100% CO 2037 cm-1 2013
CO(B)asym. CO(B)ter. CO(B)edge 1950
1856 1860
1812 1796
(L)edge band at 2013 cm-1 in 100% CO were further deconvoluted into higher and lower wavenumber components for more detailed analyses.39 The band at 1950 cm-1 developed in 100% CO was assigned to asymmetric bridge-bonded CO on (100) terraces.39 Potential dependences of all the five bands in 1% CO were determined from the spectra shown in Figure 1A. They are shown in Figure 3B, together with the CO stripping CV, A, in which the band intensity of CO(L)edge is a sum of intensities of the two bands shown in Table 1 and CO(B) is a sum of the CO(B)ter. and CO(B)edge The band intensities of CO(L)edge and CO(B)sym. decrease almost linearly with increasing potential up to 0.8 V, with the rate of the decrease being higher for CO(L)edge than CO(B)sym. The continuous intensity decrease of CO(L)edge is discerned directly in Figure 1B as the CO(L)edge peak is losing its intensity with increasing potentials as already summarized in the preceding part. Intensity of the CO(L)terrace remains almost constant up to ca. 0.8 V but decreases sharply at higher potentials, accompanied by a concomitant intensity decrease of CO(L)edge and CO(B) sym. The results shown in Figure 3B suggest that oxidative removal of CO(L)edge and CO(B)sym. precedes oxidation of the overall adsorbed CO layer, which gives rise to the main CO oxidation current peak at 0.737 V observed in Figure 3A. It is difficult, however, to identify the oxidation current of the CO(L)edge and CO(B)sym. in the CV below 0.6 V. Another possibility for such a band intensity decrease of CO(L)edge and CO(B)sym. in the absence of appreciable oxidation current is the desorption or conversion of these COs to CO(L)terrace at lower potentials. In fact, the intensity of CO(L)terrace increases slightly in the same potential region as seen in Figure 3B. Desorption of CO at potentials in the hydrogen region should be accompanied by negative current due to electrochemical hydrogen adsorption, which is not observed, however, in Figure 3 A. Conversion of bridge-bonded CO to atop CO at more positive potentials has been generally recognized,23a,b,36,54 whereas a site shift from CO(L)edge to CO(L)terrace has never been reported. However, it was postulated during CO oxidation on Pt(332) and Pt(322) to interpret the oxidation pathway of CO(L)edge in terms of its conversion to CO on the (111) terrace and subsequent fast diffusion to active step edge sites.26 The issue of site shift from CO(L)edge to CO(L)terrace is currently under investigation in our laboratory and will be reported elsewhere.55 In summary, the band intensity decrease of CO(L)edge and CO(B)sym. in the absence of appreciable oxidation current, as evidenced in Figure 3B, is interpreted in terms of site shift from these CO to CO(L)terrace. On the other hand, above ca. 0.6 V we have a sharp current increase at the onset of the main current peak of CO oxidation, as shown in Figure 3A. The continuing decrease of band intensities of CO(L)edge and CO(B)sym under an almost constant intensity of CO(L)terrace can no longer be interpreted in terms of the site conversion discussed above, but their preferential oxidation over CO(L)terrace is to be concluded at the onset of the main CO stripping peak. In other words, such a conversion of CO(L)edge and CO(B)sym. to CO(L)terrace at the lower potentials triggered oxidation at the step edge sites and further oxidation of the CO adlayer. We conducted further analysis of the oxidation process of CO adsorbed on the Pt/C electrode at 0.05 V in 100% CO. Figure (54) Chang, Si-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142 (55) Sato, T.; Kunimatsu, K.; Uchida H.; Watanabe M. in preparation.
4 and Figure 5A show the ATR-FTIRAS spectra and the CO stripping CV, respectively, observed simultaneously at 5 mV/s between 0.05 and 1.2 V for CO adsorbed at 0.05 V in 100% CO. The CO coverages, θCO, calculated from the charge ratio, QCO/ 2SQH, is 0.99 as already stated. The two ν(CO) bands located at 2040 cm-1 and 1863 cm-1, respectively, are assigned to the CO(L) and CO(B), respectively, from their ν(CO) frequencies. We have another clear band located at 1950 cm-1 which is assigned to the CO(B)asym on (100) terraces.39 All the CO bands shift to higher wavenumbers with increasing potential up to ca. 0.8 V. This is followed by a sharp intensity decrease due to the CO oxidation at potentials greater than ca. 0.8 V. We can note a few points from these spectra. First, for CO(L), there is no clearly separated peak of CO(L)edge, but we can note a broad tail at the lower wavenumber side of the main peak ascribed to the CO(L)ter. The reason for the disappearance of a clear band of the CO(L)edge, which was seen at 2016 cm-1 for CO adsorbed in 1% CO, is most likely a result of dipole-dipole coupling between CO(L)edge and CO(L)terrace which led to a transfer of intensity from the former to the latter.10,13,56 Second, we see no other bands below 1500 cm-1 related to the CO oxidation as shown in Figure 4. We conducted curve fitting of the spectra observed during the CV measurements to determine the potential dependences of CO(L)ter and CO(B)edge, respectively, as well as of CO(B)asym. The result of such curve fitting of the spectrum at 0.05 V is shown in Figure 2B. All the bands assigned were already summarized in Table 1. Potential dependence of the band intensities of CO(L)ter. and CO(L)edge, CO(B)sym and CO(B)asym are presented in Figure 5B together with the CO stripping CV, A. For the COs adsorbed in 100% CO, the band intensities of CO(B)sym and CO(B)asym are significantly higher than those found for 1% CO already shown in Figure 3B. We can note a gradual but continuous decrease in the band intensity of CO(L)edge with increasing potential. Such a continuous decrease of the band intensity of CO(L)edge was also noted for CO adsorbed in 1% CO and was interpreted in terms of its site shift to CO(L)terrace in the preceding part. Accordingly, the similar result on Pt-B can be interpreted in the same way. Furthermore, the band intensity decrease of CO(B)asym starts around 0.5 V, at which the CO oxidation current becomes recognizable as shown in Figure 5A. The band intensity of CO(B)asym decreases linearly with potential above 0.7 V, while the current peak of CO oxidation is located at 0.762 V. Band intensity decreases of all the other bands become significant above 0.85 V. We can summarize the oxidation of COs adsorbed in 1% and 100% CO as follows. The oxidation of CO(L)edge starts from a relatively low potential first. This is followed by the CO(B)asym oxidation, which starts around 0.5 V and becomes the main reaction in the potential region around the peak current in the CV. At potentials higher than ca. 0.85 V, the oxidation of the overall CO adlayer takes place leading to steep decreases in the band intensities of all the adsorbed COs. In the above discussion, the intensity decrease of an adsorbed CO is interpreted in terms of its decrease of fractional coverage. This may not always be the case for a highly compressed CO adlayer on Pt in which strong dipole-dipole coupling affects the band intensities as well as peak frequencies of adsorbed CO.10,13,56 We will discuss this point in more detail in what follows. The disappearance of a clear shoulder of CO(L)edge in the spectra for CO adsorbed in 100% CO as noted in Figure 2B and Figure 4 was already interpreted in terms of such vibrational coupling (56) Hollins, P. Surf. Sci. Rep. 1992, 16, 51.
Adsorption/Oxidation of CO on Pt Catalyst
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Figure 7. (A) CO stripping CV of Pt-B catalyst (10 µg Pt/cm2) supported on a Au film surface in 0.1 M HClO4 observed at 5 mV/s. CO adsorbed at 0.05 V in 100% CO followed by 60 min N2 purging. (B) Potential dependence of the band intensities of CO(L)terrace, CO(L)edge, CO(B)sym., and CO(B)asym. The intensities were derived from spectra in Figure 6A by curve-fitting analysis. Figure 6. (A) Potential dependence of ATR-IR spectra observed simultaneously with the CO stripping CV shown in Figure 7A. CO adsorbed on 10 µg Pt/cm2 Pt-B electrode at 0.05 V in 100% CO followed by 60 min N2 purging. (B) Curve-fitting analysis of the ν(CO) bands of CO adsorbed on the Pt-B electrode at 0.05 V.
between CO(L)edge and CO(L)terrace which led to a transfer of intensity from the former to the latter. In such a case, an intensity decrease of CO(L)edge could lead to a similar decrease of band intensity of its high-frequency partner, CO(L)terrace, without change in its fractional coverage. However, the band intensity of CO(L)terrace remains almost unchanged but slightly increases below ca. 0.5 V despite the continuous decrease of CO(L)edge intensity for CO adsorbed in 1% and 100% CO, respectively, as shown in Figure 3B and Figure 5B. In the preceding part, we have ascribed this to conversion from CO(L)edge to CO(L)terrace in the absence of appreciable oxidation current. This suggests that an expected intensity decrease of CO(L)terrace due to a decreasing effect of the vibrational coupling with CO(L)edge is compensated by the effect of its surface density increase arising from such a site conversion at more positive potentials. The simultaneous intensity decrease of CO(L)terrace and CO(L)edge above 0.5 V in Figure 5B for CO adsorbed in 100% CO may be interpreted in terms of decreasing dipole coupling effect overlapped with decrease of their fractional coverages. For the CO(B)asym., in Figure 5B, a significant decrease of its band intensity above 0.5 V may also be affecting the band intensities of CO(L)s, as their ν(CO) frequencies are close enough for their engagement in the vibrational coupling. What we can say for sure, however, is that the CO(B)asym. exhibits the most significant intensity decrease due to its oxidation from 0.5 V in the CO adlayer, although a quantitative estimation of its coverage decrease is not possible. The conclusion on the earlier oxidation of the CO(B)asym. is further supported on Pt black catalyst on which it is a predominant adsorbed species as judged from its intense band at 1956 cm-1 shown in Figure 6B. 3.2.2. Oxidation of Adsorbed COs on Pt Black Catalyst. We demonstrated a higher reactivity of CO(B)asym than other bridgebonded COs and CO(L)ter in the oxidation of COs adsorbed in 100% CO on Pt/C. The reactivity of CO(B)asym was further examined with a Pt black catalyst, Pt-B, which accommodates a much higher population of CO(B)asym compared with Pt/C
catalyst.39 Adsorption of CO was conducted on Pt-B catalyst at 0.05 V during 60 min of 100% CO bubbling followed by 60 min N2 purging. We compared the spectra before and after 60 min N2 purging. The bands at 2033 cm-1, 1960 cm-1, and 1857 cm-1 observed after 60 min of 100% CO bubbling were assigned to CO(L), CO(B)asym, and CO(B)sym, respectively, in our previous report.39 A decrease of band intensity is commonly observed for CO(L) and CO(B)asym upon N2 purging. This is more significant for CO(L), and consequently, the CO(B)asym band becomes the predominant one. The intensity of the CO(B)sym band shows a slight increase after N2 purging, maybe due to a partial transition from CO(L) and/or CO(B)asym. After the N2 purging, the potential was scanned at 5 mV/s from 0.05 to 1.2 V, acquiring spectra for every 1 s. The background single beam spectrum was observed at 0.05 V before CO bubbling. Figure 6A shows potential dependences of CO spectra observed simultaneously with the CO stripping CV, which is shown in Figure 7A. From the CO stripping CV, we had θCO ) 0.91. We can note that the peak potential in the CV, 0.61-0.64 V, is close to that of polycrystalline Pt and ca. 0.1 V lower than that of Pt/C depicted in Figures 3A and 5A. The larger particle size, 10-15 nm, of Pt-B compared to Pt/C, 2.6 nm, is most likely responsible for the polycrystallinelike behavior,30a,31,32b,49 although a minor CO stripping current persists up to 1.2 V. The major characteristics of the CO stripping CV did not change in the presence or absence of Nafion coating (data not shown). All the bands shift to higher wavenumbers with increasing potential accompanied by the intensity changes. The curve fitting was applied to the spectrum observed at each potential to examine the potential dependences of the adsorbed CO species: an example of such curve fitting for the spectrum at 0.05 V is shown in Figure 6B. The bands at 2029 cm-1, 2005 cm-1, 1855 cm-1, and 1821 cm-1 are assigned to CO(L)ter., CO(L)edge, CO(B)ter., and CO(B)edge, whereas the ones at 1956 cm-1 and 1916 cm-1 are classified as CO(B)asym on (100) terraces.39 The band intensities of CO(L)ter., CO(L)edge, CO(B)asym, and CO(B)sym are plotted as a function of potential in Figure 7B, where CO(B)sym. is not separated into CO(B)ter. and CO(B)edge, since the intensity of the latter was negligibly small. We can note that the band intensities of CO(B)asym and CO(L)edge decrease continuously with increasing potential between 0.05 and 0.6 V
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and they drop sharply in the potential region beyond the prepeak in Figure 7A. Behaviors of CO(B)sym and CO(L)ter. are quite different; their intensities remain almost constant up to 0.8 V, above which they decrease sharply. These observation led us to conclude that the oxidation of CO(L)edge and CO(B)asym is responsible for the small pre-wave current below 0.55 V as well as for the main oxidation current with a peak at 0.61 V. The oxidation of CO(L)ter. and CO(B)sym contributes to the current after the main peak in CV. Thus, the higher reactivity of CO(L)edge and CO(B)asym concluded on Pt/C is confirmed on Pt-B as well. 3.2.2.1. Lack of Agreement Between CV and ATR-FTIR Data. We noticed some lack of agreement between CV and ATRFTIR data as described below. The main current peak of CO stripping CV on Pt-B is almost finished at 0.75 V as shown in Figure 7A, whereas band intensities of adsorbed CO continue to decrease above 0.75 V and diminish around 1.1 V. Moreover, the band intensity decrease of CO(L)terrace and CO(B) starts around 0.8 V, i.e., after the main peak of the CV is finished. The apparent lack of agreement between the main current peak of CV and the band intensity data may be related to the presence of oxidation current after the main current peak, which is seen as the difference between the first and second sweeps of CV in Figure 7A. The oxidation charge above 0.75 V amounts to ca. 20% of the total charge under the CO stripping CV. Electrochemically, the oxidation charge associated with the main current peak and that above 0.75 V are clearly separated, whereas the band intensity decrease cannot be separated in a similar manner due to the involvement of all the CO species in their oxidation at high potentials. We can note a similar lack of agreement between the CO stripping peak in CV and the band intensity data of CO on Pt/C as shown in Figure 3A,B for 1% CO and Figure 5A,B for 100% CO data. Overall, however, the band intensities of CO become zero around 1.1 V at which point the CO oxidation is completed as judged by the CO stripping CV. In this sense, the IR data are in agreement with the corresponding CO stripping CV. It appears, however, as if IR is seeing CO which requires higher overpotentials than the majority of the CO in the catalyst layer. This point will be discussed in more detail below. We investigated the influence of a few possible factors contributing to the disagreement between CV and IR data. The effect of the Nafion coating to retard CO oxidation on Pt nanoparticles reported by Malevich et al.57 is not likely to be an origin of the disagreement between IR and CV data discussed here, as we found no similar effect of Nafion on CO spectra39 as well as CO stripping CV on Pt black. We checked to determine whether there is any influence of mass transfer in the Nafioncoated Pt/C catalyst layer to affect the CO stripping CV. For this purpose, we compared the CVs for CO adsorbed on Pt/C at 0.05 V under 1% CO atmosphere observed at 5 and 50 mV/s to see if the peak current is proportional to the sweep rate, which is a hallmark of a surface reaction. We found that the peak current is almost proportional to the sweep rate and confirmed further that the integrated CO stripping charge was not affected by the sweep rate. On the other hand, a positive shift of the peak potential and considerable broadening are noticed at 50 mV/s. The trend is in general agreement with the behavior of Pt particles supported on gold and glassy carbon and is interpreted as being caused by the slow diffusion of CO on the Pt particles.50 This suggests that the process associated with CV is not affected by mass transfer in the catalyst layer but is controlled by surface reaction. (57) Malevich, D.; Li, J.; Chung, M. K.; McLaughlin, C.; Schlaf, M.; Lipkowski, J. J. Solid State Electrochem. 2005, 9, 267.
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We will discuss another factor which is inherent to the ATRIR method, in which the IR beam sees probably only a portion of the Pt/C catalyst supported on gold. The typical particle size of the carbon support, KETJEN BLACK (381 m2/g carbon), is ca. 50 nm, whereas according to the analysis by Osawa et al.,58 the sensitivity of ATR-SEIRAS is highest on the Au surface and diminishes quickly over a distance of a few monolayers toward the interior of the electrolyte solution. Our rough estimate based on the atop CO band peak intensity shown in Figure 2B observed in 100% CO suggests that the Pt/C catalyst adjacent to the Au surface and sampled by IR beam is ca. 1/5-1/10 of the entire catalyst layer on Au. This means that the interfacial change assessed by IR band intensity data could be different from that derived from CV if the behavior of the Pt/C in direct contact with Au differs from the overall behavior of the catalyst layer. In such a case, however, the IR data are no more representative of the whole catalyst layer. On the other hand, a much higher overpotential than that for polycrystalline Pt for CO stripping was reported for small Pt particles (2-5 nm size) deposited directly on gold.49 The overpotential is Pt coverage-dependent, i.e., the behavior of thicker deposits approaches that of polycrystalline Pt, although some of the distinct properties are also present. Such bulk-deviating properties of Pt particles supported on gold were interpreted in terms of their single-crystalline nature. However, the issue was left an open question.49 It appears that there is a close similarity between the distinct property of the Pt particles deposited on gold and that of Pt/C and Pt-B adjacent to the Au substrate as assessed by ATR-IRAS in our present study. A high overpotential for CO oxidation is commonly observed in both cases. Unfortunately, however, the origin of such distinct electrochemical behavior of the small Pt catalysts directly in contact with the gold substrate is still unclear. A considerable reduction of methanol oxidation activity on Pt with a small amount of gold adatoms reported by Watanabe and Motoo59 suggests a specific effect of gold on the Pt substrate. Probably, the dissociation of water molecules on Pt to produce OH(ads.) needed for oxidation of the methanol adsorbate, CO(ads.), is retarded in the presence of gold adatoms. This is an important issue to be investigated further. Comparison of ATRIRAS data with CV should be carefully examined in view of the above situation. This is probably a problem inherent to in situ ATR-IRAS evaluation of the Pt catalysts supported on gold. Such a disagreement between CV and IR data for Pt/C supported on gold was not apparently present in the IRAS study utilizing external reflection geometry.32b,60 This is well-understood because the IR beam passes through the overall catalyst layer and data representative of all the catalysts supported on Au could be obtained. The influence of the catalyst directly in contact with Au would be a minor one. Recently, we discovered that such a lack of agreement between CV and band intensity data is virtually absent for CO directly adsorbed on chemically deposited Pt film on a Si ATR prism. This will be reported elsewhere.61 As for the discussion of active Pt sites for CO oxidation based on the ATR-IRAS data developed below, we believe this is not affected by the situation described above. The Pt/C catalyst directly in contact with Au and seen by IR would behave in a similar manner even if its potential-dependent behavior differed from the overall catalyst layer to some extent. 3.2.2.2. Active Sites and CO Species in the Oxidation of Adsorbed COs. As described in the preceding parts, we have (58) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (59) Watababe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 259. (60) Stimming, U. Private communication. (61) Kunimatsu, K.; Sato, T.; Uchida H.; Watanabe, M. to be submitted to Electrochim. Acta.
Adsorption/Oxidation of CO on Pt Catalyst Table 2. Summary of Active CO Species in the Oxidation of CO Adsorbed on Pt/C and Pt-B CO(B):CO(B)edge and CO(B)ter active CO species Pt/C(θCO:0.69) Pt/C(θCO:1.0) Pt/C(θCO:0.91)
pre-peak region
onset of main current peak
CO(L)edge, CO(B)sym. CO(L)edge CO(L)edge, CO(B)asym.
CO(L)edge, CO(B)sym. CO(L)edge, CO(B)asym. CO(L)edge, CO(B)asym.
conducted detailed analysis of the potential dependences of the spectra of adsorbed COs observed under linear potential sweep conditions on Pt/C and Pt-B. We noted that the active Pt sites and CO species in the oxidation of adsorbed COs depended significantly on the nature of the catalyst. The results are summarized in Table 2. We can conclude that CO(L)edge, CO(B)asym, and CO(B) sym. are the key active CO species in the pre-peak region of CO oxidation, as well as in the main current peak region on both Pt/C and Pt-B catalysts. Conversion of CO(L)edge to CO(L)terrace at the lower potentials plays an important role to trigger oxidation at the step edge sites and further oxidation of the CO adlayer at higher potentials. At the final stage of CO oxidation in the potential region beyond ca. 0.8 V, all adsorbed COs are oxidized, simultaneously, including CO(L)ter. which is the least active one. The preferential oxidation of CO(L)edge and CO(B)asym in the present work on Pt catalysts is a new finding, although such an oxidation of atop-CO on defect sites was reported recently on Pd-modified quasi-single-crystal gold film electrodes by ATR-SEIRAS.43 This is contrasted with the proposed CO oxidation mechanism on Pt(335) in UHV22 and Pt(443), Pt(332), and Pt(322) electrodes with (111) terraces and (110) and (100) steps, respectively, in 0.1 M H2SO4.26,27 In UHV, it was demonstrated22 that CO adsorbed on the (111) terraces is more reactive compared to that on the (100) step edge sites and is oxidized by oxygen atoms adsorbed in the step sites. The higher reactivity of CO on (111) terraces than that on the steps was also reported on Pt(112).62 On the stepped single-crystal Pt electrodes, Pt(332) and Pt(322), with (111) terraces and (110) or (100) steps, respectively,26,27 oxidation of adsorbed CO, with initial CO coverage of 0.33-0.34, was conducted by linear potential sweep voltammetry at 0.5 and 1.0 mV/s while acquiring the ν(CO) spectra at the same time. Band splitting of atop CO into two clearly resolved features at 2050-2060 cm-1 and 2025-2030 cm-1 corresponding to CO on the (111) terraces and (100) steps was observed upon initial CO oxidation. This was interpreted in terms of lifting the vibrational coupling between the CO at (111) terraces and (100) steps. Such band splitting was less pronounced on Pt(332) with (110) steps. On the basis of the observation, it was concluded that CO adsorbed on the (111) terraces reacts first, leaving CO on either (100) or (110) steps behind. The alternative possibility of the first oxidation of CO adsorbed on step sites was dismissed by the argument that fast diffusion of CO from terrace sites would keep the local CO arrangement at the step sites unchanged, where no band splitting would be expected in such an alternative case. It was concluded that the Langmuir-Hinshelwood reaction between oxygen-containing species adsorbed at the step edge sites and COs on (111) terraces took place first. The lower reactivity of CO on the steps was rationalized by taking account of the higher adsorption energy compared to CO on the terrace. Our result summarized in Table 2, however, demonstrates that CO(L)edge is a common active CO species on Pt/C and PtB, while CO(L)terr is least active. Apparently, the supply of CO from the terrace to the step edges by fast diffusion as concluded (62) Szabo, A.; Henderson, M. A.; Yates, J. T., Jr. J. Chem. Phys. 1992, 96, 6191.
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on the stepped Pt single-crystal electrodes26,27 was not noted in our present report. This may be interpreted in terms of slower diffusion of CO with respect to its oxidation rate on such Pt nanoparticles with a high density of defect sites on which CO diffusion is retarded.50,63 The issue of CO transfer to active step edge sites during CO oxidation needs more insight in terms of the surface structure of the catalyst. The CO(B)asym has been proven to be most reactive in the CO oxidation for the first time on Pt/C and Pt-B at near-saturation CO coverage in our present report. The discrepancy observed between the stepped single crystals and the Pt nanoparticles highly dispersed on carbon should be further investigated in order to correlate the behavior of CO on the model surfaces and practical fuel cell catalysts. 3.2.3. Role of Surface Water in the Oxidation of CO. We have found significant changes in the spectral regions of ν(OH) vibrations of water molecules upon CO adsorption on Pt/C and Pt-B during 100% CO bubbling.39 Sharp and broad ν(OH) bands grow around 3630-3640 cm-1 and 3370-3430 cm-1, respectively, on Pt/C and Pt/B. No bands ascribed to water molecules desorbed from the Pt surface were observed. They were assigned to the ν(OH) vibrations of non-hydrogen-bonded and hydrogenbonded water molecules, respectively, coadsorbed with CO. The presence of such non-hydrogen-bonded water molecules coadsorbed with CO was first observed on Pt by ATR-SEIRAS40 and later on PtRu.44,45 It should be noted, however, that the nonhydrogen-bonded water molecules were first reported on Au electrodes in the absence of adsorbed CO64,65 and potentialdependent interconversion between non-hydrogen-bonded and hydrogen-bonded water molecules was proposed. We investigated the behavior of these water molecules during CO oxidation by analyzing potential dependences of ν(OH) bands shown in Figure 8A and B, which correspond to potential dependences of ν(CO) bands of adsorbed COs shown in Figure 1A and Figure 6A on Pt/C and Pt-B, respectively. In Figure 8, however, we used a spectrum at 1.2 V as the reference at which the Pt surface became free from adsorbed COs. Such choice of the reference allows detailed analysis of the adsorbed water molecules during the CO oxidation process, as shown below. The ν(OH) spectra have two positive going bands around 3640 cm-1 and 3500 cm-1 and a negative going one around 3015 cm-1. The former bands are ascribed to the ν(OH) vibrations of non-hydrogen-bonded water molecules and the latter two to hydrogen-bonded ones, respectively, coadsorbed with CO. The ν(OH) frequencies are slightly different from those found upon CO adsorption by using the background at 0.05 V before bubbling CO. This is an influence of the new reference which has given rise to the negative going band around 3015 cm-1. The significance of this band will be discussed later. Integrated band intensities of the three bands at each potential were derived by curve fitting analysis similar to the previous work,39 and their potential dependences are presented in Figure 9A,B for Pt/C and Pt-B, respectively. Note that the intensities of the non-hydrogen-bonded water molecules at ca. 3640 cm-1 on both Pt/C and Pt-B and hydrogen-bonded ones at 3500 cm-1 on Pt/C are multiplied by a factor of 10. The three bands show distinctive potential dependence as discussed below. On Pt/C, the intensity of the non-hydrogen-bonded water commences to decrease around 0.65 V, at which the oxidation of the overall CO adlayer is initiated as shown by Figure 5A,B. This implies that the non-hydrogen-bonded water molecules are directly involved in the oxidation of adsorbed COs. Such a water species to oxidize adsorbed CO was found on sputtered PtRu (63) Kobayashi, T.; Babu, P. K.; Panakkattu, K.; Chung, J. Ho.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. C 2007, 111, 7078. (64) Kunimatsu, K.; Bewick, A. Ind. J. Technol. 1986, 24, 407-412. (65) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664.
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Figure 10. Potential dependence of the ν(OH) frequency of the non-hydrogen-bonded water molecules on Pt/C and Pt-B. The frequency data were derived from Figure 8A,B by curve-fitting analysis.
Figure 8. Changes of the ν(OH) region observed simultaneously with the CO stripping CV on (A) Pt/C and (B) Pt-B shown in Figure 5A and Figure 7A, respectively. The spectrum acquired at the end of the positive going sweep, 1.2 V, was used as the reference for both A and B.
H2O and CO are not direct proof to conclude the nature of the oxygen-containing species to oxidize adsorbed CO to CO2. However, its high ν(OH) frequency and its Stark shift with potential presented in Figure 10 allow us to conclude that such non-hydrogen-bonded water molecules are adsorbed on Pt and isolated from each other by adsorbed CO. We believe, therefore, that the band intensity data combined with such a nature of the non-hydrogen-bonded water molecules offers a reasonable support to its active involvement in the oxidation of adsorbed CO as an oxygen-containing species. It is not clear, however, if such non-hydrogen-bonded water molecules dissociate to give rise to adsorbed OH species which may actually react with adsorbed COs at the last step of their oxidation. Such OH species would give rise to a ν(OH) band with O-H stretching frequency above 3700 cm-1 if they are not hydrogen-bonded as shown for OH on Si(111).66 However, such a ν(OH) band has never been observed on Pt electrodes or in the present study. The fact that we see no bands ascribed to adsorbed OH is reasonable as long as the first step to activate water to produce adsorbed OH is the rate-determining step preceding the following fast oxidation reaction between the adsorbates on neighboring Pt sites by a Langmuir-Hinshelwood-type mechanism
H2O-Pt(subs) f HO-Pt(subs) + H+ +eHO-Pt(subs) + CO-Pt(subs) f 2Pt(subs) + CO2 + H+ +e-
Figure 9. Potential dependence of the ν(OH) band intensities of the non-hydrogen-bonded, 3642 cm-1 and 3645 cm-1, and hydrogenbonded, 3500 cm-1 and 3015 cm-1, water molecules on (A) Pt/C and (B) Pt-B. The intensity data were derived from Figure 8A and B by curve-fitting analysis. All the wavenumbers given in the figures correspond to those at the lowest potential 0.1 V. Intensities of the non-hydrogen-bonded water molecules on both Pt/C and Pt-B and hydrogen-bonded ones at 3500 cm-1 on Pt/C are multiplied by a factor of 10.
bimetallic catalysts in our laboratory,44,45 a Pd nanofilm electrodeposited on a chemically deposited Au film,42 and a Pdmodified quasi-single-crystal gold film electrodes43 by ATRSEIRAS. On the other hand, the simultaneous decrease of the band intensities of non-hydrogen-bonded H2O and adsorbed CO during the CO stripping CV measurements could be interpreted in an alternative way. It may be argued that such non-hydrogenbonded H2O produced by CO should disappear when CO is removed from the Pt surface and that the band intensity data of
In this case, the non-hydrogen-bonded H2O is a precursor of adsorbed OH, with the lifetime of the latter being short and its steady-state surface concentration practically zero. On the other hand, Roth et al. reported recently an increase of surface OH during operating conditions of PEM-type DMFC with a PtRu/C anode catalyst by in situ X-ray absorption measurements.67 Such an increase of OH was observed concomitantly with a decrease of adsorbed CO. However, it is difficult to compare the result with those of CO oxidation on Pt/C and Pt black catalysts reported in the present study. Yajima et al. reported ν(OH) and δ(HOH) bands at ca. 3620 cm-1 and 1616 cm-1, respectively, of nonhydrogen-bonded water molecules during methanol oxidation on a PtRu sputtered film44,45 by ATR-SEIRAS. Contrary to OH reported on PtRu/C,67 the non-hydrogen-bonded water decreased with a decreasing amount of adsorbed CO. It was concluded that the adsorbed OH produced by dissociation of the non-hydrogenbonded water molecules at Ru sites reacts with adsorbed CO, although a ν(OH) band ascribable to OH itself was not detected. The role of surface water and OH on PtRu is an important issue in the bifunctional mechanism46 of methanol oxidation and should (66) Ibach, H.; Wagner, H.; Bruchmann, D. Solid State Commun. 1982, 42, 457. (67) Roth, C.; Benker, N.; Buhrmester, T.; Mazurek, M.; Loster, M.; Fuess, H.; Koningsberger, D. C.; Ramaker, D. E. J. Am. Chem. Soc. 2005, 127, 14607.
Adsorption/Oxidation of CO on Pt Catalyst
be investigated further on highly dispersed Pt-based bimetallic catalysts on carbon. Such a study is currently underway in our laboratory. On the other hand, behavior of the hydrogen-bonded water molecules is different. Intensity of the band at 3500 cm-1 decreases continuously with increasing potential. This suggests that the water species disappears from the Pt surface at higher potentials and is not involved predominantly in the oxidation of the major adsorbed COs. Intensity of the band at 3015 cm-1 becomes more negative, i.e., decreases, with increasing potential up to ca. 0.7 V, above which it increases again during the oxidation of adsorbed COs. In summary, continuous desorption of the two hydrogen-bonded water species precedes the overall oxidation of the CO adlayer. The Pt surface becomes dominated by the hydrogen-bonded water molecules as the non-hydrogen-bonded water molecules are consumed in the CO oxidation beyond 0.7 V. Almost the same conclusion can be derived with Pt-B based on the data presented in Figure 9B. On Pt-B, however, decrease of the band intensity of the non-hydrogen-bonded water molecules takes place above 0.5 V, where the band intensity decrease of CO(L)edge and CO(B)asym becomes significant as shown in Figure 7B. Below 0.5 V, the band intensity of the non-hydrogen-bonded water molecules increases with increasing potential, while hydrogen-bonded ones lose their band intensity in the same potential region. This could be interpreted in terms of conversion of the hydrogen-bonded water molecules to the non-hydrogenbonded ones by an effect of applied potential. On Pt/C, the band intensity of the non-hydrogen-bonded water molecules shows a similar tendency to increase between 0.05 and 0.6 V, where hydrogen-bonded water molecules are losing their band intensity. Therefore, we can conclude such hydrogen-bonded/non-hydrogenbonded conversion on Pt/C and Pt-B. On the other hand, we have found different behaviors of the non-hydrogen-bonded water molecules on Pt/C and Pt-B as shown in Figure 10, which shows potential dependences of their ν(OH) frequency. On Pt-B, the frequency increases linearly with potential with a slope of 21 cm-1/V, whereas such linearity is lost on Pt/C above 0.7 V where the band intensity of the water decreases significantly due to its consumption in CO oxidation. The ν(OH) band behavior of non-hydrogen-bonded water molecules on Pt/C is very similar to that produced by adsorption of methanol on a chemically deposited Pt film.41 The Stark tuning rate of the ν(OH) band was not reported in the literature, and we estimated it from their data (Figure 4(b)).41 We had ca. 20 cm-1/ V, which is in close agreement with the slope presented in Figure 10. A slightly higher Stark tuning rate, 25 cm-1/V, was reported on a Pd nanofilm electrodeposited on a chemically deposited Au film.42 The origin of the significant downshift of ν(OH) frequency observed on Pt/C above 0.7 V could be reduced dipole-dipole coupling effect among the oscillating O-H dipoles due to decrease of its surface concentration during CO oxidation.41 However, the ν(OH) frequency of the non-hydrogen-bonded water molecules is almost unchanged during CO adsorption at 50 mV; i.e., 3630 cm-1 and 3645 cm-1, respectively, on Pt/C and Pt-B as shown in Figure 10 in our previous report.39 This suggests that such a coupling effect is almost absent among the non-hydrogenbonded water molecules coadsorbed with CO. Another wellknown factor to reduce the ν(OH) frequency is hydrogen-bonding formation. Therefore, we interpret the large red shift of the ν(OH) frequency of the non-hydrogen-bonding water molecules during its consumption in CO oxidation in terms of increased hydrogen bonding with water molecules adsorbed on Pt sites, which have become free from CO molecules. On Pt-B, however, the linearity between the ν(OH) frequency and potential is not
Langmuir, Vol. 24, No. 7, 2008 3599
Figure 11. Potential dependence of the ν(CO) frequency of the adsorbed COs, (θCO ) 0.99), on the Pt/C electrode. (A) CO(L)ter and CO(L)edge, (B) CO(B)asym, CO(B)sym, and CO(B)edge. The frequency data were derived from Figure 4 by curve-fitting analysis. The numbers given for the linear lines are the Stark tuning rates in cm-1/ V.
affected during oxidation of the overall CO adlayer. This implies that the mechanism to increase hydrogen-bonding character of the O-H bonds as discussed above for Pt/C is absent on Pt-B. Such a difference between Pt/C and Pt-B may be related to the different nature of the major adsorbed CO species on the two catalysts, CO(L) and CO(B)asym, respectively. It was concluded that CO(L)ter, CO(L)edge, and CO(B)sym are responsible to formation of the non-hydrogen-bonded water molecules whereas CO(B)asym plays a major role in producing hydrogen-bonded water molecules coadsorbed with COs.39 On Pt-B, therefore, oxidation of the major adsorbed species, CO(B)asym, would not affect the nature of the non-hydrogen-bonded water molecules, while on Pt/C oxidation of CO(L)s, the major adsorbed species, may affect as experimentally observed, as shown in Figure 10. 3.2.4. Potential Dependence of ν(CO) Frequency and CO Adsorption Site. Potential dependence of ν(CO) frequency of adsorbed CO on electrodes has been known as the vibrational Stark effect. Its dependence on adsorption types, i.e., atop vs bridge bondings,54,68-70 and Pt particle size30a,32c,34 has been reported mostly on platinum electrodes. We were interested in examining potential dependences of the ν(CO) frequency of adsorbed COs in terms of terrace vs edge sites and symmetric vs asymmetric bridge-bonded geometries, as such analysis has never been done before. We have got a series of data obtained by curve-fitting analysis on Pt/C and Pt-B catalyst, which allows us to conduct such an examination. Figures 11 and 12 show potential dependence of ν(CO) frequency of adsorbed COs on Pt/C and Pt-B catalysts, respectively. The data were obtained by analyzing potential dependences of spectra presented in Figure 1A and Figure 9A for CO adsorbed on Pt/C and Pt-B in 100% CO, followed by N2 purging. It should be noted, however, that ν(CO) frequencies of CO(L)edge and CO(B)edge on Pt/C were derived from more detailed analyses as conducted in the previous report.39 On Pt/C, we have found that the Stark tuning rates of CO(L) at the terrace and edge sites are almost the same, 35 cm-1/V. The tuning rate is very close to 30 cm-1/V commonly (68) Weaver, M. J. Appl. Surf. Sci. 1993, 67, 147. (69) Koper, M. T. M.; Van Santen, R. A. J. Electroanal. Chem. 1999, 476, 64. (70) Wasileski, S. A.; Weaver, M. J.; Koper, M. T. M. J. Electroanal. Chem. 2001, 500, 344.
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Figure 12. Potential dependence of the ν(CO) frequency of the adsorbed COs, (θCO ) 0.91), on the Pt-B electrode. (A) CO(L)ter and CO(L)edge, (B) CO(B)asym and CO(B)sym. The frequency data were derived from Figure 6A by curve-fitting analysis. The numbers given for the linear lines are the Stark tuning rates in cm-1/V.
reported for atop COs on Pt electrodes.1,7,54,68 A downshift of the ν(CO) frequency of the CO(L)s by ca. 10 cm-1 is commonly observed above 0.9 V for CO(L)ter and CO(L)edge. This is the potential region where the band intensities, i.e., fractional coverages, of CO(L)s decrease sharply due to their electrochemical oxidation, as shown in Figure 5B. Such a downshift of the ν(CO) frequency of the CO(L)s during their electrochemical oxidation was first reported for CO adsorbed at 0.05 V on a Pt disk electrode1 and interpreted in terms of decreasing dipoledipole coupling among the oscillating C-O dipoles, which is reduced with decrease of CO coverage.56 However, the downshift by only 10 cm-1 is considerably smaller than that found on the polycrystalline Pt disk electrode, on which a 30 cm-1 downshift was observed during oxidation of atop CO adsorbed at 0.05 V. This suggests more significant atop CO island formation on Pt/C than on the polycrystalline Pt. The effect of adsorbed anions upon CO stripping, as already seen in Figure 1A, may lead to compression of adsorbed CO as proposed by Alenz et al.32b for Pt/C and Stamenkovic et al.52 for Pt(111). This could facilitate island formation of CO leading to increased dipole-dipole coupling. Deviation from the linear Stark shift of C-O stretching frequency of atop CO with potential would be small, as experimentally observed. The tuning rates of the bridge-bonded COs range from 41 cm-1/V for CO(B)asym to 56 cm-1/V for CO(B)edge. The higher Stark tuning rate of the bridge-bonded COs than atop COs is in agreement with previous reports on bulk platinum electrodes,54,68-70 and can be interpreted in terms of a higher degree of back-donation from the Pt d-orbital to the CO 2π* orbital, induced by the double layer field, at the higher coordinated Pt sites. The observed tuning rate of CO(B)asym is 41 cm-1/V, which is in between the tuning rates of CO(L)s and CO(B)sym. This is reasonable for its adsorption geometry and provides a further support to the assignment of CO(B)asym on (100) terraces discussed extensively in our previous report.39 The downshift of the ν(CO) frequency of the CO(B)s during their oxidation is much less pronounced than that of CO(L)s as shown in Figure 11B. This is particularly noted for CO(B)asym and CO(B)sym, which show almost constant ν(CO) frequencies during their oxidation. This suggests significant island formation of CO(B)asym and CO(B)sym on Pt/C. On Pt-B, Figure 12, we have found very
Kunimatsu et al.
similar results to Pt/C discussed above. The tuning rates which are similar to CO(L)ter and CO(L)edge, i.e., 28 and 29 cm-1/V, respectively, and higher for CO(B)s than CO(L)s are concluded as shown in Figure 12B. The intermediate tuning rate for CO(B)asym between CO(L)s and CO(B)sym. can be concluded as well on Pt/C and Pt-B catalyst. We could not determine an accurate tuning rate of the CO(B)edge, as its ν(CO) band is very weak and broad, as shown in Figure 6B. The ν(CO) frequencies of the atop COs and CO(B)s on Pt-B exhibit small and little downshifts, respectively, during their oxidation as presented in Figure 12. This is common to the case of Pt/C described above, and island formation of COs is also suggested on Pt-B. The island formation commonly suggested for CO(L) and CO(B) during their oxidation on Pt/C and Pt-B may be interpreted as due to compression by adsorbed anions as already discussed above. We have noticed a few more interesting points in regard to dependences of ν(CO) frequencies on the adsorption site geometry as described below. The ν(CO) frequency of the terrace site atop CO is higher than that of the edge site atop CO by almost 20 cm-1 on both Pt/C and Pt-B as shown in Figure 11 and Figure 12. This is also seen in Figure 1 for CO adsorbed in 1% CO on Pt/C. For CO(B)sym., the difference is 27-34 cm-1 as shown in Figure 11 for Pt/C, with the higher difference being seen at the lower potential side. Such a ν(CO) frequency difference between the terrace and edge site atop COs are almost in agreement with the previous reports on Pt small particles with average size varying from 1.1 nm to 10.5 nm supported on silica powder17 and on Pt(11 1 1) and Pt(711) electrodes with (100) terraces separated by (111) steps.21 On the other hand, the ν(CO) frequency of the CO(B)asym is higher than that of the CO(B)sym by ca. 100 cm-1 as seen in Figure 11 and Figure 12 on Pt/C and Pt-B, respectively. The difference is significantly larger than that found on Pt(11 11), Pt(711),21 and Pt(100)5b electrodes, on which the ν(CO) frequency of the CO(B)asym was higher than that of the CO(B)sym by ca. 20-60 cm-1. The higher ν(CO) frequency difference between CO(B)asym and CO(B)sym found on Pt/C and Pt-B in the present work is not ascribed to difference in CO coverage. The coverage was below 0.521 and 0.77,5b while it is 0.99 and 0.91 on Pt/C and Pt-B, respectively. We noticed that an increase of the ν(CO) frequency of the CO(B)asym during the 100% CO bubbling for 1 h was only ca. 10 cm-1 .39 The origin of much higher ν(CO) frequency of the CO(B)asym found on Pt/C and Pt-B than Pt(100)5b and related stepped surfaces21 is not clear. This is contrasted to the almost similar ν(CO) frequency of the CO(B)sym to that usually observed on Pt electrodes. Furthermore, the ν(CO) frequency of the CO(L)s on Pt/C and Pt-B is significantly lower than that usually found on polycrystalline and Pt single-crystal electrodes.39 It is necessary to conduct further studies to investigate factors such as particle size effect, which may be contributing to the ν(CO) frequencies of the various kinds of adsorbed COs with different adsorption geometries on Pt/C.
4. Conclusion The ν(CO) spectra of the atop and bridge-bonded COs on Pt/C and Pt-B were resolved into the bands of COs adsorbed at terrace and step edge sites by curve-fitting analysis. The simultaneous adsorption of CO at terrace and step edge sites was confirmed for both atop and symmetric bridge-bonded COs by these analyses. In 100% CO, a band assigned to asymmetric bridge-bonded CO, CO(B)asym, located around 1950-1960 cm-1 developed on both Pt/C and Pt-B; on the latter, the band has become the predominant one. Potential dependence of the ν(CO) bands of COs at terrace and step edge sites as well as of the CO(B)asym were determined
Adsorption/Oxidation of CO on Pt Catalyst
by detailed curve-fitting analysis of the spectra observed during a linear potential sweep between 0.05 and 1.2 V at 5 mV/s. Preferential desorption/oxidation of the atop CO adsorbed at the step edge site is commonly concluded on both Pt/C and Pt-B. However, CO(B)asym is the most reactive species at the higher CO coverage near the saturation whose band intensity commences to decrease already around 0.5 and 0.3 V, whereas the main current peak in the CO stripping CV was located around at 0.80 and 0.65 V on Pt/C and Pt-B, respectively. Potential dependences of the ν(OH) bands of water molecules coadsorbed with CO were analyzed throughout the potential region between 0.05 and 1.2 V. The ν(OH) band around 36423645 cm-1, which is assigned to non-hydrogen-bonded water molecules coadsorbed with CO, exhibits a linear increase with potential with a Stark tuning rate of ca. 20 cm-1/V. It is concluded that the water is the oxygen-containing species to oxidize adsorbed CO. Analysis of the lower-wavenumber region down to 1200 cm-1 was conducted on both Pt/C and Pt-B, but we have found no evidence to suggest the existence of chemical species other than
Langmuir, Vol. 24, No. 7, 2008 3601
CO and water molecules within the sensitivity of the ATR-FTIR measurements employed in the present study. The stark tuning rates of the ν(CO) bands of the COs at the terrace and step edge sites are almost independent of the adsorption geometry for both atop and bridge-bonded COs. However, CO(B)asym exhibits tuning rates of 41 cm-1/V and 37 cm-1/ V on Pt/C and Pt-B, respectively, which are in between the rates of atop and symmetric bridge-bonded COs. It is concluded that this is further support to the assignment of the asymmetric bridgebonded COs with ν(CO) vibration around 1950-1960 cm-1 at 0.05 V. Acknowledgment. This work was supported by the Leading Project on “Next Generation Fuel Cells” of Ministry of Education, Science, Sport and Culture of Japan, to which we are greatly acknowledged. We thank also Dr. Hiroshi Yano for determining the particle size distribution of the catalyst employed in the present study. LA702441X
