Infrared Spectroscopic Detection of CO Formed at Step and Terrace

J. Phys. Chem. 1995,99, 3419-3422

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Infrared Spectroscopic Detection of CO Formed at Step and Terrace Sites on a Corrugated Electrode Surface Plane during Methanol Oxidation Jungwon Shin and Carol Korzeniewski* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 Received: December 20, 1994@

A platinum single crystal electrode with ordered step and terrace structures was used to probe site-dependent aspects of methanol surface chemistry. Infrared spectral features of the adsorbed CO reaction intermediate were measured in parallel experiments with Pt(335) (Pt(s)-[4( 111) x (loo)]) and Pt( 111) electrodes. Experiments confirm that methanolic CO formation is inhibited at Pt( 111) at potentials in the hydrogen adsorption region, and they reveal that methanol dissociative chemisorption proceeds to a limited extent in this potential region at the corrugated Pt(335) surface plane, where independent vibrational spectral features for CO at step and terrace sites were detected.

Introduction The catalytic oxidation of methanol at noble metal electrodes is a technologically important process. Complete oxidation to COZ and water yields six electrons per molecule of methanol, making it attractive for use in fuel A primary complication is the formation of adsorbed intermediates, such as carbon monoxide (CO), which block catalytic surface sites and thereby inhibit energy-producing pathways. An important strategy for optimizing methanol oxidation rates has involved the study of reactions at well-characterized electrode surface^.^-^ These include basic investigations of methanol surface chemistry at well-ordered surface planes of single crystal materials (Refs 2, 3 and 4b-9 and references therein). A fruitful approach involving the use of single crystal electrodes has been to examine the effect of surface corrugation on the rate and specificity of methanol oxidation pathway^.^.^^^.^ Recent work has compared methanol oxidation processes at flat (1 11) and (100) surface planes and stepped (1 10) and high index surface plane^.^ Structurally well-defined surfaces allow systematic studies of surface stereochemical influences, and the stepped surfaces also serve as models for practical catalyst^,'^-'^ which typically contain a high defect density. In the case of methanol oxidation, voltammetric and chronoamperometric studies at single crystal platinum electrodes have shown that the fastest reaction rates occur at clean, highly corrugated surfaces but that these surfaces rapidly deactivate through poisoning mechanism^.^ At least for the low index surface planes of platinum, methanolic surface deactivation processes have been examined in greater detail by using spectroscopic methods. In particular, infrared spectroscopy has been an important tool for investigating methanol surface electrochemistry under reaction conditions (cf. refs 2 , 7, 8, and 15). These experiments detect the C-0 stretching vibrational features of adsorbed CO intermediates. Spectra indicate the onset potential for methanolic CO poison formation, the coordination environment of the poison (e.g., terminal or bridging), and the extent of CO aggregation (island formation). At Pt( 11l), Pt( loo), and Pt( 110) electrodes, infrared spectroscopy has been applied to probe methanol surface electrochemistry in aqueous electrolyte solutions.2~7~s~16-20 The methanolic CO surface coverage is low to moderate on all three

* To whom correspondence @

should be addressed. Abstract published in Advance ACS Abstracts, March 1, 1995.

0022-365419512099-3419$09.0010

surface planes. The CO surface coordination is almost exclusively terminal, and the energy of these vibrational features indicates that CO aggregates form on the surface, but only to a moderate e ~ t e n t . It ~ .is~thought that the formation of extended CO islands may be impeded by the presence of other adsorbed partial oxidation products, which are undetectable by in situ surface infrared spectroscopy technique^.^,^ The dissociative chemisorption of methanol is inhibited at a clean platinum electrode when it is poised at potentials in the classical hydrogen adsorption region3,s,21,2z (the range of potentials where adsorbed hydrogen forms through reduction of protons in solution), and at Pt( 11l), methanolic CO spectral features do not appear until the electrode potential is switched to values just positive of this region, where hydrogen desorption occurs.8 In the present study, infrared spectroscopy is used to probe methanol surface chemistry at a highly corrugated platinum surface plane. The surface employed, Pt(335) (Pt(335) Pt(s)-[4( 111) x (loo)]), was originally used to support infrared spectroscopic studies of carbon monoxide adsorption at defect sites on small platinum catalyst particle^.^^,^^ It has since become a model substrate for vibrational studies of sitedependent adsorption processes at both gas-solid and liquidsolid interfaces. 13~14.23-36 The present report draws upon the Pt(335)lCO knowledge base to examine conditions for methanolic CO formation at surface step and terrace sites. Infrared spectroscopy was used to examine the electrochemical oxidation of methanol at Pt( 111) and Pt(335) electrodes under reaction conditions. At the atomically flat (1 11) surface plane, methanolic CO formation commenced at potentials just positive of the hydrogen adsorption region, in agreement with earlier studies. In contrast, low coverages of methanolic CO formed on the corrugated (335) surface plane at potentials that coincide with stable hydrogen adsorption, and well-resolved spectral features for CO at step and terrace sites were detected. A methanolic CO bridging feature was also observed at Pt(335). This band appears at energies close to the CO bridging feature on Pt(100) and likely arises from CO at (100) symmetry sites, which occur on the face of step edges. This study is the first to detect site-dependent processes originating from methanol oxidation at a corrugated electrode surface plane. Experimental Section Single crystal materials were obtained from commercial sources with the orientation specified to within &lo. The

0 1995 American Chemical Society

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3420 J. Phys. Chem., Vol. 99, No. 11, 1995

Pt(335) single-crystal disk (Aremco) was 7 mm in diameter by 2 mm thick. The Pt(111) single crystal disk (Cornell University Materials Preparation Facility) was 9 mm in diameter by 3 mm thick. The orientation of each disk was verified in our laboratory 2957 by X-ray back-diffraction. To facilitate electrical contact, a thin platinum wire was spotwelded to the back of each single crystal disk. Immediately before each experiment, the single crystal electrode was annealed in a hydrogen flame and subsequently cooled in an argon atmosphere saturated with water vapor. This process served to clean and order the oriented surface plane of the ~ r y s t a l . ~ ~ , ~ ~ Specific details of the procedure used in our laboratory are described in ref 14. Following this process, the single crystal 20137 disk was transferred under a protective layer of ultrapure water to an electrochemical cell that contained a freshly prepared 0.1 M perchloric acid (Aldrich, redistilled, 99.999% purity) electrolyte solution. AU solutions were prepared from distilled water that was further purified by using a Bamstead Nanopure I1 2 i o o 1900 2100 1900 cartridge system followed by oxidation processing under Wavenumber (cm-1) ultraviolet irradiation (Barnstead Organic-pure). The surface order and surface cleanliness of the single crystal electrode were Figure 1. Infrared spectra of CO formed during the oxidation of methanol at (A) Pt(335) and (B) Pt(ll1) electrodes. The electrodes checked immediately after transfer by cyclic voltammetry. were in an aqueous 0.1 M HC104 electrolyte solution containing 100 The electrochemical cell used for infrared spectroscopic mM methanol. Spectra were obtained while the potential was stepped experiments has been described previously.26 A platinum in sequence from an initial value of -0.25 V. The reference spectrum counter electrode and a saturated calomel electrode (SCE) was obtained at -0.25 V. Spectral resolution is 4 cm-'. reference were used. The cell potential was controlled by using a Pine AFRDE4 potentiostat (Pine Instruments, Grove City, PA). Just prior to spectral acquisition, methanol was admitted to the cell by injecting the required volume (-100 pL) into the perchloric acid electrolyte solution. During methanol addition, the working electrode was positioned against the cell window and the potential was held fixed at -0.25 V. The solution was mixed by gentle agitation with a syringe, and then the single crystal working electrode was exposed to the methanol solution 2200 2000 1800 by pulling the electrode away from the window. After about Wavenumber (cm') 5- 10 s of exposure to the bulk methanol solution, the thin layer was reformed. Infrared spectIa were collected with a Digilab FTS-40 Fourier transform infrared spectrometer equipped with a liquid nitrogen cooled narrow band MCT detector.26 Spectra were obtained at the indicated potentials by stepping the working electrode potential in sequence from a reference value of -0.25 V. Single beam spectra were computed from the average of 1024 interferograms. Spectral resolution was set to 4 cm-l. 2200 2000 1800 Voltammetric studies of methanol oxidation were conducted Wavenumber (cm') in a three-electrode cell using a meniscus configuration. A platinum wire counter electrode and SCE reference were used. Figure 2. Infrared spectra of CO formed during the oxidation of methanol at platinum electrodes in aqueous 0.1 M HC104 electrolyte The reference electrode was located in a separate compartment solutions containing methanol: (A) Pt(335) in 50 mM methanol and and was connected to the main cell chamber through a wetted (B) Pt( 111) in 200 mM methanol. Spectral acquisition parameters are glass stopcock in contact with a Luggin capillary. As for the same as in Figure 1. spectroelectrochemical studies, the electrode was maintained at -0.25 V in a perchloric acid electrolyte solution as an aliquot At -0.1 V, CO spectral features are essentially undetectable of methanol was added. at Pt( 111), whereas three weak, but well-resolved, features For all experiments, potentials are reported with respect to appear at Pt(335). The lack of measurable CO adsorption at the SCE. Pt( 111) is consistent with previous studies, which show that

4

Results and Discussion Infrared spectra of CO formed from the oxidation of methanol at Pt(335) and Pt( 111) electrodes are shown in Figure 1. As described in the Experimental Section, methanol was admitted to each cell while the working electrode was held in fresh electrolyte solution at a potential of -0.25 V, and spectral acquisition began after methanol addition. Spectra were collected in sequence at the indicated potentials starting from -0.25 V, in the classical hydrogen adsorption region.

there is strong inhibition of methanolic CO formation on Pt(ll1) at potentials in the classical hydrogen adsorption For Pt(335), the appearance of weak CO spectral features at -0.1 V indicates that this surface plane is more effective than Pt( 111) in promoting methanol dissociative chemisorption under these conditions. To explore this behavior further, methanolic CO formation was examined as a function of the methanol concentration in solution. Figure 2 shows some results of this study. Panel A displays the spectral features of CO formed on a Pt(335) electrode in a perchloric acid electrolyte

Letters solution containing 50 mM methanol. Compared to the spectra in Figure 1, spectra in Figure 2A were obtained using a methanol concentration that was a factor of 2 lower, and a weak CO feature still appears at -0.1 V (2043 cm-', Figure 2A). Panel B of Figure 2 shows the spectral features of CO formed on Pt( 111) in a perchloric acid electrolyte solution containing 200 mM methanol. This methanol concentration is a factor of 2 greater than that used in obtaining spectra in Figure 1, and CO spectral features are still undetectable on Pt(ll1) at -0.1 V. In both cases (Figure 2, A and B), CO formation commences at more positive potentials, as expected (see below).8,21,22This concentration study provides further evidence for the inhibition of methanolic CO formation at Pt( 111) and the promotion of methanol dissociative chemisorption at Pt(335), at potentials in the classical hydrogen adsorption region. Another important characteristic of the low-potential (-0.1 V) Pt(335) spectrum shown in Figure 1 is that the bands which appear are associated with the vibrations of CO at different structural sites on the (335) surface plane. These three features can be assigned through comparison with vibrational spectra of CO adlayers formed on Pt(335) by direct CO dosing.13~14,23-34 Spectral features at 2009 and 2059 cm-' arise from CO in terminal coordination environments; the lower energy feature has been assigned to a vibrational mode of edgeCO and the higher energy feature to a terrace-CO mode.13~14~23.24~26-34 The band at about 1860 cm-I arises from CO at bridging coordination sites. It is near in energy to the bridging feature observed after direct CO dosing onto Pt( and Pt(335)14326electrodes. The latter study showed that the bridging CO feature was assignable to CO at (100) symmetry sites, which exist on the face of the Pt(335) step. In the case of methanolic CO, bridging features appear at Pt(100) elect r o d e ~ , ~ , 'but ~,'~ not at Pt(ll1) electrodes.*J' Thus, it is likely that the 1860 cm-l methanolic CO bridging feature at Pt(335) (Figure 1A) arises from edge-CO vibrations. Also significant is the wide separation of the terminal CO bands that appear in the low-potential (-0.1 V) Pt(335) spectrum. For CO adlayers formed by direct CO dosing, these features are not as easily distinguished, and there is considerable intensity transfer into the higher energy mode through dynamic dipole-dipole coupling interactions between molecules at edge and terrace site^.^^,*^ The highly resolved features that appear for methanolic CO indicate that molecules at edge and terrace sites are more weakly coupled, possibly because other adsorbed reaction intermediates disrupt the close interactions that occur on an otherwise clean platinum surface. At more positive potentials, methanolic CO spectral features appear on both Pt( 111) and Pt(335) electrodes. For Pt(l1 l), methanolic CO is initially detected at 0.0 V, just positive of the hydrogen adsorption region. A weak feature first appears at 2037 cm-' (Figure lB), which becomes more intense and shifts to higher energy with increasing electrode potential. This response has been observed in earlier studies8 and is characteristic of methanolic CO adlayer growth. The spectral shifts arise in part from dipole-dipole coupling interactions, which exist between neighboring adsorbates and become stronger as the adlayer density increases. Similar methanolic adlayer growth occurs at Pt(335) (Figure 1A). In this case, strong intermolecular coupling destroys the near independence of edgeand terrace-CO vibrational modes.13~14~23~24.26-34 As a consequence, the low-energy feature associated with edge-CO vibrations loses intensity and shifts to higher energy with increasing CO surface coverage, until only a single atop feature appears. The latter mode is associated with the coupled motion of both edge and terrace s p e ~ i e s . ~ ~ , ~ Although ~ , ~ ~ . ~the ~ ,potential ~~-~~

J. Phys. Chem., Vol. 99, No. 11, 1995 3421

-0.2 .

.

0.2

'

0.6

-0.2

.

. . 0.2

0.6

E W vs. SCE Figure 3. Voltammetry of the Pt(335) (A) and Pt(ll1) (B) electrode in 0.1 M HC104 electrolyte solution (top) and 0.1 M HC104 electrolyte solution containing 50 mM methanol (bottom). The voltammograms were recorded using a sweep rate of 50 mV/s.

range is more limited, these general trends are also observed in the spectra shown in Figure 2. Finally, the effect of electrode surface structure on the methanol oxidation rate was studied by cyclic and linear sweep voltammetry (Figure 3). Near the top of each panel in Figure 3, cyclic voltammograms for Pt( 111) and Pt(335) in methanolfree electrolyte solution are shown. Hydrogen adsorption/ desorption features appear at negative potentials, and the response across the full potential range is consistent with previously reported cyclic voltammograms of Pt( 111) (cf. refs 4b, 5, 26, 37-39, and 41) and Pt(335)26in aqueous perchloric acid electrolyte solutions. Linear sweep voltammograms obtained in solutions with methanol appear at the bottom of each panel. Although methanol concentrations differ among various experimental studies, the current density at the peak of the Pt( 111) methanol oxidation wave (ca. 1.1 mA/cm2) is within the range of values reported r e ~ e n t l y . ~For ~ ,Pt(335), ~ the linear sweep voltammetry shown in Figure 3 indicates that the current density for methanol oxidation at this surface is greater than at Pt( 11l),for equivalent potentials. This is consistent with a trend observed by Adzic? where for methanol oxidation the lowest current densities occurred at Pt( 111) and generally increased with increasing surface corrugation. The magnitude of the current density at Pt(335) is somewhat lower than expected in view of the extent of surface corrugation; for example, the current density measured at Pt( 110) under similar conditions is more than 20 times largerS5More extensive investigations using a series of structurally related stepped surfaces are needed to address this issue further.

Conclusions The reported infrared spectroscopic experiments confirm that methanolic CO formation is inhibited at Pt( 111) at potentials in the classical hydrogen adsorption region, and they reveal that the corrugated Pt(335) surface plane promotes methanol dissociative chemisorption in this potential region. This study also shows that independent vibrational bands associated with methanolic CO at different structural sites (e.g., step and terrace sites) on the (335) surface plane can be detected by controlling

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3422 J. Phys. Chem., Vol. 99, No. 11, 1995 the electrode potential and the methanol concentration in solution. This information is useful for probing site-dependent aspects of methanol surface electrochemistry.

Acknowledgment. This work was supported by the Office of Naval Research. References and Notes (1) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (2) Beden, B.; Leger, J.-M.; Lamy, C. In Modem Aspects of Electrochemistry; Bockris, J. 0..Conway, B. E., White, R. E. Eds.; Plenum: New York, 1992; Vol. 22, p 97. (3) Franaszczuk, K.; Herrero, E.; Zelenay, P.; Wieckowski, A,; Wang, J.; Masel, R. I. J. Phys. Chem. 1992, 96, 8509. (4) (a) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J . Phys. Chem. 1993,97, 12020. (b) Markovic, N.; Ross, P. N. J. Electroanal. Chem. 1992, 330, 499. (5) Herro, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (6) Adzic, R. In Modem Aspects of Electrochemistry; White, R. E., Bockris, J. O., Conway, B. E., Eds.; Plenum Press: New York, 1990; Vol. 21, p 163. (7) Chang, S. C.; h u n g , L. W. H.; Weaver, M. J. J . Phys. Chem. 1990, 94, 6013. (8) Chang, S. C.; Ho, Y.; Weaver, M. J. Su$. Sei. 1992, 265, 81. (9) Sun, S. G.; Clavilier, J. J . Electroanal. Chem. 1987, 236, 95. (.lo) Somorjai, G. A. Chemistry in Two Dimensions: Suqaces; Cornel1 University Press: Ithaca, NY, 1981. (11) Somorjai, G. A. J. Phys. Chem. 1990, 94, 1013. (12) Somorjai, G. A. Su$. Sci. 1991, 242, 481. (13) Greenler, R. G.; Burch, K. D.; Kretzschmar, K.; Klauser, R.; Bradshaw, A. M.; Hayden, B. E. Surf. Sci. 1985, 153, 338. (14) Kim, C. S.; Korzeniewski, C.; Tornquist, W. J. J. Chem. Phys. 1994, 100, 628. (15) Korzeniewski, C.; Severson, M. W. Spectrochim. Acta, in press. (16) Juanto, S.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1987, 237, 119. (17) Beden, B.; Juanto, S.; Leger, J. M.; Lamy, C. J . Electroanal. Chem. 1987, 238, 323. (18) Beden, B.; Hahn, F.; Lamy, C.; Leger, J. M.; Tacconi, N. R. d.; Lezna, R. 0.;Arvia, A. J. J. Electroanal. Chem. 1989, 261, 401. (19) Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem. 1989, 93, 7218.

(20) Morallon, E.; Vazquez, 3. L.; Perez, J. M.; Beden, B.; Hahn, F.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1993, 344, 289. (21) Matsuda, S.; Kitamura, F.; Takahashi, M.; Ito, M. J . Electroanal. Chem. 1989, 274, 305. (22) Iwasita, T.; Nart, F. C. J. Electroanal. Chem. 1991, 317, 291. (23) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf. Sci. 1985, 149, 394. (24) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. C h m . Phys. 1994, 101, 9113. (25) Wang, H.; Tobin, R. G.; Lambert, D. K. J . Chem. Phys. 1994,101, 4277. (26) Kim, C. S.; Tornquist, W. J.; Korzeniewski, C. J. Phys. Chem. 1993, 97, 6484. (27) Luo, J. S.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. J . Chem. Phys. 1993, 99, 1347. (28) Xu, J.; Yates, J. T., Jr. J . Chem. Phys. 1993, 99, 725. (29) Jansch, H. J.; Xu, J.; Yates, T. T., Jr. J . Chem. Phys. 1993, 99, 721. (30) Luo, J. S.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. Surf. Sei. 1992, 274, 53. (31) Xu, J.; Henriksen, P.; Yates, J. T., Jr. J . Chem. Phys. 1992, 97, 5250. (32) Lambert, D. K.; Tobin, R. G. Surf. Sci. 1990, 232, 149. (33) Luo, J.-S.; Tobin, R. G.; Lambert, D. K.; Wagner, F. T.; Moylan, T. E. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 469. (34) Leibsle, F. M.; Sorbello, R. S.; Greenler, R. G. Surf. Sci. 1987, 179, 101. (35) Greenler, R. G.; Leibsle, F. M.; Sorbello, R. S. Phys. Rev. E 1985, 32, 8431. (36) Greenler, R. G.; Dudek, J. A.; Beck, D. E. Surf. Sci. 1984, 145, u53. (37) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (38) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (39) Zurawski, D.; Rice, L.; Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 230, 221. (40) Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 227, 259. (41) Wasberg, M.; Palaikis, L.; Wallen, S.; Kamrath, M.; Wieckowski, A. J . Electroanal. Chem. 1988, 256, 51. (42) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095. JP943352Z