Pattern Formation during CO Electrooxidation on Thin Pt Films Studied

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2008, 112, 9548–9551 Published on Web 06/11/2008

Pattern Formation during CO Electrooxidation on Thin Pt Films Studied with Spatially Resolved Infrared Absorption Spectroscopy Richard Morschl,† Johannes Bolten,†,| Antoine Bonnefont,‡,⊥ and Katharina Krischer*,†,§ Physik Department, Technische UniVersita¨t Mu¨nchen, 85748 Garching, Germany, and Institut de Chimie, CNRS et UniVersite´ de Strasbourg, 4 rue Blaise Pascal, BP 1032, 67070 Strasbourg Cedex, France ReceiVed: May 06, 2008; ReVised Manuscript ReceiVed: May 29, 2008

Infrared absorption spectroscopy in the attenuated total reflection configuration is utilized to obtain spatially resolved information on the metal liquid interface under reaction conditions. This is demonstrated with in situ infrared measurements of the CO coverage on thin Pt films during CO electrooxidation under galvanostatic control conditions. The formation of two stationary domains exhibiting different CO coverages is observed. The relative sizes of the domains adjust with the applied current while the density of adsorbed CO molecules in each domain is independent of it. I. Introduction In situ infrared absorption spectroscopy is one of the few tools available to obtain information about molecules adsorbed at the solid-liquid interface.1 IR absorption experiments under steady reaction conditions became possible with the introduction of attenuated total reflection (ATR) IR absorption spectroscopy.2 Here, a thin metal film deposited on the base of an optical prism serves as the working electrode of an electrochemical cell. The IR beam is directed through the prism to the back of the electrode from where it is reflected, the evanescent wave interacting with the molecules at the interface. This technique considerably widened the applicability of IR measurements for kinetic and mechanistic studies (see, e.g., ref 3). ATR-IR absorption spectroscopy could even be successfully applied to oscillatory electrode reactions.4 So far, a restriction of this technique has been that it yielded only spatially averaged signals, whereas information about the spatial distribution of adsorbates could not be obtained. The latter information is in particular valuable when studying systems that exhibit nonlinear phenomena, such as bistability and oscillations, since they often go along with spatial pattern formation.5 One prominent example for which pattern formation on the electrode surface has been predicted is the electrooxidation of CO.6,7 In this letter we demonstrate that employing a focal plane array (FPA) detector it is possible to obtain spatially resolved information about adsorbates with a spatial resolution of some 10 µm during CO electrooxidation. More specifically, we show that under galvanostatic conditions CO oxidation does not occur uniformly on the electrode surface. Rather, two domains form, one with a densely packed CO adlayer and a reactive one that is practically free of adsorbed CO. * To whom correspondence should be addressed. E-mail: krischer@ ph.tum.de. URL: http://www.physik.tu-muenchen.de/lehrstuehle/E19/ AG%20Krischer.html. † Technische Universita ¨ t Mu¨nchen. ‡ CNRS et Universite ´ de Strasbourg. § E-mail: [email protected]. | E-mail: [email protected]. ⊥ E-mail: [email protected].

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Figure 1. Optical path of the IR beam in the experimental setup. The radiation is totally reflected at an angle of 60° at the solid/liquid interface and probes a region of 2.5 × 4.3 mm2.

II. Experimental Section A. Setup for Spatially Resolved IR Measurements. The key component of our setup for spatially resolved IR measurements is a MCT focal plane array (FPA) detector placed in an IMAC macro chamber (both Bruker Optics, Germany). A Bruker Tensor 27 Fourier transform spectrometer was hooked to the IMAC chamber. The FPA detector consists of an quadratic array of 64 × 64 IR sensitive elements with a size of 40 × 40 µm2, enabling the simultaneous acquisition of 4096 interferograms. An imaging optics between the sample (in our case the working electrode) and the detector maps the probed surface region onto the FPA. Since an interferogram of a specific electrode position is associated with a certain detector pixel, spatially resolved probing of the metal liquid interface becomes possible. The spectrometer was operated at 4 cm-1 resolution, and 40 interferograms were added for a spectrum. The experiments were carried out in the ATR configuration. Details of ATR-IR absorption spectroscopy are described in refs 8 and 9. Figure 1 depicts the optical path of our current setup. Infrared radiation from a MIR source was first guided through an optical prism to the interface via a gold mirror at an incident angle of 60°. At the interface, the IR beam experienced attenuated total reflection, and a second gold mirror directed it through the imaging optics to the detector. With this guidance of the IR beam, a region of 2.5 × 4.3 mm2 located at the center of the Pt film is projected onto the detector.  2008 American Chemical Society

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Figure 2. Electrochemical cell used for IR measurements. A chemically deposited Pt film was used as the working electrode. The radiation totally reflected at the metal/liquid interface is analyzed.

In a transmission experiment, the detector optics images a 2.5 × 2.5 mm2 region of the sample plane 1:1 onto the FPA detector. Since the probed region of the Pt film should be imaged exactly onto the detector, the ATR setup was optimized in a way such that the optical path length of the light through the prism exhibited a minimal deviation from the one of the transmission configuration. A monocrystalline, p-type Si prism (OEC GmbH, Germany, 1 Lambda) with the shape of a truncated cone served as the ATR element. Two sides of the prism were cut at an angle of 60° with respect to the totally reflecting plane and acted as windows for the IR radiation. A thin platinum film (circular shape, 4.56 cm2) was deposited on the totally reflecting plane of the prism with an electroless process according to Osawa’s method described in ref 10. Information about the spatial distribution of the CO coverage was obtained by integrating the CO peaks of the spectra recorded for each detector pixel. Since different regions of our Pt films show different strengths of the absorption signal, each pixel is normalized to its value at complete saturation. The integrated and normalized absorption peak is plotted vs the electrode position in a 2D pseudocolor plot. All spectra are displayed in absorbance units (A ) -log(I/ Iref)), where I and Iref are the intensities of the infrared radiation reflected at the Pt/electrolyte interface with and without CO, respectively. B. Electrochemical Setup. The electrochemical cell was hooked to the reflecting base of the optical prism as depicted in Figure 2. It consisted of two glass compartments and three silicone tubes connecting both. With this two-part cell a socalled “bubble lift” convection11 can be realized, providing an enhanced and defined convective transport of the electrolyte. It is accomplished by attaching a glass capillary with a funnel on one side to the middle silicone tube. Gas pumped into the cell at its bottom bubbles up in the outer two tubes and takes the electrolyte with it. While the gas leaves the cell at its top, the electrolyte flows back to the cell’s bottom in the middle tube. The arrows inside the tubes in Figure 2 indicate the flow of electrolyte. Before any measurement, the glass components were cleaned in hot HNO3 vapor and carefully rinsed with Millipore water. The silicone tubes were cleaned with Millipore water. A Pt wire in the lower compartment and a reversible hydrogen electrode (RHE) in the upper compartment were used as counter and reference electrodes, respectively. The Pt film on the Si prism served as the working electrode and was attached to the cell by

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Figure 3. Integrated CO peak plotted versus space for the following currents: 0.3, 1.4, 1.5, 1.7, 1.75, and 1.8 mA (left to right and top to bottom). Red indicates a high CO coverage, and blue a CO free surface. Note that the blue region in the middle of the lower edge indicates an area where the film hardly showed any CO peak even for complete coverage. Therefore, this region was set to 0 in all images.

sandwiching an O-ring. The geometrical area of the Pt film in contact with the electrolyte was approximately 175 mm2 . The Pt film was contacted with a Pt wire wrapped around a Teflon ring (see Figure 2). A FHI potentiostat was used for the potentiostatic measurements. Its analogue output voltages were digitized by a National Instruments data acquisition board (BNC-2090) and transferred to a personal computer. The galvanostatic measurements were carried out with a Jaissle potentiostat (PGU 10V-1A-IMP-S) together with its related software (EcmWin). The electrolyte was a 0.5 M H2SO4 solution prepared with Milli-Q water and 96% H2SO4 (Merck suprapure). In order to obtain information about the quality of the Pt film electrode, cyclic voltammograms were recorded in an argon (Air Liquide Germany, 4.8N) purged electrolyte, and IR reference spectra were taken at different potentials. Good quality films exhibited the typical cyclovoltammetric signature of polycrystalline Pt electrodes. Afterward, the electrolyte was saturated with carbon monoxide (Air Liquide Germany, 4.7 N), and several spectra were recorded under potential control at 140 mV vs RHE. A completely covered surface was assumed whenever the shift in the peak position of linearly bonded CO due to the coverage dependent dipole-dipole interaction vanished. III. Results and Discussion Spatially resolved in situ IR measurements at the Pt electrolyte interface during the electrooxidation of CO were carried out under galvanostatic control conditions. Figure 3 depicts 2D pseudocolor plots of the integrated CO peak intensities versus space for different preset currents. A rainbow color scale is used for the presentation with red indicating a high CO coverage and blue a low one. For applied currents up to 1.3 mA, the values of CO peak intensities exhibit only slight deviations from the ones of the surface with maximum CO coverage. Thus, this region of the electrode is still completely covered by CO. When the preset current is further increased up to 1.8 mA, the probed area shows two different regions: one in which the integrated intensity is still unchanged compared to the completely covered surface and one in which no CO peak is

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Figure 4. Spectra of detector pixel that correspond to (a) a covered, (b) an intermediate, and (c) an uncovered state. The CO peaks of the high covered and the intermediate state are located at 2099 cm-1.

discernible anymore, i.e., this region is almost free of CO. The higher the set current, the larger the region that is practically free of adsorbed CO. Finally, for currents higher than 1.8 mA the low-coverage state spreads over the entire probed region. A closer inspection of the images reveals that some exhibit a quite broad transition region between the CO covered and almost free portions of the electrode. Here, the integrated absorbance of the CO peak takes on an intermediate value. Further information about the CO adlayer can be obtained from the wavenumber of the maxima of vibrational CO peaks at different positions on the electrode on the one hand, and from mean spectra averaged over all detector pixels, on the other hand. Representative spectra of three individual detector pixels that correspond to electrode positions in a high-coverage, an intermediate, and a low-coverage state, are depicted in Figure 4. The CO peaks of the first two spectra are both located at 2099 cm-1, the one of the intermediate state showing a reduced height. These peaks are attributed to linearly bonded COad. In our galvanostatic experiments, we never found an indication of bridge bonded or multifold-bonded COad. In the spectrum of the pixel corresponding to the low-coverage state (Figure 4c) a CO peak is not discernible at all, confirming the interpretation that in this region hardly any CO is adsorbed. Figure 5 compares averaged spectra of all detector pixels of patterned states for different applied currents to averaged spectra obtained during adsorption experiments at a fixed potential of 140 mV (RHE). In the latter, several spectra were recorded after starting to purge the Ar-saturated electrolyte with CO. These experiments always yielded uniform images of the CO coverage, reflecting a homogeneous CO coverage of the electrode. Furthermore, the peak position shifts from 2071 to 2087 cm-1 with progressing time (Figure 5a), and the peak intensity increases. This increase signifies an increasing coverage, while the peak shift can be attributed to an increasing dipole-dipole interaction between adsorbed CO molecules with increasing coverage. Thus, these two features support the picture of a uniformly increasing CO coverage during CO adsorption at a low, fixed potential. In the case of our patterned states (Figure 5b) the situation is completely different. The peak position in the average spectra does not change for increasing currents; rather, for all applied

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Figure 5. Average spectra of all detector pixel: (a) for a homogeneously covered surface at a potential of 140 mV for two different times after the CO supply was started. The spectrum depicted as a solid line was recorded after the dashed one. (b) For patterned states at 0.1 (solid), 1.4 (dashed), 1.5 (dotted), and 1.7mA (dashed-dotted). The peaks in panel a are located at 2087 (solid line) and 2071 cm-1 (dashed line). The peaks of all patterned states have their maximum at 2099 cm-1 for approximately the same electrode potential of 580 mV.

current densities, it is located at 2099 cm-1, while the peak intensity decreases. We can thus conclude that all adsorbed CO molecules experience the same local CO coverage. Yet, the fraction of the electrode surface that is covered with CO decreases with increasing current, which results in a decreased peak height. We also observe that the peak position coincides with the one of a fully CO-covered surface. Note that also the peak positions of intermediate states are at 2099 cm-1 (cf. Figure 4) suggesting that also for this state the coverage density of adsorbed CO is the same as for the high-coverage state. This might be due to one of the following reasons: Fluctuations in the electrolyte transport can cause fluctuations in the domain’s sizes. The reduced height stems then from temporal changes in the extension of the high coverage state during the accumulation of the interferograms added for one spectrum. Another possible explanation is the formation of micro islands in the high covered state in the transition region with an extension smaller than the area probed by one pixel (∼40 × 80 µm2). In this case, positions in the high-coverage and the lowcoverage state contribute to the pixel’s absorption spectrum. Although, the images of the CO coverage obtained for a certain applied current after 20 s and 5 and 12 min differed only slightly from each other, without further analysis we do not want to exclude the possibility of an ongoing formation of the patterned state as an explanation of the reduced peak height. Taking all our observations together, we arrive at the following consistent picture for galvanostatic CO oxidation: Under galvanostatic conditions two domains form on the electrode, one of them being fully covered by COad and for this reason having a negligible reaction rate, the other one being practically uncovered by CO, the reaction occurring at a diffusion limited rate. The relative fraction of the two domains adjusts such that the ‘reactive domain’ can deliver the preset current. This interpretation can be further elucidated by looking at potentiostatic and galvanostatic current-potential curves. Figure 6 depicts a cyclic voltammogram (CV) of the Pt film in a CO saturated electrolyte taken at 20 mV/s. In the potential region between approximately 0.5 and 0.6 V, the CV exhibits a pronounced hysteresis between a low current and a high current

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J. Phys. Chem. C, Vol. 112, No. 26, 2008 9551 region that is to a first approximation independent of the applied current and homogeneous in space. The local properties of the two domains coincide with those of the two stable states in the bistable region under potentiostatic conditions when a voltage is applied equal to the value the electrode potential takes on under galvanostatic conditions. IV. Conclusion

Figure 6. Cyclic voltammogram of the Pt film in a CO saturated electrolyte (solid line) taken at 20 mV/s and potentials for different applied currents during galvanostatic measurements (red crosses).

branch. Meanwhile, it is well established that this hysteresis originates from a bistability in the polarization curve.12 In a bistable system, two stable steady states are separated by an unstable one. The latter connects the ends of the low and high current branches, and thus the bistable behavior is related to an S-shaped polarization curve. Naively, one would expect that under current control and intermediate current values the electrode potential takes on a value of the potentiostatically unstable branch at the chosen current density. However, the measurements show that this is not the case: The red crosses in Figure 6 are current-potential pairs measured under galvanostatic control. Obviously, independent of the applied current the electrode potential takes on a value that lies in the middle of the bistable region at approximately 580 mV. Pattern formation in electrochemical systems with an Sshaped current-potential characteristic under galvanostatic control was investigated theoretically for a two variable model system in one spatial dimension.13 Our experimental findings are, in fact, in agreement with the theoretical predictions, corroborating further the above given interpretation: The theoretical analysis predicts that under galvanostatic conditions the middle uniform state of the S-shaped polarization curve is unstable with respect to spatial fluctuations, the electrode splitting into two stationary domains, one with a high current density and one with a low current density. The local properties of the two states are independent of applied current; when continously changing the preset current from 0 to the diffusion limited value only the relative areas of the domains change from a completely covered to a totally uncovered state. Furthermore, as long as the spatial domains exist, the potential drop over the double layer takes on an intermediate value inside the bistable

The introduction of spatially resolved infrared absorption spectroscopy opens the possibility to probe inhomogeneities in adsorption layers, and thus reaction rates, on meso- and macroscopic scale. The present study demonstrates that this technique reveals novel information even on a well studied system namely CO electrooxidation on Pt under current control: Whenever the preset current is set to an intermediate value between the ones corresponding to completely covered and a totally uncovered surface two stationary domains form. One domain exhibits a high-coverage state and the other one a low-coverage state. When changing the applied current only the relative areas of the domains change, while the density of adsorbed CO of both states is independent of it. These measurements are in excellent agreement with existing theoretical studies of systems exhibiting an S-shaped current potential characteristic. Acknowledgment. We thank P. Bauer for fruitful discussions. This work was in part supported by the Deutsche Forschungsgemeinschaft (Project KR1189/10). References and Notes (1) Rodes, A. Perez J. M. and Aldaz, A. Handbook of Fuel Cells, Vol. 2; Vielstich, W., Lamm A., Gasteiger, H. A., Eds.; Wiley: Chichester, U.K., 2003; Chapter 16. (2) Osawa, M. Handbook of Vibrational Spectroscopy; Wiley: Chichester, 2002; pp 785-799. (3) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. J. Angew. Chem., Int. Ed. 2006, 45, 981. (4) Samjeske, G.; Osawa, M. Angew. Chem., Int. Ed. 2005, 44, 5694. (5) Krischer, K.; Mazouz, N.; Grauel, P. Angew. Chem., Int. Ed. 2001, 40, 851. (6) Bonnefont, A.; Varela, H.; Krischer, K. ChemPhysChem. 2003, 4, 1260. (7) Krischer K. AdVances in Electrochemical Science and Engineering; Kolb D. M., Alkire R. C., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (8) Ataka, K.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (9) Osawa, M.; Ataka, K.; Yoshii, K.; Yotsuyanagi, T.; Electron, J. Spectrosc. Relat. Phenom. 1993, 64/65, 371. (10) Miki, A.; Ye, S.; Senzaki, T.; Osawa, M. J. Electroanal. Chem. 2004, 563, 23. (11) da Fonseca, C.; Ozanam, F.; Chazalviel, J.-N. Surf. Sci. 1996, 365, 1. (12) Koper, M. T. M.; Schmidt, T. J.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2001, 105, 8381. (13) Krischer, K.; Mazouz, N.; Fla¨tgen, G. J. Phys. Chem. B 2000, 104, 7545.

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