A High Pressure PM-IRRAS Study of CO and O2 Coadsorption and

Jun 28, 2008 - Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, UMR 5182, CNRS, UniVersité de Lyon, 46 Allée d'Italie, F-69364 Lyon Cedex 06, ...
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J. Phys. Chem. C 2008, 112, 10862–10867

A High Pressure PM-IRRAS Study of CO and O2 Coadsorption and Reactivity on PtSn Alloy Surfaces Ce´line Dupont,†,‡ David Loffreda,† Franc¸oise Delbecq,† Francisco Jose´ Cadete Santos Aires,‡ Eric Ehret,‡ and Yvette Jugnet*,‡ Laboratoire de Chimie, Ecole Normale Supe´rieure de Lyon, UMR 5182, CNRS, UniVersite´ de Lyon, 46 Alle´e d’Italie, F-69364 Lyon Cedex 06, France, Institut de recherches sur la catalyse et l’enVironnement de Lyon, UMR 5256, CNRS, UniVersite´ de Lyon, 2 AVenue Albert Einstein, F-69626 Villeurbanne Cedex, France ReceiVed: March 19, 2008; ReVised Manuscript ReceiVed: May 21, 2008

The adsorption of CO on the two terminations, p(2 × 2) and (3 × 3) R30°, of the bulk Pt3Sn(111) alloy is studied by high pressure polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The influence of O2 is also investigated. While no differences are observed when CO is chemisorbed alone, the behavior of the two terminations clearly differs in presence of O2. The presence of oxygen induces modifications on the (3 × 3)R30° surface. Finally we study how the interaction between O2 and the surface modifies the reactivity. Hence we show that the p(2 × 2) termination is more efficient than the (3 × 3) R30° for CO oxidation. However this termination remains more efficient than pure platinum. I. Introduction Platinum has been considered as an efficient fuel cell catalyst over the years for methanol or for hydrogen oxidation. These two reactions involve the presence of carbon monoxide: as an intermediate during methanol dehydrogenation1 or as an impurity contained in hydrogen combustible. In both cases, CO acts as a poison which weakens the catalyst activity because of its high affinity for platinum. A promising way to improve the catalyst tolerance toward CO poisoning has been proposed several times by different authors; it consists in replacing the pure metal platinum by bimetallic alloys such as PtRu,2–4 PtMo,5 or PtSn.6,7 The alloying of platinum with a second metal may have two important consequences: an electronic (or ligand) effect which reduces the strength of CO adsorption on platinum or a selective adsorption effect leading to a bifunctional mechanism in which CO adsorbs preferentially on Pt atoms whereas the oxidizing species is selectively bound to the second metal. Both effects have been confirmed, in particular on PtSn alloys, by the studies of CO and O chemisorption. In fact, theoretical and experimental investigations 6–9 have shown that CO interacts essentially with Pt, and that the adsorption strength is lower than on pure platinum surfaces. They have also proved that oxygen interacts preferentially with tin. Regarding the activity of bimetallic surfaces toward CO oxidation, an enhanced efficiency of Pt-based bimetallic alloys has been obtained for PtRu10 and PtSn.10–12 Thus a more accurate study of the chemisorption properties of these alloys is of great interest in order to improve our understanding of fundamental mechanisms, and in order to design new active, resistant and selective catalysts. In this work, we will focus on the coadsorption and reactivity of carbon monoxide and oxygen under elevated pressures on PtSn surfaces studied by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The Pt3Sn(111) * Corresponding author. E-mail: [email protected]. † Laboratoire de Chimie, Ecole Normale Supe ´ rieure de Lyon, UMR 5182, CNRS, Universite´ de Lyon. ‡ Institut de recherches sur la catalyse et l’environnement de Lyon, UMR 5256, CNRS, Universite´ de Lyon.

Figure 1. Top views of (a) the p(2 × 2) and the (b) (3 × 3) R30° terminations of the Pt3Sn(111) alloy. The surface layers of these two terminations contain 25 and 33 at. % of tin for the p(2 × 2) and the (3 × 3) R30° termination respectively. Platinum atoms are reported in yellow; tin atoms are in green.

bulk alloy exhibits two surfaces, the p(2 × 2) and the (3 × 3) R30° terminations which will be compared in this study. These two surface structures have already been described13 and are represented in Figure 1.They differ by Pt and Sn atomic arrangements, leading to specific adsorption sites, and by tin concentration in the surface layer: 25 and 33 at. % Sn for the p(2 × 2) and for the (3 × 3) R30° structures, respectively. Beyond the single vibrational study of CO chemisorption, the final aim is to determine the activity of PtSn catalysts regarding CO oxidation and more precisely to check the influence of the surface structure. To do so, we expose in a first part the observations for the adsorption of CO alone under high pressure conditions, then we briefly report on the coadsorption of O2 and CO under high pressure. Finally a preliminary study of reactivity is performed, under catalytic conditions to estimate the activity of these two different alloy surfaces for the CO oxidation. Those results are compared to those obtained with Pt(111) under identical experimental conditions. II. Experimental Setup The experiments were conducted in a ultra high vacuum (UHV) system composed of three chambers separated by gate

10.1021/jp802416f CCC: $40.75  2008 American Chemical Society Published on Web 06/28/2008

A High Pressure PM-IRRAS Study valves. The central one is used for sample preparation and surface characterization. It is equipped with an ion gun, an electron gun for sample annealing, a mass spectrometer, a dual Mg/Al X-ray source and an hemispherical analyzer for XPS investigations. The base pressure is in the 10-10 Torr range. On one side of the instrumental setup, a chamber dedicated to low pressure chemisorption studies is equipped with a ELS3000 (LK technologies) high resolution electron energy loss spectrometer (HREELS). On the other side of the preparation chamber, the sample can be transferred into a small stainless steel reactor placed inside an infrared environment and previously described.14 Briefly, it has a size of about 1 L and it is equipped with two ZnSe IR windows. Pressure inside the reactor can be varied from the low 10-9 Torr range up to atmosphere. A leak valve between the reactor and the mass spectrometer allows gas sampling and characterization of the reactants and products. PM-IRRAS spectra are obtained from a Thermo Nicolet NEXUS Fourier transform infrared spectrometer in specular reflection mode under grazing incidence (8° relative to the surface). The incident beam is intercepted by a ZnSe polarizer and a 100 MHz photoelastic modulator (PEM-90 from HINDS Instruments) allowing very rapid polarization changes. The polarizer allows the selection of the p- (in the plane of incidence) or of the s- (perpendicular to the plane of incidence) component of the IR beam. The PEM is orientated in such a way that the incident light is polarized in a plane which is at 45° to its main axis. After reflection on the surface, the IR beam is focused onto the mercury cadmium telluride (MCT) detector by an optical lens. After demodulation, two spectra are obtained corresponding to (p + s) and (p - s) signals. By combining these spectra, the gas phase (insensitive to polarization effects) and the adsorbate phase (orientated by the surface and thus very sensitive to polarization effects) can be separated. The surface signal is obtained from the ratio of these spectra (p - s)/(p + s) while the gas phase signal is obtained from the “s” signal alone. Spectra collected at various pressures are then normalized by the corresponding reference spectrum obtained under vacuum. The spectra are collected with a spectral resolution of 4 cm-1 with 1024 scans coadded. The catalytic performances are deduced from mass spectroscopy measurements. For quantification purposes, sensitivity factors and fragmentation coefficients have been determined separately for pure reactants (CO, O2) and product (CO2). The activity, expressed in mol · cm-2 · s-1, is obtained from the slope of the curve P(CO2) ) f(t) determined for conversion rates lower than 50%. The Pt3Sn(111) sample is a small disk of 0.8 cm diameter. The sample was prepared following the procedure of Bardi et al.13 and its cleanliness and stoichiometry were controlled by XPS. Both surfaces were prepared by classical series of sputtering and annealing cycles. The p(2 × 2) is obtained after annealing at 1000-1100 K while the (3 × 3) R30° is obtained at 600 K. Each annealing lasts for 20 min in order to reach the segregation equilibrium. A quantification of our XPS results after the cleaning procedure has been performed along the normal to the crystal from the Pt 4f and Sn 3d core level area. In fact the p(2 × 2) termination corresponds to a simple truncation of the bulk crystal. The mean value of tin concentration normalized to that of the p(2 × 2) taken at 25 at. % reaches only 16.5 at. % in the case of the (3 × 3) R30°. Although these values are integrated over the analyzed depth, they clearly indicate a tin depletion in the surface sublayers of the (3 × 3) R30° in agreement with Bardi and co-workers.13

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Figure 2. PM-IRRAS spectrum of CO chemisorption on Pt(111) at room temperature and under 10-1 Torr CO.

For comparison, a few experiments have been performed on Pt(111). This sample is a small disk of 1 cm diameter. It was also prepared by series of sputtering and annealing cycles. The sputtering was carried out during 10 min and the annealing during 5 min at 1000 K. After the first cycle a supplementary annealing at 900 K under oxygen pressure (10-7 Torr) was done to eliminate carbon traces. CO gas was first passed through a molecular sieve (5A 4-8) and then in a liquid nitrogen trap in order to prevent any Ni carbonyl adsorption and decomposition on the surface. The CO gas container (Linde 3.7) was equipped with a Ni free valve (Air Liquide). No manometer was used for introduction in the reactor. CO2 (Linde 4.5) and O2 (alphagas N55) were only passed through a liquid nitrogen trap. III. Results A. CO Chemisorption. As a reference point, we report in Figure 2 a normalized PM-IRRAS spectrum measured on Pt(111) in contact with 10-1 Torr of CO at room temperature. This spectrum shows an intense and positive band at 2096 cm-1 characteristic of terminally bonded CO and a broad band at 1860-1880 cm-1 assigned to bridge bonded CO. The CO stretching band positions are in good agreement with those reported by Andersen et al.15 in a closely related experiment. The main difference comes from the procedure of background subtraction. In our case, we have chosen to subtract the reference spectrum (characteristic of the “clean surface”) from the spectrum measured under 10-1 Torr CO. However the clean surface spectrum is measured about 1 h after the last annealing of the sample and after sample transfer to the reactor. Under such conditions, it is not possible to get a surface free of CO. So, the band which appears as negative in the spectrum at 2084 cm-1 corresponds to the residual CO which has chemisorbed while the sample was cooling back to room temperature after the last annealing. This phenomenon increases the difficulties to obtain quantitative results. The CO adsorption is now studied on the two terminations of Pt3Sn(111) sample. Figure 3 shows the CO stretching spectral region of successive PM-IRRAS spectra recorded at room temperature after an exposure of 1000 L (1 L ) 10-6 Torr · s) dosed at 10-5 Torr (a) and after cumulative and increasing pressures from 10-2 Torr (b) to 10-1 Torr (c), 1 Torr (d), 10 Torr (e), and 100 Torr (f). Spectra (a) have been recorded under dynamic conditions whereas spectra (b-f) have been registered

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Figure 3. Pressure dependence of PM-IRRAS spectra for CO adsorbed at room temperature on Pt3Sn(111)-p(2 × 2) and Pt3Sn(111)-(3 × 3) R30°. (a) Spectra have been obtained after an exposure of 1000 L. Other spectra have been registered under static conditions: (b) 10-2 Torr; (c) 10-1 Torr; (d) 1 Torr; (e) 10 Torr; (f) 100 Torr; (g) after evacuation of the reactor.

under static conditions. The reactor is then evacuated and new spectra (g) are recorded. According to Figure 3, the two terminations have almost the same behavior toward CO adsorption. After a 1000 L exposure, a small band characteristic of CO adsorbed on top platinum is observed at 2077 cm-1 on both surfaces. This band is not very well-defined, especially on the p(2 × 2) structure, and it is of relatively small intensity. By opposition to our previous HREELS observations on the p(2 × 2) structure9 no multibonded CO is detected when experiments are performed under high pressure. This may result from different surface coverage conditions or from different sensitivity between HREELS and PM-IRRAS. When increasing the static pressure from 10-2 Torr to 100 Torr, this band becomes sharper and more intense. The intensity of this band is increased by about 170% on the p(2 × 2) structure and by about 30% on the (3 × 3) R30° as soon as 10-2 Torr of CO are introduced. For higher exposure, the intensity change is not significant anymore. Although Pt(111) and Pt3Sn(111) crystals do not have exactly the same size, rendering the quantification difficult, the intensities measured on Pt3Sn(111) are small compared to those measured on Pt(111) (see Figure 2). Three main reasons can explain these lower intensities measured on the alloy surface: the smaller size of the sample, the reduced number of Pt atoms in the surface layer of the Pt3Sn alloy and the weakening of CO adsorption already observed7 in presence of tin. The ν(CO) band shifts progressively toward 2089 cm-1, a value close to that measured on Pt(111) (see Figure 2). The observed shift is small (less than 12 cm-1) and depends on termination and pressure. In agreement with our previous HREELS and DFT investigation9 of CO adsorption on Pt3Sn(111)-p(2 × 2), this shift is linked to the increase of the CO surface coverage. Moreover, this order of magnitude is consistent with previous results of CO chemisorption on Pt(111).15,16

Finally, just after evacuation of the reactor down to 10-7-10-6 Torr, new spectra are recorded (Figure 3(g)). There is still some CO remaining. Compared to the measurements performed under high pressure (Figure 3, spectra (b-f)), the simultaneous decrease of the intensity and the redshift of the band are consistent with a lowering of the CO coverage. The small differences of intensity and band position observed between before (spectra (a)) and after (spectra (g)) evacuation of the reactor can be assigned to the difference of pressure at which measurements have been performed: in the 10-9 Torr range before high pressure experiments and only in the 10-7-10-6 Torr range just after evacuation of the reactor. In the following, before investigating CO oxidation, we will study the coadsorption of CO and O2 following two procedures. In the first case labeled below as “CO + O2 coadsorption”, CO is introduced first and let in contact with the surface. During that period, the stability of chemisorbed CO is checked and preliminary PM-IRRAS spectra are recorded; afterwhile oxygen is introduced. In the second procedure “O2 + CO coadsorption” the order is reversed; O2 is introduced first, then CO is added. B. CO+O2 Coadsorption. The influence of O2 on CO predosed on both surfaces is now studied at room temperature and represented in Figure 4. A first PM-IRRAS spectrum (see Figure 4(a)) is registered under 10-1 Torr of CO, without O2. After 90 min under 10-1 Torr CO, the same spectrum is obtained (Figure 4(b)) indicating that no evolution occurs with time. To follow the influence of oxygen, a stoichiometric pressure of O2 is first introduced (Figure 4(c)), then this pressure is increased to 1 Torr (Figure 4(d)). Finally the sample is left under the mixture of 10-1 Torr of CO and 1 Torr of O2 for 2 h at room temperature (Figure 4(e)). Contrary to results relative to the adsorption of CO alone, here clear differences between the two surfaces are observed in terms of band shape and intensity. The CO stretching band is only very slightly shifted for both surfaces

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Figure 4. Influence of O2 on preadsorbed CO on Pt3Sn(111)-p(2 × 2) and Pt3Sn(111)-(3 × 3) R30°. All PM-IRRAS spectra have been recorded at room temperature, under static conditions: (a) 10-1 Torr CO; (b) same as (a) after 90 min; (c) same as (b) + 10-1 Torr O2; (d) 10-1 Torr CO + 1 torr O2; (e) same as (d) after 2 h.

after the introduction of oxygen. On the p(2 × 2) surface, no change is observed except for a small shift toward higher wavenumbers and a slight decrease (less than 10%) after 2 h of contact. This effect may be a sign of the beginning of CO oxidation. On the contrary, for the (3 × 3) R30° termination, the ν(CO) intensity decreases by about 30% as soon as 10-1 Torr of oxygen are added (Figure 4(c)). This effect is instantaneously enhanced by about 10% when the oxygen pressure is increased to 1 Torr. This loss of intensity is linked to the CO desorption which is more effective when O2 pressure is increased. This indicates that the adsorptions of O2 and CO are more competitive on this surface. After 2 h of contact, beyond the progressive decrease, a broadening of the ν(CO) stretching band is noticed on the (3 × 3) R30° termination. Although no structural characterization of the surface has been done, a surface destructuration could be the origin of this effect. Such a broadening is not observed on the p(2 × 2). Hence this structural modification could reflect a larger interaction between O2 and the Pt3Sn(111)-(3 × 3) R30° surface. C. O2 + CO Coadsorption. Finally CO adsorption is studied after O2 has been introduced in the reactor. The results are reported in Figure 5. After a preliminary introduction of 10-1 Torr of O2, as expected, no particular band is observed in the PM-IRRAS spectra (a) whatever the surface. However, it does not mean that oxygen does not interact with the surface. In fact bands which might be involved in O2 adsorption are out of the PM-IRRAS wavenumber range (4000-800 cm-1). Then 10-2 Torr of CO (Figure 5 (b)) are added into the reactor. On the p(2 × 2) surface, a sharp band appears whose intensity and

position are comparable to those observed when CO alone was introduced in the reactor (see Figure 3 (b), (c)). The increase of the oxygen pressure to 1 Torr (see Figure 5 (d)) does not induce any noticeable change. This confirms the stability of CO on this surface. The situation is different when looking now at the behavior of the (3 × 3) R30°. In presence of 10-1 Torr O2, the IR band appearing after introducing 10-1 Torr CO is of small intensity, very broad, and not well defined. This could result from a small CO coverage on a disordered surface perturbed by oxygen. The behavior of oxygen under elevated pressures (in the range of 10-2 Torr) has already been studied by Jerdev et al.17 on ordered p(2 × 2) and (3 × 3) R30° Sn/Pt(111) surfaces between 380-425 K. The topmost layer of these surfaces is similar to those of the bulk Pt3Sn(111) that we are studying here. Jerdev et al. show that after a fast oxygen uptake on both surfaces up to a coverage of about 0.5 monolayer (ML), the kinetics of oxygen accumulation is lowered until saturation (1.2 and 1.4 ML respectively for the p(2 × 2) and the (3 × 3) R30°). It takes more than 1 h to get saturation at 380 K under 0.02 Torr O2. According to these results, the kinetics of O2 accumulation is faster on the (3 × 3) R30° termination. Hence the introduction of O2 before CO has more consequences on this termination than on the p(2 × 2). D. A Step Further toward Reactivity. In summary, our present results have shown, at room temperature, a high stability of CO in presence of oxygen on the p(2 × 2) structure. On the contrary, CO is partially desorbed by oxygen which interacts

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Figure 5. Adsorption of CO on O2 precovered Pt3Sn(111)-p(2 × 2) and Pt3Sn(111)-(3 × 3) R30° surfaces at room temperature. PM-IRRAS spectra correspond to cumulative pressure: (a) 10-1 Torr of O2; (b) addition of 10-2 Torr of CO; (c) addition of 9 × 10-2 Torr of CO; (d) addition of 9 × 10-1 Torr of O2.

with the Sn surface atoms in the case of the (3 × 3) R30° structure. In agreement with previous studies,6,17 the strong affinity between tin and oxygen species has been demonstrated here again. The next step consists now in performing the CO oxidation reaction at higher temperature on these two surfaces. The following approach has been used: 10-1 Torr of CO and 1 Torr of O2 have been introduced in the reactor at room temperature, then the sample has been progressively heated to 425 K; this temperature was reached after 25 min. The activity has been monitored by the CO2 evolution in mass spectra recorded after successive gas samplings from the reactor to the quadrupole mass spectrometer. The evolution of CO2 partial pressure during this reaction is reported in Figure 6 for both terminations. Zero on the time scale corresponds to the moment when oxygen is introduced and heating started. At this point CO has already been introduced. From this graph, a clear difference appears between the two PtSn surfaces: CO2 production is much more efficient on the p(2 × 2) termination. After 200 min, the p(2 × 2) surface has converted 75% of CO into CO2 against 25% only for the (3 × 3) R30° termination. Activities of 5.5 × 10-14 and 9.3 × 10-13 mol · cm-2 · s-1 have been estimated on the p(2 × 2) and the (3 × 3) R30° respectively for conversion rates lower than 50%. PM-IRRAS spectra measured during the reaction show on both surfaces a very small ν(CO) band at 2080-2085 cm-1 indicating a low CO coverage on the surface. Previous experimental8,10,11,18 and theoretical12 studies have proved the high efficiency of PtSn alloys compared to pure platinum. For comparison, the same experiment has been performed on the Pt(111) sample leading to an activity of 4.2 × 10-13 mol · cm-2 · s-1, confirming this result. However regarding differences between the two PtSn terminations this statement

Figure 6. Evolution of CO2 partial pressure (Torr) as a function of time during CO oxidation at 425 K. Here, 10-1 Torr of CO and 1 Torr of O2 are introduced at room temperature. Three surfaces are compared: the two terminations of Pt3Sn(111), p(2 × 2) and (3 × 3) R30°, and pure platinum Pt(111).

has to be detailed. If the presence of tin improves the CO oxidation, it appears that changes in the surface structure modify this reaction. As explained before, on the (3 × 3) R30° termination, a preliminary adsorption of O2 involves great changes in CO adsorption. Although no direct PM-IRRAS characterization is available for O2 interaction with the surface, the evolution of CO adsorption indicates a strong oxygen induced modification of the (3 × 3) R30° termination. A thorough explanation of the difference of activity between (3 × 3) R30° and p(2 × 2) terminations requires a detailed analysis of the reaction mechanism from a theoretical kinetic

A High Pressure PM-IRRAS Study point of view, which is in progress. XPS measurements performed after reaction at 425 K and evacuation of the reactor do not exhibit any change of the tin concentration on the p(2 × 2) structure. A very small increase from 16.5 to 17.2 at. % is observed on the (3 × 3) R30° and attributed to a slight segregation of tin during the reaction. This is consistent with the hypothesis of a stronger interaction of oxygen with the (3 × 3) R30° involving a segregation of tin and thus a modification of the surface structure. IV. Conclusion In summary, the influence of the surface structure of PtSn alloys on CO adsorption and CO/O2 coadsorption is investigated by means of a PM-IRRAS study. The consequences on reactivity are also tackled. According to our results, CO chemisorption under high pressure has the same behavior, whatever the termination. The conclusions differ when we take into account the coadsorption with O2. We can now distinguish both terminations. On the p(2 × 2) termination, O2 does not desorb CO whereas on the second structure, O2 chemisorption is more competitive with previously adsorbed CO. The investigation of CO chemisorption on a O2-precovered surface leads to the same distinction between the two terminations. The presence of O2 has more salient consequences on the (3 × 3) R30° termination. Hence the interaction between oxygen and the surface seems to induce some modifications of the Pt3Sn(111) structure. These surface changes lead either to CO desorption or, if O2 is preadsorbed, to CO chemisorption inhibition. These chemisorption results have an influence on reactivity. From our preliminary study of CO oxidation, clear discrepancies appear between the two PtSn terminations and pure platinum. The presence of tin improves the reactivity compared to Pt(111) for both structures. Between the two alloys, the p(2 × 2) termination is much more efficient than the (3 × 3) R30°. At this stage, only a detailed kinetic analysis might explain these

J. Phys. Chem. C, Vol. 112, No. 29, 2008 10867 discrepancies in the catalytic behavior. As a conclusion, CO oxidation should be performed preferentially on the Pt3Sn(111)p(2 × 2) termination when model surfaces are considered. Acknowledgment. The authors thank the Pr. U. Bardi group (Universita di Firenze, Italy) for the loan of their Pt3Sn(111) sample. References and Notes (1) Ishikawa, Y.; Liao, M. S.; Cabrera, C. R. Surf. Sci. 2000, 463, 66. (2) Christoffersen, E.; Liu, P.; Ruban, A.; Skiver, H. L.; Norskov, J. K. J. Catal. 2001, 199, 123. (3) Koper, M. T. M.; Shubina, T. E.; van Santen, R. A. J. Phys. Chem. B 2001, 106, 686. (4) Liao, M. S.; Cabrera, C. R.; Ishikawa, Y. Surf. Sci. 2000, 445, 267. (5) Ji, Z. and; Li, J. Q. Chem. Phys. Lett. 2006, 424, 111. (6) Shubina, T. E.; Koper, M. T. M. Electrochim. Acta 2002, 47, 3621. (7) Liu, P.; Logadottir, A.; Norskov, J. K. Electrochim. Acta 2003, 48, 3731. (8) Stamenkovic, V. R.; Arenz, M.; Blizanac, B. B.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. Surf. Sci. 2005, 576, 145. (9) Dupont, C.; Loffreda, D.; Delbecq, F.; Jugnet, Y. J. Phys. Chem. C 2007, 111, 8524. (10) Avgouropoulos, G.; Ioannides, T. Appl. Catal. B: EnV. 2005, 56, 77. (11) Schubert, M. M.; Kahlich, M. J.; Feldmeyer, G.; Huttner, M.; Hackenberg, S.; Gasteiger, H. A.; Behm, R. J. Phys. Chem. Chem. Phys. 2001, 3, 1123. (12) Dupont, C.; Jugnet, Y.; Loffreda, D. J. Am. Chem. Soc. 2006, 128, 9129. (13) Ceelen, W. C. A. N.; van der Gon, A. W. D.; Reijme, M. A.; Brongersma, H. H.; Spolveri, I.; Atrei, A.; Bardi, U. Surf. Sci. 1998, 406, 264. (14) Shorthouse, L. J.; Jugnet, Y.; Bertolini, J. C. Catal. Today 2001, 70, 33. (15) Andersen, M.; Johansson, M.; Chorkendorff, I. J. Phys. Chem. B 2005, 109, 10285. (16) Rupprechter, G.; Dellwig, T.; Unterhalt, H.; Freund, H. J. J. Phys. Chem. B 2001, 105, 3797. (17) Jerdev, D. I.; Koel, B. E. Surf. Sci. 2001, 492, 106. (18) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 8945.

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