Interaction of Coadsorbed CO and Deuterium on a Bimetallic, Pt

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Interaction of Coadsorbed CO and Deuterium on a Bimetallic, Pt Monolayer Island Modified Ru(0001) Surface Heinrich Hartmann, Joachim Bansmann, Thomas Diemant, and R. Jürgen Behm J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp504409s • Publication Date (Web): 05 Jun 2014 Downloaded from http://pubs.acs.org on June 10, 2014

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The Journal of Physical Chemistry

Interaction of Coadsorbed CO and Deuterium on a Bimetallic, Pt Monolayer Island Modified Ru(0001) Surface H. Hartmann, J. Bansmann, T. Diemant, and R.J. Behm* Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany

Abstract We have investigated the coadsorption of CO and deuterium on structurally well defined bimetallic, Pt monolayer island modified Ru(0001) surfaces, focussing on the interactions between the coadsorbed species and their impact on the adsorption and desorption characteristics. Temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS) measurements after adsorption at 90 K reveal considerable differences between adlayers formed by pre-adsorption of deuterium and subsequent saturation by CO or in the reverse way. We demonstrate that these differences are caused by the limited mobility of adsorbed CO at low temperatures and spill-over of COad from Pt monolayer areas to Ru(0001) areas upon heating, e.g., during a TPD measurement. The interplay between energetics, including the presence of weakly adsorbing Pt monolayer sites and strongly adsorbing Ru(0001) sites as well as interactions between coadsorbed species, and the onset of COad spill-over on the adsorption and desorption behavior of deuterium, which results in complex deuterium desorption spectra, is illustrated and discussed. Keywords: Coadsorption, Adsorbate-adsorbate interactions, Surface modification, carbon monoxide, hydrogen, platinum, Ru(0001) Submitted to J. Phys Chem. C: 05.05.2014 Special Issue ‘Festschrift John C. Hemminger’

Corresponding author: *E-mail: [email protected]. Tel.: +49-731-502-5450. Fax: +49-731-502-5452.

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1 Introduction The interaction of hydrogen and CO with bimetallic PtRu surfaces has attracted considerable interest due to the suitability of these systems as electrocatalysts and anode material in low temperature Polymer Electrolyte Membrane Fuel Cells (PEMFCs).1 Their superior CO tolerance compared to pure platinum anode catalysts

2

made them attractive as catalysts for

the conversion of CO contaminated H2-rich fuel gas,3 which is generated in the reformate process of hydrocarbons, as well as for the direct conversion of methanol in Direct Methanol Fuel Cells (DMFCs).4;5 While for pure platinum anode materials the catalytic activity is strongly reduced by the presence of site blocking COad, bimetallic PtRu electrode surfaces were supposed to enable oxidative COad removal already at lower potentials than possible on Pt via a bifunctional mechanism.2 In this mechanism, the Ru sites of the surface are supposed to enable dissociative adsorption of H2O already at lower potentials than Pt, facilitating the subsequent oxidation of CO adsorbed on Pt sites to CO2 by reaction with OHad adsorbed on Ru sites. More recently, temperature programmed desorption (TPD) studies on the interaction of CO with bimetallic PtRu surfaces

6-11

introduced another possible explanation for the improved

CO tolerance. According to these studies, electronic ligand and strain effects cause a significant reduction of the binding strength of CO adsorbed on Pt monolayer islands on Ru(0001) and on monolayer PtRu/Ru(0001) surface alloys, which contributes to the improved CO tolerance by enhancing the desorption of COad under steady-state conditions. However, deuterium adsorption studies conducted on these surfaces

11;12

revealed a similar decrease of

the adsorption energy for deuterium, which leads to the question whether the latter effect, resulting in a lower steady-state Had coverage on the bimetallic surface in the PEMFC, and therefore in a first order approximation in a lower H2 oxidation current, would overcompensate the improvements in the CO tolerance expected from the lower CO


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adsorption energy. This is background for the present work, where we investigated the coadsorption of CO and hydrogen and the interactions between these coadsorbed species on a Ru(0001) surface modified by monolayer Pt islands. Before turning to the results of this study, we will briefly summarize previous findings on the coadsorption of CO and hydrogen on Ru(0001) and Pt(111) surfaces and on monolayer PtRu/Ru(0001) surface alloys as reference. The coadsorption of CO and hydrogen (deuterium) on Ru(0001) has been the subject of several experimental and theoretical studies.13-21 TPD experiments by Peebles et al. 13 demonstrated that neither pre-adsorbed COad nor Dad species are displaced by exposure to the respective other species at 100 K. Shifts of the thermal desorption states in D2 TPD spectra to lower temperatures indicated a repulsive interaction between the coadsorbed species COad and Dad. This observation was supported by Mak et al.,19 who explained a decrease of the Had diffusion coefficient with increasing CO coverage in coadsorbed layers by repulsive adsorbate-adsorbate interactions and formation of segregated islands of the coadsorbed species. While adsorption of deuterium on a saturated COad layer was found to be impossible, appreciable quantities of CO could be adsorbed in the reverse case, into a saturated Had layer, at 100 K.13;20;22 Additionally, CO was found to completely displace pre-adsorbed hydrogen from the surface at 250 K.16 Riedmüller et al. reported that adsorption of CO on a perfect hydrogen layer with a coverage of 1 ML is an activated process with an activation barrier of ≥ 25 kJ mol-1.14;15 It was also shown that imperfections in the Had layer open an alternative non-activated adsorption pathway which results in the formation of small isolated CO islands in the Had layer.16 For the reverse case of adsorption of hydrogen into a closed CO adlayer, Ueta et al. calculated an activation barrier of ca. 40 kJ mol-1.18 IR experiments showed that the band of CO coadsorbed with hydrogen experiences a small shift to higher wave numbers.22;23 Based on density functional theory (DFT) calculations, this shift can be attributed to electrostatic interactions within the



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coadsorbed layer for small amounts of CO molecules embedded in a hydrogen layer.23 There were no indications for the formation of reaction products during coadsorption of CO and molecular hydrogen. In contrast, Mitchell et al. reported the formation of formyl and formaldehyde species upon exposure of a CO saturated Ru(0001) surface to atomic hydrogen at sample temperatures below 130 K.24 These products decomposed upon annealing, leading to the desorption of molecular CO and hydrogen. Recent molecular beam experiments by Ueta et al. found evidence for the formation of stabilized Dx-COy species on the surface for medium COad pre-coverages and higher D2 beam energies.21 For coadsorption of CO and hydrogen on Pt(111), an early study by Baldwin indicated that it is possible to form a CO-H adsorption complex on the surface.25 Later studies, however, concluded that the two adsorbate species form segregated islands on Pt(111) as a consequence of repulsive adsorbate-adsorbate interactions between COad and Had.26-28 Similar to Ru(0001), coadsorption of hydrogen and CO caused a temperature shift of the H2 desorption peak in the TPD spectra. While no displacement of hydrogen by post-exposure of a Had layer to CO was observed, adsorption of considerable amounts of CO was possible on a Had saturated Pt(111) surface at 100 K.29 Extensive studies have been conducted to investigate various aspects of the coadsorption of CO and deuterium on PtRu/Ru(0001) surface alloys.8;20;22 From a scanning tunneling microscopy (STM) study with atomic resolution it is known that the surface of this model system consists of Pt and Ru atoms randomly intermixed in the topmost layer, independent of the concentrations of the respective surface atoms.30 Similar to the pure metal surfaces, the presence of coadsorbed CO leads to a significant destabilization of Dad. Increasing the fraction of Pt atoms in the surface alloy causes an additional destabilization and displacement of Dad from the surface already upon CO exposure at 100 K. The different surface morphology of the bimetallic surface investigated in the present work, consisting of monolayer Pt islands on ACS Paragon Plus Environment

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Ru(0001), can provide additional insight into the interactions of coadsorbed CO and hydrogen species on bimetallic PtRu surfaces. The results of a study on the individual adsorption of either CO or deuterium on Pt submonolayer film covered Ru(0001) surfaces have been published previously.11 TPD and IRRAS experiments showed that the adsorption energies of both species were considerably lower on the Pt islands compared to adsorption on Ru(0001) or Pt(111). Increasing Pt coverages were found to strongly lower the deuterium sticking probability and saturation coverage. Conversely, while the adsorption energy of CO was significantly weaker on the Pt parts of the surface, the sticking probability and saturation coverage were little affected by an increasing Pt coverage. Furthermore, these studies revealed a distinct kinetic effect in CO adsorption.31;32 While for Ru(0001) CO adsorption is well known to saturate at 0.68 monolayers (ML), it was found that spill-over of CO adsorbed on the Pt monolayer islands to the Ru(0001) areas may increase the local COad density in these areas up to a maximum value above 0.8 ML. Spill-over is kinetically hindered, however, at low temperatures, and activated only at ~150 K, e.g., during a thermal desorption experiment, which causes distinct modifications in the CO TPD spectra recorded on these bimetallic Pt/Ru(0001) surfaces. In this paper, we will present results on the coadsorption of deuterium and CO on a monolayer Pt island modified Ru(0001) surface with approximately 0.3 monolayers (ML) Pt coverage. In the first part, we deal with the mutual influence of the coadsorbed species on each other, as well as their distribution on the Pt monolayer and Ru(0001) parts of the surface following different exposure sequences at 90 K (sections 3.1, 3.2). In the second part, we elucidate the effect of increasing temperature on the coadsorbate layer (section 3.3), and the role of spill-over processes during adsorption at elevated temperatures, during annealing of the adlayer or during the heating ramp in the course of TPD measurements (section 3.4). This



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will include also a comparison with the results of similar experiments on PtRu/Ru(0001) surface alloys.8;20;22

2 Experimental The experiments were performed in a standard UHV system (base pressure of about 1×10-10 mbar) equipped with facilities for temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy (XPS). A Ru(0001) single crystal (8 mm diameter) was used as substrate for the preparation of the bimetallic surfaces. It can be cooled by liquid N2 to 90 K or heated by electron bombardment up to 1700 K. The Ru(0001) single crystal was cleaned by standard procedures described before.11 The surface cleanness after preparation was checked by a TPD spectrum of a COad saturated Ru(0001) substrate, which reacts sensitively to surface contaminations. The Pt submonolayer films on Ru(0001) were prepared by deposition of Pt (substrate temperature 300 K, deposition rate of approximately 0.1 ML s-1), using an electron beam evaporator (Omicron, EFM 3), and subsequent annealing to 700 K for 60 s. STM measurements on samples prepared by this procedure showed the formation of pseudomorphic monolayer Pt islands (average island diameter at Pt0.3-ML/Ru(0001): ~18 nm) which coalesce at higher Pt coverages.30;33 Since the Pt films on Ru(0001) have been annealed to 700 K, no further significant changes in the structure of the Pt monolayer islands are observed during TPD measurements up to 550 K. Gas exposures to CO and D2 were carried out by backfilling the chamber via a glass tube pointing into the direction of the sample. Therefore the effective gas pressures on the sample are higher (by about a factor of 5 for CO and a factor of 2 for D2) than those measured by the ion gauge; the exposures were corrected for this. For the TPD measurements, gas dosing was usually started at 90 K. The sample was heated in a linear heating ramp of 5 K s-1, partial


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pressures were recorded by a quadrupole mass spectrometer (Pfeiffer Vacuum, QMS 200), which was in line of sight to the sample during desorption. The QMS was shielded by a cap with an aperture of 4 mm against undesired gas contributions from the sample holder or the sample edges. The distance between cap aperture and sample could be adjusted reproducibly by an electrical contact. The IR measurements were performed using a Bruker Tensor 27 spectrometer with a liquid N2-cooled mercury-cadmium-telluride (MCT) detector. Typically, 2000 scans were co-added with a resolution of 4 cm-1.

3 Results and Discussion 3.1 D2 adsorption on a 0.25 ML Pt/Ru(0001) surface pre-covered with COad In this section we will present results on the coadsorption of CO and deuterium on a Pt monolayer island modified Ru(0001) surface with a Pt coverage of 0.25 ML (Pt0.25ML/Ru(0001),

where the sample was first exposed to different doses of CO, followed by

exposure to 40×10-6 mbar·s of D2 (adsorption temperature 90 K). This exposure was found to be sufficient to saturate the bimetallic surface on a COad free surface. Figure 1 shows the respective D2 (Fig. 1a) and CO (Fig. 1b) TPD spectra. The CO TPD spectra are identical to those recorded without coadsorption of deuterium from a comparable surface.11 The two high temperature peaks (at 380 and 450 K) of the CO TPD spectrum are related to CO desorption from the Ru(0001) areas of the surface, with the peak at 380 K resulting from repulsive COad – COad interactions at local COad coverages on the Ru(0001) areas of the surface exceeding 0.33 ML.34 The left part of the spectrum with another desorption peak located at 290 K results mainly from CO desorption from the Pt monolayer islands on the bimetallic surface.11 For all CO exposures, D2 desorption is essentially completed before the onset of the CO desorption, and the adsorption of D2 at 90 K causes no displacement of COad species.



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The effect of pre-adsorbed CO on the adsorption properties of deuterium in this Pt coverage range is very similar to that on an unmodified Ru(0001) surface: For the largest CO exposure (2.55×10-6 mbar·s, θCO = 0.33 ML) which still allows for an appreciable Dad coverage (θD = 0.11 ML), D2 desorbs in a single peak with its maximum at ~290 K. This is by ~100 K lower in temperature than the desorption peak obtained for the desorption of a similar Dad coverage without coadsorbed CO from this surface (dashed line, ~390 K, see 11). With decreasing CO pre-exposure, the peak intensity grows, reflecting an increasing Dad coverage. Different from deuterium desorption from a pure deuterium adlayer without coadsorbed CO, the peak does not shift to lower temperature with increasing Dad coverage, but remains fixed in temperature. Formally, this would correspond to a COad induced change from second order desorption kinetics to first order desorption kinetics for deuterium adsorption, which is unlikely to happen. The change in desorption properties can be easily explained, however, if deuterium desorption takes place from Dad islands in a surrounding COad phase, whose local Dad coverage is constant. With decreasing COad coverage, only the total area of these Dad islands increases. Even for second order desorption kinetics this phase separation would result in a constant temperature of the desorption peak, independent of the Dad coverage before the onset of desorption. In contrast, for desorption from a COad free surface the peak shifts to lower temperature with increasing Dad coverage, as expected for second order desorption kinetics. The effect of coadsorbed CO on the deuterium adsorption and desorption behavior is reflected by the loss of desorption intensity at the high temperature side of the D2 desorption peak when comparing to a TPD spectrum recorded after exposure to 40×10-6 mbar·s of pure D2 (dashdotted lines). The different shape of the desorption peaks as compared to desorption from a pure Dad layer can tentatively be explained by interaction with the surrounding COad phase. We speculate that this is, to a certain extent, compressed by coadsorbed Dad. With increasing


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deuterium desorption, the COad phase can relax, which increases the two-dimensional pressure on the Dad phase and this way results in a narrower deuterium desorption peak. The desorption intensity in the low-temperature region (desorption from Pt islands) is considerably lower compared to the TPD spectra from a pure Dad adlayer. A clear desorption peak is detected only for Dad coverages above 0.45 ML. At this Dad coverage the D2 TPD peak maximum is located at 135 K; for the next lower COad pre-coverage (0.06 ML), which allows for a Dad coverage of 0.6 ML, the peak maximum shifts to 140 K. In agreement with results obtained on Pt-free Ru(0001), deuterium adsorption is inhibited on a surface covered by a saturated CO adlayer, and no COad displacement is observed. Even though CO desorption on the Pt part of the surface takes place at much lower temperatures than on Ru(0001), the onset of desorption is still ~100 K above the adsorption temperature. The total coverages reached this way, normalized to the saturation coverages reached on the Pt-free Ru(0001) surface, are listed in Table 1. We used this normalization to indicate the adsorption sites that are available in principle, although from energetic reasons they are only partly occupied for deuterium adsorption on the Pt monolayer islands at 90 K. As evident from the table, the total coverage never exceeds 100%. Hence, the adsorbate density does not give any indications for the formation of a new adsorbate species or an adsorption complex, but is compatible with a layer of coadsorbed COad and Dad species. The same coadsorption sequence was also investigated using IR spectroscopy. The respective D2 and CO coverages are given in Figure 2. Spectra recorded from the pure CO adlayers before exposure to D2 are presented for comparison (dashed lines). The main bands can be assigned to CO on top of Pt monolayer islands on Ru(0001) (2087 cm-1), CO on edge positions of the Pt islands (2062-2070 cm-1), and CO on Ru(0001) (2030-2055 cm-1).7 Especially the CO band on Ru(0001) shows a pronounced blue shift with increasing COad coverage and a much smaller blue shift in the case of additional adsorption of deuterium. ACS Paragon Plus Environment


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Similar to the results on a Pt-free Ru(0001) surface,11;22 the coadsorption of D2 with ~0.05 ML of pre-adsorbed CO causes a blue-shift of the IR band. The extent of this up-shift (17 cm-1) is, however, much larger than for the unmodified Ru(0001) surface (~7 cm-1). With increasing COad pre-coverage, this blue-shift decreases continuously and disappears for high COad coverages. In analogy to adsorption on Ru(0001), this shift of the IR band is tentatively attributed to interactions between the coadsorbed species. As in the case of pure CO adsorption on Pt/Ru(0001), the Pt-CO band is only detected at higher COad coverages, with intensities never above those found for a pure CO adlayer of similar coverage (dashed lines).11 This demonstrates that there was no deuterium post-adsorption induced displacement of COad from Ru to Pt sites. Considering the higher adsorption energies of CO on the Ru and Pt sites compared to those of deuterium, this result agrees with expectations. In addition, the mobility of COad on the surface at 90 K is too low to enable site exchange processes (see refs. 31;32

). Smaller effects caused by the onset of COad mobility during the heating ramp in the

TPD experiments will be discussed in section 3.4. In total, deuterium post-adsorption on a COad pre-covered Pt0.25-ML/Ru(0001) surface results in filling up available adsorption sites which are not blocked by pre-adsorbed CO, with the (normalized) total coverage obtained this way never exceeding the saturation coverages of the individual adsorbate species. The highly mobile Dad species will first fill up the accessible Ru(0001) sites, before Pt monolayer sites are populated as well. Because of the low adsorption energy on the latter sites, only a small fraction of them is populated, and this decreases rapidly with increasing COad pre-coverage. This results in a coadsorption behavior very similar to that on the Pt-free Ru(0001) surface.

3.2 CO adsorption on a 0.3 ML Pt/Ru(0001) surface pre-covered with Dad


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Next, we will present TPD and IR results on the coadsorption of CO and deuterium in the reversed sequence. In this case, a bimetallic surface with 0.3 ML Pt on Ru(0001) (Pt0.3ML/Ru(0001))

was pre-covered by an increasing amount of Dad, followed by a CO exposure of

30×10−6 mbar·s. This CO exposure is sufficient to saturate the surface with COad in the absence of pre-adsorbed Dad.11 Figure 3 shows D2 (Fig. 3a) and CO TPD (Fig. 3b) spectra recorded from the coadsorbed Dad + COad layer on this Pt modified Ru(0001) surface. Similar to the findings for the reversed dosing sequence (see preceding section), the CO TPD spectra are essentially identical to those obtained in earlier experiments for pure CO adsorption experiments on Pt/Ru(0001) (without coadsorption of deuterium – see dotted line).11 The only change compared to CO adsorption on a Dad free surface is a lower COad coverage, which decreases with increasing amount of pre-adsorbed Dad. This closely resembles also previous findings for the coadsorption of deuterium and CO on pure Ru(0001).22 A comparison of the temperature ranges of CO and D2 desorption shows that the former one only starts after the latter is (almost) completed, i.e., there is essentially no Dad on the surface any more once COad desorption starts. This explains the absence of any changes in the CO TPD spectra after coadsorption compared to pure CO adsorption. For the smallest D2 exposure (0.25×10-6 mbar s), resulting in a Dad pre-coverage of 0.12 ML, the respective COad coverage is about 85% of the saturation coverage in the absence of Dad (θCO(sat.) = 0.62 ML).11 For the highest D2 exposure used in this experiment (40×10-6 mbar s, θD = 0.75 ML), it is still possible to adsorb approximately 0.25 ML of CO into the Dad adlayer. A complete overview on the respective coverages obtained in these experiments is given in Table 2. Depending on the Dad pre-coverage, CO desorption in the TPD spectrum occurs either only from Ru sites (larger Dad pre-coverages = smaller COad coverages) or from both Pt and Ru sites (smaller Dad pre-coverages = larger COad coverages). It is important to ACS Paragon Plus Environment


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note, however, that the resulting TPD peaks do not reflect the distribution of the COad molecules directly after adsorption at 90 K, but the situation at the respective desorption temperature, which can be influenced by adsorbate migration in the course of the TPD run.31;32 This point will be addressed in more detail below, together with the IR results. In contrast to the CO TPD spectra, the modifications of the Dad adsorption properties upon post-adsorption of COad are more pronounced and more clearly revealed by the D2 TPD spectra (Fig. 3a). Before describing and discussing these results in more detail, we will briefly discuss the D2 TPD spectrum for a pure saturated D adlayer (dash-dotted line). This spectrum shows a broad peak with a maximum around 320 K (β1 peak) and a shoulder at ~380 K (β2, peak), which reflects desorption from the Ru(0001) areas of the surface, and a second, much smaller peak at lower temperature with a maximum around 140 K (γ peak), which originates from D2 desorption from the Pt monolayer areas of the surface. Again, a more detailed description is given in ref. 11. Returning to the coadsorption experiments, at lower Dad pre-coverages there is only a single deuterium desorption state with a maximum at T ≈ 200 K, located in the temperature regime between D2 desorption from Ru(0001) areas and Pt monolayer areas in the case of pure deuterium adsorption.11 For higher D2 pre-exposures (beginning with 4×10-6 mbar·s), an additional high temperature state is observed at ~290 K, i.e., in the same temperature range where the desorption maximum of the pure Dad layer is detected at highest Dad coverages. After the largest D2 exposure of 40×10-6 mbar·s, also a small low temperature peak appears in the temperature range 120 - 150 K, which is attributed to deuterium desorption from the Pt islands (γ state) (see above). Comparison of the intensity in this low temperature state with that in the low temperature peak obtained without CO post-adsorption (dashed line) as well as comparison of the total intensities in these D2 TPD spectra (D2 exposure 40×10−6 mbar·s, Table 2) indicate that there is no measurable displacement of Dad from the surface upon postACS Paragon Plus Environment

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adsorption of CO. One should note, however, that the maximum (local) Dad coverage reached on the Pt islands at 90 K is relatively low (θD ≈ 0.3 ML) even without CO coadsorption, i.e., the total amount of Dad obtained in this measurement before CO post-adsorption should be below 0.1 ML, which means that smaller changes due to Dad displacement would not be detected. This will be discussed further in section 3.3. Desorption in the two states at higher temperature is assigned to D2 desorption from Ru sites, with some overlap in the transition region between the peaks. This assessment is also backed by a comparison of the integrated intensities of the TPD spectra after the maximum D2 exposure with and without coadsorbed CO; the intensity in the two desorption states at higher temperature in the coadsorption spectra (above ~150 K) almost equals the intensity and hence the Dad coverage of the high temperature desorption state in the D2 TPD spectrum recorded upon pure deuterium adsorption (beginning at ~220 K). The main D2 desorption peak at 200 K appears at significantly lower temperatures compared to the TPD peak maximum without coadsorption with CO. This shift amounts to almost 200 K when comparing spectra after the smallest D2 exposure (0.25×10−6 mbar·s). Moreover, there is even a down-shift by 40 – 50 K relative to coadsorption of CO and D2 on pure Ru(0001) after similar exposures (D2 exposure: 0.24×10−6 mbar·s, CO exposure: 30×10−6 mbar·s). It is also significantly lower than the D2 desorption peak detected for desorption from the coadsorbed layer produced by deuterium post-adsorption, where the maximum of the main desorption peak was at about 290 K. The results of IRRAS measurements conducted at 90 K after the same dosing sequence are shown in Fig. 4. For a better understanding, the respective COad and Dad coverages (determined by TPD) are given in the figure. Furthermore, we also show IR spectra of pure CO adlayers with similar coverage (dashed lines). The IR spectra for coadsorbed COad and Dad (full lines) generally show two bands located at 2080 - 2090 cm-1 and 2025 - 2060 cm-1, ACS Paragon Plus Environment


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which are related to CO adsorbed on Pt island and Ru(0001) sites, respectively.7;11;35 For small coverages of pure COad (dashed lines in the lower part of Fig. 4), another band appears at an intermediate position of ~2065 cm-1, which grows on the expense of the high frequency band. This band was attributed to CO molecules adsorbed at the edge sites of the Pt islands, which are energetically more favorable than the sites in the inner of the Pt islands.7 Starting with high D2 exposures (and thus high Dad coverages), the IR spectra recorded after CO post-adsorption mainly show a strong COad band located at the position assigned to Pt-CO (~2087 cm-1),35 indicating a high COad coverage on the Pt islands, and a much smaller band related to CO adsorption on Ru(0001) sites (2030-2033 cm-1). The position of the latter agrees very well with the results of IR measurements on non-modified Ru(0001), after the same adsorption procedure at 100 K.22 From this similarity we conclude that the local environment of the COad in the coadsorbed layer on the Ru(0001) parts of the bimetallic surface is comparable to that on the non-modified Ru(0001) surface under similar coverage conditions, which for small COad coverages had been described as consisting of small COad islands embedded in the Dad layer.16;22 With decreasing Dad pre-coverage, the intensity of the CO-Ru band increases and the band shifts to higher wave number, from 2030 to 2055 cm-1, while the CO-Pt band changes very little, neither in wave number nor in intensity. Irrespective of the Dad pre-coverage, it is always observed at the same frequency (2089 cm-1) and shows already for the spectrum with the smallest COad coverage of 0.25 ML almost the same intensity (95%) as for the highest COad coverage of 0.56 ML (with or without coadsorbed Dad). Here it should be noted that the coverage dependent shift of the CO band on a Pt monolayer on Ru(0001) is quite small, with a change from 2084 to 2091 cm-1 when going from small COad coverage to saturation.11 These observations are compatible with a predominant adsorption of CO on the Pt monolayer islands, which we would expect from the preferential occupation of adsorption sites on the Ru(0001) areas by (pre-)adsorbed deuterium. The latter results from the higher


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adsorption energy of Dad on these sites together with the high mobility of Dad on the surface even at 90 K.11 For coadsorbed layers, the intensity of the CO-Ru band is considerably smaller than after pure CO adsorption, especially upon post-exposure after high D2 pre-exposures, in good agreement with the preferential adsorption of CO on the Pt monolayer sites proposed above. Following the above arguments, we assume that up to the local saturation coverage of 1 ML the preadsorbed Dad is mainly located on the Pt-free Ru(0001) areas of the bimetallic surface. Even for the highest D2 exposure, the Pt islands are only partially covered with Dad, the maximum (local) Dad coverage reached on the Pt islands in the absence of COad at 90 K is only ~0.3 ML. Our proposal of preferential CO adsorption on the Pt islands is further backed by the observation of Riedmüller et al. that CO adsorption on a dense, pre-adsorbed Dad layer on Ru(0001) is strongly hindered.14;15 The tendency for preferential CO adsorption on the Pt islands with their dilute Dad layer increases with higher Dad pre-coverage, while adsorption on the Ru(0001) areas of the surface is kinetically inhibited more and more.

3.3 Temperature induced modifications of CO + D adlayers on a bimetallic Pt/Ru(0001) surface Since it is likely that the distribution of the respective species on the surface after adsorption at 90 K is mainly determined by the surface distribution of the pre-adsorbed Dad and thus does not reflect the situation during deuterium desorption, we investigated the effects of increasing sample temperature on the coadsorbate layers by IRRAS measurements. Similarly to the measurements shown in the preceding chapter, the sample was first exposed to 40×10−6 mbar·s of D2, then to 30×10−6 mbar·s CO (both at 90 K). In the following, we present a series of IR spectra which were recorded in 10 K steps with increasing temperature (Fig. 5), using a 0.25 ML Pt island modified Ru(0001) surface.



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The first spectrum, measured directly after gas exposure at 90 K, is very similar to the result from the surface with 0.3 ML Pt presented in Fig. 4 (see above) with two bands at 2033 and 2088 cm-1, which are related to CO adsorbed on Ru(0001) and Pt monolayer sites, respectively. The CO-Ru band shows a marked broadening and decreasing intensity in the temperature range from 90 to 150 K, while no change is observed for the CO-Pt band. The corresponding D2 TPD spectrum (right part of Fig. 5), which was collected in a second experiment on this surface after the same gas exposure sequence as described above, but with a continuous heating ramp, shows that an equivalent ~0.05 ML of Dad has desorbed in this temperature range. The change in the CO-Ru band shape would be compatible with a restructuring of COad islands, and the formation of such COad islands has been described by Riedmüller et al. for comparable coadsorption experiments on Ru(0001).16 Previous measurements for a similar coadsorption experiment as performed here, but using a Pt-free Ru(0001) surface, however, revealed a broad CO-Ru band from the very beginning (adsorption at 90 K), which only sharpened at higher temperature (see below). The reason for this discrepancy is not yet known and still under investigation. Upon reaching 160 K, the CO-Pt band shows a sharp decrease in intensity, until it vanishes almost completely at 200 K. Concomitantly, the low and broad CO-Ru band develops into a sharp band located at a slightly higher wave number (2038 cm−1) than upon adsorption. The IR results indicate that the majority of CO molecules adsorbed on the Pt island sites diffuses to the Ru(0001) areas, concomitant with the onset of the first major D2 desorption feature at 160 K (Fig. 5b). A further increase in temperature above 200 K causes a continuous shift of the CO-Ru band to lower wave numbers (2029 cm−1 at 300 K). Since no CO desorption is observed in this temperature range, this red-shift of the IR band cannot be explained by a decreasing COad coverage and is instead attributed to decreasing repulsive interactions in the coadsorbate layer on the Ru parts of the surface due to the decreasing amount of coadsorbed


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Dad (see Fig. 5b). Accordingly, the band position is constant in the temperature range of 300 K – 350 K, where no more D2 desorption is observed. Finally, above 350 K, the IR band displays the down-shift and intensity loss expected for a decreasing COad coverage due to CO desorption in this temperature range.

3.4 Thermally activated spill-over effects in CO + D adlayers on a bimetallic 0.3 ML Pt/Ru(0001) surface In the previous sections, we demonstrated that the D2 desorption behavior differs significantly depending on the order of adsorption, pre-adsorption of CO followed by post-saturation with deuterium or pre-adsorption of deuterium followed by post-saturation with CO. While in the first case we find a characteristic desorption peak with a maximum at ~290 K, the reverse adsorption sequence results in desorption at significantly lower temperatures, in a peak with its maximum at ~200 K. In both cases the peaks do not shift significantly with coverage. Both peaks were related to desorption from the Pt-free Ru(0001) areas of the Pt0.3-ML/Ru(0001) or Pt0.25-ML/Ru(0001) surfaces, respectively. Deuterium desorption from the Pt monolayer areas, in contrast, occurs in a low temperature peak or low temperature shoulder (γ peak) with its maximum at 140 – 150 K. In contrast to deuterium desorption, CO desorption was essentially identical in both cases, hence it is independent of the adsorption sequence. Most likely, the CO adlayer has reached a thermodynamic equilibrium (on the surface) at the onset of CO desorption. There are at least two possible explanations for the differences in the deuterium desorption behavior: i) The deuterium belonging to the desorption peak at 200 K was adsorbed on the Ru(0001) part at 90 K and remains there during desorption. In the mixed adlayer resulting on the Ru(0001) areas upon deuterium pre-adsorption and subsequent saturation with CO, the



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interactions between coadsorbed species are sufficiently strong to initiate deuterium desorption at ~150 K, resulting in a D2 desorption peak with a maximum at 200 K, while in the reverse case the effective interactions in the resulting mixed adlayer are much less pronounced and lead to a desorption peak with a maximum at 290 K. ii) Deuterium detected in the desorption feature was originally adsorbed on the Ru(0001) areas of the surface at 90 K, but is displaced from there either to the Pt monolayer islands or into vacuum (direct desorption) by COad molecules which migrate from the Pt monolayer islands to the Ru(0001) areas and displace the Dad species. This process, which may occur either during the CO exposure or once the migration of the COad species is thermally activated, e.g., during the heating ramp in the TPD experiments, is schematically shown in Fig. 6. In this mechanism, spill-over of COad from Pt monolayer island areas to Ru(0001) areas is responsible for deuterium desorption, and the 200 K peak would reflect the thermal activation of COad spill-over in the mixed adlayer rather than the lowering of the effective adsorption energy of deuterium due to adsorbate – adsorbate interactions in the mixed phase. Thermodynamic driving force for the COad spill-over process is the much higher adsorption energy of CO on the Ru(0001) areas compared to the Pt monolayer sites (Fig. 6d). Here it should be noted that a similar spill-over process of CO adsorbed on Pt monolayer island modified Ru(0001) surfaces was reported before in TPD measurements recorded after low temperature CO adsorption (90 K).31;32 In that case, spill-over of COad was activated at ~150 K, and it was found to result in significant modifications of the CO TPD spectra. It should further be noted that for adsorption at 90 - 100 K deuterium desorption from the Pt monolayer islands starts right at the adsorption temperature, which means that additional deuterium moved to these sites will desorb instantaneously above a critical Dad coverage.


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The activation of spill-over of COad from Pt monolayer island sites is identified by the IRRA spectra presented in section 3.3 (Fig. 5a). Different from previous findings for a pure CO adlayer on the bimetallic surface,31;32 where COad spill over was activated at 150 K, it occurs at slightly higher temperature in the high coverage mixed adlayer in these experiments, in the range 160 – 200 K. The D2 TPD spectrum recorded on a similarly prepared mixed adlayer (Fig. 5b) furthermore demonstrates that deuterium desorption in the 200 K peak is correlated with the spill-over of COad. These data seem to support the second mechanism proposed above, i.e., deuterium displacement by thermally activated spill-over of COad. In that case, the 290 K peak observed upon adsorption in the reverse sequence, pre-adsorption of CO and subsequent saturation with deuterium, reflects the situation and the adsorbate-adsorbate interactions in the mixed adlayer much better, since in this case there is much less spill over of COad from the Pt monolayer sites (see Fig. 6a). It should be noted that COad spill-over is not fully absent, since for CO preadsorption at 90 K the mobility of COad is very low, and COad transfer from the Pt monolayer islands to the Ru(0001) areas is inhibited. But there is no preferential adsorption of CO on the Pt monolayer islands, as found for deuterium pre-adsorption and subsequent saturation with CO (section 3.2). Hence, COad spill-over is expected to cause some deuterium desorption in the low temperature range (180 – 230 K), but not to dominate the deuterium desorption behavior. This also explains the observation of a 290 K deuterium desorption peak for desorption from mixed adlayer prepared by pre-adsorption of large deuterium coverages and subsequent saturation with CO (Fig. 6c). In that case, the spill-over of COad will displace part of the Dad on the Ru(0001) areas, but not all. The remaining fraction will desorb from the mixed CO + D adlayer which is formed also upon pre-adsorption of CO and subsequent saturation with deuterium, in a peak at 290 K, in perfect agreement with experimental observations. Finally, ACS Paragon Plus Environment


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for very high Dad pre-coverages (see Fig 6b), there is also desorption from the Pt monolayer islands in the shoulder at ~150 K (Fig. 3a). If these mechanistic ideas are correct, one would expect that adsorption at slightly higher temperatures, where spill-over of adsorbed CO is activated, would cause significant changes in the TPD spectra. This was tested in an additional experiment, where we investigated the influence of an increase of the CO exposure or a change of the adsorption temperature on the adsorption behavior and displacement of Dad by CO during CO post-adsorption. In Fig. 7a we show two D2 TPD spectra and the related CO TPD spectra recorded from a sample with 0.3 ML Pt on Ru(0001) after saturation with D2 (40×10-6 mbar·s D2) and two different exposures of CO at 90 K. The full line illustrates deuterium desorption after an exposure of 30×10-6 mbar·s CO, as it was used in the experiments before (section 3.2), while the dashdotted spectrum was recorded after a much higher CO exposure (75×10-6 mbar·s CO). The differences between both D2 TPD spectra are relatively small: the D2 TPD display a slightly lower Dad coverage (0.69 ML instead of 0.74 ML Dad) after the higher CO exposure, mainly due to a loss of desorption intensity at the high temperature end of the desorption spectrum at around 300 K, which according to our above interpretation results from deuterium desorption from the mixed adlayer on the Ru(0001) areas, while the low-temperature side of the spectrum remains unchanged. The CO TPD spectra in turn show a small additional shoulder at the low-temperature side of the spectrum around 400 K after the higher exposure and the COad coverage increases from 0.22 ML to 0.27 ML. The situation changes drastically when using the higher adsorption temperature of 150 K (Fig. 7b). Already after an exposure of 15×10-6 mbar·s CO, which is only half of the exposure used in the previous experiments (Fig. 3), the D2 TPD shows significant deviations from the result at 90 K, namely a strong decrease of the intensity in the high temperature state at around 300 K. The Dad coverage decreases to 0.47 ML and it decreases even more with increasing CO ACS Paragon Plus Environment

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exposure, the respective COad and Dad coverages are listed in Table 3. Finally, after 150×10-6 mbar·s CO, the Dad coverage is reduced to only 0.07 ML. The COad coverage, on the other hand, increases to 0.66 ML. A further increase in the CO exposure to 225×10-6 mbar·s does not lead to any further changes of the adsorbate coverages. Similar to adsorption at 90 K, the peak maximum of the main D2 desorption feature is again at ~200 K; in this case it shifts to slightly lower temperature with decreasing Dad coverage, with a common leading edge. Considering our above explanation of this D2 desorption peak, where we attributed this to displacement of Dad due to thermally activated spill-over of COad from Pt monolayer sites to Ru(0001) sites, the present results must mean that the COad spill-over is rather slow at 150 K, leaving enough COad on Pt monolayer sites that can spill-over to the Ru(0001) areas during subsequent desorption to create the 200 K peak. This agrees well with the IR data in Fig. 5, which also indicated that COad spill-over on the mixed adlayer covered Pt0.3-ML/Ru(0001) surface occurs mainly in the temperature range 180 – 200 K. In contrast, for a pure CO adlayer COad migration was found to be activated at about 150 K.31;32 After larger CO exposures, the D2 TPD spectra show an interesting small feature at higher temperature (~270 K), which was not observed after dosing at 90 K, and whose intensity remains constant at CO exposures of ≥ 75 10-6 mbar s. At lower CO exposures, this feature is hidden by the high temperature D2 desorption peak at slightly higher temperature (290 K), which we assume to result from deuterium desorption from a mixed layer on the Ru(0001) surface areas (see also section 3.1). Under present adsorption conditions, a high COad coverage is created on the Pt monolayer islands, as the Ru(0001) areas are still largely blocked by Dad. Slow spill-over of COad during adsorption at 150 K has displaced most of the Dad from the Ru(0001) areas, but apparently not all. During the heating ramp of the subsequent desorption experiment a small amount of Dad can still be displaced, as evident from the first peak at 200 K, but not all. We assume that for the remaining dispersed Dad



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recombination is hindered, e.g., the mobility of the Dad in the CO adlayer is not sufficient for recombination. Accordingly we attribute this feature to Dad atoms which are trapped in the COad layer on the Ru(0001) part of the surface. This idea is supported by results reported by Mak et al.,19 who determined the Had diffusion coefficient in coadsorbate layers at 260 K with varying COad coverage (from 0 to 0.2 ML) using laser induced thermal desorption. They found that already at these rather low COad coverages the diffusion coefficient decreases significantly with increasing COad coverage. According to that picture, these Dad atoms seem to become mobile only at higher temperature, after the onset of COad desorption, when the CO coverage is reduced. Finally, it should be noted that the effects related to spill-over of COad are specific for bimetallic PtRu surfaces structured on a larger scale, such as the Pt monolayer island Ru(0001) surfaces. They are not expected and have not been observed experimentally for bimetallic surfaces structured on an atomic scale such as PtRu/Ru(0001) surface alloys.20;22 In the latter case, the distances between different sites are too small to inhibit occupation of a neighbored, more stable site, at least at present temperatures. For these surfaces, coadsorption of CO and deuterium is dominated by a combination of substrate – adsorbate interactions and adsorbate – adsorbate interactions, where in particular the former ones will vary significantly with the local surface composition, due to electronic ligand and strain effects and geometric ensemble effects.

4 Conclusions We have systematically investigated the coadsorption and desorption behavior of deuterium and CO on a structurally well defined bimetallic PtRu surface, interested in particular in the interplay between substrate induced and adsorbate induced (adsorbate – adsorbate interactions) modifications of the adsorption and desorption energetics and kinetics. Using a


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Ru(0001) surface modified by extended Pt monolayer islands with a coverage of ~0.3 ML as model system, and TPD and IRRAS as main method, we arrive at the following conclusions on the interaction of COad and Dad with this surface: 1.

Most important, the properties of the mixed adlayer formed upon coadsorption of CO and deuterium on the Pt0.3-ML/Ru(0001) surface depend sensitively not only on the relative amounts of the coadsorbed species, Dad or COad, but also on a number of other parameters, in particular on the temperature and on the order of adsorption.

2.

In mixed Dad + COad layers, energetics favor the adsorption of CO on the more strongly adsorbing Ru(0001) sites rather than on the weakly adsorbing Pt0.3-ML/Ru(0001) sites since the difference in binding energy between the two sites is larger for COad than for Dad. This principal preference is modified by adsorbate – adsorbate interactions. Furthermore, reaching it may be hindered or even inhibited by kinetic limitations.

3.

For adsorption at low temperatures (90 K), displacement of Dad or COad by postadsorption of the respective other species is not possible. Furthermore, deuterium adsorption is inhibited by a saturated CO adlayer. In contrast, CO adsorption is possible on a Pt/Ru(0001) surface saturated with adsorbed deuterium. Up to ~0.3 ML CO can be adsorbed on the Dad saturated Pt0.3-ML/Ru(0001) surface. This is explained by a combination of i) CO adsorption on the Pt monolayer islands, where deuterium adsorption at 90 K results in a rather low local Dad saturation coverage of ~0.3 ML, and ii) the formation of a high density mixed adlayer by insertion of CO molecules in defects of the (1×1) Dad adlayer after saturation, exceeding a total relative coverage of 1.0 ML on the Ru(0001) surface areas.

4.

Post-adsorption of deuterium (post-saturation with Dad) on a surface pre-covered with CO adlayers of different coverages at 90 K leads only to slight shifts of the CO-Ru band position to higher wave number. Apart of this, the sequential occupation of first



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predominantly Ru and then Pt sites with increasing COad coverage is maintained. In contrast, post-adsorption of CO (saturation exposure) on surfaces pre-covered with deuterium adlayers of different coverages at 90 K results in a reversal of this sequence. Since Ru sites are partly or completely blocked by pre-adsorbed Dad at this temperature, CO is mainly adsorbed on the Pt islands. Additionally, CO adsorption is also possible to a smaller extent on the Ru part of the surface, either if the local Dad layer has not reached saturation or analogously to the process described in the item above. The distinct differences between the two cases are associated with kinetic limitations, specifically with the limited mobility of COad on these surfaces. 5.

Upon annealing of the mixed CO + D adlayer, e.g., during a TPD run or in temperature dependent IRRAS measurements (Fig. 5), COad surface mobility in the adlayer is activated at ~160 – 200 K (Fig. 6). Spill-over of COad from the Pt/Ru(0001) sites to the more strongly adsorbing Ru(0001) sites causes the displacement and desorption of deuterium from this part of the surface with a peak maximum at ~200 K (Fig. 6b–d). While COad is at first mainly located on the metastable Pt/Ru(0001) sites, IR spectra indicate an increase of COad on Ru(0001) sites in the temperature range 180 - 200 K. Hence, in this case the desorption temperature is determined by kinetic effects, by the onset of COad mobility, rather than by the modified binding energy of Dad. For the opposite case, pre-adsorption of CO and post-adsorption of deuterium, D2 desorption takes place in a narrow peak with a maximum at ~290 K (Fig. 6a), which is almost identical with the peak position for D2 desorption in the absence of COad (β1 state). Repulsive interactions between Dad and COad and segregation into islands are held responsible for the constant peak temperature, independent of Dad coverage, and the narrower width of the desorption peak for D2 desorption from the mixed adlayer.


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6.

For adsorption at 150 K and higher, Dad can be displaced from the surface by postadsorption of CO from the Pt0.3-ML/Ru(0001) surface, while for the Pt free Ru(0001) this is observed only for much higher adsorption temperature. The more facile displacement of adsorbed deuterium can be explained by the fact that D2 desorption from the Pt monolayer islands occurs already in the temperature range 90 – 150 K, while on Ru(0001) it starts only at 180 - 200 K. Displacement of Dad from Ru(0001) sites to Pt/Ru(0001) sites and eventually to desorption is driven by COad spill-over (see 2. and Fig. 6b -d).

Overall, both COad – Dad interactions and Pt surface modification have pronounced effects on the adsorption characteristics of CO and deuterium. The Pt monolayer island modified Ru(0001) surface can be considered as an intermediate case between Ru(0001), where only pure Ru sites are available, and PtRu/Ru(0001) monolayer surface alloys, where neighbored Pt and Ru sites are much more frequent and where due to ligand and ensemble effects a much larger variety of surface sites is available. Accordingly, the destabilization of Dad due to coadsorption with COad is even more pronounced on PtRu/Ru(0001) surface alloys than on the 0.3 ML Pt island modified Ru(0001) surface, and because of the close proximity of the different sites, spill-over effects are absent on the latter surfaces.

Acknowledgements This work was supported by the State Foundation Baden-Württemberg within the Network ‘Functional Nanostructures’ (project B1) and by the Deutsche Forschungsgemeinschaft via Research Group 1376 (Be 1201/18-1).



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References (1) Petrii, O. A. Pt-Ru Electrocatalysts for Fuel Cells: A Representative Review. J. Solid State Electrochem. 2008, 12, 609-642. (2) Watanabe, M.; Motoo, S. Electrocatalysis by Ad-Atoms. Part III. Enhancement of the Oxidation of Carbon Monoxide on Platinum by Ruthenium Ad-Atoms. J. Electroanal. Chem. 1975, 60, 275-283. (3) Ralph, T. R.; Hogarth, M. P. Catalysis for Low Temperature Fuel Cells. Part II: The Anode Challenges. Platinum Metals Rev. 2002, 46, 117-135. (4) Watanabe, M.; Motoo, S. Electrocatalysis by Ad-Atoms. Part II. Enhancement of the Oxidation of Methanol on Platinum by Ruthenium Ad-Atoms. J. Electroanal. Chem. 1975, 60, 267-273. (5) Hogarth, M. P.; Ralph, T. R. Catalysis for Low Temperature Fuel Cells. Part III: Challenges for the Direct Methanol Fuel Cell. Platinum Metals Rev. 2002, 46, 146-164. (6) Buatier de Mongeot, F.; Scherer, M.; Gleich, B.; Kopatzki, E.; Behm, R. J. CO Adsorption and Oxidation on Bimetallic Pt/Ru(0001) Surfaces – A Combined STM and TPD/TPR Study. Surf. Sci. 1998, 411, 249-262. (7) Schlapka, A.; Käsberger, U.; Menzel, D.; Jakob, P. Vibrational Spectroscopy of CO Used as a Local Probe to Study the Surface Morphology of Pt on Ru(001) in the Submonolayer Regime. Surf. Sci. 2002, 502-503, 129-135. (8) Diemant, T.; Hager, T.; Hoster, H. E.; Rauscher, H.; Behm, R. J. Hydrogen Adsorption and Coadsorption with CO on Well-Defined Bimetallic PtRu Surfaces –– A Model Study on the CO Tolerance of Bimetallic PtRu Anode Catalysts in Low Temperature Polymer Electrolyte Fuel Cells. Surf. Sci. 2003, 541, 137-146. (9) Schlapka, A.; Lischka, M.; Gross, A.; Käsberger, U.; Jakob, P. Surface Strain versus Substrate Interaction in Heteroepitaxial Metal Layers: Pt on Ru(0001). Phys. Rev. Lett. 2003, 91, 016101 (1-4). (10) Rauscher, H.; Hager, T.; Diemant, T.; Hoster, H.; Buatier de Mongeot, F.; Behm, R. J. Interaction of CO with Atomically Well-Defined PtxRuy/Ru(0001) Surface Alloys. Surf. Sci. 2007, 601, 4608-4619.


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(11) Hartmann, H.; Diemant, T.; Bansmann, J.; Behm, R. J. Interaction of CO and Deuterium with Bimetallic, Monolayer Pt-Island/Film Covered Ru(0001) Surfaces. Phys. Chem. Chem. Phys. 2012 , 14, 10919-10934. (12) Diemant, T.; Rauscher, H.; Behm, R. J. Interaction of Deuterium with Well-Defined PtxRu1-x/Ru(0001) Surface Alloys. J. Phys. Chem. C 2008, 112, 8381-8390. (13) Peebles, D. E.; Schreifels, J. A.; White, J. M. The Interaction of Coadsorbed Hydrogen and Carbon Monoxide on Ru(001). Surf. Sci. 1982, 116, 117-134. (14) Riedmüller, B.; Ciobîca, I. M.; Papageorgopoulos, D. C.; Berenbak, B.; van Santen, R. A.; Kleyn, A. W. The Dynamic Interaction of CO with Ru(0001) in the Presence of Adsorbed CO and Hydrogen. Surf. Sci. 2000, 465, 347-360. (15) Riedmüller, B.; Ciobîca, I. M.; Papageorgopoulos, D. C.; Frechard, F.; Berenbak, B.; Kleyn, A. W.; van Santen, R. A. CO Adsorption on Hydrogen Saturated Ru(0001). J. Chem. Phys. 2001, 115, 5244-5251. (16) Riedmüller, B.; Papageorgopoulos, D. C.; Berenbak, B.; van Santen, R. A.; Kleyn, A. W. ‘Magic’ Island Formation of CO Coadsorbed with H on Ru(0001). Surf. Sci. 2002, 515, 323-336. (17) Groot, I. M. N.; Juanes-Marcos, J. C.; Diaz, C.; Somers, M. F.; Olsen, R. A.; Kroes, G. J. Dynamics of Dissociative Adsorption of Hydrogen on a CO-Precovered Ru(0001) Surface: A Comparison of Theoretical and Experimental Results. Phys. Chem. Chem. Phys. 2010, 12, 1331-1340. (18) Ueta, H.; Groot, I. M. N.; Gleeson, M. A.; Stolte, S.; McBane, G. C.; Juurlink, L. B. F.; Kleyn, A. W. CO Blocking of D2 Dissociative Adsorption on Ru(0001). ChemPhysChem 2008, 9, 2372-2378. (19) Mak, C. H.; Deckert, A. A.; George, S. M. Effects of Coadsorbed Carbon Monoxide on the Surface Diffusion of Hydrogen on Ru(001). J. Chem. Phys. 1988, 89, 5242-5250. (20) Diemant, T.; Bansmann, J.; Rauscher, H. Coadsorption of Hydrogen and CO on Hydrogen Pre-Covered PtRu/Ru(0001) Surface Alloys. ChemPhysChem 2010, 11, 14821490. (21) Ueta, H.; Groot, I. M. N.; Juurlink, L. B. F.; Kleyn, A. W.; Gleeson, M. A. Evidence of Stable High-Temperature Dx-CO Intermediates on the Ru(0001) Surface. J. Chem. Phys. 2012, 136, 114710 (1-6). ACS Paragon Plus Environment


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(22) Diemant, T.; Rauscher, H.; Bansmann, J.; Behm, R. J. Coadsorption of Hydrogen and CO on Well-Defined Pt35Ru65/Ru(0001) Surface Alloys — Site Specificity vs. Adsorbate–Adsorbate Interactions. Phys. Chem. Chem. Phys. 2010 , 12, 9801-9810. (23) Wang, R. L. C.; Kreuzer, H. J.; Jakob, P.; Menzel, D. Lateral Interactions in Coadsorbate Layers: Vibrational Frequency Shifts. J. Chem. Phys. 1999, 111, 21152122. (24) Mitchell, W. J.; Xie, J.; Jachimowski, T. A.; Weinberg, W. H. Carbon Monoxide Hydrogenation on the Ru(001) Surface at Low Temperature Using Gas-Phase Atomic Hydrogen: Spectroscopic Evidence for the Carbonyl Insertion Mechanism on a Transition Metal Surface. J. Am. Chem. Soc. 1995, 117, 2606-2617. (25) Baldwin, V. H.; Hudson, J. B. Coadsorption of Hydrogen and Carbon Monoxide on (111) Platinum. J. Vac. Sci. Technol. 1983, 8, 49-52. (26) Lenz, K.; Poelsema, B.; Bernasek, S. L.; Comsa, G. Lateral Distribution of Coadsorbed H and CO on Pt(111) Studied by TEAS. Surf. Sci. 1987, 189/190, 431-437. (27) Bernasek, S. L.; Lenz, K.; Poelsema, B.; Comsa, G. Formation of Islands Consisting of Repelling Adsorbates. Surf. Sci. 1987, 183, L319-L324. (28) Hoge, D.; Tüshaus, M.; Bradshaw, A. M. Island Formation During CO/H Coadsorption on Pt(111) Studied by IR Reflection-Absorption Spectroscopy. Surf. Sci. 1988, 207, L935-L942. (29) Peebles, D. E.; Creighton, J. R.; Belton, D. N.; White, J. M. Coadsorption of Hydrogen and Carbon Monoxide on Nickel and Platinum. J. Catal. 1983, 80, 482-485. (30) Hoster, H. E.; Bergbreiter, A.; Erne, P. M.; Hager, T.; Rauscher, H.; Behm, R. J. PtxRu1-x/Ru(0001) Surface Alloys — Formation and Atom Distribution. Phys. Chem. Chem. Phys. 2008, 10, 3812-3823. (31) Hartmann, H.; Diemant, T.; Behm, R. J. Kinetically Limited CO Adsorption: Spill-Over as a Highly Effective Adsorption Pathway on Bimetallic Surfaces. ChemPhysChem 2013, 14, 3801-3805. (32) Hartmann, H.; Diemant, T.; Behm, R. J. Spill-Over Effects on Bimetallic Pt/Ru(0001) Surfaces. Top. Catal. 2013, 56, 1333-1344.


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(33) Diemant, T.; Bergbreiter, A.; Bansmann, J.; Hoster, H. E.; Behm, R. J. From Adlayer Islands to Surface Alloy: Structural and Chemical Changes on Bimetallic PtRu/Ru(0001) Surfaces. ChemPhysChem 2010, 11, 3123-3132. (34) Pfnür, H.; Feulner, P.; Menzel, D. The Influence of Adsorbate Interactions on Kinetics and Equilibrium for CO on Ru(001). II. Desorption Kinetics and Equilibrium. J. Chem. Phys. 1983, 79, 4613-4623. (35) Jakob, P.; Schlapka, A. CO Adsorption on Epitaxially Grown Pt Layers on Ru(0001). Surf. Sci. 2007, 601, 3556-3568.



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Table 1: Relative Dad and COad saturation coverages (θD/θD,sat and θCO/θCO,sat) on a monolayer Pt island modified Ru(0001) surface (with 0.25 ML Pt) together with the total relative coverage (sum of θD/θD,sat and θCO/θCO,sat) for the coadsorbate system described in Figs. 1 and 2. We used the corresponding saturation coverages of the two adsorbates on pure Ru(0001) (θD,sat = 1.0, θCO,sat = 0.68) as reference for relative values. The respective maximum coverages for pure Dad and COad on the bimetallic Pt0.25-ML/Ru(0001) surface are even smaller (θD,sat=0.85 and θCO,sat=0.67).

CO pre-exp. / 10-6 mbar·s 0 0.15 0.45 1.05 1.5 2.25 4.5 30

θCO / ML

θCO / θCO,sat

-0.06 0.12 0.23 0.33 0.49 0.63 0.67

-0.09 0.18 0.34 0.49 0.72 0.93 0.99

θD / θD,sat (θD,sat =1) 0.85 0.60 0.47 0.27 0.11 0.04 0.02 0.01


Total relative coverage / % 85 69 65 61 60 76 95 100

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Table 2: Relative Dad and COad saturation coverages (θD/θD,sat and θCO/θCO,sat) on a monolayer Pt island modified Ru(0001) surface (with 0.3 ML Pt) together with the total relative coverage (sum of θD/θD,sat and θCO/θCO,sat) for the coadsorbate system described in Figs. 3 and 4. We used the corresponding saturation coverages of the two adsorbates on pure Ru(0001) (θD,sat = 1.0, θCO,sat = 0.68) as reference for relative values. The respective maximum coverages for pure Dad and COad on the bimetallic Pt0.3-ML/Ru(0001) surface are even smaller (θD,sat = 0.75 and θCO,sat = 0.62). The last line gives the Dad coverage for 40×10-6 mbar s without CO post-adsorption.

D2 exposure / 10-6 mbar·s 0.25 0.55 1.2 4.0 40 40

θD / ML (θD,sat =1) 0.12 0.26 0.44 0.68 0.75 0.77

θCO / ML

θCO / θCO,sat

0.56 0.49 0.37 0.27 0.25 --

0.82 0.72 0.54 0.40 0.36 --


Total relat. coverage / % 94 98 99 108 110 77


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Table 3: Relative Dad and COad saturation coverages (θD/θD,sat and θCO/θCO,sat) on a monolayer Pt island modified Ru(0001) surface (with 0.3 ML Pt) together with the total relative coverage (sum of θD/θD,sat and θCO/θCO,sat) for the coadsorbate system described in Fig. 6 after adsorption at 150 K. We used the corresponding saturation coverages of the two adsorbates on pure Ru(0001) (θD,sat = 1.0, θCO,sat = 0.68) as reference for relative values. The respective maximum coverages for pure Dad and COad on the bimetallic Pt0.30-ML/Ru(0001) surface after adsorption at 150 K are smaller (θD,sat = 0.69 and θCO,sat = 0.68).

CO pre-exp. / 10-6 mbar·s 0 15 30 75 105 150 225

θCO / ML

θCO / θCO,sat

-0.32 0.41 0.56 0.59 0.67 0.67

-0.46 0.60 0.82 0.87 0.99 0.99

θD / θD,sat (θD,sat =1) 0.69 0.47 0.37 0.19 0.13 0.07 0.07


Total relative coverage / % 69 93 97 101 100 106 106

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Figure captions Fig. 1

D2 and CO TPD spectra recorded on a 0.25 ML Pt monolayer island modified Ru(0001) surface after pre-exposure to increasing amounts of CO, followed by 40×10-6 mbar·s D2. The upper panel (a) shows D2 TPD results, the lower panel (b) CO TPD results (CO exposures / 10-6 mbar·s: 0.15, 0.45, 1.05, 1.5; 2.25, 4.5, 30). Spectra recorded after exposure to 40×10-6 mbar·s D2 (dashed-dotted lines) and 0.2×10-6 mbar·s D2 (dashed line) without coadsorption of CO are added for comparison.

Fig. 2

IR spectra recorded on a 0.25 ML Pt monolayer island modified Ru(0001) surface after pre-exposure to increasing amounts of CO, followed by 40×10-6 mbar·s D2. Colors and exposures correspond to the TPD spectra in Fig. 1. IR spectra recorded on pure CO adlayers with similar COad coverage are added for comparison (dashed lines).

Fig. 3

D2 and CO TPD spectra recorded on a 0.3 ML Pt monolayer island modified Ru(0001) surface after pre-exposure to increasing amounts of D2, followed by 30×10-6 mbar·s CO. The upper panel (a) shows D2 TPD results, the lower panel (b) CO TPD results (D2 exposures / 10-6 mbar·s: 0.25, 0.55, 1.2, 4.0, 40). Spectra recorded after exposure to 0.2×10-6 mbar·s (dashed line) and 40×10-6 mbar·s D2 (dash-dotted line) or to 30×10-6 mbar·s CO (dotted line) are added for comparison.

Fig. 4

IR spectra recorded on a 0.3 ML Pt monolayer island modified Ru(0001) surface after pre-exposure to increasing amounts of D2, followed by 30×10-6 mbar·s CO. Colors and exposures correspond to the TPD spectra in Fig. 3. IR spectra recorded on pure CO adlayers with similar COad coverage are added for comparison (dashed lines).



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Fig. 5

(a) IR spectra recorded after an exposure to 40×10−6 mbar·s D2 followed by 30×10−6 mbar·s CO on a 0.25 ML Pt monolayer island modified Ru(0001) surface at 90 K (initial coverage: 0.76 ML D + 0.24 ML CO) and stepwise heating (10 K steps), (b) D2 TPD spectrum recorded after the same exposure sequence.

Fig. 6

Schematic representation of the mixed CO + D adlayer on a Pt monolayer island modified Ru(0001) surface for different adsorption sequences and at different characteristic coverages (right 90 K, middle 150 K, left 200 K). a) Pre-adsorption of CO at 90 K, followed by saturation with deuterium at the same temperature (results in D2 TPD peaks at 140 K and 290 K); b) pre-adsorption of very large amounts of deuterium at 90 K, followed by saturation with CO at the same temperature, where the total amount of COad is less than what is needed to completely displace preadsorbed deuterium from Ru(0001) surface areas, and deuterium still populates Pt monolayer island sites (results in D2 TPD peaks at 140 K, 200 K and 290 K); c) preadsorption of large amounts of deuterium at 90 K, followed by saturation with CO at the same temperature, where the total amount of COad is less than what is needed to completely displace pre-adsorbed deuterium from Ru(0001) surface areas upon spillover from Pt monolayer island sites (results in D2 TPD peaks at 200 K and 290 K); d) as c), but pre-adsorption of smaller amounts of deuterium, such that spill-over of COad leads to complete displacement of adsorbed deuterium from the Ru(0001) surface areas (results in a D2 TPD peak at 200 K) (green: Dad initially adsorbed on Pt monolayer sites, blue: COad initially adsorbed on Pt monolayer sites, magenta: Dad initially adsorbed on Ru(0001) sites, brown: COad initially adsorbed on Ru(0001) sites.

Fig. 7

TPD spectra recorded on a 0.3 ML Pt monolayer island modified Ru(0001) surface after an exposure to 40×10−6 mbar·s D2 followed by increasing CO exposures at ACS Paragon Plus Environment

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temperatures of (a) 90 K and (b) 150 K. The upper panels show D2 TPD results, the lower panels CO TPD results. CO exposures / 10−6 mbar·s: (90 K) 30 (full line), 75 (dash-dotted line); (150 K) 15, 30, 75, 105, 150, 225. Dashed lines: spectra of the respective

adsorbates

without

coadsorption

recorded

40×10-6 mbar·s D2 or to 30×10-6 mbar·s CO.


after

exposure

to


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Figure 1


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Figure 2 ACS Paragon Plus Environment

H. Hartmann et al.


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Figure 3


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T = 90 K

T = 150 K

T = 200 K D D

D D D D

O C

O O C D C D

D

O C

O C

Ru(0001)

a)

O C

Ru(0001)

b)

O D D D D D D C

O C

Ru(0001) D D

O C

D D

D D

O D D D D D D C

D

Ru(0001)

Ru(0001)

(290 K)

O O O O D C D D C C C

O O D C D D C

D D O O C D D C

D D

D

O C

(290 K) O C

O C

Ru(0001) D D D D

O C

O C

O C

O O D D D D D C C

O C

O C

Ru(0001)

c)

O C

O O O D C C C

O O D D D D D C C

Ru(0001)

O O C C

Ru(0001) D D

O C

O C

O C D

d)

D

O C

Ru(0001)

O C

O C

O C

O C

O C D

O D C

O C

O C

O O O O O O C C C C C C

Ru(0001)

Ru(0001)

Figure 6

H. Hartmann et al.



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Table of Content (TOC) graphic

(140 K)

(200 K) (290 K)

D D

O D D C

O D D D D D D C



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Interaction of Coadsorbed CO and Deuterium on a Bimetallic, Pt

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