Graphene-Supported Pd Nanoclusters Probed by Carbon Monoxide

Article pubs.acs.org/JPCC

Graphene-Supported Pd Nanoclusters Probed by Carbon Monoxide Adsorption Karin Gotterbarm, Carina Bronnbauer, Udo Bauer, Christian Papp,* and Hans-Peter Steinrück Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The adsorption of CO on graphene-supported Pd nanoparticles was studied in situ with high-resolution synchrotron-based X-ray photoelectron spectroscopy. At 150 K, CO adsorbs mainly in bridge and 3-fold-hollow sites. The nanoparticles are considered as a mixture of low-index facets. The variation of the amount of deposited Pd revealed identical CO adsorption behavior for all investigated cases, confirming a similar average cluster size over a wide range of Pd coverages. The desorption characteristics were studied with temperature-programmed XPS. The observed desorption maxima at 230 and 430 K are in good agreement with temperature-programmed desorption data on stepped Pd single crystals. At 500 K, CO is completely desorbed from the Pd clusters. The adsorption and desorption of CO are found to be not fully reversible as the Pd particles undergo restructuring upon heating.

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there is no effective step preference on this surface.16 Highresolution electron energy loss spectroscopy (HREELS) and low electron energy diffraction (LEED) measurements on Pd(510) showed that CO adsorption initially occurs on terraces.18 Gerber et al. studied the influence of CO on Pt nanocluster arrays deposited on graphene/Ir(111) at room temperature.22 They found Smoluchowski ripening for Pt clusters smaller than 10 atoms. For larger clusters, CO exposure leads to cluster transformation toward a more three-dimensional shape. We report in situ high-resolution synchrotron-based X-ray photoelectron spectroscopy (HR-XPS21) studies of the adsorption of CO on graphene-supported Pd nanocluster arrays. High energy and time resolution enable us to gain insight into the adsorption sites as well as a detailed understanding of the thermal desorption process.

INTRODUCTION Graphene−palladium nanocomposites1 are intriguing catalyst materials with promising applications in catalytic C−C crosscoupling,2,3 oxidation of alcohols4 and formic acid,5 and for hydrogen sensing.6 The synthesis from exfoliated graphite and wet-chemical dispersion of Pd particles is well-suited to produce scalable amounts of these catalysts. However, in order to investigate the fundamental properties of Pd nanoparticles on graphene in detail, it is necessary to study better defined systems. The template effect of the intrinsic graphene Moiré pattern on certain single crystal substrates can be used to produce regular arrays of nanoclusters with a narrow size distribution.7−10 We recently reported the fabrication of such Pd nanoclusters on graphene grown on Rh(111).11 One very effective method to characterize nanoparticles is to investigate their adsorption properties. CO is an important probe molecule in surface science and has been studied in detail on various Pd single crystal surfaces. On Pd(100)12 and Pd(110),13 CO adsorbs mainly on bridge sites, while on Pd(111)14 mainly 3-fold hollow and on top sites are occupied for high CO coverages. Moreover, the Pd 3d5/2 core level is very sensitive to CO adsorption. It has been shown that the Pd 3d5/2 binding energy increases with the number of bonds to a CO molecule, i.e., its coordination to substrate atoms.12,14 CO adsorption has also been studied on a number of stepped Pd single crystal surfaces, e.g., Pd(210),15 Pd(211),16,17 Pd(311),15 and Pd(510).18 Such experiments on regularly stepped surfaces, i.e., with defined defects, are a first step toward the adsorption on clusters, which exhibit a variety of steps, kinks, etc. However, in contrast to results on stepped Pt surfaces,19−21 both experimental and theoretical results point to a small influence of the steps on the adsorption behavior. Density functional theory calculations of CO adsorbed on Pd(211) yield similar adsorption energies for terrace and step sites, confirming that © XXXX American Chemical Society

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EXPERIMENTAL DETAILS XPS measurements were performed at beamline U49/2-PGM1 of Helmholtz-Zentrum Berlin in a transportable UHV apparatus23 consisting of two chambers. An Omicron EA 125 U7 HR hemispherical electron energy analyzer, a quadrupole mass spectrometer, an electron beam evaporator, and a threestage supersonic molecular beam are connected to the analyzer chamber. LEED optics, a sputter gun, a quartz-crystal microbalance (QCM), and a second metal evaporator opposite to the QCM are situated in the preparation chamber. The Rh sample is cooled with liquid nitrogen and heated resistively. With this setup, sample temperatures between 100 and 1400 K can be achieved. An additional tungsten filament is mounted at Received: August 21, 2014 Revised: October 6, 2014

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dx.doi.org/10.1021/jp508454h | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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the back of the sample to apply linear heating ramps of 0.5 K/s during XPS measurements. These experiments are referred to as temperature-programmed XPS (TP-XPS).24 All XP spectra shown were recorded at normal emission. C 1s and Rh 3d core level spectra were measured with a photon energy of 380 eV, at a total energy resolution of 170 meV. Pd 3d5/2 and O 1s/Pd 3p3/2 spectra were recorded with 450 and 650 eV photon energy, at total energy resolutions of 180 and 230 meV, respectively. Acquisition times are typically 10 s or less per spectrum. All spectra are referenced to the Fermi level and are corrected with a linear background. XPS signals were fitted with asymmetric Doniach−Sunjic functions convoluted with a Gaussian function.25 The fitting parameters are listed in detail in Tables S1 and S2 of the Supporting Information. Graphene was grown via chemical vapor deposition of 2 × 10−8 mbar propene at 920 K.26 Pd was deposited onto the sample by electron beam evaporation at 150 K. CO was dosed at 150 K a background pressure of 6 × 10−9 mbar. CO exposures are given in langmuirs (L). Pd coverages were calibrated with the QCM. Carbon coverages were calibrated using a reference structure of CO on Rh(111), which exhibits a (2 × 2) LEED pattern with a coverage of 0.75 ML.27 One ML of adsorbate thereby corresponds to one carbon atom (i.e., CO molecule) per Rh surface atom.

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RESULTS We recently reported on the formation of regular Pd nanoclusters produced by evaporating palladium onto a graphene/Rh(111) template.11 In the present study, Pd nanocluster arrays were probed by CO adsorption. Selected C 1s spectra during the adsorption of CO on 0.7 ML of Pd at 150 K are shown in Figure 1a. The thick black spectrum was recorded directly after deposition of Pd. It shows the two characteristic signals for graphene on Rh(111), at 285.10 and 284.50 eV, resulting from different interaction strengths of the C atoms in the Moiré structure with the substrate.26,28 After dosing CO (thin black spectra), a signal at 285.90 eV evolves. With increasing CO exposure, this signal increases and shifts to higher binding energies. Finally, after exposure to 1.3 L of CO (blue spectrum), the peak saturates and is located at 286.00 eV. In comparison to the first spectrum, the intensities of the two graphene signals are slightly damped after saturation with CO. Note that CO does not adsorb on pristine graphene/Rh(111) under the conditions applied here (data not shown). The asymmetry and full width at half-maximum (fwhm) of the CO signal vary with increasing CO coverage. Therefore, it was not possible to obtain satisfying fitting results using only one CO contribution with constant fitting parameters. Instead, the CO signal is described by two contributions: CO1 at 285.90 eV and CO2 at 286.06 eV. Typical fits are shown in Figure S1, and the fitting parameters are listed in Table S1 of the Supporting Information. The quantitative analysis is shown in Figure 2a. The coverage of CO1 (285.90 eV, red circles) increases linearly with CO exposure, and after 0.5 langmuir a coverage of 0.22 ML is reached. When further increasing the exposure, the second CO component, CO2 (286.06 eV, purple circles), starts to increase until its saturation coverage of 0.08 ML is reached at 1.3 L. At the same time, the coverage of CO1 decreases to 0.18 ML. As the surface of the Pd nanoparticles has to be understood as a mixture of steps and differently oriented facets, it is not trivial to exactly distinguish the adsorption sites of CO. However, the relative binding energy shifts can give a good idea of the degree of coordination. In comparison to XPS

Figure 1. Adsorption of 6 × 10−9 mbar of CO on 0.7 ML of Pd/ graphene/Rh(111) at 150 K: (a) selected C 1s spectra during CO adsorption; (b) selected Pd 3d5/2 spectra during CO adsorption.

data of CO on Pd(111)14 and (100),12 the C 1s binding energy of CO1 (285.90 eV) corresponds to higher coordinated CO molecules (i.e., step and hollow sites) and CO2 (286.06 eV) corresponds to lower coordinated sites such as bridge and possibly top sites. Considering the small separation of only 0.16 eV between CO1 and CO2, the latter is rather unlikely. The complex mixture of adsorption sites on the particles and the overlap of the asymmetric tail of the graphene signals with the CO signals do not allow to obtain more information on the adsorption of CO from the C 1s data. Selected Pd 3d5/2 spectra taken during the adsorption of CO on 0.7 ML of Pd at 150 K are shown in Figure 1b. Note that this experiment was conducted after one complete CO adsorption−desorption cycle. The effects of this will be discussed later. The thick black spectrum of the clean Pd particles shows an asymmetric signal with the peak maximum at 335.1 eV and a fwhm of 1.0 eV. Already small amounts of CO lead to a significant change in the shape of the Pd 3d5/2 signal. After an exposure to 0.03 langmuir of CO (first thin black spectrum), an additional shoulder at 335.4 eV develops, and the Pd signal at 335.1 eV consisting of the bulk and surface contributions decreases. The new signal is assigned to Pd atoms interacting with CO. With increasing CO exposure, the Pd− CO signal increases further and shifts to higher binding energies while the Pd signal at 335.1 eV decreases further. After saturation with CO, two clearly separated signals are observed B



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Figure 2. Quantitative analysis obtained from fitting of the experiments shown in Figure 1 and Figure 4; (a−c) adsorption of CO on 0.7 ML of Pd; (d−f) TP-XPS of CO on 0.7 ML of Pd. Typical fits are shown in Figures S1, S2, and S4, and the quantitative analysis of the graphene signals is shown in Figure S3.

(blue spectrum). The signal at 335.9 eV is assigned to Pd atoms interacting with CO and the one at 335.1 eV is assigned to Pd atoms in the bulk of the nanoparticles. Pd 3d5/2 spectra with signals at similar binding energies have been observed for COsaturated Pd(111),14 (110),13 and (100)12 surfaces. The Pd 3d5/2 surface core level shift is known to be very sensitive to CO adsorption: It has been demonstrated to be directly proportional to the number of bonds a Pd atom forms with a CO molecule.12,14 A shift of approximately 1.0 eV thereby corresponds to one Pd atom coordinated to one CO molecule. This means for example that Pd atoms in contact with one

bridge-coordinated CO molecule exhibit a shift of 0.5 eV with respect to the bulk contribution, since this one CO molecule is bound to two Pd atoms. Accordingly, Pd atoms in contact with two bridged CO molecules exhibit a shift of 1.0 eV (see Table 1). Before we discuss the results of the quantitative analysis, we will briefly explain the Pd 3d5/2 contributions that we used to describe the spectra. All fitting parameters are listed in Table S2, and typical fits are shown in Figure S2 of the Supporting Information. In agreement with previous work,11,29 we used two surface components, S1 and S2, in addition to the bulk C



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the differences between the CO-induced contributions observed in the C 1s and Pd 3d5/2 core levels. For both, the binding energy shifts relate to the degree of coordination of the respective molecule or atom. In the C 1s region, CO molecules bonded at step sites and 3-fold hollow sites induce the CO1 species, while lower coordinated molecules in bridge sites and top sites lead to the additional CO2 signal. From the surface core level shifts of the Pd 3d5/2 region (see Table 1), we assign the Pd−COα signal to Pd atoms coordinated to one 3-fold hollow-bound CO molecule and to Pd atoms forming a 3-fold step site coordinated to one CO molecule. Furthermore, the Pd−COβ signal corresponds to Pd atoms either bound to one CO molecule in a bridge site or to two CO molecules in 3-fold hollow sites. In the same way, the Pd−COγ contribution is assigned to Pd atoms coordinated to either two CO molecules bound in bridge sites or one CO molecule on top. In this model, the coverages of the C 1s and Pd 3d5/2 species cannot be translated directly into one another, since one type of CO molecule can be associated with multiple contributions in the Pd region. For example, a CO molecule adsorbed in a bridge site (CO2) may be bound to two inequivalently coordinated Pd atoms: one Pd which is bound to a second CO bridge (CO2) molecule leading to a Pd−COγ contribution and one Pd which is not coordinated otherwise thus leading to a Pd−COβ contribution. From this assignment of the adsorption sites and from the good agreement of the spectral shape of our data with single crystal data, we conclude that the surface of the graphene-supported Pd nanoparticles mainly consists of (111) and possibly some other low indexed facets such as (100) facets. The size of the nanoparticles of only ∼3 nm11 also leads to a considerable fraction of the adsorption sites to be at steps. Since we have no further information on the geometry of the step sites, we cannot clearly assign them to one of the Pd−CO species. To investigate the influence of the Pd cluster density, nanocluster samples with different Pd amounts were prepared and exposed to CO. Figure 3 shows C 1s, O 1s/Pd 3p3/2, and Pd 3d5/2 spectra of three different Pd layers saturated with CO. The spectra for the sample with the lowest Pd amount of 0.4 ML are shown in black, for 0.7 ML of Pd in blue (see also Figure 1), and for 1.2 ML of Pd in red. The C 1s spectra (see Figure 3a) show three asymmetric signals: the CO signal at 286.0 eV and the two graphene signals, C1 at 285.1 eV and C2 at 284.6 eV. For higher Pd coverages (0.7 and 1.2 ML), the C2 signal is shifted to 284.5 eV. With increasing Pd coverage, the graphene signals are more attenuated and therefore decrease in intensity. The intensity ratio of C1 to C2 is slightly different for 1.2 ML of Pd (red spectrum) due to a different ratio already for the pristine graphene layer. The CO signal intensity rises with the Pd coverage due to the higher surface area available to CO. The O 1s and Pd 3p3/2 spectra are displayed in Figure 3b. The Pd 3p3/2 signal appears at 532.0 eV and therefore overlaps with the CO O 1s signal in all displayed spectra. All three O 1s/Pd 3p3/2 spectra exhibit a broad, asymmetric signal at 532.0 eV, with a shoulder at 533.5 eV that includes the overlapping signals of the Pd 3p3/2 and two O 1s contributions. For higher Pd coverages, the Pd 3p3/2 signal and both CO signals increase, leading to a similar peak shape (fwhm of 2.5 eV) for all three Pd coverages. Figure 3c shows the CO-saturated Pd 3d5/2 spectra. Note that the Pd 3d5/2 spectra displayed here were recorded after the first CO adsorption experiment (which included a heating step, see below) in contrast to Figure 1b, which corresponds to adsorption directly after the preparation

Table 1. Relative Pd 3d5/2 Binding Energies of Pd−CO Contributionsa EB [eV] this work Surnev et al.,14 Pd(111) Andersen et al.,12 Pd(100)

bulk, 335.04 bulk, 334.89 bulk, 334.95

dE to bulk [eV] α

Pd−CO + 0.36 hollow 1b + 0.36

Pd−COβ + 0.66 bridge 1b + 0.51 bridge 1b + 0.55

hollow 2b + 0.70

Pd−COγ + 1.04 top + 1.04 bridge 2b + 1.00

a

Shifts are calculated with respect to the bulk contribution (average values). b1 and 2 denote the number of CO molecules coordinated to one Pd atom in the same kind of adsorption site.

component, to describe the clean Pd particles. To describe the interaction with CO, we employed three components with different binding energy shifts relative to the bulk signal: Pd− COα, Pd−COβ, and Pd−COγ. Because of the proportionality of the binding energy shifts to the number of coordinated CO molecules, the assignment of adsorption sites in the Pd 3d5/2 data is less ambiguous than in the C 1s data. For comparison, the relative binding energy shifts in this work, and the works of Andersen et al.12 and Surnev et al.14 are listed in Table 1. The shift of 0.36 eV of Pd−COα translates to about one-third of a bond to one CO molecule. We therefore assign Pd−COα to Pd atoms coordinated to one CO molecule in a 3-fold hollow position. The binding energy shift of Pd−COβ lies between that of Pd atoms bound to two CO molecules in hollow sites on Pd(111) (“hollow 2” in Table 1) and Pd atoms bound to one CO molecule in a bridge site on both Pd(100) and Pd(111) (“bridge 1”). Pd−COβ might therefore be assigned as mixture of such atoms. In the same way, the Pd−COγ contribution most likely corresponds to a mixture of Pd atoms bound to CO molecules in top sites and of Pd atoms bound to two CO molecules in bridge sites. The very similar binding energy shifts of on-top and 2-fold bridge-coordinated Pd atoms (see Table 1) do not allow us to distinguish between them. Note that considering the size of the nanoparticles of ∼3 nm,11 it is probable that step sites make up a considerable fraction of the adsorption sites, which were not distinguishable in our analysis. The results of the fitting procedure are shown in Figure 2b,c. During CO exposure, the bulk and the two surface signals (black and gray diamonds, Figure 2c) decrease. The intensity of the bulk component decreases to 54% of its original value after an exposure of 0.3 langmuir. Until 0.5 langmuir, the two surface signals have vanished. Simultaneously, all three CO-induced contributions increase (see Figure 2b). The increase is steepest for Pd−COα (blue circles) and flattest for Pd−COγ (green circles). The Pd−COα signal, assigned to Pd atoms interacting with 1/3 of a CO molecule, reaches its maximum intensity at 0.1 langmuir CO exposure and subsequently decreases. This is due to the increasing CO population on the Pd particles leading to a more dense packing and thus to Pd atoms that interact with more than only one CO molecule. The Pd−COβ contribution (orange circles) peaks at 0.3 langmuir and then decreases until it stays constant when CO saturation is reached for exposure higher than 0.6 langmuir. At this exposure, the Pd−COα signal has completely vanished and Pd−COγ is the highest populated species. This is again attributed to the more dense packing. Here the interpretation is that all surface Pd atoms interact with 0.5 or more CO molecules because the signal due to interaction with only one 3-fold bond CO has disappeared (blue circles). At this point we want to elucidate D



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core levels, we conclude that the amount of deposited Pd does not influence the CO adsorption mode in the range of deposited Pd amounts investigated here. This confirms our earlier finding that for higher Pd amounts the cluster density increases rather than the average cluster size.11 After CO adsorption at 150 K a heating ramp of 0.5 K/s was applied to the sample while continuously recording XP spectra. Selected C 1s spectra during this temperature-programmed XPS experiment are shown in Figure 4a. The blue spectrum at

Figure 4. Thermal stability of CO on 0.7 ML of Pd/graphene/ Rh(111): (a) selected C 1s spectra, (b) selected Pd 3d5/2 spectra; both collected at the denoted temperatures.

150 K corresponds to the blue spectrum in Figure 1a after saturation with CO. After heating to 300 K (green spectrum in Figure 4a), the CO signal decreased slightly by 0.05 ML and is shifted to lower binding energies by 60 meV. This corresponds to a surface with only CO1 adsorbed on the clusters, while CO2 at 286.06 eV is either desorbed or transformed to CO1 at 285.90 eV. This leads to the observed shift and decrease in intensity. At the same time, both graphene signals increase. Since the observed increase in intensity is higher than the intensity loss due to damping (see Figure 1a and Figure S3 in the Supporting Information), we attribute this increase to an additional effect, namely a restructuring of the Pd particles upon heating.11 As the particles become more three-dimensional, less graphene is covered with Pd and therefore the graphene signals increase. Upon heating to 400 K (orange spectrum in Figure 4a), both graphene signals increase even

Figure 3. CO adsorbed on different amounts of Pd on graphene/ Rh(111): (a) C 1s spectra, (b) O 1s/Pd 3p3/2 spectra, (c) Pd 3d5/2 spectra of CO saturated Pd layers at 150 K. Black spectra: 0.4 ML of Pd (1.1 L of CO), blue spectra: 0.7 ML of Pd (1.3 L of CO), red spectra: 1.2 ML of Pd (1.7 L of CO).

of the Pd clusters. The effect of the number of adsorption− desorption cycles will be discussed in detail along with the Pd 3d5/2 TP-XPS data later on. All three Pd 3d5/2 spectra consist of a Pd bulk signal at 335.1 eV and a larger Pd−CO signal at 335.9 eV. With increasing Pd amount, both signals increase equally in intensity, indicating that also the number of available adsorption sites for CO increases. From the similarity of the spectral shape and binding energies for all three investigated E



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and only minor amounts of Pd−COβ remain on the surface. At 500 K, the bulk signal is a little smaller, and the surface signals are larger than before adsorption at 150 K. This results in a broader peak shape due to the elevated temperature, as already mentioned before. As also observed for the adsorption process, the Pd 3d5/2 spectra are very similar to those of CO on Pd single crystal surfaces. On both Pd(111)14 and Pd(100),12 adsorption of CO at elevated temperatures leads to a shift of the CO-induced Pd 3d5/2 signal to lower binding energies similar to the obervations herein. Decomposition of the spectra at 300 K on Pd(111) and at 350 K on Pd(100) reveals less-CO induced Pd 3d species than at 150 K. The similarities of the desorption behavior of CO demonstrate again that the Pd nanoparticles consist of a mixture of (111) and (100) facets.

more, indicating further restructuring of the Pd particles. At the same time, the CO1 signal at 285.90 eV decreases further due to desorption. Finally, at 500 K it has vanished since CO has desorbed from the surface. Between 400 and 500 K, the graphene signals at 285.1 and 284.6 eV show no further increase, but only the typical temperature-induced broadening reported recently.26 The quantitative evaluation of the heating experiment is shown in Figure 2d. The CO2 species (purple circles) vanishes until 250 K. Since the coverage of CO1 increases in this temperature regime, the observed decrease of CO2 is partly due to desorption and partly due to conversion to CO1. For the more stable CO1 species, desorption starts around 250 K and is finished until 480 K. Numerical differentiation of the total CO amount yields desorption maxima at 230 and 430 K. We find these temperatures to be independent of the amount of deposited Pd. They are also in agreement with thermal desorption spectra of CO on stepped Pd surfaces, where after adsorption at 300 K the dominant desorption maximum was found at 450 K with a heating rate of 2 K/s.17 This confirms that the investigated Pd nanoparticles can be interpreted as a mixture of different low-index facets together with step sites connecting these facets. Figure 4b shows Pd 3d5/2 spectra of a similar TP-XPS experiment measured after a second adsorption of CO onto the Pd clusters (see Figure 1b). These second adsorption experiments differ slightly from the first adsorption experiments due to the restructuring of the Pd particles upon heating. The particles become more three-dimensional and therefore exhibit lower surface areas. Thus, we find that the amount of CO is lower by 25% as compared to the first experiment on the same clusters. The blue spectrum at 150 K (Figure 4b) corresponds to the blue spectrum in Figure 1b after saturation with CO. It shows a Pd−CO signal at 335.9 eV, with 1.1 eV fwhm, and a smaller Pd bulk signal at 335.1 eV. Upon heating to 300 K (green spectrum), the Pd−CO signal shifts to 335.6 eV and becomes narrower. Additionally, the intensity at 335.1 eV increases. The fwhm of the green spectrum is 1.0 eV. As CO desorbs, the average number of Pd bonds to CO decreases, resulting in the observed shift to lower binding energies. At 400 K, the Pd−CO signal is shifted to even lower binding energy, and the bulk peak further increased. This leads to a peak maximum at 335.4 eV and an overall fwhm of 1.0 eV. The continuous desorption of CO between 300 and 400 K leads to the further shift and increase of the clean Pd peak. Finally, at 500 K the peak maximum is shifted back to 335.1 eV, since CO has completely desorbed from the Pd particles at this temperature. Notice that the fwhm of 1.2 eV is broadened in comparison to the spectrum at 150 K before CO adsorption (see Figure 1b, thick black spectrum). This effect is attributed to the elevated sample temperature. After cooling down to 150 K, a fwhm of 1.0 eV was observed again. The quantitative analysis of this experiment is shown in Figure 2e,f. Typical fits are shown in Figure S4 of the Supporting Information. At 150 K, Pd−COβ and Pd−COγ (orange and green circles in Figure 2e) coexist, and there is no Pd−COα (blue circles) on the surface. Up to 250 K, the signals of Pd−COβ and Pd−COα increase while Pd−COγ decreases. At 350, K Pd−COγ has completely vanished, and the intensity of Pd−COα has reached its maximum. At this temperature, the Pd−COβ signal has decreased to half of its maximum intensity. Simultaneously, the bulk and surface signals (black and gray diamonds, Figure 2f) start to increase again as the Pd surface becomes less covered with CO. Finally, until 500 K Pd−COα completely vanishes,

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CONCLUSIONS We investigated the adsorption and thermal stability of CO on graphene-supported Pd nanoclusters in situ with highresolution XPS. Upon adsorption at 150 K, we find at least three CO-induced species in the Pd 3d5/2 region and two in the C 1s region. In comparison to single crystal data, we conclude that CO adsorbs mainly in hollow and bridge sites. The particles consist mainly of (111) facets and possibly some other low-index facets as well as step sites. Several samples with varying Pd amount were prepared and CO was adsorbed until saturation. We found the same adsorption behavior for all investigated cases, substantiating our previous finding that deposition of higher Pd amounts results in increased cluster density rather than cluster size. The adsorption of CO on graphene-supported Pd nanoparticles is reversible upon heating to 500 K, since CO is desorbed completely at this temperature. TP-XPS experiments revealed two desorption maxima at 230 and 430 K, in agreement with earlier TPD data on stepped Pd crystals.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Fitting parameters, typical fits, and the quantitative analysis of the graphene signals. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel +49-9131-8527326; e-mail [email protected] (C.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank HZB for the allocation of synchrotron radiation beamtime and BESSY staff for support during beamtime. This work was supported by SFB 953 “Synthetic Carbon Allotropes”. K. Gotterbarm thanks Fonds der Chemischen Industrie for her Ph.D. grant.

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