Adsorption Dynamics of CO on Silica-Supported Cu Clusters: A

Jul 17, 2011 - Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States ...... Henry , C. R. ...
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Adsorption Dynamics of CO on Silica-Supported Cu Clusters: A Molecular Beam Scattering Study M. Komarneni, J. Shan, and U. Burghaus* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States ABSTRACT: The adsorption kinetics and dynamics of CO on silica-supported Cu clusters were studied with thermal desorption spectroscopy and molecular beam scattering. In addition, the electronic properties of the Cu clusters and sample cleanliness were characterized with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Physical vapor deposition was used to deposit the clusters; according to XPS, the Cu clusters remain metallic. The CO impact energy, surface temperature, and Cu coverage dependence of the adsorption dynamics can be discussed in the framework of the capture zone model. The XPS/AES and thermal desorption spectroscopy (TDS) data are consistent with a standard growth mode of Cu on silica: a nucleation phase is followed by a cluster growth phase until thick and rather smooth Cu films are formed.

1. INTRODUCTION The use of nanoparticles for promoting catalytic processes is a major thrust in academia and industry.1 4 How the size, rim, shape, and chemical composition of metal clusters affect their catalytic activity are the major questions currently explored.5 8 As part of an ongoing project utilizing electron beam lithography9 11 to study supported metal clusters, we report here about physical vapor deposition (PVD) of copper on silica as a reference system. In addition, studies on supported copper model catalysts are motivated by the importance of the methanol synthesis and water gas shift reaction (see, e.g., refs 12 17). Furthermore, metallization of silica has applications in microelectronics.18 Among other silica-supported systems,19 PVD of copper on amorphous silica films was studied by Xu and co-workers18,20 22 and by others23 26 with Cu thermal desorption spectroscopy (TDS),18,23 X-ray photoelectron spectroscopy (XPS),21 Fourier transform infrared spectroscopy (FTIR),22,24 and on-air scanning tunneling microscopy (STM).20 Other deposition techniques have been employed too.27,28 Although Cu-on-silica was among the first model catalysts studied in detail,20 molecular beam scattering experiments have not, to the best of our knowledge, been conducted. Cu clusters start to desorb at 1000 K without intermixing with the support, although a minor Cu species, probably an oxide, is apparently formed at greater temperatures and was discussed in the framework of metal support interactions.18,28 Interesting and important for the discussion of the data described later is that the adsorption probability of Cu on silica, SCu, at room temperature is unusually low and amounts to only approximately 0.2 in the monolayer deposition range of copper. The adsorption probability of Cu on the Cu clusters (near saturation of the support with Cu clusters) may approach one, however.22,29 The Cu growth mode on silica r 2011 American Chemical Society

was studied in detail with FTIR and STM. Briefly, depositing Cu at low temperatures (100 K) results in highly dispersed 2D clusters that are characterized by a single CO TDS peak and a single IR band. Upon annealing at 300 K, 3D clusters form. This room-temperature morphology was also considered in our project. The annealed Cu deposits were characterized by several IR absorption bands and at least two CO TDS peaks indicating a Cu cluster ensemble consisting of various different copper surfaces.22 At low Cu exposures, a typical cluster size amounts to 10 30 Å, whereas at large Cu coverages, a continuous copper grain structure (∼200 Å) has been reported.20,22

2. EXPERIMENTAL PROCEDURES Experimental Setup. The measurements were conducted with a home-built, triply differentially pumped molecular beam system.30 The supersonic beam was attached to a scattering chamber housing two mass spectrometers (one for beam scattering experiments and one for a time-of-flight (TOF) beam characterization). In addition, a double pass cylindrical mirror analyzer equipped with an electron gun for Auger electron spectroscopy (AES) and an X-ray source for X-ray photoelectron spectroscopy (XPS) was mounted on the scattering chamber. The usual tools, including a sputter gun, home-built metal evaporators, and an atomic hydrogen source, were also mounted on the scattering chamber. The project focused on the molecular beam scattering experiments. Received: May 31, 2011 Revised: July 11, 2011 Published: July 17, 2011 16590

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The Journal of Physical Chemistry C AES/XPS. For XPS, the Mg KR line (at 1253.6 eV) was used with a pass energy of 50 eV of the analyzer. The XPS spectra were referenced with respect to the O 1s line at a binding energy (BE) of 532.9 eV.31 Charging effects, however, result in XPS peak shifts up to 2.6 eV. The electron energy for AES amounted typically to 2 keV with a modulation voltage of 2 eV. Uncertainties reported for AES intensities are based on the signal-to-noise ratio of the spectra. Adsorption Transients. The impact energy, Ei, of the CO molecules was varied within (0.09 0.98) eV by using a pure beam and by seeding 3% CO in He combined with a variation of the nozzle temperature within 300 700 K. Ei was determined with TOF. Initial adsorption probabilities, S0, and coveragedependent adsorption probabilities, S(Θ), were collected using the King and Wells uptake technique. The uncertainties in S0 amounted to (0.05 as estimated from the signal-to-noise ratio of the transients. To reduce the uncertainties, for some experiments, several independent measurements were averaged reducing the statistical error to (0.02. TDS. TDS data were collected with a shielded mass spectrometer in close proximity to the sample. In addition, CO was dosed onto the surface with the molecular beam system suppressing possible background effects further. The sample temperature, Ts, was decreased to ∼90 K. The reading of the thermocouple was calibrated in situ within (5 K by recording the condensation peaks of the alkanes. The heating rate for TDS amounted to approximately 1.7 K/s. Cu Deposition, Sample Cleaning. Copper was deposited at room temperature on silica by a Cu doser consisting of a 0.25 mm O.D. high-purity (99.9% from Goodfellow) Cu wire wrapped by a 0.15 mm O.D. W wire that was heated resistively. Several different silica supports and a Cu doser were used throughout the project. The clusters were never annealed above 350 K. A Si(111) wafer (from MEMS nanotechnology Exchange, VA, United States) with a 1 μm thick amorphous oxide layer was used as the support. The sample was cleaned with several Ar+ sputtering cycles (2 μA, 2 keV, 20 h). As described previously,9 sputtering not only removes contaminations but also reduces silica to silicon. Therefore, the sample was annealed in oxygen (2  10 5 mbar) for a total of eight hours to oxidize the support. Blind Experiments. Blind experiments were collected on bare silica supports.9,32 Fully oxidized and partially reduced supports were considered. No significant uptake of CO (consistent with ref 21) was seen in beam scattering experiments at the lowest accessible surface temperature. Results of the blind experiments are depicted below together with the data gathered on the supported Cu clusters.

3. PRESENTATION OF THE RESULTS AND DISCUSSION 3.1. Characterization of the Silica Support. The main object of this study is characterization of the adsorption dynamics of CO on silica-supported Cu clusters. However, the surface cleanliness and the growth of the Cu clusters were monitored with XPS. Figure 1A depicts examples of AES survey scans collected while cleaning the silica support. Different supports were used throughout the project. However, very similar AES and XPS data were obtained. As evident, initially present carbon impurities were reduced below the detection limit of the AES system. A very weak feature (AES N to O ratio of below 2%) corresponding to nitrogen remained even after extensive cleaning of the support. The inset in Figure 1A depicts AES spectra within the silica

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Figure 1. (A) AES spectra of the silica support before cleaning (dotted line) and after cleaning (solid line). The inset shows the AES silicon region of silica and partially reduced silica. (B) XPS spectra of the silica support after cleaning. The inset shows the XPS Si 2p peak after cleaning.

region. The absence of a Si0 AES line (expected at 92 eV)33 indicates a fully oxidized support. This fact can be important, depending on the details of a given project, since silicon is chemically quite active, but silica is an inert support. However, in the case of CO as a probe molecule, even partially reduced silica does not adsorb CO in significant amounts down to adsorption temperatures of 90 K.32,34 Similarly, Figure 1B shows a complete XPS scan of the cleaned surface, which is in agreement with the reference spectra of silica.35 Again, a minor nitrogen feature remained. Because of the XPS sensitivity factors, 0.477 for N 1s and 0.283 for Si 2p,35 the uncorrected N 1s intensity appears larger than it really is. The inset depicts the silica 2p region. The peak position of the Si 2p line and the fact that only a single peak was seen indicate again a fully oxidized support consistent with the AES data. 3.2. Characterization of the Supported Cu Cluster Growth. Figure 2A summarizes the AES data collected as a function of Cu deposition time, χCu (see lower scale). Depicted is the ratio of the Cu-to-O AES peaks as a function of χCu. Examples of the corresponding spectra are shown in Figure 2B. The O AES signal originates from the support. Two sets of independent experiments are shown in Figure 2A. Different silica supports and 16591

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Figure 3. XPS Cu 2p region for different copper exposures on silica at 300 K.

Figure 2. (A) Ratio of copper-to-oxygen AES peak intensity as a function of copper exposure. Two dosers with different copper fluxes have been used. The open squares are the result of a doser with a large flux, and the solid circles are the result of a doser with a smaller flux. For clarity, the exposure time of the second doser is enlarged by a factor of 10. (The original data are shown as an inset.) The upper scale gives the estimated Cu coverage in monolayer (ML) with 1 ML = 7.8  1015 Cu atoms/cm2). (B) AES spectra. Copper region as a function of copper exposure on silica. The inset shows the AES oxygen region.

Cu doser were used. To compensate for differences in the Cu flux, the exposure times were multiplied by a scaling factor. As evident, the shape of the curves is identical. Therefore, the exact Cu flux is unimportant. (The inset depicts the data using the original exposure time scale for the second Cu doser. The exposure scale on the main panel is the original scale of the first Cu doser. Accordingly, the Cu fluxes of the different doser differ by a factor of ∼10.) Initially, a linear increase in the AES signals is seen (section I) up to a critical deposition time where the AES intensities increase rather suddenly (section II) to a saturation level (section III). This type of curve shape is uncommon for supported cluster systems. On most metal oxide (see, e.g., refs 9 and 30) and metal36 supports, the slope of AES versus χCu curves does not change as dramatically as seen here for the silica support. As the

height (or film thickness) of the Cu clusters increases, the AES intensities typically start to increase slower than initially because of screening effects. Thus, simply (two) linear segments in AES growth curves would be expected (cf., e.g., Figure 1 in ref 15) but not a steplike shape. Inspecting the AES spectra (see Figure 2B) does not reveal any unusual features. The AES spectra before, at, and after the step in the AES versus χCu curves look very similar except for variations in the peak intensities (as summarized in Figure 2A). All peaks shown in Figure 2B can be assigned to copper.37 Similarly, the O AES signal from the silica support (see the inset in Figure 2B) decreases as χCu increases as expected. Similarly, XPS spectra (see Figure 3) recorded as a function of χCu do not provide a direct explanation for the rather uncommon shape of the growth curve. As evident from Figure 3, which depicts the spectra of the Cu 2p XPS region, the Cu clusters remain metallic for the entire Cu exposure range. In contrast, copper oxides would give rise to well-known shakeup satellite features in XPS.38 Thus, we can rule out Cu-oxide formation. This is an important and nontrivial result since nanometer size clusters may have a distinct affinity for oxide formation as evident for other systems (see, e.g., ref 39). XPS core-level shifts are difficult to interpret as vastly debated in the literature. The very small and positive (from 939 to 940 eV) shift of the Cu XPS line position is common for supported cluster systems;40 however, here it may simply be related to charging effects. The low-adsorption temperatures safely rule out an intermixing and alloy formation. Unusual growth behavior for Cu-on-silica has been reported before in a Cu TDS study.18 An unusually low adsorption probability of Cu on silica, SCu, was seen, whereas the adsorption probability of Cu-on-Cu should approach one.18 Thus, we assign the steplike feature seen here in AES data (Figure 2A) to variations in the adsorption probability of copper on silica. We conclude that below χCu < 60 min, Cu preliminarily decorates the bare silica support. Here, according to ref 18, SCu is about 0.2 for the room-temperature deposition of copper. Once the nucleation sites are filled (section I in Figure 2A) and the Cu clusters start to grow (section II in Figure 2A), SCu jumps up to values close to one because now Cu is primarily anchored on Cu islands. This certainly leads to an abrupt increase in AES intensities, when the 16592

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Figure 5. Typical example of uptake curves (King and Wells technique) for bare supports and the model catalyst (Cu exposure time = 60 min).

Figure 4. TDS spectra of CO on silica-supported Cu clusters with (A) 110.5 min and (B) 2.5 min exposure time of Cu. CO is dosed with the molecular beam; the CO exposure time, χCO, is indicated.

Cu flux is kept constant. Although detailed STM data in the submonolayer coverage regime appear unavailable, the AES and XPS data are consistent with a standard cluster growth of Cu on silica (nucleation f 2D/3D cluster growth f film formation). This conclusion is supported by the CO TDS data shown in Figure 4. At small (Figure 4A) and large (Figure 4B) but otherwise constant Cu deposition, the CO exposure, χCu, was varied. According to a prior study about Cu/silica,22 CO TDS data for Cu deposits annealed above room temperature consist of two peaks similar to the data shown here. Desorption temperatures of 160 and 180 K were reported,22 which are quite close to our CO TDS peaks at 150 and 200 K. The different TDS peaks indicate chemically distinct adsorption sites and Cu cluster growth. Unfortunately, TDS data for submonolayer Cu coverages have not been previously reported for the Cu/silica system. However, also in the nucleation regime (section I), different adsorption sites will be present, including the rim, corner, and terrace sites of the Cu clusters. For example, for Cu/ ZnO(0001) (ref 15), various CO TDS peaks were seen independent of χCu. Thus, the CO TDS data are consistent with the cluster growth of copper. That the CO TDS peak positions shift with CO exposure only for large amounts of predeposited copper is plausible. Only then

will the CO coverages be large enough to induce significant lateral interactions of CO. The direction of the peak shift indicates repulsive lateral interactions as expected. In fact, the integrated intensity of the CO TDS peaks at large Cu exposure was by a factor of 3 larger than the one for small Cu exposure (Figure 4). Therefore, we conclude that section I (Figure 2 A) is the nucleation regime of the Cu cluster formation, section II is assigned to the cluster growth regime, and in section III, the surface is covered by a film composed of large 3D clusters. The step feature in Figure 2A is caused by a rapid change in the Cu adsorption probability. Accordingly, a deposition time of χCu = 60 min may be assigned to one monolayer (ΘCu = 1 ML) of copper meaning that the nucleation phase of the Cu growth is completed and S0Cu f 1. In other words, the step feature is used as a calibration point. Accordingly, we added a Cu coverage scale to Figure 2A (see upper scale) estimated as described in the following. (Again, the same growth curve is seen for different Cu fluxes, see Figure 2A, i.e., the exact value of the Cu flux does not affect any conclusions.) Unfortunately, we do not have the equipment to directly measure the Cu flux, but according to ref 29, the change from low (S0Cu ∼ 0.35, ref 29) to large (S0Cu ∼ 1.0, ref 29) adsorption probabilities of Cu corresponds to a Cu coverage of ∼7.8  1015 Cu atoms/cm2. In other words, at the step feature in Figure 2A, ΘCu ∼ 7.8  1015 Cu = 1 ML atoms are adsorbed. If we assume that S0Cu remains constant at 0.35 up to that coverage (corresponding to Δt = 60 min, Figure 2A lower scale), the Cu flux, F, is given by F = Θ/S0Cu Δt = 6.2  1012 Cu atoms/(cm2 s). The estimated flux appears consistent with prior studies18,21,29 using similar Cu doser. (In refs 18 and 21, S0Cu was, however, estimated as 0.2 which would result in a flux of 1.1  1013 Cu atoms/(cm2 s).) The Cu flux is constant, but beyond the step feature in Figure 2A, the adsorption probability increases to S0Cu ∼ 1.0. Therefore, the Cu coverage at the end of the adsorption trace (at χCu = 120 min) in Figure 2A will amount to ∼4.0 ML (=F  S0CuΔt + 1 with S0Cu = 1.0 and Δt = 60 min). No carbon residuals were seen in AES/XPS after CO was dosed on the model catalyst. Thus, molecular adsorption/ desorption is concluded.41 3.3. Typical Adsorption Transients. The adsorption dynamics of CO on Cu/silica was characterized by collecting adsorption 16593

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Figure 6. Initial adsorption probability of CO on silica-supported copper clusters as a function of copper exposure.

transients with molecular beam scattering.42 A typical example is depicted in Figure 5 for a bare silica support, a partially reduced silica support, and an exposure of χCu = 60 min (1 ML) of copper on silica. Shown is the CO pressure in the scattering chamber as a function of CO exposure time, t. For the clean supports, only a step function is evident when a beam flag is opened at t = 0 s. However, for the model catalyst, the CO pressure increases initially fast but afterward approaches the saturation level more slowly. The pressure increase is delayed since CO starts to adsorb on the catalyst surface. Once the surface is saturated, all CO molecules will be backscattered since thermal desorption and CO condensation can be neglected at the chosen adsorption temperature (∼90 K). Therefore, when scaling the data properly, the adsorption transient corresponds directly to 1 S with S for the coverage (time) dependent adsorption probability. In particular, for t = 0, the initial adsorption probability, S0, is directly available from the transients. The area above the transient and below the saturation level corresponds to the number of adsorbed CO molecules. Therefore, it is evident that the CO coverage on the bare supports is very small since the transient consists of a nearly perfect step function. (This conclusion is consistent with prior work in which different measuring techniques were used.21) Integrating the transient results in the CO coverage, Θ, dependent adsorption probability, S(Θ). It is customary to normalize the CO coverage obtained at the lowest surface temperature to one (ΘCO = 1 ML) and to scale the saturation coverage at greater temperatures accordingly. The absolute CO coverage is not easily available experimentally. 3.4. Initial Adsorption Probabilities of CO: Cluster Size Effects. Figure 6 depicts S0 as a function of χCu for constant surface temperature, Ts, and CO impact energy, Ei. S0 initially increases linearly and levels out at S0 = 0.6 for χCu ∼ 20 min (0.33 ML). This is a typical result for supported cluster systems and can be discussed in the framework of the so-called capture zone model.43 The experiments were conducted at an adsorption temperature of negligible CO coverage on the bare support (cf., Figure 5). However, even in that case, the lifetime of the probe molecule on the surface can be sufficiently long enough to allow for diffusion of the CO molecules from the support to the metal deposits. So-called capture/collection zone models (CZM) have been applied by Rumpf et al.43 and by others (see, e.g., refs 7 and

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44) to account for these kinetic effects. Therefore, we have to consider at least the following adsorption/desorption pathways: (1) thermal desorption from the deposits or the support, (2) direct adsorption on the support or the metal deposits, (3) trapping of the adsorbates on the support and subsequent diffusion to the deposits, and (4) trapping of the adsorbates on the deposits and subsequent diffusion to the support. (Steps 3 and 4 are sometimes also called spillover or inverse spillover effects although these terms are historically used for dissociative adsorption.) The importance of pathway 1 obviously depends on the surface temperature; for Ts < 100 K (cf., Figure 4), it can be neglected. Pathway 3 is the most likely scenario for most of the data shown here since the Cu coverage is small. The size of the capture zone is given by a balance of thermal desorption and diffusion rates, or in other words, the ratio of the surface residence time and the characteristic diffusion time determines the size of the capture zone around an isolated cluster. For a cluster ensemble, the capture zones eventually overlap. Increasing the surface temperature will decrease the capture zone because the surface residence time and the diffusion time decrease. The initial increase in S0 as a function of Cu coverage (Figure 6) can be explained by an increase in the total capture zones formed around all Cu deposits together while increasing the Cu coverage. If the coverage is large enough, the capture zones start to overlap, and a further increase in catalyst activity (S0) cannot be expected in the framework of the simple CZM. Thus, judged by the initial reactivity toward CO adsorption, a model catalyst with χCu = 10 min (0.17 ML) is as active as one consisting of large 3D Cu clusters (see Figure 6). The steps seen in Figure 2A and Figure 6 do not coincide. In Figure 2A, the stepwise signal increase is related to variations in the adsorption probabilities of Cu on Cu/silica. The step feature in Figure 6 is, however, a kinetics effect caused by the diffusion and trapping of CO molecules along the support and subsequent adsorption on the Cu clusters. In contrast to the CZM, at large deposition times, there appears to be a trend of decreasing S0 with increasing deposition time (see the solid line in Figure 6). This small variation in S0 may be a dynamic effect caused by variations in the energy-transfer processes depending on the morphology of the Cu layer formed. According to the AES data (Figure 2A) for χCu > 50 min (0.83 ML), large Cu clusters form that assemble most likely a closed film of copper rather than dispersed clusters for χCu > 90 min (2.5 ML). It appears plausible that this thick Cu film is smoother (less corrugated) than dispersed clusters. Considering a simple hard sphere model, a less corrugated surface would lead to less efficient energy-transfer processes and smaller S0 than a highly corrugated surface (see, e.g., ref 45). Therefore, S0 would decrease when dispersed clusters (χCu ∼ 50 s) are compared with a smoother film (χCu ∼ 90 min, 2.5 ML). This small trend also appears consistently in the data discussed in the following; it is, however, within the experimental uncertainties. S0 versus cluster deposition time curves (“growth curves”, Figure 6) have also been determined for other supported cluster systems. For example, for Au/silica, an entirely different shape is seen.9 11 For gold, the reactivity of the clusters dramatically changes with their size resulting in more complex growth curves. Thus, these types of data can indeed reflect differences in cluster growth morphologies and reactivity trends. 3.5. Initial Adsorption Probabilities of CO: Temperature Effect. Nonactivated adsorption of CO would not lead to any temperature effects in S0; see, e.g., ref 46. However, similarly to 16594

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Figure 7. Initial adsorption probability of CO on Cu/silica as a function of the surface temperature. Data for three different Cu exposures are shown as indicated. The lines are included as a guide for the eye.

Figure 9. Coverage dependence of the adsorption probability of CO on Cu/silica for (A) small and (B) large Cu coverages. The impact energy of CO, Ei, was varied. Figure 8. Initial adsorption probability of CO on Cu/silica as a function of the impact energy. The data for Cu(111) are from ref 47.

the cluster size effects described above, the capture zones surrounding the supported clusters generate a temperature dependence of S0 that reflects the diffusion kinetics of the system. Typically, the activation energies for adsorption/desorption and diffusion vary in sympathy with the adsorption temperature, Ts. Both the surface residence time and the diffusion time decrease as Ts increases. Therefore, the size of the capture zones decreases with Ts, and S0 decreases with Ts, even for nonactivated molecular adsorption. This type of scenario is evident from Figure 7. Depicted is S0 as a function of Ts at constant Ei and parametric in Cu deposition time. S0 indeed decreases (for constant χCu) with Ts because of the decrease in the capture zone. (See the lines in Figure 7 added as a guide for the eye.) As described in the last sections (3.3 and 3.4), a cluster size effect is evident while varying χCu. S0 initially increases with χCu (see curves for 2.5 min (0.04 ML) and 60.5 min (1 ML) deposition time) because of an increase in the capture zone as the dispersion and number of Cu clusters increase. For large χCu, again a trend of a small decrease in S0 is evident (see curves for 60.5 min (1 ML) and 110.5 min (3.7 ML) Cu deposition time).

As discussed in section 3.4, we relate this to variations in the energy-transfer processes caused by morphology changes in the Cu cluster ensemble. 3.6. Initial Adsorption Probabilities of CO: Impact Energy Effect. The energy, Ei, dependence of S0 at low Ts is shown in Figure 8 for different Cu deposition times. For χCu = 110.5 min (3.7 ML), two independent experiments collected from two different silica wafers are shown using also a different Cu doser. The data match perfectly. (Thus, the Cu flux is unimportant for our discussion.) Reference data for a copper single crystal are also included (from ref 47). Most obvious is the decrease in S0 with Ei. This result is expected for nonactivated molecular adsorption of CO.48 In simple terms, the larger the Ei, the larger the speed of the probe molecules when hitting the surface and the smaller the interaction time with the surface. Therefore, the efficiency of the energy-transfer processes (for molecular adsorption) decreases with Ei , hence, S0 decreases with Ei. At constant Ei, the changes seen in S0 with varying deposition time and cluster size are within (or close) to the experimental uncertainties. (Larger effects have been seen for other supports.15) It is not too surprising that S0 values match at low Ei (independent of χCu) since the details of the energy-transfer processes will not be important when a slow CO molecule 16595

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The Journal of Physical Chemistry C impacts the surface. At large Ei, the variations seen in S0 are larger because here the details of the energy-transfer processes become crucial in determining the fate of the probe molecule (adsorption vs desorption). Here, the cluster size effects, already discussed, are evident. Consider, for example, the variation in S0 with χCu at Ei = 0.85 eV: S0 initially increases with χCu, as expected, since the number and size of the clusters increase. Therefore, the total capture zone and S0 increase. When approaching the limit of a thicker Cu film, S0, however, decreases. Our proposal that the latter is related to the smoothness of the Cu film (as opposed to a highly corrugated ensemble of small clusters) is supported by the fact that S0(Ei) at large Cu deposition times matches the data obtained (by a different group47) for a Cu(111) single crystal. 3.7. Coverage Dependence of CO. Examples for the CO coverage, Θ, dependence of the adsorption probabilities, S(Θ), are given in Figure 9. Shown are S(Θ) curves as a function of Ei (at low Ts) for small (Figure 9A) and large (Figure 9B) χCu. The capture zone model (CZM) assumes trapping of adsorbates on the surface and diffusion to the clusters as high-energy adsorption sites, that is, the CZM is a special case of a precursor model. Therefore, Kisliuk-like shapes of S(Θ) curves are expected and were indeed seen here (Figure 9). Since the molecules are trapped in an extrinsic/intrinsic precursor, S(Θ) remains constant up to saturation of the catalyst with CO. At that point, the adsorption probability drops to zero. S0 = S(Θ f 0) scales with Ei, Ts, and χCu as already described. Finally, one should compare different Cu model catalysts.12 Restricting the discussion to molecular beam scattering projects, we are aware of only one study on CO adsorption on Cu model catalysts; Cu/ZnO(0001) was studied in refs 15 and 49. (Prior spectroscopic and kinetics studies on Cu model catalysts were already described above.) Other metal and support combinations were certainly investigated and are reviewed in refs 7, 44, and 50 52. At large Ei, S0 is actually slightly larger (ΔS0 ∼ 0.1) on Cu/ silica than on Cu/ZnO(0001) (ref 30). The latter is the traditional methanol synthesis model system. Large S0 values are typically a prerequisite of large catalytic activity in surface reactions. At low Ei, the Cu/ZnO(0001) system is, however, significantly more reactive toward CO adsorption than Cu/silica with S0 approaching one. Thermal impact energies are indeed most important for applications.

4. SUMMARY The adsorption of Cu/silica has been studied with spectroscopic, kinetic, and dynamics (molecular beam scattering) techniques. The morphology of the system has been characterized before by other groups.18,20 22 In agreement with prior studies, our TDS, XPS, and AES data indicate that Cu clusters nucleate on the silica support for low Cu deposition times forming highly dispersed and small clusters. The nucleation phase is followed by a cluster growth regime before at large Cu deposition time thicker Cu films form. According to the beam scattering data, the Cu films appear rather smooth. In addition to this standard growth mode, unusual Cu growth dynamics is seen in AES data that can be explained by the coverage-dependent adsorption probabilities of Cu. The Cu clusters remain metallic at ultrahigh vacuum conditions. The adsorption dynamics of CO on Cu/silica can be modeled within the framework of the capture zone model (CZM). Accordingly, CO molecules adsorb on the silica support from

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where they diffuse to the high-binding energy sites, that is, the Cu clusters. Thus, the CZM is a precursor model consistent to the Kisliuk-like shapes of the coverage-dependent adsorption probabilities seen here. The capture zone’s size decreases as the surface temperature, Ts, increases, which explains the decrease observed in the initial adsorption probability, S0, with Ts. Similarly, the total area of the capture zone increases as the number of Cu clusters increases, that is, S0 increases with Cu deposition time as indeed observed. For large Cu coverages, S0 approaches the value of Cu single crystals indicating that less corrugated Cu films form. S0 decreases with impact energy as expected for molecular and nonactivated adsorption.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; fax: 701.231.8831.

’ ACKNOWLEDGMENT Financial support by an NSF-CAREER award (CHE0743932) is acknowledged. A supplemental equipment grant (for acquiring the second-hand XPS system) from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, is acknowledged (Project DE-FG02-08ER15987). ’ REFERENCES (1) Nanocatalysis (NanoScience and Technology); Heinz, U., Landman, U., Eds.; Springer: Berlin, Germany, 2007; ISBN 978-3-540-74551-8. (2) Nanotechnology in Catalysis; Zhou, B., Hermans, S., Somorjai, G. A., Eds.; Springer series: nanostructure science and technology; Berlin, Germany, 2004; ISBN 0-306-48323-8. (3) Thomas, J. M.; Thomas, W. J. Principles and practice of heterogeneous catalysis; VCH: Weinheim, Germany, 2005; ISBN 3-527-29239-X. (4) Lee, I.; Albiter, M. A.; Zhang, Q.; Ge, J.; Yin, Y.; Zaera, F. Phys. Chem. Chem. Phys. 2011, 13, 2449. (5) Choudhary, T. V.; Goodman, D. W. Top. Catal. 2002, 21, 25. (6) Liu, C. J.; Burghaus, U.; Besenbacher, F.; Wang, Z. L. ACS Nano 2010, 4, 5517. (7) Henry, C. R. Surf. Sci. Rep. 1998, 31, 235. (8) Ertl, G.; Freund, H. J. Phys. Today 1999, 1, 32. (9) Kadossov, E.; Cabrini, S.; Burghaus, U. J. Mol. Catal. A: Chem. 2010, 321, 101. (10) Kadossov, E.; Burghaus, U. Chem. Phys. Lett. 2009, 483, 250. (11) Kadossov, E.; Burghaus, U. Catal. Lett. 2010, 134, 228. (12) Askgaard, T. S.; Norskov, J. K.; Ovesen, C. V.; Stoltze, P. J. Catal. 1995, 156, 229. (13) Bowker, M.; Houghton, H.; Waugh, K. C. J. Chem. Soc., Faraday Trans. 1981, 77, 3023. (14) Hansen, J. B. In Handbook of Heterogeneous Catalysis; Ertl, G., Kn€ozinger, H., Sch€uth, F., Weitkamp, J., Eds., Wiley-VCH: Weinheim, Germany, 1997; Vol. 1. (15) Wang, J.; Burghaus, U. J. Chem. Phys. 2005, 123, 184716. (16) Wang, J.; Funk, S.; Burghaus, U. Catal. Lett. 2005, 103, 219. (17) Wang, J.; Funk, S.; Burghaus, U. J. Chem. Phys. 2005, 123, 204710. (18) Xu, X.; Goodman, D. W. Appl. Phys. Lett. 1992, 61, 1799. (19) Lu, J.-L.; Kaya, S.; Weissenrieder, J.; Gao, H. J.; Shaikhutdinov, S.; Freund, H. J. Surf. Sci. Lett. 2006, 600, L153. (20) Xu, X.; Vesecky, S. M.; Goodman, D. W. Science 1992, 258, 788. (21) Xu, X.; He, J. W.; Goodman, D. W. Surf. Sci. 1993, 284, 103. (22) Xu, X.; Goodman, D. W. J. Phys. Chem. 1993, 97, 683. (23) Piercea, D. E.; Burnsa, R. P.; Gabrielb, K. A. Thin Solid Films 1991, 206, 340. 16596

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