Polarization-Dependent Infrared Spectroscopy of Adsorbed Carbon

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Polarization Dependent Infrared Spectroscopy of Adsorbed Carbon Monoxide to Probe the Surface of a Pd/Cu(111) Single Atom Alloy Christopher M Kruppe, Joel D Krooswyk, and Michael Trenary J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01227 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 22, 2017

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The Journal of Physical Chemistry

Polarization Dependent Infrared Spectroscopy of Adsorbed Carbon Monoxide to Probe the Surface of a Pd/Cu(111) Single Atom Alloy Christopher M. Kruppe, Joel D. Krooswyk, Michael Trenary* Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607 *[email protected]

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ABSTRACT Single atom alloys can be formed by depositing a low coverage of active metal onto the surface of another metal. Few experimental techniques permit verification that the deposited metal exists as single-atom sites on the host metal surface. Here we use polarization dependent-reflection absorption infrared spectroscopy (PD-RAIRS) to characterize the surface of a Pd/Cu(111) single atom alloy through the use of CO as a probe molecule. In the presence of 1 × 10-2 Torr of CO at 300 K significant coverages of CO are only achieved when Pd is present on the surface. The Pd coverage at the surface is determined with RAIRS from the C-O stretch peak areas while the total amount of Pd in the first few atomic layers is measured with Auger electron spectroscopy. Isolated Pd atoms in Cu(111) are revealed with RAIRS by the C-O stretch peak of CO bound ontop of a Pd atom. The appearance of C-O stretch peaks due to bridge site CO occurs at Pd coverages where Pd atoms start to agglomerate. An isosteric heat of adsorption for CO of 32 kJ/mol was measured with PD-RAIRS for CO in dynamic equilibrium with the Pd/Cu(111) surface and provides new information on the bonding properties of isolated Pd atoms in the Cu(111) host.


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Introduction The use of metal alloys greatly expands the range of catalytic properties over those available with single-metal catalysts. Alloys show promise for more efficient reactions, tailored selectivity, and lowered cost. All of these can be achieved through controlling the composition of the alloy. Of particular interest is the use of a small amount of active noble metal alloyed into a cheaper and more inert host metal.1-3 Dilution of the active metal lowers material cost and introduces structures on the surface that change the selectivity towards certain products. To measure the properties of such nanoscale systems, surface science methods must be utilized. A more detailed understanding of the influence of lattice parameters (strain effects), electronic interactions (ligand effects), and morphology of the alloys (ensemble effects) on the basic surface properties should provide insights for preparing bimetallics and other alloys for catalytic applications. In the system of interest, Pd/Cu(111), ensemble effects have been shown to be of critical importance in selective hydrogenation reactions.4-6 When a small amount of Pd is deposited on a Cu(111) single crystal, a single atom alloy (SAA) can be formed where Pd atoms are dispersed along the step edges of the Cu(111) surface.7 Even 1% of Pd (0.01 ML) on Cu(111) has been shown by scanning tunneling microscopy (STM) to dissociate H2, which does not happen on Cu(111) unless high pressures and temperatures are used.8 Furthermore, Tierney et al. found through density function theory (DFT) calculations that for low Pd coverages the most energetically favorable configuration consists of isolated Pd atoms surrounded by Cu atoms.8 The dissociative sticking probability of H2 on the Pd/Cu(111) SAA surface was recently measured to be 2×10–3 for a 1% Pd coverage.9 This measured sticking probability is similar to that estimated in a molecular dynamics study based on DFT of the same system in which the 3 ACS Paragon Plus Environment


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barrier to dissociative adsorption was found to be 0.25 eV.10 Although this barrier is higher than the value of 0.02 eV obtained in an earlier calculation,8 the essential role of isolated Pd atoms in the Cu(111) surface in promoting H2 dissociation is clear from these studies. Furthermore, this ability to split H2 has a profound effect on the catalytic activity of the material, including its selectivity, which depends on the Pd coverage. Pd/Cu alloys are studied for their applications in hydrogen purification11, nitrate reduction12, CO2 hydrogenation to methanol13, and acetylene hydrogenation.4-6,14 Developing methods to characterize this model system and other SAA systems is crucial for establishing their potential for catalytic applications. Studies of the basic surface properties of metal alloys are often best performed in ultrahigh vacuum (UHV) and at low temperatures. Under such conditions, techniques such as scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD) can provide valuable information. However, to examine the surface under more catalytically relevant conditions, it is useful to correlate information from these methods with information from a technique, such as infrared (IR) spectroscopy, that can be used both in UHV and at ambient pressures (AP). In metal nanoparticle (NP) research, CO has been used to qualitatively probe the state of the surface through the sensitivity of the C-O stretch frequency to different metals and to different binding sites. This use of CO has been specifically applied to the surface structure of Pd/Cu and other SAA nanoparticles.14-17 For such purposes, it would be better to compare to IR data for CO adsorbed on a single-crystal SAA of well-defined surface structure such as used here, which leads to much sharper C-O stretch peaks than are typically obtained with NP SAAs, where site heterogeneity results in broad peaks. The fact that our IR spectra are obtained in ambient pressures of CO, similar to the CO pressures used for characterizing the NPs, further aids in


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making the comparisons more directly relevant. In a similar fashion, Liu et al. have shown how UHV surface science studies of metal single crystals and their alloys can be applied to their analogous alloy nanoparticles.15 Based on STM and TPD results for Pt/Cu(111) and on IR and mass spectrometry for Pt-Cu nanoparticles supported on Al2O3, they found that both types of SAA were much more resistant to CO poisoning than pure Pt. These results demonstrate the utility of surface science studies of single-crystal SAAs for establishing the basic properties of more practical forms of SAA catalysts. All of this is critically important for the Pd/Cu system as the formation of specific Pd structures is dependent on the local concentration of Pd.18 Here we have used polarization dependent-reflection absorption infrared spectroscopy (PD-RAIRS) of adsorbed CO to probe the surface of a Pd/Cu(111) SAA. Because CO only adsorbs on the Pd atoms at room temperature, the CO IR signal provides a way to quantify the surface Pd coverage. The sensitivity of this method is greater than can be achieved with Auger electron spectroscopy. Furthermore, when clusters containing two or more Pd atoms form, CO can bridge bond between the Pd atoms as revealed by a distinctly lower C-O stretch frequency. The appearance of bridge-site CO in the RAIR spectrum thus reveals the Pd coverage where a transition from isolated Pd atoms to two-dimensional Pd agglomeration occurs. Comparison of the C-O stretch frequency of CO on top of a Pd atom surrounded by Cu atoms in the SAA with the C-O stretch frequency of CO on top of a Pd atom in Pd(111) can reveal the extent to which electronic properties of a Pd atom are influenced by the surrounding atoms. Furthermore, by using PD-RAIRS to quantify the CO coverage in dynamic equilibrium with gas phase CO, the isosteric heat of adsorption of CO on the Pd/Cu(111) SAA was measured to provide further information on the adsorption properties of isolated Pd atoms incorporated into a Cu host.



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Experimental The experiments were conducted in a UHV system consisting of an upper analysis chamber and a lower ambient pressure IR cell. The sample is transferred into the IR cell through spring loaded Teflon seals that isolate the analysis chamber during ambient pressure experiments. The analysis chamber houses a LK Technologies RVL2000 Auger-LEED system and a Pfeiffer Prisma quadrupole mass spectrometer. The IR experiments were performed with a Bruker 70v FTIR. The system is described in detail elsewhere.19 The Cu(111) crystal (99.9999%, Princeton scientific) was cleaned with repeated Ar sputtering (1KeV, 10 µA) and annealing (950 K). The Cu(111) crystal has two spark-eroded holes that allows for Ta wires to be passed through and cradle the crystal in position. The surface was checked for cleanliness using AES. Pd was deposited onto the Cu(111) crystal at 380 K via a resistive evaporator with a thin Pd wire (99.99%, Alpha Aesar) wrapped around a W wire. Evaporating the Pd with the Cu(111) surface at 380 K creates a 50:50 surface:subsurface Pd/Cu alloy that is composed of mainly single Pd atoms dispersed around step edges of the Cu(111) surface as shown with STM by Baber et al.7 Deposition rates of ~0.006 ML/min allowed for controlled submonolayer Pd coverages. The Pd coverages are based on IR results, which yields the Pd coverage only at the surface. The sample was translated into the ambient pressure cell for PD-RAIRS measurements. This technique is described in detail elsewhere20 and was used to distinguish IR features of surface species from those of gas phase molecules during ambient pressure experiments. The yaxis label of the RAIRS figures indicates the type of spectra taken. Spectra with

∆

correspond to

PD-RAIR measurements where s-polarized spectra were subtracted from the p-polarized spectra to yield surface contributions only. All RAIR spectra were taken using 1024 cm-1 scans and 4 cm-1 resolution. Carbon monoxide (99.99% Matheson Tri-Gas) and hydrogen gas (Praxair, 6 ACS Paragon Plus Environment

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99.999%) were passed through a liquid N2 cold trap prior to exposure and checked for purity by mass spectrometry. Results Low temperature RAIRS of CO on Cu(111) and Pd/Cu(111) RAIRS of CO on Cu(111) has been studied extensively in UHV at low temperatures21-26 and a direct comparison with the previous work can reveal the extent to which a low coverage of Pd alters the properties of Cu(111). A comparison of RAIR spectra of CO on clean Cu(111) and on a Cu(111) surface with 0.030 ML of Pd is shown in Figure 1a and b, respectively. At 100 K, the terminal site peak appears at 2074 cm-1 and reaches its maximum intensity after a 1.5 L exposure. Increasing the exposure to 10 L results in a reduction of the terminal site peak, while two bridge site peaks appear at 1816 and 1833 cm-1. This progression is in accordance with previous studies of CO on Cu(111).21 The results of Figure 1 demonstrate that CO adsorbs on the Cu sites of Pd/Cu(111) just as it does on clean Cu(111) and is not affected by a low coverage of Pd. Spectra taken after annealing the surface saturated with CO in UHV are shown in Figure 2. For Cu(111) without Pd and with 0.030 and 0.085 ML of Pd, an anneal from 100 K to 110 K results in the complete loss of the bridge site CO as well as an increase in intensity of the on-top site CO. The resulting peak at 2073 cm-1 is at a similar position to the peak found after a 1.5 L exposure of CO at 100 K. In the CO TPD experiments on Cu(111) presented in the supplementary information of Marcinkowski et al.,27 CO is found to desorb in three major peaks near 110, 130, and 175 K with a trailing edge extending to 225 K. When 0.01 ML of Pd is present on the surface, a small peak at 270 K appears in the spectrum that can be assigned to CO desorbing from Pd. This peak begins just before 250 K. Figure 2 shows that annealing to 230 K 7 ACS Paragon Plus Environment


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for 2 minutes appears to completely desorb CO from clean Cu(111), but an increasing amount of CO remains on the surface as the Pd coverage increases. With an expanded intensity scale, some residual CO can be detected in the spectra for the Pd-free surface, which presumably comes from CO readsorption from the background. Therefore, annealing the surface after adsorption at low temperatures to determine the amount of CO adsorbed on Pd and hence to determine the Pd coverage is not ideal. However, the observation of only CO at the Pd on-top sites, and not on the Pd bridge sites, indicates that only isolated Pd atoms, rather than dimers or other Pd aggregates, are present at these Pd coverages. This is consistent with previous studies of CO adsorption on Pd on Cu(111) where on-top CO dominates the spectra for only the thinnest Pd films.28 Room temperature RAIRS of CO on Cu(111) and Pd/Cu(111) From previous studies it is known that CO does not adsorb on Cu(111) at room temperature and our results are consistent with this. However, as Figure 3a shows, exposure of CO to 0.14 ML Pd/Cu(111) yields a peak at 2064 cm-1 that increases with exposure, but is still quite weak after 10 L. This is in contrast to a previous study where a small exposure of CO to Pd thin films on Cu(111) resulted in multiple peaks in the IR spectra stemming from CO in multiple adsorption sites.28 The only peak seen for UHV exposures from Figure 3a is for CO at the Pd ontop sites. Figure 3b shows that exposing the surface to an ambient pressure of CO results in gas phase CO peaks at 2115 and 2176 cm-1 as well as the peak due to CO adsorbed on Pd atoms at 2068 cm-1. Upon evacuation, Firgure 3c shows that the 2068 cm-1 peak loses most of its intensity and continues to decrease as the CO slowly desorbs. The absence of peaks due to CO bound at Pd bridge sites at higher pressures indicates that there are no Pd pairs or other Pd aggregates that can accommodate bridge bound CO. The progression from UHV exposures to the ambient pressure CO shows that CO is only weakly 8 ACS Paragon Plus Environment

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bound to the 0.14 ML Pd/Cu(111) surface. In contrast, CO adsorbs on Pd(111) at room temperature in UHV to a saturation coverage of 0.6 ML.29-30 To more directly contrast the behavior of CO on 0.14 ML Pd/Cu(111) and Cu(111), Figure 4 shows p- and s-polarized spectra and the difference between the two for Cu(111) in the presence of 1 × 10-2 Torr of CO. The difference spectrum reveals a weak peak at 2070 cm-1 due to CO adsorbed on the Pd-free Cu(111) surface. Comparison of the intensity of this peak to the intensity of the peak for 0.33 ML CO on Cu(111) at low temperature, where on-top sites are saturated (largest signal for ontop site), implies that only 0.0015 ML of CO can adsorb on the Cu(111) surface at room temperature and this pressure. Mudiyanselage et al. have also observed a 2070 cm-1 peak on Cu(111) at 300 K in the presence of gas phase CO, which was similarly weak for similar CO pressures.22 The weakness of the CO peak for the Pd-free surface thus indicates that the large peak observed in Figure 3b for the Pd/Cu(111) surface under an ambient CO pressure of 1 × 10-2 Torr is due to CO adsorbed directly on the Pd atoms in the Cu(111) surface. We can also exclude the possibility that CO might be adsorbed on surface Cu atoms that are perturbed by subsurface Pd. This is based on observations by Marcinkowski et al., who used pulses from their STM tip to selectively remove CO molecules from the Pd/Cu(111) surface and found that the CO was bound only to surface Pd atoms.27 Coverage and Morphology of Pd/Cu(111) The observations in Figure 3 can be used to quantify the surface Pd coverage from the CO peak area. The p-polarized spectrum from Figure 4 is then used to subtract out the small amount of CO bound to the Cu and the s-polarized spectrum is used to subtract out the gas phase CO signal to yield the PD-RAIR spectra in Figure 5. The Pd coverage is estimated by taking the ratio of the peak areas on Pd/Cu(111) to the peak area found for CO on Cu(111) at low



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temperature. For quantitative purposes, the 1.5 L exposure of CO at 100 K was used as this yields the highest peak for the terminal site before the onset of bridge site occupation, which is known from the literature to correspond to a CO coverage of 0.33 ML.21 The Pd coverages determined this way from the CO spectra in Figure 5 are compared in Table 1 with the Pd coverages determined by the Auger electron spectra. The details of obtaining quantitative coverages from AES are presented in the supporting information. The inset of Figure 5 shows that a less than 0.01 ML coverage of Pd can be easily distinguished not only from the baseline, but also from the amount of CO that is stabilized on the Pd-free Cu(111) surface under the same conditions. In contrast, Pd coverages less than 0.05 ML could not be detected with AES, demonstrating its lower sensitivity. The discrepancy between the coverages calculated by the two methods is related to their different probing depths. While the electrons detected with AES come from both surface and subsurface Pd atoms, RAIRS detects only CO bound to surface Pd atoms. Thus the Pd coverages determined by RAIRS should be lower than the coverages from AES, which is consistent with the values in Table 1. At the two highest Pd coverages of 0.21 and 0.23 ML, the IR spectra indicate that CO occupies Pd bridge sites. Figure S3 shows PD-RAIR spectra immediately after evacuating the IR cell and after 1 hour in UHV for four different Pd coverages. Further information on the properties of the SAA Pd/Cu(111) surfaces are provided by H2 TPD results presented in the supplementary data, which show a single H2 desorption peak for SAA Pd/Cu(111) surfaces. The desorption peak shifts to lower temperatures with increasing Pd coverage, which is a trend also observed by Sykes and coworkers.4,7,18,27 The low coverage determined from AES, the single desorption peaks from H2 TPD, and the lack of bridge site CO in the PD-RAIR spectra at room temperature are all indicative of isolated Pd atoms on the Cu(111) surface.


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Heat of adsorption of CO on Pd/Cu(111) surfaces Adsorption isotherms were obtained from PD-RAIRS by using the C-O stretch peak areas as a measure of the equilibrium CO coverage under a static pressure of CO. Spectra showing the terminal site CO stretch as a function of pressure at four different temperatures for a surface Pd coverage of 0.035 ML are shown in the supporting information. The reversibility of adsorption was verified by changing the temperature under a static pressure of CO versus evacuating the cell between each temperature where the pressure dependence of the coverage was measured. Auger spectra taken after the isotherm measurements showed that no carbon had been deposited under the conditions used. This is important as at higher CO pressures, some carbon deposition was observed. Following the example of Truong et al.,31 Figure 6 shows isotherms plotted as CO RAIRS peak areas versus PCO. From the equilibrium pressure needed to achieve a given coverage (a given value of PCO) at a given temperature, the Clausius-Clapeyron equation ∆ (ln ) =− (1⁄ ) implies that plots of ln(PCO) versus 1/T should yield straight lines with slopes proportional to the isosteric heats of adsorption. Such a plot is shown in Figure 7 for a fixed CO coverage of 0.03 ± 0.003 ML. For the four temperatures used for the isotherms of Figure 6, this is the only CO coverage that was common to the four isotherms, precluding determination of the CO coverage dependence of ∆H. The experiments were repeated for Pd coverages of 0.0048, 0.018, 0.024, and 0.25 ML, although at the latter coverage Pd aggregation was evident. All Pd coverages gave similar heats of adsorption of around 32 kJ/mol. In the Clausius-Clapeyron equation, the temperature is assumed to be the same for both the gas and the surface. This would be the case for high pressures where the mean free path is 11 ACS Paragon Plus Environment


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short relative to the size of the crystal so that gas molecules in the immediate vicinity of the surface will be thermally equilibrated with it. For the pressure range of 1.5×10–3 to 5×10–2 Torr that we used for the isotherms in Figure 6, the mean free path is calculated to vary from 33 to 1 mm, which is at least a factor of three smaller than the dimensions of our IR cell. Therefore, the gas molecules that collide with the surface are likely equilibrated with it and to therefore have the same temperature as the surface. For pressures low enough that the mean free path is long relative to the size of the IR cell, the gas molecules would be at the temperature of the chamber walls, which is room temperature. The Clausius-Clapeyron equation can still be used, provided the actual pressure is replaced by an effective pressure, which is equal to the pressure needed to produce the same surface collision rate that would pertain when the gas temperature equaled the surface temperature. Since the collision rate for a given pressure is inversely proportional to the ∗ square root of the temperature, a plot of ln ( ) vs 1/T, where the effective pressure, P*, is given

by 300 ⁄ =

∗

would correct for the difference in temperature of the surface and the cell walls, assumed to be at 300 K. The plot using these corrected pressures yields a ∆H of 31 kJ/mol, which within the uncertainly is the same value obtained with the uncorrected pressures. Discussion Vibrational Properties of CO on Cu(111) and Pd/Cu(111) Our RAIR spectra of CO on Pd/Cu(111) provide insights into the extent to which the chemisorption properties of the Pd and Cu atoms in the SAA differ from surface atoms on


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Cu(111) and Pd(111). Table 2 summarizes the vibrational frequencies from RAIRS and SFG32 studies of CO on Cu(111), Pd/Cu(111) and various Pd surfaces from this work and from the literature. At low temperature and low CO exposures, CO adsorbs on the Cu sites of the 0.030 ML Pd/Cu(111) surface producing a spectrum essentially the same as that reported elsewhere21 for Cu(111), with an on-top frequency of 2068 cm-1 and bridge-site peaks at 1833 and 1816 cm-1. This demonstrates that the Cu atoms of the Pd/Cu(111) surface are not perturbed by the presence of isolated Pd atoms. In contrast, at 300 K where CO occupies primarily Pd sites, the CO RAIR spectrum is substantially different from CO on Pd(111). The most significant difference is that for the 0.14 ML Pd/Cu(111) SAA, only on-top Pd sites are occupied, as expected when only isolated Pd atoms are present. In UHV, Bradshaw and Hoffmann used RAIRS to show that CO first occupies three-fold bridge sites on Pd(111) up to 0.33 ML, followed by occupation of the two-fold bridge sites at 0.5 ML, and only for higher coverages achieved under an ambient CO pressure of 1 × 10-5 Torr is an on-top CO stretch observed at 2092 cm-1. For thin films of Pd on Cu(111), Wadayama et al.28 observed bridge-site CO with RAIRS for all film thicknesses, but for the thinnest films, on-top CO was also observed even for low CO exposures at 300 K. The occupancy of on-top sites of the thin Pd films even for low CO coverages at 300 K suggests that energetic factors that favor only bridge-site occupancy at low CO coverages on Pd(111) are altered by the presence of the Cu(111) substrate.28 Our observation, however, of on-top CO in the absence of bridge-site CO is unique among the Pd surfaces considered, including those of Pd nanoparticles. As the results presented in Table 2 indicate, the exact value of ν(CO) for CO at the on-top sites of Pd(111) depends on the conditions used, but the reported values are generally greater than 2064-2068 cm-1, the value that we observe for CO on the Pd atoms of Pd/Cu(111).



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Furthermore, our results show little shift with CO coverage of ν(CO) for CO on Pd/Cu(111), in contrast to the results of Szanyi et al.33 where ν(CO) of on-top CO varies from about 2064 to 2110 cm-1 as the CO coverage is increased in an ambient of 1 × 10-6 Torr by lowering the temperature to 100 K. The lack of a significant shift of ν(CO) with CO coverage on Pd/Cu(111) is likely due to the fact that the Pd atoms are separated from each other thereby minimizing dipole-dipole interactions. A comparison of ν(CO) of on-top CO on Pd(111) and Pd/Cu(111) in the absence of coupling effects is difficult as the on-top CO on Pd(111) is always accompanied by CO at bridge sites, so that while coupling effects on Pd/Cu(111) can be ruled out in the limit of zero CO coverage, it is impossible to observe on-top CO on Pd(111) under such conditions. Heat of adsorption of CO on Pd/Cu(111) surfaces The measurement of the heat of adsorption of CO assumes a dynamic equilibrium between gas-phase and adsorbed CO. This assumption in turn requires that adsorption be reversible. Irreversible adsorption could occur if the CO dissociates to an appreciable extent or if the surface undergoes irreversible reconstruction. Furthermore, the area of the RAIRS peak for on-top CO was assumed to be proportional to the CO coverage. Satisfying these assumptions constrained the temperature and pressure range that could be used in the measurements. For clean Cu(111), higher CO pressures can lead to reconstruction of the surface34 and irreversible adsorption. For pressures low enough to avoid reconstruction, lower temperatures would be needed for significant CO coverages to be achieved, but at low temperatures CO adsorption occurs irreversibly. For these reasons, measurement of the heat of adsorption for the clean surface was not feasible. For the Pd/Cu(111) surface, temperatures of 300 K and above were used to avoid adsorption on the Cu sites. The upper temperature range of 420 K was chosen as higher temperatures would have required higher CO pressures in order to achieve a measureable 14 ACS Paragon Plus Environment

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CO coverage, and such high pressures could lead to reconstruction and irreversible adsorption. For example, it is known that CO can induce Pd segregation from a Pd/Cu alloy.35 For the temperatures and pressures used for the Pd/Cu(111) surfaces, CO dissociation is not expected as it has been shown that Pd(111) and supported Pd nanocatalysts are resistant to carbon deposition from CO until higher pressures and temperatures (~140 Torr and > 600 K).36 A further limitation on the upper temperature was the need to avoid diffusion of Pd into the subsurface region, which was found to occur above 500 K in an STM study.37 Our measured value of the heat of adsorption of CO on Pd/Cu(111) did not depend on the coverage of Pd. This is consistent with the idea that for the relatively low Pd coverages used, the Pd atoms are far enough apart that repulsion between CO molecules is negligible. Therefore the measured value should reflect the CO-Pd bond strength. However, the value obtained of 32 kJ/mol seems anomalously low. For example, the activation energy for desorption, Ed, can be estimated from the desorption temperature as described by Redhead.38 Using a peak desorption temperature of 270 K for CO on Pd/Cu(111) as measured by Marcinkowski et al.,27 a heating rate of 1 K/s, and a pre-exponential of ν = 1013 s-1, yields Ed = 72 kJ mole. Although our Pd coverage of 0.035 ML is somewhat higher than the value of 0.01 ML used by Marcinkowski et al., we would not expect the factor of three difference in Pd coverage to have such a big effect, even if the Pd is distributed differently in the two cases. In the supplementary information to their study, Marcinkowski et al., used a kinetic Monte Carlo method to simulate their CO and H2 TPD results and they found that an activation energy of 89 kJ/mol and a pre-exponential of 2.1 × 1016 s-1 gave a simulated CO TPD peak that matched the experimental peak at 270 K.27 The simulations were performed for Pd atoms distributed on a single Cu(111) terrace and therefore did not allow for the influence of steps or for possible accumulation of Pd near the steps. The difference between



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the isosteric ∆H and Ed from TPD can be compared to the case of CO on Cu(100), where a desorption temperature of 175 K, a heating rate of 0.7 K/s, and an assumed ν of 1013 s-1, yields Ed = 46 kJ/mol.39 This is considerably lower than the value of ∆H of 72 kJ/mol in the limit of zero coverage measured by Truong et al.31 by the same pressure-dependent RAIRS method that we have used. The higher value for ∆H than for Ed for CO on Cu(100) could be explained by a reversible restructuring of the surface in the presence of elevated pressures of CO, a possibility suggested by the recent STM work of Eren et al.34 It is also possible that the expectation that Ed for desorption, based on a non-equilibrium kinetics experiment, should equal the ∆H for adsorption, which is a thermodynamic quantity measured under equilibrium, is not valid. The two quantities are equal only if the kinetic barrier to adsorption is zero. A barrier to adsorption of CO on Pd/Cu(111) equal to 40 kJ/mol would therefore reconcile the Ed and ∆H values. However, to the best of our knowledge, CO adsorption on metals is not an activated process and there is no reason to expect a barrier on the Pd/Cu(111) SAA. It is also possible that the assumption of a pre-exponential of 1013 used to estimate the Ed from the desorption temperature is not valid. However, for the rate constant for desorption at 270 K with Ed = 72 kJ/mol and ν = 1013 s-1 to be equal to the rate constant for Ed = 32 kJ/mol would require a pre-exponential of ν = 105 s-1 and there is no obvious reason for such a low value. A heat of adsorption of 32 kJ/mol would correspond to a desorption temperature of about 130 K, which is the temperature where CO desorption begins for the highest CO coverages on Cu(111).40 Our results can therefore be rationalized by assuming that the intermolecular repulsions that destabilize CO adsorption at high coverages under UHV conditions, also destabilize CO adsorption under dynamic equilibrium due to the presence of transiently adsorbed CO on the Cu(111) sites of the Pd/Cu(111) surface.


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Conclusion PD-RAIRS, AES, and TPD have been used to establish the presence, coverage, and morphology of Pd deposited onto a Cu(111) surface. The Pd coverage up to 0.2 ML can be quantified in two ways; from AES peak-to-peak ratios, and from CO peak areas measured with PD-RAIRS under ambient pressures at room temperature. Above 0.2 ML of Pd, the presence of bridge site CO reveals the formation of Pd aggregates. An isosteric heat of adsorption for CO on the Pd/Cu(111) surface of 32 kJ/mol was measured using PD-RAIRS and was found to be independent of the coverage of both CO and Pd. This anomalously low adsorption energy is likely associated with repulsive interactions between impinging gas phase molecules and ads orbed CO. Supporting Information Description Auger electron spectra for Pd coverage, Pd coverage determined by RAIRS, H2 TPD from Pdfree and Pd/Cu(111) surfaces, RAIR spectra of CO with high Pd coverages, PD-RAIR spectra obtained for use in heat of adsorption calculations. Acknowledgement We gratefully acknowledge financial support from the National Science Foundation (CHE – 1464816)



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Eren, B.; Zherebetskyy, D.; Patera, L. L.; Wu, C. H.; Bluhm, H.; Africh, C.; Wang, L.W.; Somorjai, G. A.; Salmeron, M. Activation of Cu(111) Surface by Decomposition into Nanoclusters Driven by CO Adsorption. Science 2016, 351, 475-478. McCue, A. J.; Anderson, J. A. CO Induced Surface Segregation as a Means of Improving Surface Composition and Enhancing Performance of CuPd Bimetallic Catalysts. J. Catal. 2015, 329, 538-546. Ozensoy, E.; Min, B. K.; Santra, A. K.; Goodman, D. W. CO Dissociation at Elevated Pressures on Supported Pd Nanoclusters. J. Phys. Chem. B 2004, 108, 4351-4357. Bach Aaen, A.; Lægsgaard, E.; Ruban, A. V.; Stensgaard, I. Submonolayer Growth of Pd on Cu(111) Studied by Scanning Tunneling Microscopy. Surf. Sci. 1998, 408, 43-56. Redhead, P. A. Thermal Desorption of Gases. Vacuum 1962, 12, 203-211. Pope, T. D.; Griffiths, K.; Norton, P. R. Surface and Interfacial Alloys of Pd with Cu(100): Structure, Photoemission and CO Chemisorption. Surf. Sci. 1994, 306, 294-312. Kirstein, W.; Krüger, B.; Thieme, F. CO Adsorption Studies on Pure and Ni-Covered Cu(111) Surfaces. Surf. Sci. 1986, 176, 505-529.

Table 1. Comparison of Pd coverages determined from Auger spectroscopy and from IR data. Data marked with an asterisk correspond to Pd coverages where IR peaks due to CO adsorbed at bridge sites are seen. Color

ML Pd Auger

ML Pd IRsurface

Brown

N/A

0.0038

Purple

0.05

0.014

Cyan

0.14

0.030

Red

0.3

0.080

Blue

0.41

0.14

Green*

0.66

0.21

Orange*

0.68

0.23


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Table 2. Summary of IR peak positions for CO bound to Pd and Cu surfaces from the literature and from this work. Surface

Exposure/pressure and temperature

-1

Adsorption site/frequency (cm ) On-top

Bridge

Cu(111), 0.14 ML Pd/Cu(111) [this work]

10 L at 100 K

2068

1833, 1816

Cu(111) [21]

10 L at 95 K

2067

1830, 1813

Threefold hollow

-6

Pd(111) [33]

1 × 10 Torr at 90 K, annealing

2110

1950

1825-1895

0.1-0.3 nm Pd film on Cu(111) [28]

1.2 L at 90 K

2067-2080

1950

1850

0.4 ML Pd/Cu(111) [this work]

10 L at 300 K

2064

-2

2068

1 × 10 Torr at 300 K > 0.5 ML Pd/Cu(111) [this work]

1 × 10 Torr at 300 K

-2

2073

1925

1890

0.1 nm Pd film on Cu(111) [28]

1.2 L at 300 K

2068

1915

1845

0.6 nm Pd film on Cu(111) [28]

1.2 L at 300 K

2090

1925

2085-2110

1930-1950

2085-2105

1972-1990

-6

Pd(111) [32] (SFG) 3.5 nm Pd NP [32] (SFG)

1 × 10 – 1000 mbar at 300 K -7 1 × 10 – 200 mbar at 300 K


1885


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Figure 1. RAIR spectra for increasing coverages of CO on surfaces at 100 K. (a) Cu(111) and (b) 0.030 ML Pd/Cu(111). The intensity scale of the spectral region below 2000 cm-1 has been expanded by a factor of 10.


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Figure 2. RAIR spectra after exposing 10 L of CO to the crystal at 100 K. The inset shows an expanded intensity scale in the 1750-1950 cm-1 region to highlight the bridge site peaks. The red dotted line is for clean Cu(111), the green line is for 0.030 ML Pd/Cu(111), and the blue line from 0.085 ML surface Pd. The spectra were obtained at 100 K after annealing to 110 and 230 K.



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Figure 3. RAIR spectra for CO exposed to 0.14 ML Pd/Cu(111) at 300 K. In Figure 3a the CO exposure was increased from 0.5 L to a total of 10 L. Figure 3b shows spectra from the sample exposed to static pressure of 1 × 10-2 Torr CO. The red spectrum overlaid in Figure 3b is from a Pd-free Cu(111) surface. The IR cell is then evacuated to UHV, where the last spectrum is taken. Figure 3c shows repeated scans (scan time = 4 minutes, scans taken immediately after each other) after the cell had been evacuated. The last spectrum shown is after annealing the sample to 500 K for two minutes.


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Figure 4. PD-RAIR spectra of 1 × 10-2 Torr CO over Cu(111). An overlay of p(blue) and s(red dotted) polarized spectra (top) and their difference (bottom). The two large peaks in the p- and s-polarized spectra correspond to the P and R branch rotational envelopes of gas phase CO. The 4 cm-1 resolution used is insufficient to resolve individual rotational lines. The results reveal that there is a slight contribution (0.0015 ML) at 2070 cm-1 from CO stabilized on the Pd-free Cu(111) surface.



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Figure 5. RAIR spectra in the presence of 1 × 10-2 Torr CO on Cu(111) at 300 K for increasing Pd coverages. Inset is used to show the difference at very low coverages. The small CO signal for CO on the Pd-free Cu(111) has been subtracted from the spectra for the Pd/Cu(111) surfaces. The listed Pd coverages are based on PD-RAIR spectra.


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Figure 6. Adsorption isotherms of 0.035 ML Pd/Cu(111) at 310 (blue), 340 (green), 380 (black), and 420 K (red).



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Figure 7. Plot of ln ( ) versus 1/T from isotherms for a constant CO coverage of 0.03 ML. The error in the heat of adsorption based on the slope of the line is ~10%.


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TOC Graphic


Polarization-Dependent Infrared Spectroscopy of Adsorbed Carbon

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