Infrared Reflection Absorption Spectroscopy Study of CO Adsorption and Reaction on Oxidized Pd(100)

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1 J. Phys. Chem. C 2008, 112, Infrared Reflection Absorption Spectroscopy Study of CO Adsorption and Reaction on Oxidized Pd(100) Feng Gao, Matthew Lundwall, and D. Wayne Goodman* Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas ReceiVed: NoVember 8, 2007; In Final Form: February 4, 2008 The adsorption, desorption, and reaction of CO on the ( 5 5)R27 surface oxide on Pd(100) grown by two methods was investigated with infrared reflection absorption spectroscopy. CO multilayer desorbs at 40 K, whereas monolayer desorption is complete at 210 K. The surface oxide formed at 575 K with a 600 Langmuir (L) exposure of O 2 exhibits a stronger interaction with CO compared with a surface oxide formed at 575 K with a 4500 L O 2 exposure, presumably due to defects in the former. The surface oxide formed with a lower exposure of O 2 exhibits enhanced reactivity with CO at 400 K. Below 60 K, O 2 blocks CO adsorption on the monolayer oxide. 1. Introduction Very recently it was discovered in our laboratory that the rate of CO oxidation on Pt group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O 2 /CO ratio above The catalytic surfaces exhibit rates 2-3 orders of magnitude greater than those observed under stoichiometric reaction conditions and similar reactant pressures 2 or previously in ultrahigh vacuum studies at any reactant conditions. 3-6 The O 2 /CO ratios required to achieve these so-called hyperactive states for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with Auger and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate one monolayer of surface oxygen with no detectable adsorbed CO. In contrast, under stoichiometric O 2 /CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and are far less active for CO oxidation. With the use of Pd(100) as a model catalyst, very recent density functional theory (DFT) calculations predict that either the ( 5 5)R27 surface oxide (with an ideal oxygen coverage of 0.8 ML) or CO-covered Pd(100) is most likely present under catalytically interesting gas-phase conditions. 7 Under the hyperactive reaction conditions where O 2 /CO ratio is substantially greater than the stoichiometric ratio, the ( 5 5)R27 surface oxide is believed to be the active surface. 1 Here we investigate CO adsorption/desorption and reaction on this surface by means of infrared reflection absorption spectroscopy (IRAS). 2. Experimental Section The experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of Torr equipped with IRAS, Auger electron spectroscopy (AES), lowenergy electron diffraction (LEED), and a quadrupole mass * To whom correspondence should be addressed. Phone: Fax: goodman@mail.chem.tamu.edu. spectrometer. The IRAS spectra were obtained using a Matheson Cygnus 100 spectrometer. The IR beam impinged the sample through CaF 2 windows with an incident angle of 85 with respect to the surface normal. The spectra were typically collected using 200 scans at a resolution of 4 cm -1 with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector, resulting in a total collection time of 2 min for each spectrum. The instrumentation and data acquisition for IRAS have been described in detail elsewhere. 8 A Pd(100) sample was mounted on a Vacuum Generators heating and cooling sample stage directly attached to a continuous flow liquid helium cryostat. The sample could be cooled to 20 K and resistively heated to 1100 K. The sample temperature was measured using a K-type thermocouple that was spotwelded to the edge of the sample. The thermocouple was not calibrated below liquid nitrogen temperature; however, according to the data provided by the manufacturer, the base temperature that can be achieved is 15 K, very close to the lowest indicated temperature of 20 K. On the basis of this the error of the sample temperature measurement is estimated to be e5 K. The liquid helium cryostat could also be filled with liquid nitrogen allowing the sample to be cooled to 80 K. The Pd(100) sample was cleaned by cycles of Ar + sputtering and annealing in O 2, which has been described previously. 9 The cleanliness of the sample was confirmed with AES and LEED. O 2 and CO were introduced into the chamber through leak valves by back-filling. The ( 5 5)R27 surfaces were formed using two methods developed previously. The first involves reacting Torr of O 2 with Pd(100) at 575 K for 10 min (600 L, 1 L ) Torr s), 10 and the second method utilizes Torr of O 2 reacting with Pd(100) at 575 K for 15 min (4500 L). 11 In the following, these two surface oxides are denoted as the 5 surface (1) and (2), respectively. 3. Results 3.1. CO Adsorption/Desorption on the 5 Pd(100) Oxide Surfaces. CO adsorption was monitored first on freshly prepared 5 surfaces at 20 K. At all CO exposures two bands are evident, one intense and sharp feature at 2142 cm -1 typical for multilayer CO 8,12,13 and a much weaker signal at 1940 cm /jp710713r CCC: $ American Chemical Society Published on Web 03/26/2008

2 6058 J. Phys. Chem. C, Vol. 112, No. 15, 2008 Gao et al. Figure 1. (a) IRAS spectra of multilayer CO as a function of exposure at 20 K on 5 surfaces (1) and (2). CO exposures are marked adjacent to each spectrum. (b) Integrated IRAS intensities as a function of CO exposure shown in (a). (c) IRAS spectra of bridging CO as a function of exposure at 20 K on 5 surface (1). CO exposures are marked adjacent to each spectrum. All spectra were recorded for 200 scans at a resolution of4cm -1. assigned to CO adsorption on bridging sites. 14 Figure 1a displays the multilayer feature on the 5 surfaces as a function of CO exposure. The left panel presents results for the 5 surface (1), and the right panel displays data for the 5 surface (2). The corresponding integrated signal areas, plotted in Figure 1b as a function of CO exposure, show that on both surfaces the CO signal intensity increases linearly with CO exposure up to 0.1 L, then increases slowly with higher exposures. Most noticeably, however, the signal intensity on the 5 surface (2) is greater at high CO exposures, especially at 0.54 L. Of course this does not mean necessarily that more CO is adsorbed on the 5 surface (2) since, according to IR surface selection rules, the adsorption geometry together with coverage plays a role in determining the signal intensity. 15 Figure 1c shows plots of the CO bridge-bond signal versus CO exposure on the 5 surface (1). The intensity of this feature indicates no significant change with CO exposure. In fact, this feature saturates prior to any CO exposure and is likely due to background CO adsorption given that following the formation of the 5 surface (1), the pressure of the chamber is rather high ( Torr). During the time interval between acquisition of the background IR spectrum (at 300 K) and measurement of the adsorbate spectra (at 20 K), sufficient background CO could very well adsorb. Similar results were found for the 5 surface (2); therefore, no data for this surface are shown. Figure 2a presents the multilayer CO feature variation as a function of annealing temperature following a 0.54 L CO

3 IRAS Study of CO on Oxidized Pd(100) J. Phys. Chem. C, Vol. 112, No. 15, Figure 2. (a) IRAS spectra recorded during annealing 0.54 L CO covered 5 surfaces to higher temperatures where the annealing temperatures are marked adjacent to each spectrum. All spectra were recorded at the indicated temperatures for 200 scans at a resolution of 4cm -1. (b) Integrated IRAS intensities as a function of annealing temperature shown in (a). exposure at 20 K. Note that all spectra are recorded at the indicated temperatures to avoid readsorption of CO desorbing from the sample and sample holder during spectra acquisition. As shown in the left panel, essentially no change occurs with respect to the CO signal intensity and frequency between 20 and 35 K for the 5 surface (1). Furthermore, there was no change in the chamber pressure during annealing suggesting no CO desorption below 35 K. A noticeable change occurs upon annealing to 40 K, i.e., the CO signal intensity increases 10% and the peak position shifts slightly to 2141 cm -1 with a slight pressure increase ( P Torr), consistent with desorption of CO at 40 K. A marked change occurs when the sample is further annealed to 43 K with a concomitant drop in the signal intensity indicating desorption of multilayer CO, accompanied by a rapid pressure increase in the vacuum chamber. Spectra for the 5 surface (2) are shown on the right panel of Figure 2a. CO behaves slightly differently on this surface, i.e., CO multilayer desorbs by 40 K with no CO signal intensity increase during further annealing. The integrated IR signal peak areas versus annealing temperature are plotted in Figure 2b. It is noteworthy that a weak band is evident at 2150 cm -1 following multilayer CO desorption on both surfaces. Presumably this band also exists at lower temperatures but is masked by the much more intense multilayer signal at 2142 cm -1.On the other hand, no variation of the bridging CO band occurs during annealing above 40 K on both surfaces (data not shown). Figure 3a displays spectra found for monolayer CO that remains with further annealing. These spectra show that the atop band shifts from 2140 to 2120 cm -1, together with a drop in signal intensity as a function of temperature. The bridging band follows the same trend during annealing. All CO features disappear at a sample temperature of 210 K. The 2150 cm -1 feature, which disappears completely above 80 K, likely is related to CO adsorption on surface silica impurities 16 and is not discussed further. The integrated signal areas (excluding the 2150 cm -1 feature) are plotted in Figure 3b allowing a direct comparison of the two surfaces. Clearly, the signal intensity from both bands is measurably higher on the 5 surface (1). Particularly between 80 and 170 K, the intensity of the atop band is approximately 2 times greater on the 5 surface (1) compared with the 5 surface (2). In contrast, the bridging band differs little from one surface to the other CO Titration of the 5 Oxide Surfaces. The results presented above illustrate the difference between the two 5 surface oxides. For multilayer CO, the signal on the 5 surface (2) is more intense compared with the 5 surface (1) (Figure 1), whereas the reverse is evident for monolayer CO (Figure 3). In order to investigate the effects of this difference on the reactivity of these two surface oxides, CO titration experiments were carried out at different temperatures. In these experiments the sample was cooled with liquid nitrogen to minimize contamination caused by desorption from the sample holder. Shown in Figure 4 are data for one set of CO titration experiments on the 5 surface (1) where each cycle consists of the following: (i) exposure to 1.2 L of CO at 80 K, (ii) followed by annealing to 300 K, (iii) cooling to 80 K, (iv) exposure to 1.2 L of CO, and (v) acquisition of an IR spectrum. It is expected that if CO oxidation reaction occurs below 300 K, the spectrum recorded after each cycle will differ from the freshly prepared surface. As shown in Figure 4, only slight changes are found among these spectra indicating that the 5 surface (1) is largely inert to low exposures of CO below 300 K, consistent with previous studies. 17,18 Essentially identical results were obtained with the 5 surface (2) (data not shown). It should be emphasized that reduction of surface oxides proceeds as low as 200 K when the CO pressure becomes sufficiently high (unpublished results). The reactivity of the surface oxides was further investigated at 400 K (Figure 5). The experiments were performed as follows: (i) dosing of the 5 surfaces with the indicated exposures of CO at 400 K, (ii) cooling to 150 K, (iii) exposure to 1.2 L of CO, and (iv) acquisition of an IR spectrum. As shown in Figure 5a, upon reaction of 5 surface (1) with 0.6 L of CO at 400 K, the signal intensity for both bands at 2130 and 1940 cm -1 decreases, and more importantly, a new CO band appears at 1994 cm -1. The signal intensity of the 2130 and 1940 cm -1 bands decreases upon reacting with additional dosed CO, and finally, after reaction with 2.4 L of CO, the only band remaining is the CO bridging feature at 1997 cm -1, indicating that the

4 6060 J. Phys. Chem. C, Vol. 112, No. 15, 2008 Gao et al. Figure 3. (a) IRAS spectra recorded during annealing monolayer CO-covered 5 surfaces to higher temperatures where annealing temperatures are marked adjacent to each spectrum. All spectra were recorded at the indicated temperatures for 200 scans at a resolution of 4 cm -1. (b) Integrated IRAS intensities as a function of annealing temperature shown in (a). surface oxide is fully reduced to Pd(100). 19 It should be pointed out that the disappearance of the 1940 cm -1 band occurs prior to that of the 2130 cm -1 band. Figure 5b depicts spectra recorded in the same experiment with 5 surface (2). Clearly the same trend is obtained, but the growth rate of the band at cm -1 is lower compared with 5 surface (1). This is highlighted in Figure 5c where the signal intensity of the cm -1 band is plotted as a function of CO exposure at 400 K. In this case, since the 1940 cm -1 feature overlaps with that at cm -1, the signal height instead of integrated signal area is displayed to represent signal intensity to avoid deconvolution-induced error CO Adsorption/Desorption on O 2 -Precovered 5 Surfaces. Experiments were carried out studying CO adsorption and desorption on O 2 -saturated oxide surfaces. The surface oxides were exposed to6lofo 2 at 20 K prior to CO adsorption to achieve O 2 saturation. A spectrum acquired for a freshly prepared 5 surface (1) exposed first to 6LofO 2 at 20 K and then to 0.02 L of CO is compared in Figure 6 with a spectrum of 0.02 L of CO adsorbed on a clean 5 surface (1). On the O 2 -precovered surfaces, the 2142 cm -1 feature is much sharper and the 1930 cm -1 band almost completely attenuated (middle spectrum), in contrast to the spectrum recorded without O 2 coadsorption (top spectrum). A subtraction of these two spectra is shown in the bottom spectrum of Figure 6. It is noteworthy that this spectrum appears to be very similar to the monolayer CO spectra of Figure 3a recorded at temperatures below 80 K. To further investigate this effect, the adsorption of various amounts of CO was carried out on both surface oxides precovered with O 2 at 20 K (see Figure 7a). At first glance, these spectra look identical to those shown in Figure 1a for the clean 5 surface oxides. However, the integrated IR signal intensities do show apparent differences (Figure 7b). On the 5 surface (1), the CO intensity is always lower on the oxygen-precovered surface until an exposure of 0.54 L where the intensity converges to the same value as on the clean surfaces. In contrast, this point is reached with a CO exposure of only slightly higher exposure (0.24 L) on the 5 surface (2).

5 IRAS Study of CO on Oxidized Pd(100) J. Phys. Chem. C, Vol. 112, No. 15, Figure 4. IRAS spectra following cycles of 1.2 L CO adsorption at 80 K f annealing to 300 K f cooling to 80 K f 1.2 L CO adsorption at 80 K on the 5 surface (1). All spectra were recorded for 200 scans at a resolution of 4 cm -1. Annealing experiments were carried out with the results shown in Figure 8 for the 5 surface (1). As shown in Figure 8a, the multilayer signal persists at 43 K, whereas on clean 5 surface (1) the multilayer has desorbed by this temperature (Figure 2a). Figure 8b presents spectra following further annealing to higher temperatures. When these data are compared to Figure 3a (left panel), a marked difference is evident between 50 and 60 K. On an O 2 -precovered surface, both bands at 2130 and 1930 cm -1 become significantly weaker at 50 K indicating that most CO has desorbed. However, by annealing to 60 K, these two bands regain their signal intensity and become similar to those on the clean 5 surface (1) at similar temperatures (Figure 3a). 4. Discussion The 5 surface oxide has been known for many years, 17,18,20-22 but only recently has the structure of this phase been clearly identified. 10,11,23 Basically, the structure can be described as a PdO(101) trilayer on Pd(100) where each unit cell contains four Pd and four O atoms. Two Pd atoms are fourfold and the other two are twofold coordinated to oxygen. Two oxygen atoms sit on top of the reconstructed Pd layer, and two reside at the interface of the Pd(100) substrate forming the so-called trilayer structure. 7 It is noteworthy that this surface oxide exhibits all three adsorption sites, namely, atop, bridging, and hollow sites, for CO as observed for Pd(100). DFT calculations predict that the bridging site is the most energetically favorable site for CO followed by the twofold oxygen coordinated Pd atop site. The hollow sites are the least favored energetically. 7 Previous studies have shown that below 35 K, CO is physisorbed on top of a more strongly bound CO monolayer on metal surfaces, such as Cu(111), 12 Cu(100), 8,13 and Pd(100), 13 and metal oxide surfaces including NiO(100) and MgO(100). 24 This physisorbed multilayer is characterized by an IR band at 2142 cm -1, 8,12,13 very close to the gas-phase value of 2143 cm Using temperature-programmed desorption, Wichtendahl et al. 24 found that multilayer CO desorbs at 30Kon NiO(100) and MgO(100). These authors also found a weak desorption state at 45 K due to desorption from surface defects. The adsorption/desorption characteristics of multilayer CO in this study are in good agreement with these earlier studies in that a 2142 cm -1 band (Figure 1) is found that disappears, i.e., desorbs, from the surface between 35 and 43 K (Figure 2). One apparent difference between the 5 (1) and (2) surfaces is that for surfaces exposed to 0.54 L of CO at 20 K, the CO multilayer signal intensity on the 5 (2) surface is measurably higher (Figure 2b). An explanation of this behavior is beyond the scope of the present study but clearly is due to differences between the two surface oxide substrates. Also noteworthy is that the CO multilayer desorption temperature on the 5 surface (1) is slightly higher than for the 5 (2) surface. This can be rationalized by a higher density of surface defects on the 5 surface (1) compared with (2) and that CO binds more strongly with defects. 24 When the 5 surface (1) is exposed to 0.54 L of CO and annealed to 40 K, the CO feature is evident at 2141 cm -1 (Figure 2a), in agreement with this conclusion. IRAS results reveal that for monolayer CO, both bridging and atop sites are occupied (Figure 3a) with the signal intensity of the atop CO sites being greater than for the bridging sites. Very recent theoretical calculations suggest that a bridging site is more energetically favored (0.31 ev difference between bridging and atop sites assuming one CO molecule in the unit cell, and 0.24 ev difference with two CO molecules in the unit cell). 7 As shown in Figure 3b, especially between 80 and 200 K, the atop CO intensity continually decreases with an increase in temperature, whereas the bridging CO yield remains relatively unchanged. This behavior supports the results of the theoretical calculation. 7 From these data the fraction of CO molecules that adsorb onto each unit cell of the 5 surfaces can be estimated. It has to be emphasized that error can result using integrated infrared area for this purpose. The top spectrum shown in Figure 5a can be used as a reference since this represents the CO-saturated Pd(100) surface. 19 With this method the CO coverage is estimated to be approximately 0.8 ML. However, a previous study by Ortega et al. 19 showed that the integrated CO signal area remains unchanged at coverages of 0.5 ML and above. The integrated signal of a CO monolayer adsorbed on 5 surfaces shown in Figure 3b therefore can be used to compare with the integrated intensity of CO on clean Pd(100). For instance, the combined area of both atop and bridging CO at 80 K on the 5 surface (1) is 0.28 au, 60% of the signal area of 0.5 ML of CO on Pd(100) ( 0.5 au). Considering the lengthening of the Pd-Pd distance on the 5 surfaces compared with Pd(100), one estimates that for a saturated CO monolayer on 5 surfaces, approximately two CO molecules occupy the unit cell consisting of four Pd and four O atoms. There are several adsorption geometries that contain two CO molecules in one such unit cell. 7 The experimental data of Figure 3a are most consistent with these two CO molecules bound at atop sites. Of course the CO saturation coverage calculated above cannot be correlated directly with the reactivity of 5 surfaces under realistic reaction conditions. Data shown in Figure 5c, acquired by monitoring the growth of the cm -1 feature, indicate that the 5 surface (1) has a higher reactivity toward CO. However, one must be very cautious about such a conclusion since, as mentioned earlier, the signal intensity depends not only on CO coverage but also on the molecular orientation. Based on LEED, in situ elevated-temperature STM, and kinetic measurements, Zheng and Altman concluded that during CO titration, the less reactive

6 6062 J. Phys. Chem. C, Vol. 112, No. 15, 2008 Gao et al. Figure 5. (a) IRAS spectra recorded on the 5 surface (1) exposed to 1.2 L of CO at 150 K after the surfaces were reacted to various amount of CO at 400 K. All spectra were recorded for 200 scans at a resolution of 4 cm -1. (b) IRAS spectra recorded on the 5 surface (2) exposed to 1.2 L of CO at 150 K after the surfaces have been reacted with various amount of CO at 400 K. All spectra were recorded for 200 scans at a resolution of4cm -1. (c) cm -1 band intensity vs CO exposure at 400 K. 5 surface oxide converts first to high density (2 2) domains and the latter are then reduced further to clean Pd(100). 18 These authors also concluded that the high-density (2 2) areas are not reduced until the 5 structure is completely removed. Our CO titration results are consistent with this conclusion. A 5 surface oxide is characterized by the coexistence of two CO bands at 2030 and 1940 cm -1. Correspondingly, the removal of these two bands occurs concomitantly with removal of the 5 structure. It is emphasized, however, that the 2030 cm -1 band is not restricted to the 5 structure and exists on p(2 2) surfaces at low temperatures in the form of an O-Pd-CO complex. 26 Furthermore, the 1990 cm -1 bridging band exists both on clean Pd(100) and on (2 2) oxygen-covered surfaces. Therefore, the complete attenuation of the 5 structure cannot be judged accurately by changes in the 2130 and 1990 cm -1 bands but, rather, can be more accurately assessed by the disappearance of the bridging CO feature 1940 cm -1. This follows because once the 5 surface is reduced, either partially to the (2 2) structure or completely to clean Pd(100), the resulting surfaces will display CO bridging features above 1990 cm -1 (not at 1940 cm -1 ), assuming that the surface is saturated with CO. 19,26 The complete removal of the 1940 cm -1 band is achieved by reaction of 1.8 L of CO with the 5 surface (1) at 400 K. However, this feature is still evident following 1.8 L CO reaction with the 5 surface (2) and even remains to an extent following reaction with 2.4 L of CO. Along with the growth of the 1990 cm -1 feature shown in Figure 5c, the removal of the 1940 cm -1 band also demonstrates the higher reactivity of the 5 surface (1). Following 1.8 and 2.4 L CO reaction with the 5 surfaces (1) and (2), respectively,

7 IRAS Study of CO on Oxidized Pd(100) J. Phys. Chem. C, Vol. 112, No. 15, Figure 6. IRAS spectra recorded after 0.02 L CO adsorption onto a clean and O 2-precovered 5 surface (1). A subtracted spectrum is also included. All spectra were recorded for 200 scans at a resolution of4cm -1. coexistence of the 2130 and 1997 cm -1 bands with no apparent 1940 cm -1 band demonstrates that only (2 2) structures remain on the surfaces. A difference in the reactivity of these two surface oxides toward CO is found (Figure 5), which warrants a discussion of the possible origins. Since the 5 surface (1) is formed using 600 L of O 2, whereas the 5 surface (2) is formed using 4500LofO 2 at the same temperature, we assume the 5 surface (2) is more homogeneous, i.e., fewer surface defects (for example, missing O atoms) with more ideal 5 domains. It is well-known that for oxide thin films frequently used as metal supports in model catalysts, different synthesis methods can result in different densities of surface defects. 27 This assumption is very consistent with the data of Figure 3 which show that, between 40 and 200 K, both atop and bridging CO exhibit higher signal intensity on the 5 surface (1). The higher desorption temperature of multilayer CO on 5 surface (1) shown in Figure 2 is also consistent with this conclusion. An alternative explanation for the reactivity differences could be partial formation of PdO bulk oxide on the 5 surface (2) since a larger O 2 exposure is used in its synthesis. Zheng and Altman 18 found that the formation of bulk PdO attenuates reactivity with CO as indicated in similar CO titration measurements using temperature-programmed reaction. If this is the case, one would expect that following the consumption of this species, the oxygen removal rate on the 5 surface (2) should become identical to the 5 surface (1). Data shown in Figure 5c, however, do not support this argument as no clear reaction induction period is evident. Therefore we conclude that no bulklike PdO species is formed during the formation of the 5 surface (2) and that the reactivity differences between the two surface oxide films can be explained as due to variations in the density of surface defects. It follows that increased binding of CO to the defect sites leads to a high coverage of CO that then gives rise to an increase in CO 2 formation. Figure 7. (a) IRAS spectra of multilayer CO as a function of exposure at20kono 2-precovered 5 surfaces (1) and (2). CO exposures are marked adjacent to each spectrum. (b) Integrated IRAS intensities as a function of CO exposure shown in (a). For comparison, data obtained from the clean 5 surfaces (shown in Figure 1b) are also included. Finally we comment on the adsorption/desorption of CO on the O 2 -precovered surfaces. As shown in Figure 6, following O 2 adsorption only the CO multilayer feature is observed. This suggests, first, that O 2 occupies the same surface sites as CO and, second, CO cannot displace monolayer O 2 adsorbed on the surface at 20 K, contrary to behavior found at higher temperatures. 26 This conclusion is valid even with higher CO exposures. As shown in Figure 7b, the CO intensity on O 2 -precovered surfaces is always lower compared with that measured on clean 5 surfaces at identical CO exposures, excluding a 0.54 L CO exposure where sufficient multilayer CO remains to preclude O 2 adsorption. The monolayer O 2 desorption temperature can also be accurately estimated from Figure 8b noting that a marked change occurs between 50 and 60 K. This can only be rationalized by three consequent steps occurring on the surface: (i) O 2 desorbs between 50 and 60 K; (ii) CO desorbs from the sample holder; (iii) CO readsorbs on the surface. Comparing Figures 2a and 8a, multilayer CO desorbs at higher temperatures on the oxygenprecovered surface. This is rather unexpected since the most energetically favorable surface sites have already been occupied by oxygen. This behavior can be rationalized nevertheless assuming some attractive interaction between adsorbed O 2 and CO.

8 6064 J. Phys. Chem. C, Vol. 112, No. 15, 2008 Gao et al. energy of monolayer CO is estimated to be kj/mol. This energy is lower than that on clean Pd or Pd surfaces with chemisorbed oxygen. This means that surface oxides are more tolerant to CO poisoning during high-pressure CO oxidation reactions, consistent with the fact these surfaces are highly reactive under realistic reaction conditions. 1 (b) More monolayer CO adsorbs on the 5 surface formed using 600 L of O 2 compared with that formed using 4500 L of O 2. Presumably this is because more surface defects are present on the surface in the former case. Consistent with this conclusion, the 5 surface formed with less O 2 shows a higher reactivity with CO at 400 K. (c) O 2 preadsorption at 20 K prevents CO adsorption on the CO monolayer and persists until O 2 desorbs at K. Acknowledgment. We gratefully acknowledge the support for this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences/Geosciences/ Biosciences, Catalysis and Chemical Transformations Program, and the Robert A. Welch Foundation. We also thank Dr. M. S. Chen for useful discussions. Figure 8. (a) IRAS spectra recorded during annealing multilayer CO adsorbed on an O 2-precovered 5 surface (1) where the annealing temperatures are marked adjacent to each spectrum. (b) Spectra recorded following annealing where each temperature is marked adjacent to each spectrum. All spectra were recorded at the indicated temperatures for 200 scans at a resolution of 4 cm Conclusions The interaction of CO with 5 surfaces grown on Pd(100) has been investigated with IRAS. The main conclusions are listed in the following: (a) CO multilayer stays on the surface oxides below 40 K. Above this temperature only a CO monolayer remains. Monolayer CO desorbs completely at 210 K. A desorption activation References and Notes (1) Chen, M. S.; Cai, Y.; Yan, Z.; Gath, K. K.; Axnanda, S.; Goodman, D. W. Surf. Sci. 2007, 601, (2) Berlowitz, P. J.; Peden, C. H. F.; Goodman, D. W. J. Phys. Chem. 1988, 92, (3) Engel, T.; Ertl, G. AdV. Catal. 1979, 28, (4) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. J. Chem. Phys. 1980, 73, (5) Madey, T. E.; Engelhardt, H. A.; Menzel, D. Surf. Sci. 1975, 48, (6) Reed, P. D.; Comrie, C. M.; Lambert, R. M. Surf. Sci. 1977, 64, (7) Rogal, J.; Reuter, K.; Scheffler, M. Phys. ReV.B2007, 75, (8) Kim, C. M.; Yi, C. W.; Goodman, D. W. J. Phys. Chem. B 2005, 109, (9) Grunze, M.; Ruppenderand, H.; Elshazly, O. J. Vac. Sci. Technol., A 1988, 3, (10) Lundgren, E.; Mikkelsen, A.; Anderson, J. N.; Kresse, G.; Schmid, M.; Varga, P. J. Phys.: Condens. Matter 2006, 18, R481. (11) Kostlnik, P.; Seriani, N.; Kresse, G.; Mikkelsen, A.; Lundgren, E.; Blum, V.; Sikola, T.; Varga, P.; Schmid, M. Surf. Sci. 2007, 601, (12) Cook, J. C.; Clowes, S. K.; McCash, E. M. J. Chem. Soc., Faraday Trans. 1997, 93, (13) Eve, J. K.; McCash, E. M. Chem. Phys. Lett. 2002, 360, (14) Hayden, B. E. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum Press: New York, 1987; Chapter 7. (15) Attard, G.; Barnes, C. Surfaces; Oxford University Press: Oxford, UK, (16) Beebe, T. P.; Gelin, P.; Yates, J. T. Surf. Sci. 1984, 148, (17) Chang, S. L.; Thiel, P. A.; Evans, J. W. Surf. Sci. 1988, 205, (18) Zheng, G.; Altman, E. I. J. Phys. Chem. B 2002, 106, (19) Ortega, A.; Hoffmann, F. M.; Bradshaw, A. M. Surf. Sci. 1982, 119, (20) Orent, T. W.; Bader, S. D. Surf. Sci. 1982, 115, (21) Chang, S. L.; Thiel, P. A. J. Chem. Phys. 1988, 88, (22) Vu, D. T.; Mitchell, K. A. R.; Warren, O. L.; Thiel, P. A. Surf. Sci. 1994, 318, (23) Todorova, M.; Lundgren, E.; Blum, V.; Mikkelsen, A.; Gray, S.; Gustafson, J.; Borg, M.; Rogal, J.; Reuter, K.; Andersen, J. N.; Scheffler, M. Surf. Sci. 2003, 541, (24) Wichtendahl, R.; Rodriguez-Rodrigo, M.; Hartel, U.; Kuhlenbeck, H.; Freund, H.-J. Phys. Status Solidi 1999, 173, (25) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand: Princeton, NJ, 1950; Vol. 1. (26) Stuve, E. M.; Madix, R. J.; Brundle, C. R. Surf. Sci. 1984, 146, (27) Kim, Y. D.; Wei, T.; Wendt, S.; Goodman, D. W. Langmuir 2003, 19,