Infrared Reflection Absorption Spectroscopy Study ... - ACS Publications

Mar 26, 2008 - Feng Gao,Matthew Lundwall, andD. Wayne Goodman*. Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, ...
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J. Phys. Chem. C 2008, 112, 6057-6064

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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 77842-3012 ReceiVed: NoVember 8, 2007; In Final Form: February 4, 2008

The adsorption, desorption, and reaction of CO on the (x5×x5)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 O2 exhibits a stronger interaction with CO compared with a surface oxide formed at 575 K with a 4500 L O2 exposure, presumably due to defects in the former. The surface oxide formed with a lower exposure of O2 exhibits enhanced reactivity with CO at 400 K. Below 60 K, O2 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 O2/CO ratio above 0.5.1 The catalytic surfaces exhibit rates 2-3 orders of magnitude greater than those observed under stoichiometric reaction conditions and similar reactant pressures2 or previously in ultrahigh vacuum studies at any reactant conditions.3-6 The O2/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 O2/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 (x5×x5)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 O2/CO ratio is substantially greater than the stoichiometric ratio, the (x5×x5)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 5 × 10-10 Torr equipped with IRAS, Auger electron spectroscopy (AES), lowenergy electron diffraction (LEED), and a quadrupole mass * To whom correspondence should be addressed. Phone: 979-8450214. Fax: 979-845-6822. E-mail: [email protected].

spectrometer. The IRAS spectra were obtained using a Matheson Cygnus 100 spectrometer. The IR beam impinged the sample through CaF2 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 O2, which has been described previously.9 The cleanliness of the sample was confirmed with AES and LEED. O2 and CO were introduced into the chamber through leak valves by back-filling. The (x5×x5)R27° surfaces were formed using two methods developed previously. The first involves reacting 1 × 10-6 Torr of O2 with Pd(100) at 575 K for 10 min (600 L, 1 L ) 1 × 10-6 Torr s),10 and the second method utilizes 5 × 10-6 Torr of O2 reacting with Pd(100) at 575 K for 15 min (4500 L).11 In the following, these two surface oxides are denoted as the x5 surface (1) and (2), respectively. 3. Results 3.1. CO Adsorption/Desorption on the x5 Pd(100) Oxide Surfaces. CO adsorption was monitored first on freshly prepared x5 surfaces at 20 K. At all CO exposures two bands are evident, one intense and sharp feature at ∼2142 cm-1 typical for multilayer CO8,12,13 and a much weaker signal at ∼1940 cm-1

10.1021/jp710713r CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008

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Figure 1. (a) IRAS spectra of multilayer CO as a function of exposure at 20 K on x5 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 x5 surface (1). CO exposures are marked adjacent to each spectrum. All spectra were recorded for 200 scans at a resolution of 4 cm-1.

assigned to CO adsorption on bridging sites.14 Figure 1a displays the multilayer feature on the x5 surfaces as a function of CO exposure. The left panel presents results for the x5 surface (1), and the right panel displays data for the x5 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 x5 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 x5 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 x5 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 x5 surface (1), the pressure of the chamber is rather high (∼1-2 × 10-9 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 x5 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

IRAS Study of CO on Oxidized Pd(100)

Figure 2. (a) IRAS spectra recorded during annealing 0.54 L CO covered x5 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 4 cm-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 x5 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 ∼ 2 × 10-10 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 x5 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

J. Phys. Chem. C, Vol. 112, No. 15, 2008 6059 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 impurities16 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 x5 surface (1). Particularly between 80 and 170 K, the intensity of the atop band is approximately 2 times greater on the x5 surface (1) compared with the x5 surface (2). In contrast, the bridging band differs little from one surface to the other. 3.2. CO Titration of the x5 Oxide Surfaces. The results presented above illustrate the difference between the two x5 surface oxides. For multilayer CO, the signal on the x5 surface (2) is more intense compared with the x5 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 x5 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 x5 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 x5 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 x5 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 x5 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

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Figure 3. (a) IRAS spectra recorded during annealing monolayer CO-covered x5 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 x5 surface (2). Clearly the same trend is obtained, but the growth rate of the band at 19911997 cm-1 is lower compared with x5 surface (1). This is highlighted in Figure 5c where the signal intensity of the 19911997 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 1991-1997 cm-1, the signal height instead of integrated signal area is displayed to represent signal intensity to avoid deconvolution-induced error. 3.3. CO Adsorption/Desorption on O2-Precovered x5 Surfaces. Experiments were carried out studying CO adsorption and desorption on O2-saturated oxide surfaces. The surface oxides were exposed to 6 L of O2 at 20 K prior to CO adsorption to achieve O2 saturation. A spectrum acquired for a freshly prepared x5 surface (1) exposed first to 6 L of O2 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 x5 surface (1). On the

O2-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 O2 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 O2 at 20 K (see Figure 7a). At first glance, these spectra look identical to those shown in Figure 1a for the clean x5 surface oxides. However, the integrated IR signal intensities do show apparent differences (Figure 7b). On the x5 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 x5 surface (2).

IRAS Study of CO on Oxidized Pd(100)

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 x5 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 x5 surface (1). As shown in Figure 8a, the multilayer signal persists at 43 K, whereas on clean x5 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 O2-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 x5 surface (1) at similar temperatures (Figure 3a). 4. Discussion The x5 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-1.25 Using temperature-programmed desorption, Wichtendahl et al.24 found that multilayer CO desorbs at ∼30 K on NiO(100) and MgO(100). These authors also found a weak

J. Phys. Chem. C, Vol. 112, No. 15, 2008 6061 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 x5 (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 x5 (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 x5 surface (1) is slightly higher than for the x5 (2) surface. This can be rationalized by a higher density of surface defects on the x5 surface (1) compared with (2) and that CO binds more strongly with defects.24 When the x5 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 x5 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 x5 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 x5 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 x5 surfaces compared with Pd(100), one estimates that for a saturated CO monolayer on x5 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 x5 surfaces under realistic reaction conditions. Data shown in Figure 5c, acquired by monitoring the growth of the 1991-1997 cm-1 feature, indicate that the x5 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

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Figure 5. (a) IRAS spectra recorded on the x5 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 x5 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 of 4 cm-1. (c) 1991-1997 cm-1 band intensity vs CO exposure at 400 K.

x5 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 x5 structure is completely removed. Our CO titration results are consistent with this conclusion. A x5 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 x5 structure. It is emphasized, however, that the ∼2030 cm-1 band is not restricted to the x5 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 x5 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 x5 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 x5 surface (1) at 400 K. However, this feature is still evident following 1.8 L CO reaction with the x5 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 x5 surface (1). Following 1.8 and 2.4 L CO reaction with the x5 surfaces (1) and (2), respectively,

IRAS Study of CO on Oxidized Pd(100)

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Figure 6. IRAS spectra recorded after 0.02 L CO adsorption onto a clean and O2-precovered x5 surface (1). A subtracted spectrum is also included. All spectra were recorded for 200 scans at a resolution of 4 cm-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 x5 surface (1) is formed using 600 L of O2, whereas the x5 surface (2) is formed using 4500 L of O2 at the same temperature, we assume the x5 surface (2) is more homogeneous, i.e., fewer surface defects (for example, missing O atoms) with more ideal x5 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 x5 surface (1). The higher desorption temperature of multilayer CO on x5 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 x5 surface (2) since a larger O2 exposure is used in its synthesis. Zheng and Altman18 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 x5 surface (2) should become identical to the x5 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 x5 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 CO2 formation.

Figure 7. (a) IRAS spectra of multilayer CO as a function of exposure at 20 K on O2-precovered x5 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 x5 surfaces (shown in Figure 1b) are also included.

Finally we comment on the adsorption/desorption of CO on the O2-precovered surfaces. As shown in Figure 6, following O2 adsorption only the CO multilayer feature is observed. This suggests, first, that O2 occupies the same surface sites as CO and, second, CO cannot displace monolayer O2 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 O2-precovered surfaces is always lower compared with that measured on clean x5 surfaces at identical CO exposures, excluding a 0.54 L CO exposure where sufficient multilayer CO remains to preclude O2 adsorption. The monolayer O2 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) O2 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 O2 and CO.

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Gao et al. energy of monolayer CO is estimated to be 50-60 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 x5 surface formed using 600 L of O2 compared with that formed using 4500 L of O2. Presumably this is because more surface defects are present on the surface in the former case. Consistent with this conclusion, the x5 surface formed with less O2 shows a higher reactivity with CO at 400 K. (c) O2 preadsorption at 20 K prevents CO adsorption on the CO monolayer and persists until O2 desorbs at 50-60 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. References and Notes

Figure 8. (a) IRAS spectra recorded during annealing multilayer CO adsorbed on an O2-precovered x5 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-1.

5. Conclusions The interaction of CO with x5 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

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