Pd(100) Model Alloy Catalysts

Feb 3, 2010 - low as ∼160 K.17-19 In order to further understand this chemistry, the ..... of O2 at 80 K followed by CO admission, or by adsorbing O...
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J. Phys. Chem. C 2010, 114, 16909–16916

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Carbon Monoxide Oxidation over Au/Pd(100) Model Alloy Catalysts† Zhenjun Li,‡ Feng Gao,§ and Wilfred T. Tysoe*,‡ Department of Chemistry and Laboratory for Surface Studies, UniVersity of WisconsinsMilwaukee, Milwaukee, Wisconsin 53211, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255 ReceiVed: NoVember 30, 2009; ReVised Manuscript ReceiVed: January 14, 2010

The oxidation of carbon monoxide is studied on a series of Au/Pd(100) alloys with various compositions that are formed by adsorbing four monolayers of gold onto a Pd(100) surface and by heating to form the alloy. The composition of the alloy surface is measured using Auger and low-energy ion scattering spectroscopies and the surface chemistry followed by temperature-programmed desorption and reflection absorption infrared spectroscopy. Carbon monoxide oxidized rapidly on Au/Pd(100) model alloy surfaces. Oxygen is found to dissociatively adsorb on alloy surfaces for gold coverages below ∼0.6 ML and relatively stable molecular oxygen species were found on the surface at gold coverages between ∼0.2 and 0.6 ML, but did not contribute to CO oxidation on these Au/Pd(100) alloys. Carbon monoxide oxidation and desorption were found to occur at almost identical temperatures, implying that the activation energies for these processes are almost identical. This suggests that the activation energy for CO oxidation is controlled by the heat of adsorption of CO; the most weakly held CO is the most easily oxidized. The most weakly adsorbed CO desorbs at between ∼100 and ∼125 K and is associated either with CO adsorbed on surface gold sites or a restructured surface associated with the presence of isolated palladium sites. 1. Introduction There has been a resurgence of interest in developing lowtemperature CO oxidation catalysts to provide CO-free hydrogen feedstocks for fuel cells1-4 and this endeavor was spurred by the discovery that supported gold nanoparticles are extremely active for this reaction.5,6 It has also been suggested that nickel-gold single crystal alloys are extremely active for CO oxidation, where carbon dioxide formation is found at 70 K.7 It was proposed, in this case, that dioxygen species provided the oxidant. Gold-palladium alloy catalysts also provide the basis for active and selective oxidation catalysts, notably in the formation of hydrogen peroxide from hydrogen and oxygen8-15 where recent density functional theory calculations invoke an adsorbed dioxygen species.16 It has also been shown that Au/ Pd(100) surface can catalyze CO oxidation at temperatures as low as ∼160 K.17-19 In order to further understand this chemistry, the reaction of carbon monoxide and oxygen is studied on a Au/Pd(100) alloy in ultrahigh vacuum. In this case, the relationship between the alloy structure and the oxidation activity is facilitated by being able to grow alloys with a wide range of compositions by evaporating a thin (∼4 monolayers, ML) film of gold onto a Pd(100) substrate and heating to form the alloys, where alloys of different compositions can be obtained merely by heating to various temperatures.20,21 The variation in the presence of various ensembles on the alloy with composition has been probed using CO titrations.21 These results suggested that next-nearest neighbor palladium sites were preferentially occupied and that CO occupied nearest-neighbor (i.e., bridge) sites at low gold coverages. As the gold coverage increased above ∼0.4 ML, isolated palladium sites appeared †

Part of the “D. Wayne Goodman Festschrift”. * Author to whom correspondence should be addressed. E-mail: [email protected]. Telephone: 414 229 5222. Fax: 414 229 5036. ‡ University of WisconsinsMilwaukee. § Texas A&M University.

where only the occupancy of next-nearest neighbor sites varied. CO was initially found to adsorb on atop palladium sites and desorbed at ∼350 to 370 K. However, as the gold coverage increases above ∼0.5 ML, CO was found to adsorb much more weakly on the surfaces and desorbed at ∼170 and 112 K. The nature of the sites at which CO is weakly held is not yet understood but was suggested to be due to some restructuring of the surface. However, in view of the observation that gold-palladium alloys are excellent CO oxidation catalysts, CO2 formation was probed with TPD following coadsorption of CO and O2 on the alloy surfaces in UHV. Since O2 has been proposed as the oxidant for H2O2 formation16 and for CO oxidation on gold-nickel alloys,7 the adsorption of oxygen on the Au/Pd(100) alloy was also examined. 2. Experimental Section The equipment used for reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) experiments has been described in detail elsewhere.22 Briefly RAIRS spectra were collected in a UHV system coupled with a Bruker Equinox spectrometer; typically each spectrum was acquired for 1000 scans at a resolution of 4 cm-1. TPD data were collected in another UHV chamber equipped with a Dycor quadrupole mass spectrometer interfaced to a computer that allowed up to five masses to be sequentially monitored in a single experiment. The mass spectrometer was enclosed in a shroud, which contained a 1-cm diameter hole to ensure that only species desorbing from the front face of the sample were detected. The sample could be cooled to 80 K in both chambers by thermal contact to a liquid-nitrogen-filled reservoir and resistively heated to ∼1200 K. The Pd(100) single crystal was cleaned using a standard procedure and its cleanliness was monitored using Auger spectroscopy and TPD collected following oxygen adsorption.23 Gold was evaporated from a small alumina tube furnace24 that

10.1021/jp911374u  2010 American Chemical Society Published on Web 02/03/2010

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allowed controlled and reproducible evaporation rates to be achieved. In order to precisely control the temperature of the gold source, and therefore its evaporation rate, a C-type thermocouple was placed into the gold pellet. The amount of gold deposited onto the surface was monitored using Auger spectroscopy from the peak-to-peak intensities of the Au NVV and Pd MNN Auger features and the monolayer coverage was gauged from breaks in the gold uptake signal.11 The gold-palladium alloy was formed by initially depositing 4 monolayers of gold and then by annealing to various temperatures for a period of five minutes in ultrahigh vacuum to produce the desired Au/Pd atomic ratio. Gold coverages on the alloy surfaces were measured with low energy ion scattering spectroscopy (LEISS).25 Unlike electron-based techniques, for example XPS and AES, which probe several top layers and yield the average composition of these layers, LEISS is only sensitive to the outmost layer of the sample therefore provides only the composition information of the first layer. The following annealing temperatures were used to obtain the various gold coverages, in parentheses; 570 (0.98 ML), 620 (0.94 ML), 680 (0.88 ML), 740 (0.81 ML), 800 (0.67 ML), 860 (0.56 ML), 920 (0.4 ML), 980 (0.24 ML), 1040 (0.13 ML), and 1100 K (0.05 ML). The carbon monoxide (Aldrich, 99.99+% purity), oxygen (Praxair, 99.9%), and 18O2 (CIL, 95% 18O2) were transferred to glass bottles and attached to the all-glass gas-handling systems of the vacuum chambers. The cleanliness of all reactants was monitored mass spectroscopically. 3. Results 3.1. Adsorption of O2 on Au/Pd(100) Alloys: TPD. The adsorption of oxygen on Pd(100) has been studied in detail previously.26-28 It has been found that oxygen dissociatively adsorbs on the surface and forms p(2 × 2) islands for coverages in excess of 0.05 ML and yields a fully developed p(2 × 2) structure at a 0.25 ML coverage at 300 K. The p(2 × 2) pattern is then gradually replaced by a c(2 × 2) structure as the oxygen coverage increases to 0.5 ML. The sticking coefficient of O2 on Pd(100) has been found to be constant prior to the full development of the p(2 × 2) structure and to decrease thereafter. Figure 1a presents O2 desorption profiles on various alloys following exposure to 5 langmuir (1 langmuir ) 1 × 10-6 Torr s) of O2 at 300 K, where top-layer gold coverages are marked adjacent to the corresponding curves. Clearly the oxygen dissociation capability decreases as the gold coverage rises. As is shown in Figure 1b, the oxygen coverage decreases rapidly with increasing gold coverage and as the gold coverage exceeds 0.6 ML, the alloy surfaces are incapable of dissociating oxygen. Experiments with higher O2 exposure (30 langmuir) yield the same conclusion (data not shown). A line is plotted through these data as a guide to the eye. In order to search for the adsorption of molecular oxygen on the surface, various Au/Pd(100) alloys were exposed to oxygen at 80 K and the 32 amu (O2) TPD profiles were collected for various alloys. Note that the maximum temperature during the desorption sweep was kept sufficiently low so that the surface composition of the alloy did not change (this is verified by LEISS measurements prior to and after TPD experiments using the same surface). The resulting profiles are displayed in Figure 2a, which reveals two molecular desorption states. The first is at ∼118 K, which is largest when the gold coverage is the lowest; this is due to molecular oxygen adsorbed on the Pd(100) surface. This has been observed previously on clean Pd(100) surface using HREELS29 and TPD.30 Note that the true desorp-

Figure 1. (a) 32 amu (oxygen) TPD data for various Au/Pd(100) alloys exposed to 5 langmuir of oxygen at 300 K collected using a heating rate of 3.7 K/s. The gold coverage is marked adjacent to the corresponding spectrum. (b) Plot of the oxygen desorption yield as a function of gold coverage measured from the desorption profiles shown in (a). The solid line is included as a guide to the eye.

tion temperature may be even lower than ∼118 K since the leading edge of the desorption trace is coincident with the adsorption temperature (80 K). In this case, the integrated area under the desorption state does not necessarily reflect the total oxygen coverage of molecular oxygen on the Pd(100) surface. An additional, more-stable molecular oxygen feature appears at ∼180 K for gold coverages between ∼0.2 and 0.4 ML. In order to verify that this desorption state is due to molecular oxygen that has not dissociated, an alloy surface containing 0.4 ML of gold was dosed with an equimolar mixture of 18O2 and 16 O2 at 80 K. The resulting 32 (16O2), 34 (16O18O), and 36 (18O2) amu spectra, collected during the same desorption sweep, are displayed in Figure 2(b). Some oxygen clearly dissociated during the heating as evidenced by the small oxygen feature at ∼800

Carbon Monoxide Oxidation

Figure 2. (a) 32 amu (oxygen) TPD data for various Au/Pd(100) alloys exposed to 30 langmuir of oxygen at 80 K collected using a heating rate of 3.7 K/s. The gold coverage is marked adjacent to the corresponding spectrum. (b) TPD data for Au/Pd(100) alloy with a gold coverage of 0.4 ML following a 30 langmuir exposure to an approximately equimolar mixture of 18O2 and 16O2 monitoring 32, 34, and 36 amu.

K and the relatively large signal at 34 amu at this temperature confirms that it is due to the recombination of adsorbed atomic oxygen. However, the 117 and 179 K peaks show only a very small 34 amu signal (that likely arises from some 16O18O contamination in the 18O2) compared to the large 32 an 36 amu signals confirming that these two low-temperature states are due to the desorption of molecular oxygen. The gold-coverage dependence of the molecular state coincides quite closely with the variation in intensity of a CO desorption feature from the alloy between ∼352 and 374 K,21 which is assigned to CO adsorption on atop palladium sites, strongly suggesting that these are also the sites for the adsorption of molecular oxygen on the alloys. 3.2. RAIRS of CO on Oxygen-Saturated Alloys. In order to explore the nature of the adsorption sites for CO adsorbed on oxygen-saturated alloys, a series of infrared spectra were collected for alloys with gold coverages below 0.5 ML that had

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16911 been exposed to 30 langmuir of oxygen at 300 K to saturate the surface. The resulting spectra are displayed in Figure 3, panels (a) (ΘAu ) 0.05 ML), (b) (ΘAu ) 0.13 ML), (c) (ΘAu ) 0.24 ML), and (d) (ΘAu ) 0.40 ML). Previous work for CO alone adsorbed on Au/Pd(100) alloys has shown that it adsorbs both on atop and bridge palladium sites where atop adsorption yields vibrational frequencies between ∼2075 and ∼2090 cm-1 and adsorption on bridge sites yields modes from ∼1900 to 1980 cm-1 where the vibrational frequency shifts to higher frequencies as the coverage increases.21 Carbon monoxide adsorption on gold sites is manifest by a CO stretching frequency of ∼2106 cm-1. Thus, at the highest gold coverages (ΘAu ) 0.40 ML), Figure 3d), CO adsorbs almost exclusively at atop sites as indicated by the intense modes between ∼2075 and 2087 cm-1, and the relatively weak bridge-site modes at ∼1971 and 1912 cm-1. The relatively large peak at ∼2109 cm-1 is due to adsorption on gold sites, and the loss of this feature on heating to ∼126 K indicates that CO desorbs from the gold sites below this temperature. This is accompanied by an increase in intensity and a frequency shift of the ∼2087 cm-1 feature. This effect is ascribed to dipole-dipole coupling between CO adsorbed on gold and palladium sites that causes the lower-frequency mode to lose intensity.31 The CO on palladium atop sites shifts slightly on heating to ∼290 K and is strongly attenuated on warming to ∼344 K indicating that CO desorbs from atop palladium sites at a little above 300 K. As the gold coverage is reduced to ΘAu ) 0.24 ML (Figure 3c), less CO is detected on gold sites, whereas the CO adsorbed on atop sites again desorbs at somewhat above 300 K. The lower gold coverage also results in a larger proportion of CO adsorbed on bridge sites from the feature at ∼1973 cm-1, and this peak shifts and loses intensity at much higher temperatures and thus desorbs above 400 K. As the gold coverage is reduced to 0.13 ML (Figure 3b), very little CO adsorption on gold sites is detected in accord with the lower gold coverage, while the CO on atop sites desorbs at a lower temperature, ∼250-290 K. The relative amount of CO that occupies bridge sites increases considerably as the gold coverage decreases and again this CO desorbs well above 400 K. This trend continues for a surface with 5% of a monolayer of gold (Figure 3a), where the coverage of CO on atop palladium sites is reduced and it desorbs at even lower temperatures, below ∼250 K. Correspondingly, the bridge-bonded features are much more intense and the CO desorbs at relatively high temperatures, above ∼475 K. 3.3. CO Oxidation on AuPd Alloys: TPD. CO oxidation on late transition metals is known to proceed via the Langmuir-Hinshelwood mechanism,32,33 i.e., by a reaction between coadsorbed CO (a) and O (a). The effect of different alloy compositions was explored by exposing various Au/ Pd(100) alloys to 10 langmuir of a 1:2 mixture of CO and oxygen (3.3 langmuir of CO and 6.6 langmuir of oxygen) and then collecting TPD data while monitoring 28 (CO), 44 (CO2) and 32 (O2) amu. The results are displayed in Figure 4 for alloys with gold coverages of (a) 0.13, (b) 0.24, and (c) 0.4 ML. Note that oxygen dissociates on all of these surfaces (Figure 1). The 28 amu fragment of carbon dioxide is ∼10% of the 44 amu signal in our spectrometer. In all cases, a substantial amount of CO2 (44 amu signal) is formed and the peak maxima in the CO2 desorption states are quite close to those found for CO desorption (28 amu signal). A similar correspondence between the CO and CO2 desorption temperatures has been found previously for CO oxidation catalyzed by Pt(111).34 Some 32 amu (O2) signal is detected at ∼120 K assigned to molecular oxygen on the Pd(100) surface,

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Figure 3. Infrared spectra of CO adsorbed on oxygen-saturated alloy surfaces as a function of CO exposure at 80 K and after heating to various temperatures. The CO exposures and annealing temperatures are displayed adjacent to the corresponding spectra, for alloys with gold coverages of (a) 0.05, (b) 0.13, (c) 0.24, and (d) 0.40 ML.

while the ∼180 K desorption states (see Figure 2) are absent. No oxygen desorption is detected between 700 and 800 K indicating that all of the adsorbed, atomic oxygen has been consumed by oxidizing CO to CO2. Several CO2 states can be identified. Note that, due to the fragmentation of CO2 in the mass spectrometer ionizer, these states also contribute to the 28 amu signal. However, in all cases, the 28 amu signal is too large to be accounted for by CO2 fragmentation alone and arise predominantly from the desorption of CO. It is also evident that the relative intensities of the various CO desorption states differ from those found on oxygen-free Au/Pd(100) alloys,21 and presumably occurs because of the influence of coadsorbed oxygen and the fact that CO is also being oxidized. At all coverages, relatively weak CO2 formation states are observed at ∼480 and ∼370 K and these have been observed previously on clean Pd(100) and designated γ2 and γ1 states, respectively.26 At a gold coverage of 0.13 ML, an additional, prominent state appears at ∼225 K, with an additional, weaker

state at ∼126 K. These desorption states are also detected for an alloy with a gold coverage of 0.24 ML but the lowtemperature states appears to shift to even lower temperatures and, when the gold coverage increases to 0.4 ML, the CO2 desorption state at ∼100 K becomes larger than the ∼225 K state. Since molecular oxygen is detected on the alloy surface (Figure 2), where it desorbs at ∼180 K, but is not seen for a surface dosed with a mixture of CO and oxygen (Figure 4), a TPD experiment is performed to check whether molecular oxygen contributes to the CO2 formation. Prior to this, for comparison, the evolution of the various CO2 formation states is explored on an alloy containing 0.24 ML of gold. The surface was first saturated with oxygen by a 10 langmuir exposure and then warmed to ∼190 K to remove any molecular oxygen from the surface. This procedure leads to an atomic oxygen coverage of ∼0.08 ML (see Figure 1). The surface was then dosed with various exposures of CO and the results are shown in Figure

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Figure 4. Temperature-programmed desorption data collected using a heating rate of 3.7 K/s for various alloys exposed to 10 langmuir of a 2:1 molar mixture of carbon dioxide and oxygen at 80 K monitoring 32 (oxygen), 44 (carbon dioxide), and 28 (carbon monoxide) amu, for alloys with gold coverages of (a) 0.13, (b) 0.24, and (c) 0.4 ML.

5a, which displays the resulting 44 amu desorption profiles as a function of CO exposure, and the CO exposures are indicated adjacent to the corresponding spectra. Four CO2 desorption peaks are found from the alloy surface precovered by ∼0.08 ML of O(a) and then saturated with CO in accord with the data shown in Figure 4, although an additional small state is identified at ∼287 K that is not easily seen in the data of Figure 4. At the lowest CO exposure (0.5 langmuir), a single CO2 desorption state develops at 380 K. Increasing the CO exposure to 1 langmuir results in the development of other states at 287 and 480 K. Further increasing the CO exposure to 2 langmuir leads to an even lower-temperature state at 232 K (as a shoulder). With CO exposures of 4 langmuir and above, a final and lowesttemperature peak develops at 114 K. A general trend is that lower-temperature CO2 formation states develop at the expense of higher-temperature ones as the CO coverage increases. These results indicate that alloying palladium with gold produces surface sites that are more active than the clean surface for CO

oxidation, in accord with results for the catalytic activity of these surfaces for CO oxidation.17,18 As displayed in Figure 5b, either by adsorbing 10 langmuir of O2 at 80 K followed by CO admission, or by adsorbing O2 at 80 K then annealing to 190 K to remove any molecular species before CO admission, or by directly adsorbing O2 at 300 K before cooling the sample (where no molecular oxygen can exist on the surface), and then adsorbing CO, the CO2 (44 amu) desorption yields, and the lineshapes of the desorption curves are almost identical. This therefore rules out any significant contribution of O2 to CO2 formation. This implies that the absence of the ∼180 K desorption state in the data shown in Figure 4, panels b and c, is due to the molecular oxygen being displaced by CO. 4. Discussion An Au/Pd alloy surface with the (100) orientation is arguably one of the simplest alloys in terms of CO adsorption where CO

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Figure 6. Energy diagram for the adsorption and oxidation of carbon monoxide by Au/Pd(100) alloys.

Figure 5. (a) 44 amu (carbon dioxide) TPD spectra for a Au/Pd(100) alloy with a gold coverage of 0.24 exposed to oxygen at 80 K, then heated to ∼190 K and then exposed to CO, where the CO exposures in Langmuirs are given adjacent to the corresponding spectrum and (b) 44 amu (carbon dioxide) TPD spectra for a Au/Pd(100) alloy with a gold coverage of 0.24 with various pretreatments, that are indicated adjacent to the corresponding spectrum, prior to exposing to 10 langmuir of CO.

only occupies bridge sites formed by nearest-neighbor palladium atoms and atop sites, which are modified by the nature of the next-nearest neighbor sites.21 As such, a combination of RAIRS and TPD allows for very detailed understanding of CO adsorption/desorption and a correlation between the nature of the adsorbed CO and its oxidation activity. This is facilitated by the observation that CO2 is formed and CO desorbs at approximately the same temperatures (Figure 4). Although molecular oxygen is detected on the surface (Figure 2), it apparently plays no role in CO oxidation (Figure 5) so that the customary Langmuir-Hinshelwood model will be adopted in which oxygen adsorbs dissociatively on the surface and CO adsorbs molecularly, and they react to form CO2 in a simple, bimolecular surface reaction. The observation that CO desorption and CO2 formation occurs at almost identical temperatures

implies that the activation energies for CO desorption and oxidation are relatively similar and the fact that the CO2 formation and CO desorption temperatures coincide for states from ∼100 to 480 K suggests that both are controlled by the heat of adsorption of CO. That is, the more weakly that CO is adsorbed on the surface, the more easily it oxidizes. The reaction enthalpy for CO (g) + 1/2O2 (g) f CO2 (g) is -67 kcal/mol.35 Thus, the minimum energy needed to form CO2 from adsorbed CO and atomic oxygen, is ∆H(ads)(CO) + ∆H(ads)(O) - 67 kcal/ mol. If the transition state to CO2 formation is identical on all alloy surfaces and is designated Eact(CO2) relative to the energy of gas-phase CO2, then, as shown in Figure 6, the overall activation energy for CO2 formation from adsorbed CO and atomic oxygen is ∆H(ads)(CO) + ∆H(ads)(O) - 67 + Eact(CO2). This value decreases with decreasing heat of adsorption of CO since CO and CO2 desorb almost simultaneously. From the above arguments, this is approximately equal to the desorption activation energy of CO. Since CO adsorption is not activated, this is equal to the heat of adsorption of CO, so that ∆H(ads)(O) ∼ 67 - Eact(CO2) and implies that the heat of adsorption of atomic oxygen should be less than 76 kcal/mol. The heat of adsorption of oxygen measured on aluminasupported palladium is between 43 and 53 kcal/mol36 and recent DFT calculations give a heat of adsorption of oxygen in the 4-fold hollow site on Pd(100) as 1.95 eV (45 kcal/mol)37 and suggests that Eact(CO2) ∼ 10-20 kcal/mol. Note that this also suggests that reducing the heat of adsorption of oxygen should further reduce the CO2 formation energy. While oxygen only adsorbs on the alloy in ultrahigh vacuum for gold coverages below ∼0.5 ML (Figure 1b), it is likely that more weakly held oxygen sites could be occupied at higher oxygen pressures on alloys with higher gold coverages. The relationship between CO desorption and oxidation allows the CO adsorption site to be related directly to the corresponding CO2 formation state. The high-temperature CO and CO2 formation states have been observed previously on clean Pd(100)26 and the γ1 state has been ascribed to CO reacting with a disordered oxygen phase and the γ2 state to the reaction of CO bound to palladium bridge sites that are unaffected by the presence of oxygen that react with isolated oxygen atoms. Additional CO desorption and CO2 formation states appear at ∼101-126, 224-232 and 370-380 K (Figures 4 and 5). These states have been observed for CO adsorbed alone on a Au/Pd(100) alloy.21 The desorption state at ∼380 K has been assigned to CO adsorbed on atop palladium sites, while the state at ∼230 K is due to the adsorption on CO on adjacent atop

Carbon Monoxide Oxidation sites, where the heat of adsorption is reduced due to repulsive interactions between coadsorbed CO molecules. The lowest-energy state, where CO2 is formed between 101 and 126 K, depending on coverage (Figures 4 and 5), coincides with a desorption state for CO on Au/Pd(100) that appears at ∼112 K. This state grows with increasing gold coverage up to ΘAu ∼ 0.67 ML and decreases in intensity at higher coverages. It could thus be due to CO adsorbed on gold sites and the increase in intensity of the CO stretching mode at ∼2109 cm-1 assigned to CO on atop gold sites (Figure 3) is in accord with this view. However, since the desorption yield from this state decreases at higher gold coverages,21 it was suggested that it was due to a restructuring of the alloy surface. The O2 TPD data shown in Figure 1 reveal that when surface gold coverage exceeds ∼0.6 ML, the oxygen dissociation capacity is completely lost. CO TPD spectra21 reveal the lack of CO desorption from bridge sites on the ΘAu ) 0.67 ML surface and this is also confirmed by the corresponding infrared data. This provides rather convincing evidence that oxygen dissociation only occurs on contiguous palladium sites. Further evidence of this assignment comes from the infrared data (Figure 3). Adsorbing 1 langmuir of CO on an oxygen precovered surface leads to a clear blue shift of the bridge-bonded CO band as compared with that on the bare surface.21 This indicates that O (a) does occupy contiguous palladium sites so that CO binding with these sites is weakened. This conclusion confirms recent discoveries by Goodman and co-workers who found that CO oxidation only occurs on Au/Pd alloy surfaces that contain contiguous palladium sites.17,18 Clearly this is due to O2 dissociation capability of such surfaces. Moreover, these studies and the findings of the present study all support the significance of the Langmuir-Hinshelwood mechanism. Room temperature CO oxidation at atmospheric pressures has been an important topic in recent years due to the need to remove small amounts of carbon monoxide from fuel cell feedstocks.3,4 Late transition metals that efficiently catalyze CO oxidation at elevated temperatures have no reactivity at room temperature. Supported gold nanoparticles show some potential; however an unacceptable rate of deactivation is still an important issue to be solved before industrial applications are considered. Supported gold nanoparticles need a properly chosen support material and particle size to activate oxygen in order to be reactive. Recently, it has been proposed that, by adding another element into gold that is capable of activating O2, the support and particle size requirements for supported gold should become much less critical. In principle, palladium meets this need because of its O2 dissociation capability. As demonstrated by the data in Figure 5, the ΘAu ) 0.24 ML alloy surface precovered with O (a) and saturated with CO forms CO2 in various states. All of the CO2 formation states, except that at ∼114 K, have been found on Pd(100).26 It is inferred above that the most probable assignment for the 114 K CO2 state is due to combination of CO adsorbed on gold and oxygen on a adjacent palladium sites. In other words, the proposal of adding palladium to gold to function as the oxygen dissociation site in combination with sites that bind CO only weakly, to lead to low-temperature CO oxidation, is indeed achievable. Under ultrahigh vacuum conditions, the present study reveals that surface the gold coverage must be less than ∼0.6 ML to allow for the existence of contiguous Pd sites where O2 dissociation occurs, at least under UHV conditions. At elevated pressures, preferential segregation of palladium to the surface is induced by the reactants, and allows an alloy surface that is completely inert in UHV to become reactive at elevated pressures.17,18

J. Phys. Chem. C, Vol. 114, No. 40, 2010 16915 Conclusions Carbon monoxide oxidized rapidly on Au/Pd(100) model alloy surfaces. Oxygen is found to dissociatively adsorb on alloy surface for gold coverages below ∼0.6 ML emphasizing the importance of the presence of contiguous palladium sites for oxygen dissociation. Relatively stable molecular oxygen species were found on the surface at gold coverages between ∼0.2 and 0.6 ML, but did not contribute to CO oxidation on this Au/ Pd(100) alloy. Carbon monoxide oxidation and desorption were found to occur at almost identical temperatures, suggesting that the activation energies for these processes are almost identical. This suggests that the activation energy for CO oxidation is controlled by the heat of adsorption of CO; the most weakly held CO is the most easily oxidized. The most weakly adsorbed CO desorbs at between ∼100 and ∼125 K and is associated either with CO adsorbed on gold sites on the surface or a restructured surface associated with the presence of isolated palladium sites. Acknowledgment. We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under Grant No. DE-FG02-92ER14289 and a Research Growth Initiative grant from the University of WisconsinsMilwaukee. References and Notes (1) Kummer, J. T. J. Phys. Chem. 1986, 90, 4747. (2) Armor, J. N. Appl. Catal., A 1999, 176, 159. (3) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. J. Catal. 1997, 171, 93. (4) Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J. J. Catal. 1999, 182, 430. (5) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 2, 405. (6) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (7) Lahr, D. L.; Ceyer, S. T. J. Am. Chem. Soc. 2006, 128, 1800. (8) Edwards, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Faraday Discuss. 2008, 138, 225. (9) Abate, S.; Centi, G.; Melads, S; Perathoner, S.; Pinna, F.; Strukul, G. Catal. Today 2005, 104, 323. (10) Samnata, C. Appl. Catal., A 2008, 350, 133. (11) Landon, P.; Collier, P. J.; Papworth, A. J.; Kiely, C. J.; Hutchings, G. J. Chem. Commun. 2002, 2058. (12) Landon, P.; Collier, P. J.; Carley, A. F.; Chadwick, D.; Papworth, A. J.; Burrows, A.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1917. (13) Solsona, B. E.; Edwards, J. K.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. Chem. Mater. 2006, 18, 2689. (14) Edwards, J. K.; Solsona, B.; Landon, P.; Carley, A. F.; Herzing, A.; Watanabe, M.; Kiely, C. J.; Hutchings, G. J. J. Mater. Chem. 2005, 15, 4595. (15) Edwards, J. K.; Solsona, B.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. J. Catal. 2005, 236, 69. (16) Ham, H. C.; Hwang, G. S.; Han, J.; Nam, S. W.; Lim, T. H. J. Phys. Chem. C 2009, 113, 12943. (17) Gao, F.; Wang, Y.; Goodman, D. W. J. Am. Chem. Soc. 2009, 131, 5734. (18) Gao, F.; Wang, Y.; Goodman, D. W. J. Phys. Chem. C 2009, 113, 14993. (19) Piednoir, A.; Languille, M.; Piccolo, L.; Valcarcel, A.; Cadete Santos. Aires, F.; Bertolini, J. Catal. Lett. 2007, 114, 110. (20) Li, Z.; Gao, F.; Wang, Y.; Calaza, F.; Burkholder, L.; Tysoe, W. T. Surf. Sci. 2007, 601, 1898. (21) Li, Z.; Gao, F.; Furlong, O.; Tysoe, W. T. Surf. Sci. 2010, 604, 136. (22) Kaltchev, M.; Thompson, A. W.; Tysoe, W. T. Surf. Sci. 1997, 391, 145. (23) Li, Z.; Gao, F.; Tysoe, W. T. Surf. Sci. 2008, 602, 416. (24) Wytenburg, W. J.; Lambert, R. M. J. Vac. Sci. Technol. A 1992, 10, 359. (25) Niehus, H.; Heiland, W.; Taglauer, E. Surf. Sci. Rep. 1993, 17, 213. (26) Stuve, E. M.; Madix, R. J.; Brundle, C. R. Surf. Sci. 1984, 146, 155.

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