Formation, Characterization, and Reactivity of Adsorbed Oxygen on

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Formation, Characterization, and Reactivity of Adsorbed Oxygen on BaO/Pt(111) Kumudu Mudiyanselage,† Donghai Mei,† Cheol-Woo Yi,‡ Jason F. Weaver,§ and Ja´nos Szanyi*,† Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K8-87, Richland, Washington 99352, United States, Department of Chemistry and Institute of Basic Science, Sungshin Women’s UniVersity, Seoul 136-742, Korea (ROK), and Department of Chemical Engineering, UniVersity of Florida, GainesVille, Florida 32611, United States ReceiVed: September 7, 2010; ReVised Manuscript ReceiVed: October 15, 2010

The formation of adsorbed O (Oad) species and their reactivities in CO oxidation on BaO/Pt(111) (at two BaO coverages) were studied with temperature programmed desorption (TPD), infrared reflection absorption (IRA), and X-ray photoelectron (XP) spectroscopies. In neither of these two systems was the Pt(111) surface completely covered with BaO. On the system with lower BaO coverage (∼45% of the Pt(111) surface is covered by BaO), two different Oad species form following the adsorption of O2 at 300 K: O adsorbed on the BaO-free Pt(111) sites (OPt) and at the Pt-BaO interface (Oint). On the system with higher BaO coverage (∼60% of the Pt(111) surface is covered by BaO), two types of Oint are seen at the Pt-BaO interface. The desorption of OPt from the BaO-free portion of the Pt(111) surface gives an O2 desorption peak with a maximum desorption rate at ∼690 K. Migration of Oint to the Pt(111) sites and their recombinative desorption give two explosive desorption features at ∼760 and ∼790 K in the TPD spectrum. The reactivities of these Oad species with CO to form CO2 follow their sequence of desorption; i.e., the OPt associated with the BaO-free Pt(111) surface, which desorbs at 690 K, reacts first with CO, followed by the Oint species at the Pt-BaO interface (first the one that desorbs at ∼760 K and finally the one that is bound the most strongly to the interface, and desorbs at ∼790 K). 1. Introduction The role of metal-oxide(support) interfaces in determining the catalytic activities of heterogeneous catalyst systems is well documented.1-10 The enhancement of the catalytic activities is generally attributed to effects of strong metal-oxide interactions,9 which can be divided into two categories, i.e., geometric and electronic.3 In addition, the reaction rate enhancement has also been attributed to spillover or migration mechanisms of adsorbed species between catalytically active phases and support.11 Previous study has also reported that the reaction rates of model catalyst systems depend on the oxidation states of the supporting oxide, free metal surface area, and the number of sites at the interface between metal and the support.4 Furthermore, the boundary between metal and a support also play an important role in catalytic reactions. There are certain catalytic reactions that take place exclusively at the boundary between metal and a support.12,13 These reactions may be slow or may not take place in the absence of a particular support-metal/metal oxide combination. For example, Boffa et al. showed that the methanation of CO2 on Rh foil with submonolayer quantities of oxides took place at the metal-oxide interface.13 In the photocatalytic degradation of methanol, interfacial sites were shown to be the active centers located at the boundary between Pd and TiO2 in a Pd/TiO2 catalyst.12 All of these studies point out the importance of specific interfacial sites and the adsorption of active species on those sites for catalytic reactions. All the concepts described in the previous section (metal-oxide interactions, metal-oxide interface or boundary between metal * Corresponding author. E-mail: [email protected]. † Pacific Northwest National Laboratory. ‡ Sungshin Women’s University. § University of Florida.

and a support, spillover, and migration mechanisms) are applicable not only to “real (high surface area) catalysts” but also to “model catalysts” including “inverse catalysts”, which, e.g., is an oxide deposited on a noble metal surface. We have been studying these types of model systems prepared by reactive layer assisted deposition (RLAD)14-19 of BaO clusters on Al2O3/ NiAl and Pt(111). In this work, we studied the adsorption of O2 on BaO/Pt(111) systems at 300 K and found that adsorbed O (Oad) atoms bind to both the BaO-free Pt (OPt) and the Pt-BaO interface (Oint) and investigated the reactivities of these Oad in CO oxidation. The presence of Oint species on the BaO/ Pt(111) systems, to the best of our knowledge, has not been reported. However, Bowker and co-workers suggested the dissociation of O2 at the defect regions of a BaO/Pt(111) system with clean Pt sites and creation of high local coverage of oxygen surroundings the BaO particles.20 Mueller et al. showed the formation of an O adlayer on the BaO/W(001)21 system and the enhancement of the adsorption probability of oxygen on W(001) by the presence of BaO. They also reported that the Oad coverage on W(001) with BaO is greater than that on bare W(001), despite the fact that a fraction of the W(001) surface was covered by BaO. The excess O on this system appeared to be bound to the W(001) substrate, as determined by ultraviolet photoelectron spectroscopy (UPS) measurements.21 There are different oxygen species (chemisorbed oxygen, subsurface, surface oxides, bulk oxides etc.) present on supported-transition-metal particle surfaces under reaction conditions.22 Oxygen can exist on Pt surfaces in a variety of forms, either as chemisorbed O or oxide (surface or bulk). Understanding the properties of Oad on model catalyst systems is essential to many heterogeneously catalyzed reactions, such as oxidations of CO, NO, and hydrocarbons. Specifically, interrogating the

10.1021/jp108541y  2010 American Chemical Society Published on Web 11/05/2010

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properties of Oad species on BaO/Pt(111), such as adsorption sites, and reactivities is important because these O species may play a crucial role in NOx storage and reduction (NSR) catalysis. In this paper, we report the formation of Oint on the Pt-BaO interface following the adsorption of O2 on BaO/Pt(111) model systems, and the reactivities of these O adatoms in the oxidation of CO. 2. Experimental Section All the experiments were performed in a combined ultrahigh vacuum (UHV) surface analysis chamber and elevated-pressure reactor/infrared reflection absorption spectroscopy (IRAS) cell system with a base pressure of less than 2.0 × 10-10 Torr [1 Torr ) 1.3332 mbar] in both chambers. The UHV chamber is equipped with X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature programmed desorption (TPD) techniques. The elevated-pressure cell is coupled with a commercial Fourier transform infrared (FT-IR) spectrometer (Bruker, Vertex 70). The Pt(111) single crystal (10 mm diameter, 2 mm thick, Princeton Scientific) used in these experiments was spot-welded onto a U-shaped Ta wire, and the sample temperature was measured by a C-type thermocouple spot-welded to the backside of the crystal. The Pt(111) crystal was cleaned by repeated cycles of Ar+ ion sputtering and annealing in O2 at 800 K. The cleanliness of the surface was verified with AES, XPS, and LEED. The BaO clusters were prepared by RLAD:14-17 first the desired amount of Ba was deposited onto a N2O4 multilayer on the Pt(111) crystal at 90 K by physical vapor deposition using a resistively heated Ba doser (SAES Getters), and then the thus formed BaNxOy layer was thermally decomposed by annealing to 1000 K. The obtained BaO film was characterized by XPS. The reactant gases for the TPD and XPS measurements were introduced into the UHV chamber through pinhole dosers and delivered to the sample surface through collimating tubes. A constant oxygen partial pressure of ∼2 × 10-9 Torr in the UHV chamber during O2 exposure was achieved by maintaining a Pback ) 3 Torr pressure in the gas manifold behind the pinhole. The oxygen exposure can conveniently be varied by changing either the O2 pressure in the gas manifold (Pback), or the exposure time at a constant pressure in the gas manifold. The same gas dosing system was set up in the elevated-pressure reactor/IRAS cell. IR spectra were collected at 4 cm-1 resolution using a grazing angle of approximately 85° to the surface normal. All the IR spectra collected were referenced to a background spectrum acquired from the clean BaO/Pt(111) sample prior to gas adsorption or following the adsorption of O2 at 300 K to prepare O-BaO/Pt(111) systems. Each spectrum presented is the average of 1024 scans, requiring a spectral acquisition time of 80 s. In cases where the sample was annealed to higher temperatures after gas exposure, the sample was cooled down to the initial temperature (where the background spectrum was taken) before the spectrum was acquired. The isothermal reaction between adsorbed oxygen and CO was carried out by exposing the O-BaO/Pt(111) systems to a constant flux of CO. In these experiments the oxygen-saturated sample was positioned directly in front of the collimating gas tube delivering the CO. The changes in the gas composition during CO exposure were monitored by a mass spectrometer, not directly in the line of site of the sample. The constant flux of CO was maintained by keeping the pressure in the gas manifold at ∼4 Torr (the exact CO flux on the sample surface is not known; however, the pressure in the chamber never exceeded 2 × 10-9 Torr).

Mudiyanselage et al. 3. Computational Methods We performed periodic density functional theory (DFT) slab calculations using the Vienna ab initio simulation package (VASP).23-26 The projector augmented wave (PAW) method combined with a plane wave basis set at a cutoff energy of 400 eV was used to describe core and valence electrons.27,28 The Perdew-Burke-Ernzerhof (PBE) functional29 was employed in all the calculations. The ground-state atomic geometries of bulk and surface were obtained by minimizing the forces on each atom to below 0.01 eV/Å. The calculated lattice parameter of face-centered cubic Pt bulk is 3.982 Å, which is in good agreement with the experimental value of 3.92 Å and previous DFT result of 3.986 Å obtained using the PW91 functional.30 The periodic Pt(111) surface was modeled using a (4 × 4) supercell with four atomic layers. A height of 15 Å in the z-direction was used to separate the surface slab and its images. Different k-point grid samplings ranging from (1 × 1 × 1) to (4 × 4 × 1) were tested for the clean Pt(111) surface and adsorbed oxygen atoms on Pt(111). We found that the (3 × 3 × 1) k-point sampling scheme was accurate enough since the calculated adsorption energy differences of oxygen atoms were less than 0.05 eV. Two model BaO/Pt(111) systems were constructed to represent the Pt(111) surface-supported BaO nanocluster catalysts. The (BaO)2 and (BaO)4 clusters cut from the cubic BaO bulk structure were initially placed on the Pt(111) surface. As shown below, large geometrical reconstructions of the supported (BaO)2 and (BaO)4 clusters were observed upon optimization. The interaction energies between the Pt(111) surface and the supported (BaO)2 and (BaO)4 clusters were the same (∼4.0 eV). The adsorption energies, Ead, of two atomic oxygens on three model substrates, i.e., Pt(111), (BaO)2/Pt(111), and (BaO)4/ Pt(111) were calculated as follows

Ead ) Esubstrate+2O - (Esubstrate + EO2) where Esubstrate + 2O is the total energy of the systems with two oxygen atoms adsorbed on the model surface slab; Esubstrate is the total energy of the optimized clean surface substrate without adsorbed O; EO2 is the energy of one oxygen molecule in vacuum (triplet). On the basis of this definition, a negative value of Ead indicates a favorable (exothermic) adsorption. 4. Results and Discussion 4.1. Adsorption of CO on Pt(111) and BaO/Pt(111). To determine the fraction of the Pt(111) surface covered by BaO clusters, we first performed TPD experiments following room temperature adsorption of CO on BaO/Pt(111). The integrated CO desorption peak areas from the BaO/Pt(111) systems were compared to those obtained from Pt(111) to estimate the fraction of BaO-free Pt(111) surface. Figure 1 shows TPD spectra of CO obtained following the adsorption of CO on clean Pt(111) and two BaO/Pt(111) systems (lower BaO coverage, system I; higher BaO coverage, system II) at 300 K. CO desorption from clean Pt(111) occurs in a single peak with maximum desorption rate at ∼378 K and a small, high temperature shoulder around 500 K, as reported previously.31 The high temperature shoulder is most likely due to the CO adsorption at step sites.31,32 CO desorption from the BaO/Pt(111) systems occurs in two unresolved desorption features centered at 378 and 518 K. IRAS experiments were performed following the adsorption of CO to identify the adsorbed species on BaO/Pt(111) and correlate them with the desorption features in the TPD spectrum

Adsorbed Oxygen on BaO/Pt(111)

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Figure 3. Possible structures of carbonite ((CO2)2-), tilted-CO, and proposed O-species (O on Pt(111) (OPt) and two types of interfacial-O (Oint)) on BaO/Pt(111).

Figure 1. CO TPD spectra obtained following the adsorption of CO on Pt(111) and BaO/Pt(111) (two different BaO coverages) systems (system I, lower BaO; system II, higher BaO coverage).

Figure 2. RAIR spectra obtained following the adsorption of CO on BaO/Pt(111) (system II) and subsequent annealing to the indicated temperatures.

shown in Figure 1. Following the adsorption of CO on BaO/ Pt(111) (system II) at 300 K, IR features appeared at 1491, 1712, 1840, and 2081 cm-1 as shown in Figure 2 (since similar results were observed with system I, here we present results obtained from system II only). The peaks at 2081 and 1840 cm-1 can readily be assigned to CO adsorbed on atop and bridging sites of Pt(111), respectively.33-36 Although the position of the peak at 1491 cm-1 is very close that of carbonate species, the formation of carbonates following the adsorption of CO on BaO/ Pt(111) is ruled out for the following reasons: (i) No evidence for CO dissociation was observed; therefore, there were no O species present on the surface to oxidize CO to form CO2, and CO could not react with BaO to form BaCO3. (ii) BaCO3 formed on this system following the adsorption of CO2 shows a very weak IR feature centered at 1387 cm-1. (iii) BaCO3 formed on this system decomposes above 700 K; however, the IR feature observed at 1491 cm-1 disappears after annealing the sample to 600 K. Similar IR features were observed previously following the adsorption of CO on oxide surfaces and were assigned to carbonite ions (CO22-).37-41 Formation of carbonite species has been observed on MgO, CaO, SrO, and BaO as a result of the interaction between CO and coordinatively unsaturated O2- ions.37-41 The formation of CO22- ions by the reaction of surface basic oxygen ions of ThO2 with CO molecules was proposed by Lamotte et al.42 This carbonite formation was also

observed on La2O3 and Pt/CeO2.43,44 On the ceria surface, three types of carbonite species were observed and one of them showed a vibrational feature at 1465 cm-1.45 Furthermore, these carbonites were reported to form via a nonredox reaction of CO with O2- surface sites.45 The carbonite species formed on CaO show IR peaks at 1480-1485, 850-890, and 717-743 cm-1.40 However, due to the CaF2 windows (transmission cutoff ∼1100 cm-1) used in the IR cell, we cannot detect IR signals at wavenumbers below 1100 cm-1. Therefore, on the basis of previously reported results we assign the IR feature at 1491 cm-1 to carbonite ions (CO22-) formed by the interaction between CO and coordinatively unsaturated O2- of BaO located at the Pt-BaO interface.40 The proposed structure for this carbonite species derived from previously reported structures is shown in Figure 3a.37,40 These carbonite species are absent when the Pt surface is completely covered by BaO. The IR peak observed at 1712 cm-1 following the adsorption of CO on BaO/Pt(111) is most likely due to a tilted CO species adsorbed through both the carbon (to Pt) and oxygen (to Ba2+) ends, as reported previously.44 The proposed structure for this tilted CO species is depicted in Figure 3b.46,47 There have been many studies reported on the formation of tilted CO on metal-oxide systems. A tilted CO species, in addition to linear and bridged CO, was identified to give broad bands in a frequency range of 1700-1765 cm-1 on Pt/CeO2 following the adsorption of CO. These features were eliminated by annealing to 573 K.44 Similar peaks were reported in the literature on Rhsupported catalysts.47 IR bands observed at ∼1725 and 1626 cm-1 on Rh-CeO2/SiO2 and Rh/SiO2, respectively, were attributed to a C- and O-bonded species located at the metal/ support interface.47 Following the adsorption of CO on Pt-Na/ CeO2, two bands were observed at 1776 and 1690 cm-1, whereas on Pt/CeO2 only the band 1690 cm-1 was present.46 These bands were also attributed to a C and O coordinated CO.46 In these species, C bonds to Pt and O bonds to Na+ (∼1776-1790 cm-1), and Cen+ (∼1690 cm-1).46 A band observed at 1728 cm-1 following the adsorption of CO on Pd-Li/SiO2 was also attributed to the C- and O-bonded CO.46 Therefore, on the basis of the wealth of these previously reported results, the IR band at 1712 cm-1 observed in our systems can be assigned to a tilted CO at the Pt-BaO interface. The intimate contact between Pt and BaO at the interface on the BaO/Pt(111) system may facilitate the coordination of both the C and O ends of the CO molecule. The CO desorption peak observed at 378 K following the adsorption of CO on BaO/Pt(111) can be assigned to CO adsorbed on BaO-free Pt(111) sites because this peak overlaps with that obtained following the adsorption of CO on Pt(111). This is further confirmed by the IR spectroscopic data shown in Figure 2, where IR features (2081 and 1840 cm-1) responsible for CO adsorbed on Pt(111) sites disappeared after annealing to 400 K. The higher temperature CO desorption peak at 518 K, which appears only in the presence of BaO, is associated

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with the BaO layer. This desorption feature is due to the CO derived from the decomposition of carbonite species and the tilted-CO adsorbed at the Pt-BaO interface. Even though deconvolution of the TPD peaks cannot be done precisely, a comparison of the underlying peak areas of the TPD spectra shows that approximately 45 and 60% of the Pt(111) surface is covered by BaO in systems I and II, respectively. This estimation is based only on the site blocking effect by BaO but electronic effects could also influence the adsorption of CO on these BaO/ Pt(111) model systems. These results, as well as IRAS data (not shown) obtained following the adsorption NO and NO2, clearly indicate that the Pt(111) surface is not completely covered with BaO, and hence that sites on the Pt(111) surface are available for the adsorption of gaseous species. Although these results provide clear measure for the fraction of BaO-free Pt(111) surface, they cannot give any information about the geometrical structures and morphologies of the deposited BaO layers. These data could only be obtained from scanning probe measurements which were not available to us in this study. 4.2. Adsorption of O2 on BaO/Pt(111) at 300 K. It is wellknown that O2 dissociates on the clean Pt(111) surface to form a p(2×2) oxygen adlayer at 300 K, corresponding to 0.25 monolayer coverage.48-51 Therefore, O2 should also dissociate to form atomic O on these BaO/Pt(111) systems, due to the presence of Pt(111) sites for the adsorption of O2 as described in the previous section. BaO covers approximately 45% of the Pt(111) surface in system I, as determined by comparison of CO TPD peak areas obtained following the adsorption of CO on Pt(111) and BaO/Pt(111) at 300 K. First we performed TPD experiments following the adsorption of O2 on system I at 300 K to identify the nature of the Oad species formed. Figure 4a shows a series of TPD spectra (mass fragment 32) obtained following the exposure of system I at 300 K to increasing amounts of O2. At the lowest O2 exposure, a single peak is observed with a maximum desorption rate at 760 K. With increasing O2 exposure, a new desorption feature appears at a lower temperature. At the highest exposure, two partially resolved features observed at 690 and 760 K, indicating the presence of at least two types of adsorbed atomic oxygen on the BaO/Pt(111) system I. The O2 TPD spectrum obtained from the p(2×2)-O layer on Pt(111), which is aligned with the low temperature desorption peak observed at ∼690 K following the highest O2 exposure on BaO/Pt(111), is also displayed in Figure 4a for comparison. It is well-known that at coverages of 0.25 monolayer or lower, oxygen desorbs from the clean Pt(111) surface with a maximum rate near 700 K.48,52 Therefore, the feature centered near 690 K can be assigned to the recombinative desorption of OPt from the Pt(111) sites. The TPD feature centered around 760 K may represent adsorbed oxygen on the BaO clusters, O of BaO2, or Oint at the Pt-BaO interface. There is no prior experimental evidence reported for the presence of adsorbed O or O2 on BaO for the BaO/Pt(111) systems.20,53 We also do not observe any evidence for the presence of BaO2 in the XP spectra as shown in Figure 4b, or 5b, which shows O 1s peaks in the range 528.9-529.4 eV, very different from the value previously reported for the O 1s binding energy in BaO2/Pt(111) (533 eV) and in powder BaO2 (533 eV).8,47,48 A binding energy of 530.8 eV was also reported for O 1s of BaO2,54 but even this value is not consistent with the one we observed (528.9-529.4 eV in Figures 4b and 5b) in this study. In addition, previously reported results have shown that the BaO2 layer formed on Pt(111) was metastable at 573 K.53 Therefore, if BaO2 formed during the exposure of the BaO/Pt(111) system to O2 at 300 K, it should decompose and release O, giving O2

Mudiyanselage et al.

Figure 4. (a) TPD spectra obtained following the adsorption of O2 on the BaO/Pt(111) (system I) at 300 K with increasing exposures (PO2(back) ∼ 3.0 Torr; tO2 ) 10, 30, 60, 180 s) for mass fragment 32 (O2). The O2 TPD spectrum (PO2(back) ∼ 3.0 Torr; tO2 ) 210 s) obtained from the p(2×2)-O on Pt(111) is also displayed in (a) for comparison. (b) O 1s XP spectra collected following exposure of BaO/Pt(111) (system I) to O2 at 300 K and subsequent annealing to the indicated temperatures (PO2(back) ∼ 3.0 Torr; tO2 ) 180 s).

feature in the TPD spectrum at lower temperature than that observed in the present study. Bowker et al. observed the irreversible transformation of metallic Ba particles deposited onto Pt(111) to BaO, and partial conversion of the thin Ba film into metastable BaO2, which was lost when the oxygen atmosphere was removed.20 It seems that the preparation method strongly influences the composition of the BaOx (BaO or BaO2) layer. In our study, we deposited Ba on a N2O4 layer at 90 K and then annealed this system to 1000 K, which leads to the formation of BaO particles. All these results allow us to rule out the formation of BaO2 under the applied experimental conditions in our study. Therefore, the sharp O2 TPD peak centered at 760 K is most probably due to Oint at the Pt-BaO interfacial sites. This conclusion is further supported by the absence of O2 desorption at this temperature (i) from the (Oad(0-0.75 monolayer)/Pt(111)) systems,55 and (ii) following the adsorption of O2 on the systems where the Pt(111) surface was completely covered by BaO (BaO(∼3 MLE)/Pt(111) and BaO(>20 MLE)/Pt(111)).18 The narrow shape of the O2 desorption feature at 760 K is attributed to the simultaneous migration of Oint from the interfacial sites to the Pt(111) sites and recombinative explosive


Figure 5. (a) TPD spectra obtained following the adsorption of O2 on the BaO/Pt(111) (system II) at 300 K with increasing exposures (PO2(back) ∼ 3.0 Torr; tO2 ) 5, 10, 15, 30, 45, 60, 120, 180 s) for mass fragment 32 (O2). The O2 TPD spectrum (PO2(back) ∼ 3.0 Torr; tO2 ) 210 s) obtained from the p(2×2)-O on Pt(111) is also displayed in (a) for comparison. (b) O 1s XP spectra collected following exposure of BaO/ Pt(111) (system II) to O2 at 300 K and subsequent annealing to the indicated temperatures.

desorption of O from Pt(111) sites. This explosive desorption occurs due to the fact that the temperature where the Oint becomes mobile is higher than the recombinative desorption temperature of OPt from Pt(111) (i.e., these Oint are strongly bound to the Pt-BaO interface). Therefore, as soon as Oint on the Pt-BaO interface are released, they reach the Pt(111) sites, recombine to O2 and desorb rapidly. Due to the high population of the Pt-BaO interface by Oint, the oxygen coverage on the BaO-free Pt sites remains appreciably high, even at temperatures higher than that of the maximum O2 desorption rate on Pt(111). The consequence of this relatively high OPt coverage is the observed explosive desorption. Similar explosive-desoption of O2 was observed for the decomposition of PtO2 domains formed following the oxidation of Pt(111) by gas phase O atoms at 450 K.52 The O2 desorption peak arising from the decomposition of PtO2 was seen to shift to higher temperature in comparison to the desorption temperature of chemisorbed Oad on Pt(111).52 Weaver et al. also found that these platinum oxide domains were less active toward the oxidation of CO than chemisorbed oxygen present at lower coverages on Pt(111).52 Although we observe similarities for the explosive desorption of O2 between PtO2 and interfacial Oint formed on BaO/Pt(111), it is unlikely that

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20199 PtO2-like domains formed under the mild conditions (O2 as oxidant, PO2 ∼ 2.0 × 10-9 Torr; 300 K) applied in our experiments. The decomposition temperature of the PtO2 domains was seen to continuously shift toward higher temperature with increasing atomic oxygen exposure, which was attributed to the formation of increasingly larger PtO2 domains. Furthermore, since the Pt substrate was slowly converting to PtO2, the desorption peak intensity never saturated. Both of these observations are in contrast to our findings: the temperature of the explosive desorption peak did not shift with O2 exposure, and it saturated before the BaO-free Pt(111) surface formed the saturated OPt layer. Also, Weaver and co-workers find that p(2×1) domains and Pt oxide chains form on Pt(111) before the growth of bulk oxide and the concomitant development of an explosive O2 TPD peak.52,56 Oxygen desorption from the p(2×1) and oxide-chain phases produces relatively broad O2 TPD features at temperatures of ∼650 and 580 K, respectively. The absence of such features in the O2 TPD spectra obtained in the present study provides strong evidence that oxidation of the Pt(111) substrate does not occur in our experiments with BaO/Pt(111). Libuda et al. have shown the formation of a thin Pd interfacial oxide layer on Pd/Fe3O4.22 They also reported the migration of O from a thin Pd interfacial oxide layer, which was stabilized by the support, to a metallic Pd surface.22 This study further showed reversible accumulation of large amounts of oxygen in the form of a thin Pd oxide layer at the metal/oxide interface of Pd/Fe3O4. Oxygen from this interfacial oxide layer was released and migrated back onto the metallic Pd surface, where it then was available for CO oxidation reaction.22 The possibility of the formation of a Pt oxide interfacial layer in our BaO/Pt(111) systems cannot be completely ruled out, although we did not detect a shoulder at ∼76.9 eV binding energy in the Pt 4f peak which would suggest the formation of Pt oxide, as it was reported by Weaver et al.,52 as well as Parkinson et al.57 on Pt(111). Even if this interfacial oxide layer was there, due to the low oxygen coverage in our system, this shoulder may not be detected. However, it is unlikely to form a Pt oxide interfacial layer in BaO/Pt(111) system under the mild conditions (O2 as an oxidant, PO2 ∼ 2.0 × 10-9 Torr; 300 K) applied in this study. The integrated area of the O2 peak in the TPD spectrum obtained following the highest O2 exposure of BaO/Pt(111) is ∼30% higher than that obtained on bare Pt(111) (p(2×2)-O layer), although a fraction of the Pt(111) surface is covered by BaO. A similar result was observed previously following the adsorption of O2 on BaO/W(001). The adsorption probability of O2 on W(001) has been shown to be enhanced by the presence of BaO and the Oad coverage on BaO/W(001) was greater than that on bare W(001), although a fraction of the W(001) surface was covered by BaO.21 The excess O on this system appeared to be bound to the W(001) substrate, as suggested by the UPS results.21 At the lowest O2 exposure of BaO/Pt(111), only the high-temperature O2 TPD feature was observed at 760 K as shown in Figure 4a. With increasing O2 exposure, a second O2 desorption feature appears around 690 K. These observations indicate that O first occupies the Pt-BaO interfacial sites, then it starts filling bare, BaO-free Pt(111) sites. Most probably, O2 dissociates on bare Pt(111) sites and then migrates to the energetically more favorable Pt-BaO interfacial sites. In this case, the higher oxygen coverage obtained on the BaO/Pt(111) surface compared with clean Pt(111) implies that the interfacial sites accommodate higher local oxygen coverages than the Pt(111) domains.

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Figure 4b shows a series of O 1s XP spectra collected following exposure of BaO/Pt(111) system I to O2 at 300 K followed by stepwise annealing to the indicated temperatures. The O 1s XP spectrum obtained following the adsorption of O2 at 300 K shows a peak centered at 529.4 eV with a shoulder at ∼531.0 eV. These O 1s peaks can be associated with Oad on Pt(111) and at the Pt-BaO interface, as well as with the O2ions in BaO. The shoulder near 531.0 eV originates from the OPt on Pt(111), since it disappears upon annealing the sample to 690 K. Additional confirmation of the assignment of this shoulder to Pt-adsorbed oxygen atoms was obtained by acquiring an O 1s XP spectrum from a BaO-free Pt(111) surface exposed to O2 at 300 K. The binding energy of this OPt species was measured at 530.0 eV, and this value was used for the deconvolution of the O 1s features (for both systems I and II) after saturation exposure of O2 (see insets in Figures 4b and 5b). Further annealing to 1000 K leads to the desorption of the entire remaining Oad, resulting in the disappearance of most of the intensity of the O 1s feature at ∼529.0 eV. The low intensity still remaining is associated with the O2- ions in the BaO particles. On the basis of the results of these TPD and XPS experiments, however, we cannot rule out the possibility of the decomposition (full or partial) of BaO at higher temperatures (i.e., above 690 K). To determine whether BaO decomposes at higher temperatures (700-1000 K), producing the O2 desorption feature at 760 K, we performed the following experiments. First, we converted the BaO/Pt(111) system completely to Ba(OH)2/ Pt(111) by exposing the sample to H2O at 300 K. As the results of our previous study have shown, H2O readily reacts with BaO to form Ba(OH)2.19 Then this Ba(OH)2/Pt(111) system was heated to 1000 K under the same conditions as in the TPD experiments discussed above for the O2-exposed sample. In the TPD spectrum, however, no desorption of O2 was observed. If BaO clusters supported on the Pt(111) substrate decompose in the 700-1000 K temperature range, we should have observed O2 desorption feature(s) in the TPD spectrum following the decomposition of Ba(OH)2 to BaO at around 500 K. Therefore, this observation rules out the decomposition of BaO during annealing to 1000 K temperature, in agreement with the results of previous studies on annealing the BaO/Al2O3/NiAl(110)16 and BaO/Cu(111)58 systems to 1000 K. Adsorption of O2 at 300 K on the BaO/Pt(111) (system II) was also performed to further explore the nature of the Oad species using TPD and XPS. Figure 5a shows a series of O2 TPD spectra obtained following the adsorption of O2 on the BaO/Pt(111) (system II) at 300 K as a function of O2 exposure. At the lowest O2 exposure studied, a single peak is observed at 794 K. With increasing O2 exposure, a new desorption feature appears around 760 K, similar to the high temperature feature observed on system I (Figure 4a). Further increase of O2 exposure leads to the development of a shoulder at around 690 K. At the highest O2 exposure, three O2 desorption features are observed at 690, 756, and 794 K, suggesting the presence of at least three types of adsorbed oxygen on this particular BaO/ Pt(111) system. In accord with system I, we assign the feature centered around 690 K to the desorption of OPt from the BaOfree Pt(111) sites, while we assign those desorbing at peak temperatures of 756 and 794 K to the Oint at the Pt-BaO interfacial sites. The origin of the appearance of two desorption features at 756 and 794 K, instead of the one feature at ∼760 K observed in system I, is most likely due to the presence of two types of Oint as schematically shown in Figure 3c. At low BaO coverages only one high-temperature O2 desorption feature appears at ∼760 K, which is assigned to the Oint at the Pt-BaO

Mudiyanselage et al. interface (at the boundary around the BaO islands on the Pt(111) surface). The second high-temperature O2 desorption feature at 794 K may be assigned to the recombinative desorption of Oint that originates from the BaO-Pt-BaO interface. These sites form as the BaO island density on the Pt(111) surface increases, and oxygen molecules dissociate on Pt occupy sites between neighboring BaO clusters. The formation of BaO2 on BaO/ Pt(111) (system II) can be ruled out for the same reasons described in the previous section for system I. Interestingly, the underlying integrated peak area of O2 TPD spectra obtained with the highest O2 exposures on systems I and II are comparable. Figure 5b shows O 1s XP spectra collected following exposure of the BaO/Pt(111) (system II) to O2 at 300 K and subsequent annealing to the indicated temperatures. The O 1s XP spectrum obtained following the adsorption of O2 at 300 K shows a peak centered at 529.3 eV with a shoulder around 531.9 eV. After annealing to 600 K, this shoulder disappears and the O 1s peak shifts to 528.9 eV, although no O2 desorption occurs on this system below 600 K temperature. The O 1s peak (528.9 eV) present in the XP spectrum represents Oad on both Pt(111), and Pt-BaO interface and the O2- in BaO. After annealing to 690 K, the O 1s peak shifts to 528.7 eV and the total intensity also reduces slightly due to the desorption of OPt from Pt(111), and probably, a small fraction of Oint from the Pt-BaO interface. Part of the remaining Oint desorbs after annealing to 760 K. Annealing to 1000 K leads to the desorption of all remaining Oint. Following an annealing step to 1000 K, an O 1s XP peak appears at 528.9 eV, characteristic of the O2- in BaO. 4.3. Simulations of Explosive O2 Desorption from BaO/ Pt(111). We find that a relatively simple kinetic model can reproduce the main features seen in O2 TPD spectra obtained from the BaO/Pt(111) surfaces that were investigated. To describe O2 desorption from system I, the model assumes that adsorbed oxygen atoms initially populate two binding states on the BaO/Pt(111) surface. One of these states corresponds to oxygen atoms adsorbed on Pt(111) domains (OPt), and the other represents more strongly bound oxygen atoms located at the perimeter of BaO clusters (“interfacial” oxygen: Oint). The model neglects the direct desorption of interfacial oxygen and instead considers that oxygen atoms migrate from interfacial sites to the Pt(111) domains and that recombinative desorption occurs only from the Pt(111) domains. The implication is that direct desorption of the interfacial oxygen atoms is energetically prohibitive compared with migration. The steps considered in the model may be represented by the following kinetic scheme,

2OPt f O2(g) Oint f OPt

(d) (m)

Here, OPt represents oxygen atoms adsorbed on Pt(111) domains and Oint represents oxygen atoms bound at the Pt-BaO interface. In the model, the total rate of desorption is set equal to the product of the local desorption rate from Pt(111) domains and the fractional area of the surface covered by these domains. We describe the total desorption rate (rd) as a second-order process using the following equation, rd ) kdΘPt2fPt where kd represents the rate coefficient for desorption, ΘPt represents the local coverage of oxygen atoms on Pt(111) domains, and fPt represents the fractional surface area of exposed Pt(111). We describe the rate of migration (rm) using the equation, rm ) km(1 - ΘPt/



Θmax Pt )Θintfint, where km represents the rate coefficient for migration, Θint represents the local oxygen coverage at interfacial sites, max represents the maximum coverage of oxygen atoms on ΘPt the Pt(111) domains, and fint represents the fractional surface area covered by interfacial sites. Migration is taken to be a firstorder process and we assume that the probability for migration scales linearly with the probability of finding an empty site on the Pt(111) domains, which is set equal to that for a randomly distributed lattice gas. (Note that the migration in this system is fundamentally different from “site hopping” of adsorbates on low index metal crystal surfaces and may better be described as a chemical reaction as chemical bonds are broken (e.g., Oint-Ba) during the course of “migration” of interfacial oxygen atoms. Therefore, the energetic of this “migration” is vastly different from that of the conventional migration by site hopping.) We assume that the rate coefficients follow the Arrhenius equation, kj ) νj exp(-Ej/RT) where νj and Ej represent the pre-exponential factor and activation energy for process j, respectively. The activation energy for migration is taken as a constant in the model, whereas we assume that the activation energy for desorption from Pt(111) decreases linearly with the local oxygen coverage according to the equation, Ed(kJ/mol) ) 233 - 101ΘPt (monolayer). Campbell et al.59 have shown previously that the atomic oxygen binding energy on Pt(111) decreases with increasing coverage, and that this dependence is well represented by a linear equation near the temperatures for desorption. We determined parameters for the Ed equation by setting νd ) 3 × 1013 s-1 monolayer-1 and optimizing the agreement between simulated and experimentally determined O2 TPD spectra obtained from Pt(111) covered initially with 0.25 monolayer of oxygen atoms. We obtain excellent agreement between the simulated and experimentally determined TPD spectra for oxygen-covered Pt(111)(not shown). The parameters that we use to describe O2 desorption from Pt(111) agree well with those reported previously.52 To simulate O2 TPD spectra obtained from the BaO/Pt(111) surface, we numerically integrated the following balance equations for the adsorbed oxygen species,

dCPt ) (rm - rd)/β dT

and

dCint ) -rm /β dT

Here, β represents the heating rate and CPt and Cint represent the total coverages of oxygen atoms adsorbed on Pt(111) domains and interfacial sites, respectively. The total oxygen coverage on the surface is given by Ctot ) CPt + Cint. We calculate the local coverages at each integration step by evaluating the equations, CPt ) ΘPtfPt and Cint ) Θintfint and assuming that the fractional surface areas fPt and fint remain constant during TPD. Figure 6a shows the simulated O2 TPD spectrum that best matches the TPD spectrum obtained experimentally from the BaO/Pt(111) surface (system I) for an initial total oxygen coverage of 0.30 monolayer. This comparison reveals that the kinetic model accurately reproduces the key features of the O2 TPD spectra obtained from system I. In particular, the initial portions of the simulated and experimental TPD traces closely overlap, and the model predicts the emergence of an explosive desorption feature. The model slightly overestimates the width of the explosive peak and also underestimates the desorption rate along the trailing edge of the spectrum. Several factors could cause these differences, including greater surface heterogeneity than considered in the model as well as additional steps in the

Figure 6. Simulated O2 TPD spectra for (a) BaO/Pt(111) (system I) with an initial oxygen coverage of 0.30 monolayer and (b) BaO/Pt(111) (system II) with an initial oxygen coverage of 0.32 monolayer. The simulated and experimental spectra are shown as red and black lines, respectively. The TPD spectra shown here were generated for the maximum oxygen coverages obtained experimentally. The model parameters used in these calculations are listed in Table 1.

actual desorption pathway. Overall, the agreement between the simulated and experimentally determined TPD spectrum is quite good, especially considering the simplicity of the model. This agreement suggests that the model accurately captures the essential kinetic processes that govern O2 desorption from the BaO/Pt(111) surface (system I). According to the model, the initial portion of the broad desorption feature centered at ∼690 K arises almost entirely from the desorption of oxygen atoms that are initially adsorbed on the Pt(111) domains. The migration rate is very low over the initial portion of the TPD curve but starts to increase rapidly with temperature above about 700 K. This rapid migration of oxygen from interfacial sites to the Pt(111) domains causes the local oxygen coverage on the Pt(111) domains to decrease more slowly with temperature over a narrow range (∼25 K), which in turn causes the O2 desorption rate to increase sharply due to the steep rise in kd with temperature. The peak maximum at 760 K is reached once the supply of interfacial oxygen starts to deplete and the local coverage on the Pt(111) domains begins decreasing quickly again. We obtained the optimum fit for system I by applying physically reasonable constraints and examining how each model parameter influences the desorption traces. For example, we find that the explosive peak becomes sharper within

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Mudiyanselage et al.

TABLE 1: Model Parameters Used in the Simulations of O2 TPD Spectra from BaO/Pt(111)a BaO/Pt(111) system I system II

Ctot(initial) (monolayer) 0.30 0.32

νm (s-1) 16

10 1016

Em (kJ/mol)

fPt

fint

ΘPt(initial) (monolayer)

Θint(initial) (monolayer)

243.7 240.5 (255.5)

0.73 0.60

0.11 0.10 (0.09)

0.265 0.215

1.0 1.0

a The values in parentheses correspond to parameters used to describe the type II interfacial oxygen that is assumed to exist on system II at max high BaO coverages. Both type I and type II oxygen are present on system II. A value of ΘPt ) 0.5 monolayer was used in all of the calculations.

increasing values of the migration prefactor. To determine an upperbound for νm, we used the transition state theory formula and assumed that the maximum prefactor would be obtained when an adsorbed oxygen atom transforms from a fully constrained state to a state with free translation in twodimensions. This calculation suggests a maximum prefactor of νm ∼ 1016 s-1 at 750 K. Table 1 lists the best fit parameters for the model. With the migration prefactor fixed, we then adjusted the migration activation energy to reproduce the peak temperature (760 K) of the explosive desorption feature. We also find that the initial values of CPt and Cint determine the relative intensities of the broad desorption feature near 690 K and the explosive peak at 760 K, respectively, while the initial local coverage (ΘPt) on Pt(111) domains has a significant influence on the leading edge of the desorption feature. For system I, we obtain the best agreement between the simulated and experimental TPD traces using a value of fPt ) 0.73, which is higher than the value of 55% estimated from the CO TPD traces. Finally, the model suggests that 0.11 monolayer of oxygen atoms is initially present at interfacial sites of system I when the total oxygen coverage is 0.30 monolayer. The BaO clusters would need to be small, though of a physically reasonable size, to accommodate this amount of oxygen at their perimeters. For example, assuming that the maximum local coverage of oxygen at the interfacial sites is 1 monolayer, the model suggests that the perimeter of the BaO-Pt(111) domains represents 11% of the total surface area, and thus nearly 40% of the exposed area of the BaO clusters. In this case, a 0.44 nm thick ring at the perimeter of a 2 nm BaO cluster would represent the interfacial area on which oxygen atoms adsorb. To simulate O2 TPD spectra for system II, we included a second type of interfacial oxygen (type II) and used a higher activation energy for migration compared with the type I interfacial oxygen. Figure 6b shows the simulated O2 TPD spectrum that best matches the TPD spectrum obtained experimentally from system II for an initial oxygen coverage of 0.32 monolayer. As expected, including the more strongly bound type II oxygen produces a second sharp peak in the simulated TPD spectrum. Since the model overestimates the widths of the explosive peaks, the maximum intensities of these peaks are smaller than those observed experimentally. The simulated and experimentally determined TPD spectra for system II exhibit good agreement overall. Similar to our findings for system I, the best fit for system II occurs for a value of fPt that is higher than that estimated from the CO TPD traces (fPt ) 0.60 vs 0.40). The origin of this difference is difficult to ascertain from the available data and may only reflect the approximate nature of the model. The success of the desorption model in reproducing the main features of the measured O2 TPD spectra lends support to the conclusion that distinct types of sites are available at the Pt-BaO interface for binding oxygen and that the release of these oxygen atoms onto the Pt(111) domains is responsible for the explosive TPD peaks.

4.4. DFT Calculations on O2 Adsorption on BaO/Pt(111). To gain more insight into the oxygen adsorption in the vicinity of supported BaO clusters, Bader charge analysis was performed for the (BaO)2/Pt(111) and (BaO)4/Pt(111) systems. Pronounced charge transfer from the supported BaO clusters toward the Pt(111) substrate was found to occur upon the deposition of BaO clusters. The supported BaO clusters are positively charged; +0.86 |e| for (BaO)2 and +1.24 |e| for (BaO)4 on the Pt(111) surface. These results indicate a higher extent of charge transfer from the larger BaO clusters toward the Pt(111) support. Bader charge analysis also showed that the charges on the Ba ions in the supported BaO clusters are the same (+1.46 |e|), regardless of the cluster size. As a result, all the electron charge transferred from the BaO clusters to the Pt(111) support come from the oxygen ions of the BaO clusters. The charges on the two oxygen atoms of the supported (BaO)2 dimer are equal (-1.02 |e|), while that for the four oxygen atoms in the supported (BaO)4 tetramer are -0.87 |e|, -1.06 |e|, -1.36 |e|, -1.37 |e|, depending on the position of the oxygen ion with respect to the Pt(111) surface (the one in the bottom, closest to the metal surface, loses more electrons). On the other hand, the Pt atoms that closely connect to the supported BaO clusters become electron-rich (-0.1 to -0.3 |e|) by accepting electrons from the clusters. Compared to the BaO-free, clean Pt(111) surface, the interfacial electronrich surface Pt atoms provide sites for oxygen adsorption with an increased electron density. Thus the second peak in our oxygen TPD profile may well originate from the recombinative oxygen desorption of Oint at the Pt-BaO boundary. This observed charge transfer from the oxygen atoms of the BaO clusters toward the Pt(111) substrate may also explain the very similar binding energies of different oxygen species (adsorbed O atoms and oxygen ions in BaO) in the XP spectrum discussed above. The calculated adsorption energies of oxygen atoms, (from the dissociation of an O2 molecule) are 2.58 and 2.47 eV on (BaO)2/Pt(111) and (BaO)4/Pt(111), respectively (Figure 7). Compared to the adsorption energy of 2.14 eV on the pure Pt(111) surface, the adsorption of oxygen atoms at the boundary between the supported BaO cluster and Pt(111) is stronger. This is consistent with our TPD results that exhibit an additional O2 desorption peak at 760 K when BaO was deposited onto the Pt(111) surface in an amount insufficient to cover the entire metal surface. 4.5. Adsorption of CO on O-BaO/Pt(111). To understand the reactivity of Oad on BaO/Pt(111), we prepared O-BaO/ Pt(111) systems following the adsorption of O2 on BaO/Pt(111) (system II) at 300 K and investigated their reactivities in the CO oxidation reaction at the temperatures specified in Figure 8. Figure 8a shows the changes in the 44 amu mass signal (CO2) as a function of CO exposure time when the O-BaO/Pt(111) sample kept at the indicated temperatures was exposed to a constant CO flux. During exposure of the O-BaO/Pt(111) to CO at 200 K, no CO2 desorption was observed. However, desorption of CO2 is instantaneous at the onset of CO exposure



Figure 7. DFT-predicted models for the adsorption of two atomic oxygens on (a) Pt(111), (b) (BaO)2/Pt(111), and (c) (BaO)4/Pt(111) surfaces. Pt atoms are in blue, Ba atoms are in green, and oxygen atoms are in red.

at 250 K and higher temperatures, and the CO2 desorption rate increases with temperature. The formation of CO2 at 250 K agrees with a previous study, which reported the CO oxidation reaction with disordered O atoms took place on Pt(111) at ∼225 and ∼260 K whereas this reaction occurred at 320 K with p(2×2)-O on Pt(111).60 Therefore, the CO2 observed during CO exposure at 250 K is derived from the reaction between CO and disordered OPt on the Pt(111) sites of BaO/Pt(111). The CO2 evolution curve obtained during the adsorption of CO at 400 K (Figure 8a) indicates the presence of three CO2 evolution features at 0-3, 3-5, and >5 s, time intervals from the onset of CO exposure. This observation of three CO2 evolution features suggests the presence of three CO2 formation regimes, which may be associated with the three types of Oad (OPt on Pt(111) sites and two types of Oint at the Pt-BaO interface) on the BaO/Pt(111) surface exhibiting fundamentally different reactivities in CO oxidation. The results of postexposure (postreaction) TPD may allow us to identify these three types of O species. The postreaction O2 TPD curve, obtained following the exposure of O-BaO/Pt(111) to CO at 400 K, does not show any O2 desorption features, indicating the absence of unreacted O species on the BaO/Pt(111) system. However, CO2 desorption

features were observed at 496 and 777 K in the postreaction TPD as shown in Figure 8c. The CO2 desorption feature at 496 K is most likely due to the formation of CO2 from the reaction between carbonite species and a small fraction of Oint responsible for the desorption feature at 795 K, which did not react during CO exposure at 400 K. The CO2 desorption feature at 777 K is due to the decomposition of BaCO3 formed from the reaction between CO2, formed in the oxidation of CO, and BaO. We observed a similar CO2 desorption feature following CO2 adsorption on BaO/Pt(111) systems due to the formation of BaCO3. A single CO2 peak at ∼773 K in the TPD spectrum was also observed following the exposure of BaO/Cu(111) to CO2 at 323 K.58 The CO2 evolution curve obtained during the adsorption of CO at 300 K (Figure 8a) indicates the presence of two CO2 evolution features in the time intervals 0-3 and 3-5 s from the onset of CO exposure. The postreaction O2 TPD traces in Figure 8b show that exposing the O-BaO/Pt(111) sample to a constant CO flux at 300 K results in the removal of O atoms that are responsible for the desorption feature at 754 K and the shoulder around 690 K. However, a fraction of Oint that is responsible for the O2 desorption at 754 K reacts with adsorbed CO to release CO2 around 388 K during annealing as

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Figure 8. (a) Changes in the 44 amu mass signal (CO2) as a function of CO exposure time as the O-BaO/Pt(111) sample kept at the indicated temperatures was exposed to a constant CO flux. TPD spectra for (b) O2, (c) CO2, and (d) CO obtained following exposure to CO at the indicated temperatures. (Mass spectrometer signal collection was started at t ) 0 s, and the CO flux was initiated at t ) 12 s.)

Mudiyanselage et al. shown in Figure 8c. Therefore, only a fraction of Oint associated with the desorption feature at 754 K and all the OPt on Pt(111) react with CO and releases CO2 during CO exposure at 300 K. In addition, a small amount of Oint and adsorbed CO react to form weakly held carbonates that decompose with the release of CO2 at around 633 K. During the exposure of CO at 200 K no CO2 evolution was observed. However, adsorbed CO at 200 K reacts with O during the temperature ramp in the postexposure TPD experiment and releases CO2 at 268 and 388 K as shown in Figure 8c. The CO2 desorbed at 268 K originates from the reaction of CO and OPt on Pt(111). The remaining CO on the system reacts with Oint associated with the desorption feature at 754 K. In addition, a small amount of CO desorbs as shown in Figure 8d. At 250 K, CO reacts with OPt on Pt(111) and evolves CO2 during CO exposure and the remaining adsorbed CO reacts during the TPD experiment and releases CO2 at 388 K. The CO2 desorption feature observed around 633 K during TPD experiments following the exposure of O-BaO/Pt(111) to CO at 200, 250, and 300 K is most likely due to the weakly held carbonate. It seems that this weakly bound carbonate is formed only in the presence of Oint. The CO2 desorption feature at 633 K was not observed following the adsorption of CO at 400 K, due to the absence of O on the BaO/Pt(111) system, although a CO2 desorption feature was observed at 496 K due to the reaction of carbonite with the residual Oint on the system. The CO2 desorption feature at 777 K originates from the decomposition of BaCO3 formed from the reaction between CO2, which derived from CO oxidation, and BaO. However, the CO2 desorption feature at ∼777 K was not observed in the TPD experiments that were performed following the adsorption of CO on O-BaO/Pt(111) at or below 300 K temperatures, indicating the absence of strongly bound BaCO3. These data show that when the Oint responsible for the desorption feature at 795 K is present following the adsorption of CO at or below 300 K temperatures, strongly bound BaCO3 is absent. Therefore, it seems that the presence of Oint either reduces the thermal stability of barium carbonate or inhibits the formation of strongly bound BaCO3. Figure 8d shows CO TPD spectra obtained following the adsorption of CO on O-BaO/Pt(111) at the indicated temperatures. Initially, at a high O coverage, there are few sites available for CO adsorption. After CO exposure at 200 K, a fraction of adsorbed CO desorbs around 300 K and the other part reacts with OPt to release CO2. Similarly, a fraction of adsorbed CO desorbs at ∼370 K and the remaining CO reacts with OPt and releases CO2 at the same temperature. After the adsorption of CO on O-BaO/Pt(111) at 300 K, unreacted CO desorbs in two features at ∼370 and ∼490 K temperatures. During the CO exposure at 300 K, a fraction of OPt reacts with CO to form CO2. This reaction makes more Pt sites available for further CO adsorption and a fraction of these adsorbed CO desorb around 370 K and the rest reacts with Oint to form CO2, which, in turn, desorbs immediately after the formation (reaction limited desorption) around 388 K as shown in Figure 8c. The CO TPD feature at 370 K is aligned with the CO desorption from clean Pt(111). Therefore, the feature at ∼370 K can be assigned to adsorbed CO on the BaO-free Pt(111) sites on the BaO/Pt(111) system. The weak feature at around 494 K is due to the carbonite species as described earlier. CO consumes almost all the Oad to form CO2 during CO exposure at 400 K. This reaction makes Pt sites as well as Pt-BaO interfacial sites available for CO adsorption. However, due to the desorption



of CO from Pt(111) sites below 400 K, only the CO associated with carbonite species desorb around 494 K as shown in Figure 8d. The data shown in Figure 8 suggest the presence of several CO oxidation regimes or pathways as well as different types of O with respect to the reactivity toward CO oxidation reaction. Although all the CO oxidation pathways cannot be characterized precisely due to the complexity of the system, three main processes and associated Oad species can be derived from the observed results as shown in the following reaction scheme.

CO + OPt f CO2 CO + OPt + Oint(I) f CO2

(250 K)

(i)

(300-350 K)

(ii) CO + OPt + Oint(I) + Oint(II) f CO2

(400-450 K)

(iii) We performed experiments to investigate the reactivities of these Oad species toward NO oxidation. However, NO was not oxidized by these Oad species under our experimental conditions as expected due to the absence of loosely bound O. Our previous study has shown that the NO oxidation reaction could only take place in the presence of weakly bound oxygen atoms, i.e., above 0.25 monolayer atomic oxygen coverage on Pt(111).55 These weakly bound oxygen atoms are more active than the strongly bound ones in the ordered p(2×2)-O/Pt(111) layer and desorb below 700 K, in contrast to the O species on BaO/Pt(111) systems, which are strongly bound and desorb at higher temperatures. We also investigated the formation of BaCO3 following the exposure of BaO/Pt(111) to a CO + O2 (1:1) mixture at 300 K. The CO TPD spectrum obtained following the exposure of the sample to a CO + O2 (1:1) mixture at 300 K (not shown) shows two features at 367 and 498 K, which can be readily assigned to the adsorbed CO on Pt sites and CO associated with carbonite and tilted CO species, similar to the assignment in section 4.1. A large fraction of OPt formed by the dissociation of O2 on the Pt surface immediately reacts with CO to form CO2, which desorbs form the Pt surface as soon as it forms at 300 K. This is also confirmed by the desorption of CO2 (not shown) while dosing CO + O2 (1:1) on BaO/Pt(111). Therefore, there is no adsorbed O on the Pt sites and no O2 desorption feature observed in the TPD spectrum. However, a small fraction of O formed on the Pt surface migrates to interfacial sites without reacting with CO. Although these O do not react with CO to form CO2 at 300 K, they combine with CO to form CO2 during annealing. A fraction of this CO2 desorbs, giving a peak at 387 K and the rest reacts with BaO to form carbonates, which decomposes giving a CO2 desorption peak at ∼780 K similarly to the carbonate layers obtained following adsorption of CO2 on BaO/Pt(111) and CO on O-BaO/Pt(111). 5. Conclusions Three different Oad species, OPt on the Pt(111) sites and two types of Oint at the Pt-BaO interface, form on BaO/Pt(111) (the Pt(111) surface is only partially covered with BaO clusters) following the adsorption of O2 at 300 K. The total O coverage obtained on BaO/Pt(111) is ∼30% higher than that on bare Pt(111) under comparable conditions although a fraction of Pt(111) surface is covered by BaO in the BaO/Pt(111) systems. The O2 desorption profiles observed for the O2-exposed BaO/ Pt(111) system can be reasonably explained by a simple model

that takes into account the desorption rate of O2 from the BaOfree Pt(111) domains, and the migration rate of Oint from the Pt-BaO interface to the BaO-free Pt(111) surface. According to the model, migration of Oint from the highly oxygen covered Pt-BaO interface to the Pt(111) domains temporarily sustains the OPt coverage during TPD, thus producing explosive O2 desorption features. DFT calculations reveal that charge donation from the BaO clusters toward the Pt substrate makes the interfacial sites energetically preferred for O adsorption in comparison to the BaO-free Pt(111) sites. The Oint gives two explosive O2 desorption peaks at ∼760 and ∼790 K temperatures in the TPD spectrum. The high temperature peak at ∼790 K appears only at higher BaO coverage. The OPt on Pt(111), which desorbs giving an O2 peak at ∼690 K in the TPD spectrum, is the most reactive O species toward CO oxidation. The Oint associated with the peak at ∼760 K is moderately reactive toward CO oxidation whereas the Oint responsible for the feature at ∼790 K is the least reactive. BaCO3 can form on these BaO/Pt(111) systems via three pathways following the adsorption of (i) CO on O-BaO/Pt(111), (ii) CO2 on BaO/ Pt(111), and (iii) CO + O2(1:1) mixture on BaO/Pt(111). However, experimental results indicate either the inhibition of the formation of strongly bound BaCO3 or destabilization of carbonates by the presence of interfacial Oint(II) at 300 K. Although NO cannot be oxidized by Oad species formed on BaO/ Pt(111) under the applied experimental conditions, these Oad species may play a significant role in NSR catalysis under practical conditions. Acknowledgment. We gratefully acknowledge the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences for the support of this work. The research described in this paper was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle Memorial Institute under contract number DE-AC05-76RL01830. C.W.Y. also acknowledges the support of this work by Sungshin Women’s University Research Grant of 2010. J.F.W. gratefully acknowledges financial support provided by the Department of Energy, Office of Basic Energy Sciences, Catalysis Science Division through grant number DEFG02-03ER15478. References and Notes (1) Park, J.; Renzas, J.; Contreras, A.; Somorjai, G. A. Top. Catal. 2007, 46, 217. (2) Haller, G. L. J. Catal. 2003, 216, 12. (3) Haller, G. L.; Resasco, D. E. AdV. Catal. 1989, 36, 173. (4) Hayek, K.; Fuchs, M.; Klo¨tzer, B.; Reichl, W.; Rupprechter, G. Top. Catal. 2000, 13, 55. (5) Hayek, K.; Kramer, R.; Paa´l, Z. Appl. Catal., A 1997, 162, 1. (6) Oudenhuijzen, M. K.; van Bokhoven, J. A.; Ramaker, D. E.; Koningsberger, D. C. J. Phys. Chem. B 2004, 108, 20247. (7) Schmidt, E.; Hoxha, F.; Mallat, T.; Baiker, A. J. Catal. 2010, 274, 117. (8) Stakheev, A. Y.; Kustov, L. M. Appl. Catal., A 1999, 188, 3. (9) Tauster, S. J. Acc. Chem. Res. 1987, 20, 389. (10) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170. (11) Sinfelt, J. H.; Lucchesi, P. J. J. Am. Chem. Soc. 1963, 85, 3365. (12) Bowker, M.; James, D.; Stone, P.; Bennett, R.; Perkins, N.; Millard, L.; Greaves, J.; Dickinson, A. J. Catal. 2003, 217, 427. (13) Boffa, A.; Lin, C.; Bell, A. T.; Somorjai, G. A. J. Catal. 1994, 149, 149. (14) Yi, C.-W.; Kwak, J. H.; Szanyi, J. J. Phys. Chem. C 2007, 111, 15299. (15) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 2134.

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