Oxygen Coverage Dependence of NO Oxidation on Pt(111) - The

Mar 12, 2009 - E-mail: [email protected]., †. Pacific Northwest National Laboratory. , ‡. Sungshin Women's University. Cite this:J. Phys. Chem...
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Oxygen Coverage Dependence of NO Oxidation on Pt(111) Kumudu Mudiyanselage,† Cheol-Woo Yi,‡ and Ja´nos Szanyi*,† Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K8-87, Richland, Washington 99352, and Department of Chemistry and Institute of Basic Science, Sungshin Women’s UniVersity, Seoul 136-742, Korea (ROK) ReceiVed: December 31, 2008; ReVised Manuscript ReceiVed: January 27, 2009

The interaction of NO with adsorbed atomic oxygen on Pt(111) was studied with temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and low-energy electron diffraction (LEED). Atomic oxygen adlayers with 0.25 and 0.75 ML coverages were prepared on a Pt(111) single crystal by dissociative chemisorption of O2 at 300 K and NO2 at 400 K, respectively. These two oxygen precovered surfaces were used to study the oxygen coverage dependence of NO oxidation at different sample temperatures. The well-ordered p(2 × 2)-O layer, corresponding to ΘO ) 0.25 ML, does not react with NO to form NO2 in the temperature range 350-500 K, in contrast to CO oxidation, which takes place readily at a sample temperature as low as 300 K. At ΘO ) 0.75 ML the NO oxidation reaction is facile, and the formation of NO2 is observed even at 150 K. However, the NO oxidation reaction completely stops as the atomic oxygen coverage drops below 0.28 ML, because all the weakly bound oxygen atoms available only at higher O coverages have been consumed. The remaining oxygen atoms are bound too strongly to the Pt(111) surface and, therefore, unable to participate in NO oxidation in the 150-500 K temperature range. 1. Introduction Understanding the mechanism of NO (the primary NOx component of the exhaust gas stream of internal combustion engines) oxidation over NOx storage and reduction (NSR), as well as selective catalytic reduction (SCR) catalysts,1 is very important in order to improve their efficiencies. Since NO oxidation is the first step in the NOx removal process over NSR catalysts, the efficiency of the NOx uptake process fundamentally depends on the NO oxidation step. In the complex NSR catalysts currently used in practice, platinum, a well-known and common oxidation catalyst, is utilized to oxidize NO to NO2. Detailed microscopic-level information on the oxidation of NO on Pt, therefore, is essential to understand and improve the NOx removal process in NSR catalysis. One approach that may lead to understanding the energetics of NO oxidation is coadsorption of NO with atomic oxygen on well-characterized Pt surfaces. Coadsorption of NO and oxygen has been studied on different transition metal surfaces, including Pt(111),2-5 using a variety of surface science techniques under ultrahigh-vacuum (UHV) conditions. However, in these studies, NO oxidation to form NO2 on Pt(111) has not been observed because the activation barrier of NO desorption is lower than that to NO oxidation, which leads to the desorption of NO before oxidation can occur.2 This is in contrast to CO oxidation, which takes place readily on Pt(111)2,6 under the same experimental conditions. Oxygen exchange reactions following the coadsorption of NO and O2 on Pt(111) have been observed previously.3,5 Swabe et al. reported that, at 145 K, molecularly adsorbed oxygen and NO are involved in the exchange reaction, which, above 270 K, takes place between NO, and the metastable oxygen atoms that do not possess a long-range order on the Pt(111) surface. However, no oxygen exchange reaction was observed when NO * Corresponding author. E-mail: [email protected]. † Pacific Northwest National Laboratory. ‡ Sungshin Women’s University.

is coadsorbed with a well-ordered p(2 × 2)-O layer. Consequently, the oxidation of NO was not observed in this study. Zhu et al. have studied the adsorption of NO on a p(2 × 2)O/Pt(111) surface by high-resolution X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD).4 Their results revealed that the presence of a p(2 × 2)-O adlayer alters the NO adsorption behavior as compared to the clean Pt(111) surface. On clean Pt(111), NO adsorption takes place on the energetically favorable fcc threefold hollow sites. Since atomic oxygen in the p(2 × 2) adlayer also occupies the fcc sites of Pt(111), NO binds to on-top sites at coverages up to 0.25 ML, and at higher coverages, NO can additionally bind to hcp sites. No evidence for NO oxidation on the p(2 × 2)-O adlayer was found in this study. The effect of surface atomic oxygen on the bonding of NO to Pt(111) has also been studied by Bartram et al.2 They showed that adsorbed atomic oxygen changes the energetics of NO adsorption sites and the nature of the Pt-NO bond. Their study also revealed that NO adsorbed on the on-top site on 0.25 and 0.75 ML O-covered surfaces in contrast to clean Pt(111) where NO preferentially adsorbed on fcc sites at lower coverages up to 0.25 ML. They also did not observe the NO oxidation reaction in this study. However, very recently, NO oxidation over a Pt(111) crystal at atmospheric pressure was reported in batch reactor experiments.7 This study showed that a 0.76 ( 0.06 ML of chemisorbed atomic oxygen coverage was present on the surface under the reaction conditions employed. A theoretical study of the NO oxidation reaction on Pt(111) has been carried out by Ovesson et al.8 They reported that the NO + O f NO2 reaction is endothermic on Pt(111) at low O coverages (ΘO < 0.25 ML), but at sufficiently high O coverages, Pt was an efficient oxidation catalyst and the reaction became exothermic. Their results also showed that O coverages higher than 0.45 ML were required for the NO + O reaction to proceed efficiently at room temperature. Our study confirms most of the general features established for the coadsorption of NO on O/Pt(111) (ΘO ) 0.25 and 0.75

10.1021/jp811520u CCC: $40.75  2009 American Chemical Society Published on Web 03/12/2009

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ML). However, for the first time, here we report the facile NO oxidation on 0.75 ML-O/Pt(111) under UHV conditions. Here, we provide a detailed description on the effect of oxygen coverage on the NO oxidation on Pt(111) and present results that confirm the predictions of a previous theoretical study on NO oxidation on Pt(111).8 In addition, we present data for the CO oxidation reaction on the 0.75 ML-O/Pt(111) sample to contrast the activity differences of CO and NO with adsorbed atomic oxygen. 2. Experimental Section All the experiments were performed in a combined UHV surface analysis chamber and elevated-pressure reactor cell with a base pressure of both chambers of less than 2.0 × 10-10 Torr. The UHV chamber is equipped with XPS, Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering spectroscopy (LEISS), and TPD techniques. The elevated-pressure cell is coupled to a commercial Fourier transform infrared (FT-IR) spectrometer (Bruker, Vertex 70) for infrared reflection absorption spectroscopy (IRAS). The Pt(111) single crystal (10 mm diameter, 2 mm thick, Princeton Scientific) used in these experiments was spotwelded onto a U-shaped Ta wire, and the sample temperature was measured by a C-type thermocouple spot-welded to the backside of the Pt 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. NO2, NO, and O2 were introduced into the UHV chamber through a pinhole doser and delivered to the sample surface through a collimating tube. The same gas dosing system was set up for gas introduction in the elevated-pressure cell. This setup allows us to expose the sample to the desired coverages by adjusting the pressure in the gas manifold (back pressure) and/or the exposure time. The back pressures and exposure times applied are reported for each experiment in the figure captions. The total pressure in either chamber has never exceeded 2 × 10-9 Torr during gas dosing. Oxygen (ultrahigh purity) was used as received, while NO2 was purified by several cycles of freeze/pump/thaw prior to use. Glass bulbs containing NO and CO were kept under liquid nitrogen for the duration of the experiments that involved these gases. IRAS spectra were collected at 4 cm-1 resolution, using a grazing angle of approximately 85° to the surface normal. Prior to each set of IRAS experiments, a background spectrum was collected from the clean or O-covered Pt(111) sample and used as the reference for the spectra collected from the adsorbate-exposed samples. Each spectrum presented is the average of 1024 scans, requiring a spectral acquisition time of ∼1 min. Throughout the course of this study, each experiment began with the preparation and characterization (XPS, LEED) of a clean Pt(111) sample, followed by the preparation of the O adlayer with the required coverage (mostly 0.25 and 0.75 ML). For the adsorption/reaction studies, the experiments were carried out in a fashion similar to the King-Wells method.9 Briefly, the O (0 to 0.75 ML)/Pt(111) sample was positioned in front of (∼1 mm away) the collimating gas dosing tube and the mass spectrometer data collection was initiated prior to any gas introduction. While the sample was kept at a constant temperature, the reactant gas (NO or CO) was introduced through the pinhole doser and the changes in the reactant/product gas concentrations were followed as a function of time. After the completion of the adsorption/reaction run, the reactant gas dosing was terminated, the sample was moved in front of the mass spectrometer, and a TPD experiment was carried out to

Figure 1. TPD profiles obtained from 0.75 and 0.25 ML-O on Pt(111). The 0.75 ML-O adlayer was obtained by NO2 adsorption at 400 K, while the 0.25 ML-O (p(2 × 2)-O) layer was prepared by O2 adsorption at 300 K. LEED images (at primary electron energy of 60 eV) taken from the 0.75 and 0.25 ML-O/Pt(111) surfaces are shown in the insets. [PNO2 ) 6.0 Torr, tNO2 ) 30 s; PO2 ) 6.0 Torr, tO2 ) 360 s.]

determine the amount of adsorbed gases on the Pt(111) surface. The heating rate used in all the TPD experiments was 2 K s-1. For the IRAS studies, the clean and O-covered (0.25 and 0.75 ML)/Pt(111) samples were prepared and characterized in the UHV surface analysis chamber, and then the sample was moved into the elevated-pressure reactor/IR cell. Here, the sample was exposed to the reactant gas (NO or CO) at the desired temperature for a certain time period, and then IR data were collected at a constant sample temperature. In cases where the sample was annealed to higher temperatures after NO exposure, the sample was cooled back to the initial temperature before the spectrum was acquired. 3. Results 3.1. Preparation and Characterization of 0.25 and 0.75 ML Atomic Oxygen Adlayers on Pt(111). An atomic oxygen layer on the Pt(111) surface can be prepared by the dissociative chemisorption of either O2 or NO2. However, the maximum atomic oxygen coverage that can be obtained with the dissociative chemisorption of O2 is 0.25 ML.10-13 The decomposition of NO2 on the Pt(111) surface is well-known to be more efficient than O2 to provide higher atomic oxygen coverages up to 0.75 ML.2,14-16 Therefore, 0.25 ML (a well-ordered p(2 × 2)-O layer) atomic oxygen layers were prepared by adsorption of O2 at 300 K, while 0.75 ML atomic oxygen coverages were obtained by adsorption of NO2 at 400 K. TPD spectra for mass 32 obtained after the O adlayer preparation by the two above-mentioned processes are shown in Figure 1. In agreement with previously reported results,12,17 the TPD spectrum from the 0.25 ML-O/ Pt(111) sample displayed one desorption feature with maximum desorption rate at ∼700 K. The LEED image of this 0.25 ML O-covered surface shows a sharp p(2 × 2) pattern (see inset in Figure 1). The TPD spectrum obtained from the 0.75 ML atomic oxygen covered Pt(111) sample shows three distinct desorption maxima. The two desorption features that are absent in the TPD spectrum of the 0.25 ML O-Pt(111) sample at 542 and 624 K are well below the desorption temperature of the p(2 × 2)-O layer. The LEED image of the 0.75 ML O-covered surface indicates that long-range order in the adlayer is disrupted due to the repulsive interactions among adsorbed atomic oxygen at O coverages higher than 0.25 ML. Similar LEED patterns have been reported for O coverages higher than 0.25 ML on Pt(111)

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Figure 2. CO oxidation on 0.75 ML-O/Pt(111) at 300 K as a function of CO exposure time. Mass spectrometer signal for (a) CO and (b) CO2 as a function of CO exposure time at 300 K; (c) TPD spectra for O2 obtained after exposure of CO at 300 K. [PNO2 ) 6.0 Torr, tNO2 ) 30 s; PCO ) 1.0 Torr, tCO ) 15, 30, 45, 180 s.]

previously.14,17,18 These 0.25 and 0.75 ML O-precovered Pt(111) surfaces were used subsequently to study the reactivity of adsorbed O atoms in the oxidation reactions of NO and CO. 3.2. CO Oxidation on 0.75 ML-O/Pt(111). In order to better understand the reactivities of O adlayers in the NO oxidation reaction, first we investigated the reactivity of a 0.75 ML atomic oxygen layer in the oxidation of CO. Although the oxidation of CO on Pt(111) is one of the most extensively studied and understood systems in UHV surface science,6,19 here we briefly discuss the reactivity of adsorbed O adlayers on Pt(111) in the oxidation of CO to establish a baseline for comparison with the NO + O reaction. Panel (a) of Figure 2 shows the changes in the 28 amu mass signal (CO) as a function of exposure time in a King-Wells type of experiment when a 0.75 ML-O/Pt(111) sample kept at 300 K was exposed to a constant CO flux. The corresponding mass 44 signal (CO2 produced by the oxidation of CO) is shown in panel (b). The production of CO2 is instantaneous at the onset of CO exposure of the 0.75 ML-O/ Pt(111) sample, and the CO2 production rate increases linearly

Mudiyanselage et al. with CO exposure time in the first ∼37 s. In the time period when the CO2 formation rate increases linearly, the CO concentration in the gas phase passes through a maximum and decreases exponentially with exposure time up to about 52 s. As the CO2 formation rate reaches a maximum and starts decreasing, the CO concentration reaches its minimum, and then increases with time-on-stream. The very broad CO2 evolution feature is followed by another, lower-intensity peak at a longer (∼88 s) CO exposure time as the concentration of CO gradually increases and reaches a constant value and the CO2 formation rate drops to zero. These results are in complete agreement with the well-established mechanism of CO oxidation on precious metal surfaces. At the high O coverage of 0.75 ML, initially there are very few adsorption sites available for CO adsorption; therefore, the CO oxidation rate (reaction between adsorbed CO and adsorbed atomic O) is slow. As a result of the reaction (consumption of some of the atomically adsorbed O), however, new adsorption sites for CO open up, and the reaction rate increases in this coverage regime. In the time period where the CO2 formation rate linearly increases, the gas-phase concentration of CO decreases, as an increasing amount of CO is being consumed in the CO + O reaction. The increasing rate of CO2 production, however, results in the depletion of atomic oxygen from the Pt(111) surface, which consequently leads to a decrease in the CO2 formation rate and an increase in the CO concentration. The presence of two CO2 evolution features in these experiments suggests that there are two CO2 formation regimes. These two regimes of CO2 production can be associated with two types of O atoms on the Pt(111) surface exhibiting fundamentally different reactivities toward CO. The result of postreaction TPD unambiguously allows us to identify these two types of O species: high-reactivity O atoms that are present on the Pt(111) surface at 0.25 ML < ΘO < 0.75 ML, and O atoms with lower reactivities at ΘO < 0.25 ML. The TPD traces of the 32 amu fragment (O2) (panel (c) of Figure 2) clearly show that exposing the 0.75 ML-O/Pt(111) sample to a constant CO flux results in first the removal of O atoms that are responsible for the evolution of the two lower-temperature desorption features (maxima at 542 and 624 K). After longer exposure times, all the O atoms that are weakly held on the Pt(111) surface were removed, and then the O atoms from the strongly held p(2 × 2)-O layer react with incoming CO molecules, but at a greatly reduced rate. It is interesting to note that the CO level reaches saturation gradually even after the CO2 production completely stopped (at ∼138 s time-on-stream). This is due to the adsorption of CO on the clean (O-free) Pt(111) surface. Postreaction TPD indeed substantiated the saturation of the Pt(111) surface by CO after all the atomically adsorbed oxygen was consumed in the CO oxidation reaction. On the other hand, neither CO nor CO2 desorption was observed after partial consumption of the 0.75 ML-O layer, i.e., at 300 K sample temperature, CO possesses sufficient mobility to find atomic oxygen on the Pt(111) surface even at relatively low O coverages. The results displayed in the three panels show that the atomic O adlayer prepared on the Pt(111) surface can completely be removed by CO oxidation at 300 K sample temperature. 3.3. Adsorption of NO on O/Pt(111) (ΘO ) 0, 0.25, and 0.75 ML). In the investigation of the interaction of NO with adsorbed atomic oxygen, first we conducted TPD experiments after exposing the clean and O-covered (0.25 and 0.75 ML) Pt(111) surfaces to NO at 200 K sample temperature. Selected TPD spectra for the 30 (NO) and 32 (O2) amu mass fragments obtained from clean Pt(111), 0.25 ML-O/Pt(111), and 0.75 ML-

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Figure 3. TPD spectra for the 30 (NO) and 32 (O2) amu mass fragments obtained from clean Pt(111) [(a) and (b)]; 0.25 ML-O/Pt(111) [(c) and (d)]; 0.75 ML-O/Pt(111) [(e) and (f)] following NO exposures at 200 K. [(a) PNO ) 4.3 Torr, tNO ) 3, 5, 30 s; (c) PNO(1) ) 6.0 Torr, tNO(1) ) 10 s; PNO(2) ) 1.5 Torr, tNO(2) ) 5 s; PNO(3) ) 0.8 Torr, tNO(3) ) 3 s; (e) PNO(1) ) 3.7 Torr, tNO(1) ) 5, 30, 90, 180 s, PNO(2) ) 2.5 Torr, tNO(2) ) 600 s.]

O/Pt(111) samples following NO exposure to different coverages are shown in Figure 3. On the clean Pt(111) surface, the TPD spectrum for the saturated NO adlayer shows two main unresolved features. The high-temperature shoulder in the spectrum has been assigned to the desorption of NO molecules bound to fcc sites, while the lower-temperature feature with a desorption rate maximum at 322 K was attributed to NO adsorbed on on-top sites.4 These two desorption features were denoted previously as β1 (low temperature) and β2 (high temperature).2,4 At low NO exposures (low NO coverages), NO preferentially adsorbs on fcc hollow sites that are energetically most favorable. As the NO coverage increases, these fcc sites become saturated with adsorbed NO, and additional NO adsorption can only take place on the energetically less favorable on-top sites. Due to the repulsive interactions among adsorbed NO molecules, the onset temperature for NO desorption decreases with increasing NO coverage. The corresponding 32 amu TPD traces obtained from the NO/Pt(111) surfaces are shown in panel b of Figure 3. These results clearly show that no oxygen desorption takes place from the clean Pt(111) after adsorption of NO at 200 K sample temperature, i.e., NO adsorbs

and desorbs molecularly from the O-free Pt(111) surface, and no evidence for NO dissociation is seen at this sample temperature. The TPD spectra recorded after adsorption of NO on the 0.25 ML O-precovered Pt(111) surface (Figure 3c) show only one desorption peak at 315 K with a very low intensity feature around 230 K. Precovering the Pt(111) surface with a 0.25 ML atomic O layer completely eliminates the high-temperature (β2) NO desorption feature. This is due to the blocking of the fcc sites by adsorbed O atoms that initially adsorb on these energetically preferred sites. However, this p(2 × 2)-O adlayer (oxygen atoms up to 0.25 ML coverage) has little effect on the β1 NO desorption state, and the intensity of this desorption feature remains practically unchanged (in comparison to the clean Pt(111) surface). The corresponding 32 amu TPD traces (O2 desorption) are shown in Figure 3d. The complete overlap of these TPD traces indicates that the atomic oxygen coverage on the 0.25 ML-O/Pt(111) surface did not change as the result of NO adsorption, i.e., no reaction between NO and the adsorbed oxygen atoms took place. The saturated NO coverage on clean Pt(111) has been reported at 0.57 ML after adsorption of

5770 J. Phys. Chem. C, Vol. 113, No. 14, 2009 NO at 250 K.20 The details of coverage-dependent NO TPD data on the p(2 × 2)-O/Pt(111) surface have been previously discussed by Zhu et al.4 They have reported that total NO and O coverage after adsorption of NO on a p(2 × 2)-O/Pt(111) surface was 0.57 ML. Since there is no significant NO desorption between 200 and 250 K, it is reasonable to assume that the NO coverage on Pt(111) at saturation would also be approximately 0.57 ML after adsorption of NO at 200 K. Using these data for the coverage estimation in our samples, at saturation NO coverage in Figure 3c, the total NO and O coverage is 0.58 ML, in excellent agreement with previously reported results.4 (The calculated NO and O coverages presented in this study are based on the integrated peak areas of the TPD spectra.) In contrast to the clean and 0.25 ML O-covered Pt(111) surfaces, no detailed data are available for coverage-dependent adsorption of NO on 0.75 ML-O/Pt(111). When NO is coadsorbed with 0.75 ML of atomic oxygen (panel e of Figure 3), the maximum desorption rate of the β1 state is shifted to lower temperature by 18 K in comparison to that on the clean Pt(111) at saturation. The β2 NO desorption state, however, disappeared completely, and a new desorption feature appeared at 233 K. This new, low-temperature NO desorption feature at 233 K, however, can mostly be assigned to NO formed from the cracking of desorbed NO2 in the mass spectrometer. This assignment is substantiated by the TPD data shown in the inset in this panel, obtained after the highest NO exposure of a 0.75 ML-O/Pt(111) surface, and shows two NO2 desorption features at 233 and 300 K. This low-temperature (233 K) desorption feature has been reported by Bartram et al.;2 however, no peak assignment was provided. The 46 amu trace in the inset also reveals that NO originated from the cracking of NO2 in the ionization chamber of the mass spectrometer also contributes to the NO desorption peak at 304 K in Figure 3e. The corresponding TPD data for the 32 amu mass fragment are displayed in Figure 3f. In stark contrast to the 0.25 ML O/Pt(111), the O2 desorption features decreased significantly after exposure of the 0.75 ML atomic oxygen layer to NO, suggesting reaction between O and NO at this high atomic oxygen coverage. At the highest NO exposure of the 0.75 MLO/Pt(111) surface, the remaining O coverage is estimated at 0.42 ML. The TPD results from the clean and atomic oxygen-covered Pt(111) surfaces established that only molecular NO adsorption/ desorption takes place on the clean and 0.25 ML O-covered Pt(111) surfaces. On the other hand, the observation of NO2 evolution and the large drop in the amount of oxygen desorbed following the exposure of this sample to NO substantiate the reaction of NO with adsorbed atomic oxygen at high O coverages (e.g., 0.75 ML) on the Pt(111) surface. Next, we discuss the results of IRAS experiments conducted on the clean and 0.75 ML O-covered Pt(111) surfaces in order to gain information about the nature of NOx species (NO and/or NO2) present on these surfaces following NO exposure at 200 K. Two series of IR spectra obtained after the exposure of clean (a) and 0.75 ML O-covered (b) Pt(111) to NO at 200 K sample temperature are shown in Figure 4. Additional spectra collected after annealing the samples with the highest NO exposure to the indicated temperatures are also displayed. All the IR spectra were collected at 200 K sample temperature. The series of IR spectra obtained after NO adsorption on the clean Pt(111) surface reveals that at the lowest NO exposure (top spectrum in Figure 4a) only one IR absorption feature is present at 1496 cm-1 and is assigned to NO adsorbed at fcc hollow sites of

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Figure 4. Series of IR spectra obtained after exposure of clean (a) and 0.75 ML-O covered (b) Pt(111) to NO at 200 K, and subsequent annealing to the indicated temperatures. All the spectra were collected at 200 K. [(a) PNO ) 3.1 Torr, tNO ) 40 s (total exposure time to reach saturation); (b) PNO ) 3.5 Torr, tNO ) 45 s.]

Pt(111).21,22 Increasing the NO coverage on the Pt(111) surface results in the appearance of another band centered at 1720 cm-1, and it represents the N-O stretching vibration of NO adsorbed on on-top sites.21-26 Note that the intensity of the 1496 cm-1 IR peak decreases when the 1720 cm-1 feature appears. The IR feature at 1496 cm-1 completely disappeared as the Pt(111) surface was saturated with NO, while the IR spectrum was dominated by a very sharp high-intensity band at 1720 cm-1. These results are consistent with sequential site occupation of NO molecules on the Pt(111) surface as the NO coverage increases. First, the energetically most favorable threefold fcc hollow sites are occupied by NO. As the NO coverage increases, on-top sites become gradually populated due to the increasing repulsion between adsorbed NO molecules. Annealing this NOsaturated Pt(111) surface to 250 K results in no change in the IR spectrum, in agreement with the above-discussed TPD data, i.e., no NO desorption takes place in the 200-250 K temperature range. Heating the sample to 300 K brings about a large decrease in the intensity of the 1720 cm-1 band and the reappearance of the 1496 cm-1 feature. (Concomitant with the decrease in the intensity of the 1720 cm-1 feature, this peak also red-shifts to 1710 cm-1 due the changes in the interactions among NO molecules as a consequence in the change in coverage.) Upon annealing the sample to 350 K, the IR absorption feature characteristic of on-top bound NO (1720 cm-1) completely disappears, and the only IR band still present in the spectrum is the one associated with the fcc-bound NO molecules (∼1480 cm-1). The IR data presented here completely support the discussion we presented above for the TPD results, i.e., NO adsorbs/desorbs molecularly from the Pt(111) surface after adsorption at 200 K. The IR spectrum recorded after the exposure of the 0.75 MLO/Pt(111) surface to NO at 200 K is displayed in Figure 4b

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Figure 5. TPD data for NO, O2, and NO2 after exposure of the 0.75 ML-O/Pt(111) surface to NO at 150 K. [PNO2 ) 6.0 Torr, tNO2 ) 30 s (to reach 0.75 ML O coverage); PNO ) 6.0 Torr, tNO ) 180 s.]

(top spectrum). This spectrum was obtained after the highest NO exposure of this sample, when no change due to NO addition was observed anymore in the IR spectrum. The spectrum is dominated by a broad peak centered at 1728 cm-1 representing the N-O stretching vibration of adsorbed NO bound to on-top sites. Besides this high-intensity IR feature, additional low-intensity bands are clearly visible as well. The IR peak at 1770 cm-1 can be assigned to bent NO on an on-top site.2 The features centered at 1272 and 1551 cm-1 can be assigned to the vibrations of adsorbed NO2 formed in the reaction of NO with oxygen atoms on Pt(111) and bound to the surface in different configurations. The peak at 1272 cm-1 can be assigned to NO2 adsorbed through the N atom as reported in a previous study on the adsorption of NO2 on O-Pt(111) by Bartram et al.15 They reported that, when NO2 adsorbed on the 0.75 ML O-covered surface, an intense peak appeared at 1270 cm-1, and it was assigned to the ONO symmetric stretch of NO2 adsorbed in a nitro configuration (bound through the N atom) with C2V symmetry. The IR feature observed at 1886 cm-1 at 200 K is most likely due to the on-top bonded NO strongly interacting with O. In the presence of atomic O, atop site-bound NO with frequencies in the range 1840-1880 cm-1 on transition metal surfaces has been reported previously.27 However, in an earlier study, IR bands at 1687 and 1845 cm-1 of a Ru complex were assigned to ν(N-O) of the linear and bent nitrosyl, respectively.28 Alternatively, this peak may be attributed to the vibration of weakly bound dinitrosyl species as reported by Friend and co-workers in their study on the adsorption of NO on highly oxidized Mo(110).29 However, in our study, the IR peak at 1886 cm-1 appeared only after annealing the NO layer adsorbed at 150 to 200 K, as shown in Figure 6. In addition, this peak disappears with the desorption of NO2 at 233 K as shown by Figures 3e and 4 suggesting an association of this IR peak with the desorption of NO2. Therefore, we assign the IR peak at 1886 cm-1 to on-top bonded NO strongly interacting with O. (Although our data are more consistent with the assignment proposed here, the assignment of this IR feature to dinitrosyl species cannot completely be ruled out either.) Upon annealing to 250 K, the IR bands at 1272, 1770, and 1886 cm-1 completely disappear, and only the features centered at 1551 and 1728 cm-1 remain. The IR peak at 1551 cm-1 in Figure 4b is assigned to bridge-bonded (µ-N,O-nitrito) NO2 formed by NO oxidation even at 200 K. The intensity of this IR feature increases slightly when the sample is heated from 200 to 250 K. However, this band was completely eliminated when the

Figure 6. A series of IR spectra obtained (a) after exposure of the 0.75 ML-O/Pt(111) surface to NO with increasing exposure at 150 K and (b) subsequent annealing to the indicated temperatures. [PNO ) 11.0 Torr, tNO ) 100 s (total time to reach saturation).]

sample was further annealed to 300 K. This observation is in agreement with the data presented in Figure 3e, which shows an NO2 desorption feature at 300 K. Although the NO coverage on this partially O-covered Pt(111) surface (i.e., after annealing to 300 K) is lower than that on the clean Pt(111) surface after the same process, here we only see the presence of an on-top bound NO (IR band at 1728 cm-1). This is due to the presence of a significant amount (more than 0.25 ML; see Figure 3f) of atomic oxygen on the Pt(111) surface, that occupies all the fcc sites. Annealing to 350 K results in the disappearance of all the adsorbed NOx-related IR features, again in complete agreement with the TPD data of Figure 3e. In order to gain further understanding of the processes taking place on the 0.75 ML O/Pt(111) surface upon its exposure to NO, we conducted IRAS and TPD experiments with this sample exposed to NO at 150 K. The traces of mass fragments 30 (NO), 32, (O2), and 46 (NO2) amu obtained from the TPD experiment conducted after the exposure of a 0.75 ML O/Pt(111) surface to NO at 150 K to full saturation are shown in Figure 5. In order to clearly see the changes in the different mass fragments, the scales are not identical for the three mass fragments. For comparison, the 32 amu trace collected from the as-prepared 0.75 ML O/Pt(111) sample is also displayed. Comparing the O2 desorption traces collected from the “clean” and NO-exposed 0.75 ML O/Pt(111) samples reveals the consumption of only a small amount of atomic oxygen from the Pt(111) surface. We can also see desorption of NO2 from the surface in the 150-310 K sample temperature range. Note the relatively “large” intensity of the NO2 desorption feature at 181 K, relative to that of the 290 K one. The TPD results suggest that NO2 can form even at 150 K sample temperature on the 0.75 ML O-Pt(111) surface or during the TPD run. IRAS data collected after the exposure of the 0.75 ML O/Pt(111) sample to NO at 150 K substantiates

5772 J. Phys. Chem. C, Vol. 113, No. 14, 2009 the formation of NO2 at this low sample temperature. Panel (a) of Figure 6 shows a series of IRAS spectra recorded from a 0.75 ML O/Pt(111) sample with increasing NO exposure. At low NO exposure, the spectrum is dominated by an IR feature centered at 1784 cm-1 and assigned to adsorbed NO in a bent configuration at this high atomic oxygen coverage. The lowintensity IR band at 1715 cm-1 reveals the presence of a small amount of on-top adsorbed NO.3,30 An IR feature with very low intensity can also be seen at 1275 cm-1 assigned to NO2 adsorbed through the N atom. The intensity of this IR feature increases gradually with increasing NO exposure, and concomitantly, the intensity of the bent NO feature (1784 cm-1) decreases somewhat. These results unambiguously prove that NO2 forms on the 0.75 ML atomic oxygen-covered Pt(111) surface at 150 K sample temperature (although the amount of NO2 formed is small, and the surface is dominated by adsorbed NO). Annealing the sample to increasingly higher temperatures brings dramatic changes in the IR spectra as shown in Figure 6b. Heating the sample to 175 K results in increases in the intensities of both the 1275 and 1730 cm-1 IR bands and in a drop in the intensity of the 1788 cm-1 feature. This suggests that at this temperature atomic oxygen can react with NO to form NO2. Some of the thus-formed NO2 desorbs (TPD trace of mass 46 in Figure 5); some are still adsorbed on the surface. When the sample temperature is raised to 200 K, the intensities of both the 1788 and 1275 cm-1 IR bands decrease dramatically. In the TPD experiment, there was a large amount of NOx (both NO and NO2) desorption in this temperature range. Concomitantly, the intensity of the 1730 cm-1 IR feature increased significantly, while two new low-intensity bands appeared at 1554 and 1888 cm-1. The increase in the intensity of the 1730 cm-1 feature is the result of the decrease in atomic oxygen coverage as NO2 formed and desorbed from the surface. The appearance of the 1554 cm-1 band (ONO asymmetric stretching vibration of adsorbed NO2 in µ-N,O-nitrito, bridge-bonded NO2, configuration on the O covered surface15) suggests the onset of another NO2 formation channel at these higher temperatures. The observation of the peak at 1888 cm-1 (bent NO strongly interacting with adsorbed atomic oxygen on the Pt(111) surface) at 200 K sample temperature indicates that rearrangement of either or both NO and O on the surface is necessary to initiate this strong NO-O interaction. Our IR data suggest that it is primarily the adsorbed NO layer that rearranges on the surface, since a large drop in the intensity of the 1788 cm-1 band (bent NO) is observed at this temperature. The empty sites required for the rearrangement of the NO layer may be created by desorption of NO2, which adsorbed in a nitro configuration, after annealing to 200 K. Similar O and NO rearrangement phenomena that leads to creating an NO-O interaction was reported in the NO + O(2 × 1)/Ru(001) system by Jakob et al.31 They interpreted their experimental results by the desorption of NO after annealing to higher temperature that led to reduced NO coverage, which in turn facilitated the rearrangement of O and NO. The disappearance of the 1888 cm-1 peak after annealing to 250 K is most probably due to the desorption of this NO as NO2 after reacting with O as we have discussed above. The intensity of the 1730 cm-1 IR band (linearly bonded NO) reaches its maximum at 225 K sample temperature, then decreases as the sample was heated to higher temperatures, and is completely absent in the spectrum recorded after annealing to 350 K. The intensity of the 1554 cm-1 band increases as the sample temperature is increased to 250 K and disappears at 300 K. The exposure of the 0.75 ML O/Pt(111) to NO at 150 and 200 K sample temperatures resulted in the formation of NO2.

Mudiyanselage et al.

Figure 7. A series of IR spectra obtained after exposure of the 0.75 ML-O/Pt(111) surface at 250 K to NO in increasing amounts and subsequent annealing to the indicated temperatures. [PNO ) 7.5 Torr, tNO ) 600 s (total time to reach saturation).]

Only one type of adsorbed NO2 was observed after the exposure of the 0.75 ML O/Pt(111) surface to NO at 150 K (represented by the IR feature at 1275 cm-1). On the other hand, two surface NO2 species were seen after NO exposure at 200 K, i.e., IR bands at 1272 and 1551 cm-1 (Figure 4b). TPD desorption results substantiated the formation of NO2 in three different channels, i.e., at three different sample temperatures (NO2 desorption at 181, 233, and 300 K). The 181 K NO2 desorption feature was only observed after the exposure of the 0.75 ML O/Pt(111) surface to NO at 150 K, and was correlated with the 1275 cm-1 IR band. The second desorption feature seen at 233 K, however, does not seem to correlate with any of the characteristic IR signatures of adsorbed NO2. It seems to be related to the 1888 cm-1 IR feature of adsorbed NO molecules strongly interacting with atomic oxygen. During the TPD run in a thermally activated process, NO and O combines to form NO2 and desorbs immediately from the surface; therefore, no IR signature of this NO2 is detected. The third NO2 formation process begins at sample temperature of around 200 K. The IR feature representing the thus-formed NO2 (1554 cm-1) is present in the spectrum collected after NO exposure of the 0.75 ML Pt(111) sample to NO at 200 K, and also in the spectra following the annealing of the NO exposed sample at 150 to 200 K. The intensity of this feature in both series of IR spectra is low. It is evident in both series of spectra that the intensity of this IR feature reaches it maximum at around 250 K annealing temperature and completely disappears when these samples are heated to 300 K. In order to further characterize this NO2 formation channel, we carried out another series of IR experiments in which the 0.75 ML O/Pt(111) sample exposed to NO at 250 K. The results of these experiments are displayed in Figure 7. First, the atomic oxygen-covered surface was exposed to increasing amounts of NO, and then the sample was annealed to the indicated temperatures. Upon exposure of the 0.75 ML O/Pt(111) sample to NO at the lowest exposure, we observe only IR features characteristic of an on-top bound NO on an O-covered Pt(111) surface (IR band at 1720-1730 cm-1). With increasing NO exposure, a broad band develops around 1547 cm-1, and its intensity increases with the NO exposure time, then saturates. The intensity of this adsorbed NO2 feature is much higher than those we have seen after NO exposure at 150 and 200 K, followed by annealing to 250 K. Annealing this sample to 300 K, however, results in the complete disappearance of this IR feature, suggesting desorption (or decomposition) of

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J. Phys. Chem. C, Vol. 113, No. 14, 2009 5773

Figure 8. TPD spectra for O2 after exposure of the 0.25 ML-O/Pt(111) surface to NO at the indicated temperatures. For comparison, a TPD curve for O2 obtained from a p(2 × 2)-O/Pt(111) surface is also displayed in this figure. [PO2 ) 9.0 Torr, tO2 ) 150 s; PNO ) 7.0 Torr, tNO ) 90 s (at every temperature).]

the adsorbed NO2. The results of TPD (Figure 9b) clearly shows desorption of NO2 in this temperature range after the exposure of the 0.75 ML O/Pt(111) sample to NO at 250 K. 3.4. Interaction of NO with Atomic Oxygen (0.25 and 0.75 ML O/Pt(111)) at Different Sample Temperatures. In the previous paragraphs, we have established that the coverage of atomic oxygen on the Pt(111) surface is critical in the reaction of NO and adsorbed oxygen atoms. At O coverages less than 0.25 ML, NO2 formation was not observed, while NO2 formed readily even at 150 K on the 0.75 ML O/Pt(111) surface. In this section, we report the results of a study in which O/Pt(111) surfaces with ΘO ) 0.25 and 0.75 ML, prepared by the same methods discussed above, were exposed to equal amounts of NO in the 250-500 K temperature range, and the amounts of surface oxygen remaining on the surface were estimated in subsequent TPD experiments. Figure 8 shows a series of TPD traces for O2 following the same NO exposure of p(2 × 2)-O/Pt(111) to NO at the indicated temperatures. For comparison, a TPD curve for O2 obtained from a p(2 × 2)-O/Pt(111) is also displayed in this figure (i.e., no NO exposure). These data clearly show that the O coverage on the Pt(111) surface does not change after the exposure of 0.25 ML atomic oxygen to NO, indicating the absence of any reaction between NO and the p(2 × 2)-O layer. The small increase in O coverage at higher temperature (450 and 500 K) NO exposures shown in Figure 8 is due to the dissociation of a small fraction of NO on Pt(111),21,32-34 although these increases are, for all practical purposes, close to the limit of the error of these measurements. Figure 9a shows TPD spectra for O2 obtained after the exposure of 0.75 ML O/Pt(111) to equal amounts of NO at the indicated sample temperatures. Reduction in the amount of desorbed O2 following the exposure of 0.75 ML O/Pt(111) sample to NO substantiates the facile reaction between O and NO at this high atomic oxygen coverage. The reaction between NO and adsorbed O atoms to form NO2 is also confirmed by the appearance of an NO2 desorption peak (m/q ) 46) at 300 K after NO exposure at 250 K, as shown in Figure 9b. Although the NO oxidation reaction can be observed even at 150 K on the 0.75 ML O/Pt(111) surface, as shown by Figure 6a, it does not proceed to the complete consumption of the atomic oxygen layer, and adsorbed oxygen is detected on the surface even after NO exposure at 450 K. In fact, the coverage of the adsorbed O that remains on the surface after exposure to NO at 450 K is

Figure 9. TPD spectra for (a) O2 and (b) NO2 after exposure of the 0.75 ML-O/Pt(111) surface to NO at the indicated temperatures. For comparison, TPD curves for O2 obtained from 0.75 and 0.25 ML-O/ Pt(111) are also displayed. [PNO ) 8.8 Torr, tNO ) 60 s (at every temperature).]

about 0.28 ML (orange curve), practically identical to what we have obtained from the p(2 × 2)-O/Pt(111) surface after NO exposure at 500 K (see Figure 8). 4. Discussion The data presented here clearly demonstrate the critical role of atomic oxygen coverage in the NO oxidation reaction on Pt(111). The NO oxidation reaction can only take place in the presence of weakly bound oxygen atoms, i.e., above 0.25 ML atomic oxygen coverage. These oxygen atoms are more active than the strongly bound ones in the ordered p(2 × 2)-O layer. Therefore, understanding the properties of adsorbed O adlayers on Pt(111), such as their structures, adsorption sites, repulsive interactions between O adatoms, and O-Pt bond strengths, is important. It is well-known that a maximum atomic oxygen coverage of 0.25 ML can be obtained on Pt(111) through the dissociative chemisorption of O2 under UHV conditions.10-13 However, atomic oxygen coverages greater than 0.25 ML (up to 0.75 ML) can be prepared by the exposure of clean Pt(111)2,14-16 to NO2 at temperatures around 400 K. NO2 readily dissociates on the Pt(111) surface leaving an oxygen atom behind as NO desorbs. Preparation of atomic oxygen coverages even higher than 0.75 ML (up to 2.4 ML) by using ozone was reported by Saliba et al.,18 and these higher oxygen coverages on Pt(111) can also be prepared by gaseous oxygen atoms.17,35 Weaver et al. reported that oxygen atoms initially populate a chemisorbed phase, then eventually lead to the growth of bulklike oxide particles through an intermediate surface oxide phase at atomic oxygen coverages higher than 0.75 ML. They also found that these oxide particles are less active toward the oxidation of CO than chemisorbed oxygen phases at lower coverages on Pt(111).17 Therefore, we expect that atomic oxygen

5774 J. Phys. Chem. C, Vol. 113, No. 14, 2009 coverages higher than 0.75 ML would also be inactive toward the NO oxidation reaction, and we only studied systems with oxygen coverages up to 0.75 ML. The TPD data obtained at an atomic oxygen coverage of 0.75 ML show three distinct desorption features, with two new features appearing at temperatures below that for desorption from the 0.25 ML coverage. Weaver et al. have denoted these three features as β1, β2, and β3, and they have estimated that the desorption activation energy decreased from 188 to 109 kJ/mol as the atomic oxygen coverage increased from 0.25 to 0.76 ML.17 Although the TPD spectrum of 0.75 ML O/Pt(111) exhibits three different O2 desorption features, previous studies have suggested that the adsorbed oxygen atoms were chemically similar.2,14 Therefore, the large decrease in desorption activation energy was attributed to the strong repulsive interaction among oxygen atoms at high coverages.14 Distinct TPD features were predicted for species desorbing from the different local configurations induced by lateral interactions.36 The multiple desorption features observed in the TPD spectrum at 0.75 ML atomic oxygen coverage may also be due to the adsorption of O on hcp sites. At ΘO > 0.25 ML, O atoms may begin to adsorb at hcp hollow sites, where the binding energy was predicted to be lower than at the fcc sites by about 50 kJ/mol.37 Jerdev et al. have also reported that O atoms start to occupy hcp sites above 0.25 ML coverages.38 They claimed that the low-temperature desorption peaks observed in the TPD spectrum at 0.75 ML atomic oxygen coverage were most likely due to the desorption of weakly bound O atoms at hcp sites. Ovesson et al. reported that the preference of the oxygen atoms to bind to fcc hollow sites over the hcp sites increases when the surface is precovered with a p(2 × 2)-O overlayer.8 Recently, Getman et al. showed that O atoms prefer to bind to fcc sites, and these fcc-bound O atoms interact repulsively with their neighbors through a combination of electronic interactions and surface relaxation effects that alter the Pt(111)-O bond strength.39 These explanations show that the origin of the decrease in the O-Pt(111) binding energy as the oxygen coverage increases above 0.25 ML is still not completely understood. Therefore, at present the continuous decrease in the O-Pt(111) binding energy can be attributed to both lateral repulsive interactions and the adsorption of O atoms on the energetically less favorable hcp hollow sites of Pt(111) at coverages above 0.25 ML. What is clear from the data presented here, as well as in previously reported experimental17,35,40 and theoretical8,39 studies, is that weakly bound chemisorbed oxygen atoms are, indeed, present in the 0.25 ML < ΘO < 0.75 ML oxygen coverage regime on Pt(111), and the higher the atomic oxygen coverage, the lower the bond strength between the Pt(111) surface and the O atoms. Even at coverages below 0.25 ML we observe that the temperature of maximum desorption rate shifts to lower temperatures as the O coverage increases, indicating the development of repulsive interactions between O atoms adsorbed on the fcc sites of Pt(111) as the p(2 × 2)-O overlayer forms.11 An estimated pairwise repulsive energy of 22 kJ/mol for oxygen atoms in the p(2 × 2)-O adlayer was reported by Yeo et al.41 Despite this repulsive interaction, our results indicate that O atoms in the well-ordered p(2 × 2) layer do not react with NO in the temperature range 350-500 K. On the other hand, these oxygen atoms readily react with CO, and all the atomic oxygen can be removed from the Pt(111) surface even at room temperature by CO oxidation (see the results reported in Figure 2). The pronounced difference between the oxidations of CO and NO reflects their fundamentally different adsorption strengths on the Pt(111) surface, with the adsorption of CO being significantly stronger than that of NO.

Mudiyanselage et al. In fact, no CO oxidation can be detected when a clean Pt(111) surface is exposed to a CO + O2 gas mixture at room temperature. Under these conditions, the Pt(111) surface is covered by adsorbed CO, effectively blocking the fcc sites where O2 dissociation takes place in the absence of CO. In order to initiate the oxidation of CO on precious metal surfaces, the sample temperature needs to be elevated where a fraction of CO desorbs opening up sites for O2 dissociation. As soon as O2 dissociates, the resulting O atoms immediately react with adsorbed CO.2,6,19 In our experiments, a 0.75 ML atomic oxygencovered Pt(111) surface was prepared first, eliminating the competition for the adsorption sites by CO and O2. This reactive atomic oxygen layer reacted with CO very effectively even at 300 K. Furthermore, the removal of atomic oxygen from the Pt(111) surface did not stop at 0.25 ML O coverage, but rather continued until all the atomic oxygen was removed by the CO oxidation reaction. The oxidation of NO requires weakly bound O atoms that are only present at ΘO > 0.25 ML. At the highest atomic oxygen coverage of this study (i.e., 0.75 ML), the reaction between NO and O is facile, and at 200 K sample temperature after the adsorption of NO on 0.75 ML O/Pt(111), only 0.42 ML of O remains on the surface. By thermal activation, this reaction can be accelerated; however, 0.28 ML of O was still on the surface after NO exposure at 400 K. Raising the sample temperature to even 500 K did not result in the complete removal of atomic oxygen layer adsorbed on the Pt(111) surface; 0.28 ML remained after NO exposure. NO2 dissociates readily on Pt(111) at 400 K, as we used this reaction to prepare the 0.75 ML O/Pt(111) surface. Therefore, one might expect that the dissociation of NO2 formed by exposing the 0.75 ML O/Pt(111) surface to NO at 400 K results in no change in the atomic oxygen coverage. However, the TPD spectrum acquired after prolonged NO exposure of the 0.75 ML O/Pt(111) surface at 400 K reveals that only 0.28 ML O remained on the surface. This result clearly indicates that NO2 desorption is preferred over dissociation under these experimental conditions in UHV. The presence of atomic oxygen on the surface lowers the NO2 desorption temperature,15 and it may contribute to the enhancement of preferential NO2 desorption. At lower sample temperatures (200-350 K), the desorption of NO2 formed in NO oxidation is preferred over dissociation, as evidenced by the low-temperature NO2 desorption features in postreaction TPD experiments, and may also be aided by the presence of atomic oxygen and adsorbed NO in significant coverages. The possibility of NO2 dissociation cannot be ruled out completely from these experimental results; however, under the conditions of our experiments NO2 desorption seems to be the dominating process, in contrast to NO2 dissociation. On the basis of the results obtained in NO exposure experiments of the 0.75 ML O/Pt(111) surface at different temperatures (IRAS) and those about the nature of the reaction products (IRAS and TPD), three NO oxidation regimes can be distinguished as shown in the scheme below: (a) NO exposure at 150 K leads to the formation of adsorbed NO2 in nitro (bound through the N atom) configuration. No NO2 desorption or dissociation occurs at this temperature. (b) NO exposure in the 200-250 K temperature range produces adsorbed NO2 in µ-N,O-nitrito (bridge-bonded NO2) configuration. At 200 K, mainly bridge-bonded NO2 with a small amount of NO2 in the nitro configuration are present on the surface (IRAS). Annealing to 225 K leads to the complete desorption of NO2 adsorbed in the nitro configuration. (c) NO exposure in the 300-450 K temperature range leads to the formation of NO2, which mostly desorbs from the surface before it can dissociate:

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J. Phys. Chem. C, Vol. 113, No. 14, 2009 5775 at 150 K

NO(ads) + O(ads) 98 NO2(ads)

(a)

at 200 - 250 K

NO(ads) + O(ads) 98 NO2(ads) + NO2(gas)v (b) at 300 - 450 K

NO(ads) + O(ads) 98 NO2(gas)v

(c)

Sample temperature plays a critical role in NO oxidation reaction on Pt(111).8 As we have discussed above, NO exposure at 200 K removes about 0.33 ML atomic oxygen, while NO exposure at 400 K removes ∼0.47 ML atomic O by NO oxidation. This temperature dependence may be due to the increase of an activation barrier of NO oxidation reaction with decreasing atomic oxygen coverage (i.e., with decreasing atomic oxygen coverage, the reactivity of the remaining oxygen atoms decrease). Upon the exposure of 0.75 ML O/Pt(111) to NO at 150 K, only a small fraction of atomic oxygen is consumed in the NO oxidation. Although increasing the sample temperature helps to overcome the activation barrier, at higher temperatures NO desorption competes with NO oxidation. Therefore, a fraction of NO adsorbed at 150 K desorbs without taking part in the NO oxidation reaction. However, when 0.75 ML O/Pt(111) is exposed to NO at higher sample temperatures (200-400 K), a higher fraction of O reacts with NO to form NO2 than at 150 K. The activation barrier can be overcome easily, and the atomic oxygen removal from the Pt(111) surface by the NO oxidation reaction proceeds to a higher extent as the sample temperature is increased. The thermal activation weakens the Pt-O bond, making the adsorbed O more susceptible to reaction with NO. However, NO exposure at 350, 400, and 450 K leads to the consumption of approximately the same amount of O by the NO oxidation reaction. One of the most important outcomes of the experimental results presented here is the confirmation of the prediction of the theoretical study reported by Ovesson et al.8 Our experimental results unambiguously show that at 0.25 ML atomic oxygen (p(2 × 2)-O layer) coverage no NO oxidation reaction takes place in the temperature range 350-500 K. On the basis of their calculations, Ovesson et al.8 predict that the NO oxidation reaction does not occur at 0.20 ML atomic oxygen coverage in the 300-500 K temperature range. They predict that 0.45 ML atomic O coverage is required for the reaction to be activated at room temperature, in almost perfect agreement with our experimental results that showed appreciable NO oxidation rates at 300 K only above 0.40 ML of atomic O coverages. They also showed that at 0.25 ML O coverage the NO oxidation reaction may occur to a significant extend at and above 500 K, which, however, was not observed under our experimental conditions. The results of our study clearly show that the presence of weakly bound oxygen atoms on the Pt(111) surface is required for the reaction of NO to form NO2. In practical applications, under highly oxidizing environments high oxygen coverages can be obtained, and hence, these weakly bound oxygen atoms can facilitate the NO oxidation reaction in NSR catalysis. Other components of the catalytic system, such as BaO and Al2O3, may have a pronounced effect on these processes. Therefore, their influences on NO oxidation in NSR catalysis need to be investigated in detail on these model systems.

5. Conclusions The results presented here indicate that weakly bound oxygen atoms are required for the NO oxidation reaction on Pt(111). Atomic oxygen coverages above 0.28 ML need to be achieved in order to obtain these active, weakly bound oxygen atoms. At 0.75 ML atomic O coverage, NO oxidation takes place even at 150 K, and increasing the sample temperature to 350 K greatly enhances the rate of this reaction. However, NO oxidation does not proceed to consume all the atomic oxygen on the Pt(111) surface at sample temperatures up to 500 K; instead, it stops as the O coverage drops below 0.28 ML. Three different NO oxidation channels on the 0.75 ML O/Pt(111) surface were identified in this study. Exposure of this surface to NO at 150 K results in the formation of adsorbed NO2 in nitro configuration (first channel). Annealing the sample exposed to NO at 150 K to 200 K or adsorption of NO at 200 K opens a second NO oxidation channel. Even though, there is no IR spectroscopic evidence for the adsorbed NO2 in this channel; desorption of NO2 confirms the presence of NO oxidation. The NO2 formed in this channel seems to originate from the weakly held NO in strong interaction with surrounding O atoms (1886 cm-1 IR feature). Exposure of the 0.75 ML O/Pt(111) surface to NO in the 200-250 K temperature range produces adsorbed NO2 in µ-N,O-nitrito (bridge-bonded NO2) configuration (∼1550 cm-1 IR feature) that desorbs around 300 K. 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 U.S. 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 2008. References and Notes (1) Gandhi, H. S.; Graham, G. W.; McCabe, R. W. J. Catal. 2003, 216, 433. (2) Bartram, M. E.; Koel, B. E.; Carter, E. A. Surf. Sci. 1989, 219, 467. (3) Sawabe, K.; Matsumoto, Y.; Yoshinobu, J.; Kawai, M. J. Chem. Phys. 1995, 103, 4757. (4) Zhu, J. F.; Kinne, M.; Fuhrmann, T.; Tra¨kenschuh, B.; Denecke, R.; Steinru¨k, H. P. Surf. Sci. 2003, 547, 410. (5) Sawabe, K.; Matsumoto, Y. Surf. Sci. 1994, 303, L385. (6) Wintterlin, J.; Volkening, S.; Janssens, T. V. W.; Zambelli, T.; Ertl, G. Science 1997, 278, 1931. (7) Smeltz, A. D.; Getman, R. B.; Schneider, W. F.; Ribeiro, F. H. Catal. Today 2008, 136, 84. (8) Ovesson, S.; Lundqvist, B. I.; Schneider, W. F.; Bogicevic, A. Phys. ReV. B 2005, 71, 115406. (9) King, D. A.; Wells, M. G. Proc. R. Soc. London, Ser. A: Math. Phys. Eng. Sci. 1974, 339, 245. (10) Gland, J. L.; Sexton, B. A.; Fisher, G. B. Surf. Sci. 1980, 95, 587. (11) Materer, N.; Starke, U.; Barbieri, A.; Do¨ll, R.; Heinz, K.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1995, 325, 207. (12) Gland, J. L. Surf. Sci. 1980, 93, 487. (13) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 1. (14) Parker, D. H.; Bartram, M. E.; Koel, B. E. Surf. Sci. 1989, 217, 489. (15) Bartram, M. E.; Windham, R. G.; Koel, B. E. Langmuir 1988, 4, 240. (16) Dahlgren, D.; Hemminger, J. C. Surf. Sci. 1982, 123, L739. (17) Weaver, J. F.; Chen, J.-J.; Gerrard, A. L. Surf. Sci. 2005, 592, 83. (18) Saliba, N. A.; Tsai, Y. L.; Panja, C.; Koel, B. E. Surf. Sci. 1999, 419, 79.

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