In Situ X-ray Photoelectron Spectroscopy of Catalytic Ammonia

Sep 4, 2008 - Yi Sun , Yan Wang , Ji Sheng Pan , Ling-ling Wang and Chang Q. Sun. The Journal of Physical Chemistry C 2009 113 (33), 14696-14701...
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J. Phys. Chem. C 2008, 112, 15382–15393

In Situ X-ray Photoelectron Spectroscopy of Catalytic Ammonia Oxidation over a Pt(533) Surface S. Gu¨nther,*,† A. Scheibe,‡ H. Bluhm,§ M. Haevecker,§ E. Kleimenov,§ A. Knop-Gericke,§ R. Schlo¨gl,§ and R. Imbihl‡ Department Chemie, Ludwig-Maximilians UniVersita¨t Mu¨nchen, Butenandtstrasse 11 E, 80377 Mu¨nchen, Germany, Institut fu¨r Physikalische Chemie and Elektrochemie, Leibniz-UniVersita¨t HannoVer, Callinstrasse 3-3a, 30167 HannoVer, Germany, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: April 15, 2008; ReVised Manuscript ReceiVed: July 18, 2008

The NH3 + O2 reaction on a Pt(533) surface has been studied in the 10-4 mbar range and close to 1 mbar pressure with in situ X-ray photoelectron spectroscopy using synchrotron radiation. The coverages of the various O- and N-containing surface species have been followed in T-cycling experiments with varying mixing ratios O2/NH3 and varying total pressure. In heating/cooling cycles hysteresis of ∼50-100 K width occur. Adsorbed NOad already decomposes at T > 350 K. Under stationary conditions, no adsorbed NO could be detected. At no time during the experiments were Pt bulk oxides formed. A shift in the surface core level component of the Pt 4f spectrum by more than 0.5 eV toward higher binding energy is attributed to Pt atoms of the (100) step edges which are coordinated to more than one oxygen atom similar to the model proposed by Wang et al. Phys. ReV. Lett. 2005, 95, 256102. 1. Introduction The catalytic oxidation of ammonia with air to NO over a Pt/Rh gauze in the Ostwald process is an important industrial reaction.2 At high temperature mainly NO is produced, whereas at lower temperature (T < 800 K) N2 is the preferred reaction product. To a small extent also N2O is formed.

4NH3 + 5O2 f 4NO + 6H2O

(R1)

4NH3 + 3O2 f 2N2 + 6H2O

(R2)

2NH3 + 2O2 f N2O + 3H2O

(R3)

Despite its importance, comparatively few single crystal studies of this reaction system have been conducted focusing on flat and stepped Pt(111) surfaces and on Pt(100).3-11 Platinum is known to catalyze ammonia decomposition,12-15 but the stepwise ammonia decomposition followed by reaction of the dissociation products with oxygen plays a minor role in ammonia oxidation. As shown by quantum chemical calculations and also supported by experiments, the major pathway involves a direct interaction of adsorbed ammonia with chemisorbed oxygen or an OH species leading to H abstraction.7,10,11,14 This so-called ammonia activation by chemisorbed oxygen or an OH species is the key step of the reaction mechanism. The main features of the reaction can be semiquantitatively reproduced with a realistic mathematical model derived from a simplified reaction mechanism.16 The reaction exhibits a dominant N2 production at low T which shifts to NO as main product at high T, because it is the availability of oxygen that controls which reaction pathway is chosen. What was missing, however, was an identification and a quantitative characterization of the adsorbate coverages under reaction conditions over a broad * Corresponding author. E-mail: [email protected]. † Ludwig-Maximilians Universita ¨ t Mu¨nchen. ‡ Leibniz-Universita ¨ t Hannover. § Fritz-Haber-Institut der Max-Planck-Gesellschaft.

pressure range. In an electron energy loss spectroscopy (EELS) study, the intermediates NH and NH2 were identified on a Pt(111) surface,6 but, first, the coverages were not quantified and, second, this study was conducted under instationary conditions at low pressure (10-8 mbar). Similarly, an X-ray photoelectron spectroscopy (XPS) study by Weststrate et al. of ammonia oxidation on Pt(410) conducted parallel to ours was carried out at very low pressure (p e 1.5 × 10-7 mbar).17 In this study, we employ XPS with a differentially pumped instrument for an in situ characterization of a Pt(533) surface under reaction conditions spanning the pressure range from 10-4 to 1 mbar. With 1 mbar one is still 3 orders of magnitude away from the pressure of 1 bar used in the industrial process, but a substantial part of the pressure gap has been bridged in this way. The measured adsorbate coverages under stationary conditions should serve as a basis for a realistic modeling of the reaction. The Pt(533) surface we employ here was characterized in previous studies.9,10 Its surface consists of four (111) terrace units followed by a (100) step, as expressed in the microfacet notation Pt-4(111) × (100).18 A stepped Pt(111) surface was chosen, because we also wanted to address the problem of the reaction-induced structural changes. A visible roughening of the Ostwald catalyst occurs within seconds and minutes but reactioninduced structural changes also take place under the very mild conditions of a UHV experiment. On Pt(533) a reversible reaction-induced doubling of the step height occurs under certain reaction conditions associated with a change in selectivity from N2 to NO production.9,10 The question as to whether the doubling of the step height on Pt(533) is actually driven by a more strongly bonded oxygen species and to what degree oxide formation occurs was an additional motivation for this XPS study. In fact, we could use the spectral signature of the O 1s, N 1s, and Pt 4f core level peaks to characterize the Pt(533) surface during reaction conditions and propose plausible models

10.1021/jp803264v CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008

Catalytic Ammonia Oxidation over a Pt(533) Surface

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for the oxygen adlayer on the single-atomic and on the doubleatomic stepped surface. 2. Experimental Section The experiments were performed at the undulator beamline U49/2-PGM1 at the synchrotron radiation facility BESSY in Berlin in a specially designed differentially pumped XPS system.19,20 The X-rays were admitted to the experimental cell through a 100-nm-thick SiNx window. The photoelectrons, emitted under normal emission, entered a differentially pumped electrostatic lens system and were focused on the entrance slit of a standard electron energy analyzer, where high vacuum conditions were maintained by an additional pumping stage. This setup allowed a variation of the total pressure in the reaction cell between 10-7 and 1 mbar. Under reaction conditions, the cell was operated as a continuous flow reactor. Reaction products were monitored with a differentially pumped quadrupole mass spectrometer (QMS). The sample temperature could be varied with infrared laser heating between 300 and 800 K. Typically heating ramps with 10 K/min were used. The Pt sample was cleaned by a combination of Ar ion sputtering/exposure to oxygen in the 10-6 mbar range and annealing to 1020 K. Since the cell was operated up to 1 mbar pressure, only a residual gas pressure of 10-8 mbar could be reached. Because of the modest background pressure of the system (especially after the usage of NH3), the sample had to be kept C-free by adjusting a sample temperature of ∼330 K and by introducing oxygen with a pressure of 10-7 to 10-6 mbar to facilitate a clean off reaction. More drastic conditions with p(O2) ) 10-5 mbar at 510 K were used to clean an already carbon-contaminated surface. 2.1. Coverage Calibration. To relate the measured core level intensities to adsorbate coverages, we performed a calibration experiment before the in situ reaction analysis after removing carbon and raising the O2 pressure to a maximum of 10-4 mbar. At this pressure the O 1s intensity reached rapidly its saturation level. A single O 1s peak at 529.7 ( 0.2 eV was found, whose binding energy (BE) agrees with that of the (2 × 2) O structure on Pt(111).21 We therefore relate the measured O 1s intensity to an oxygen coverage of ∼0.25 ML (section 3.3.1), which is the known saturation level for Pt(111) for p < 10-5 mbar.21,22 We use this calibration for the O 1s intensity also as calibration for all other adsorbates whose coverages we calculate via their known atomic cross sections. As a consequence, all given coverages are atomic coverages (i.e., if a molecule like a hydrocarbon molecule contains n C atoms, then the corresponding molecular coverage is obtained from the atomic coverage by division through n). In our case, the C 1s peaks (not shown) of the initially carbon-contaminated surface are rather broad (the fwhm ranges between 1.5 and 2 eV), indicating a mixture of different species. We therefore can only use atomic C-coverages because the nature of the C-contaminants is unknown. To maintain a constant analyzer transmission, the photon energy for the detection of each core level (O 1s, N 1s, C 1s, and Pt 4f) was varied in such a way that the photoelectrons for each core level left the surface with equal kinetic energy of ∼300 eV. Thus, only the synchrotron light intensity variation upon photon energy change had to be taken into account, which was separately measured with the help of a photodiode with known quantum efficiency. A further advantage of this procedure is that the signal damping due to the scattering of the emitted photoelectrons with gas-phase molecules at p g 10-2 mbar is equal for each probed core level, and thus even at high pressure the coverage of different adsorbate species on Pt(533)

Figure 1. NH3 adsorption at 300 K: (a) 0, (b) 1, and (c) 20 min after reaching p(NH3) ) 1 × 10-4 mbar. Four different N 1s species at 399.7 ( 0.3, 398.2 ( 0.2, 397.3 ( 0.2, and 396.7 ( 0.2 eV are identified. They are related to different adsorbates as indicated in the figure (see text).

can be determined by evaluating the relative intensities of their corresponding core levels. At pressures above 10-1 mbar, we could detect contributions of gas-phase molecules to the recorded N 1s and the O 1s spectra; namely, we could identify the O 1s spectrum of molecular oxygen and the N 1s spectrum of ammonia in the gas phase. Since only the adsorbate species are relevant, the gas-phase parts of the spectra are not displayed here. 3. Results 3.1. Adsorption Experiments. 3.1.1. NH3 Adsorption. Figure 1 shows the development of the N 1s spectrum after introducing 1 × 10-4 mbar NH3 at 300 K. We can deconvolute the spectrum into four species at 399.7 ( 0.3, 398.2 ( 0.2, 397.3 ( 0.2, and 396.7 ( 0.2 eV.23 The first species can be assigned to adsorbed molecular NH3 in agreement with the literature value of 399.9 eV for NH3 on Pt(111).24,25 We attribute the 398.2 eV component to adsorbed NHx (x ) 1, 2). The third component at 397.3 eV is clearly due to Nad in fair agreement with literature values for Nad on Pt(111) between 397.6 and 397.7 eV.26-28 We also note a species at 396.7 eV whose peak area grows continuously throughout the experiment. This species was found to be linked to presence of carbon contamination on the surface and therefore is named NC. Probably through

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Figure 2. NO adsorption at 300 K and NO decomposition during slight annealing at p(NO) ) 5 × 10-6 mbar. The N1s and O 1s spectra are shown at T ) (a) 300, (b) 350, and (c) 375 K. As reference, the O 1s spectrum from the oxygen-saturated Pt(533) surface (p(O2) ) 1 × 10-4 mbar at 470 K) and the N 1s spectrum from NH3 adsorption on Pt(533) (p(NH3) ) 1 × 10-4 mbar at 300 K) are displayed as well. Note that the O 1s spectra of the oxide and of NOad have approximately the same binding energy. The Oox component refers to a Si-related oxide (see text).

adsorption from the residual gas this carbon-related species accumulated at ptot < 10-4 mbar under nonoxidizing conditions. We tentatively attribute the 396.7 eV peak to a CN species on Pt(533). Lindquist et al. reported an N 1s binding energy of 397.0 eV for CN on Pt(111), which is 0.3 eV higher than the value found here, but atomic nitrogen experiences a similar shift to higher energies on Pt(111).29 We did not attempt to fit two separate species for NH2 and NH, because all acquired N 1s spectra did not provide wellresolved data necessary for unambiguous fitting of two separate components. The apparent shift of the NH3 peak toward lower binding energy in Figure 1c might indicate the population of another peak between the fitted component for NH3 and NHx, but we decided not to introduce speculative components for the N 1s deconvolution. The NH3 decomposition visible in the spectra of Figure 1a-c is most likely due to beam damaging effects. It is known that an ammonia layer on Pt(111) is sensitive to electron beam irradiation, which partially decomposes the molecules to NHx (x ) 0, 1, 2).25 Such beam damaging effect of 50 eV electrons may become significant when exceeding a fluence of 5 × 1016 electrons/cm2. In our experiments, we typically exposed the sample to 1015 photons/(s × cm2). Assuming a similar cross section for photons and electrons, beam damaging should be already expected after X-ray exposure of 1-min duration. Indeed, similar effects were reported in ref 17. While NH3 decomposition is reported to occur on clean Pt(111) above 600 K,6,13 this temperature is lowered already to 400 K for Pt(100).12 In the presence of Oad, decomposition may occur already below 300 K.6,17 Therefore, also defects and reaction with residual oxygen might contribute to the NH3 decomposition shown in Figure 1.

3.1.2. NO Adsorption. Figure 2 displays the N 1s and O 1s spectra obtained after introducing NO at p(NO) ) 5 × 10-6 mbar at 300 K followed by subsequently recorded spectra at slightly elevated temperatures. As reference, the N 1s spectrum after NH3 adsorption and the O 1s spectrum of the oxygensaturated Pt(533) surface are included in the plot. In the N 1s spectra of NO, we can clearly distinguish three species at 397.3, 400.4, and 402.3 eV. The small peak of the first component originates from adsorbed NHx, which results from residual gas adsorption of NH3. The coverage of this species slowly decreased upon further NO adsorption. The species at 400.4 eV can be assigned to NO adsorbed on (111) terraces consistent with the values of 400.4 eV found for NO/Pt(111) in ref 28 and 400.7 eV reported in ref 27. With increasing exposure after saturation of the 400.7 eV state, the component at 402.3 eV starts to grow (not shown here). We assign this species to NO adsorbed at the (100) steps, because binding energies have been found in the literature for NO/ Pt(100) (401 eV in ref 30) higher than those for Pt(111). The fact that NO adsorption lifts the hex reconstruction of the Pt(100) surface complicates the assignment of different adsorbate species. NO decomposition on Pt is highly structure-sensitive. On Pt(111), practically no decomposition occurs in a TPD experiment with adsorbed NO, whereas more than 60% dissociates on Pt(100).31,32 Figure 2 shows that NO decomposes on the Pt(533) surface already upon slight heating. We attribute this to the high reactivity of (100) steps on Pt(533). The decomposition of the NO molecule can be nicely followed by the O 1s peak as demonstrated in Figure 2. Initially, a disordered NOad layer is present as reflected by an O 1s binding energy of 531.1 eV.27,28 Above 350 K, the Oad species with a BE of 529.7 eV

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Figure 3. N 1s and O 1s spectra (upper panels) and the extracted adsorbate coverages on Pt(533) (lower panel) during ammonia oxidation at p(NH3) ) 1 × 10-4 mbar with a mixing ratio p(O2)/p(NH3) ) 3:1. The arrows in the lower plot indicate at which temperatures the spectra shown in the upper panels were taken. The temperature was increased stepwise by applying a ramp of ∼10 K/min and subsequently keeping the temperature fixed for several (2-5) minutes to acquire the spectra.

is populated at the expense of molecular NOad (see the reference spectrum in Figure 2). The corresponding N 1s spectra indicate that after annealing nearly no N-containing species are present on the surface. Since a (2 × 2) N layer on Pt(111) was reported to be stable up to 500 K before desorption occurs, one can assume that coadsorption effects may have facilitated the desorption of nitrogen. This is supported by the observation that the coadsorption of NH3 and O2 on Pt(111) led to N2 release into in the gas phase already at 373 K.26 For the further characterization of the reaction system, it is important that the NO-related N 1s components can be well separated from the NHx species originating from the NH3 adsorption. 3.2. NH3 + O2 Reaction. 3.2.1. Reaction Conditions in the 10-4 mbar Range. Figure 3 shows a compilation of N 1s and O 1s spectra, taken during the stationary ammonia oxidation over Pt(533) at p(NH3) ) 1 × 10-4 mbar and a mixing ratio O2/NH3 ) 3:1. The measured intensities were converted in

adsorbate coverages as explained in the Experimental Section. A coverage plot is displayed in the lower panel of Figure 3. The total coverage of N-containing adsorbates amounts to ∼0.25 ML in the range 300 K < T < 500 K. In this temperature range, ∼0.2 ML of carbon was present on the surface. Above 500 K, the total coverage decreased rapidly and carbon vanished completely. NH3,ad was detectable between 300 and 470 K, but the dehydrogenated species NHx (x ) 1, 2), Nad, and NC always covered the major part of the surface. We believe that at least above 400 K the NH3 decomposition reflects the “true” reaction (oxygen) assisted dehydrogenation of the ammonia molecule. As will be shown in the next paragraph, the amount of NH3,ad drops significantly when we strongly raise the O2 partial pressure. This effect should not be observed if NH3 decomposition were initiated only by X-ray irradiation. The assignment of the different components in the N 1s spectra to the dehydrogenated species NHx, Nad, and NC is identical to the NH3 adsorption experiment in Figure 1. The

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Figure 4. Top panel: Temperature dependence of the N2 and NO production rate during heating cycle of the catalytic NH3 oxidation over Pt(533) (taken from ref 10; see text). Middle panel: The adsorbate coverages during heating. Bottom panel: The adsorbate coverages during cooling. Experimental conditions: p(NH3) ) 1 × 10-4 mbar, mixing ratio p(O2)/p(NH3) ) 3:1. The heating was performed stepwise as outlined in the caption of Figure 3.

NC component at 396.7 eV is linked to the presence of carbon on the surface. This component completely vanishes if the C contamination drops significantly above ∼480 K. For reasons given above, it was not attempted to distinguish between NH2,ad and NHad, because the N 1s spectra allowed the unambiguous fitting of only one additional component at 398.2 ( 0.2 eV (although the large fwhm of the NHx component in Figure 3b might indicate the presence of two species). Since DFT calculations show that NHad is the more stable adsorbate as compared to NH2,ad,14 we attribute the main contribution of the NHx signal to NHad. The O 1s spectra reveal a very low coverage of oxygen species during the whole experiment displayed in Figure 3. The component of the O 1s spectrum at 529.7 eV corresponds to chemisorbed oxygen, Oad. The other component at ∼532.0 eV represents an oxide, Oox, which is related to a Si contamination of the Pt crystal. This species slowly accumulates on the surface as will be discussed below. At low temperature, a small amount of Oad is present on the surface. In the T range between 500 and 600 K the surface is completely depleted of Oad, and above 600 K Oad can again be found on the surface. In Figure 4, we relate the surface coverages to the catalytic reactivity of the Pt(533) surface. In the upper panel, the N2 and the NO production measured during running a heating ramp under similar reaction conditions is displayed (data taken from ref 10). In the middle panel, the coverage plot of Figure 3 is

Gu¨nther et al. repeated. The sudden drop of the total coverage at ∼480 K coincides with the initiation of the reaction. After ignition of the N2 production, no more NH3,ad (but some NHx) is found on the surface. At the maximum of the N2 production rate, only Nad is present, indicating that desorption of nitrogen, Nad + Nad f N2(g), is the rate-limiting step in N2 formation. NO formation is detectable as soon as Oad appears again beyond the temperature range of the Oad depleted surface. When the NO production exceeds the N2 formation, almost exclusively oxygen species are present on the surface. At no time during the TPR experiment is NOad found. At high temperature, apparently NO desorption is much faster than NO formation. At lower temperature where NO desorption is slow,27,28 we believe that NO decomposition prevents the accumulation of larger quantities of NO. As shown in section 3.1.2, dissociation of NO occurs already at T < 370 K. The lower panel of Figure 4 displays the adsorbate coverages during the cooling part of a T-cycle experiment. The Oox component (BE ≈ 532.0 eV) accumulates on the surface because its formation is irreversible under our reaction conditions. The comparison with the heating part reveals a hysteresis of ∼50-100 K width. In fact, in ref 10 hysteresis effects were found in the catalytic rates and their origin was traced back to reversible restructuring of the surface. Although the heating and cooling of the experiment in Figures 3 and 4 were conducted rather slowly (heating ≈ 80 min, cooling ≈ 120 min), transient effects cannot be excluded. In addition, the buildup of carbon and oxide contaminations might play a role so that we cannot definitely assign the hysteresis to a restructuring of the surface. Figure 5 shows the N 1s and the O 1s spectra and corresponding adsorbate coverages when we increase the oxygen partial pressure in the TPR experiment such that an O2/NH3 ) 10:1 mixing ratio (MR) is reached. Qualitatively, the same development of the N 1s spectra is observed but now the total coverage drop sets in at a slightly lower temperature. Below this onset, the relative contribution of the NHx signal is significantly stronger as compared to the TPR experiment at a MR ) 3:1 (Figures 3 and 4). In addition, Oad is present already at 380 K. Since the experiment of Figure 5 was performed after the run with MR ) 3:1 Oox has accumulated. 3.2.2. Reaction Conditions Close to 1 mbar. The T-cycling experiments described in the previous paragraph were repeated after raising the total pressure to close to 1 mbar, corresponding to a partial pressure increase by a factor of 550. The resulting N 1s and O 1s spectra are displayed in Figure 6. Carbon accumulation due to adsorption from the residual gas is now nearly negligible. As a consequence, the NC component in the N 1s spectra has almost vanished. The drop in the total coverage that indicates the onset of catalytic activity is now observed already between 430 and 470 K. On the other hand, monitoring the reaction products with the help of the differentially pumped QMS confirmed significant N2 production above ∼520 K. A considerable oxygen coverage is already present at low temperature before ignition of ammonia oxidation. Both observations are apparently a consequence of the higher total pressure resulting in a stronger self-cleaning of the reacting surface as compared to the experiments in the 10-4 mbar range. Another difference we note is that NH3, NHx, and Nad species remain detectable up to much higher temperatures. This is in line with the finding that the N2 production rate does not drop as rapidly at high temperatures as is the case for low pressure reactions (e.g., Figure 4). The extension of the N2 production channel toward higher pressure is consistent with the findings summarized in a recent review article.11 We also

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Figure 5. Adsorbate coverages on Pt(533) during ammonia oxidation in the 10-4 mbar range with a large excess of oxygen in the gas phase. Left panels: Evolution of adsorbate coverages during heating and cooling. Right panels: selected N 1s and O 1s spectra during the heating branch. The corresponding temperatures are indicated in the coverage plot. Experimental conditions: p(NH3) ) 1 × 10-4 mbar, mixing ratio p(O2)/p(NH3) ) 10:1. The heating was performed in steps. Each temperature increase was achieved by applying a temperature ramp of ∼10 K/min followed by several minutes waiting time at constant temperature for acquiring the core level spectra.

realize that now a rather large part of the surface is covered with an oxidic species Oox characterized by an O 1s BE of ∼532.2 eV. Obviously, this species accumulates on the surface with each additional heating/cooling cycle. The coverage of Oad slowly decays followed by a steep increase above T > 570 K. In principle, the same course was seen in the 10-4 mbar range but now it is much more pronounced. A hysteresis in the coverages of ∼50 K width is reduced compared to 10-4 mbar, but still visible in the Oad coverage. 3.3. Analysis of the Pt 4f Core Level Spectra. 3.3.1. Oxygen Adsorption. Figure 7 displays the Pt 4f spectra taken during exposure to oxygen, with p(O2) being varied from 10-5 to 0.9 mbar. The upper panel displays the spectrum during a cleaning procedure at T ) 510 K and p(O2) ) 10-5 mbar, leading to a nearly clean surface. Only a minor quantity of Oox was detectable. Panels b and c display the corresponding Pt 4f spectra during O2 adsorption at p(O2) ) 10-4 and 0.9 mbar, respectively. In agreement with literature data, we determined the position of the Pt 4f bulk peak at 70.95 ( 0.05 eV. For deconvolution, we used a Doniach-Sunjic line shape with a Lorentzian width of 0.32 eV and an asymmetry of 0.20. The overall energy resolution of 0.25 eV at the Fermi edge is reflected in a typical Gaussian line width of 0.30 eV. The observed spin orbit splitting of 3.33 eV between the Pt 4f7/2 and the Pt 4f5/2 is in agreement with literature data.33 At surface core levels we had to introduce three components: one shifted by +0.39 eV toward lower binding energy (scl_a) and two shifted toward higher binding energy by -0.20 eV (scl_b) and -0.55 to -0.65 eV (scl_c), respectively. The scl_a component is the known surface core level originating from surface atoms on Pt(111) not coordinated to any adsorbate.1,21,28 Consequently, it is most pronounced in the

Pt 4f spectrum of the clean surface. The -0.2 eV shifted scl_b species belongs to surface Pt atoms coordinated to one electronegative adsorbate atom. The strongly shifted surface core level scl_c is found when the surface Pt atoms are more than single coordinated with respect to oxygen. Such a state has been prepared on Pt(111) by establishing a high NO coverage of 0.5 and 0.75 ML, respectively.28 Recently, Wang et al. reported on the formation of a 1D surface Pt-oxide, PtO2, after dosage of 500 L of O2 at 310 K on Pt(332), a surface consisting of (111) terraces separated by (111) steps.1 It was argued that the occurrence of a -0.5 eV shifted component of Pt 4f should indicate the presence of Pt atoms along the (111) steps, which are coordinated to more than one oxygen atom each. The high local oxygen coverage along the step edges could be used to explain the high measured oxygen coverage, which was 1.7 times as large as that on Pt(111)-(2 × 2) O. The Pt 4f spectrum of the (almost) adsorbate-free Pt (533) surface (Figure 7a) consists of the expected scl_a and the bulk core level component. Additionally, a tiny scl_c component is present that might be due to a small Si contamination found on the Pt(533) crystal which attracts oxygen and which produces locally a high oxygen coverage. Note that this species corresponds to a coverage well below 0.1 ML, consistent with the detected minor quantity of Oox. Considering the Pt 4f spectra if Figure 7b,c, we can use the presence of the three surface core levels to propose a tentative model for the oxygen adlayer on Pt(533). The (2 × 2) O of Pt(111) with oxygen adsorbed in fcc sites contains noncoordinated and single-coordinated surface Pt atoms, resulting in scl_a and scl_b components for Pt 4f.1,21 The presence of a wellresolved third component, scl_c, demonstrates that higher

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Figure 6. Adsorbate coverages on Pt(533) during ammonia oxidation in the 1 mbar range with a large excess of oxygen in the gas phase. Left panels: Evolution of adsorbate coverages during heating and cooling. Right panels: selected N 1s and O 1s spectra during the heating branch. The corresponding temperatures are indicated in the coverage plot. Experimental conditions: p(NH3) ) 5.5 × 10-2 mbar, mixing ratio p(O2)/p(NH3) ) 10:1. The heating was performed in steps. Each temperature increase was achieved by applying a temperature ramp of ∼10 K/min followed by several minutes waiting time at constant temperature for acquiring the core level spectra.

coordinated Pt atoms exist on the surface. We propose a structure model of the oxygen-covered Pt(533) surface displayed in Figure 8. Here, we take into account that under reaction conditions and also in pure oxygen a reversible doubling of the step height of the Pt(533) surface may occur. During annealing in 10-6 mbar oxygen, a doubling of the step height was observed in the similar Pt(557) surface in the temperature range between 550 and 800 K.34 During the ammonia oxidation over Pt(533) in the 10-4 mbar range with a 1:1 ratio O2/NH3, the double-atomic step height was reported to be present during heating from 590 to 770 K and with a 10:1 ratio during heating from 500 to 650 K.9,10 In the current investigation, we had no possibility to verify this restructuring process by LEED. For the 10-4 mbar range, an assignment can therefore be only made tentatively, and for the range around 1 mbar, no information about possible restructuring effects is available. Starting with a model of Pt (533) with single-atomic steps in Figure 8a, we put oxygen in the fourfold hollow site along the (100) step and in the threefold fcc sites on the (111) terraces, keeping the local coverage on the (111) plane at θO ) 0.25. The most symmetric arrangement is displayed in Figure 8a. The indicated unit cell contains eight surface Pt atoms and three oxygen atoms corresponding to a coverage of 0.375 ML. For simplicity, we count the size of the indicated unit cell as eight Pt atoms, although one might use a projection on effective (111) terrace sites yielding noninteger numbers for surface atoms as in ref 1. Note that in the proposed model only one out of eight Pt atoms is not coordinated to oxygen, in agreement with the low

intensity of the scl_a component in the Pt 4f spectrum shown in the middle panel of Figure 7. Comparing the amount of single- and double-coordinated Pt atoms, one would expect the scl_c core level component to be about 4/3 more intense than the scl_b component. This is clearly not the case as seen in Figure 7. The intensity ratio scl_c/scl_b equals 0.77 instead of the expected 1.33, which indicates that only 58% of the doublecoordinated sites are populated. With (0.58 × 2 + 1) ) 2.16 O atoms per unit cell a coverage of 0.27 ML results. For the sake of simplicity, we use a coverage of 0.25 ML for calibration as noted in the Experimental Section. We want to point out that the Pt(533) surface during adsorption of oxygen does not reflect a perfect adlayer as if prepared after oxygen dosing in UHV, but rather reflects a surface in dynamic equilibrium between oxygen adsorption and clean off reaction of contaminants. Therefore, we expect a considerable amount of disorder and deviations from the ideal coverage. As shown in the lower panel of Figure 7, upon raising the oxygen pressure to 0.9 mbar, the ratio of the scl_c and scl_b component equals 1.25, which is not too far from the expected value of 1.33. During oxygen adsorption, only surface core level shifts occur. At no time were Pt 4f peak shifts detected that would indicate the formation of bulk Pt-oxides (PtO with Pt 4f7/2 at 72.3 eV and PtO2 with Pt 4f7/2 at 74.1 eV).35,36 For the double-atomic step structure shown in Figure 8b, we have to fill all fourfold sites along the (100) steps to keep the same oxygen coverage and the same fcc (2 × 2) O adatom arrangement on the (111) terrace as in the model of Figure 8a. Thus, along the (100) steps a 1D surface PtO2 oxide is created, very similar to the one proposed by Wang et al. for the (111)

Catalytic Ammonia Oxidation over a Pt(533) Surface

Figure 7. Scaled Pt 4f spectra of the Pt(533) crystal obtained during (a) a cleaning treatment at p(O2) ) 1 × 10-5 mbar and T ) 510 K (the surface is almost adsorbate free), (b) during oxygen adsorption at 340 K and p(O2) ) 1 × 10-4 mbar, and (c) oxygen adsorption at 410 K and p(O2) ) 0.9 mbar. Four different components are used to fit each peak of the doublet: bulk and three surface core levels (scl_a, scl_b, and scl_c). The surface core level slc_a accounts for Pt not coordinated to oxygen, scl_b for Pt atoms coordinated to one oxygen atom, and scl_c for surface Pt atoms coordinated to more than one oxygen atom.

steps of a Pt (322) surface.1 In this 1D oxide, each surface Pt atom of the (100) steps is coordinated to four oxygen atoms. While the amount of single-coordinated Pt surface atoms remains the same in the two models, in Figure 8b twice as many noncoordinated Pt atoms exist. This would lead to a pronounced scl_a peak that is not observed in the spectra of Figure 7. Furthermore, in Figure 8a 50% of the surface Pt atoms are coordinated to two O atoms, whereas in model Figure 8b this number only amounts to 25%. Thus, for double-atomic steps one would expect that the height of the scl_c component in the Pt 4f spectrum to be reduced by a factor of 2, while an additional small, further shifted surface core level component should be present due to 12.5% fourfold coordinated surface Pt atoms in Figure 8b. Clearly, the spectra displayed in Figure 7b,c match better with the single-atomic stepped surface in Figure 8a than with the double-atomic steps in Figure 8b. This result is consistent with previous LEED studies because the spectra in Figure 7a,b were taken at lower temperatures than the 550 K beyond which the doubling of the step height occurs in pure oxygen.34 The fact that the scl_c component in Figure 7c had to be shifted slightly to higher binding energy might indicate that at 0.9 mbar Pt surface atoms exist that are coordinated to more than two oxygen atoms, but due to the complete absence of the scl_a component the Pt 4f spectrum still is more compatible with the adsorbate model of the single stepped

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15389 surface of Figure 8a, in accordance with the applied low temperature of 410 K. 3.3.2. Stationary Ammonia Oxidation. To detect Pt-oxide formation, Pt 4f spectra were recorded during ammonia oxidation in the 10-4 mbar and in the 0.6 mbar range. Figure 9a displays the Pt 4f spectra obtained during ammonia oxidation at 420 and 770 K with p(NH3) ) 1 × 10-4 mbar and MR ) 3:1 (see corresponding experiment of Figures 3 and 4). Figure 9b shows the spectra recorded at 440 and 770 K with p(NH3) ) 5.5 × 10-2 mbar and MR ) 10:1 (see corresponding experiment of Figure 6). The Pt 4f spectra at p(NH3) ) 1 × 10-4 mbar and with MR ) 10:1 (Figure 5) are almost identical to the ones obtained with an MR ) 3:1 and therefore not reproduced here. It should be mentioned that all Pt 4f spectra can be well fitted with the same components used for oxygen adsorption. This means that no bulk Pt bulk oxides are formed during ammonia oxidation, whereas surface Pt-oxides might contribute to the spectra. The scl_c component in the Pt 4f spectrum grows with coverage, and therefore this component is larger at lower temperature as compared to high temperature, and it also grows with increasing pressure as shown by the data in Figure 9. At low pressure and T ) 420 K (lower left panel of Figure 9), one finds the scl_a component that corresponds to uncoordinated Pt (i.e., a adsorbate free surface Pt atoms). This component is absent at elevated pressure for T ) 440 K. At a temperature of 770 K, the adsorbate coverage is low at both low and elevated total pressures. The scl_a component is therefore more pronounced than at 420 K/440 K (upper panels of Figure 9). Similar to the Pt 4f spectrum in the upper panel of Figure 7 again a tiny scl_c component is present in the upper spectra of Figure 9 (obtained at 770 K). It is again attributed to the observed Si contamination on the Pt(533) surface and will be discussed further below. The intensity of the scl_b component in the upper right panel representing Pt surface atoms single coordinated to oxygen reflects the presence of chemisorbed oxygen, Oad, at 770 K and high pressure, which is in fact observed (Figure 6). What is most surprising is that the scaled Pt 4f spectra in the upper panel of Figure 9 are very similar, despite a difference in the Oox concentration from below 0.03 ML (left) to 0.81 ML (right).37 Since the Pt 4f peak shape is hardly affected by the accumulated Oox species, we attribute the Oox signal with an O 1s BE at 532 eV to a strained SiOx layer on top of the Pt substrate. We also monitored the binding energy of the Si 2p core level to confirm this assignment. The binding energy of Si 2p in elemental Si lies at 98 eV, and values between 103.3 and 103.6 eV were reported for SiO2.33 The 102.2-102.5 eV BE measured here is attributed to strained SiOx in agreement with ref 38 where similar binding energies were reported. It is further supported by the fact that the Oox peak is linked to the presence of Si as contaminant and that its intensity scales with the amount of Si. The formation of a Pt-oxide can be ruled out since the Pt 4f peak remained unaffected. In addition, according to the literature, Pt-oxides are characterized by an O 1s binding energy of 530.3- 530.5 eV,35,36 which is well separated from the BE of ∼532.0 eV measured here. The small scl_c component in the Pt 4f spectrum that persists even under reducing conditions is assigned to the interface Pt/SiOx because this interface represents a ternary Pt/Si/O phase where a high local oxygen concentration can cause a shift in the Pt 4f signal. The low intensity of the scl_c component even in the upper right panel of Figure 9 (where a nominal concentration of 0.81 ML Oox was accumulated) can be explained by the fact that most probably

15390 J. Phys. Chem. C, Vol. 112, No. 39, 2008

Gu¨nther et al.

Figure 8. Tentative ball models for the adsorption layer of chemisorbed oxygen on a Pt(533) surface single-atomic steps (a) and a doubleatomic steps (b). The unit cells and the coordination numbers of the Pt surface atoms are indicated.

Figure 9. Scaled Pt 4f spectra of the Pt(533) crystal obtained during stationary ammonia oxidation. (a) At 420 and 770 K, p(NH3) ) 1 × 10-4 mbar, and MR p(O2)/p(NH3) ) 3:1 (corresponding to Figures 3 and 4). (b) At 440 and 770 K, p(NH3) ) 5.5 × 10-2 mbar, and MR p(O2)/p(NH3) ) 10:1 (corresponding to Figure 6). Every spectrum was fitted by applying the same components as the ones used for the data set of the Pt(533) surface in oxygen atmosphere plotted in Figure 7: bulk- and three surface core levels (scl_a, scl_b, and scl_c).

the SiOx phase is present in islands rather than in a flat homogeneous layer on the Pt(533) surface. Thus, the main Oox intensity in the O 1s peak originates from 3D SiOx islands above the Pt/SiOx interface, as is evidenced by the depth profiling we performed by varying the photon energy. By changing the photon energy from 830 to 1330 eV, we change the kinetic energy and thus the escape depth of the emitted photoelectrons. As demonstrated by Figure 10 obtained from a strongly “oxidized” surface, the Oox component in the 1330 eV spectrum is about 20% larger than that in the 830 eV spectrum. The ratio of the Oox/Oad intensity would only then be independent of the photon energy if both signals would originate from the same escape depth, for example, if the

oxide would be contained in the first layer as is the case for chemisorbed oxygen. Using the values for the escape length of the emitted photoelectrons in Pt of λ ) 6 Å for hν ) 830 eV and 11.2 Å for hν ) 1330 eV,39 we calculate that the Oox species is deposited as a 3D oxide on top of the metallic Pt substrate with an average height of about four atomic layers. Instead of covering about 80% of the metal surface as calculated for an oxidic film of monolayer thickness, only ∼30% of the Pt(533) surface is covered by the SiOx phase. The change of the total Pt 4f intensity obtained from a sample with or without a high amount of accumulated Oox seems to be consistent with this interpretation. Using the atomic photoionization cross sections for Si 2p and Pt 4f, we can

Catalytic Ammonia Oxidation over a Pt(533) Surface

Figure 10. “Depth profiling” of the oxygen species Oad and Oox by acquiring the O 1s spectrum at 830 and 1330 eV from the Pt(533) surface after the high pressure ammonia oxidation experiment was performed and a nominal amount of 0.81 ML of Oox was accumulated. The plots are scaled such that the Oad component in both spectra has the same intensity. It is well visible that the Oox component of the 1330 eV spectrum is about 20% enhanced, indicating that the Oox species is distributed in a several-layers-thick phase.

estimate the amount of Si and obtain the stoichiometry of SiOx with x ranging between 1 and 2. Furthermore, we can compare the relative intensity drop of the Pt 4f spectra of two samples under similar conditions, but with or without a high amount of accumulated Oox (e.g., the two upper Pt 4f spectra displayed in Figure 9). Therefore, we scale the corresponding spectra to the same background intensity and thus remove intensity variations due to different alignment and electron transmission mainly due to the different electron damping of the low or high gas-phase pressure in the differentially pumped reactor. (Here, we assume that the electron damping of the Pt 4f peak equals the one of the background electrons.) In doing so, one can obtain an estimate of the relative intensity variation of the corresponding Pt 4f peaks. In the case where a nominal amount of 0.81 ML Oox was accumulated (i.e., 4 ML high islands were covering ∼30% of the surface), the corresponding Pt 4f spectrum was found to be about 28% smaller with respect to the one obtained from a surface with about 0.03 ML Oox. This number is in fair agreement with the one obtained from the O 1s depth profiling experiment. 4. Discussion In our study, a significant amount of Si contamination was present on the surface, but since the Si is presumably bound in 3D islands which cover a certain part of the Pt surface (estimated 30%) but do not influence the remaining surface the validity of the data should not be affected by the SiOx contamination. This is of course a crucial assumption but it is supported by the data, which give no indication that any of the spectroscopic features we relate to adsorbates on the Pt surface are influenced by the SiOx contamination. We also compared the stationary reaction rates vs temperature of the silicon oxide-contaminated Pt(533) surface with the rates on a clean surface and found no qualitative differences. In particular, the position of the rate maxima, the shape of the curves, and the onset of NO formation remained unchanged. All this supports the conclusion that the SiOx is present as stable islands which block a certain fraction of the Pt surface but leave the rest unchanged. The high thermodynamic stability of silicon oxide will make SiOx practically inert under the reaction conditions employed here. Different from the role of cerium oxide in the automotive catalytic converter, it will not act as an oxygen storage material releasing or absorbing oxygen depending on the reaction conditions. Furthermore, the observed moderate drop in activity upon SiOx

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15391 accumulation seems to be consistent with the estimated unselective blocking of about 30% of the surface rather than decorating specifically the step edges and thereby blocking the most active sites in ammonia decomposition. On the basis of quantum chemical calculations and experiments, there is more or less general agreement that in catalytic ammonia oxidation ammonia decomposes via direct interaction with adsorbed oxygen or an OH species.7,10,14,40 Although the skeleton mechanism for this reaction has thus been established, it remains to be verified whether the proposed intermediates exist on the Pt surface and to quantify their coverages. This latter point, which is particularly important for a realistic modeling of the reaction, has been achieved in this study. The following reaction sequence can be formulated.

NH3,ad + Oad f NH2,ad + OHad

(R4)

NH2,ad + Oad f NHad + OHad

(R5)

NHad + Oad f Nad + OHad

(R6)

Oad + Had f OHad + *

(R7)

NH3,ad + OHad f NH2,ad + H2O(g) + *

(R8)

NH2,ad + OHad f NHad + H2O(g) + *

(R9)

NHad + OHad f Nad + H2O(g) + *

(R10)

OHad + Had f H2O(g) + 2*

(R11)

2OHad f Oad + H2O(g) + *

(R12)

The existence of NHx (x ) 1, 2) intermediates was demonstrated for ammonia oxidation on Pt(111) in an EELS study by Mieher and Ho.6 They found that, after sequential dosing of O2 and NH3 at T ) 85 K followed by heating, the NHx species were visible up to 300 K, whereas Nad remained detectable up to 650 K.6 An OH species was only seen at T < 285 K. An HNO has thus far not been detected in any reaction experiment. Weststrate et al.17 reported on the identification of NH3ad NH2ad, NHad, and Nad during the ammonia oxidation over Pt(410) at pressures below 1.5 × 10-7 mbar, although the exact peak position of the NH2ad species is not evident from the N 1s spectrum they present. They identified as oxygen-containing adsorbates NOad, two types of chemisorbed oxygen, and at T < 200 K a further species, which was assigned to H2Oad. The qualitative picture of the thermal stability of the different NHx (x ) 0-3) adspecies is similar to the one reported by Mieher et al.6 In our experiments, we could not find any evidence for an OH species similar to the conclusions from Weststrate et al.,17 but this is in line with our expectations because this reactive intermediate will rapidly be consumed either through reaction with NHx (x ) 1-3) or in the water-forming reactions R8R12. A substantial fraction of the surface is covered by NHx (x ) 1, 2) intermediates. Surprisingly, their coverage increases with increasing oxygen content in the gas atmosphere as demonstrated by Figures 35. In a large excess of oxygen, the NHx coverage can reach up to 0.2 ML as shown by Figure 5. This result can be explained with the activation of ammonia decomposition by oxygen leading to the accumulation of NHx (x ) 1, 2) intermediates on a reactive surface at not too high temperatures. The energy profile for the nonactivated and the Oad/OHadactivated decomposition of ammonia was calculated for Pt(111) and Pt(211) in a DFT study by Offermans et al.14 The main result was that both Oad and OHad lower the energy barrier for ammonia decomposition, but whereas Oad only facilitates the abstraction of the first H atom from ammonia (eq R4), OHad

15392 J. Phys. Chem. C, Vol. 112, No. 39, 2008 lowers the activation barriers for all H-abstracting steps The bonding strength of NHx (x ) 1, 2) was found to increase with decreasing number of hydrogen atoms, x. Energetically, the dominating intermediate should therefore be NH. Since we cannot differentiate between NH and NH2 in our spectra, we cannot verify this point but overall all our data agree with the proposed mechanism and the calculated energy profile. The main conclusions we can draw with respect to the pressure gap are the following ones: (i) At high pressure (1 mbar), the N intermediates are detectable up to much higher temperature than at low pressure (10-4 mbar). (ii) At high pressure, chemisorbed oxygen is also present in the low T regime before ignition of the reaction. (iii) As a consequence of (ii), the self-cleaning of the surface is enhanced at high pressure and the formation of carboncontaining contaminations is strongly reduced. (iv) At no time was bulk Pt-oxide formed in our experiments, but the formation of highly O-coordinated Pt-step atoms or even a 1D surface oxide localized at steps is consistent with the Pt 4f spectra we recorded. In contrast to Si-oxide, the C contamination is a dynamic contamination whose amount changes with the reaction conditions. In the reactive state and, in particular, at 1 mbar, this contamination plays hardly any role but carbon can accumulate as shown in Figure 3 if the surface is not active. The increase of the carbon signal correlates with the appearance of an N 1s signal at 396.7 eV, which we tentatively relate to the formation of a CN species on Pt(533) (section 3.1.1). Since the C coverage is much larger than the NC coverage (Figures 4 and 5), it is clear that only a small part of the surface carbon can be present as a CN species. The majority of the carbon therefore has to be attributed to hydrocarbon species CXHY or to graphitic or carbidic carbon. The presence of more than one carbon species on the surface is in agreement with the already mentioned fact that the observed C 1s spectra are rather broad (fwhm ≈ 1.5-2 eV). For ethylene adsorption on Pt(111), a saturation coverage of 0.25 ML was reported for T ) 120 K.41,42 Compared with this coverage, the atomic carbon coverage of 0.2 ML, which is reached at maximum on our surface, seems to indicate that a fairly large part of the Pt(533) surface is blocked by carbon. If, however, we take into account that also higher hydrocarbons such as benzene are likely to be involved and that the carbon contamination might grow as 3D islands, then the percentage of the surface that is blocked would be much smaller. Irrespective of the precise nature of the carbon contamination, we see that the carbon is very efficiently removed after ignition of the reaction. At 1 mbar, no carbon is left beyond 470 K, and at 10-4 mbar, the surface is carbon free above 550 K. Apparently, the active state of the surface is hardly influenced by the carbon contamination. Under reaction conditions, no NO was detectable on the surface, even at 1 mbar. Since NO is a reaction product, this intermediate has to exist but its stationary concentration will depend on the relative rates of its formation and removal. Apparently at low temperature (350 K < T < 580 K), NO decomposition (which sets in already at 350 K) prevents the formation of a larger NO concentration, whereas at T > 600 K, a high desorption rate keeps the intermediate concentration below our detection limit. Weststrate et al. showed the formation of NOad after dosing NH3 on an oxygen-precovered Pt(410) surface at 200 K. This species desorbs upon heating between 430 and 530 K.17 In contrast to our experiments, they also detected NOad under stationary reaction conditions below 600 K. A possible explanation for the different findings is that (100)

Gu¨nther et al. steps which are present on Pt(533) but not on Pt(410) exhibit a particularly high activity in dissociating NO. The small stationary NO coverage on Pt(533) explains the absence of N2O formation in our experiments. With few exceptions, N2O is typically not detected in UHV experiments,17 but at higher pressure (p > 1 mbar) it is found as a byproduct of NO/N2 formation in ammonia oxidation. The spectra we recorded did not allow us to verify whether a doubling of the step height occurs under reaction conditions. Since the oxygen coverage is rather low in the temperature regime of 600-800 K (Figures 36) where a doubling of the step height should take place, the changes in the Pt 4f spectrum are too subtle to judge whether double- or single-atomic steps are present. Nevertheless, the formation of a 1D surface Ptoxide along the (100) steps as suggested in Figure 8b might well be the driving force for the observed doubling of the step height on Pt(533) and related surfaces.9,34,43,44 Legare et al. in fact observed a shift of one component of Pt 4f by 1.2 eV toward higher BE after adsorbing 100 L of O2 on Pt(533) at 700 K. They proposed the formation of a “superficial Pt-oxide”, similar to PtO, which expands into subsurface layers.43 The locally high oxygen concentration of the 1D oxide sketched in Figure 8b might also explain the observed change in selectivity from N2 toward NO formation which accompanies the doubling in step height since the oxygen atoms in the 1D oxide remain reactive.1,9,10 The existence of this 1D oxide under reaction conditions, however, needs to be verified in future experiments. 5. Conclusions In the pressure range 10-4 to ∼1 mbar, we could detect chemisorbed oxygen and the nitrogen-containing species NH3 and NHx (x ) 0, 1, 2) during catalytic ammonia oxidation on Pt(533). No NOad and no OHad were found under reaction conditions. The absence of NOad at low temperatures was attributed to a particularly high activity of (100) steps in dissociating NO. The coverage of the NHx (x ) 1-3) species was found to increase with growing oxygen content in the gas atmosphere reaching up to 0.2 ML. This unexpected relation was attributed to the role of Oad in activating ammonia, thus causing the enrichment of NHx intermediates. With increasing total pressure, the amount of carbon contamination was strongly reduced. At no time during our experiments were bulk Pt-oxides detectable. We observed, however, that Pt surface atoms coordinated to more than one oxygen atom. These Pt atoms might be due to a 1D oxide phase localized along step edges. Linked to a Si contamination, we found an oxygen species with low reactivity characterized by an O 1s energy of ∼532.0 eV. This species is assigned to an unreactive strained SiOx phase (with x ) 1-2) that presumably grows in islands and covers part of the Pt surface. Acknowledgment. This work was supported by the DFG under the priority program 1091 “Bridging the gap between ideal and real systems in heterogeneous catalysis”. The BESSY staff is gratefully acknowledged for the continuous support of the in situ XPS measurements. References and Notes (1) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmidt, M.; Hammer, B.; Andersen, J. N. Phys. ReV. Lett. 2005, 95, 256102. (2) Chilton, T. H. The Manufacture of Nitric Acid by the Oxidation of Ammonia; Chemical Engineering Progress Monograph Series 3; American Institute of Chemical Engineers: New York, 1960. (3) Gland, J. L.; Korchak, V. N. J. Catal. 1978, 53, 9.

Catalytic Ammonia Oxidation over a Pt(533) Surface (4) Gland, J. L.; Kollin, E. B. J. Vac. Sci. Technol. 1981, 18, 604. (5) Asscher, M.; Guthrie, W. L.; Lin, T.-H.; Somorjai, G. A. J. Phys. Chem. 1984, 88, 3233. (6) Mieher, W. D.; Ho, W. Surf. Sci. 1995, 322, 151. (7) Bradley, J. M.; Hopkinson, A.; King, D. A. J. Phys. Chem. 1995, 99, 17032. (8) Kim, M.; Pratt, S. J.; King, D. A. J. Am. Chem. Soc. 2000, 122, 2409. (9) Scheibe, A.; Gu¨nther, S.; Imbihl, R. Catal. Lett. 2002, 86, 33. (10) Scheibe, A.; Lins, U.; Imbihl, R. Surf. Sci. 2005, 577, 1. (11) Imbihl, R.; Scheibe, A.; Zeng, Y.; Gu¨nther, S.; Kraehnert, R.; Kondratenko, V. A.; Baerns, M.; Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Phys. Chem. Chem. Phys. 2007, 9, 3522. (12) Bradley, J. M.; Hopkinson, A.; King, D. A. Surf. Sci. 1997, 371, 255. (13) Guthrie, W. L.; Sokol, J. D.; Somorjai, G. A. Surf. Sci. 1981, 109, 390. (14) Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Surf. Sci. 2006, 600, 1714. (15) Gohndrone, J. M.; Olsen, C. W.; Backman, A. L.; Gow, T. R.; Yagasaki, E.; Masel, R. I. J. Vac. Sci. Technol., A 1986, 7, 1986. (16) Scheibe, A.; Hinz, M.; Imbihl, R. Surf. Sci. 2005, 576, 131. (17) Weststrate, C. J.; Bakker, J. W.; Rienks, E. D. L.; Vinod, C. P.; Matveev, A. V.; Gorodetskii, V. V.; Nieuwenhuys, B. E. J. Catal. 2006, 242, 184. (18) van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1980, 92, 489. (19) Ogletree, D. F.; Bluhm, H.; Lebedev, G.; Fadley, C. S.; Hussain, Z.; Salmeron, M. ReV. Sci. Instrum. 2002, 73, 3872. (20) Bluhm, H.; Ha¨vecker, M.; Knop-Gericke, A.; Kiskinova, M.; Schlo¨gl, R.; Salmeron, M. MRS Bull. 2007, 32, 1022. (21) Puglia, C.; Nilsson, A.; Hernnas, B.; Karis, O.; Bennich, P.; Martensson, N. Surf. Sci. 1995, 342, 119. (22) Norton, P. R.; Davies, J. A.; Jackmann, T. E. Surf. Sci. 1982, 122, L593. (23) The given energy positions together with the error bars indicate that the energy intervals within the corresponding N 1s peaks were allowed to vary to provide consistent fit results for all N 1s spectra of our investigations. Therefore, the center energy position and the confidence

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