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J. Phys. Chem. C 2010, 114, 19734–19743
Influence of Steps on the Adsorption and Thermal Evolution of SO2 on Clean and Oxygen Precovered Pt Surfaces R. Streber,† C. Papp,*,† M. P. A. Lorenz,† O. Ho¨fert,† W. Zhao,† S. Wickert,‡ E. Darlatt,‡ A. Bayer,† R. Denecke,‡ and H.-P. Steinru¨ck†,§ Lehrstuhl fu¨r Physikalische Chemie II, UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany, Willhelm-Ostwald-Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Leipzig, Linne´strasse 2, 04103 Leipzig, Germany, and Erlangen Catalysis Resource Center (ECRC), UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ReceiVed: June 29, 2010; ReVised Manuscript ReceiVed: September 15, 2010
We investigated the adsorption and reaction of SO2 on clean and oxygen precovered, flat, and regularly stepped Pt surfaces, Pt(111), Pt(322), and Pt(355) by in situ high resolution X-ray photoelectron spectroscopy. Induced by the steps and/or the coadsorbed oxygen, several new SO2 species are observed, in addition to the two known SO2 species on Pt(111). On all investigated surfaces, a hit and stick mechanism is found for the adsorption, and no diffusion from terrace to step sites occurs at low temperatures. For the oxygen free stepped surfaces, a higher reactivity toward SO2 dissociation/disproportionation is found compared to Pt(111). On all three oxygen precovered Pt surfaces some SO3 is already formed directly upon SO2 exposure at low temperatures. Heating of this mixed O/SOx layers first results in oxidation of more SO2 to SO3, and at higher temperatures in subsequent oxidation of SO3 to SO4, which finally decomposes above 450 K. The different reactions show a significant influence of the steps. 1. Introduction Defects often play a decisive role in catalysis. Many relevant heterogeneously catalyzed chemical reactions take place on small active particles, whose surface properties are governed by surface defects, i.e., low coordinated sites.1,2 This is mainly related to pronounced differences in the geometric and electronic structure of these sites as compared to flat surfaces of perfect single crystals, which are typically investigated in surface science. The resulting differences in the potential energy surfaces lead to changed bond strengths and, if existing, activation energies for adsorption. Poisoning of active sites is a major issue in large scale applications of heterogeneously catalyzed reactions,3 but also of significant fundamental interest in understanding the underlying principles and mechanisms. Sulfur and its oxides are common impurities in crude oil; as such they are known to poison platinum containing catalysts. The resulting decrease of activity, e.g., in automotive catalysts,3-7 is of great economic and ecological importance. Due to this high relevance the adsorption and reaction of SO2 on platinum single crystal surfaces has been the subject of numerous studies in the past, which aimed at a fundamental understanding of the underlying processes.8-22 However, to our knowledge no studies on the interaction of SO2 on stepped surfaces, including all transition metal surfaces, have been published so far, although vicinal surfaces with a regular arrangement of monatomic steps are well-suited model systems to study the role of surface defects with various surface science methods. The surfaces studied here * To whom correspondence should be addressed. E-mail: christian.papp@ chemie.uni-erlangen.de. † Lehrstuhl fu¨r Physikalische Chemie II, Universita¨t Erlangen-Nu¨rnberg. ‡ Willhelm-Ostwald-Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Leipzig. § Erlangen Catalysis Resource Center (ECRC), Universita¨t ErlangenNu¨rnberg.
could help to elucidate the relevant sites for adsorption and reaction of SO2. In the literature, there is general agreement that on clean Pt(111) SO2 adsorbs molecularly in two different adsorption geometries at 120 K, namely flat lying and upright standing SO2. These two geometries have been identified from their different binding energies in high resolution X-ray photoelectron spectroscopy (HR-XPS)16 and from near edge X-ray absorption fine structure (NEXAFS) measurements16 and have been confirmed later on by density functional theory (DFT) based calculations by Lin et al.13 A low energy electron diffraction (LEED) study of this system showed no long-range order.11 In temperature programmed desorption (TPD) experiments on Pt(111) SO2 is found to desorb around 370 K for small initial coverage, whereas for higher initial coverage the main desorption peak shifts to ∼320 K.8,11,20 Upon heating SO2 multilayers to 300 K, the formation of SO and SO4 species has been proposed based on high resolution electron energy loss spectroscopy (HREELS)19 data; however, from HR-XPS, S, and SO4 have been identified as surface species16 under comparable conditions, with the SO4 species being stable up to at least 370 K. Different from these experiments, heating of a monolayer of SO2 leads to nearly complete desorption of intact SO2, with only very small amounts dissociating and forming S and SO4.16,18 The thermal evolution of SO2 on O-precovered Pt(111) was studied by TPD; the corresponding spectra showed a SO2 desorption peak at 320 K and also significant desorption of SO2 and/or SO3 above 500 K8,20,21 (note that SO3 is difficult to identify in the mass spectrometer, as the dominant cracking fragment is SO223). This high temperature peak was associated with the decomposition of SO4. An HR-XPS study by Lee et al. identified two SO2 species on oxygen precovered Pt(111),12 with binding energy values corresponding well to those on clean Pt(111).16 In the same study, the formation of an SOx species is reported at ∼200 K, which already starts to decompose at 280
10.1021/jp105994f 2010 American Chemical Society Published on Web 10/28/2010
Adsorption and Thermal Evolution of SO2 K, leaving only atomic S on the surface at 340 K,12 which is in contrast to the conclusions derived by TPD (see above). In a very recent HR-XPS study of SO2 adsorption on the same system, we also observed the two well-known contributions of lying and standing SO2.18 In addition, a third oxygen-induced SO2 species was found and it was shown that formation of some SO3 already occurs upon adsorption below 130 K. Upon heating to 290 K, more SO2 is oxidized to SO3 and at higher temperatures subsequent oxidation of 46% of the initial amount of SO2 and SO3 to SO4 was observed. In agreement with the TPD results, SO4 decomposed and SOx desorbed above 450 K.18 To our knowledge no study of SO2 adsorption on regularly stepped Pt surfaces has been published so far. There is only one combined Auger electron spectroscopy (AES) and TPD study on polycrystalline Pt foil,24 which might contain a high amount of defects, although their nature was ill defined. In that study, Wu et al. found several desorption states for SO2 and on the oxygen precovered surface also for SO3, even at temperatures as high as 1400 K. In this work, we present the first study of adsorption and reaction of SO2 on clean and oxygen precovered regularly stepped Pt(322) and Pt(355) surfaces and compare these results to measurements on flat Pt(111), which have been published by us recently.18 We use HR-XPS to identify the various SOx species formed upon adsorption and during heating. SO2 adsorbs molecularly on the flat and stepped clean Pt surfaces. While on Pt(111) SO2 mostly desorbs intact upon heating, on the stepped surfaces significant amounts of SO3 and S are found. On all oxygen precovered surfaces some SO3 is already formed upon adsorption at low temperature. During heating SO2 reacts to SO3, which is subsequently oxidized to SO4. For all processes, a significant influence of the steps is observed. 2. Experimental Section The measurements were performed using a transportable apparatus (for details see ref 25) at the BESSY II synchrotron radiation facility in Berlin, Germany, at beamline U49/2 PGM1. The analysis chamber is equipped with a supersonic molecular beam setup. The XP spectra were measured using an energy analyzer (Omicron EA 125 HR U7) that is mounted in the plane of the synchrotron ring, at an angle of 50° with respect to the incoming synchrotron radiation. S 2p spectra were collected with a total resolution of 140 meV at a photon energy of 260 eV and a data acquisition time of 10 s per spectrum. All binding energies were referenced to the Fermi edge. Using this procedure, the S 2p3/2 contribution of lying SO2 exhibits a value of 165.4 eV. The reproducibility of binding energy values within this study is (0.03 eV; the calibration of the absolute binding energy scale compared to other studies has an uncertainty of typically (0.4 eV (compare to ref 18). The Pt(322) and Pt(355) samples were oriented with their macroscopic surface normal pointing toward the electron analyzer and the monatomic steps perpendicular to the plane formed by the electron analyzer and incoming X-ray beam. To study the thermal evolution of SO2 temperature programmed XPS (TP-XPS) experiments were performed with a linear heating ramp of 0.5 K/s. Details of sample preparation and cleaning can be found in ref 26. The Pt(355) and Pt(322) surfaces both have five atom rows wide (111) terraces with monatomic steps of different orientations, namely (111) and (100), respectively. For the SO2 adsorption experiments the surfaces were exposed to a SO2 background pressure of 1 to 6 × 10-9 mbar, while XP spectra were recorded continuously. For saturating the surfaces with oxygen, the supersonic molecular beam was used to dose O2.
J. Phys. Chem. C, Vol. 114, No. 46, 2010 19735 A total of 60 L of O2 were dosed on Pt(111) at temperatures below 150 K. Subsequently, the surface was heated to 300 K, leading to a p(2 × 2) overlayer of atomic oxygen with a coverage of 0.25 ML.27,28 The O 1s spectra of the so prepared oxygen layers were then used for calibrating all other oxygen coverages. To saturate the stepped surfaces with oxygen, Pt(355) was exposed to 720 L and Pt(322) to 900 L of oxygen, at a sample temperature of 300 K. This leads to oxygen coverages of 0.54 ( 0.04 ML for Pt(355) and 0.33 ( 0.03 ML for Pt(322).29 The sulfur coverages were calibrated by comparison to S 2p spectra collected with a photon energy of 380 eV from a p(2 × 2) overlayer of S on Pt(111) with a coverage of 0.25 ML;30 this layer was prepared by dosing H2S and subsequent heating to 700 K with H2 desorbing and S remaining on the surface. 3. Results and Discussion 3.1. Adsorption of SO2 on Stepped Pt Surfaces. First, we briefly reconcile the adsorption of SO2 on a Pt(111) surface.18 Figure 1a shows selected S 2p spectra collected successively during dosing of SO2 below 130 K. Two sharp, well-separated doublets can be seen with the S 2p3/2 (S 2p1/2) components at 164.5 (165.7) eV and 165.4 (166.6) eV, which grow with increasing exposure. The separation of the 2p3/2 and 2p1/2 peaks is 1.22 ((0.03) eV and their intensity ratio is 2:1, as expected from the spin multiplicity (this holds for all SOx species (x ) 0-4) observed in this study, i.e., independent of the chemical environment). The doublet with the S 2p3/2 (2p1/2) component at 164.5 (165.7) eV was assigned to flat lying SO2 with the molecular plane parallel to the surface, the one at 165.4 (166.6) eV as upright standing SO2 with the molecular plane perpendicular to the surface.12,13,16,18 The S 2p3/2 peaks are marked with an orange (lying: SO2(ly)) and a pink (standing: SO2(st)) line in Figure 1a. During SO2 adsorption the overall shape of the spectra does not change, as is particularly evident when comparing the spectra for exposures of 0.07 and 0.17 L (dotted and bold black lines in Figure 1a, respectively). This means that both species are adsorbing with a constant ratio.18 Due to the sensitivity of SO2 to beam damage induced by synchrotron radiation, some minor amounts of SO and S are formed,12,18 with the S 2p3/2 (2p1/2) components at 163.5 (164.7) eV and 162.2 (163.4) eV, respectively. Figure 1b shows selected spectra collected during SO2 adsorption on Pt(322) at 155 K. Again, the two doublets assigned to flat lying SO2(ly) and upright standing SO2(st), indicated by orange and pink lines, respectively, are the dominating features of the spectra. However, significantly more SO2 is bound in the upright standing than in the flat lying geometry as compared to Pt(111). From the comparison of spectra recorded at similar total coverage (bold black spectra in Figure 1, parts a and b, respectively), it is evident that for Pt(322) the peaks at 165.5 (166.7) eV due to upright standing SO2(st), exhibit a shoulder at the low binding energy side, at 165.2 (166.4) eV (see also Supporting Information, Figure SI1); the position of the corresponding S 2p3/2 contribution is marked with a purple line in Figure 1b. This contribution is assigned to an additional SO2 species, SO2(step-st), bound to the low coordinated Pt atoms at the (100) oriented steps of the Pt(322) surface; due to its energetic position close to that of the upright standing SO2(st) it is assumed to have a similar upright standing geometry. Note that the adsorption geometry cannot be determined from the XP spectra. Additional investigations with other techniques (like NEXAFS or HREELS) are necessary, which are only available for the two SO2 species on clean Pt(111). Hence, the labeling of all other SO2 species has
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Figure 1. S 2p spectra collected during SO2 dosing on (a) Pt(111) at T < 130 K, (b) Pt(322) at T ) 155 K, (c) Pt(355) at T ) 100 K, (d) oxygen precovered Pt(111) at T < 130 K, (e) oxygen precovered Pt(322) at T ) 155 K, and (f) oxygen precovered Pt(355) at T ) 100 K. All spectra were recorded with a photon energy of 260 eV in normal emission geometry. The color-coded vertical lines indicate the energetic positions of the S 2 p3/2 peaks of the SOx species present on the surface. The intensity scale is the same for all experiments.
to be understood as tentative assignments. As for clean Pt(111), the ratio of the three SO2 species on Pt(322) is not influenced by coverage, as is concluded from their unchanged relative intensities with increasing exposure (compare, e.g., dotted and bold black line in Figure 1b). In Figure 1c, the spectra for SO2 adsorption on Pt(355) at 100 K are shown. Again, the peaks for SO2(ly) at 164.5 (165.7) eV and SO2(st) at 165.5 (166.7) eV are the most prominent features. On Pt(355), significantly more SO2 is bound in the flat lying than in the upright standing geometry, opposite to the situation on Pt(322). The comparison between Pt(111) and Pt(355) shows again the additional shoulder at 165.2 (166.4) eV, due to a third, step induced SO2 species. Furthermore, a very small contribution of a forth SO2 species is detected at 164.4 (165.6) eV, i.e., at the low binding energy side of the SO2(ly) peaks (dark red line in Figure 1c), which becomes prominent during heating (see below). From the energetic positions these two additional species are assigned to SO2(stepst), as on Pt(322), and SO2(step-ly), respectively. For higher SO2 exposures another doublet at 166.8 (168.0) eV appears, due to the formation of SO2 multilayers (spectrum for 1.03 L indicated as thin dashed curve in Figure 1c). As the multilayers are very sensitive toward synchrotron radiation induced beam damage, significant amounts of SO and S are found at these high SO2 exposures. As for the other two surfaces, the shape of the XP spectra does not change with increasing exposure (see, e.g., dotted and bold black line in Figure 1c), indicating that also on Pt(355) all SO2 species adsorb in a constant ratio. Step specific species have been identified by XPS also for other adsorbates, like CO (refs 26 and 31, and references
therein), CH3,32 NO,33 and benzene.34 In contrast to SO2, where one step species is found on Pt(322) and two on Pt(355), for CO two step species (on top and bridge bound) exist on Pt(322) and only one (on top) on Pt(355). An explanation for this reversed behavior might be that CO prefers low coordinated sites, whereas the highest possible coordination is favored for S. For CH3, only one step adsorption site could be observed on both surfaces. The spectra in Figure 1a-c were fitted for a quantitative analysis (for details see Supporting Information Figures SI-1 and SI-2): The corresponding results are displayed in Figure 2a-c, respectively, where the coverage of the different SOx species is plotted versus the SO2 exposure. Note that all coverages are derived neglecting photoelectron diffraction effects, which can be different for the SOx species (further discussion see below). For Pt(111), both the flat lying SO2(ly) and upright standing SO2(st) adsorb with an almost constant ratio, until multilayer adsorption starts at ∼0.35 L.18 Surprisingly, this is also true for the two stepped surfaces, where the step-induced SO2 species also grow steadily with exposure. This behavior of SO2 is very different from that of the other adsorbates such as CO [ref 26, and references therein], CH3,32 and benzene,34 where step sites are predominantly occupied at low coverages and terrace sites only at higher coverages. This suggests a hit and stick mechanism for SO2 adsorption on the investigated Pt surfaces at low temperatures, in contrast to the other adsorbates, where surface diffusion from terrace to step sites readily occurs. For Pt(111), Lin et al.14 calculated the activation energy for self-
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Figure 2. Quantitative analysis of the spectra depicted in Figure 1 on (a) clean Pt(111), (b) clean Pt(322), (c) clean Pt(355), (d) oxygen precovered Pt(111), (e) oxygen precovered Pt(322), and (f) oxygen precovered Pt(355).
diffusion (site change) from fcc upright standing to hcp flat lying SO2 to be 42 kJ/mol (for further discussion, see Section 3.2). Apart from providing new adsorption sites, the steps also influence the ratio of upright standing SO2(st) and flat lying SO2(ly) on the terraces:. At an exposure of ∼0.3 L 55% SO2(ly) and 45% SO2(st) are observed on Pt(111). On Pt(322), the upright standing SO2 terrace species dominates with 64% SO2(st), 22% SO2(ly) and 14% SO2(step-st); in contrast, on Pt(355) the flat lying terrace species has the highest occupation, with 59% SO2(ly), 29% SO2(st), 8% SO2(step-ly) and 4% SO2(step-st). This indicates that on the two stepped surfaces, due to the difference in step orientation, not only different step adsorption sites are available (one for Pt(322), two for Pt(355)), but also the preferred adsorption geometry on the terrace is different. For all three surfaces, the sum of all SO2 species in the first (chemisorbed) layer is 0.25 ( 0.02 ML, indicating that the presence of steps on the surface does not change the overall number of adsorption sites available for SO2. This is different from the behavior of other adsorbates: CO exhibits lower saturation coverages on the stepped surfaces compared to Pt(111),26 while atomic oxygen shows higher saturation coverages of 0.33 ( 0.03 ML for Pt(322) and 0.54 ( 0.04 ML for Pt(355) compared to 0.25 ML ( 0.02 ML on Pt(111)29,35 at 300 K. In this first part, we analyzed the influence of steps on the adsorption of SO2 on Pt. On both Pt(322) and Pt(355) new, stepinduced SO2 species are observed, and the ratio of upright standing and flat lying SO2 species on the (111) terraces is found to depend on the step orientation. The simultaneous occupation of all SO2 species on all studied surfaces points at a general hit and stick behavior for the low temperatures investigated, also
in the presence of steps. In the next section, the influence of steps on desorption and reaction of SO2 upon heating will be discussed. 3.2. Thermal Evolution of SO2 on Stepped Pt Surfaces. To investigate the thermal evolution of the adsorbed SO2 layers, we performed TP-XPS experiments with a constant heating rate of 0.5 K/s. Figure 3a-c shows selected S 2p spectra as waterfall plots, recorded during heating the SO2 layers on Pt(111), Pt(322), and Pt(355), respectively. For Pt(111) and Pt(322), these layers correspond to those formed in the adsorption experiments discussed in Section 3.1 (Figure 1a,b). For Pt(355), a separate new layer with a smaller amount of SO2 was prepared by dosing 0.11 L. This was necessary, since the adsorption experiment in Figure 1c yielded thick multilayers, which display significant beam damage and also a different thermal evolution (see Polcˇik et al.16). In addition, the dominating S 2p levels of the multilayers overlap with those of SO3 and SO4, hindering a clear analysis of the latter. In Figure 3d, a color-coded density plot of all S 2p spectra collected during heating of SO2 on Pt(355) is depicted, which allows to derive more precise information about the thermal behavior than from Figure 3c. For Pt(111), we start with the two terrace SO2 species, SO2(ly) and SO2(st) with S 2p3/2 (2p1/2) binding energies of 164.5 (165.7) eV and 165.4 (166.6) eV, respectively, at low temperatures. Upon heating, the intensity of SO2(st) increases at the expense of SO2(ly), which is most evident when comparing the orange spectrum for 122 K with the yellow one for 262 K in Figure 3a. At higher temperatures, SO2 is desorbing and only a small amount of S with peaks at 162.2 (163.4) eV is left on the surface. For Pt(322), the shape of the spectra does not change significantly until ∼250 K, indicating that the ratio of the SO2 species does not change. At higher temperatures (the green
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Figure 3. Selected S 2p spectra of the TP-XPS experiments shown as waterfall plots on (a) clean Pt(111), (b) clean Pt(322), (c) clean Pt(355), (d) as a color coded density plot of the experiment shown in (c), waterfall plots of (e) oxygen precovered Pt(111), (f) oxygen precovered Pt(322), (g) oxygen precovered Pt(355), and (h) as a color coded density plot of the experiment shown in (g). All spectra were recorded with a photon energy of 260 eV in normal emission geometry. The vertical lines indicate the energetic positions of the S 2 p3/2 peaks of the various species. Note that the intensity scales of the experiments are not identical.
spectrum in Figure 3b), a new S 2p doublet emerges at 166.0 (167.2) eV, which is assigned to SO3.18,36 Simultaneously, the formation of elemental sulfur is observed, with the binding energies of 162.2 (163.4) eV typical of S adsorbed on terrace sites.37 At 500 K, S is the only (S- containing) species left on the surface. At this temperature, the peak is broader than at 350 K, pointing toward a partial diffusion of S to the step sites. Further heating to 600 K (not shown) leads to a shift to 162.4 (163.6) eV, indicating that S has moved to the (100) steps.37
For Pt(355) comparison of the spectra at 99 K (orange) and 271 K (yellow) in Figure 3c reveals again a conversion of SO2 species on the surface. In contrast to Pt(111), the intensity of SO2(ly) increases and furthermore a binding energy shift from 164.5 (165.7) eV to 164.4 (165.6) eV occurs, which is better visible in Figure 3d. This shift is attributed to a migration from flat lying SO2 from the terraces to the steps, i.e., from SO2(ly) to SO2(step-ly). In addition, also a small amount of S forms at terrace sites, i.e., with peaks at 162.2 (163.4) eV. Further heating
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Figure 4. Quantitative analysis of the spectra of Figure 3(a) to (c): total coverage (bold black line), sum of all SO2 species (red diamonds), S coverage (yellow circles), SO3 coverage (green triangles), and SO2 multilayer coverage (thin gray line) of TP-XPS experiments of SO2 on the (a) clean Pt(111) surface, (b) clean Pt(322) surface, (c) clean Pt(355) surface; coverages of SO2(ly) (orange lying rhombs), SO2(st) (pink standing rhombs), SO2(step-ly) (dark red lying open rhombs), and SO2(step-st) (purple open standing rhombs) on the (d) clean Pt(111) surface, (e) clean Pt(322) surface, (f) clean Pt(355) surface.
up to ∼350 K (green spectrum in Figure 3c) leads to formation of both SO3 and S. The binding energy of S at 162.0 (163.2) eV now indicates S adsorbed at the (111) steps.37 The quantitative analysis of the TP-XPS experiments in Figure 3 is depicted in Figure 4. Panels a-c show the total amounts of SO2 (sum over all different SO2 species without multilayer), SO3, S and the total sum of all SOx species (x ) 0-3) plotted versus the temperature, for all three surfaces. For Pt(111) we find, as was already evident from the spectra in Figure 3a, that most of the initially adsorbed SO2 (0.25 ML) desorbs between 200 and 350 K; only minor amounts dissociate and/or disproportionate forming S (0.01 ML, ∼4%) and SO3 (∼1%).18 In contrast, on Pt(322) only ∼70% of the initially adsorbed amount of 0.25 ML SO2 desorbs and a significant part reacts to form 0.04 ML S and 0.04 ML SO3 at 300 K. On Pt(355) 0.04 ML of S and a maximal amount of 0.07 ML of SO3 are formed at 340 K. Note that on this surface, SO2 dissociates and/or disproportionates completely with no SO2 desorption. This is attributed to the lower initial coverage of only 0.12 ML used in this experiment (we expect SO2 desorption when starting with a saturated layer, i.e., 0.25 ML). The data clearly show that on both stepped surfaces a significant SO2 dissociation occurs upon heating, with the resulting O reacting with SO2 to form SO3. The small reactivity toward SO2 dissociation on Pt(111) could be due to some remaining defects always present also on well prepared flat surfaces and/or minor damage from the intense X-rays. In the following, we compare the two stepped surfaces and their thermal behavior in detail: On Pt(322), the reaction of SO2
to SO3 starts at 230 K with a steep increase of the SO3 coverage to 0.04 ML until 285 K. Thereafter, the SO3 coverage remains constant before it starts to decrease around 320 K (see Figure 4b). S starts to form at ∼200 K, reaching its maximum value of 0.04 ML at 285 K. On Pt(355) some SO2 dissociation and the resulting S formation occurs already below 180 K. Between 200 and 280 K, the sulfur coverage remains constant and thereafter a stronger increase is observed until 340 K. The oxidation of SO2 to SO3 only starts at 250 K (see Figure 4c). The increase of SO3 is slower than on Pt(322), but a higher maximum coverage of 0.07 ML is reached at 340 K. At this temperature also, the maximal S coverage is obtained and no more SO2 is found on the surface. Above 340 K desorption of SO3 starts and at 440 K only S is left on the surface. The comparison of the two stepped surfaces shows that the major increase in S coverage and the onset of SO3 formation occur at lower temperatures on Pt(322) than on Pt(355); this is also true for the temperature, where the maximum SO3 coverage is reached, 285 vs 340 K, respectively. This suggests that SO2 dissociation and oxidation of SO2 to SO3 is faster on Pt(322), i.e. in the presence of (100) steps. The reason for the formation of some S already below 180 K on Pt(355) is not clear. Note that since the TP-XPS experiment for Pt(355) did not start with a fully covered surface, we cannot fully rule out coverage dependent effects during this reaction. The reactivity of steps toward other molecules has also been found to depend on the step orientation: The Pt(355) surface, i.e., (111) oriented steps, is more reactive toward oxidation of CO and S18,35 and also the dehydrogenation of CH3 to CH occurs
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at lower temperatures on Pt(355) than on Pt(322).32 In contrast to that the Pt(322) surface, i.e., (100) steps, has been found to be far more reactive toward NO dissociation.33,38 As one dissociating SO2 molecule yields one S and two O atoms, twice the amount of SO3 as S could be formed. While on Pt(355) significantly more SO3 than S is observed, this is not the case for Pt(322). One possible explanation is that part of the SO3 desorbs at the temperature of its formation; alternatively, not all O has to react with SO2, but a fraction could also react with S above 400 K. This would be in line with the observation that on both stepped surfaces the S coverage decreases slightly above 400 K, which is attributed to reaction of S with oxygen from the SO2 dissociation. They react to SOx (x ) 1-3), which is instantly desorbing at this temperatures. As a consequence, at 600 K only 0.04 ML sulfur and no oxygen (within the margin of errors) is left on both stepped surfaces, compared to only 0.01 ML of S and no oxygen on Pt(111) at 400 K. The analysis of Figure 4c shows a slight rise of the total S coverage (black solid line) between 250 and 340 K, i.e., when SO3 is formed on the surface. This is an apparent increase due to differences in photoelectron diffraction for SO2 and SO3 at the low kinetic energies of the photoelectrons used in this study (∼100 eV). A similar behavior has been observed previously in a study of S oxidation for SO3/SO4.36 Note that photoelectron diffraction effects generally cannot be avoided when using low kinetic energies and therefore impose a certain error to all denoted coverages. The thermal evolution of the different SO2 species on the three surfaces is depicted in Figure 4d-f. For Pt(111), we find that ∼60% of flat lying SO2(ly) is converted into upright standing SO2(st) until 220 K. Above this temperature, the remaining SO2(ly) coverage is constant until 250 K and thereafter both SO2 species desorb.18 A simple Redhead analysis39 of the interconversion (self-diffusion) of SO2 from the lying to the upright standing adsorption geometry yields an activation energy of ∼52 kJ/mol. In this analysis, we assumed a first order process with a preexponential factor of 1013 s-1 and used the inflection point of the increase of the SO2(st) coverage at 193 K (Figure 4d, heating rate 0.5 K/s) as estimate for the rate maximum. This high activation energy explains the observed hit and stick behavior on Pt(111), i.e., the fact that at low temperatures the ratio of the occupation of different sites does not change with coverage (see above). As the large barrier also hinders the diffusion on the (111) terrace, SO2 molecules impinging on terraces cannot reach the energetically favorable step adsorption sites on Pt(322) and Pt(355), in contrast to the above-mentioned studies on other small molecules such as CO or CH3.26,32 On Pt(322) no interconversion of SO2 species occurs. Upon heating the total SO2 coverage slightly decreases up to 225K (Figure 4b). From Figure 4e, it is evident that this is due to desorption of first flat lying SO2(ly) and then step related SO2(step-st). The coverage of upright standing SO2(st) remains almost constant until 225 K. From 225 to 300 K, a steep decrease of the total SO2 coverage is observed, which is related to desorption and also reaction of all three SO2 species to SO3. Above 300 K, only some SO2(step-st) is present on the surface, which desorbs until 350 K, making this step species the thermally most stable surface species on Pt(322). On Pt(355), all partial SO2 coverages display only small changes up to 175 K. Thereafter, until 275 K a complete conversion of SO2(st), SO2(step-st) and SO2(ly) into SO2(steply), is observed, yielding a maximum coverage of 0.08 ML for
Streber et al. this most stable SO2 species. This value for the lying step species appears to be a reasonable step coverage, as for the much smaller CO molecule a step coverage of 0.15 ML was found on this surface.26 Further heating leads to oxidation to SO3, which is completed at 350 K. The TP-XPS experiments show that steps significantly influence the thermal evolution of SO2 on Pt. On Pt(355), we find a ratio of ∼2:1 lying to standing SO2 species at 210 K, while for Pt(322) and Pt(111) this ratio is between 1:3 and 1:4. The (100) steps on Pt(322) lead to the formation of upright standing SO2 step species on the surface, but no significant interconversion of different SO2 species occurs upon heating. In contrast to that, the (111) steps on Pt(355) stabilize the flat lying SO2 step species, with all other SO2 species being converted into this most stable species. On both stepped surfaces, the step related SO2 is the thermally most stable SO2 species. A Redhead analysis of the hopping barrier on Pt(111) explains the hit and stick behavior observed in the adsorption experiments. 3.3. Adsorption of SO2 on O Precovered Surfaces. An alternative route for changing surface properties is the preadsorption of oxygen. Figure 1d shows selected S 2p spectra recorded during adsorption of SO2 on Pt(111) precovered with 0.25 ML of atomic oxygen. Both SO2 species known from clean Pt(111) are present: flat lying SO2(ly) with the S 2p3/2 (2p1/2) peaks at 164.5 (165.7) eV, marked by an orange line, and upright standing SO2(st) with peaks at 165.5 (166.7) eV, marked by a pink line. The comparison to spectra with a similar coverage on clean Pt(111) (bold black spectra in Figure 1a,d) shows two additional doublets for the oxygen precovered surface. The doublet at 165.2 (166.4) eV, marked by a purple line, is assigned to an oxygen induced species, SO2(ox)18 and the one at 166.0 (167.2) eV, marked by a green line, is assigned to SO3.18,36 Above 0.17 L, also a doublet due to SO2 multilayers at 166.8 (168.0) eV and very small contributions due to SO at 163.5 (164.7) eV and S at 162.2 (163.4) eV are present. The comparison to clean Pt(111) further reveals that the ratio of SO2(st) to SO2(ly) changes from ∼1:1 on the clean to ∼2:1 on the O precovered surface. Similar to the clean surface the overall shape of the S 2p spectra does not change with increasing coverage, indicating a hit and stick adsorption mechanism. In Figure 1e,f selected spectra collected during the adsorption of SO2 on oxygen precovered Pt(322) at 155 K and Pt(355) at 100 K are shown, respectively. In both cases, the spectral shape remains again unchanged. All species found on oxygen precovered Pt(111) are also present on the oxygen precovered Pt(322) surface. However, their relative intensities are different. The peaks assigned to flat lying SO2(ly), indicated by the orange line, are very small. The doublet at 165.2 (166.4) eV, assigned to the oxygen induced SO2(ox) species (purple line) has a comparable height as the doublet of the upright standing SO2(st), marked by the pink line. Interestingly, a doublet at 165.2 (166.4) eV (purple line) is also observed on clean Pt(322) (i.e., without oxygen being present) in Figure 1b and also on O precovered Pt(111) (i.e., with no steps) in Figure 1d, with identical binding energies. Thus, it is not possible to determine if on the O precovered Pt(322) surface this doublet is induced by the preadsorbed oxygen or the steps, named SO2(step/ox-st). Apparently, preadsorbed oxygen and step sites have the same electronic and/or geometric influence on the SO2, at least concerning the XPS binding energies. Most likely, the doublet contains contributions of oxygen and also of step influenced SO2 species. The detailed analysis further reveals (see also Supporting Information, Figure SI-2a) that this doublet has a shoulder at the lower binding energy side at 165.0 (166.2) eV,
Adsorption and Thermal Evolution of SO2 which is assigned to an additional oxygen induced SO2 species, which must be bound in a slightly modified geometry. As this species is only present on this surface, it must be influenced by a combination of oxygen and the (100) steps and is therefore denoted as SO2(step/ox-2). On oxygen precovered Pt(355) all species found on oxygen precovered Pt(111) are also visible (Figure 1f). The second step species SO2(step-ly), which was seen on clean Pt(355) at 164.4 (165.6) eV, is absent. Figure 2d-f displays the quantitative analysis of the data in Figure 1d-f. As expected from the unchanged spectral shape during adsorption, all SOx species increase with nearly constant coverage ratios for all investigated surfaces, up to the starting point of multilayer adsorption. For Pt(111), the relative coverages at 0.17 L amount to 44% upright standing SO2(st), 25% flat lying SO2(ly), 12% oxygen-influenced SO2(ox) and 15% SO3.18 At higher exposures, the formation of multilayers begins.18 On Pt(322) SO3 and the step or oxygen-influenced SO2(step/ ox-st) species have a large contribution also at low total coverages. At an exposure of 0.18 L, we find 23% SO3 formed directly upon adsorption, 29% SO2(step/ox -st), 23% SO2(step/ ox-2), 21% SO2(st) and only 3% SO2(ly). For higher exposures (>0.24 L), the coverages of SO3 and oxygen or step influenced SO2 increase stronger than the other SOx species. On Pt(355) at 0.12 L (onset of multilayer formation) SO3 is the dominant species, with 36% of the total coverage. The relative SO2 coverages amount to 25% SO2(ly), 23% SO2(st) and 16% SO2(step/ox-st). In contrast to Pt(322), a significantly larger amount of SO2(st) and SO2(ly) is found, i.e., SO2 not influenced by oxygen or steps. At first sight this seems surprising, since the oxygen precoverage on Pt(355) is higher than on Pt(322) (0.54 ML vs 0.33 ML, respectively). A possible explanation is that oxygen is distributed in a different manner on the two stepped surfaces: on Pt(355) 0.40 ML of oxygen are bound to the (111) steps and form a one-dimensional oxide,29,35,40 with the terraces not fully occupied. On Pt(322), the oxygen atoms form a (2 × 2) overlayer on the terraces and the oxygen density at the steps is comparable to that on the terraces, which leads to a more even distribution of oxygen over the whole surface.35 This leads to a different “oxygen density” on the terraces for the two stepped surfaces and affects the fraction of SO2 influenced by the preadsorbed oxygen. Despite these differences, we find some general similarities in the impact of preadsorbed oxygen on the adsorption of SO2: First, the SO2 saturation coverage is decreased as compared to the clean surfaces, where the same saturation value of ∼0.25 ML was found for all Pt surfaces studied here. On the O precovered surfaces the corresponding values for the SOx coverages are 0.20 ML for Pt(111),18 0.18 ML for Pt(322) and 0.14 ML for Pt(355). This decrease is attributed to a reduction in available adsorption sites for SO2, caused by the preadsorbed oxygen. The higher saturation coverages of oxygen (0.25, 0.33, and 0.54 ML, respectively) block a higher fraction of the stepped surfaces. In addition, the comparison to the clean surfaces shows that the amount of flat lying SO2(ly) is diminished to a higher extent than by the steps. In other words, oxygen seems to selectively block more adsorption sites for flat lying than for upright standing SO2 species. On Pt(111) 0.25 ML of O reduce the SO2 saturation coverage by 0.05 ML (from 0.25 to 0.20 ML). Thus, five oxygen atoms are needed to block the adsorption of one SO2 molecule. Similar values are calculated for the stepped surfaces (4.7 (322); 4.5 (355)). The need of five oxygen atoms
J. Phys. Chem. C, Vol. 114, No. 46, 2010 19741 to block one SO2 adsorption site is quite remarkable, since 0.25 ML of oxygen reduces the CO coverage to 50% from 0.5 to 0.25 ML on Pt(111). One oxygen atom blocks one (bridge) adsorption site in this case.41 The second similarity in the oxygen induced changes is that SO3 is formed immediately upon adsorption of SO2 on all three surfaces at low temperatures (
