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SO2 adsorption and thermal evolution on clean and oxygen precovered Pt(1 1 1)
Chemical Physics Letters 494 (2010) 188–192
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
SO2 adsorption and thermal evolution on clean and oxygen precovered Pt(1 1 1) R. Streber a, C. Papp a,*, M.P.A. Lorenz a, O. Höfert a, E. Darlatt c, A. Bayer a, R. Denecke c, H.-P. Steinrück a,b a
Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany Erlangen Catalysis Resource Center (ECRC), Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany c Willhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstraße 2, 04103 Leipzig, Germany b
a r t i c l e
i n f o
Article history: Received 23 February 2010 In final form 3 June 2010 Available online 8 June 2010
a b s t r a c t We studied the adsorption and reaction of sulfur dioxide on clean and oxygen precovered platinum surfaces by in situ high resolution X-ray photoelectron spectroscopy and discuss earlier, contradicting results. On the clean surface we find flat lying and upright standing SO2, adsorbing in similar amounts at low temperature. Upon heating, the ratio changes strongly favoring the standing SO2; subsequently almost all SO2 desorbs. On the oxygen precovered surface the standing SO2 is preferably populated and even at low temperatures a direct reaction of SO2 to SO3 occurs. Upon heating, SO2 oxidation to SO3 and subsequently to SO4 is found. Ó 2010 Elsevier B.V. All rights reserved.
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 [1– 5], 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 many studies in the past, which aimed at a fundamental understanding of the underlying processes [6–19]. However, despite these efforts the complex chemistry of sulfur oxides on platinum surfaces is still not completely understood, with some of the results even being contradictory. There is consensus in the literature that on clean Pt(1 1 1) SO2 adsorbs molecularly in two different conformations at 120 K. The two species have been identified from their different binding energies in high resolution X-ray photoelectron spectroscopy (HR-XPS) [14], and from near edge X-ray absorption fine structure (NEXAFS) measurements they have been assigned as flat lying and upright standing SO2 [14]. Density functional theory (DFT) based calculations by Lin et al. [13] confirm these experimental results. Temperature programmed desorption (TPD) measurements of SO2 on Pt(1 1 1) show that for small initial coverages SO2 desorbs around 370 K and for higher initial coverages the main desorption peak is found at 320 K [6,9,17]. Upon heating SO2 multilayers to room temperature the formation of SO and SO4 species has been proposed by high resolution electron energy loss spectroscopy (HREELS) [16]; on the other hand from HR-XPS, the formation of S and SO4 has been reported [14] under comparable conditions, with the SO4 species being stable up to at least 370 K. On oxygen precovered Pt(1 1 1) again two SO2 species have been identified by HR-XPS [10], with binding energy values corre* Corresponding author. E-mail address: [email protected] (C. Papp). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.06.007
sponding well to those on clean Pt(1 1 1) [14]; in the same study the formation of an SOx species is found at 200 K, which the authors assign to SO4, although it starts to already decompose at 280 K, leaving only atomic S on the surface at 340 K [10]; interestingly the S 2p3/2 binding energy of this SO4 species is not consistent with values reported in the literature [14,19]. The thermal evolution of SO2 on O-precovered Pt(1 1 1) was also 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 K [6,17,18] (note that SO3 is difficult to identify in the mass spectrometer, as the dominating cracking fragment is SO2 [20]). This high temperature peak was associated with the decomposition of SO4. These TPD results are in contradiction to the HR-XPS study reporting SO4 decomposition to be completed at much lower temperatures. Because of these conflicting results, we revisited the adsorption and reaction of SO2 on a clean and an oxygen precovered Pt(1 1 1) surface and performed a detailed in situ X-ray photoelectron spectroscopy study. This technique allows the identification of adsorbed species during adsorption and also during heating of the surface, with the latter method being called temperature programmed (TP)-XPS [21]. The experiments were performed using a transportable apparatus [22] at the third generation synchrotron source BESSY II in Berlin, Germany at beamline U49/2 PGM-1. In order to obtain very high resolution in the S 2p region, we decided to use a comparably low photon energy of 260 eV. This has the advantage of also being able to identify minority species, but the disadvantage that at the low kinetic energy of 95 eV photoelectron diffraction effects play a role, hampering the quantitative analysis (see below). The overall resolution in the S 2p region was 140 meV, as determined by fitting the Fermi edge. XP spectra were collected in 10 s per spectrum at normal emission. SO2 was dosed from the background at pressures of 2–5 109 mbar. S
R. Streber et al. / Chemical Physics Letters 494 (2010) 188–192
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coverages were calibrated by comparison to S 2p spectra from a p(2 2) (0.25 ML) overlayer of S on Pt(1 1 1), prepared by dosing of H2S and subsequent heating to 700 K with H2 desorbing and S remaining on the surface [23]. Fig. 1a shows selected S 2p spectra recorded during SO2 adsorption at 130 K on clean Pt(1 1 1). 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. The separation of the 2p3/2 and 2p1/2 peaks is 1.2 eV and their ratio in intensity is 2:1, as expected from the spin multiplicity (this holds for all SOx species (x = 0–4) observed in this study). During SO2 adsorption the overall shape of the spectra does not change, i.e. the two S 2p3/2 components have always nearly the same height. The doublet at 164.5 (165.7) eV is assigned to a flat lying SO2 species, the one at 165.4 (166.6) eV to the upright standing SO2 species, according to literature [14]. Only at high coverages a very weak additional feature at 166.8 (168.0) eV can be seen, which is assigned to the adsorption of multilayers. Furthermore, at 162.2 (163.4) eV a minor contribution is visible, which is assigned to elemental sulfur [14,24] generated by beam damage. To investigate the influence of preadsorbed oxygen, the Pt(1 1 1) surface was saturated with oxygen at 130 K and subsequently heated to 300 K. This procedure gives a p(2 2) oxygen overlayer with a oxygen coverage of 0.25 ± 0.02 ML [25,26]; the oxygen coverage was checked by XPS, collected with a photon energy of 650 eV. In Fig. 1b selected S 2p spectra recorded during SO2 adsorption at 130 K on this oxygen precovered surface are shown. Again the two doublets at 164.5 (165.7) eV and 165.4 (166.6) eV, due to flat lying and upright standing SO2, respectively, are the most prominent features, similar to the clean surface. However, the comparison of spectra with a similar coverage (see bold black and dotted black spectra in Fig. 1) shows that the spectral shape is very different from that of the spectra without O precoverage. This is mainly due to the significantly smaller intensity of the flat
lying SO2 (164.5/165.7 eV) on the O-precovered surface. Furthermore, two additional doublets are found, indicating two additional S containing species: The doublet with the S 2p3/2 (S 2p1/2) component at 166.0 (167.2) eV, marked by the green lines in Fig. 1b, is assigned to SO3, in agreement to literature [24]; note that for this species the 2p1/2 component (167.2 eV) is better resolvable, since the 2p3/2 peak (166.0 eV) overlaps with other peaks. The other new doublet at 165.2 (166.4) eV, marked by the purple lines in Fig. 1b, is tentatively assigned to a third SO2 species in a different environment, due to lateral interactions with the preadsorbed oxygen (see also Supporting Information). Similar to clean surface, on the oxygen precovered surface the shape of the spectra initially does not change with increasing SO2 dose. Only for high SO2 doses, a doublet at 166.7 (167.9) eV due to SO2 multilayers is observed, as on clean Pt(1 1 1). A close look at the low binding energy region reveals that a very small amount of atomic S, as evident from the doublet at 162.2 (163.4) eV is present; an additional small species at 163.5 (164.7) eV is assigned to SO; this assignment is based on its binding energy value between S and the SO2 species. This SO species is only formed when SO2 multilayers are present and most likely resulting from beam damage due to the high intensity of the synchrotron radiation [10]. For a quantitative analysis the spectra in Fig. 1a and b were fitted (for details see Supporting Information) and the corresponding results are displayed in Fig. 2a and b for clean and oxygen precovered Pt(1 1 1), respectively. On clean Pt(1 1 1), both the flat lying and the upright standing SO2 species are occupied simultaneously, with nearly identical intensities and a total saturation coverage of 0.25 ML. This value was obtained by comparing the S 2p area with the one of a S p(2 2) superstructure on Pt(1 1 1); note that all coverages are derived neglecting photoelectron diffraction effects, which can be different for the SOx species (further discussion see below). At exposures higher than 0.3 L the adsorption of SO2 in multilayers begins.
Fig. 1. Selected S 2p spectra collected during adsorption of SO2 on (a) Pt(1 1 1) and (b) oxygen precovered Pt(1 1 1). The long vertical lines indicate the energetic positions of the 2p3/2 peaks, the short ticks those of the 2p1/2 peaks of the SOx species present on the surface. Photon energy: 260 eV; detection at normal emission.
Fig. 2. Quantitative analysis of the spectra in Fig. 1: (a) adsorption on the clean Pt(1 1 1) surface; (b) adsorption on the oxygen precovered surface.
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On O-precovered Pt(1 1 1) the three SO2 species and the SO3 species increase with nearly constant relative coverages up to an exposure of 0.16 L (Fig. 2b). The relative coverages at 0.16 L amount to 42% upright standing SO2, 25% flat lying SO2, 16% oxygen-influenced SO2 and 16% SO3. At higher exposures, the formation of multilayers begins; at the same time the signals due to upright standing and flat lying SO2 rise slightly while those of oxygen-influenced SO2 and SO3 show a stronger increase. At 0.43 L the total coverage of the three SO2 and the SO3 species is 0.20 ML. The minor amounts of S and SO (below 1%) observed during the SO2 adsorption experiments are not shown in Fig. 2 for clarity (however, they correspond to first points of the TP-XPS experiments in Fig. 5). The immediate formation of SO3 indicates reaction of SO2 and oxygen already at low coverages and temperatures. Lin et al. investigated this reaction by density functional theory (DFT) calculations [12]. They found that the reaction of adsorbed SO2 and O via a Langmuir–Hinshelwood mechanism has an activation energy of 42 kJ/mol and the direct reaction of gas phase SO2 with preadsorbed oxygen via a Eley–Rideal mechanism has a lower value of only 25 kJ/mol. This could contribute to the significant oxidation of SO2 even below 130 K. Summarizing the adsorption behavior of SO2 on clean and oxygen precovered Pt(1 1 1) surface, we find that on the clean surface almost all SO2 adsorbs intact and in a constant, almost 1:1 ratio between flat lying and upright standing molecules; on the oxygen precovered surface a part of the SO2 reacts to SO3 already at 130 K and the ratio of flat lying to upright standing SO2 is strongly shifted towards the upright standing SO2 species. To investigate the thermal evolution of the adsorbed layers we performed temperature programmed XPS experiments with a constant heating rate of 0.5 K/s. Fig. 3a shows selected S 2p spectra recorded during heating of the SO2/Pt(1 1 1) layer, Fig. 3b a colorcoded density plot of all S 2p spectra, which allows to derive more
detailed information. The corresponding quantitative analysis is found in Fig. 5a. After desorption of the multilayers below 170 K, the doublet at 165.4 (166.6) eV due to standing upright SO2 grows at the expense of that at 164.5 (165.7) eV due to flat lying SO2, so that 60% of the flat lying SO2 is converted to upright standing SO2 at 220 K. This conversion is in good agreement with a previous HR-XPS study by Polcˇik et al., who find a stable layer of predominantly standing SO2 upon adsorption at 212 K, but could not resolve the small amount of remaining lying SO2 [14]. At higher temperatures the amount of standing SO2 decreases (Fig. 5a), while the amount of lying SO2 stays constant until 250 K, before it also starts to decrease and desorbs till 320 K. The remaining standing SO2 desorbs until 360 K, leaving a small amount of S on the surface, which amounts to less than 10% of the initially adsorbed SO2, again in agreement with the results of Polcˇik et al. [14]. In addition minor amounts of SO3 (<0.01 ML) are also formed around 280 K and disappear again at 380 K. Interestingly, Polcˇik et al. find that upon heating of thick SO2 multilayers a stable SO4 species is observed around 330 K [14], which is not the case in this study as the temperature dependent experiment started only with the onset of multilayers (0.05 ML). The thermal evolution of the SO2 layer on O-precovered Pt(1 1 1) was also followed by TP-XPS. Fig. 4a shows selected S 2p spectra, Fig. 4b the color-coded density plot of all S 2p spectra and Fig. 5b the corresponding quantitative analysis. After desorption of the multilayers until 150 K, the doublets due to oxygeninfluenced SO2 (165.2/166.4 eV) and upright standing SO2 (165.4/ 166.6 eV) decrease in intensity until they have vanished at 300 K due to desorption or reaction. The doublet due to flat lying SO2 (164.5/165.7 eV) first increases slightly until 170 K and then starts to decrease around 250 K. The quantitative analysis indicates that a part of the upright standing or oxygen-influenced SO2 is converted to flat lying SO2 and that no SO2 is found on the surface at 300 K.
Fig. 3. Selected S 2p spectra of the TP-XPS experiment on the (a) clean surface shown as (a) waterfall plot and (b) color-coded density plot to the experiment shown in (a). Photon energy: 260 eV; detection at normal emission. The long vertical lines indicate the energetic positions of the 2p3/2 peaks of the various species, the short ticks those of the 2p1/2 peaks.
Fig. 4. Selected S 2p spectra of the TP-XPS experiment on the oxygen precovered surface shown as (a) waterfall plot and (b) color-coded density plot to the experiment shown in (a). Photon energy: 260 eV; detection at normal emission. The long vertical lines indicate the energetic positions of the 2p3/2 peaks of the various species, the short ticks those of the 2p1/2 peaks.
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Fig. 5. Quantitative analysis of the spectra of Figs. 3 and 4: (a) TP-XPS of SO2 on the clean Pt(1 1 1) surface; (b) TP-XPS of SO2 on the oxygen precovered surface.
Upon heating, also the SO3 species (166.0/167.2 eV) increases in intensity and reaches its maximum at 280 K (see green spectrum in Fig. 4). Between 230 and 300 K the total S 2p intensity decreases significantly, indicating desorption of SOx species around 270 K. Around 300 K a new doublet at 166.8 (168.0) eV shows up, which is assigned to SO4 by comparison to literature [14,24]. This indicates that on the oxygen precovered surface oxidation of SO2 to SO3 and subsequently SO4 occurs in significant amounts. Above 450 K SO4 is decomposing to SO3, which desorbs; the resulting oxygen subsequently reacts with sulfur to SO2 or SO3, which again desorbs. At 520 K there is no S containing species left on the surface, only a small amount of oxygen (0.05 ML) remains. The detailed inspection of Fig. 5b reveals that even below 300 K small amounts of SO4, but also SO are present on the surface, which were formed during desorption of the multilayers, possibly due to synchrotron radiation damage. The small amount of SO4 only shows up in the quantitative analysis after the multilayers have desorbed. Since the XPS peaks of SO2 multilayers and of SO4 have similar binding energies, the exact onset of SO4 formation can not be determined. The SO signal decreases at 280 K and at the same time the S signal starts to grow indicating dissociation of SO. If we now compare the results of the two TP-XPS experiments on clean and oxygen precovered Pt(1 1 1) we find that on the clean surface a conversion of 60% of flat lying to upright standing SO2 takes place above 150 K, whereas on the oxygen precovered surface some conversion occurs in the opposite direction, from standing and oxygen-influenced to lying SO2. On the clean surface no SO2 is found above 350 K, mainly due to desorption and to a much lesser extent (<10%) to disproportionation to S and SO3. On the oxygen precovered surface no SO2 is found on the surface at 290 K, due to desorption of SOx and/or reaction of SO2 to SO3; up to this temperature the total coverage is reduced by 30%, and 0.11 ML SO3 have been formed. Further heating leads to the sub-
sequent nearly complete oxidation to SO4 which starts to decompose at 450 K to finally be gone at 510 K. The total amount of SO4 formed by oxidation of SO2 is 0.09 ML or 46% of the initial amount of SO2 and SO3. As one needs two oxygen atoms for the oxidation of SO2 to SO4 the amount of 0.25 ML of oxygen initially present on the surface would be sufficient for this complete conversion. However, the fact that we also observe some atomic S indicates some dissociation/disproportionation of SO2 and/or partly SO3 leading to S and SO4. From the fact that after heating to 520 K only 0.05 ML oxygen is left on the surface, i.e. 0.20 ML oxygen is consumed, we have to conclude that the main desorption product is SO3 rather than SO2. However, we have to note here that from previous studies of S oxidation we know that diffraction of the low kinetic energy photoelectrons can play a major role in the quantitative analysis of different SOx species [24], imposing a certain error on the absolute coverage values for the different species because of the strongly differing local surrounding (see also [24,27]). Note that recombinative oxygen desorption occurs at much higher temperatures [6]. These results for the oxygen precovered surface are in strong contradiction to the HR-XPS study of Lee et al., who found only one SOx species [10,28], which disappeared from the surface until 320 K and was assigned to SO4. Interestingly, the thermal stability of this species and its binding energy mimic the behavior of the species that was assigned to SO3 in our study. To compare different XPS studies, we use the S 2p3/2 binding energy of flat lying SO2, which is clearly observed in all studies, as reference and calculate the binding energy difference to the other SOx species (with x = 0– 4); this has the advantage that different calibrations of the absolute energy scale do not influence the analysis. For the second SO2 species, SO3 and S we find that the values are in very good agreement (±0.2 eV) for all three HR-XPS studies on Pt(1 1 1) [10,14] and also with the values for SO2 found on Pt(1 1 0) [19]. However, for SO4, the difference of 1.4 eV observed by Lee et al. deviates significantly (by 0.9–1.1 eV) from all other studies [14,19] and the present study (see Table 1), but fits very well to the values reported for SO3 (0.1 eV). Also the thermal stability of the SOx species observed by Lee et al. matches the behavior of SO3 observed in this study, and is in contradiction to the thermal stabilities of SO4 deduced from TPD studies [6,17]. Lee et al. do not observe any S containing species other than elemental sulfur above 340 K. We find that at this temperature the further oxidation of SO3 to SO4 occurs and SOx is observed up to temperatures of 500 K. In addition, there is also a contradiction in the quantitative analysis in the paper of Lee et al. After multilayer desorption they find a total SO2 coverage of 0.26 ML, which is oxidized quantitatively to finally yield 0.26 ML of SO4; this would require 0.52 ML atomic oxygen; however only 0.25 ML could have been available, which is very close to the amount required to form quantitatively SO3 instead of SO4. Altogether, this leads to the assumption that the oxygen coverage in the case of Lee et al. [14] was even significantly lower than
Table 1 Relative S 2p binding energies of SOx species found on platinum surfaces.
DESO2 ð1yÞ=SOx in eV
SOx species S
SO
SO2 (ly)
SO2 (st)
SO2 (ox)
SO3
SO4
This work SO2/O/Pt(1 1 1)
2.3
1.0
0.9
0.7
1.5
2.3
Polcˇik [14] SO2/Pt(1 1 1)
2.5
Zebisch [19] SO2/Pt(1 1 0)
2.7
Lee [10] SO2/O/Pt(1 1 1)
2.6
164.5 0 164.0 0 163.7 0 164.1 0
1.1
0.8 0.9 0.9
2.5 1.5
2.3 1.4
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0.25 ML. This would explain the absence of SO4 formation, which is stable up to 500 K, and also the absence of SOx desorption at higher temperatures, which was found in TPD studies [6,17,18] and in the present study. At this point we want to address the accuracy of the denoted coverages in previous studies and also in our study. The coverages of all S containing species on the clean and oxygen precovered surface were determined by comparison to the (2 2) S layer with the well known coverage of 0.25 ML. For the clean surface we find a SO2 coverage of 0.24 ML after heating to 200 K. When we use the O 1s spectrum corresponding to the (2 2) O layer as reference, we obtain a larger value of 0.38 ML; the corresponding values for the oxygen precovered surface are 0.20 ML (S 2p) and 0.32 ML (O 1s). The amount of 0.38 ML SO2 for the clean surface is in reasonable agreement with the value 0.43 ML reported by Polcˇik et al. upon adsorption at 212 K, which was also derived by comparison to the O 1s spectrum [14]. The difference is attributed to differences in photoelectron diffraction at the low kinetic energies used. After careful considerations we decided to use the coverage calibration based on the sulfur XP spectra due to the following reasons: (1) From steric reasons a coverage of 0.4 ML of flat lying and upright standing SO2 appears too large; (2) larger changes in the local environment are expected for oxygen than for sulfur in the adsorbed molecular species as compared to the atomic species; (3) the O 1s-derived large value for the oxygen precovered surface of 0.32 ML SO2 would not be in line with the fact that 46% of it (0.15 ML) are converted to SO3 and subsequently to SO4, which would require 0.30 ML atomic oxygen, with 0.25 ML being available. Nevertheless, one has to state that we cannot rule out that the actual coverages might be somewhat different from the denoted ones due to photoelectron diffraction effects, which might also be different for different SOx species. This is for instance also indicated by the fact that the exact ratio between flat lying and upright standing species in our study is not identical to the one observed previously [10,14], which is attributed to the different photon energies used to measure the S 2p spectra, i.e. 260 eV here and 400 eV in Ref. [10]. In summary, we investigated the adsorption and reaction of SO2 on clean and oxygen precovered surfaces with high resolution in situ X-ray photoelectron spectroscopy. The adsorption on the clean surface shows a flat lying and an upright standing SO2 species adsorbing in a constant ratio. This ratio changes significantly in the coadsorption situation, showing mainly the upright standing SO2 geometry. In addition a third SO2 species induced by the coadsorbed oxygen is observed. Interestingly, a significant amount of SO2 directly reacts to SO3 upon adsorption at 130 K, showing the high reactivity of this system. The thermal evolution on the clean surface shows a temperature-induced conversion of the flat lying SO2 into the upright standing SO2, but less than 10% disproportionation to S and SO3. For the surface precovered with oxygen, 46% of the initially adsorbed SO2 are converted to adsorbed SO3 and then subsequently to SO4, which is stable on the surface up to 500 K.
This study is giving a complete and unifying picture on the reaction and adsorption of SO2 on Pt(1 1 1), allowing to understand the adsorption properties at low temperatures, the thermal evolution and the influence of preadsorbed oxygen. Acknowledgement This work has been supported by the Excellence Cluster ‘Engineering of Advanced Materials’ granted to the University of Erlangen-Nuremberg. We also thank the BMBF for their financial support through grant 05 ES3XBA/5 and the BESSY staff for their support during beamtime. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2010.06.007. References [1] G. Ertl, H. Knözinger, J. Weitkamp, Handbook of Heterogeneous Catalysis, VCH, Weinheim, 1997. [2] J.A. Rodriguez, D.W. Goodman, Surf. Sci. Rep. 14 (1991) 1. [3] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [4] K.C. Taylor, Catal. Rev. Sci. Eng. 35 (1993) 457. [5] J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VHC, New York, 1997. [6] S. Astegger, E. Bechtold, Surf. Sci. 122 (1982) 491. [7] M. Höfer, S. Hillig, H.-W. Wassmuth, Vacuum 41 (1990) 102. [8] U. Köhler, H.-W. Wassmuth, Surf. Sci. 117 (1982) 668. [9] U. Köhler, H.-W. Wassmuth, Surf. Sci. 126 (1983) 448. [10] A.F. Lee, K. Wilson, A. Goldoni, R. Larciprete, S. Lizzit, Surf. Sci. 513 (2002) 140. [11] X. Lin, K.C. Hass, W.F. Schneider, B.L. Trout, J. Phys. Chem. B 106 (2002) 12575. [12] X. Lin, W.F. Schneider, B.L. Trout, J. Phys. Chem. B 108 (2004) 13329. [13] X. Lin, W.F. Schneider, B.L. Trout, J. Phys. Chem. B 108 (2004) 250. [14] M. Polcik, L. Wilde, J. Haase, B. Brena, G. Comelli, G. Paolucci, Surf. Sci. 381 (1997) L568. [15] J.A. Rodriguez, T. Jirsak, S. Chaturvedi, J. Hrbek, J. Am. Chem. Soc. 120 (1998) 11149. [16] Y.-M. Sun, D. Sloan, D.J. Alberas, M. Kovar, Z.-J. Sun, J.M. White, Surf. Sci. 319 (1994) 34. [17] K. Wilson, C. Hardacre, C.J. Baddeley, J. Lüdecke, D.P. Woodruff, R.M. Lambert, Surf. Sci. 372 (1997) 279. [18] K. Wilson, C. Hardacre, R.M. Lambert, J. Phys. Chem. 99 (1995) 13755. [19] P. Zebisch, M. Stichler, P. Trischberger, M. Weinelt, H.-P. Steinrück, Surf. Sci. 371 (1997) 235. [20] C.G. Vayenas, H.M. Saltsburg, J. Catal. 57 (1979) 296. [21] A. Baraldi, G. Comelli, S. Lizzit, D. Gocco, G. Paolucci, R. Rosei, Surf. Sci. 367 (1996) L67. [22] R. Denecke, M. Kinne, C.M. Whelan, H.-P. Steinrück, Surf. Rev. Lett. 9 (2002) 797. [23] R.J. Koestner, M. Salmeron, E.B. Kollin, J.L. Gland, Surf. Sci. 172 (1986) 668. [24] R. Streber, C. Papp, M.P.A. Lorenz, A. Bayer, R. Denecke, H.-P. Steinrück, Angew. Chem. Int. Ed. 48 (2009) 9743. [25] M. Kinne, T. Fuhrmann, J.F. Zhu, C.M. Whelan, R. Denecke, H.-P. Steinrück, J. Chem. Phys. 120 (2004) 7113. [26] H. Steininger, S. Lehwanld, H. Ibach, Surf. Sci. 123 (1982) 1. [27] M. Kinne et al., J. Chem. Phys. 117 (2002) 10852. [28] A.F. Lee, K. Wilson, A. Goldoni, R. Larciprete, S. Lizzit, Catal. Lett. 78 (2002) 379.
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SO2 adsorption and thermal evolution on clean and oxygen precovered Pt(1 1 1) R. Streber a, C. Papp a,*, M.P.A. Lorenz a, O. Höfert a, E. Darlatt c, A. Bayer a, R. Denecke c, H.-P. Steinrück a,b a
Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany Erlangen Catalysis Resource Center (ECRC), Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany c Willhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstraße 2, 04103 Leipzig, Germany b
a r t i c l e
i n f o
Article history: Received 23 February 2010 In final form 3 June 2010 Available online 8 June 2010
a b s t r a c t We studied the adsorption and reaction of sulfur dioxide on clean and oxygen precovered platinum surfaces by in situ high resolution X-ray photoelectron spectroscopy and discuss earlier, contradicting results. On the clean surface we find flat lying and upright standing SO2, adsorbing in similar amounts at low temperature. Upon heating, the ratio changes strongly favoring the standing SO2; subsequently almost all SO2 desorbs. On the oxygen precovered surface the standing SO2 is preferably populated and even at low temperatures a direct reaction of SO2 to SO3 occurs. Upon heating, SO2 oxidation to SO3 and subsequently to SO4 is found. Ó 2010 Elsevier B.V. All rights reserved.
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 [1– 5], 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 many studies in the past, which aimed at a fundamental understanding of the underlying processes [6–19]. However, despite these efforts the complex chemistry of sulfur oxides on platinum surfaces is still not completely understood, with some of the results even being contradictory. There is consensus in the literature that on clean Pt(1 1 1) SO2 adsorbs molecularly in two different conformations at 120 K. The two species have been identified from their different binding energies in high resolution X-ray photoelectron spectroscopy (HR-XPS) [14], and from near edge X-ray absorption fine structure (NEXAFS) measurements they have been assigned as flat lying and upright standing SO2 [14]. Density functional theory (DFT) based calculations by Lin et al. [13] confirm these experimental results. Temperature programmed desorption (TPD) measurements of SO2 on Pt(1 1 1) show that for small initial coverages SO2 desorbs around 370 K and for higher initial coverages the main desorption peak is found at 320 K [6,9,17]. Upon heating SO2 multilayers to room temperature the formation of SO and SO4 species has been proposed by high resolution electron energy loss spectroscopy (HREELS) [16]; on the other hand from HR-XPS, the formation of S and SO4 has been reported [14] under comparable conditions, with the SO4 species being stable up to at least 370 K. On oxygen precovered Pt(1 1 1) again two SO2 species have been identified by HR-XPS [10], with binding energy values corre* Corresponding author. E-mail address: [email protected] (C. Papp). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.06.007
sponding well to those on clean Pt(1 1 1) [14]; in the same study the formation of an SOx species is found at 200 K, which the authors assign to SO4, although it starts to already decompose at 280 K, leaving only atomic S on the surface at 340 K [10]; interestingly the S 2p3/2 binding energy of this SO4 species is not consistent with values reported in the literature [14,19]. The thermal evolution of SO2 on O-precovered Pt(1 1 1) was also 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 K [6,17,18] (note that SO3 is difficult to identify in the mass spectrometer, as the dominating cracking fragment is SO2 [20]). This high temperature peak was associated with the decomposition of SO4. These TPD results are in contradiction to the HR-XPS study reporting SO4 decomposition to be completed at much lower temperatures. Because of these conflicting results, we revisited the adsorption and reaction of SO2 on a clean and an oxygen precovered Pt(1 1 1) surface and performed a detailed in situ X-ray photoelectron spectroscopy study. This technique allows the identification of adsorbed species during adsorption and also during heating of the surface, with the latter method being called temperature programmed (TP)-XPS [21]. The experiments were performed using a transportable apparatus [22] at the third generation synchrotron source BESSY II in Berlin, Germany at beamline U49/2 PGM-1. In order to obtain very high resolution in the S 2p region, we decided to use a comparably low photon energy of 260 eV. This has the advantage of also being able to identify minority species, but the disadvantage that at the low kinetic energy of 95 eV photoelectron diffraction effects play a role, hampering the quantitative analysis (see below). The overall resolution in the S 2p region was 140 meV, as determined by fitting the Fermi edge. XP spectra were collected in 10 s per spectrum at normal emission. SO2 was dosed from the background at pressures of 2–5 109 mbar. S
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coverages were calibrated by comparison to S 2p spectra from a p(2 2) (0.25 ML) overlayer of S on Pt(1 1 1), prepared by dosing of H2S and subsequent heating to 700 K with H2 desorbing and S remaining on the surface [23]. Fig. 1a shows selected S 2p spectra recorded during SO2 adsorption at 130 K on clean Pt(1 1 1). 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. The separation of the 2p3/2 and 2p1/2 peaks is 1.2 eV and their ratio in intensity is 2:1, as expected from the spin multiplicity (this holds for all SOx species (x = 0–4) observed in this study). During SO2 adsorption the overall shape of the spectra does not change, i.e. the two S 2p3/2 components have always nearly the same height. The doublet at 164.5 (165.7) eV is assigned to a flat lying SO2 species, the one at 165.4 (166.6) eV to the upright standing SO2 species, according to literature [14]. Only at high coverages a very weak additional feature at 166.8 (168.0) eV can be seen, which is assigned to the adsorption of multilayers. Furthermore, at 162.2 (163.4) eV a minor contribution is visible, which is assigned to elemental sulfur [14,24] generated by beam damage. To investigate the influence of preadsorbed oxygen, the Pt(1 1 1) surface was saturated with oxygen at 130 K and subsequently heated to 300 K. This procedure gives a p(2 2) oxygen overlayer with a oxygen coverage of 0.25 ± 0.02 ML [25,26]; the oxygen coverage was checked by XPS, collected with a photon energy of 650 eV. In Fig. 1b selected S 2p spectra recorded during SO2 adsorption at 130 K on this oxygen precovered surface are shown. Again the two doublets at 164.5 (165.7) eV and 165.4 (166.6) eV, due to flat lying and upright standing SO2, respectively, are the most prominent features, similar to the clean surface. However, the comparison of spectra with a similar coverage (see bold black and dotted black spectra in Fig. 1) shows that the spectral shape is very different from that of the spectra without O precoverage. This is mainly due to the significantly smaller intensity of the flat
lying SO2 (164.5/165.7 eV) on the O-precovered surface. Furthermore, two additional doublets are found, indicating two additional S containing species: The doublet with the S 2p3/2 (S 2p1/2) component at 166.0 (167.2) eV, marked by the green lines in Fig. 1b, is assigned to SO3, in agreement to literature [24]; note that for this species the 2p1/2 component (167.2 eV) is better resolvable, since the 2p3/2 peak (166.0 eV) overlaps with other peaks. The other new doublet at 165.2 (166.4) eV, marked by the purple lines in Fig. 1b, is tentatively assigned to a third SO2 species in a different environment, due to lateral interactions with the preadsorbed oxygen (see also Supporting Information). Similar to clean surface, on the oxygen precovered surface the shape of the spectra initially does not change with increasing SO2 dose. Only for high SO2 doses, a doublet at 166.7 (167.9) eV due to SO2 multilayers is observed, as on clean Pt(1 1 1). A close look at the low binding energy region reveals that a very small amount of atomic S, as evident from the doublet at 162.2 (163.4) eV is present; an additional small species at 163.5 (164.7) eV is assigned to SO; this assignment is based on its binding energy value between S and the SO2 species. This SO species is only formed when SO2 multilayers are present and most likely resulting from beam damage due to the high intensity of the synchrotron radiation [10]. For a quantitative analysis the spectra in Fig. 1a and b were fitted (for details see Supporting Information) and the corresponding results are displayed in Fig. 2a and b for clean and oxygen precovered Pt(1 1 1), respectively. On clean Pt(1 1 1), both the flat lying and the upright standing SO2 species are occupied simultaneously, with nearly identical intensities and a total saturation coverage of 0.25 ML. This value was obtained by comparing the S 2p area with the one of a S p(2 2) superstructure on Pt(1 1 1); note that all coverages are derived neglecting photoelectron diffraction effects, which can be different for the SOx species (further discussion see below). At exposures higher than 0.3 L the adsorption of SO2 in multilayers begins.
Fig. 1. Selected S 2p spectra collected during adsorption of SO2 on (a) Pt(1 1 1) and (b) oxygen precovered Pt(1 1 1). The long vertical lines indicate the energetic positions of the 2p3/2 peaks, the short ticks those of the 2p1/2 peaks of the SOx species present on the surface. Photon energy: 260 eV; detection at normal emission.
Fig. 2. Quantitative analysis of the spectra in Fig. 1: (a) adsorption on the clean Pt(1 1 1) surface; (b) adsorption on the oxygen precovered surface.
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On O-precovered Pt(1 1 1) the three SO2 species and the SO3 species increase with nearly constant relative coverages up to an exposure of 0.16 L (Fig. 2b). The relative coverages at 0.16 L amount to 42% upright standing SO2, 25% flat lying SO2, 16% oxygen-influenced SO2 and 16% SO3. At higher exposures, the formation of multilayers begins; at the same time the signals due to upright standing and flat lying SO2 rise slightly while those of oxygen-influenced SO2 and SO3 show a stronger increase. At 0.43 L the total coverage of the three SO2 and the SO3 species is 0.20 ML. The minor amounts of S and SO (below 1%) observed during the SO2 adsorption experiments are not shown in Fig. 2 for clarity (however, they correspond to first points of the TP-XPS experiments in Fig. 5). The immediate formation of SO3 indicates reaction of SO2 and oxygen already at low coverages and temperatures. Lin et al. investigated this reaction by density functional theory (DFT) calculations [12]. They found that the reaction of adsorbed SO2 and O via a Langmuir–Hinshelwood mechanism has an activation energy of 42 kJ/mol and the direct reaction of gas phase SO2 with preadsorbed oxygen via a Eley–Rideal mechanism has a lower value of only 25 kJ/mol. This could contribute to the significant oxidation of SO2 even below 130 K. Summarizing the adsorption behavior of SO2 on clean and oxygen precovered Pt(1 1 1) surface, we find that on the clean surface almost all SO2 adsorbs intact and in a constant, almost 1:1 ratio between flat lying and upright standing molecules; on the oxygen precovered surface a part of the SO2 reacts to SO3 already at 130 K and the ratio of flat lying to upright standing SO2 is strongly shifted towards the upright standing SO2 species. To investigate the thermal evolution of the adsorbed layers we performed temperature programmed XPS experiments with a constant heating rate of 0.5 K/s. Fig. 3a shows selected S 2p spectra recorded during heating of the SO2/Pt(1 1 1) layer, Fig. 3b a colorcoded density plot of all S 2p spectra, which allows to derive more
detailed information. The corresponding quantitative analysis is found in Fig. 5a. After desorption of the multilayers below 170 K, the doublet at 165.4 (166.6) eV due to standing upright SO2 grows at the expense of that at 164.5 (165.7) eV due to flat lying SO2, so that 60% of the flat lying SO2 is converted to upright standing SO2 at 220 K. This conversion is in good agreement with a previous HR-XPS study by Polcˇik et al., who find a stable layer of predominantly standing SO2 upon adsorption at 212 K, but could not resolve the small amount of remaining lying SO2 [14]. At higher temperatures the amount of standing SO2 decreases (Fig. 5a), while the amount of lying SO2 stays constant until 250 K, before it also starts to decrease and desorbs till 320 K. The remaining standing SO2 desorbs until 360 K, leaving a small amount of S on the surface, which amounts to less than 10% of the initially adsorbed SO2, again in agreement with the results of Polcˇik et al. [14]. In addition minor amounts of SO3 (<0.01 ML) are also formed around 280 K and disappear again at 380 K. Interestingly, Polcˇik et al. find that upon heating of thick SO2 multilayers a stable SO4 species is observed around 330 K [14], which is not the case in this study as the temperature dependent experiment started only with the onset of multilayers (0.05 ML). The thermal evolution of the SO2 layer on O-precovered Pt(1 1 1) was also followed by TP-XPS. Fig. 4a shows selected S 2p spectra, Fig. 4b the color-coded density plot of all S 2p spectra and Fig. 5b the corresponding quantitative analysis. After desorption of the multilayers until 150 K, the doublets due to oxygeninfluenced SO2 (165.2/166.4 eV) and upright standing SO2 (165.4/ 166.6 eV) decrease in intensity until they have vanished at 300 K due to desorption or reaction. The doublet due to flat lying SO2 (164.5/165.7 eV) first increases slightly until 170 K and then starts to decrease around 250 K. The quantitative analysis indicates that a part of the upright standing or oxygen-influenced SO2 is converted to flat lying SO2 and that no SO2 is found on the surface at 300 K.
Fig. 3. Selected S 2p spectra of the TP-XPS experiment on the (a) clean surface shown as (a) waterfall plot and (b) color-coded density plot to the experiment shown in (a). Photon energy: 260 eV; detection at normal emission. The long vertical lines indicate the energetic positions of the 2p3/2 peaks of the various species, the short ticks those of the 2p1/2 peaks.
Fig. 4. Selected S 2p spectra of the TP-XPS experiment on the oxygen precovered surface shown as (a) waterfall plot and (b) color-coded density plot to the experiment shown in (a). Photon energy: 260 eV; detection at normal emission. The long vertical lines indicate the energetic positions of the 2p3/2 peaks of the various species, the short ticks those of the 2p1/2 peaks.
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Fig. 5. Quantitative analysis of the spectra of Figs. 3 and 4: (a) TP-XPS of SO2 on the clean Pt(1 1 1) surface; (b) TP-XPS of SO2 on the oxygen precovered surface.
Upon heating, also the SO3 species (166.0/167.2 eV) increases in intensity and reaches its maximum at 280 K (see green spectrum in Fig. 4). Between 230 and 300 K the total S 2p intensity decreases significantly, indicating desorption of SOx species around 270 K. Around 300 K a new doublet at 166.8 (168.0) eV shows up, which is assigned to SO4 by comparison to literature [14,24]. This indicates that on the oxygen precovered surface oxidation of SO2 to SO3 and subsequently SO4 occurs in significant amounts. Above 450 K SO4 is decomposing to SO3, which desorbs; the resulting oxygen subsequently reacts with sulfur to SO2 or SO3, which again desorbs. At 520 K there is no S containing species left on the surface, only a small amount of oxygen (0.05 ML) remains. The detailed inspection of Fig. 5b reveals that even below 300 K small amounts of SO4, but also SO are present on the surface, which were formed during desorption of the multilayers, possibly due to synchrotron radiation damage. The small amount of SO4 only shows up in the quantitative analysis after the multilayers have desorbed. Since the XPS peaks of SO2 multilayers and of SO4 have similar binding energies, the exact onset of SO4 formation can not be determined. The SO signal decreases at 280 K and at the same time the S signal starts to grow indicating dissociation of SO. If we now compare the results of the two TP-XPS experiments on clean and oxygen precovered Pt(1 1 1) we find that on the clean surface a conversion of 60% of flat lying to upright standing SO2 takes place above 150 K, whereas on the oxygen precovered surface some conversion occurs in the opposite direction, from standing and oxygen-influenced to lying SO2. On the clean surface no SO2 is found above 350 K, mainly due to desorption and to a much lesser extent (<10%) to disproportionation to S and SO3. On the oxygen precovered surface no SO2 is found on the surface at 290 K, due to desorption of SOx and/or reaction of SO2 to SO3; up to this temperature the total coverage is reduced by 30%, and 0.11 ML SO3 have been formed. Further heating leads to the sub-
sequent nearly complete oxidation to SO4 which starts to decompose at 450 K to finally be gone at 510 K. The total amount of SO4 formed by oxidation of SO2 is 0.09 ML or 46% of the initial amount of SO2 and SO3. As one needs two oxygen atoms for the oxidation of SO2 to SO4 the amount of 0.25 ML of oxygen initially present on the surface would be sufficient for this complete conversion. However, the fact that we also observe some atomic S indicates some dissociation/disproportionation of SO2 and/or partly SO3 leading to S and SO4. From the fact that after heating to 520 K only 0.05 ML oxygen is left on the surface, i.e. 0.20 ML oxygen is consumed, we have to conclude that the main desorption product is SO3 rather than SO2. However, we have to note here that from previous studies of S oxidation we know that diffraction of the low kinetic energy photoelectrons can play a major role in the quantitative analysis of different SOx species [24], imposing a certain error on the absolute coverage values for the different species because of the strongly differing local surrounding (see also [24,27]). Note that recombinative oxygen desorption occurs at much higher temperatures [6]. These results for the oxygen precovered surface are in strong contradiction to the HR-XPS study of Lee et al., who found only one SOx species [10,28], which disappeared from the surface until 320 K and was assigned to SO4. Interestingly, the thermal stability of this species and its binding energy mimic the behavior of the species that was assigned to SO3 in our study. To compare different XPS studies, we use the S 2p3/2 binding energy of flat lying SO2, which is clearly observed in all studies, as reference and calculate the binding energy difference to the other SOx species (with x = 0– 4); this has the advantage that different calibrations of the absolute energy scale do not influence the analysis. For the second SO2 species, SO3 and S we find that the values are in very good agreement (±0.2 eV) for all three HR-XPS studies on Pt(1 1 1) [10,14] and also with the values for SO2 found on Pt(1 1 0) [19]. However, for SO4, the difference of 1.4 eV observed by Lee et al. deviates significantly (by 0.9–1.1 eV) from all other studies [14,19] and the present study (see Table 1), but fits very well to the values reported for SO3 (0.1 eV). Also the thermal stability of the SOx species observed by Lee et al. matches the behavior of SO3 observed in this study, and is in contradiction to the thermal stabilities of SO4 deduced from TPD studies [6,17]. Lee et al. do not observe any S containing species other than elemental sulfur above 340 K. We find that at this temperature the further oxidation of SO3 to SO4 occurs and SOx is observed up to temperatures of 500 K. In addition, there is also a contradiction in the quantitative analysis in the paper of Lee et al. After multilayer desorption they find a total SO2 coverage of 0.26 ML, which is oxidized quantitatively to finally yield 0.26 ML of SO4; this would require 0.52 ML atomic oxygen; however only 0.25 ML could have been available, which is very close to the amount required to form quantitatively SO3 instead of SO4. Altogether, this leads to the assumption that the oxygen coverage in the case of Lee et al. [14] was even significantly lower than
Table 1 Relative S 2p binding energies of SOx species found on platinum surfaces.
DESO2 ð1yÞ=SOx in eV
SOx species S
SO
SO2 (ly)
SO2 (st)
SO2 (ox)
SO3
SO4
This work SO2/O/Pt(1 1 1)
2.3
1.0
0.9
0.7
1.5
2.3
Polcˇik [14] SO2/Pt(1 1 1)
2.5
Zebisch [19] SO2/Pt(1 1 0)
2.7
Lee [10] SO2/O/Pt(1 1 1)
2.6
164.5 0 164.0 0 163.7 0 164.1 0
1.1
0.8 0.9 0.9
2.5 1.5
2.3 1.4
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0.25 ML. This would explain the absence of SO4 formation, which is stable up to 500 K, and also the absence of SOx desorption at higher temperatures, which was found in TPD studies [6,17,18] and in the present study. At this point we want to address the accuracy of the denoted coverages in previous studies and also in our study. The coverages of all S containing species on the clean and oxygen precovered surface were determined by comparison to the (2 2) S layer with the well known coverage of 0.25 ML. For the clean surface we find a SO2 coverage of 0.24 ML after heating to 200 K. When we use the O 1s spectrum corresponding to the (2 2) O layer as reference, we obtain a larger value of 0.38 ML; the corresponding values for the oxygen precovered surface are 0.20 ML (S 2p) and 0.32 ML (O 1s). The amount of 0.38 ML SO2 for the clean surface is in reasonable agreement with the value 0.43 ML reported by Polcˇik et al. upon adsorption at 212 K, which was also derived by comparison to the O 1s spectrum [14]. The difference is attributed to differences in photoelectron diffraction at the low kinetic energies used. After careful considerations we decided to use the coverage calibration based on the sulfur XP spectra due to the following reasons: (1) From steric reasons a coverage of 0.4 ML of flat lying and upright standing SO2 appears too large; (2) larger changes in the local environment are expected for oxygen than for sulfur in the adsorbed molecular species as compared to the atomic species; (3) the O 1s-derived large value for the oxygen precovered surface of 0.32 ML SO2 would not be in line with the fact that 46% of it (0.15 ML) are converted to SO3 and subsequently to SO4, which would require 0.30 ML atomic oxygen, with 0.25 ML being available. Nevertheless, one has to state that we cannot rule out that the actual coverages might be somewhat different from the denoted ones due to photoelectron diffraction effects, which might also be different for different SOx species. This is for instance also indicated by the fact that the exact ratio between flat lying and upright standing species in our study is not identical to the one observed previously [10,14], which is attributed to the different photon energies used to measure the S 2p spectra, i.e. 260 eV here and 400 eV in Ref. [10]. In summary, we investigated the adsorption and reaction of SO2 on clean and oxygen precovered surfaces with high resolution in situ X-ray photoelectron spectroscopy. The adsorption on the clean surface shows a flat lying and an upright standing SO2 species adsorbing in a constant ratio. This ratio changes significantly in the coadsorption situation, showing mainly the upright standing SO2 geometry. In addition a third SO2 species induced by the coadsorbed oxygen is observed. Interestingly, a significant amount of SO2 directly reacts to SO3 upon adsorption at 130 K, showing the high reactivity of this system. The thermal evolution on the clean surface shows a temperature-induced conversion of the flat lying SO2 into the upright standing SO2, but less than 10% disproportionation to S and SO3. For the surface precovered with oxygen, 46% of the initially adsorbed SO2 are converted to adsorbed SO3 and then subsequently to SO4, which is stable on the surface up to 500 K.
This study is giving a complete and unifying picture on the reaction and adsorption of SO2 on Pt(1 1 1), allowing to understand the adsorption properties at low temperatures, the thermal evolution and the influence of preadsorbed oxygen. Acknowledgement This work has been supported by the Excellence Cluster ‘Engineering of Advanced Materials’ granted to the University of Erlangen-Nuremberg. We also thank the BMBF for their financial support through grant 05 ES3XBA/5 and the BESSY staff for their support during beamtime. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2010.06.007. References [1] G. Ertl, H. Knözinger, J. Weitkamp, Handbook of Heterogeneous Catalysis, VCH, Weinheim, 1997. [2] J.A. Rodriguez, D.W. Goodman, Surf. Sci. Rep. 14 (1991) 1. [3] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [4] K.C. Taylor, Catal. Rev. Sci. Eng. 35 (1993) 457. [5] J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VHC, New York, 1997. [6] S. Astegger, E. Bechtold, Surf. Sci. 122 (1982) 491. [7] M. Höfer, S. Hillig, H.-W. Wassmuth, Vacuum 41 (1990) 102. [8] U. Köhler, H.-W. Wassmuth, Surf. Sci. 117 (1982) 668. [9] U. Köhler, H.-W. Wassmuth, Surf. Sci. 126 (1983) 448. [10] A.F. Lee, K. Wilson, A. Goldoni, R. Larciprete, S. Lizzit, Surf. Sci. 513 (2002) 140. [11] X. Lin, K.C. Hass, W.F. Schneider, B.L. Trout, J. Phys. Chem. B 106 (2002) 12575. [12] X. Lin, W.F. Schneider, B.L. Trout, J. Phys. Chem. B 108 (2004) 13329. [13] X. Lin, W.F. Schneider, B.L. Trout, J. Phys. Chem. B 108 (2004) 250. [14] M. Polcik, L. Wilde, J. Haase, B. Brena, G. Comelli, G. Paolucci, Surf. Sci. 381 (1997) L568. [15] J.A. Rodriguez, T. Jirsak, S. Chaturvedi, J. Hrbek, J. Am. Chem. Soc. 120 (1998) 11149. [16] Y.-M. Sun, D. Sloan, D.J. Alberas, M. Kovar, Z.-J. Sun, J.M. White, Surf. Sci. 319 (1994) 34. [17] K. Wilson, C. Hardacre, C.J. Baddeley, J. Lüdecke, D.P. Woodruff, R.M. Lambert, Surf. Sci. 372 (1997) 279. [18] K. Wilson, C. Hardacre, R.M. Lambert, J. Phys. Chem. 99 (1995) 13755. [19] P. Zebisch, M. Stichler, P. Trischberger, M. Weinelt, H.-P. Steinrück, Surf. Sci. 371 (1997) 235. [20] C.G. Vayenas, H.M. Saltsburg, J. Catal. 57 (1979) 296. [21] A. Baraldi, G. Comelli, S. Lizzit, D. Gocco, G. Paolucci, R. Rosei, Surf. Sci. 367 (1996) L67. [22] R. Denecke, M. Kinne, C.M. Whelan, H.-P. Steinrück, Surf. Rev. Lett. 9 (2002) 797. [23] R.J. Koestner, M. Salmeron, E.B. Kollin, J.L. Gland, Surf. Sci. 172 (1986) 668. [24] R. Streber, C. Papp, M.P.A. Lorenz, A. Bayer, R. Denecke, H.-P. Steinrück, Angew. Chem. Int. Ed. 48 (2009) 9743. [25] M. Kinne, T. Fuhrmann, J.F. Zhu, C.M. Whelan, R. Denecke, H.-P. Steinrück, J. Chem. Phys. 120 (2004) 7113. [26] H. Steininger, S. Lehwanld, H. Ibach, Surf. Sci. 123 (1982) 1. [27] M. Kinne et al., J. Chem. Phys. 117 (2002) 10852. [28] A.F. Lee, K. Wilson, A. Goldoni, R. Larciprete, S. Lizzit, Catal. Lett. 78 (2002) 379.