Mechanism of Sulfur Poisoning and Storage: Adsorption and Reaction

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Mechanism of Sulfur Poisoning and Storage: Adsorption and Reaction of SO2 with Stoichiometric and Reduced Ceria Films on Cu(111) Markus Happel,† Yaroslava Lykhach,*,† Nataliya Tsud,‡ Tomas Skala,§ Kevin C. Prince,§,|| Vladimír Matolín,‡ and J€org Libuda†,^ †

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Lehrstuhl f€ur Physikalische Chemie II, Friedrich-Alexander-Universit€at Erlangen-N€urnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holesovickach 2, 18000 Prague, Czech Republic § Sincrotrone Trieste SCpA, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy IOM, Strada Statale 14, km 163.5, 34149 Basovizza-Trieste, Italy ^ Erlangen Catalysis Resource Center, Friedrich-Alexander-Universit€at Erlangen-N€urnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ABSTRACT: We have performed a systematic synchrotron radiation photoelectron spectroscopy (SR-PES) study of the interaction of SO2 with stoichiometric CeO2(111) and reduced CeO2 x films supported on Cu(111). Resonant photoelectron spectroscopy (RPES) was utilized to monitor changes in the cerium oxidation states. Adsorption of SO2 at 150 and 300 K results mostly in formation of sulfite (SO32 ) species on both reduced and stoichiometric ceria. Other species such as sulfate (SO 4 2 ), SO 2 chemisorbed at cationic sites, anionic SO2 , atomic sulfur, surface sulfides, and bulk oxy-sulfides were detected. The origin of these species is discussed in relation to the changes in the oxidation state of ceria cations at the surface. The thermal evolution of the species formed upon adsorption at 150 and 300 K was studied systematically on both the stoichiometric and the reduced films. It was found that the sulfite partially desorbs to release SO2 and partially decomposes. In general, the reduced CeO2 x surface was found to be much more active toward cleavage of S O bonds, as indicated by a higher amount of oxy-sulfide species produced. The decomposition pathways can be monitored via changes of the ceria oxidation state and are discussed in relation to the stoichiometry of the ceria surface.

1. INTRODUCTION Sulfur oxides (SOx) are among the most efficient poisons in heterogeneous catalysis. In many processes the influence of SOx on the catalytic activity is detrimental, such as for threeway catalysts where SOx removal from the exhaust stream represents a major challenge.1 5 The high affinity of many catalytic materials toward SOx can, however, also be utilized for sulfur removal. For example, regenerable SOx traps using suitable trapping materials can be introduced upstream of the catalytic converter.6,7 Ceria-based materials are potential candidates for such applications.8 10 The technological importance of ceria and its interaction with SO2 has stimulated intense research in the field.11 21 Contradictory results with regard to the nature of the formed species, their stability, and the decomposition pathways on ceria have been published. Often it is reported that sulfur dioxide has a negative effect on the oxygen-storage capacity of ceria due to the formation of strongly bound sulfate (SO42 ) species.17 Formation of sulfates (SO42 ) has been reported following adsorption of SO2 on ceria powders18 20 and polycrystalline films.19 21 At the same time, sulfites (SO32-) have been observed mostly on reduced (CeO2 x, Ce2O3+x) polycrystalline19 or metallic films.19 r 2011 American Chemical Society

Rodriguez et al.19 concluded that, in the presence of oxygen vacancies, the formation of sulfite is favored at the expense of sulfate formation. Overbury et al.,21 on the other hand, showed that the only species formed by adsorption of SO2 on stoichiometric CeO2(111)/Ru(0001)21 thin film is the sulfite. These experimental results were recently corroborated in a theoretical study by Lu et al.,22 who provided a clear picture of the interaction of SO2 with stoichiometric and reduced (111) and (110) surfaces of ceria. The latter authors questioned the direct formation of sulfate on stoichiometric CeO2(111) mainly based on structural arguments, as the distance between two adjacent oxygen surface atoms is too long to accommodate the surface sulfate. Instead it was suggested that sulfate formation should occur from sulfites by creation of an oxygen vacancy nearby, a process that should involve substantial thermal activation. In agreement with this, Smirnov et al.20 demonstrated formation of sulfate during SO2 adsorption above 600 K on the samples where sulfite was the dominant species below 500 K. However, Received: July 22, 2011 Revised: September 2, 2011 Published: September 06, 2011 19872

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The Journal of Physical Chemistry C

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Figure 1. S 2p spectra during SO2 adsorption on stoichiometric CeO2(111)/Cu(111) at 150 (a) and 300 K (b). Fitted curves are shown as continuous lines. The spectra were acquired at photon energy 220 eV. The intensities of the spectral components as a function of SO2 exposure are displayed for 150 (c) and 300 K (d).

it should be noted that this conclusion is in disagreement with the studies of Rodriguez et al.,13,14 who reported formation of sulfates on the surface of stoichiometric single crystal CeO2(111), even upon adsorption of SO2 at 150 K. In general, it should be noted that some contradictory results in these studies could arise from difficulties associated with charging effects on oxide single crystals or powder samples, inaccurate binding energy (BE) calibration, and a high risk of radiation damage when using synchrotron radiation, especially

on insulator surfaces. In fact, a close inspection of the literature reveals a wide variation of BEs reported for sulfate and sulfite species on ceria.13 15,18 21 The complexity of SO2 interaction with ceria under oxidizing and reducing conditions has been demonstrated by Ferriz et al.23 These authors presented a Ce S O phase diagram showing a variety of compounds formed depending on the stoichiometry of ceria and the partial pressures of oxygen and sulfur dioxide. The complex set of compounds includes sulfates, oxy-sulfides, 19873

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The Journal of Physical Chemistry C and bulk cerium sulfide. The tendency for oxy-sulfide and sulfide formation increases with an increasing degree of ceria reduction.23 Facile decomposition of SO2 with formation of cerium oxy-sulfide was also reported by other authors,13,14,21 similar to the generation of atomic sulfur19 on reduced ceria. Liu et al.15 studied SO2 adsorption on Ce1 xZrxO2(111) model catalysts by means of synchrotron radiation photoelectron spectroscopy (SR-PES). They observed an even more complex set of compounds upon adsorption of SO2 on highly reduced surfaces. They concluded that Ceδ+ species (δ e 3) play a key role in the cleavage of S O bonds, which leads to a higher amount of atomic sulfur formed on reduced surfaces. To provide a detailed understanding of the complex sulfur-oxo chemistry on ceria surfaces, careful studies on well-defined model surfaces are clearly required. In the present work, we report the results of a systematic SR-PES and resonant photoelectron spectroscopy (RPES) study of the interaction of SO2 with stoichiometric and reduced ceria thin films on Cu(111).24 27 In a unique fashion, these experiments provide combined information on the sulfur oxygen surface species and on the related changes in the ceria surface oxidation state, thus, yielding a much clearer picture of the SO2/ceria surface chemistry.

2. EXPERIMENTAL SECTION High-resolution SR-PES was performed at the Materials Science Beamline at the Elettra synchrotron facility in Trieste, Italy. The radiation source was a bending magnet that produces synchrotron light in the energy range of 21 1000 eV. The ultrahigh vacuum end-station (base pressure 1 10 10 mbar) was equipped with a multichannel electron energy analyzer (Specs Phoibos 150), a rear view Low Energy Electron Diffraction (LEED) optics, an argon sputter gun, and a gas inlet system. The basic setup of the chamber includes a dual Mg/Al X-ray source used for the energy calibration of the synchrotron light and off-line work. Additionally, electron-beam evaporator for Ce deposition was installed. A single crystal Cu(111) disk (MaTecK) was used as a substrate to grow the stoichiometric (CeO2) and partially reduced (CeO2 x) thin films of ceria. First, Cu(111) was cleaned by several cycles of Ar+ sputtering (at 300 K for 60 min) and annealing (723 K for 5 min) until no traces of carbon or any other contaminant were found in the photoelectron spectra. Epitaxial layers of CeO2 were prepared on clean Cu(111) by physical vapor deposition of Ce metal (Goodfellow, 99.99%) in an oxygen atmosphere (pO2 = 5 10 7 mbar, Linde, 99.995%) at 523 K, followed by annealing of the films at 523 K in an oxygen atmosphere at the same pressure for 10 min. The preparation method24 27 yields a continuous, stoichiometric CeO2(111) film with a thickness in the range of 1.9 2.1 nm, as determined from the attenuation of the Cu 2p3/2 intensity. LEED observations on the prepared films confirm the epitaxial growth of CeO2(111) with the characteristic (1.5 1.5) superstructure in relation to the Cu(111) substrate.26 According to our previous STM studies, the flat CeO2(111) terraces are separated by a characteristic step structure and are partially covered by small ceria aggregates.24 Partially reduced ceria films (CeO2 x) were prepared by exposing stoichiometric CeO2 films to 15 L (1 L = 1.33 10 6 mbar s) of methanol by backfilling the chamber at a sample temperature of 700 K, followed by annealing to 700 K in UHV for 25 min. The procedure yielded partially reduced CeO2 x films, where x was in the range of 0.30 0.32. Facile reduction of CeO2 films most likely

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results from its surface morphology, giving rise to enhanced reactivity.28,29 Valence band spectra were acquired at three different photon energies, 115.0, 121.4, and 124.8 eV, and core level spectra of S 2p were acquired at 220 eV from CeO2 x/Cu(111) samples. The binding energies (BEs) in the spectra acquired by means of synchrotron radiation were calibrated with respect to the Fermi level. Additionally, Al Kα radiation (1486.6 eV) was used to measure the core levels of O 1s, Ce 3d, and Cu 2p3/2. All spectra were acquired at constant pass energy and at an emission angle for the photoelectrons of 20 and 0 with respect to the sample normal while using the X-ray source or synchrotron radiation, respectively. The total spectral resolutions achieved were 1 eV with Al Kα (1486.6 eV) and 150 200 meV with synchrotron radiation. The core level spectra of S 2p were fitted with Voigt profile after subtraction of a linear background. During the experiment the sample temperature was controlled by a DC power supply passing a current through the Ta wires holding the sample. Temperatures were measured by a K-type thermocouple attached to the rear surface of the sample. Stable temperature and fast cooling after the annealing steps were achieved by simultaneous resistive heating and cooling of the manipulator with liquid nitrogen. The investigated samples were exposed to total doses of SO2 (Linde, 99.98%) of 2 and 50 L at 150 and 300 K, respectively. SO2 was dosed by backfilling the UHV chamber.

3. RESULTS AND DISCUSSION 3.1. SO2 Adsorption on Stoichiometric CeO2/Cu(111) at 150 and 300 K. The evolution of the S 2p spectra obtained from

stoichiometric CeO2(111)/Cu(111) films after exposure to increasing doses of SO2 at 150 and 300 K is displayed in Figure 1a,b. Three main components are observed in the S 2p region after exposure. For a SO2 dose of 2 L at 150 K, these features are centered at 166.9, 166.1, and 162.4 eV, while for a dose of 50 L at 300 K the components are situated at 166.6, 162.8, and 161.7 eV (BEs are given for the 2p3/2 component only). These peaks dominate the spectra throughout the whole range of exposures. The high BE feature we attribute to the formation of surface sulfites (SO32-), in accordance with previously reported BE values for sulfites (see, e.g., refs 13 15, 18 21, and 30). The sulfite is the principal species formed by chemisorption of SO2 at an oxygen anion site and formally results from a Lewis acid base interaction, that is, without any change of the formal oxidation states. Sulfites have been identified on various ceria surfaces, including thin polycrystalline films,18 21 well-ordered CeO2(111)/Ru(0001),21 and on CeO2(111) single crystals.13,14 Besides sulfite formation, sulfate (SO42 ) species have often been reported on the ceria films after exposure to SO2, also under the conditions similar to those applied in our work.14,19 A recent density functional theory (DFT) study by Lu et al.,22 however, came to the conclusion that the formation of sulfates from adsorbed SO2 is highly unfavorable on the ideal CeO2(111) surface. In agreement, the present experiments clearly show that sulfate formation does not occur on regular CeO2(111) sites. Nonetheless, the presence of a small fraction of sulfate species cannot be fully excluded on the basis of the present S 2p spectra. These sulfate species could, for example, be formed at defect sites or steps, which are present at a moderately high concentration on CeO2(111)/Cu(111).31,32 Sulfate signals are usually 19874


The Journal of Physical Chemistry C found at BEs 1.1 1.5 eV higher than those of the sulfites.14,18 20 We attempted to fit the SO42 /SO32 region with two S 2p components separated by 1.0 eV. The obtained contributions from sulfite and sulfate to the S 2p region (2 L, 150 K) are shown in the Figure 1a (dashed lines). The fit is satisfactory if the intensity ratio of the SO42 /SO32 components does not exceed 0.06. We conclude that SO42 formation represents only a very minor reaction channel on stoichiometric CeO2(111), both at 150 K and at 300 K. The peak at 166.1 eV emerges in the S 2p spectra only at 150 K. Absence of this peak at 300 K indicates that it may be associated with weakly interacting species on ceria. In agreement with the literature,21 we assign this peak to SO2 chemisorbed at cerium cationic sites. Similar signals were also found on other thin oxide films and single crystals below 150 K, for example, on UO2 x/Mo,33 Cr2O3(0001)/Pt(111),34 ZnO/Pt(111),35 ZnO(0001)-Zn,35 and TiO2(110).30 The small peaks at 162.4 (150 K) and 162.8 eV (300 K) are in the BE range reported for atomic sulfur on ceria.15,19 Finally, a weak feature at 161.7 eV emerges in the S 2p spectra at 300 K only. Taking into account the absence of this feature at 150 K, we assume that higher adsorption temperatures are essential for formation of this species. This indicates an activated decomposition step, potentially including the ceria support, and we, therefore attribute this feature to a sulfide (S2 ) species. The low binding energy position of the peak matches the values reported for sulfide or oxy-sulfide15 (CeO2 xSx), where sulfur is found in the oxidation state 2 . The atomic sulfur and oxy-sulfide in the S 2p spectra on stoichiometric ceria are rather weak and they were not observed by Overbury et al.21 and Rodriguez et al.14,19 in their studies on ceria powders and CeO2(111). This suggests that SO2 decomposition may be closely related to the specific nature of defect sites present on the ceria surface. One possible pathway for the formation of atomic sulfur on stoichiometric ceria is via overall disproportionation (e.g., 3 SO2 + 2 O2 f 2 SO42 + S). It should be pointed out that net disproportionation may involve complex reaction pathways and intermediates. For example, the primary formation of sulfates could lead to formation of reduced ceria sites, at which further reaction steps may proceed. For instance, Liu et al. suggested the formation of oxy-sulfides upon decomposition of sulfates on stoichiometric ceria surfaces.15 In section 3.3 we will provide further evidence that sulfate formation could be involved in the formation of surface sulfide. The intensities of the above-discussed surface species as a function of SO2 exposure are shown in Figure 1c,d for adsorption at 150 and 300 K. The total amount of sulfur-related species formed at 150 K is about 35% larger than that at 300 K. All features display similar exposure dependence and saturate simultaneously (at about 0.8 L for 150 K and at about 5 L for 300 K). This observation suggests that the surface species formed are largely immobile and no interconversion occurs under the present conditions. Additional information on the oxidation state of the ceria films was obtained from RPES experiments. This method provides utmost surface sensitivity and has been applied previously in a series of studies on the surface chemistry of ceria surfaces.36 38 It is based on the calculation of the so-called resonance enhancement ratio (RER), which is determined from the relative intensities of Ce3+ and Ce4+ resonant features in the valence band spectrum. The RER is a direct measure of the surface ratio

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Figure 2. RER as a function of SO2 exposure on stoichiometric CeO2(111)/Cu(111) (bottom traces) and reduced CeO2 x/Cu(111) (top traces) at 150 (squares) and 300 K (circles).

of Ce3+ and Ce4+ and is commonly used to quantify the degree of ceria reduction.36 The evolution of the RER during adsorption of SO2 on stoichiometric CeO2(111)/Cu(111) at 150 and 300 K is plotted in Figure 2. Immediately after preparation, very low RER values are found (∼0.1), indicating the nearly perfect stoichiometry of the ceria thin film after preparation. Upon SO2 exposure we observe a weak increase of the RER, indicating a minor reduction of the ceria film. The effect is slightly more pronounced at 150 K than at 300 K. We suggest that this effect may be due to the formation of a small amount of sulfates at defect sites such as steps or small ceria aggregates, which are often found on the surface of the ceria thin film (see refs 24 and 31 for more details). According to Lu et al.,22 stable sulfates could easily form on other ceria surfaces (e.g., (110)) but not on CeO2(111). Creation of the sulfate leads to the reduction of two neighboring Ce4+ ions. The very weak reduction and absence of a clear sulfate feature in the S 2p region indicates, however, that the amount of sulfates formed is very small and spontaneous formation of sulfates clearly does not occur on stoichiometric CeO2(111) at low temperature. 3.2. SO2 Adsorption on Reduced CeO2 x/Cu(111) at 150 and 300 K. The development of the S 2p spectra on reduced CeO2 x/Cu(111) upon exposure to increasing doses of SO2 at 150 and 300 K is shown in Figure 3a,b. The films of reduced ceria with average stoichiometry of approximately CeO1.69 were prepared via methanol-mediated reduction (see Experimental Section for details of the procedure). In general, the spectral shapes could be reproduced by four S 2p doublets at 150 K and five S 2p doublets at 300 K. Their assignment will be discussed in the following. The development of the corresponding features is shown in Figure 3c,d as a function of SO2 exposure at 150 and 300 K. The saturation exposures of SO2 are similar to those for the stoichiometric samples at the corresponding temperatures (∼0.8 L for 150 K; ∼5 L for 300 K). Several species observed on the reduced CeO2 x/Cu(111) surface could be straightforwardly identified by comparison with the spectra obtained on the stoichiometric ceria (compare with Figure 1a,b). We assign the dominant components at 167.2 eV (150 K) and 166.9 eV (300 K) primarily to a surface SO32 19875



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Figure 3. S 2p spectra during SO2 adsorption on reduced CeO2 x/Cu(111) at 150 (a) and 300 K (b). The spectra were acquired at photon energy 220 eV. The intensities of the spectral components is displayed as a function of SO2 exposure at 150 (c) and 300 K (d).

species. As discussed in the previous section, the high BE tail of this feature may, however, also contain a contribution from SO42 . It should be noted that the SO32 /SO42 peak displays a substantial shift toward higher BE (167.2 eV) for the lower adsorption temperature. In addition, a broadening is observed in comparison with the spectra taken at 300 K and on the stoichiometric ceria film. This suggests that the SO42 contribution is largest for low-temperature adsorption on the reduced film. Similar to the procedure described for the stoichiometric ceria

film (see section 3.1), a fit could provide an estimate for the upper limit of the sulfate contribution to the region. It was found that the SO42 /SO32 does not exceed 0.15, assuming a peak separation between sulfite and sulfate of ΔBE = 1.0 eV. The corresponding fit and the individual components are shown in the S 2p spectrum (2 L, 150 K) in Figure 3a (dashed lines). The presence of multiple components in the S 2p spectrum at high binding energy was also reported by Overbury et al.,21 but these authors suggested formation of more than one type of sulfite 19876



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Figure 4. S 2p spectra during annealing of stoichiometric (a) and reduced (b) ceria pre-exposed to a total dose of 2 L of SO2 at 150 K. Fitted curves are shown as continuous lines. The spectra were acquired at photon energy 220 eV.

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Figure 5. Intensities of the spectral components and RERs during annealing of stoichiometric (a, c) and reduced (b, d) ceria, pre-exposed to a total dose of 2 L of SO2 at 150 K.

rather than considering sulfates. It is noteworthy that we observe a pronounced narrowing of the SO42 /SO32 component in the S 2p spectra following adsorption of SO2 at 300 K in comparison to adsorption at 150 K. This narrowing and shift to lower BE is accompanied by the emergence of a new peak at 160.7 eV, which we identify as an oxy-sulfide species. These observations may be interpreted to signify that one pathway for SO2 decomposition involves the initial formation of surface sulfates which at higher temperature may then undergo decomposition to sulfides. The transformation of sulfates to oxy-sulfides has previously been observed by Liu et al. on ceria-zirconia thin films.15 However, we will show in the following section that two different types of sulfates are formed on reduced ceria (see section 3.3). For sulfates formed at lower temperature, temperature-dependent experiments discussed in the following section show in fact no clear evidence for decomposition to sulfides, whereas this process is clearly observed for those sulfate species formed at elevated temperature. In analogy to the experiments on the stoichiometric film, we associate the peak at 166.1 eV (150 K) with weakly chemisorbed SO2, possibly at cerium cationic sites. Also, an atomic surface sulfur species is identified in the spectra at BEs of 162.1 (150 K) and 162.0 eV (300 K). The BE differences between the species

formed on the reduced and the stoichiometric sample can be attributed to the different chemical environment and electrostatic potential. Generally, the amount of atomic sulfur that is formed on the reduced ceria upon adsorption of SO2 at 300 K is considerably larger than that at 150 K. Most importantly it is, however, also much greater than the amount of decomposition products formed on the stoichiometric film. This observation is in accordance with the earlier literature15 that points to a crucial role of Ce3+ centers in S O bond cleavage. Apart from those species that are also observed on stoichiometric ceria, new features emerge on the reduced film. The corresponding peaks are located at BEs of 163.3 (150 K), 163.4 (300 K), and 164.4 eV (300 K). In previous work, numerous species such as SO+, S + O, SO, or Sn (n = 2, 8) have been associated with similar features21,33,39 Because the observed BEs are considerably higher than that of atomic sulfur but lower than the typical values for sulfites, we may consider intermediate oxidation states between +1 and +3. In their recent DFT study, Lu et al.22 identified the formation of anionic SO2 species on reduced CeO2 x(111) as a likely reaction pathway. This species is formed by insertion of one oxygen atom of the SO2 into the oxygen vacancy followed by transfer of one electron from the Ce3+ center to the LUMO of SO2. Such SO2 species may represent a 19878


The Journal of Physical Chemistry C first intermediate species on the pathway to S O bond cleavage. So far, no experimental evidence for the SO2 species has ever been reported on ceria, but their existence has been clearly proven on many other oxides including MgO,40 ZnO,41 TiO2,42 supported NiO/Al2O3,43,44 and MoO3/Al2O3.43 Based on these arguments, we tentatively assign the features at intermediate BE to anionic SO2 species. However, it should be pointed out that most likely more than one species is formed on the present model surface (observed BEs of 163.3, 163.4, and 164.4 eV). These BE differences could be caused either by the changes in the chemical nature of the species or by different coordination, for example, at defect or step sites. The development of the RER as a function of SO2 exposure for the reduced CeO2 x/Cu(111) is displayed in Figure 2. A massive reoxidation of the film is observed for the reaction at 150 K. At 300 K the effect is even more pronounced. This observation clearly demonstrates that the main reaction channel on the partially reduced ceria is not described by a net disproportionation, but by a net reduction of SO2. Several individual reaction channels may contribute to this reoxidation process, such as the formation of the anionic SO2 species, discussed above, the formation of atomic S, and the formation of sulfides and oxy-sulfides. At low reaction temperature (150 K), the formation of sulfides is suppressed similarly to the decomposition of the SO42 intermediate (see discussion above). This results in a weaker reoxidation at 150 K, as indicated by the RPES data. 3.3. Thermal Stability of the Species Formed by Adsorption of SO2 at 150 K on Stoichiometric CeO2(111)/Cu(111) and Reduced CeO2 x/Cu(111). In the next step, we investigate the thermal stability of the surface species formed on the surfaces of stoichiometric CeO2(111)/Cu(111) and reduced CeO2 x/ Cu(111) during adsorption of SO2 at 150 K. A series of S 2p spectra obtained during stepwise annealing of the samples are shown in Figure 4a,b. The spectra obtained after a total dose of 2 L of SO2 at 150 K are plotted at the top of the corresponding graphs. As discussed in the previous section, the features identified in these spectra are (i) SO32 (CeO2(111)/Cu(111): 166.9 eV; CeO2 x/Cu(111): 167.2 eV), containing a minor contribution from (ii) SO42-, especially on CeO2 x/Cu(111), (iii) chemisorbed SO2 at cationic sites (166.1 eV), (iv) a species with intermediate oxidation state, tentatively assigned to SO2 (163.3 eV), and (v) an atomic sulfur species (162.1 eV, 162.4 eV). The development of the corresponding peak intensities during stepwise annealing of the sample is plotted in Figure 5a,b. The related evolution of the RER is given in Figure 5c,d. Upon annealing, we observe a gradual decrease of the intensity of the dominating peak associated with SO32 , which finally vanishes on both samples at about 540 K. For the reduced ceria film, the SO32 /SO42 region shows characteristic changes of the spectral shape. Up to temperatures of 200 K, a narrowing of the peak is observed, which points toward the loss of a weakly bound SO42 species. At temperatures above 400 K a new feature in the same high BE region appears (at approximately 167.5 eV BE). We assign this feature to a second surface sulfate species with higher thermal stability. This species is formed as an intermediate via oxidation of SO32 . We will come back to this species in the discussion of the decomposition mechanism. The peak associated with SO2 adsorbed at cationic sites disappears from the S 2p spectra on both samples at about 280 K, indicating a relatively weakly chemisorbed species. For the atomic sulfur species on stoichiometric ceria at a BE of 162.4 eV,

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a peculiar behavior is observed: Before the atomic sulfur signal disappears from the spectra at about 440 K, its S 2p peak shifts to 163.0 eV, which is close to the BE value for adsorption on stoichiometric ceria at 300 K (162.8 eV). We conclude that this atomic species becomes mobile upon annealing and migrates to more favorable adsorption sites. Similar phenomena were observed by Zhao et al.45 during decomposition of SOx on TiO2 x(110). It is noteworthy that formation of the surface sulfide and oxysulfide species, the signals of which appear at lower BE, requires substantially higher reaction temperatures than formation of surface sulfur. On the stoichiometric ceria the corresponding peaks at 161.6 and 160.9 eV only appear at temperatures of 400 K and above. On the partially reduced films, the corresponding features (160.9 and 160.4 eV) are already observed around 300 K. This suggests that on the reduced film additional decomposition channels with lower activation energy are available. A clear identification of the numerous sulfur species at lower BE is complicated due to the fact that their values differ strongly on the reduced and the oxidized film. Despite the much lower BE on the reduced film, we assign the features at 160.9 and 160.4 eV to sulfide/oxy-sulfide species. With increasing temperature, the low BE components (CeO2(111)/Cu(111): 160.9 eV; CeO2 x/ Cu(111): 160.4 eV) are transformed into the higher BE components (CeO2(111)/Cu(111): 161.6 eV; CeO2 x/Cu(111): 160.9 eV). We attribute the low BE components to surface S2 species and the appearance of the higher BE components to thermally activated formation of bulk oxy-sulfides. As seen in Figure 5a,b, 20 and 60% of the total amount of the surface sulfur is converted to oxy-sulfides at 600 K on stoichiometric and reduced ceria, respectively. SO32 decomposition dominates on reduced ceria and SO2 desorption dominates on the stoichiometric film. The fact that at low temperature the surface is predominately covered by SO32 implies that the major decomposition channel is described by a net conversion of SO32 to surface sulfides and oxy-sulfides via a surface reaction, that is, without involvement of any further gas phase species. This transformation from SO32 to S2 may, however, involve different pathways and intermediates. From the RPES experiments and the temperature-dependent SR-PES data, some information on these pathways can be extracted. One aspect involves SO42 as a potential intermediate. As discussed above, we observe indications for the loss of weakly adsorbed SO42 species on reduced ceria at temperatures up to 200 K. It is noteworthy that the loss of these species upon heating is accompanied by a very weak reoxidation of the ceria only (see Figure 5d) and hardly any formation of surface S2 (see Figure 4b). These observations strongly suggest that this weakly bound SO42 is not the primary intermediate for sulfide formation. We cannot exclude the possibility that a minor fraction of this species decomposes to surface sulfur species (162.1 eV) but the weak reoxidation suggests that the major fraction of this species is either reconverted to SO32 or desorbs. The second intermediate observed on the reduced ceria film is the anionic SO2 species, associated with the peak at 163.3 eV. Indeed, this species disappears in a temperature region between 300 and 450 K, which coincides with the temperature window in which the formation of surface sulfides is largest (see Figure 5b). As a final intermediate, we observe a second strongly bound SO42 species. This species is visible on the reduced film between 420 and 540 K, a temperature region that is clearly above the stability window of the anionic SO2 . This observation 19879



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Figure 6. S 2p spectra during annealing of stoichiometric (a) and reduced (b) ceria pre-exposed to a total dose of 50 L of SO2 at 300 K. Fitted curves are shown as continuous lines. The spectra were acquired at photon energy 220 eV.

suggests that the SO42 species may be formed as a consecutive reaction involving decomposition products of SO2 /SO32 . We may speculate that part of the oxygen released during SO32 / SO2 decomposition intermittently reacts with SO32 to form surface SO42 . The remaining oxygen leads to reoxidation of the ceria support, as indicated by the RER, see Figure 5d. Finally, the high-temperature sulfate species also decomposes at temperatures exceeding 450 K. The total amount of sulfur does not decrease in the temperature window between 420 and 580 K (see Figure 5b). This suggests that decomposition of the high-temperature surface sulfate leads to formation of S2 rather than to desorption of SO3 (or SO2 and O2). Similar decomposition channels for sulfates have been suggested by Liu et al.15 for SO2 on Ce1 xZrxO2(111) films. Finally, on both samples, surface sulfides are converted into bulk oxy-sulfides. At 600 K, the bulk oxy-sulfide is the only species

on both the stoichiometric and reduced ceria. Upon annealing to even higher temperatures, the intensity of the S 2p signal from the bulk oxy-sulfide decays rapidly, which is associated with the diffusion of S2 into deeper layers. 3.4. Thermal Stability of the Species Formed by Adsorption of SO2 at 300 K on Stoichiometric CeO2(111)/Cu(111) and Reduced CeO2 x/Cu(111). We briefly describe the thermal behavior of the samples after exposure to SO2 at 300 K. The corresponding spectra of the S 2p obtained after adsorption of 50 L of SO2 at 300 K are shown in Figures 1b and 3b. Development of the S 2p spectral regions during stepwise annealing of the samples is displayed in Figure 6a,b. The spectral features and their thermal behavior can be straightforwardly understood based on the discussion in the previous section. The most prominent species on both surfaces is SO32 , and in addition, 19880



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during decomposition of SO32 to sulfides. At higher temperatures (up to 550 K), these surface sulfates also decompose to form sulfides. (7) Atomic sulfur is formed in small amounts on stoichiometric and reduced ceria, even at low reaction temperatures. This reaction channel is most likely associated with defect sites. (8) Surface sulfides (S2 ) are formed as an intermediate species of the decomposition processes. Sulfide formation starts at 300 K on reduced ceria and at substantially higher temperature (approximately 400 K) on stoichiometric CeO2(111). At temperatures around 550 K, these species are converted into bulk sulfide species, that is, cerium oxy-sulfide. At temperatures exceeding 600 K, the mobility of the S2 is sufficiently high to facilitate diffusion into deeper layers, which leads to a depletion of the sulfide concentration in the surface layers. (9) Interaction of stoichiometric CeO2(111) with SO2 gives rise to very minor changes of the cerium oxidation state only. Interaction of reduced ceria with SO2, on the other hand, leads to strong reoxidation, even at temperatures as low as 150 K. Further reoxidation occurs at temperatures above 300 K and is associated with decomposition of SO32 to surface S2 .

atomic sulfur and surface sulfides are observed. The main difference is related to the fact that the amount of atomic sulfur at 300 K is larger due to the larger SO2 dose (50 L) and higher reaction temperature. With respect to the assignment of the other features, we refer to sections 3.1 and 3.2. Similar to the situation for adsorption at 150 K (see section 3.3), we observe that the reactivity of the reduced film is much larger than that of the stoichiometric ceria. In the temperature range up to 550 K, SO32 mainly decomposes on CeO2 x/Cu(111), whereas mainly desorption is observed on CeO2(111)/Cu(111). The feature at intermediate BE, which we assigned to SO2 , decomposes at temperatures up to 500 K. Again, it is followed by the formation of more sulfates. Similar to the low-temperature adsorption experiments, these are formed primarily on the reduced film in a temperature window between 450 and 600 K. Decomposition occurs with intermediate formation of surface sulfides. The final product is the bulk oxy-sulfide formed around 550 K on the reduced film. Similar to the observations for low-temperature SO2 adsorption, the transformation of the surface sulfide to the bulk sulfide is less sharp for the stoichiometric film (compare Figures 4 and 6), most likely due to the fact that only a small amount of sulfide is generated on stoichiometric ceria.

4. CONCLUSIONS We have performed a systematic SR-PES study of the adsorption and reaction of SO2 on stoichiometric CeO2(111) and reduced CeO2 x ceria films supported on Cu(111). The thermal stability of all surface species formed was investigated in dependence of the adsorption temperature and stoichiometry of the samples. In parallel with core level photoelectron spectroscopy, RPES was used to monitor changes in the cerium oxidation state. The most important observations are summarized as below: (1) Sulfite (SO32-) is the dominant species formed on both the stoichiometric CeO2(111) and reduced ceria surfaces upon adsorption at 150 and 300 K. (2) Upon thermal treatment, sulfite partially decomposes and partially desorbs in the form of SO2. At temperatures up to 300 K, desorption dominates, and at temperatures exceeding 400 K, decomposition is the dominating process. (3) The reactivity of reduced ceria for SO2 decomposition is much larger than for the stoichiometric surface. Up to 60% of the adsorbed SO2 is observed to undergo decomposition on the reduced ceria surface, whereas decomposition represents a minority channel (20%) on stoichiometric CeO2(111). (4) Species at intermediate BEs are observed in the photoelectron spectrum, which are associated with the formation of anionic SO2 . These species are identified as an intermediate of SO2 decomposition. Conversion of these anionic SO2 species to surface sulfides occurs at temperatures exceeding 300 K. (5) Sulfate (SO42 ) is formed in small amounts, in particular, on the reduced ceria surface. Temperature-dependent experiments suggest formation of two different types of sulfates. A weakly bound sulfate species is formed already at 150 K. However, this species decomposes at temperatures below 200 K. (6) A second more strongly bound sulfate (SO42 ) species is formed on reduced ceria at temperatures exceeding 400 K. This species is formed as a product of a consecutive reaction, which most likely involves the oxygen released

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +49-9131-8528867.

’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) and by the Ministry of Education of the Czech Republic (LC06058 and LA08022). We acknowledge additional support from the DFG within the excellence cluster “Engineering of Advanced Materials” in the framework of the Excellence Initiative. M.H. gratefully acknowledges support by a grant of the “Fonds der Chemischen Industrie”. In addition, travel support by the DAAD is acknowledged. ’ REFERENCES (1) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419–449. (2) Matsumoto, S. I. Catal. Today 2004, 90, 183–190. (3) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. Catal. Today 1996, 27, 63–69. (4) Hilaire, S.; Sharma, S.; Gorte, R. J.; Vohs, J. M.; Jen, H.-W. Catal. Lett. 2000, 70, 131–135. (5) Boaro, M.; de Leitenburg, C.; Dolcetti, G.; Trovarelli, A.; Graziani, M. Top. Catal. 2001, 16, 299–306. (6) Happel, M.; Desikusumastuti, A.; Sobota, M.; Laurin, M.; Libuda, J. J. Phys. Chem. C 2010, 114, 4568–4575. (7) Carlsson, P. A.; Happel, M.; Kylhammar, L.; Libuda, J.; Gr€onbeck, H.; Skoglundh, M. Appl. Catal., B 2009, 91, 679–682. (8) Kim, G. Ind. Eng. Chem. Res. 1982, 21, 267–274. (9) Kylhammar, L.; Carlsson, P. A.; Ingelsten, H. H.; Gr€onbeck, H.; Skoglundh, M. Appl. Catal., B 2008, 84, 268–276. (10) Waqif, M.; Pieplu, A.; Saur, O.; Lavalley, J. C.; Blanchard, G. Solid State Ionics 1997, 95, 163–167. 19881


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Mechanism of Sulfur Poisoning and Storage: Adsorption and Reaction

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